- Email: [email protected]
Experimental study of condensation heat transfer on hydrophobic vertical tube
International Journal of Heat and Mass Transfer 120 (2018) 305–315
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Experimental study of condensation heat transfer on hydrophobic vertical tube Mattacaud Ramachandralal Rajkumar a,⇑, Arjunan Praveen a, Radhakrishnan Arun Krishnan a, Lazarus Godson Asirvatham b, Somchai Wongwises c,d,⇑ a
Advanced Thermo Fluid Research Lab, Department of Mechanical Engineering, College of Engineering, Trivandrum 695016, Kerala, India Department of Mechanical Engineering, Karunya University, Tamil Nadu, India Fluid Mechanics, Thermal Engineering and Multiphase Flow Research Lab. (FUTURE), Department of Mechanical Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, Bangkmod, Bangkok, Thailand d The Academy of Science, The Royal Institute of Thailand, Sanam Suea Pa, Dusit, Bangkok 10300, Thailand b c
a r t i c l e
i n f o
Article history: Received 22 September 2017 Received in revised form 4 December 2017 Accepted 4 December 2017
Keywords: Dropwise condensation Hydrophobic vertical tube Degree of sub-cooling Droplet velocity Contact angle hysteresis
a b s t r a c t Condensation heat transfer on surfaces can be enhanced by altering the surface topography. Surface modification technology can promote dropwise condensation which can exhibit higher heat transfer rate than filmwise condensation. This paper presents experimental data on the condensation of steam on vertical bare copper tubes and lead coated copper tubes for degree of sub-cooling in the range 0.5 °C DT 20 °C. The chemical texture of the tube surface was altered by coating with lead of thickness 10 mm and the physical texture of the surface was transformed by providing four grooves each having equal depths 0.10 mm, 0.15 mm and 0.30 mm. The condensation heat transfer characteristics of the tube surface is explained based on contact angle hysteresis and sliding velocity of the droplet. The results of the study reveal that for the tubes tested the average condensation heat transfer coefficient decreases with increase in degree of subcooling. It is also found that for copper tubes, providing grooves aids condensation heat transfer for the range of sub-cooling. However, for lead coated copper tube with/without grooves the heat transfer performance at DT < 2 °C shows marked difference in contrast to DT > 2 °C. Ó 2017 Published by Elsevier Ltd.
1. Introduction Vapour condensation has been a process of significant interest due to its applications in the field of power generation, water desalination and HVAC [1–3]. Mainly, there are two types of surface condensation processes- filmwise (FWC) and dropwise condensation (DWC). Typically filmwise condensation is observed on hydrophilic surfaces which has high surface energy resulting in high wettability. However, on hydrophobic surfaces with low surface energy the liquid condenses in the form of droplets causing dropwise condensation. Recently researchers have tried several modifications on surfaces to promote dropwise condensation, which lead to higher heat transfer rates compared to filmwise condensation. Dropwise condensation on surfaces can be achieved by
⇑ Corresponding authors at: Fluid Mechanics, Thermal Engineering and Multiphase Flow Research Lab. (FUTURE), Department of Mechanical Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, Bangkmod, Bangkok, Thailand (S. Wongwises). E-mail addresses: [email protected] (M.R. Rajkumar), [email protected]. th (S. Wongwises). https://doi.org/10.1016/j.ijheatmasstransfer.2017.12.019 0017-9310/Ó 2017 Published by Elsevier Ltd.
changing the surface morphology and surface chemistry [4]. Even though dropwise condensation leads to higher heat transfer, in spite of sustained research over the last decade the mechanism behind the formation of droplet and its transport is still not well understood and remains a challenge. The role of changing the chemical texture of the surface, so as to promote dropwise condensation (DWC) has been investigated by many researchers [5–12]. It is also important to mention here that few of the researchers [13–15] have also studied the influence of physical texturing on surfaces to enhance condensation heat transfer. However, recently a combination of chemical texturing along with physical texturing as a method of enhancing condensation heat transfer has also been well documented in literature [16]. A study was conducted by Izumi et al. [17] on condensation heat transfer characteristics of a vertical copper plate surface with round shaped grooves coated with ethanol solution of oleic acid. The study revealed that there exist an optimum groove width corresponding to maximum heat transfer. Experiments were conducted by Lara et al. [18] on vertical naval brass and copper plates with Ni-P-PTFE and without coatings. Ni-P-PTFE coated brass and copper plates showed better heat transfer performance.
306
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
Nomenclature A Cp d0 di hc kCu L _ m Q R r Ssurf T Tco Tci Two
effective condensation area, constant in Eq. (3) [m2] specific heat at constant pressure, [kJ/kg K] outside diameter of the cylindrical test specimen, [m] inside diameter of the cylindrical test specimen, [m] condensation heat transfer coefficient, [kW/m2 K] thermal conductivity of copper tube, [W/m K] length of the cylindrical test specimen, [m] mass flow rate of cooling water, [kg/s] heat transfer rate of cooling water, [kW] dependent variable distance measured radially, [m] surface entropy, [J/m2 K] temperature, [K] outlet temperature of cooling water, [K] inlet temperature of cooling water, [K] average outer wall surface temperature of copper tube, [K]
Experiments were conducted on dimpled titanium plate and vertical grooved copper plate by Lara et al. [19]. Copper was coated with lead containing and lead free Ni-P-PTFE while the titanium plate was kept bare. The overall heat transfer coefficient was experimentally measured for all cases. Condensation heat transfer of steam on three vertical titanium plates was investigated by Baojin et al. [20] by tuning the surface energy. Two titanium plates used for the tests were subjected to modification which include chemical etching on one plate and the other subjected to a combination of chemical etching and hydrogen peroxide solution treatment. It was observed that on unmodified surface, both filmwise and dropwise condensation was found to coexist, whereas filmwise condensation was achieved on plate modified with chemical etching and Dropwise condensation was achieved on plate subjected to combination of chemical etching and hydrogen peroxide solution treatment. Lee et al. [21] conducted experiments on micro/nano scale porous surfaces made by polyphenylene sulphide (PPS), and PTFE based polymer coatings, SAM and etching on the plain surfaces of steam condenser tubes. It was found that etched surface gives the highest transfer coefficient due to its lowest surface energy. Condensation heat transfer outside horizontal plain and finned copper tubes with different surface wettability was experimentally studied by Hu et al. [22]. Investigations were conducted on plain copper tube, hydrophobic copper tube, hybrid (hydrophilichydrophobic) finned copper tube, hybrid (hydrophilic-super hydrophobic) finned tube. The tests were conducted in presence of non-condensable gases and also in vacuum. It was reported that in the presence of non-condensable gases hybrid (hydrophilicsuper hydrophobic) hybrid finned tube achieved the highest condensation heat transfer performance. However, in vacuum hybrid (hydrophilic-hydrophobic) finned copper tube showed the highest condensation heat transfer performance. Huang et al. [23] compared the condensation heat transfer performance on a pure copper surface with a super hydrophobic-modified copper surface. The super hydrophobic surface was prepared using a hydrogen peroxide immersion and fluorosilane polymer coating. It was found that super hydrophobic modified copper surface performed better than the pure copper surface. Condensation heat transfer on tube bundles having three different surface structures plain, 2-D finned and 3-D finned tubes was tested by Hu et al. [24]. Comprehensive study on the effect of Reynolds number of cooling water, Reynolds number of air vapour mixture and volume fraction of water vapour
Twi Tvap DT Usurf V
a1 a2
ɣ
rR
rXi Xi h
average inner wall surface temperature of copper tube, [K] vapour temperature, [K] degree of sub-cooling, (Tvap-Two), [K] surface internal energy, [J/m2] volume flow rate, [l/h] receding contact angle, [deg] advancing contact angle, [deg] surface free energy, [N/m] standard deviation of independent variable, 12 Pn @R 2 2 rR ¼ i¼1 @Xi rXi standard deviation of dependent variable dependent variable contact angle hysteresis, (a2-a1)
on convective condensation heat transfer coefficient and number of tube rows was conducted. In addition to the above techniques, different types of geometrically enhanced tubes have also been used for many years to enhance condensation heat transfer. These tubes may be simple two dimensional rectangular integral fin tubes, three dimensional pin fin tubes or wire rapped tubes. The condensation heat transfer on these types of tubes has received considerable attention in heat transfer research because of its wide industrial application especially in industrial condensers. Ali and Briggs [25,26] reported data for the condensation of R-113 and ethylene glycol at near atmosphere pressure and low velocity on five three dimensional pin fin tubes. They studied the effect of circumferential pin spacing and thickness on the condensate retention on the pin fin tubes. The results of the study showed that among all the pin fin tubes considered in this work, for R 113, the best performing pin fin tube gave a heat transfer enhancement of 14% higher than the equivalent two dimensional integral fin tube, having the same fin root diameter, longitudinal fin spacing, thickness and fin height. However, for ethylene glycol, the best performing pin fin tube gave a heat transfer enhancement of 20% higher than the equivalent two dimensional integral fin tube. The same authors [27] extended their work to systematically study the influence of pin geometry on enhancement of heat transfer during condensation of ethylene glycol on eleven different pin fin tubes and plain tube. It was reported that the best performing pin fin tube gave a heat transfer enhancement of 5.5: 17% higher than obtained from optimised two dimensional fin tube reported in literature and 24% higher than equivalent two dimensional integral fin tube. Ali and Briggs [28] further reported the experimental data for condensation of ethylene glycol and R113 at near atmospheric pressure and low velocity on three identical pairs of pin fin tubes made of copper, brass and bronze. The study revealed that copper pin fin tubes showed better heat transfer performance in comparison with brass and bronze tubes. The above experimental studies of Ali and Briggs have shown that heat transfer performance of integral pin fin tubes are superior than equivalent integral fin tubes. It is worth to mention here that the major factor that affects the heat transfer from integral pin fin tubes is condensation retention or flooding as reported by few researchers [29–31]. Ali and Briggs [32] measured the liquid retention angles on 15 rectangular pin fin tubes under static conditions (without condensation) using water, ethylene glycol and R-113. It was found that condensate retention
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
angles for all pin fin tubes were larger than equivalent integral fin tubes. An expression for condensate retention angle on pin fin tubes was also proposed and found to agree with measured retention angles to within 15%. Ali and Abubaker [33] performed experiments to study the effect of vapour velocity on six horizontal pin fin tubes and equivalent integral fin tube. using water ethylene glycol and R-141b. It was observed that for all tubes tested, condensate retention angle increased with increase in vapour velocity. Also the circumferential pin spacing was found to be a significant parameter affecting the condensate retention angle regardless the effect of vapour velocity. Experimental study on condensation of water, ethylene glycol and R-141b on integral fin tubes, horizontally oriented in a vertical wind tunnel was conducted by Ali and Amanat [34]. Eight tubes of different dimensions were used with fixed fin root diameter and the internal diameter of the tubes were kept as 12.7 mm and 8.0 mm respectively. The study showed that retention angle is a strong function of fluid properties, geometric properties and vapour Reynolds number. A semi empirical correlation was brought out which showed excellent agreement with present experimental data. Ali and Briggs [35] developed a simple semi empirical correlation accounting for the combined effect of gravity and surface tension for condensation of steam, ethylene glycol and R113 on horizontal pin fin tubes. The developed model gave good overall agreement with the experimental data. Detailed comparison between the model and experimental data indicated that the model satisfactorily predicts the dependence of heat transfer enhancement on both geometric variables and fluids. Ali and Qasim [36] reported data for the condensation of steam on instrumented horizontal tube wrapped with 0.8 mm and 1.0 mm diameter wires made of copper, aluminium and brass with varying pitches of 2.0 mm, 4 mm and 6 mm. It was reported that the effect of thermal conductivity is evident for all tested pitches, however more dominant in the case of 2.0 mm pitch. Also, for both tested wires the best heat transfer enhancement compared to the plain tube are found at a pitch of 4.0 mm for all the cases of wire rapped tubes. The effect of circumferential pin thickness was investigated by Ali and Abubacker [37] on retention angle as a function of vapour velocity for four pin fin tubes using water, ethylene glycol and R-141b. The results of the study provided a guidelines that for the cases where the retention angle is less than 90 at low velocity (=0m/s) the use of pin fin tubes would not provide much help in reducing condensate retention than integral fin tubes for higher vapour velocities. However for cases where retention angle is larger than 90 at low velocity pin fin tubes shows promising role in controlling the condensate retention than integral fin tubes at higher vapour velocities. Experimental data was reported by Ali et al. [38] for the condensation heat transfer on square wires wrapped on horizontal tube. The square wires considered in the study were brass and copper of 0.8 mm and 1.0 mm square cross section with pitches 2.0, 4.0 mm and 6.0 mm respectively. For all the tubes tested the highest enhancement ratio were found to be at 4.0 mm pitch square wire wrapped tube. Ali et al. [39] performed experiments to delve into the condensate retention on eight horizontal integral finned tubes with different fin spacing but same root diameter. Condensation was simulated with low approaching zero vapour velocity of condensate using water, ethylene glycol and R141-b. The major highlight of this work was that they could provide substantial data base for theoretical studies on condensation on low finned tubes and this data would help in the design of compact heat exchangers. Analytical model was developed by Ali [40] to predict the condensate flooding on the horizontal pin-fin tubes during free convection condensation. The developed model is a pivotal step towards the development of a comprehensive heat transfer model to predict condensing coefficient on horizontal pin fin tubes. Heat transfer measurements have been conducted by Ali et al. [41] for condensation of
307
steam – ethanol mixture on vertical downward flow over horizontal water cooled low finned copper tube. The study revealed that condensate retention between fins was found lower, compared with that of pure steam condensation, resulting from lower surface tension due to addition of ethanol. From the above literature it can be concluded that surface modifications augment condensation heat transfer by accelerating the condensate removal. It is noteworthy that most of the previous researchers have investigated the impact of surface modifications on the heat transfer characteristics of vertical plates and horizontal tubes. The condensation heat transfer performance of vertical plates conducted by earlier investigators have one side of the plate subjected to condensing environment while the other side is exposed to cooling water. In the case of vertical tubes, steam condenses over the entire tube surface and cooling water is circulated inside the tube. However, studies relating to the influence of surface modification on the condensation heat transfer characteristics of vertical tubes are not found elsewhere in literature. The vertical tube placed in condensing environment finds application in vertical shell and tube condensers in refrigerating systems. In the present work, the main objective is to thoroughly investigate the condensation heat transfer performance of vertical tubes placed in steam condensation environment. Also, investigations have been conducted to study the influence of physical (providing grooves) and chemical texturing (coating with lead) on the condensation heat transfer performance of the vertical tubes. The above said objectives have been achieved by performing experiments on cylindrical vertical copper tubes placed in steam condensation environment. 2. Surface tension and contact angle hysteresis The condensation heat transfer performance of any surface depends on dynamics of the liquid droplet residing on surfaces. The droplet will remain stable when surface tension forces balance the gravity force. The size of the droplet corresponding to this equilibrium condition termed as capillary length has a major influence on heat transfer [16]. As the surface tension forces decreases the capillary length decreases and therefore force of gravity dominates and the droplet falls off easily enhancing the heat transfer from the surface [42,43]. It is important to mention here that, surface tension forces per unit length is also equivalent to surface energy per unit area. The surface energy can be related to surface entropy and surface internal energy by the equation given below [8]
psurf ¼ U surf TSsurf
ð1Þ
The reduction in surface energy can be achieved either by increasing the surface entropy or by decreasing the surface internal energy. In the present investigation, the surface topography modifications, of the tube surface, in the form of coating with lead and providing grooves, will improve the surface roughness as stated by [44]. Increase in surface roughness increases the surface entropy. It is important to mention here that, since changes in surface topography has been done at atmospheric conditions, the surface internal energy remains same. Therefore, in the above equation the combined effect of surface entropy and surface internal energy is to decrease the surface energy. Stated differently, the reduction in surface energy results in lowering the surface tension forces per unit length, allowing the droplet to fall off easily from surfaces. However, the shape of liquid droplet residing on any surface is greatly influenced by gravity forces. If a liquid droplet resides under a horizontal surface, the gravity forces and surface tension forces balance each other and creates an equilibrium shape for the droplet and the droplet will have a constant contact angle all around its perimeter i.e., Ideal contact angle hysteresis defined
308
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
as the difference between advancing (a2) and receding(a1) contact angles (see Fig. 1) [45], is zero. However, when the surface is turned vertical the droplet will deform to balance the gravity forces. This deformation results in non -zero contact angle hysteresis. It is worth to mention here that, the dependence of surface tension forces on contact angle hysteresis has been well documented by earlier researchers [46–50]. A small contact angle hysteresis will cause the droplet to slide off easily because of reduced surface tension forces. 3. Experimental methodology 3.1. Experimental setup The schematic of the experimental set-up is shown in Fig. 2. The test section consists of a cylindrical test specimen of inner diameter £16.5 mm and outer diameter £18.5 mm and length 180 mm made of copper, fixed vertically in a transparent condensation chamber of dimensions £200 mm and height 240 mm. Steam supplied from the steam generator is allowed to pass through an orifice and is admitted into the lower entrance of the condensation chamber. The steam admission is done such that the steam condenses uniformly over the test specimen. The condensation is visualised using high speed digital video camera (HiSpec 2, Fastec Imaging, USA). To measure the droplet velocity a marking is provided at periphery of the grooved tube at a distance of 5 cm from the beginning point of the groove. From the digital images of the droplet, the time taken by the droplet to travel a distance of 5 cm was measured, that is from the groove beginning to 5 cm marking point. Using this data, the droplet mobility is quantified. The temperature of steam in the steam generator is regulated with a controller unit. The volume flow rate of steam is adjusted with a ball valve and the steam pressure is measured with a pressure gauge. Another pressure gauge is located further downstream of the pipe to ensure that there is no pressure drop in the steam pipe. The steam pipe connecting the steam generator and condensation chamber is insulated to prevent any heat loss to the surroundings. The temperature of the steam in the condensation chamber is measured by K-type thermocouple of accuracy ±0.5 K. Steam is allowed to condense on a vertical cylindrical copper test tube of inner diameter
16.5 mm and outer diameter 18.5 mm and length 180 mm. The copper tube is fabricated in such a way that the bottom portion of the copper tube is integral with the lateral surface of the tube. Whereas, the upper end has a removable threaded brass cap having provisions for circulation of cooling water. This arrangement is detailed in the exploded view of copper tube depicted in the schematic diagram shown in Fig. 2. When the open end of the tube is closed with threaded brass cap, the arrangement will act as enclosed copper tube. The copper tube is placed vertically and fixed with a flange on to the frame of the experimental set up. The surface temperature of the test specimen is measured by six different K-type thermocouples, three on one side placed equidistant, and the other three located diametrically opposite (Fig. 2). The thermocouples are placed to the test specimen by drilling a hole of diameter 0.5 mm and depth 0.5 mm. All the thermocouples are fixed to the measuring points using highly conducting thermobond (Fabrica, India Ltd). The cooling water temperature is recorded by thermocouples placed at the inlet and outlet of the cooling water passage. In addition to this a thermocouple is placed inside the condensation chamber to measure the steam temperature. All the thermocouples are calibrated before they are fixed at respective locations. The test surface is cooled by circulating water from an overhead tank. The overhead tank is connected to the rotameter by £20 mm flexible pipe. The outlet of the rotameter is connected to the copper tube provided in the brass cap by a £6.25 mm flexible pipe. The cooling water flow is adjusted with valve and the flow rate is measured with rota meter (Eureka, India Ltd). All the thermocouples are connected to a PC based data acquisition system (Keithley, India Ltd). All the measurements are taken under steady state conditions. Details are shown in the front view and back view of the experiment set up photograph in Fig. 3. Experiments were conducted on different test specimen (vertical copper tube with/ without grooves and lead coated vertical copper tube with/without grooves). The bare copper tube was coated with lead by electrodeposition process. In this process, copper tube was kept as cathode and lead electrode was used as anode. The two electrodes were completely immersed in methane sulphonate electrolyte and current was supplied approximately for 15 min. During this time interval, lead was deposited on copper kept as cathode. To ensure that the thickness of lead coating was 10 mm, the sample was taken out and the thickness of coating was measured using digital outside micro meter (Mitutoyo, India Ltd). To provide grooves on the vertical tube, precision milling operation was performed with the help of vertical milling machine. An end mill cutter of 3 mm diameter was used to provide grooves on the surface of vertical tube for depth 0.10 mm, 0.15 mm and 0.30 mm keeping the number of grooves as four. 3.2. Data reduction In this experiment, condensation heat transfer coefficient (CHTC) was calculated from the flow rate and temperature rise of cooling water and is expressed in Eq. (2). For calculation purpose the volume flow rate of cooling water was converted to mass flow rate assuming that density of water is constant.
_ p ðT co T ci Þ Q ¼ mC
ð2Þ
Q AðT v ap T wo Þ
ð3Þ
hc ¼
Fig. 1. Contact angle hysteresis(h) of a falling droplet.
where A ¼ pd0 L In the case of grooved vertical tube, it was difficult to measure the actual condensation area accurately. Therefore, to calculate this a model of grooved vertical tube was created in commercial
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
309
Fig. 2. Schematic diagram of the experimental set up.
a. Front view
b. Rear view Fig. 3. Photograph of the experimental set up.
modelling software, ProE [51]. The surface area of the grooved vertical tube was calculated using the measure option available in ProE software. This area was employed in evaluating the surface condensation heat transfer coefficient mentioned in Eq. (3) for grooved vertical tubes. The average outer tube surface temperature was estimated from the measured average tube inner surface temperature. Since the vertical cylinder is hollow and exposed to fluids at different temperatures on its inner and outer surface, for steady state conditions with no heat generation, the appropriate form of heat equation [52] is given by
1 d dT kCu r ¼0 r dr dr
ð4Þ
Eq. (4) can be solved using appropriate boundary conditions (assuming kCu as constant) to obtain radial temperature distribution associated with radial conduction through the cylinder wall. The rate at which energy is conducted across the cylinder surface (Fourier’s law) and is given by
qr ¼ kcu r
dT dr
ð5Þ
310
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
The temperature distribution obtained from eq, 4 can be used in conjunction with Eq. (5) to obtain the following expression for the outlet temperature of the cooling water.
T wo ¼
Qlnðdo =di Þ þ T wi 2pLkcu
ð6Þ
PN Ti where T wi ¼ Ni¼1 , N = 6. The difference between measured average inner surface temperature and estimated average outer surface temperature was found to be 0.24 °C for bare copper tube. The same methodology was extended to tube coated with lead and the difference in temperature at low degree of sub cooling was found to be only 0.53 °C. The difference in estimated and measured tube surface temperature in both cases was marginal, therefore the measured surface temperature recorded by the thermocouples Twi were used in the calculation of degree of sub cooling (DT = Tvap Two), which in turn was used in the calculation of surface condensation heat transfer coefficient (Eq. (3)).
Fig. 4. Comparison of average surface condensation heat transfer coefficient(hc) with degree of sub-cooling (DT) for bare copper tube.
3.3. Experimental uncertainty
4. Results and discussions
Errors in measured data causes uncertainty in experimental results [53]. The uncertainty in experimental measurements (coolant flow rate, vapour temperature, surface temperature of specimen, diameter and length of the specimen, temperature of cooling water) propagates into the dependent parameters (heat carried away by cooling water, surface area and heat transfer coefficient) and this is calculated by the procedure outlined in [54]. The uncertainty in experimental measurement’s viz: coolant flow rate is ±2% of the measured flow rate (specified by the manufacturer Eureka India Ltd), temperature measurements using K type thermocouple with uncertainty 0.5 °C, diameter and length of the specimen with uncertainty of 0.02 mm (digital Vernier-Yuri India Ltd). The uncertainty has been estimated to a confidence interval of 68.27%, corresponding to a coverage factor of one. The main uncertainty correspond to the uncertainty in the measurement of heat carried away by cooling water and condensation heat transfer coefficient (hc). The estimated values of uncertainty for high (DT = 3.11) and low (DT = 0.7) degree of sub-cooling are reported in Table 1. Uncertainties of the estimated values for the entire range of sub-cooling was evaluated but not shown here due to brevity, but depicted as error bars in Figs. 4, 7 and 9. It can be inferred from Table. 1 and Figs. 4, 7 and 9 that uncertainties becomes larger at low degree of sub-cooling in comparison to high degree of subcooling. At low degree of sub-cooling the large uncertainty is mainly due to rapidly changing surface temperature in the analysed area. This transient behaviour is induced by fluctuations of the heat transfer resistance of growing, departing or entrained droplets. Since the standard deviations of the measured temperature is used as the input values for uncertainty calculations, the uncertainty will be larger at low degree of sub-cooling. However, as the degree of sub-cooling increases nucleation site increases and more droplets are formed. These droplets may coalesce and form a fluid film on the tube surface reducing the fluctuations in heat transfer area resulting in lower uncertainty bars.
The principal interest in this work is to investigate the influence of modifications in surface topography on condensation of steam on copper tube and lead coated copper tube. Experiments were conducted on bare copper tube and lead coated copper tube with/without grooves placedin steam condensation environment. The number of grooves was fixed as four and the groove depth considered in the study were 0.10 mm, 0.15 mm and 0.30 mm, width 3.0 mm. It is worth mentioning here that earlier studies by Izumi et al. [17] on condensation heat transfer from grooved vertical plates, reported that there exists an optimum groove width of 2–3 mm at which condensation heat transfer was maximum. Therefore in the present study we have also kept the groove width as 3.0 mm. The degree of sub-cooling was varied from 0.5 °C to 20 °C by adjusting the cooling water flow rate from 70 lph to 110 lph. This sub-cooling range is commonly encountered in refrigeration application devices. The photograph of the tested tubes showed that, the condensation behaviour of steam on the tubes were different, at DT < 2, steam condenses as droplets on the vertical tube surface. However at DT > 2, these droplets coalesce and form liquid film on the tube surface. Therefore, in this work the condensation behaviour is explained by dividing the sub-cooling limit DT < 2 as low degree and DT > 2 as high degree of subcooling.
4.1. Heat transfer characteristics of bare copper tube Experiments were conducted for the condensation of steam on vertical bare copper tube and grooved copper tube by varying the degree of sub-cooling. The condensation heat transfer coefficient calculated based on Eq. (3), presented in Fig. 4 reveals that grooved copper tubes have superior condensation heat transfer characteristics compared to bare copper tubes for all ranges of sub-cooling. Providing grooves on copper tubes increases the surface roughness
Table 1 Uncertainty of the various estimated parameters (Lead coated copper tube). Parameters Measured
Uncertainty
Estimated
d
l
V
T
mm
mm
l/h
°C
0.02
0.02
2%
0.5
A
0.11%
Q
hc
DT = 0.7
DT = 3.11
DT = 0.7
DT = 3.11
5%
3%
15%
35%
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
311
Fig. 5. Contact angle hystersis (h) of cooper tube with/without grooves.
Fig. 6. Comparison of contact angles.
Fig. 7. Comparison of average surface condensation heat transfer coefficient(hc) with degree of sub-cooling (D T) for copper and lead coated copper tube (Without grooves).
thereby increasing the surface entropy. The dependence of surface energy or surface tension forces on surface entropy has already been discussed in Section 2. However, the behaviour of grooved surface shows marked changes depending on the degree of sub-cooling as reported in Table 2. At low degree of sub-cooling
(DT < 2) the contact angle hysteresis (h) (see Section 2) has a major influence on the condensation heat transfer. To study this, the contact angle hysteresis of vertical copper tube with/without grooves was measured by injecting 10mL of water droplet inside the groove by keeping the test specimen vertical. The image of the droplet residing on the surface of the vertical tube was captured using high speed camera (HiSpec 2, Fastec Imaging, USA) and the image was analysed by Image Processing and Analysis software (Image J). The contact angle hysteresis illustrated in Fig. 5 reveals that contact angle hysteresis of grooved copper tubes are lower than copper tube without groove. Also by revisiting Fig. 5 it can be inferred that as the groove depth increases the contact angle hysteresis increases. Higher contact angle hysteresis, improves surface tension and therefore the droplet cannot slide off from the surface easily. In this case, copper tube with 0.10 mm groove depth has the minimum contact angle hysteresis (h = 49.28 deg) in contrast to copper tubes of depth 0.15 mm and 0.30 mm. Therefore, droplet residing in the 0.10 mm groove depth will be able to slide off easily. To substantiate this, the droplet velocity was measured using the images from the high speed digital video camera. The values of droplet sliding velocity reported in Table 3 justifies that the maximum droplet velocity is for copper tube with groove depth 0.10 mm, which corresponds to minimum contact angle hysteresis. Therefore, at low degree of sub-cooling (DT < 2) copper tube with a groove depth of 0.10 mm outperforms bare copper tube and grooved copper tube.
312
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
Table 2 Average surface condensation heat transfer coefficient, hc (kW/m2 K). Specimen
Groove depth, mm 0.00
0.10
0.15
0.30
Low degree of sub-cooling, DT=1.5 °C Copper tube Lead coated copper tube
47.66 95.51
75.36 110.81
68.56 105.53
58.56 76.34
High degree of sub-cooling, DT = 3 °C Copper tube Lead coated copper tube
22.67 48.93
30.48 42.84
35.96 41.46
27.10 32.19
Table 3 Droplet sliding velocity, m/s. Specimen
Groove depth, mm 0.00
0.10
0.15
0.30
Low degree of sub-cooling, DT=1.5 °C Copper tube Lead coated copper tube
0.26 0.66
0.56 1.20
0.50 0.95
0.36 0.59
High degree of sub-cooling, DT = 3°C Copper tube Lead coated copper tube
0.22 0.63
0.46 0.58
0.47 0.55
0.30 0.50
It can be inferred from Table 2, that at high degree of subcooling (DT > 2) Copper tube of groove depth 0.15 mm shows better heat transfer performance. At high degree of sub-cooling, the nucleation site density is larger and therefore larger concentration of liquid droplet is seen on the tube surface. The high concentration of droplets may lead to coalescence of droplets and eventually can flood the copper tube of groove depth 0.10 mm. The subsequent droplet coalescence and flooding of hydrophilic copper tube of depth 0.10 mm results in fall in droplet sliding velocity compared to the droplet velocity for 0.15 mm as reported in Table 3. This drop-in droplet sliding velocity is responsible for poor condensation heat transfer performance for copper tube of groove depth of 0.10 mm in comparison with groove depth of 0.15 mm showing a deterioration of 17.97% in heat transfer coefficient (Table 2). However, a comparison of heat transfer coefficient for copper tube with groove depth of 0.15 mm and 0.30 mm, indicates that the heat transfer performance of vertical copper tube with 0.15 mm groove depth is superior to 0.30 mm depth. As the depth increases the expanded area also increases. This increase in expanded area causes an increase in wall friction which eventually results in reduced droplet velocity as seen in Table 3. Therefore, the condensation heat transfer performance of hydrophilic copper tube of 0.30 mm falls below that of 0.15 mm groove depth with 24.92% reduction in heat transfer coefficient (Table 2). 4.2. Heat transfer characteristics of lead coated copper tube To ensure that lead coated copper tube is more hydrophobic than bare copper tube, the static contact angle of the droplet residing on the horizontal copper tube and lead coated copper tube was measured and analysed using Image J software and is presented in Fig. 6. It can be inferred from the Fig. 6 that the contact angle of lead coated copper tube is greater than bare copper tube, making the lead coated copper tube as more hydrophobic. Experiments were conducted on lead coated copper tube, for same range of sub-cooling, as that considered in the case of bare copper tube. The variation in average surface condensation heat transfer coefficient depicted in Fig. 7, indicates that lead coated copper tube outperforms bare copper tube for all range of sub-cooling with a maximum enhancement of approximately 115.84%. To explain this the SEM images (FESEM, Nova NanoSEM 450, India Ltd) of both the tubes (Fig. 8) were taken at 10,000 magnification. The images
clearly establish that lead coated surface has more roughness and therefore higher surface entropy in comparison to bare copper surface. Consequently, the lead coated copper tube has less surface energy or surface tension force than bare copper tube. As a result, liquid droplets residing on the lead coated copper tube will have a greater mobility than bare copper tube. The droplet velocity tabulated in Table 3 reveals that droplet residing in the lead copper tube has greater velocity than that residing in bare copper tube. The high sliding velocity of the droplet is responsible for higher condensation heat transfer from lead coated copper tube in comparison with bare copper tube. The results clearly indicated that providing grooves and coating independently on bare copper tube enhances condensation heat transfer. Keeping this in view, the combined effect of the above two parameters is investigated by providing grooves and coating on bare copper tube. The experiments were then conducted for the same range of sub-cooling as that in the case of bare copper tube. It can be seen from the Fig. 9 that degree of sub-cooling has a significant influence on the condensation heat transfer of lead coated copper surface with/without grooves. At low degree of subcooling the contact angle hysteresis presented in Fig. 10 indicates that the minimum contact angle hysteresis (h = 30.85 deg) is for lead coated copper tube of groove depth of 0.10 mm relative to lead coated copper tube without grooves and groove depth 0.15 mm and 0.30 mm. The droplet sliding velocity in Table 3 also indicates a higher velocity for lead coated copper tube of groove depth 0.10 mm and therefore higher heat transfer coefficient (Table 2). At high degree of sub-cooling (DT > 2 °C), the presence of grooves is detrimental to the heat transfer performance of lead coated copper tube. At high degree of sub-cooling the heat transfer coefficient is found to decrease with increase in groove depth as reported in Table 2. This behaviour can be attributed to the increase in nucleation site density. Therefore, concentration as well as size of the droplets increases as seen in the photograph of the concentration of droplet concentration depicted in Fig. 11. These factors contribute to increase in surface tension forces which eventually decrease the droplet sliding velocity. In addition to this the increase in expanded area due to larger groove depth further reduces velocity of droplet because of higher sliding friction (see Table 3). The reduced droplet velocity is responsible for the degraded heat transfer performance of grooved hydrophobic copper tube in comparison with hydrophobic copper tube.
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
313
Fig. 8. SEM images of copper and lead coated copper tube (without grooves).
points corresponding to groove depths of 0.15 and 0.30 mm for a particular value of DT degree of sub cooling. This clearly indicates that the heat transfer performance of the vertical tube at low degree of sub-cooling is not significantly affected by the groove depth. 5. Conclusion Experimental data are reported for the condensation of steam on vertical bare copper tube and lead coated copper tube with and without grooves. The number of grooves was fixed as 0.10 mm, 0.15 mm and 0.30 mm and the width as 3.0 mm. The cooling water flow rate was varied so as to maintain degree of sub-cooling in the range 0.5 °C DT 20 °C. The salient conclusions from the study are:
Fig. 9. Comparison of average surface condensation heat transfer coefficient(hc) with degree of sub-cooling (DT) for lead coated copper tube with/without grooves.
Further insight in to the condensation behaviour of grooved vertical tubes (bare copper tube and lead coated copper tube) can be envisaged from Figs. 4, 7 and 9. It can be seen from these figures that at low degree of sub-cooling (DT < 2 °C)error bar for a particular groove depth (say 0.10 mm) will overlap with the data
The presence of grooves on the vertical tube surface enhances condensation heat transfer due to reduced surface tension which in turn resulted in condensate acceleration. Degree of sub-cooling has a major influence on heat transfer performance of grooved copper tube. At low of degree of sub-cooling (DT < 2°C), the condensation performance weakens continuously with increase in groove depth. At high degree of sub-cooling (DT > 2 °C), copper tube of depth 0.15 mm outperforms copper tube of depth 0.10 mm and 0.30 mm.
Fig. 10. Contact angle hysteresis (h) of lead coated copper tube with/without grooves.
314
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
Fig. 11. Condensate formation at low and high degree of sub cooling.
Lead coated copper tubes shows better heat transfer performance than bare copper tubes for the range of sub-cooling considered in the study. The heat transfer performance of lead coated copper tube with and without grooves is significantly influenced by degree of sub-cooling. At low degree (DT < 2 °C) of sub-cooling lead coated copper tube with a groove 0.10 mm has maximum condensation heat transfer coefficient of all the cases considered in the present study. At high degree of sub-cooling(DT > 2 °C) the presence of grooves demotes the condensation heat transfer performance of lead coated vertical copper tubes. Conflict of interest statement We would like to say that we and our institution don’t have any conflict of interest and don’t have any financial or other relationship with other people or organizations that may inappropriately influence the author’s work. Acknowledgements The fifth author acknowledges the support provided by the ‘‘Research Chair Grant” National Science and Technology Development Agency (NSTDA), the Thailand Research Fund (TRF) and King Mongkut’s University of Technology Thonburi through the ‘‘KMUTT 55th Anniversary Commemorative Fund”. References [1] A. Dehbi, S. Guentay, A model for the performance of a vertical tube condenser in the presence of noncondensable gases, Nuclear Eng. Des. 177 (1997) 41–52. [2] Akili D. Khawaji, Ibrahim K. Kutubkhanah, Jong-Mihn Wie, Advances in seawater desalination technologies, Desalination 221 (2008) 47–69. [3] Man-Hoe Kim, Clark W. Bullard, Air-side performance of brazed aluminium heat exchangers under dehumidi-fying conditions, Int. J. Refrig. 25 (2002) 924– 934. [4] N. Miljkovic, E.N. Wang, Condensation heat transfer on super hydrophobic surfaces, MRS Bull. 38 (5) (2013) 397–406.
[5] T. Haraguchi, R. Shimada, S. Kumagai, The effect of polyvinylidene chloride coating thickness on promotion of dropwise steam condensation, Int. J. Heat Mass Transfer. 34 (12) (1991) 3047–3054. [6] G. Koch, D.C. Zhang, A. Leipertz, Condensation of steam on the surface of hard coated copper discs, Heat Mass Transfer 32 (1997) 149–156. [7] Xuehu Ma, Jiabin Chen, Xu. Dunqi, Jifang Lin, Chunsheng Ren, Zhenhu Long, Influence of processing conditions of polymer film on dropwise condensation heat transfer, Int. J. Heat Mass Transfer 45 (2002) 3405–3411. [8] M.H. Rausch, A. Leipertz, A.P. Frba, Dropwise condensation of steam on ion implanted titanium surfaces, Int. J. Heat Mass Transfer 53 (2010) 423–430. [9] A. Bani Kananeh, M.H. Rausch, A.P. Froba, A. Leipertz, Experimental study of dropwise condensation on plasma-ion implanted stainless steel tubes, Int. J. Heat Mass Transfer 49 (2006) 5018–5026. [10] E. Kalyuzhny, F.A. Kulacki, Condensation of FC-72, Int. J. Heat Mass Transfer 69 (2014) 337–342. [11] Bong June Zhang, Cheng Kuok, Kwang Jin Kim, Taeseon Hwang, Hyungkee Yoon, Dropwise steam condensation on various hydrophobic surfaces: Polyphenylene sulphide (PPS), polytetrafluoroethylene (PTFE), and selfassembled micro/Nano silver (SAMS), Int. J. Heat Mass Transfer 89 (2015) 353–358. [12] S. Vemuri, K.J. Kim, An experimental and theoretical study on the concept of dropwise condensation, Int. J. Heat Mass Transfer 49 (2006) 649–657. [13] Rin Yun, Jaehyeok Heo, Yongchan Kim, Film condensation heat transfer characteristics of R134a on horizontal stainless steel integral-fin tubes at low heat transfer rate, Int. J. Refrig. 32 (2009) 865–873. [14] Abhra Chatterjee, Melanie M. Derby, Yoav Peles, Michael K. Jensen, Enhancement of condensation heat transfer with patterned surfaces, Int. J. Heat Mass Transfer 71 (2014) 675–681. [15] Pallab Sinha Mahapatra, Aritra Ghosh, Ranjan Ganguly, Constantine M. Megaridis, Key design and operating parameters for enhancing dropwise condensation through wettability patterning, Int. J. Heat Mass Transfer 92 (2016) 877–883. [16] Sameer Khandekar, Krishnamurthy Muralidhar, dropwise Condensation on Inclined Textured Surfaces, Springer Briefs in Thermal Engineering and Applied Science. [17] M. Izumi, S. Kumagai, R. Shimada, N. Yamakawa, Heat transfer enhancement of dropwise condensation on a vertical surface with round shaped grooves, Exp. Therm. Flu. Sci. 28 (2e3) (2004) 243–248. [18] Orge R. Lara, Mark T. Holtzapple, Experimental investigation of dropwise condensation on hydrophobic heat exchangers part I: dimpled-sheets, Desalination 278 (2011) 165–172. [19] Orge R. Lara, Mark T. Holtzapple, Experimental investigation of dropwise condensation on hydrophobic heat exchangers. Part II: Effect of coatings and surface geometry, Desalination 280 (2011) 363–369. [20] Qi Baojin, Xu. Zhang Li, Sun Yan Hong, Experimental study on condensation heat transfer of steam on vertical titanium plates with different surface energies, Exp. Therm. Flu. Sci. 35 (2011) 211–218. [21] Kuok Cheng SangsooLee, M.D. Viljar Palmre, Mainul Hossain Bhuiya, Kwang J. Kim, Bong JuneZhang Hyung-keeyoon, Heat transfer measurement during dropwise condensation using micro/nano scale porous surface, Int. J. Heat Mass Transfer 65 (2013) 619–626.
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315 [22] H.W. Hu, G.H. Tang, D. Niu, Experimental investigation of condensation heat transfer on hybrid wettability finned tube with large amount of noncondensable gas, Int. J. Heat Mass Transfer 85 (2015) 513–523. [23] Ding-Jun Huang, Tzong-ShyngLeu, Condensation heat transfer enhancement by surface modification on a monolithic copper heat sink, App. Therm. Eng. 75 (2015) 908–917. [24] H.W. Hu, G.H. Tang, D. Niu, Experimental investigation of convective condensation heat transfer on tube bundles with different surface wettability at large amount of non-condensable gas, App. Therm. Eng. 100 (2016) 699–707. [25] H.M. Ali, A. Briggs, Condensation of R-113 on pin-fin tubes: effect of circumferential pin thickness and spacing, Heat Transfer Eng. 33 (3) (2012) 205–212. [26] H.M. Ali, A. Briggs, Condensation of ethylene glycol on pin-fin tubes: effect of circumferential pin spacing and thickness, App. Therm. Eng. 49 (2012) 9–13. [27] H.M. Ali, A. Briggs, Enhanced condensation of ethylene glycol on single pin-fin tubes: effect of pin geometery, ASME J. Heat Transfer 134 (2012) 0115031– 115038. [28] H.M. Ali, A. Briggs, Condensation heat transfer on pin-fin tubes: Effect of thermal conductivity and pin height, Appl. Therm. Eng. 60 (2012) 465–471. [29] D.I. Katz, R.E. Hope, S.C. Datsko, Liquid retention on finned tubes, Dept. of Eng. Res., Univ. of Michigan, 1946, Project M592. [30] P.J. Marto, An evaluation of film condensation on horizontal integral fin tubes, Trans. ASME. J. Heat Transfer 110 (1988) 1287–1305. [31] A. Briggs, J.W. Rose, An evaluation of models for condensation heat transfer on low finned tube, J. Enhanc. Heat Transf. 6 (1999) 51–60. [32] H.M. Ali, A. Briggs, An investigation of condensate retention on pin-fin tubes, App. Therm. Eng. 63 (2014) 503–510. [33] H.M. Ali, M. Abubaker, Effect of vapour velocity on condensate retention on horizontal pin-fin tubes, Energy Convers. Manage. 86 (2014) 1001–1009. [34] H.M. Ali, A. Ali, Measurements and semi-emperical correlation for condensate retention on horizontal integral-fin tubes: Effect of vapour velocity, Appl. Therm. Eng. 71 (2014) 24–33. [35] H.M. Ali, A. Briggs, A semi-emperical model for free convection condensation on horizontal pin-fin tubes, Int. J. Heat Mass Transfer 81 (2015) 157–166. [36] H.M. Ali, M.Z. Qasim, Free convection condensation of steam on horizontal wire wrapped tubes: Effect of wire thermal conductivity, pitch and diameter, Appl. Thermal. Eng. 90 (2015) 207–214. [37] H.M. Ali, M. Abubaker, Effect of circumferential pin thickness on condensate retention as a function of vapour velocity on horizontal pin-fin tubes, Appl. Thermal. Eng. 91 (2015) 245–251.
315
[38] H.M. Ali, M.Z. Qasim, M. Ali, Free convection condensation heat transfer of stem on horizontal square wire wrapped tubes, Int. J. Heat Mass Transfer 98 (2016) 350–358. [39] H.M. Ali, H. Ali, M. Ali, S. Imran, M.S. Kamran, F. Farukh, Effect of condensate flow rate on retention angle on horizontal low finned tubes, Therm. Sci. (2016) 1–10. [40] H.M. Ali, An analytical model for prediction of condensate flooding on horizontal pin-fin tubes, Int. J. Heat Mass Transfer 106 (2017) 1120–1124. [41] H. Ali, M.S. Kamran, H.M. Ali, F. Farukh, S. Imran, H.S. Wang, Marangoni condensation of steam-ethanol mixtures on a horizontal low-finned tube, Appl. Thermal Eng. 117 (2017) 366–375. [42] R.N. Leach, F. Stevens, S.C. Langford, J.T. Dickinson, Dropwise condensation: experiments and simulations of nucleation and growth of water drops in a cooling system, Langmuir 22 (2006) 8864–8872. [43] A. Leipertz, Dropwise condensation, in: P. Stephan (Ed.), VDI Heat Atlas VDIGVC, second ed., Springer, Germany, 2006, pp. 933–937. [44] Q. Zhao, B.M. Burnside, dropwise condensation of steam on ion implanted condenser surfaces, Heat. Recov. Syst. CHP 14 (5) (1994) 525–534. [45] Pierre-Gilles de Gennes, Francoise Brochard-Wyart and David Quere Capillarity and Wetting Phenom-ena: Drops, Bubbles, Pearls, Waves, first ed., Springer, New York, 2004. [46] C. Furmidge, G, The sliding drop on solid surfaces and a theory for spray retention, J. Colloid. Sci 17 (1962) 309–324. [47] Öner Didem, Thomas J. McCarthy, Ultra hydrophobic surfaces. Effects of topography length scales on wettability, Langmuir 16 (20) (2000) 7777–7782. [48] X.H. Ma, X.D. Zhou, Z. Lan, Y.M. Li, Y. Zhang, Condensation heat transfer enhancement in the presence ofnon-condensable gas using the inter facial effect of dropwise condensation, Int. J. Heat Mass Transf. 51 (2008) 1728– 1737. [49] S. Kim, K. Kim, J, dropwise condensation suitable for super hydrophobic surfaces, ASME J. Heat Transf. 133 (8) (2011) 0815021–0815028. [50] K. Rykaczewski, Micro droplet growth mechanism during water condensation on super hydrophobic sur-faces, Langmuir 28 (2012) 7720–7729. [51] ProE, Wildfire 4.0, Fundamentals of Heat and Mass Transfer, fifth ed., John Wiley and sons, Singapore, 2002. [52] F.P. Incropera, D.P. Dewitt, Fundamentals of Heat and Mass Transfer, fifth ed., John Wiley and sons, Singapore, 2002. [53] Ernest O. Doebelin, Dhanesh N. Manik, Measurement Systems, sixth ed., McGraw-Hill Company, New Delhi, 2012. [54] S.J. Kline, F.A. Mcklintock, Describing uncertainties in single sample experiments, Mech. Eng. (1953) 3.
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Experimental study of condensation heat transfer on hydrophobic vertical tube Mattacaud Ramachandralal Rajkumar a,⇑, Arjunan Praveen a, Radhakrishnan Arun Krishnan a, Lazarus Godson Asirvatham b, Somchai Wongwises c,d,⇑ a
Advanced Thermo Fluid Research Lab, Department of Mechanical Engineering, College of Engineering, Trivandrum 695016, Kerala, India Department of Mechanical Engineering, Karunya University, Tamil Nadu, India Fluid Mechanics, Thermal Engineering and Multiphase Flow Research Lab. (FUTURE), Department of Mechanical Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, Bangkmod, Bangkok, Thailand d The Academy of Science, The Royal Institute of Thailand, Sanam Suea Pa, Dusit, Bangkok 10300, Thailand b c
a r t i c l e
i n f o
Article history: Received 22 September 2017 Received in revised form 4 December 2017 Accepted 4 December 2017
Keywords: Dropwise condensation Hydrophobic vertical tube Degree of sub-cooling Droplet velocity Contact angle hysteresis
a b s t r a c t Condensation heat transfer on surfaces can be enhanced by altering the surface topography. Surface modification technology can promote dropwise condensation which can exhibit higher heat transfer rate than filmwise condensation. This paper presents experimental data on the condensation of steam on vertical bare copper tubes and lead coated copper tubes for degree of sub-cooling in the range 0.5 °C DT 20 °C. The chemical texture of the tube surface was altered by coating with lead of thickness 10 mm and the physical texture of the surface was transformed by providing four grooves each having equal depths 0.10 mm, 0.15 mm and 0.30 mm. The condensation heat transfer characteristics of the tube surface is explained based on contact angle hysteresis and sliding velocity of the droplet. The results of the study reveal that for the tubes tested the average condensation heat transfer coefficient decreases with increase in degree of subcooling. It is also found that for copper tubes, providing grooves aids condensation heat transfer for the range of sub-cooling. However, for lead coated copper tube with/without grooves the heat transfer performance at DT < 2 °C shows marked difference in contrast to DT > 2 °C. Ó 2017 Published by Elsevier Ltd.
1. Introduction Vapour condensation has been a process of significant interest due to its applications in the field of power generation, water desalination and HVAC [1–3]. Mainly, there are two types of surface condensation processes- filmwise (FWC) and dropwise condensation (DWC). Typically filmwise condensation is observed on hydrophilic surfaces which has high surface energy resulting in high wettability. However, on hydrophobic surfaces with low surface energy the liquid condenses in the form of droplets causing dropwise condensation. Recently researchers have tried several modifications on surfaces to promote dropwise condensation, which lead to higher heat transfer rates compared to filmwise condensation. Dropwise condensation on surfaces can be achieved by
⇑ Corresponding authors at: Fluid Mechanics, Thermal Engineering and Multiphase Flow Research Lab. (FUTURE), Department of Mechanical Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, Bangkmod, Bangkok, Thailand (S. Wongwises). E-mail addresses: [email protected] (M.R. Rajkumar), [email protected]. th (S. Wongwises). https://doi.org/10.1016/j.ijheatmasstransfer.2017.12.019 0017-9310/Ó 2017 Published by Elsevier Ltd.
changing the surface morphology and surface chemistry [4]. Even though dropwise condensation leads to higher heat transfer, in spite of sustained research over the last decade the mechanism behind the formation of droplet and its transport is still not well understood and remains a challenge. The role of changing the chemical texture of the surface, so as to promote dropwise condensation (DWC) has been investigated by many researchers [5–12]. It is also important to mention here that few of the researchers [13–15] have also studied the influence of physical texturing on surfaces to enhance condensation heat transfer. However, recently a combination of chemical texturing along with physical texturing as a method of enhancing condensation heat transfer has also been well documented in literature [16]. A study was conducted by Izumi et al. [17] on condensation heat transfer characteristics of a vertical copper plate surface with round shaped grooves coated with ethanol solution of oleic acid. The study revealed that there exist an optimum groove width corresponding to maximum heat transfer. Experiments were conducted by Lara et al. [18] on vertical naval brass and copper plates with Ni-P-PTFE and without coatings. Ni-P-PTFE coated brass and copper plates showed better heat transfer performance.
306
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
Nomenclature A Cp d0 di hc kCu L _ m Q R r Ssurf T Tco Tci Two
effective condensation area, constant in Eq. (3) [m2] specific heat at constant pressure, [kJ/kg K] outside diameter of the cylindrical test specimen, [m] inside diameter of the cylindrical test specimen, [m] condensation heat transfer coefficient, [kW/m2 K] thermal conductivity of copper tube, [W/m K] length of the cylindrical test specimen, [m] mass flow rate of cooling water, [kg/s] heat transfer rate of cooling water, [kW] dependent variable distance measured radially, [m] surface entropy, [J/m2 K] temperature, [K] outlet temperature of cooling water, [K] inlet temperature of cooling water, [K] average outer wall surface temperature of copper tube, [K]
Experiments were conducted on dimpled titanium plate and vertical grooved copper plate by Lara et al. [19]. Copper was coated with lead containing and lead free Ni-P-PTFE while the titanium plate was kept bare. The overall heat transfer coefficient was experimentally measured for all cases. Condensation heat transfer of steam on three vertical titanium plates was investigated by Baojin et al. [20] by tuning the surface energy. Two titanium plates used for the tests were subjected to modification which include chemical etching on one plate and the other subjected to a combination of chemical etching and hydrogen peroxide solution treatment. It was observed that on unmodified surface, both filmwise and dropwise condensation was found to coexist, whereas filmwise condensation was achieved on plate modified with chemical etching and Dropwise condensation was achieved on plate subjected to combination of chemical etching and hydrogen peroxide solution treatment. Lee et al. [21] conducted experiments on micro/nano scale porous surfaces made by polyphenylene sulphide (PPS), and PTFE based polymer coatings, SAM and etching on the plain surfaces of steam condenser tubes. It was found that etched surface gives the highest transfer coefficient due to its lowest surface energy. Condensation heat transfer outside horizontal plain and finned copper tubes with different surface wettability was experimentally studied by Hu et al. [22]. Investigations were conducted on plain copper tube, hydrophobic copper tube, hybrid (hydrophilichydrophobic) finned copper tube, hybrid (hydrophilic-super hydrophobic) finned tube. The tests were conducted in presence of non-condensable gases and also in vacuum. It was reported that in the presence of non-condensable gases hybrid (hydrophilicsuper hydrophobic) hybrid finned tube achieved the highest condensation heat transfer performance. However, in vacuum hybrid (hydrophilic-hydrophobic) finned copper tube showed the highest condensation heat transfer performance. Huang et al. [23] compared the condensation heat transfer performance on a pure copper surface with a super hydrophobic-modified copper surface. The super hydrophobic surface was prepared using a hydrogen peroxide immersion and fluorosilane polymer coating. It was found that super hydrophobic modified copper surface performed better than the pure copper surface. Condensation heat transfer on tube bundles having three different surface structures plain, 2-D finned and 3-D finned tubes was tested by Hu et al. [24]. Comprehensive study on the effect of Reynolds number of cooling water, Reynolds number of air vapour mixture and volume fraction of water vapour
Twi Tvap DT Usurf V
a1 a2
ɣ
rR
rXi Xi h
average inner wall surface temperature of copper tube, [K] vapour temperature, [K] degree of sub-cooling, (Tvap-Two), [K] surface internal energy, [J/m2] volume flow rate, [l/h] receding contact angle, [deg] advancing contact angle, [deg] surface free energy, [N/m] standard deviation of independent variable, 12 Pn @R 2 2 rR ¼ i¼1 @Xi rXi standard deviation of dependent variable dependent variable contact angle hysteresis, (a2-a1)
on convective condensation heat transfer coefficient and number of tube rows was conducted. In addition to the above techniques, different types of geometrically enhanced tubes have also been used for many years to enhance condensation heat transfer. These tubes may be simple two dimensional rectangular integral fin tubes, three dimensional pin fin tubes or wire rapped tubes. The condensation heat transfer on these types of tubes has received considerable attention in heat transfer research because of its wide industrial application especially in industrial condensers. Ali and Briggs [25,26] reported data for the condensation of R-113 and ethylene glycol at near atmosphere pressure and low velocity on five three dimensional pin fin tubes. They studied the effect of circumferential pin spacing and thickness on the condensate retention on the pin fin tubes. The results of the study showed that among all the pin fin tubes considered in this work, for R 113, the best performing pin fin tube gave a heat transfer enhancement of 14% higher than the equivalent two dimensional integral fin tube, having the same fin root diameter, longitudinal fin spacing, thickness and fin height. However, for ethylene glycol, the best performing pin fin tube gave a heat transfer enhancement of 20% higher than the equivalent two dimensional integral fin tube. The same authors [27] extended their work to systematically study the influence of pin geometry on enhancement of heat transfer during condensation of ethylene glycol on eleven different pin fin tubes and plain tube. It was reported that the best performing pin fin tube gave a heat transfer enhancement of 5.5: 17% higher than obtained from optimised two dimensional fin tube reported in literature and 24% higher than equivalent two dimensional integral fin tube. Ali and Briggs [28] further reported the experimental data for condensation of ethylene glycol and R113 at near atmospheric pressure and low velocity on three identical pairs of pin fin tubes made of copper, brass and bronze. The study revealed that copper pin fin tubes showed better heat transfer performance in comparison with brass and bronze tubes. The above experimental studies of Ali and Briggs have shown that heat transfer performance of integral pin fin tubes are superior than equivalent integral fin tubes. It is worth to mention here that the major factor that affects the heat transfer from integral pin fin tubes is condensation retention or flooding as reported by few researchers [29–31]. Ali and Briggs [32] measured the liquid retention angles on 15 rectangular pin fin tubes under static conditions (without condensation) using water, ethylene glycol and R-113. It was found that condensate retention
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
angles for all pin fin tubes were larger than equivalent integral fin tubes. An expression for condensate retention angle on pin fin tubes was also proposed and found to agree with measured retention angles to within 15%. Ali and Abubaker [33] performed experiments to study the effect of vapour velocity on six horizontal pin fin tubes and equivalent integral fin tube. using water ethylene glycol and R-141b. It was observed that for all tubes tested, condensate retention angle increased with increase in vapour velocity. Also the circumferential pin spacing was found to be a significant parameter affecting the condensate retention angle regardless the effect of vapour velocity. Experimental study on condensation of water, ethylene glycol and R-141b on integral fin tubes, horizontally oriented in a vertical wind tunnel was conducted by Ali and Amanat [34]. Eight tubes of different dimensions were used with fixed fin root diameter and the internal diameter of the tubes were kept as 12.7 mm and 8.0 mm respectively. The study showed that retention angle is a strong function of fluid properties, geometric properties and vapour Reynolds number. A semi empirical correlation was brought out which showed excellent agreement with present experimental data. Ali and Briggs [35] developed a simple semi empirical correlation accounting for the combined effect of gravity and surface tension for condensation of steam, ethylene glycol and R113 on horizontal pin fin tubes. The developed model gave good overall agreement with the experimental data. Detailed comparison between the model and experimental data indicated that the model satisfactorily predicts the dependence of heat transfer enhancement on both geometric variables and fluids. Ali and Qasim [36] reported data for the condensation of steam on instrumented horizontal tube wrapped with 0.8 mm and 1.0 mm diameter wires made of copper, aluminium and brass with varying pitches of 2.0 mm, 4 mm and 6 mm. It was reported that the effect of thermal conductivity is evident for all tested pitches, however more dominant in the case of 2.0 mm pitch. Also, for both tested wires the best heat transfer enhancement compared to the plain tube are found at a pitch of 4.0 mm for all the cases of wire rapped tubes. The effect of circumferential pin thickness was investigated by Ali and Abubacker [37] on retention angle as a function of vapour velocity for four pin fin tubes using water, ethylene glycol and R-141b. The results of the study provided a guidelines that for the cases where the retention angle is less than 90 at low velocity (=0m/s) the use of pin fin tubes would not provide much help in reducing condensate retention than integral fin tubes for higher vapour velocities. However for cases where retention angle is larger than 90 at low velocity pin fin tubes shows promising role in controlling the condensate retention than integral fin tubes at higher vapour velocities. Experimental data was reported by Ali et al. [38] for the condensation heat transfer on square wires wrapped on horizontal tube. The square wires considered in the study were brass and copper of 0.8 mm and 1.0 mm square cross section with pitches 2.0, 4.0 mm and 6.0 mm respectively. For all the tubes tested the highest enhancement ratio were found to be at 4.0 mm pitch square wire wrapped tube. Ali et al. [39] performed experiments to delve into the condensate retention on eight horizontal integral finned tubes with different fin spacing but same root diameter. Condensation was simulated with low approaching zero vapour velocity of condensate using water, ethylene glycol and R141-b. The major highlight of this work was that they could provide substantial data base for theoretical studies on condensation on low finned tubes and this data would help in the design of compact heat exchangers. Analytical model was developed by Ali [40] to predict the condensate flooding on the horizontal pin-fin tubes during free convection condensation. The developed model is a pivotal step towards the development of a comprehensive heat transfer model to predict condensing coefficient on horizontal pin fin tubes. Heat transfer measurements have been conducted by Ali et al. [41] for condensation of
307
steam – ethanol mixture on vertical downward flow over horizontal water cooled low finned copper tube. The study revealed that condensate retention between fins was found lower, compared with that of pure steam condensation, resulting from lower surface tension due to addition of ethanol. From the above literature it can be concluded that surface modifications augment condensation heat transfer by accelerating the condensate removal. It is noteworthy that most of the previous researchers have investigated the impact of surface modifications on the heat transfer characteristics of vertical plates and horizontal tubes. The condensation heat transfer performance of vertical plates conducted by earlier investigators have one side of the plate subjected to condensing environment while the other side is exposed to cooling water. In the case of vertical tubes, steam condenses over the entire tube surface and cooling water is circulated inside the tube. However, studies relating to the influence of surface modification on the condensation heat transfer characteristics of vertical tubes are not found elsewhere in literature. The vertical tube placed in condensing environment finds application in vertical shell and tube condensers in refrigerating systems. In the present work, the main objective is to thoroughly investigate the condensation heat transfer performance of vertical tubes placed in steam condensation environment. Also, investigations have been conducted to study the influence of physical (providing grooves) and chemical texturing (coating with lead) on the condensation heat transfer performance of the vertical tubes. The above said objectives have been achieved by performing experiments on cylindrical vertical copper tubes placed in steam condensation environment. 2. Surface tension and contact angle hysteresis The condensation heat transfer performance of any surface depends on dynamics of the liquid droplet residing on surfaces. The droplet will remain stable when surface tension forces balance the gravity force. The size of the droplet corresponding to this equilibrium condition termed as capillary length has a major influence on heat transfer [16]. As the surface tension forces decreases the capillary length decreases and therefore force of gravity dominates and the droplet falls off easily enhancing the heat transfer from the surface [42,43]. It is important to mention here that, surface tension forces per unit length is also equivalent to surface energy per unit area. The surface energy can be related to surface entropy and surface internal energy by the equation given below [8]
psurf ¼ U surf TSsurf
ð1Þ
The reduction in surface energy can be achieved either by increasing the surface entropy or by decreasing the surface internal energy. In the present investigation, the surface topography modifications, of the tube surface, in the form of coating with lead and providing grooves, will improve the surface roughness as stated by [44]. Increase in surface roughness increases the surface entropy. It is important to mention here that, since changes in surface topography has been done at atmospheric conditions, the surface internal energy remains same. Therefore, in the above equation the combined effect of surface entropy and surface internal energy is to decrease the surface energy. Stated differently, the reduction in surface energy results in lowering the surface tension forces per unit length, allowing the droplet to fall off easily from surfaces. However, the shape of liquid droplet residing on any surface is greatly influenced by gravity forces. If a liquid droplet resides under a horizontal surface, the gravity forces and surface tension forces balance each other and creates an equilibrium shape for the droplet and the droplet will have a constant contact angle all around its perimeter i.e., Ideal contact angle hysteresis defined
308
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
as the difference between advancing (a2) and receding(a1) contact angles (see Fig. 1) [45], is zero. However, when the surface is turned vertical the droplet will deform to balance the gravity forces. This deformation results in non -zero contact angle hysteresis. It is worth to mention here that, the dependence of surface tension forces on contact angle hysteresis has been well documented by earlier researchers [46–50]. A small contact angle hysteresis will cause the droplet to slide off easily because of reduced surface tension forces. 3. Experimental methodology 3.1. Experimental setup The schematic of the experimental set-up is shown in Fig. 2. The test section consists of a cylindrical test specimen of inner diameter £16.5 mm and outer diameter £18.5 mm and length 180 mm made of copper, fixed vertically in a transparent condensation chamber of dimensions £200 mm and height 240 mm. Steam supplied from the steam generator is allowed to pass through an orifice and is admitted into the lower entrance of the condensation chamber. The steam admission is done such that the steam condenses uniformly over the test specimen. The condensation is visualised using high speed digital video camera (HiSpec 2, Fastec Imaging, USA). To measure the droplet velocity a marking is provided at periphery of the grooved tube at a distance of 5 cm from the beginning point of the groove. From the digital images of the droplet, the time taken by the droplet to travel a distance of 5 cm was measured, that is from the groove beginning to 5 cm marking point. Using this data, the droplet mobility is quantified. The temperature of steam in the steam generator is regulated with a controller unit. The volume flow rate of steam is adjusted with a ball valve and the steam pressure is measured with a pressure gauge. Another pressure gauge is located further downstream of the pipe to ensure that there is no pressure drop in the steam pipe. The steam pipe connecting the steam generator and condensation chamber is insulated to prevent any heat loss to the surroundings. The temperature of the steam in the condensation chamber is measured by K-type thermocouple of accuracy ±0.5 K. Steam is allowed to condense on a vertical cylindrical copper test tube of inner diameter
16.5 mm and outer diameter 18.5 mm and length 180 mm. The copper tube is fabricated in such a way that the bottom portion of the copper tube is integral with the lateral surface of the tube. Whereas, the upper end has a removable threaded brass cap having provisions for circulation of cooling water. This arrangement is detailed in the exploded view of copper tube depicted in the schematic diagram shown in Fig. 2. When the open end of the tube is closed with threaded brass cap, the arrangement will act as enclosed copper tube. The copper tube is placed vertically and fixed with a flange on to the frame of the experimental set up. The surface temperature of the test specimen is measured by six different K-type thermocouples, three on one side placed equidistant, and the other three located diametrically opposite (Fig. 2). The thermocouples are placed to the test specimen by drilling a hole of diameter 0.5 mm and depth 0.5 mm. All the thermocouples are fixed to the measuring points using highly conducting thermobond (Fabrica, India Ltd). The cooling water temperature is recorded by thermocouples placed at the inlet and outlet of the cooling water passage. In addition to this a thermocouple is placed inside the condensation chamber to measure the steam temperature. All the thermocouples are calibrated before they are fixed at respective locations. The test surface is cooled by circulating water from an overhead tank. The overhead tank is connected to the rotameter by £20 mm flexible pipe. The outlet of the rotameter is connected to the copper tube provided in the brass cap by a £6.25 mm flexible pipe. The cooling water flow is adjusted with valve and the flow rate is measured with rota meter (Eureka, India Ltd). All the thermocouples are connected to a PC based data acquisition system (Keithley, India Ltd). All the measurements are taken under steady state conditions. Details are shown in the front view and back view of the experiment set up photograph in Fig. 3. Experiments were conducted on different test specimen (vertical copper tube with/ without grooves and lead coated vertical copper tube with/without grooves). The bare copper tube was coated with lead by electrodeposition process. In this process, copper tube was kept as cathode and lead electrode was used as anode. The two electrodes were completely immersed in methane sulphonate electrolyte and current was supplied approximately for 15 min. During this time interval, lead was deposited on copper kept as cathode. To ensure that the thickness of lead coating was 10 mm, the sample was taken out and the thickness of coating was measured using digital outside micro meter (Mitutoyo, India Ltd). To provide grooves on the vertical tube, precision milling operation was performed with the help of vertical milling machine. An end mill cutter of 3 mm diameter was used to provide grooves on the surface of vertical tube for depth 0.10 mm, 0.15 mm and 0.30 mm keeping the number of grooves as four. 3.2. Data reduction In this experiment, condensation heat transfer coefficient (CHTC) was calculated from the flow rate and temperature rise of cooling water and is expressed in Eq. (2). For calculation purpose the volume flow rate of cooling water was converted to mass flow rate assuming that density of water is constant.
_ p ðT co T ci Þ Q ¼ mC
ð2Þ
Q AðT v ap T wo Þ
ð3Þ
hc ¼
Fig. 1. Contact angle hysteresis(h) of a falling droplet.
where A ¼ pd0 L In the case of grooved vertical tube, it was difficult to measure the actual condensation area accurately. Therefore, to calculate this a model of grooved vertical tube was created in commercial
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
309
Fig. 2. Schematic diagram of the experimental set up.
a. Front view
b. Rear view Fig. 3. Photograph of the experimental set up.
modelling software, ProE [51]. The surface area of the grooved vertical tube was calculated using the measure option available in ProE software. This area was employed in evaluating the surface condensation heat transfer coefficient mentioned in Eq. (3) for grooved vertical tubes. The average outer tube surface temperature was estimated from the measured average tube inner surface temperature. Since the vertical cylinder is hollow and exposed to fluids at different temperatures on its inner and outer surface, for steady state conditions with no heat generation, the appropriate form of heat equation [52] is given by
1 d dT kCu r ¼0 r dr dr
ð4Þ
Eq. (4) can be solved using appropriate boundary conditions (assuming kCu as constant) to obtain radial temperature distribution associated with radial conduction through the cylinder wall. The rate at which energy is conducted across the cylinder surface (Fourier’s law) and is given by
qr ¼ kcu r
dT dr
ð5Þ
310
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
The temperature distribution obtained from eq, 4 can be used in conjunction with Eq. (5) to obtain the following expression for the outlet temperature of the cooling water.
T wo ¼
Qlnðdo =di Þ þ T wi 2pLkcu
ð6Þ
PN Ti where T wi ¼ Ni¼1 , N = 6. The difference between measured average inner surface temperature and estimated average outer surface temperature was found to be 0.24 °C for bare copper tube. The same methodology was extended to tube coated with lead and the difference in temperature at low degree of sub cooling was found to be only 0.53 °C. The difference in estimated and measured tube surface temperature in both cases was marginal, therefore the measured surface temperature recorded by the thermocouples Twi were used in the calculation of degree of sub cooling (DT = Tvap Two), which in turn was used in the calculation of surface condensation heat transfer coefficient (Eq. (3)).
Fig. 4. Comparison of average surface condensation heat transfer coefficient(hc) with degree of sub-cooling (DT) for bare copper tube.
3.3. Experimental uncertainty
4. Results and discussions
Errors in measured data causes uncertainty in experimental results [53]. The uncertainty in experimental measurements (coolant flow rate, vapour temperature, surface temperature of specimen, diameter and length of the specimen, temperature of cooling water) propagates into the dependent parameters (heat carried away by cooling water, surface area and heat transfer coefficient) and this is calculated by the procedure outlined in [54]. The uncertainty in experimental measurement’s viz: coolant flow rate is ±2% of the measured flow rate (specified by the manufacturer Eureka India Ltd), temperature measurements using K type thermocouple with uncertainty 0.5 °C, diameter and length of the specimen with uncertainty of 0.02 mm (digital Vernier-Yuri India Ltd). The uncertainty has been estimated to a confidence interval of 68.27%, corresponding to a coverage factor of one. The main uncertainty correspond to the uncertainty in the measurement of heat carried away by cooling water and condensation heat transfer coefficient (hc). The estimated values of uncertainty for high (DT = 3.11) and low (DT = 0.7) degree of sub-cooling are reported in Table 1. Uncertainties of the estimated values for the entire range of sub-cooling was evaluated but not shown here due to brevity, but depicted as error bars in Figs. 4, 7 and 9. It can be inferred from Table. 1 and Figs. 4, 7 and 9 that uncertainties becomes larger at low degree of sub-cooling in comparison to high degree of subcooling. At low degree of sub-cooling the large uncertainty is mainly due to rapidly changing surface temperature in the analysed area. This transient behaviour is induced by fluctuations of the heat transfer resistance of growing, departing or entrained droplets. Since the standard deviations of the measured temperature is used as the input values for uncertainty calculations, the uncertainty will be larger at low degree of sub-cooling. However, as the degree of sub-cooling increases nucleation site increases and more droplets are formed. These droplets may coalesce and form a fluid film on the tube surface reducing the fluctuations in heat transfer area resulting in lower uncertainty bars.
The principal interest in this work is to investigate the influence of modifications in surface topography on condensation of steam on copper tube and lead coated copper tube. Experiments were conducted on bare copper tube and lead coated copper tube with/without grooves placedin steam condensation environment. The number of grooves was fixed as four and the groove depth considered in the study were 0.10 mm, 0.15 mm and 0.30 mm, width 3.0 mm. It is worth mentioning here that earlier studies by Izumi et al. [17] on condensation heat transfer from grooved vertical plates, reported that there exists an optimum groove width of 2–3 mm at which condensation heat transfer was maximum. Therefore in the present study we have also kept the groove width as 3.0 mm. The degree of sub-cooling was varied from 0.5 °C to 20 °C by adjusting the cooling water flow rate from 70 lph to 110 lph. This sub-cooling range is commonly encountered in refrigeration application devices. The photograph of the tested tubes showed that, the condensation behaviour of steam on the tubes were different, at DT < 2, steam condenses as droplets on the vertical tube surface. However at DT > 2, these droplets coalesce and form liquid film on the tube surface. Therefore, in this work the condensation behaviour is explained by dividing the sub-cooling limit DT < 2 as low degree and DT > 2 as high degree of subcooling.
4.1. Heat transfer characteristics of bare copper tube Experiments were conducted for the condensation of steam on vertical bare copper tube and grooved copper tube by varying the degree of sub-cooling. The condensation heat transfer coefficient calculated based on Eq. (3), presented in Fig. 4 reveals that grooved copper tubes have superior condensation heat transfer characteristics compared to bare copper tubes for all ranges of sub-cooling. Providing grooves on copper tubes increases the surface roughness
Table 1 Uncertainty of the various estimated parameters (Lead coated copper tube). Parameters Measured
Uncertainty
Estimated
d
l
V
T
mm
mm
l/h
°C
0.02
0.02
2%
0.5
A
0.11%
Q
hc
DT = 0.7
DT = 3.11
DT = 0.7
DT = 3.11
5%
3%
15%
35%
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
311
Fig. 5. Contact angle hystersis (h) of cooper tube with/without grooves.
Fig. 6. Comparison of contact angles.
Fig. 7. Comparison of average surface condensation heat transfer coefficient(hc) with degree of sub-cooling (D T) for copper and lead coated copper tube (Without grooves).
thereby increasing the surface entropy. The dependence of surface energy or surface tension forces on surface entropy has already been discussed in Section 2. However, the behaviour of grooved surface shows marked changes depending on the degree of sub-cooling as reported in Table 2. At low degree of sub-cooling
(DT < 2) the contact angle hysteresis (h) (see Section 2) has a major influence on the condensation heat transfer. To study this, the contact angle hysteresis of vertical copper tube with/without grooves was measured by injecting 10mL of water droplet inside the groove by keeping the test specimen vertical. The image of the droplet residing on the surface of the vertical tube was captured using high speed camera (HiSpec 2, Fastec Imaging, USA) and the image was analysed by Image Processing and Analysis software (Image J). The contact angle hysteresis illustrated in Fig. 5 reveals that contact angle hysteresis of grooved copper tubes are lower than copper tube without groove. Also by revisiting Fig. 5 it can be inferred that as the groove depth increases the contact angle hysteresis increases. Higher contact angle hysteresis, improves surface tension and therefore the droplet cannot slide off from the surface easily. In this case, copper tube with 0.10 mm groove depth has the minimum contact angle hysteresis (h = 49.28 deg) in contrast to copper tubes of depth 0.15 mm and 0.30 mm. Therefore, droplet residing in the 0.10 mm groove depth will be able to slide off easily. To substantiate this, the droplet velocity was measured using the images from the high speed digital video camera. The values of droplet sliding velocity reported in Table 3 justifies that the maximum droplet velocity is for copper tube with groove depth 0.10 mm, which corresponds to minimum contact angle hysteresis. Therefore, at low degree of sub-cooling (DT < 2) copper tube with a groove depth of 0.10 mm outperforms bare copper tube and grooved copper tube.
312
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
Table 2 Average surface condensation heat transfer coefficient, hc (kW/m2 K). Specimen
Groove depth, mm 0.00
0.10
0.15
0.30
Low degree of sub-cooling, DT=1.5 °C Copper tube Lead coated copper tube
47.66 95.51
75.36 110.81
68.56 105.53
58.56 76.34
High degree of sub-cooling, DT = 3 °C Copper tube Lead coated copper tube
22.67 48.93
30.48 42.84
35.96 41.46
27.10 32.19
Table 3 Droplet sliding velocity, m/s. Specimen
Groove depth, mm 0.00
0.10
0.15
0.30
Low degree of sub-cooling, DT=1.5 °C Copper tube Lead coated copper tube
0.26 0.66
0.56 1.20
0.50 0.95
0.36 0.59
High degree of sub-cooling, DT = 3°C Copper tube Lead coated copper tube
0.22 0.63
0.46 0.58
0.47 0.55
0.30 0.50
It can be inferred from Table 2, that at high degree of subcooling (DT > 2) Copper tube of groove depth 0.15 mm shows better heat transfer performance. At high degree of sub-cooling, the nucleation site density is larger and therefore larger concentration of liquid droplet is seen on the tube surface. The high concentration of droplets may lead to coalescence of droplets and eventually can flood the copper tube of groove depth 0.10 mm. The subsequent droplet coalescence and flooding of hydrophilic copper tube of depth 0.10 mm results in fall in droplet sliding velocity compared to the droplet velocity for 0.15 mm as reported in Table 3. This drop-in droplet sliding velocity is responsible for poor condensation heat transfer performance for copper tube of groove depth of 0.10 mm in comparison with groove depth of 0.15 mm showing a deterioration of 17.97% in heat transfer coefficient (Table 2). However, a comparison of heat transfer coefficient for copper tube with groove depth of 0.15 mm and 0.30 mm, indicates that the heat transfer performance of vertical copper tube with 0.15 mm groove depth is superior to 0.30 mm depth. As the depth increases the expanded area also increases. This increase in expanded area causes an increase in wall friction which eventually results in reduced droplet velocity as seen in Table 3. Therefore, the condensation heat transfer performance of hydrophilic copper tube of 0.30 mm falls below that of 0.15 mm groove depth with 24.92% reduction in heat transfer coefficient (Table 2). 4.2. Heat transfer characteristics of lead coated copper tube To ensure that lead coated copper tube is more hydrophobic than bare copper tube, the static contact angle of the droplet residing on the horizontal copper tube and lead coated copper tube was measured and analysed using Image J software and is presented in Fig. 6. It can be inferred from the Fig. 6 that the contact angle of lead coated copper tube is greater than bare copper tube, making the lead coated copper tube as more hydrophobic. Experiments were conducted on lead coated copper tube, for same range of sub-cooling, as that considered in the case of bare copper tube. The variation in average surface condensation heat transfer coefficient depicted in Fig. 7, indicates that lead coated copper tube outperforms bare copper tube for all range of sub-cooling with a maximum enhancement of approximately 115.84%. To explain this the SEM images (FESEM, Nova NanoSEM 450, India Ltd) of both the tubes (Fig. 8) were taken at 10,000 magnification. The images
clearly establish that lead coated surface has more roughness and therefore higher surface entropy in comparison to bare copper surface. Consequently, the lead coated copper tube has less surface energy or surface tension force than bare copper tube. As a result, liquid droplets residing on the lead coated copper tube will have a greater mobility than bare copper tube. The droplet velocity tabulated in Table 3 reveals that droplet residing in the lead copper tube has greater velocity than that residing in bare copper tube. The high sliding velocity of the droplet is responsible for higher condensation heat transfer from lead coated copper tube in comparison with bare copper tube. The results clearly indicated that providing grooves and coating independently on bare copper tube enhances condensation heat transfer. Keeping this in view, the combined effect of the above two parameters is investigated by providing grooves and coating on bare copper tube. The experiments were then conducted for the same range of sub-cooling as that in the case of bare copper tube. It can be seen from the Fig. 9 that degree of sub-cooling has a significant influence on the condensation heat transfer of lead coated copper surface with/without grooves. At low degree of subcooling the contact angle hysteresis presented in Fig. 10 indicates that the minimum contact angle hysteresis (h = 30.85 deg) is for lead coated copper tube of groove depth of 0.10 mm relative to lead coated copper tube without grooves and groove depth 0.15 mm and 0.30 mm. The droplet sliding velocity in Table 3 also indicates a higher velocity for lead coated copper tube of groove depth 0.10 mm and therefore higher heat transfer coefficient (Table 2). At high degree of sub-cooling (DT > 2 °C), the presence of grooves is detrimental to the heat transfer performance of lead coated copper tube. At high degree of sub-cooling the heat transfer coefficient is found to decrease with increase in groove depth as reported in Table 2. This behaviour can be attributed to the increase in nucleation site density. Therefore, concentration as well as size of the droplets increases as seen in the photograph of the concentration of droplet concentration depicted in Fig. 11. These factors contribute to increase in surface tension forces which eventually decrease the droplet sliding velocity. In addition to this the increase in expanded area due to larger groove depth further reduces velocity of droplet because of higher sliding friction (see Table 3). The reduced droplet velocity is responsible for the degraded heat transfer performance of grooved hydrophobic copper tube in comparison with hydrophobic copper tube.
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
313
Fig. 8. SEM images of copper and lead coated copper tube (without grooves).
points corresponding to groove depths of 0.15 and 0.30 mm for a particular value of DT degree of sub cooling. This clearly indicates that the heat transfer performance of the vertical tube at low degree of sub-cooling is not significantly affected by the groove depth. 5. Conclusion Experimental data are reported for the condensation of steam on vertical bare copper tube and lead coated copper tube with and without grooves. The number of grooves was fixed as 0.10 mm, 0.15 mm and 0.30 mm and the width as 3.0 mm. The cooling water flow rate was varied so as to maintain degree of sub-cooling in the range 0.5 °C DT 20 °C. The salient conclusions from the study are:
Fig. 9. Comparison of average surface condensation heat transfer coefficient(hc) with degree of sub-cooling (DT) for lead coated copper tube with/without grooves.
Further insight in to the condensation behaviour of grooved vertical tubes (bare copper tube and lead coated copper tube) can be envisaged from Figs. 4, 7 and 9. It can be seen from these figures that at low degree of sub-cooling (DT < 2 °C)error bar for a particular groove depth (say 0.10 mm) will overlap with the data
The presence of grooves on the vertical tube surface enhances condensation heat transfer due to reduced surface tension which in turn resulted in condensate acceleration. Degree of sub-cooling has a major influence on heat transfer performance of grooved copper tube. At low of degree of sub-cooling (DT < 2°C), the condensation performance weakens continuously with increase in groove depth. At high degree of sub-cooling (DT > 2 °C), copper tube of depth 0.15 mm outperforms copper tube of depth 0.10 mm and 0.30 mm.
Fig. 10. Contact angle hysteresis (h) of lead coated copper tube with/without grooves.
314
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315
Fig. 11. Condensate formation at low and high degree of sub cooling.
Lead coated copper tubes shows better heat transfer performance than bare copper tubes for the range of sub-cooling considered in the study. The heat transfer performance of lead coated copper tube with and without grooves is significantly influenced by degree of sub-cooling. At low degree (DT < 2 °C) of sub-cooling lead coated copper tube with a groove 0.10 mm has maximum condensation heat transfer coefficient of all the cases considered in the present study. At high degree of sub-cooling(DT > 2 °C) the presence of grooves demotes the condensation heat transfer performance of lead coated vertical copper tubes. Conflict of interest statement We would like to say that we and our institution don’t have any conflict of interest and don’t have any financial or other relationship with other people or organizations that may inappropriately influence the author’s work. Acknowledgements The fifth author acknowledges the support provided by the ‘‘Research Chair Grant” National Science and Technology Development Agency (NSTDA), the Thailand Research Fund (TRF) and King Mongkut’s University of Technology Thonburi through the ‘‘KMUTT 55th Anniversary Commemorative Fund”. References [1] A. Dehbi, S. Guentay, A model for the performance of a vertical tube condenser in the presence of noncondensable gases, Nuclear Eng. Des. 177 (1997) 41–52. [2] Akili D. Khawaji, Ibrahim K. Kutubkhanah, Jong-Mihn Wie, Advances in seawater desalination technologies, Desalination 221 (2008) 47–69. [3] Man-Hoe Kim, Clark W. Bullard, Air-side performance of brazed aluminium heat exchangers under dehumidi-fying conditions, Int. J. Refrig. 25 (2002) 924– 934. [4] N. Miljkovic, E.N. Wang, Condensation heat transfer on super hydrophobic surfaces, MRS Bull. 38 (5) (2013) 397–406.
[5] T. Haraguchi, R. Shimada, S. Kumagai, The effect of polyvinylidene chloride coating thickness on promotion of dropwise steam condensation, Int. J. Heat Mass Transfer. 34 (12) (1991) 3047–3054. [6] G. Koch, D.C. Zhang, A. Leipertz, Condensation of steam on the surface of hard coated copper discs, Heat Mass Transfer 32 (1997) 149–156. [7] Xuehu Ma, Jiabin Chen, Xu. Dunqi, Jifang Lin, Chunsheng Ren, Zhenhu Long, Influence of processing conditions of polymer film on dropwise condensation heat transfer, Int. J. Heat Mass Transfer 45 (2002) 3405–3411. [8] M.H. Rausch, A. Leipertz, A.P. Frba, Dropwise condensation of steam on ion implanted titanium surfaces, Int. J. Heat Mass Transfer 53 (2010) 423–430. [9] A. Bani Kananeh, M.H. Rausch, A.P. Froba, A. Leipertz, Experimental study of dropwise condensation on plasma-ion implanted stainless steel tubes, Int. J. Heat Mass Transfer 49 (2006) 5018–5026. [10] E. Kalyuzhny, F.A. Kulacki, Condensation of FC-72, Int. J. Heat Mass Transfer 69 (2014) 337–342. [11] Bong June Zhang, Cheng Kuok, Kwang Jin Kim, Taeseon Hwang, Hyungkee Yoon, Dropwise steam condensation on various hydrophobic surfaces: Polyphenylene sulphide (PPS), polytetrafluoroethylene (PTFE), and selfassembled micro/Nano silver (SAMS), Int. J. Heat Mass Transfer 89 (2015) 353–358. [12] S. Vemuri, K.J. Kim, An experimental and theoretical study on the concept of dropwise condensation, Int. J. Heat Mass Transfer 49 (2006) 649–657. [13] Rin Yun, Jaehyeok Heo, Yongchan Kim, Film condensation heat transfer characteristics of R134a on horizontal stainless steel integral-fin tubes at low heat transfer rate, Int. J. Refrig. 32 (2009) 865–873. [14] Abhra Chatterjee, Melanie M. Derby, Yoav Peles, Michael K. Jensen, Enhancement of condensation heat transfer with patterned surfaces, Int. J. Heat Mass Transfer 71 (2014) 675–681. [15] Pallab Sinha Mahapatra, Aritra Ghosh, Ranjan Ganguly, Constantine M. Megaridis, Key design and operating parameters for enhancing dropwise condensation through wettability patterning, Int. J. Heat Mass Transfer 92 (2016) 877–883. [16] Sameer Khandekar, Krishnamurthy Muralidhar, dropwise Condensation on Inclined Textured Surfaces, Springer Briefs in Thermal Engineering and Applied Science. [17] M. Izumi, S. Kumagai, R. Shimada, N. Yamakawa, Heat transfer enhancement of dropwise condensation on a vertical surface with round shaped grooves, Exp. Therm. Flu. Sci. 28 (2e3) (2004) 243–248. [18] Orge R. Lara, Mark T. Holtzapple, Experimental investigation of dropwise condensation on hydrophobic heat exchangers part I: dimpled-sheets, Desalination 278 (2011) 165–172. [19] Orge R. Lara, Mark T. Holtzapple, Experimental investigation of dropwise condensation on hydrophobic heat exchangers. Part II: Effect of coatings and surface geometry, Desalination 280 (2011) 363–369. [20] Qi Baojin, Xu. Zhang Li, Sun Yan Hong, Experimental study on condensation heat transfer of steam on vertical titanium plates with different surface energies, Exp. Therm. Flu. Sci. 35 (2011) 211–218. [21] Kuok Cheng SangsooLee, M.D. Viljar Palmre, Mainul Hossain Bhuiya, Kwang J. Kim, Bong JuneZhang Hyung-keeyoon, Heat transfer measurement during dropwise condensation using micro/nano scale porous surface, Int. J. Heat Mass Transfer 65 (2013) 619–626.
M.R. Rajkumar et al. / International Journal of Heat and Mass Transfer 120 (2018) 305–315 [22] H.W. Hu, G.H. Tang, D. Niu, Experimental investigation of condensation heat transfer on hybrid wettability finned tube with large amount of noncondensable gas, Int. J. Heat Mass Transfer 85 (2015) 513–523. [23] Ding-Jun Huang, Tzong-ShyngLeu, Condensation heat transfer enhancement by surface modification on a monolithic copper heat sink, App. Therm. Eng. 75 (2015) 908–917. [24] H.W. Hu, G.H. Tang, D. Niu, Experimental investigation of convective condensation heat transfer on tube bundles with different surface wettability at large amount of non-condensable gas, App. Therm. Eng. 100 (2016) 699–707. [25] H.M. Ali, A. Briggs, Condensation of R-113 on pin-fin tubes: effect of circumferential pin thickness and spacing, Heat Transfer Eng. 33 (3) (2012) 205–212. [26] H.M. Ali, A. Briggs, Condensation of ethylene glycol on pin-fin tubes: effect of circumferential pin spacing and thickness, App. Therm. Eng. 49 (2012) 9–13. [27] H.M. Ali, A. Briggs, Enhanced condensation of ethylene glycol on single pin-fin tubes: effect of pin geometery, ASME J. Heat Transfer 134 (2012) 0115031– 115038. [28] H.M. Ali, A. Briggs, Condensation heat transfer on pin-fin tubes: Effect of thermal conductivity and pin height, Appl. Therm. Eng. 60 (2012) 465–471. [29] D.I. Katz, R.E. Hope, S.C. Datsko, Liquid retention on finned tubes, Dept. of Eng. Res., Univ. of Michigan, 1946, Project M592. [30] P.J. Marto, An evaluation of film condensation on horizontal integral fin tubes, Trans. ASME. J. Heat Transfer 110 (1988) 1287–1305. [31] A. Briggs, J.W. Rose, An evaluation of models for condensation heat transfer on low finned tube, J. Enhanc. Heat Transf. 6 (1999) 51–60. [32] H.M. Ali, A. Briggs, An investigation of condensate retention on pin-fin tubes, App. Therm. Eng. 63 (2014) 503–510. [33] H.M. Ali, M. Abubaker, Effect of vapour velocity on condensate retention on horizontal pin-fin tubes, Energy Convers. Manage. 86 (2014) 1001–1009. [34] H.M. Ali, A. Ali, Measurements and semi-emperical correlation for condensate retention on horizontal integral-fin tubes: Effect of vapour velocity, Appl. Therm. Eng. 71 (2014) 24–33. [35] H.M. Ali, A. Briggs, A semi-emperical model for free convection condensation on horizontal pin-fin tubes, Int. J. Heat Mass Transfer 81 (2015) 157–166. [36] H.M. Ali, M.Z. Qasim, Free convection condensation of steam on horizontal wire wrapped tubes: Effect of wire thermal conductivity, pitch and diameter, Appl. Thermal. Eng. 90 (2015) 207–214. [37] H.M. Ali, M. Abubaker, Effect of circumferential pin thickness on condensate retention as a function of vapour velocity on horizontal pin-fin tubes, Appl. Thermal. Eng. 91 (2015) 245–251.
315
[38] H.M. Ali, M.Z. Qasim, M. Ali, Free convection condensation heat transfer of stem on horizontal square wire wrapped tubes, Int. J. Heat Mass Transfer 98 (2016) 350–358. [39] H.M. Ali, H. Ali, M. Ali, S. Imran, M.S. Kamran, F. Farukh, Effect of condensate flow rate on retention angle on horizontal low finned tubes, Therm. Sci. (2016) 1–10. [40] H.M. Ali, An analytical model for prediction of condensate flooding on horizontal pin-fin tubes, Int. J. Heat Mass Transfer 106 (2017) 1120–1124. [41] H. Ali, M.S. Kamran, H.M. Ali, F. Farukh, S. Imran, H.S. Wang, Marangoni condensation of steam-ethanol mixtures on a horizontal low-finned tube, Appl. Thermal Eng. 117 (2017) 366–375. [42] R.N. Leach, F. Stevens, S.C. Langford, J.T. Dickinson, Dropwise condensation: experiments and simulations of nucleation and growth of water drops in a cooling system, Langmuir 22 (2006) 8864–8872. [43] A. Leipertz, Dropwise condensation, in: P. Stephan (Ed.), VDI Heat Atlas VDIGVC, second ed., Springer, Germany, 2006, pp. 933–937. [44] Q. Zhao, B.M. Burnside, dropwise condensation of steam on ion implanted condenser surfaces, Heat. Recov. Syst. CHP 14 (5) (1994) 525–534. [45] Pierre-Gilles de Gennes, Francoise Brochard-Wyart and David Quere Capillarity and Wetting Phenom-ena: Drops, Bubbles, Pearls, Waves, first ed., Springer, New York, 2004. [46] C. Furmidge, G, The sliding drop on solid surfaces and a theory for spray retention, J. Colloid. Sci 17 (1962) 309–324. [47] Öner Didem, Thomas J. McCarthy, Ultra hydrophobic surfaces. Effects of topography length scales on wettability, Langmuir 16 (20) (2000) 7777–7782. [48] X.H. Ma, X.D. Zhou, Z. Lan, Y.M. Li, Y. Zhang, Condensation heat transfer enhancement in the presence ofnon-condensable gas using the inter facial effect of dropwise condensation, Int. J. Heat Mass Transf. 51 (2008) 1728– 1737. [49] S. Kim, K. Kim, J, dropwise condensation suitable for super hydrophobic surfaces, ASME J. Heat Transf. 133 (8) (2011) 0815021–0815028. [50] K. Rykaczewski, Micro droplet growth mechanism during water condensation on super hydrophobic sur-faces, Langmuir 28 (2012) 7720–7729. [51] ProE, Wildfire 4.0, Fundamentals of Heat and Mass Transfer, fifth ed., John Wiley and sons, Singapore, 2002. [52] F.P. Incropera, D.P. Dewitt, Fundamentals of Heat and Mass Transfer, fifth ed., John Wiley and sons, Singapore, 2002. [53] Ernest O. Doebelin, Dhanesh N. Manik, Measurement Systems, sixth ed., McGraw-Hill Company, New Delhi, 2012. [54] S.J. Kline, F.A. Mcklintock, Describing uncertainties in single sample experiments, Mech. Eng. (1953) 3.