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Heat transfer of swirling impinging jets with TiO2-water nanofluids
Accepted Manuscript Title: Heat transfer of swirling impinging jets with TiO2 -water nanofluids Authors: K. Wongcharee, V. Chuwattanakul, S. Eiamsa-ard PII: DOI: Reference:
S0255-2701(16)30271-9 http://dx.doi.org/doi:10.1016/j.cep.2017.01.004 CEP 6913
To appear in:
Chemical Engineering and Processing
Received date: Revised date: Accepted date:
6-8-2016 22-12-2016 15-1-2017
Please cite this article as: K.Wongcharee, V.Chuwattanakul, S.Eiamsa-ard, Heat transfer of swirling impinging jets with TiO2-water nanofluids, Chemical Engineering and Processing http://dx.doi.org/10.1016/j.cep.2017.01.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Heat transfer of swirling impinging jets with TiO2-water nanofluids
K. Wongcharee1, V. Chuwattanakul2 and S. Eiamsa-ard3,* 1
Department of Chemical Engineering, Faculty of Engineering,
Mahanakorn University of Technology, Bangkok 10530, Thailand 2
Department of Food Engineering, Faculty of Engineering
King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand 3
Department of Mechanical Engineering, Faculty of Engineering,
Mahanakorn University of Technology, Bangkok 10530, Thailand E-mail address: [email protected] *Corresponding author
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Graphical highlight
Research highlights
Heat transfer of swirling impinging jets with nanofluids is investigated.
The nanofluid with concentration of 2.0 %vol. gives the highest heat transfer rate.
The maximum Nusselt number is achieved at y/w = 6.0.
Abstract An experimental work has been carried out to study the heat transfer of swirling impinging jets (SIJs) with TiO2-water nanofluids using thermochromic liquid crystal technique. Experiments were carried out at constant jet-to-target spacing ratio (L/D) of 1.0 for Reynolds numbers from 5000 to 20,000 by using nanofluids with TiO2 concentrations of 0.5%, 1.0%, 1.5%, 2.0% and 2.5% by volume, twisted tapes with twist ratios (y/W) of 4.0, 5.0, 6.0 and 7.0. Conventional impinging jets (CIJs) and swirling impinging jets (SIJs) with pure water were also tested, for comparison. At similar operation conditions, swirling impinging jets (SIJs) offer superior heat transfer to conventional impinging jets. It is also found that the nanofluids with concentrations of 0.5%, 1.0%, 1.5% and 2.0% by volume give higher Nusselt numbers than the base fluid (water) while the one with concentration of 2.5% shows opposite result. Over the studied range, the optimum condition where the maximum Nusselt number is achieved at TiO2-water nanofluids with concentration of 2.0% by volume and y/W = 6.0.
Keywords: Heat transfer, swirl impinging jet, twisted tape, nanofluid
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Nomenclature A
heat transfer surface area, m2
cp
specific heat, J kg-1 K-1
D
inner diameter of the nozzle, m
h
heat transfer coefficient, W m-2 K-1
I
supplied electrical current, A
k
thermal conductivity, W m-1 K-1
L
jet-to-target spacing, m
Nu
Nusselt number mass flow rate, kg s-1
Pr
Prandtl number heat transfer rate, W
R
electrical resistance of the heater sheet
Re
Reynolds number
T
local temperature, K mean temperature, K
t
thickness of thermochromic liquid crystal sheet, m
U
mean axial flow velocity, m s-1
W
twisted tape width, m
y
twisted tape pitch, m
Greek symbols
TLC
emissivity coefficient
volume fraction of nanoparticle
density of fluid, kg m-3
dynamic viscosity of fluid, kg s-1 m-1
Stefan-Boltzman constant
3
subscripts j
bulk
nf
nanofluid
np
nanoparticle
s
surrounding
w
wall/water
1. Introduction Development of electronic industry has offered electronic devices with faster speeds and more compact features. Therefore, the high-heat-flux management in compact spaces is necessary. A jet impingement is one of the most powerful cooling techniques to remove high-heat-flux from a heated surface because of its high localized heat transfer rates. Conventional impinging jets (CIJs) have been extensively investigated [1-5]. Jet impingement has been also adopted in many food-processing operations for example drying, freezing, toasting and baking, etc. In spite of their heat transfer enhancement, non-uniform radial distributions of local surface heat transfer limit their applications. With attempts to improve radial uniformity of heat transfer, swirling impinging jets (SIJs) have been proposed [5-9]. The concept of swirling impingement is introducing tangential flow components into the main flow or inducing swirling flow. Swirling impinging jets can be induced by inserting swirl generators into jet nozzle. Huang and El Genk [5] applied solid swirl generators which had four narrow flow slots in an air jet system. Their experiments covered the following ranges: (1) swirl angles () of 15, 30 and 45, (2) jetto-target spacings (L) of 12.7, 25.4, 50.8 and 76.2 mm corresponding to jet-to-target spacing ratios (L/D) of 1, 2, 4 and 6, respectively. The optimum condition was found at swirl angle () of 15 and jet spacing of 50.8 mm (or jet-to-target spacing ratio, L/D = 4). At the condition, both Nusselt number and radial uniformity were improved as compared to those of the conventional impinging jets (CIJs). Lee et al. [6] enhanced heat transfer of impinging air jets
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using vane-type swirl generators with four different swirl numbers of 0.0, 0.21, 0.44 and 0.77, corresponding to the angles between swirl van and vane axis of 0.0, 15, 30 and 45, respectively. The experiments were carried out for jet-to-target spacing ratios (L/D) ranging from 2 to 10. Their results revealed that at the small jet-to-target spacing ratio (L/D = 2), the mean Nusselt numbers associated with swirling impinging jets at all swirl numbers were higher than those associated with the conventional impinging jet. The maximum Nusselt number was obtained at the swirl number of 0.21 and the jet-to-target spacing ratio of 2. However, the most uniform distribution of Nusselt number was found at the swirl number of 0.77 and the jet-totarget spacing ratio of 10. Yang et al. [7] also reported that radial uniformity was improved as nozzle-to-surface distance increased. Nanan et al. [8] investigated the heat transfer of swirling impingement air jets induced by twisted tape inserts at different twist ratios (y/w) and jet-totarget spacing ratios (L/D). It was found that at small jet-to-target spacing ratios (L/D = 2 and 4), swirling impinging jets showed better performance than conventional impinging jets in heat removing while an adverse effect was observed at large jet-to-target spacing ratios (L/D = 6 and 8). Ianiro and Cardone [9] determined the influence of the swirl number on the wall heat transfer distribution at a constant Reynolds number of 28,000, for different swirl numbers and jet-to-target spacing ratios. They found that swirl jets gave lower heat transfer rates but better uniformity of heat transfer than the conventional one.
Cooling fluids also play an important role on heat transfer enhancement in high performance and compact electronic systems. Air has limited potential for such an application due to its low thermal conductivity and heat capacity. Although, conventional cooling liquids such as water, ethylene glycol and cooling oils have much higher thermal conductivity than air but their cooling capabilities are still insufficient for the requirement of high-heat-flux removal. In recent years, innovative heat transfer fluids namely nanofluids have been extensively utilized and investigated [10-23]. Nanofluids are suspensions of nanosized particles (1-100 nm) in
5
conventional heat transfer fluids or base fluids. In common, nanofluids have higher thermal conductivities and consequently greater heat transfer performances than the base fluids. However, the use of a nanofluid with excess particle loading can result in the decrease of heat transfer due to its high viscosity. Therefore, optimizing nanoparticle concentration of nanofluid is an important issue.
Nanofluids were applied in jet impingements [21-23]. Nguyen et al. [21] augmented heat transfer of a submerged impinging jet by using Al2O3-water nanofluid. Their results showed that the maximum surface heat transfer coefficient was achieved at a moderate jet-to-target spacing of 5 mm and nanofluid concentration of 2.8% by volume. Nanofluid with high particle concentration (6.0% by volume) was not suitable for the heat transfer enhancement due to its high viscosity. Li et al. [22] employed Cu nanoparticles with two different average sizes of 25 nm diameter and 100 nm in preparation of Cu-water nanofluids. The experimental results showed that the nanofluids yielded remarkably higher convective heat transfer coefficient than the base fluid. Tie et al. [23] studied the heat transfer of jet arrays impingement using Cu/water nanofluid as the working fluid. Cu-nanoparticle concentration varied from 0.17 to 0.64 by volume while dispersant sodium dodecyl benzoic sulfate (SDBS) concentration varied from 0.05 wt% to 0.1 wt%. Their experimental results revealed that heat transfer coefficient was improved by increasing the concentration of nanofluid. Although, the application of nanofluids in conventional jet impingement cooling systems has been reported in literature, to the best of our best knowledge, the use of nanofluids in swirling jet impingement is rarely found. The present work aims to the study heat transfer of TiO2-water nanofluid in swirling jet impingement in order to improve the uniformity of surface heat transfer, as compared to that in the conventional jet impingement. TiO2 has been chosen for nanofluid preparation due to its commercial availability and high thermal conductivity. Swirling impinging jets are induced by twisted tape inserts because of the ease of their fabrication and installation. The study was carried out at different twist ratios (y/W = 4.0, 5.0, 6.0 and 7.0), spacing ratios (L/D = 1, 2, 3 and
6
4) and nanofluid concentrations (0.5%, 1.0%, 1.5%, 2.0% and 2.5% by volume) in order to find the optimum condition for heat transfer enhancement.
2. Experimental apparatus The experimental setup with a closed fluid loop (Fig. 1(a-b)) mainly consisted of a fluid reservoir with a chilling system, an ultrasonic vibrator, an impinging chamber, a flow meter, a control valve, RTD (Resistance Temperature Detector), a data logger, a watt meter (Fluke clamp meter model: 325). An impingement system consisted of a jet nozzle and an impinged plate (Fig. 2). The RTD was installed to measure the jet temperature issued form the nozzle. A jet nozzle diameter was 8 mm (D). The impinged target consisted of the following parts (from top to bottom): (1) an electric heater, (2) a thin stainless steel sheet (Omega: KH Series-606/10), and (3) a thermochromic liquid crystal sheet (Omega: LCS-86-KIT) facing to digital camera. The thin stainless steel sheet was 250 mm wide, 250 mm long and 0.15 mm thick while the electric heater sheet had the same area with a thickness of 0.254 mm.
The changing of
temperature fields from the thermochromics liquid crystal sheet in each test run were recorded by the digital camera (Nikon model: D5100) which placed under the impinging chamber.
Twisted tapes were made of aluminum sheets (7 mm wide, 46 mm long and 0.8 mm thick). The tapes were prepared at three twist lengths (y) of 35, 42 and 49 mm, corresponding to twist-towidth ratios (y/W) of 4.0, 5.0, 6.0 and 7.0. A twisted tape was inserted into the nozzle to introduce swirling flow to the impinging jet. Note that the experiments without twisted tape (conventional impinging jets) were also carried out for comparison with swirling impinging jets (SIJs).
TiO2 nanoparticles with particle sizes ranging from 30 to 50 nm were received from Nanostructured Amorphous Material, Inc, USA. TiO2 particles were mixed with hexamethyldisilazane with a mass ratio of 2:1. Then, the mixture was sonicated using ultrasonic
7
vibrator for 1 h and dried. The functionalized TiO2 particles were dispersed in distilled water (the base fluid) by physical mixing and then the suspensions were sonicated for 5 h. TiO2 water/nanofluids were prepared at different concentrations of 0.5%, 1.0%, 1.5%, 2.0% and 2.5% by volume. The nanofluid stability was evaluated by sedimentation visualization and found that all nanofluids were stable for several days without any trace of visible sedimentation as shown in Fig. 1(b). The thermophysical parameters of nanoparticles and nanofluids are given in Table 1. Experiments were carried out for Reynolds numbers ranging from 5000 to 20,000.
3. Data Processing The input electrical power supplied to heated surface in term of heat flux can be expressed as
(1) where I , R and A are the supplied electrical current, the electrical resistance of the heater sheet and the heat transfer surface area. The input electrical power supplied to heated surface was kept constant at 120 to 194 W/m2 for all experiments. During the experiments, there were heat losses through the thin stainless steel sheet and thermochromic liquid crystal sheet on the rear of the impinged target, in forms of natural convection, radiation and conduction. The heat loss due to natural convection can be written as (2) where Tw is an average of local wall temperature obtained from the color information on TLC sheet and Ts is a surrounding temperature and hc is the natural convective heat transfer coefficient from horizontal surface to surrounding. The coefficient can be obtained from the empirical equation [25] which can be expressed as 0.67 Ra 0.25 hc 0.68 [ 1 ( 0.492 / Pr)9 / 16 ] 4 / 9
A radiation loss can be written as
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D knf
(3)
,
(4)
where is the Stefan-Boltzman constant and TLC = 0.9 is the emissivity coefficient for TLC sheet [26].
The effect of lateral heat conduction because of the temperature gradient within the thin stainless steel sheet (TSS) and TLC sheet can be expressed as (5)
,
where kTLC and kTSS are the thermal conductivities of TLC sheet and thin stainless steel sheet (TSS), respectively.
Consequently, the local force convective heat transfer coefficient (h), due to jet impingement can be evaluated from energy balance in small element of impinged plate which is in conjunction with TLC sheet as (6) (7)
The free convection losses due to (
) and radiation (
) through TLC sheet
were respectively around 11.7% and 12.4% of total heat flux. The lateral heat conduction (
) within impinged plate can be negligible (less than 1% of total heat flux). The heat
losses were found to be similar to those found by Geers et al. [26].
The local Nusselt number can be calculated from Nu
hD k
where D is an inner diameter of jet nozzle, and k is the thermal conductivity of fluid.
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(8)
The Reynolds number based on the inner diameter of jet nozzle is given by Re = UD/
(9)
In the present study, U is the fluid velocity and µ is the viscosity of fluid jet. Reynolds number in Eq. (8) was applied for both base fluid and nanofluids. The calculation was based on the properties of the used fluid. In each experiment, the volume flow rate was adjusted to achieve the required Reynolds number. The volume flow rates of the base fluid and nanofluids were different at the same Reynolds number.
The thermophysical properties of nanofluids including density, specific heat, viscosity and thermal conductivity were calculated using nanoparticle volume concentration (), properties of base fluid and nanoparticles. The density of nanofluid was calculated using the simple mixing rule for a mixture: nf (1 ) w np
(10)
The specific heat of the nanofluid was calculated from: c p , nf
np c p , np 1 w c p , w nf
(11)
The experimental validation by Pak and Cho [27] and Xuan and Roetzel [28] indicated that these equations are suitable for nanofluid property evaluation. The thermal conductivity of the nanofluid was calculated using Maxwell model [29] as shown in Eq. (12) which is recommended for homogeneous and low concentration liquid-solid mixtures having randomly dispersed, uniformly sized and non interacting spherical particles [30]. knf kw
knp 2kw 2 (knp kw ) knp 2kw (knp kw )
(12)
Viscosity of nanofluids was calculated via the general Einstein’s formula [31]. nf w (1 )
Where = 2.5, as recommended for hard spheres [30].
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(13)
The uncertainty of the Nusselt number can be expressed as 2 2 2 Nu 1 Nu h Nu D Nu k a Nu Nu h D k
where h
0.5
0.5
2 2 h D h D
(14)
q Tw Ts
The uncertainties of Nusselt number evaluated using the method recommended by Kline and McClintock [32] range from 4.2% to 5.8% which are slightly higher than the ones shown in the previous works [26, 33]. The uncertainties of Reynolds number, temperature measurement and velocity of the fluid are within ±4.7%, ±0.5% and ±2.3%, respectively.
4. Experimental Results 4.1 Validation test of present results The verification test of present system was performed by comparing the present heat transfer results of conventional impinging jet (CIJ) and swirling impinging jets (SIJ) with those reported by Nanan et al. [8], Yang and Lai [34] and Nuntadusit et al. [35] under the same testing condition. The comparison is shown in Fig. 3(a-b). The comparison indicates that the mean Nusselt numbers (Nu) of both the conventional impinging jet (CIJ) and the swirling impinging jets (SIJ) are comparable to those of the previously published works. Therefore, the present system is reasonably reliable.
4.2 Conventional impinging jets with different jet-to-target spacing ratios and swirling impinging jets with different twist ratios Figure 4 shows the effect of jet-to-target spacing ratio (L/D) on Nusselt number distribution of conventional impinging jets. Note that the results shown in this subsection are obtained by using water as the working fluid. At all jet-to-target spacing ratios, high Nusselt numbers or heat transfer rate areas are found around the center of the impinged plate or the stagnation location. The intensity of Nusselt number is higher at a smaller jet-to-target spacing ratio due to
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the stronger impingement on the plate. This is in agreement with the results in Fig. 5, as average Nusselt number considerably increases by decreasing jet-to-target spacing ratio. Therefore, the experiments for swirling impinging jets were carried out at only the smallest jet-to-target spacing ratio (L/D = 1.0). The average Nusselt numbers of swirling impinging jets at different twist ratios and Reynolds numbers are shown in Fig. 6. The results of the conventional impinging jets at similar conditions are also given, for comparison. Obviously, Nusselt number increases with increasing Reynolds number because more massive fluid with stronger turbulence intensity impinges on the surface. At a given Reynolds number, all swirling impinging jets (SIJs) offer superior heat transfer to the conventional ones (CIJs). These results are in conflict with those reported by Ianiro and Cardone [9]. The difference is that the jets in the present work were injected at short jet-to-target spacing (L/D = 1.0) while the jets in the work by Ianiro and Cardone [9] were injected at large jet-to-target spacings (L/D = 2.0 to 10.0). Therefore, the jet weakening due to swirling effect in the present work is insignificant. Consequently, the arrival velocity on impinged plate is still high. Under such a condition, swirling motion is able to enhance heat transfer by spreading to cover larger impinged area and entrainment of surrounding fluid to the existing jet.
For swirling impinging jets, Nusselt number slightly increases with increasing twist ratio (y/W) from 4.0 to 6.0. However, the opposite trend was found with increasing twist ratio (y/W) from 6.0 to 7.0. The results indicate that the twist ratio (y/W) of 6.0 is the optimum geometry of twisted tape insert which gives the best trade-off between increased impingement area and decreased impingement velocity caused by swirling effect or jet spreading.
4.3 Effect of nanofluid concentration The effect of nanofluid concentration on Nusselt number distribution for CIJ at L/D = 1.0 is shown in Fig. 7. The high Nusselt number in red area around stagnation point becomes more intense when nanofluid concentration increases from 0.5% to 2.0% by volume. This can be
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attributed to the better heat transfer by the fluid with a higher thermal conductivity or a lower thermal resistance. However, the size of an impinged area (the area in green, yellow and red) becomes smaller as nanofluid concentration increases. Because the fluid fluidity is diminished as fluid viscosity increases. It should also be mentioned that increasing nanofluid concentration from 2.0% to 2.5% by volume results in the significant reduction of both Nusselt number around stagnation point and the size of an impinged area. This suggests that the influence of viscosity overcomes that of thermal conductivity at high nanofluid concentration.
The effect of nanofluid concentration on average Nusselt number is also shown in Figs. 8 and 9. The results of pure water (the base fluid) are also given as the reference case for comparison. The average Nusselt number (Nu) results in Figs. 8 and 9 show that Nusselt number tends to increase as nanofluid concentration increases from 0.5% to 2.0% by volume, for both swirling and conventional jets. However, the opposite trend was found as nanofluid concentration increases from 2.0% to 2.5% by volume. For both CIJs and SIJs, the nanofluids with concentrations of 0.5%, 1.0%, 1.5% and 2.0% by volume yield higher average Nusselt numbers than pure water. This is attributed to the superior convection facilitated by the higher thermal conductivities of the nanofluids as compared to that of water. However, the nanofluid with concentration of 2.5% by volume yields comparable Nusselt number to (or slightly lower than) water because the nanofluid with excessive nanoparticles has high viscosity which suppressed the fluidity and heat transfer efficiency of the fluid. The similar explanation can also be applied for the lower Nusselt number of the nanofluid with concentration of 2.5% as compared to that of the ones with concentrations of 0.5%, 1.0%, 1.5% and 2.0% by volume (Figs. 8 and 9).
For nanofluids with TiO2 concentrations with concentrations of 0.5%, 1.0%, 1.5% and 2.0% by volume, swirling impinging jet (SIJ) offer superior heat transfer than the conventional one (CIJ) at given nanofluid concentration. This can be explained that the working fluids with low to moderate viscosities are capable to spread over large areas, due to a swirling effect. However,
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the nanofluid with high concentration (2.5% by volume), swirling impinging jet yields lower Nusselt number than the conventional one. It is possible that the high viscosity of the nanofluid retards the flow in tangential direction. In addition, the interplay between the effects of swirl flow and high viscosity causes the lower onset velocity of the swirling impinging jet, as compared to that of the conventional one. For the investigated range, the optimum condition is achieved at nanofluid concentration of 2.0% by volume and twist ratio (y/W) of 6.0.
5. Conclusions Experimental study has been performed to compare the heat transfer of swirling impinging jets (SIJs) with that of conventional impinging jets (CIJs) by using TiO2-water nanofluids and also pure water (based fluid) as the working fluids. The influences of TiO2-water nanofluid concentrations ( = 0.5%, 1.0%, 1.5%, 2.0% and 2.5% by volume), twist ratios of twisted tapes (y/W = 4.0, 5.0, 6.0 and 7.0) were investigated. The major findings can be drawn as follows. 1. At similar operation conditions, swirling impinging jets (SIJs) offer superior heat transfer to conventional impinging jets (CIJs). 2. Nanofluids with concentrations of 0.5%, 1.0%, 1.5% and 2.0% by volume give higher Nusselt numbers than the base fluid (water) while the one with concentration of 2.5% shows opposite result. 3. Over the studied range, the optimum condition where the maximum Nusselt number is achieved at TiO2-water nanofluids with concentration of 2.0% by volume and y/W = 6.0.
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CHAMBER WITH IMPINGED PLATE
Computer
Variac transformer Volt meter 220 V
Amp meter Digital camera Light
Light
Fluke clamp meter
RTD Electrical heater sheet
Thermochromics crystal sheet
Chamber with impingement plate
Twisted Tape Swirl Generator DARK ROOM
Data logger
Rotameter
Nozzle fitted with twisted tape
y/w = 4.0 Cold fluid y/w = 5.0 Control valve y/w = 6.0 Ball valve
Cold fluid
y/w = 7.0 Water chiller
Cold fluid tank Ultrasonic vibrator and nano fluid
Pump
(a) schematic diagram
= 0.5%V = 1.0%V = 1.5%V = 2.0%V = 2.5%V
= 0.5%V = 1.0%V = 1.5%V = 2.0%V = 2.5%V
Just after preparation
Seven days after preparation
(b) Photographs of TiO2-water nanofluids at different concentrations Fig. 1. Experimental setup and TiO2-water nanofluids. (continued)
19
D
y w
Nozzle pipe Twisted tape Thin electric heater sheet
Z
l
250 mm x 250 mm
Thin electric heater sheet Thin stainless steel sheet
L
Thermochromic liquid crystal sheet
W 250 mm
Fig. 2. Details of impingement plate.
20
(a) conventional impinging jet (CIJ)
(b) swirling impinging jet (SIJ) Fig. 3. Validation test of the conventional impinging jet (CIJ) and swirling impinging jets (SIJ) at ratio of jet-to-plate/nozzle diameter (L/D) of 1.0 and Re = 20,000 (y/W = 3.0 for SIJ).
21
Nu -90
-90
-80
-80
-70
-70
-60
-60
-50
Y/D
Y/D
Nu
-50
-40
-40
-30
-30
-20
-20
-10
-10
X/D L/D = 1
X/D L/D = 2 Nu
-90
-90
-80
-80
-70
-70
-60
-60
-50
Y/D
Y/D
Nu
-50
-40
-40
-30
-30
-20
-20
-10
-10
X/D L/D = 3
X/D L/D = 4
Fig. 4. Effect of ratio of jet-to-plate/nozzle diameter on Nusselt number distribution
120 Re=5000 Re=10000 Re=15000 Re=20000
110
CIJ
Nusselt number
100 90 80 70 60 50 40 0
1
2
3
4
5
L/D
Fig. 5. Effect of ratio of jet-to-plate/nozzle diameter on the average Nusselt number.
22
120 SIJ, Re=5000 SIJ, Re=10000 SIJ, Re=15000 SIJ, Re=20000
Nusselt number
110
CIJ, Re=5000 CIJ, Re=10000 CIJ, Re=15000 CIJ, Re=20000
100
90
80
70
60
3
4
5
6
7
8
y/W
Fig. 6. Effect of twist ratio on average Nusselt number at L/D = 1.0.
23
Nu
Nu
-100
-100
-90
-90
-90
-80
-80
-80
-70
-70
-70
-60
-60
Y/D
-100
Y/D
Y/D
Nu
-60
-50
-50
-50
-40
-40
-40
-30
-30
-30
-20
-20
-20
X/D 0.5%V
X/D 1.0%V
X/D 1.5%V Nu
-100
-100
-90
-90
-80
-80
-70
-70
-60
Y/D
Y/D
Nu
-60
-50
-50
-40
-40
-30
-30
-20
-20
X/D 2.0%V
X/D 2.5%V
Fig. 7. Effect of nanofluid concentration on Nusselt number distribution for CIJ at L/D = 1.0.
24
120 CIJnf, Re=5000 CIJnf, Re=10000 CIJnf, Re=15000 CIJnf, Re=20000
Nusselt number
110
CIJ, Re=5000 CIJ, Re=10000 CIJ, Re=15000 CIJ, Re=20000
100
90
80
70
60 0.0
0.0.5
1.0
1.5
2.0
2.5
3.0
Concentration (%V)
Fig. 8. Effect of nanofluid concentration on average Nusselt number for CIJ at L/D = 1.0.
140 SIJnf, Re=5000 SIJnf, Re=10000 SIJnf, Re=15000 SIJnf, Re=20000
130
SIJ, Re=5000 SIJ, Re=10000 SIJ, Re=15000 SIJ, Re=20000
Nusselt number
120 110 100 90 80 70 0.0
0.0.5
1.0
1.5
2.0
2.5
3.0
Concentration (%V)
Fig. 9. Effect of nanofluid concentration on average Nusselt number for SIJ at L/D = 1.0.
25
Table 1: Thermophysical parameters of nanoparticles and nanofluids
(a) Based fluid
water
(b) Nanoparticle
TiO2
(c) Nanoparticle diameter, (nm)
30 to 50
(d) Nanoparticle thermal conductivity coefficient, knp (W/m K)
13.7
(e) Nanoparticle specific heat, cnp (J/kg K)
685
(f) Nanoparticle density, np (kg/m3)
4170
(g) Nanofluid concentration, (% by volume)
0.5, 1.0, 1.5, 2.0 and 2.5
(h) TiO2 water/nanofluid density, nf (kg/m3)
1016-1075
(i) TiO2 water/nanofluid specific heat, cnf (J/kg K)
3779-4038
(j) TiO2 water/nanofluid thermal conductivity coefficient, knf (W/m K)
0.62-0.65
(k) TiO2 water/nanofluid viscosity, nf (mPa s)
0.88-0.93
26
S0255-2701(16)30271-9 http://dx.doi.org/doi:10.1016/j.cep.2017.01.004 CEP 6913
To appear in:
Chemical Engineering and Processing
Received date: Revised date: Accepted date:
6-8-2016 22-12-2016 15-1-2017
Please cite this article as: K.Wongcharee, V.Chuwattanakul, S.Eiamsa-ard, Heat transfer of swirling impinging jets with TiO2-water nanofluids, Chemical Engineering and Processing http://dx.doi.org/10.1016/j.cep.2017.01.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Heat transfer of swirling impinging jets with TiO2-water nanofluids
K. Wongcharee1, V. Chuwattanakul2 and S. Eiamsa-ard3,* 1
Department of Chemical Engineering, Faculty of Engineering,
Mahanakorn University of Technology, Bangkok 10530, Thailand 2
Department of Food Engineering, Faculty of Engineering
King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand 3
Department of Mechanical Engineering, Faculty of Engineering,
Mahanakorn University of Technology, Bangkok 10530, Thailand E-mail address: [email protected] *Corresponding author
1
Graphical highlight
Research highlights
Heat transfer of swirling impinging jets with nanofluids is investigated.
The nanofluid with concentration of 2.0 %vol. gives the highest heat transfer rate.
The maximum Nusselt number is achieved at y/w = 6.0.
Abstract An experimental work has been carried out to study the heat transfer of swirling impinging jets (SIJs) with TiO2-water nanofluids using thermochromic liquid crystal technique. Experiments were carried out at constant jet-to-target spacing ratio (L/D) of 1.0 for Reynolds numbers from 5000 to 20,000 by using nanofluids with TiO2 concentrations of 0.5%, 1.0%, 1.5%, 2.0% and 2.5% by volume, twisted tapes with twist ratios (y/W) of 4.0, 5.0, 6.0 and 7.0. Conventional impinging jets (CIJs) and swirling impinging jets (SIJs) with pure water were also tested, for comparison. At similar operation conditions, swirling impinging jets (SIJs) offer superior heat transfer to conventional impinging jets. It is also found that the nanofluids with concentrations of 0.5%, 1.0%, 1.5% and 2.0% by volume give higher Nusselt numbers than the base fluid (water) while the one with concentration of 2.5% shows opposite result. Over the studied range, the optimum condition where the maximum Nusselt number is achieved at TiO2-water nanofluids with concentration of 2.0% by volume and y/W = 6.0.
Keywords: Heat transfer, swirl impinging jet, twisted tape, nanofluid
2
Nomenclature A
heat transfer surface area, m2
cp
specific heat, J kg-1 K-1
D
inner diameter of the nozzle, m
h
heat transfer coefficient, W m-2 K-1
I
supplied electrical current, A
k
thermal conductivity, W m-1 K-1
L
jet-to-target spacing, m
Nu
Nusselt number mass flow rate, kg s-1
Pr
Prandtl number heat transfer rate, W
R
electrical resistance of the heater sheet
Re
Reynolds number
T
local temperature, K mean temperature, K
t
thickness of thermochromic liquid crystal sheet, m
U
mean axial flow velocity, m s-1
W
twisted tape width, m
y
twisted tape pitch, m
Greek symbols
TLC
emissivity coefficient
volume fraction of nanoparticle
density of fluid, kg m-3
dynamic viscosity of fluid, kg s-1 m-1
Stefan-Boltzman constant
3
subscripts j
bulk
nf
nanofluid
np
nanoparticle
s
surrounding
w
wall/water
1. Introduction Development of electronic industry has offered electronic devices with faster speeds and more compact features. Therefore, the high-heat-flux management in compact spaces is necessary. A jet impingement is one of the most powerful cooling techniques to remove high-heat-flux from a heated surface because of its high localized heat transfer rates. Conventional impinging jets (CIJs) have been extensively investigated [1-5]. Jet impingement has been also adopted in many food-processing operations for example drying, freezing, toasting and baking, etc. In spite of their heat transfer enhancement, non-uniform radial distributions of local surface heat transfer limit their applications. With attempts to improve radial uniformity of heat transfer, swirling impinging jets (SIJs) have been proposed [5-9]. The concept of swirling impingement is introducing tangential flow components into the main flow or inducing swirling flow. Swirling impinging jets can be induced by inserting swirl generators into jet nozzle. Huang and El Genk [5] applied solid swirl generators which had four narrow flow slots in an air jet system. Their experiments covered the following ranges: (1) swirl angles () of 15, 30 and 45, (2) jetto-target spacings (L) of 12.7, 25.4, 50.8 and 76.2 mm corresponding to jet-to-target spacing ratios (L/D) of 1, 2, 4 and 6, respectively. The optimum condition was found at swirl angle () of 15 and jet spacing of 50.8 mm (or jet-to-target spacing ratio, L/D = 4). At the condition, both Nusselt number and radial uniformity were improved as compared to those of the conventional impinging jets (CIJs). Lee et al. [6] enhanced heat transfer of impinging air jets
4
using vane-type swirl generators with four different swirl numbers of 0.0, 0.21, 0.44 and 0.77, corresponding to the angles between swirl van and vane axis of 0.0, 15, 30 and 45, respectively. The experiments were carried out for jet-to-target spacing ratios (L/D) ranging from 2 to 10. Their results revealed that at the small jet-to-target spacing ratio (L/D = 2), the mean Nusselt numbers associated with swirling impinging jets at all swirl numbers were higher than those associated with the conventional impinging jet. The maximum Nusselt number was obtained at the swirl number of 0.21 and the jet-to-target spacing ratio of 2. However, the most uniform distribution of Nusselt number was found at the swirl number of 0.77 and the jet-totarget spacing ratio of 10. Yang et al. [7] also reported that radial uniformity was improved as nozzle-to-surface distance increased. Nanan et al. [8] investigated the heat transfer of swirling impingement air jets induced by twisted tape inserts at different twist ratios (y/w) and jet-totarget spacing ratios (L/D). It was found that at small jet-to-target spacing ratios (L/D = 2 and 4), swirling impinging jets showed better performance than conventional impinging jets in heat removing while an adverse effect was observed at large jet-to-target spacing ratios (L/D = 6 and 8). Ianiro and Cardone [9] determined the influence of the swirl number on the wall heat transfer distribution at a constant Reynolds number of 28,000, for different swirl numbers and jet-to-target spacing ratios. They found that swirl jets gave lower heat transfer rates but better uniformity of heat transfer than the conventional one.
Cooling fluids also play an important role on heat transfer enhancement in high performance and compact electronic systems. Air has limited potential for such an application due to its low thermal conductivity and heat capacity. Although, conventional cooling liquids such as water, ethylene glycol and cooling oils have much higher thermal conductivity than air but their cooling capabilities are still insufficient for the requirement of high-heat-flux removal. In recent years, innovative heat transfer fluids namely nanofluids have been extensively utilized and investigated [10-23]. Nanofluids are suspensions of nanosized particles (1-100 nm) in
5
conventional heat transfer fluids or base fluids. In common, nanofluids have higher thermal conductivities and consequently greater heat transfer performances than the base fluids. However, the use of a nanofluid with excess particle loading can result in the decrease of heat transfer due to its high viscosity. Therefore, optimizing nanoparticle concentration of nanofluid is an important issue.
Nanofluids were applied in jet impingements [21-23]. Nguyen et al. [21] augmented heat transfer of a submerged impinging jet by using Al2O3-water nanofluid. Their results showed that the maximum surface heat transfer coefficient was achieved at a moderate jet-to-target spacing of 5 mm and nanofluid concentration of 2.8% by volume. Nanofluid with high particle concentration (6.0% by volume) was not suitable for the heat transfer enhancement due to its high viscosity. Li et al. [22] employed Cu nanoparticles with two different average sizes of 25 nm diameter and 100 nm in preparation of Cu-water nanofluids. The experimental results showed that the nanofluids yielded remarkably higher convective heat transfer coefficient than the base fluid. Tie et al. [23] studied the heat transfer of jet arrays impingement using Cu/water nanofluid as the working fluid. Cu-nanoparticle concentration varied from 0.17 to 0.64 by volume while dispersant sodium dodecyl benzoic sulfate (SDBS) concentration varied from 0.05 wt% to 0.1 wt%. Their experimental results revealed that heat transfer coefficient was improved by increasing the concentration of nanofluid. Although, the application of nanofluids in conventional jet impingement cooling systems has been reported in literature, to the best of our best knowledge, the use of nanofluids in swirling jet impingement is rarely found. The present work aims to the study heat transfer of TiO2-water nanofluid in swirling jet impingement in order to improve the uniformity of surface heat transfer, as compared to that in the conventional jet impingement. TiO2 has been chosen for nanofluid preparation due to its commercial availability and high thermal conductivity. Swirling impinging jets are induced by twisted tape inserts because of the ease of their fabrication and installation. The study was carried out at different twist ratios (y/W = 4.0, 5.0, 6.0 and 7.0), spacing ratios (L/D = 1, 2, 3 and
6
4) and nanofluid concentrations (0.5%, 1.0%, 1.5%, 2.0% and 2.5% by volume) in order to find the optimum condition for heat transfer enhancement.
2. Experimental apparatus The experimental setup with a closed fluid loop (Fig. 1(a-b)) mainly consisted of a fluid reservoir with a chilling system, an ultrasonic vibrator, an impinging chamber, a flow meter, a control valve, RTD (Resistance Temperature Detector), a data logger, a watt meter (Fluke clamp meter model: 325). An impingement system consisted of a jet nozzle and an impinged plate (Fig. 2). The RTD was installed to measure the jet temperature issued form the nozzle. A jet nozzle diameter was 8 mm (D). The impinged target consisted of the following parts (from top to bottom): (1) an electric heater, (2) a thin stainless steel sheet (Omega: KH Series-606/10), and (3) a thermochromic liquid crystal sheet (Omega: LCS-86-KIT) facing to digital camera. The thin stainless steel sheet was 250 mm wide, 250 mm long and 0.15 mm thick while the electric heater sheet had the same area with a thickness of 0.254 mm.
The changing of
temperature fields from the thermochromics liquid crystal sheet in each test run were recorded by the digital camera (Nikon model: D5100) which placed under the impinging chamber.
Twisted tapes were made of aluminum sheets (7 mm wide, 46 mm long and 0.8 mm thick). The tapes were prepared at three twist lengths (y) of 35, 42 and 49 mm, corresponding to twist-towidth ratios (y/W) of 4.0, 5.0, 6.0 and 7.0. A twisted tape was inserted into the nozzle to introduce swirling flow to the impinging jet. Note that the experiments without twisted tape (conventional impinging jets) were also carried out for comparison with swirling impinging jets (SIJs).
TiO2 nanoparticles with particle sizes ranging from 30 to 50 nm were received from Nanostructured Amorphous Material, Inc, USA. TiO2 particles were mixed with hexamethyldisilazane with a mass ratio of 2:1. Then, the mixture was sonicated using ultrasonic
7
vibrator for 1 h and dried. The functionalized TiO2 particles were dispersed in distilled water (the base fluid) by physical mixing and then the suspensions were sonicated for 5 h. TiO2 water/nanofluids were prepared at different concentrations of 0.5%, 1.0%, 1.5%, 2.0% and 2.5% by volume. The nanofluid stability was evaluated by sedimentation visualization and found that all nanofluids were stable for several days without any trace of visible sedimentation as shown in Fig. 1(b). The thermophysical parameters of nanoparticles and nanofluids are given in Table 1. Experiments were carried out for Reynolds numbers ranging from 5000 to 20,000.
3. Data Processing The input electrical power supplied to heated surface in term of heat flux can be expressed as
(1) where I , R and A are the supplied electrical current, the electrical resistance of the heater sheet and the heat transfer surface area. The input electrical power supplied to heated surface was kept constant at 120 to 194 W/m2 for all experiments. During the experiments, there were heat losses through the thin stainless steel sheet and thermochromic liquid crystal sheet on the rear of the impinged target, in forms of natural convection, radiation and conduction. The heat loss due to natural convection can be written as (2) where Tw is an average of local wall temperature obtained from the color information on TLC sheet and Ts is a surrounding temperature and hc is the natural convective heat transfer coefficient from horizontal surface to surrounding. The coefficient can be obtained from the empirical equation [25] which can be expressed as 0.67 Ra 0.25 hc 0.68 [ 1 ( 0.492 / Pr)9 / 16 ] 4 / 9
A radiation loss can be written as
8
D knf
(3)
,
(4)
where is the Stefan-Boltzman constant and TLC = 0.9 is the emissivity coefficient for TLC sheet [26].
The effect of lateral heat conduction because of the temperature gradient within the thin stainless steel sheet (TSS) and TLC sheet can be expressed as (5)
,
where kTLC and kTSS are the thermal conductivities of TLC sheet and thin stainless steel sheet (TSS), respectively.
Consequently, the local force convective heat transfer coefficient (h), due to jet impingement can be evaluated from energy balance in small element of impinged plate which is in conjunction with TLC sheet as (6) (7)
The free convection losses due to (
) and radiation (
) through TLC sheet
were respectively around 11.7% and 12.4% of total heat flux. The lateral heat conduction (
) within impinged plate can be negligible (less than 1% of total heat flux). The heat
losses were found to be similar to those found by Geers et al. [26].
The local Nusselt number can be calculated from Nu
hD k
where D is an inner diameter of jet nozzle, and k is the thermal conductivity of fluid.
9
(8)
The Reynolds number based on the inner diameter of jet nozzle is given by Re = UD/
(9)
In the present study, U is the fluid velocity and µ is the viscosity of fluid jet. Reynolds number in Eq. (8) was applied for both base fluid and nanofluids. The calculation was based on the properties of the used fluid. In each experiment, the volume flow rate was adjusted to achieve the required Reynolds number. The volume flow rates of the base fluid and nanofluids were different at the same Reynolds number.
The thermophysical properties of nanofluids including density, specific heat, viscosity and thermal conductivity were calculated using nanoparticle volume concentration (), properties of base fluid and nanoparticles. The density of nanofluid was calculated using the simple mixing rule for a mixture: nf (1 ) w np
(10)
The specific heat of the nanofluid was calculated from: c p , nf
np c p , np 1 w c p , w nf
(11)
The experimental validation by Pak and Cho [27] and Xuan and Roetzel [28] indicated that these equations are suitable for nanofluid property evaluation. The thermal conductivity of the nanofluid was calculated using Maxwell model [29] as shown in Eq. (12) which is recommended for homogeneous and low concentration liquid-solid mixtures having randomly dispersed, uniformly sized and non interacting spherical particles [30]. knf kw
knp 2kw 2 (knp kw ) knp 2kw (knp kw )
(12)
Viscosity of nanofluids was calculated via the general Einstein’s formula [31]. nf w (1 )
Where = 2.5, as recommended for hard spheres [30].
10
(13)
The uncertainty of the Nusselt number can be expressed as 2 2 2 Nu 1 Nu h Nu D Nu k a Nu Nu h D k
where h
0.5
0.5
2 2 h D h D
(14)
q Tw Ts
The uncertainties of Nusselt number evaluated using the method recommended by Kline and McClintock [32] range from 4.2% to 5.8% which are slightly higher than the ones shown in the previous works [26, 33]. The uncertainties of Reynolds number, temperature measurement and velocity of the fluid are within ±4.7%, ±0.5% and ±2.3%, respectively.
4. Experimental Results 4.1 Validation test of present results The verification test of present system was performed by comparing the present heat transfer results of conventional impinging jet (CIJ) and swirling impinging jets (SIJ) with those reported by Nanan et al. [8], Yang and Lai [34] and Nuntadusit et al. [35] under the same testing condition. The comparison is shown in Fig. 3(a-b). The comparison indicates that the mean Nusselt numbers (Nu) of both the conventional impinging jet (CIJ) and the swirling impinging jets (SIJ) are comparable to those of the previously published works. Therefore, the present system is reasonably reliable.
4.2 Conventional impinging jets with different jet-to-target spacing ratios and swirling impinging jets with different twist ratios Figure 4 shows the effect of jet-to-target spacing ratio (L/D) on Nusselt number distribution of conventional impinging jets. Note that the results shown in this subsection are obtained by using water as the working fluid. At all jet-to-target spacing ratios, high Nusselt numbers or heat transfer rate areas are found around the center of the impinged plate or the stagnation location. The intensity of Nusselt number is higher at a smaller jet-to-target spacing ratio due to
11
the stronger impingement on the plate. This is in agreement with the results in Fig. 5, as average Nusselt number considerably increases by decreasing jet-to-target spacing ratio. Therefore, the experiments for swirling impinging jets were carried out at only the smallest jet-to-target spacing ratio (L/D = 1.0). The average Nusselt numbers of swirling impinging jets at different twist ratios and Reynolds numbers are shown in Fig. 6. The results of the conventional impinging jets at similar conditions are also given, for comparison. Obviously, Nusselt number increases with increasing Reynolds number because more massive fluid with stronger turbulence intensity impinges on the surface. At a given Reynolds number, all swirling impinging jets (SIJs) offer superior heat transfer to the conventional ones (CIJs). These results are in conflict with those reported by Ianiro and Cardone [9]. The difference is that the jets in the present work were injected at short jet-to-target spacing (L/D = 1.0) while the jets in the work by Ianiro and Cardone [9] were injected at large jet-to-target spacings (L/D = 2.0 to 10.0). Therefore, the jet weakening due to swirling effect in the present work is insignificant. Consequently, the arrival velocity on impinged plate is still high. Under such a condition, swirling motion is able to enhance heat transfer by spreading to cover larger impinged area and entrainment of surrounding fluid to the existing jet.
For swirling impinging jets, Nusselt number slightly increases with increasing twist ratio (y/W) from 4.0 to 6.0. However, the opposite trend was found with increasing twist ratio (y/W) from 6.0 to 7.0. The results indicate that the twist ratio (y/W) of 6.0 is the optimum geometry of twisted tape insert which gives the best trade-off between increased impingement area and decreased impingement velocity caused by swirling effect or jet spreading.
4.3 Effect of nanofluid concentration The effect of nanofluid concentration on Nusselt number distribution for CIJ at L/D = 1.0 is shown in Fig. 7. The high Nusselt number in red area around stagnation point becomes more intense when nanofluid concentration increases from 0.5% to 2.0% by volume. This can be
12
attributed to the better heat transfer by the fluid with a higher thermal conductivity or a lower thermal resistance. However, the size of an impinged area (the area in green, yellow and red) becomes smaller as nanofluid concentration increases. Because the fluid fluidity is diminished as fluid viscosity increases. It should also be mentioned that increasing nanofluid concentration from 2.0% to 2.5% by volume results in the significant reduction of both Nusselt number around stagnation point and the size of an impinged area. This suggests that the influence of viscosity overcomes that of thermal conductivity at high nanofluid concentration.
The effect of nanofluid concentration on average Nusselt number is also shown in Figs. 8 and 9. The results of pure water (the base fluid) are also given as the reference case for comparison. The average Nusselt number (Nu) results in Figs. 8 and 9 show that Nusselt number tends to increase as nanofluid concentration increases from 0.5% to 2.0% by volume, for both swirling and conventional jets. However, the opposite trend was found as nanofluid concentration increases from 2.0% to 2.5% by volume. For both CIJs and SIJs, the nanofluids with concentrations of 0.5%, 1.0%, 1.5% and 2.0% by volume yield higher average Nusselt numbers than pure water. This is attributed to the superior convection facilitated by the higher thermal conductivities of the nanofluids as compared to that of water. However, the nanofluid with concentration of 2.5% by volume yields comparable Nusselt number to (or slightly lower than) water because the nanofluid with excessive nanoparticles has high viscosity which suppressed the fluidity and heat transfer efficiency of the fluid. The similar explanation can also be applied for the lower Nusselt number of the nanofluid with concentration of 2.5% as compared to that of the ones with concentrations of 0.5%, 1.0%, 1.5% and 2.0% by volume (Figs. 8 and 9).
For nanofluids with TiO2 concentrations with concentrations of 0.5%, 1.0%, 1.5% and 2.0% by volume, swirling impinging jet (SIJ) offer superior heat transfer than the conventional one (CIJ) at given nanofluid concentration. This can be explained that the working fluids with low to moderate viscosities are capable to spread over large areas, due to a swirling effect. However,
13
the nanofluid with high concentration (2.5% by volume), swirling impinging jet yields lower Nusselt number than the conventional one. It is possible that the high viscosity of the nanofluid retards the flow in tangential direction. In addition, the interplay between the effects of swirl flow and high viscosity causes the lower onset velocity of the swirling impinging jet, as compared to that of the conventional one. For the investigated range, the optimum condition is achieved at nanofluid concentration of 2.0% by volume and twist ratio (y/W) of 6.0.
5. Conclusions Experimental study has been performed to compare the heat transfer of swirling impinging jets (SIJs) with that of conventional impinging jets (CIJs) by using TiO2-water nanofluids and also pure water (based fluid) as the working fluids. The influences of TiO2-water nanofluid concentrations ( = 0.5%, 1.0%, 1.5%, 2.0% and 2.5% by volume), twist ratios of twisted tapes (y/W = 4.0, 5.0, 6.0 and 7.0) were investigated. The major findings can be drawn as follows. 1. At similar operation conditions, swirling impinging jets (SIJs) offer superior heat transfer to conventional impinging jets (CIJs). 2. Nanofluids with concentrations of 0.5%, 1.0%, 1.5% and 2.0% by volume give higher Nusselt numbers than the base fluid (water) while the one with concentration of 2.5% shows opposite result. 3. Over the studied range, the optimum condition where the maximum Nusselt number is achieved at TiO2-water nanofluids with concentration of 2.0% by volume and y/W = 6.0.
14
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18
CHAMBER WITH IMPINGED PLATE
Computer
Variac transformer Volt meter 220 V
Amp meter Digital camera Light
Light
Fluke clamp meter
RTD Electrical heater sheet
Thermochromics crystal sheet
Chamber with impingement plate
Twisted Tape Swirl Generator DARK ROOM
Data logger
Rotameter
Nozzle fitted with twisted tape
y/w = 4.0 Cold fluid y/w = 5.0 Control valve y/w = 6.0 Ball valve
Cold fluid
y/w = 7.0 Water chiller
Cold fluid tank Ultrasonic vibrator and nano fluid
Pump
(a) schematic diagram
= 0.5%V = 1.0%V = 1.5%V = 2.0%V = 2.5%V
= 0.5%V = 1.0%V = 1.5%V = 2.0%V = 2.5%V
Just after preparation
Seven days after preparation
(b) Photographs of TiO2-water nanofluids at different concentrations Fig. 1. Experimental setup and TiO2-water nanofluids. (continued)
19
D
y w
Nozzle pipe Twisted tape Thin electric heater sheet
Z
l
250 mm x 250 mm
Thin electric heater sheet Thin stainless steel sheet
L
Thermochromic liquid crystal sheet
W 250 mm
Fig. 2. Details of impingement plate.
20
(a) conventional impinging jet (CIJ)
(b) swirling impinging jet (SIJ) Fig. 3. Validation test of the conventional impinging jet (CIJ) and swirling impinging jets (SIJ) at ratio of jet-to-plate/nozzle diameter (L/D) of 1.0 and Re = 20,000 (y/W = 3.0 for SIJ).
21
Nu -90
-90
-80
-80
-70
-70
-60
-60
-50
Y/D
Y/D
Nu
-50
-40
-40
-30
-30
-20
-20
-10
-10
X/D L/D = 1
X/D L/D = 2 Nu
-90
-90
-80
-80
-70
-70
-60
-60
-50
Y/D
Y/D
Nu
-50
-40
-40
-30
-30
-20
-20
-10
-10
X/D L/D = 3
X/D L/D = 4
Fig. 4. Effect of ratio of jet-to-plate/nozzle diameter on Nusselt number distribution
120 Re=5000 Re=10000 Re=15000 Re=20000
110
CIJ
Nusselt number
100 90 80 70 60 50 40 0
1
2
3
4
5
L/D
Fig. 5. Effect of ratio of jet-to-plate/nozzle diameter on the average Nusselt number.
22
120 SIJ, Re=5000 SIJ, Re=10000 SIJ, Re=15000 SIJ, Re=20000
Nusselt number
110
CIJ, Re=5000 CIJ, Re=10000 CIJ, Re=15000 CIJ, Re=20000
100
90
80
70
60
3
4
5
6
7
8
y/W
Fig. 6. Effect of twist ratio on average Nusselt number at L/D = 1.0.
23
Nu
Nu
-100
-100
-90
-90
-90
-80
-80
-80
-70
-70
-70
-60
-60
Y/D
-100
Y/D
Y/D
Nu
-60
-50
-50
-50
-40
-40
-40
-30
-30
-30
-20
-20
-20
X/D 0.5%V
X/D 1.0%V
X/D 1.5%V Nu
-100
-100
-90
-90
-80
-80
-70
-70
-60
Y/D
Y/D
Nu
-60
-50
-50
-40
-40
-30
-30
-20
-20
X/D 2.0%V
X/D 2.5%V
Fig. 7. Effect of nanofluid concentration on Nusselt number distribution for CIJ at L/D = 1.0.
24
120 CIJnf, Re=5000 CIJnf, Re=10000 CIJnf, Re=15000 CIJnf, Re=20000
Nusselt number
110
CIJ, Re=5000 CIJ, Re=10000 CIJ, Re=15000 CIJ, Re=20000
100
90
80
70
60 0.0
0.0.5
1.0
1.5
2.0
2.5
3.0
Concentration (%V)
Fig. 8. Effect of nanofluid concentration on average Nusselt number for CIJ at L/D = 1.0.
140 SIJnf, Re=5000 SIJnf, Re=10000 SIJnf, Re=15000 SIJnf, Re=20000
130
SIJ, Re=5000 SIJ, Re=10000 SIJ, Re=15000 SIJ, Re=20000
Nusselt number
120 110 100 90 80 70 0.0
0.0.5
1.0
1.5
2.0
2.5
3.0
Concentration (%V)
Fig. 9. Effect of nanofluid concentration on average Nusselt number for SIJ at L/D = 1.0.
25
Table 1: Thermophysical parameters of nanoparticles and nanofluids
(a) Based fluid
water
(b) Nanoparticle
TiO2
(c) Nanoparticle diameter, (nm)
30 to 50
(d) Nanoparticle thermal conductivity coefficient, knp (W/m K)
13.7
(e) Nanoparticle specific heat, cnp (J/kg K)
685
(f) Nanoparticle density, np (kg/m3)
4170
(g) Nanofluid concentration, (% by volume)
0.5, 1.0, 1.5, 2.0 and 2.5
(h) TiO2 water/nanofluid density, nf (kg/m3)
1016-1075
(i) TiO2 water/nanofluid specific heat, cnf (J/kg K)
3779-4038
(j) TiO2 water/nanofluid thermal conductivity coefficient, knf (W/m K)
0.62-0.65
(k) TiO2 water/nanofluid viscosity, nf (mPa s)
0.88-0.93
26