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Heat transfer, pressure drop and fouling studies of multi-walled carbon nanotube nano-fluids inside a plate heat exchanger
Accepted Manuscript Heat transfer, pressure drop and fouling studies of multi-walled carbon nanotube nano-fluids inside a plate heat exchanger M.M. Sarafraz, F. Hormozi PII: DOI: Reference:
S0894-1777(15)00319-2 http://dx.doi.org/10.1016/j.expthermflusci.2015.11.004 ETF 8627
To appear in:
Experimental Thermal and Fluid Science
Received Date: Revised Date: Accepted Date:
29 June 2015 28 October 2015 1 November 2015
Please cite this article as: M.M. Sarafraz, F. Hormozi, Heat transfer, pressure drop and fouling studies of multiwalled carbon nanotube nano-fluids inside a plate heat exchanger, Experimental Thermal and Fluid Science (2015), doi: http://dx.doi.org/10.1016/j.expthermflusci.2015.11.004
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Heat transfer, pressure drop and fouling studies of multi-walled carbon nanotube nano-fluids inside a plate heat exchanger M.M. Sarafraz1*, a, F. Hormozi1, b 1
Faculty of Chemical, Petroleum and Gas Engineering, Semnan University, Semnan, Iran
[*corresponding author] a
E-mail: [email protected]
Tel: +989120976870 b
E-mail: [email protected]
Tel: +989123930495
Abstract: This work presents the results of an experimental research on the heat transfer and pressure drop characteristics of multi-walled carbon nanotube, (MWCNT) aqueous nano-fluids inside a plate heat exchanger, (PHE) with the consideration of fouling formation of carbon nanotubes, (CNTs). Influence of operating parameters such as flow rate (700
Keywords: heat transfer, multi-walled carbon nanotube, pressure drop, plate heat exchanger, friction factor, fouling
1. Introduction: With the continuous progress of process intensification in thermal engineering systems and energy developments, demands for high efficient heat exchangers and cooling systems have been dramatically increased. Since heat exchangers have wide applications in industries, special attention has been paid for the intensification of the heat transfer in this media. For this purpose, there are passive and active techniques, which have been utilized by several researchers to obtain the better thermal performance and energy efficiency. Although there are variety types of heat exchangers with the wide applications in refrigerants, automotive, aerospace, cooling systems and micro-electronics, plate heat exchanger, PHE has been one of the major interests of researchers not only because of its anomalous heat transfer area, but also due to its lower size (compactness) in comparison with other types of heat exchangers. In fact, flat plate heat exchangers are cost-effective devices, which can exchange a large quantity of heat in a small space with extensive heat transfer area [1]. Nano-fluids are considered as not only a passive technique, but also a promising way, which open a new window to the future of advanced thermal fluid science. Nano-fluid is comprised of the solid particles with average size of less than 100 nm dispersing in a base fluid with poor thermal conductivity such as water, ethylene glycol or engine oils. Normally, these solid particles are metal oxides or carbon-based nanotubes. Carbon nanotubes, CNTs have also been regarded as wonderful materials with the special mechanical and thermo-physical properties such as anomalous thermal conductivity and heat capacity [2]. Due to the advantages of nano-fluids, many efforts have been made to investigate the potential application of nanofluids inside the heat exchanging media. For instance, in a set of experiments conducted by Sadr et al. [3], thermal performances of nano-fluids in some industrial heat exchangers were evaluated at three mass concentrations of 2%, 4%, and 6% of SiO2–water nano-fluids. These experiments were established to compare the overall heat transfer coefficient and pressure drop of water and nano-fluids in plate and shell-and-tube heat exchangers. Experimental results showed both augmentation and deterioration of heat transfer coefficient for nano-fluids depending on the flow rate and nano-fluid concentration through the heat exchangers. They explained that the reason for
the enhancement was due to the counter effect of the changes in thermo-physical properties of fluids (e.g. viscosity) together with the fouling on the contact surfaces in the heat exchangers. The measured pressure drop revealed an increase in pressure drop in comparison with the base fluid. In another study, Huang et al. [4] investigated the heat transfer and pressure drop characteristics of aqueous alumina and multi-walled carbon nanotube, (MWCNT) water-based nano-fluids in a chevron-type PHE. They found that the heat transfer can be improved by utilizing the nano-fluids, while a little heat transfer enhancement was observed based on a constant flow velocity. The heat transfer deterioration of MWCNT/water nano-fluids was more intensive than alumina/water nano-fluids due to the relatively large viscosity increase of MWCNT/water nano-fluids. In contrast to results published by Huang et al. [4], Goodarzi et al. [5] demonstrated a significant enhancement in convective heat transfer for MWCNT/water nanofluid inside a heat exchanger. Tiwari et al. [6] put an effort to compare the heat transfer performances of various nano-fluids including CeO2, alumina, Titana and SiO2 in a PHE for different volumetric flow rates and concentrations. The optimum concentrations for different nano-fluids were determined, which yielded the maximum heat transfer improvement over the base fluid. They also showed that CeO2/water nano-fluid presented the best thermal performance (maximum performance index enhancement of 16%) with comparatively lower optimum concentration (0.75 vol. %) within the studied nano-fluids. Prasad et al. [7] conducted experiments to study the turbulent forced convection heat transfer and friction of alumina–water nano-fluid flowing through a concentric tube U-bend heat exchanger. The experiments were performed in the Reynolds number ranged from 3000 to 30000 and nano-fluid concentrations of 0.01%, 0.03%. The results indicated that an increase in Reynolds number and Prandtl number yields to an increase in the average Nusselt number, and augmentation of thermal conductivity in the nano-fluid and subsequently an enhancement in the heat transfer coefficient. The empirical correlations for the Nusselt number and friction factor were obtained as functions of the Reynolds number, Prandtl number, volume concentration and aspect ratio of inserts. Sarafraz et al. [8] investigated the thermal performance and efficiency of a heat exchanger working with biological silver/water nano-fluids. Results demonstrated that the forced convection heat transfer could be intensified in case of using nano-fluid inside the heat exchanger. Rate of intensification can be improved by increasing the mass particle loading and volumetric flow rate. Similar results for intensification of forced convection heat transfer in plate heat exchangers can be found in the
literature [9-17]. In above-explained researches, most of authors have presented the positive influence of nano-fluids on the forced convective heat transfer and approved potential application of nano-fluids in heat exchangers. For carbon nanotubes, there are few studies in the literature, which imply the outstanding thermal conductivity of this nano-fluid and their outstanding convective coefficient [18-25]. Therefore, it can be expected that by using the carbon nanotube nano-fluid as a working fluid inside a highefficient heat exchanger such as chevron-type plate heat exchanger, considerable enhancement of heat transfer may be achieved. In this work, a set of experiments has been performed to quantify the overall heat transfer coefficient of multi-walled carbon nanotube nano-fluid and the pressure drop inside a PHE. For this purpose, experiments were conducted at different flow rates, volumetric concentrations and inlet temperatures of nano-fluids. Fouling of nano-fluid inside the heat exchanger is also measured by defining a parameter so-called fouling thermal resistance.
2. Experimental: 2.1.Test rig: Fig. 1 schematically represents the test facility, which consists of three main sections: 1) Two fluid circulation units including nano-fluid loop (red: hot line) and water loop (blue: cold line). 2) Measurement instruments including the inlet and outlet RTDs, Pressure transmitters and tank thermocouples. 3) Main test section (a counter-current chevron-type plate heat exchanger). Each of circulation units has been equipped with a reservoir tank, which holds the working fluid. Water is the working fluid for water loop and MWCNT/water is the working fluid for nano-fluid loop. In addition, for each loop, a centrifugal pump has been utilized to circulate the fluid inside the system. In order to measure the fluid flow, an ultrasonic flow meter (manufactured by Flownetix) has been installed in the loop of each fluid. In order to apply the heat into the system, an AC bolt heater (manufactured by Cetal Co.) has been mounted inside the nano-fluid tank. For water loop, a refrigerant cooler (working with R-134a) is installed inside the tank for chilling the warm water. Temperature of both of tanks was controlled by a PID controller (manufactured by Autonics). Pipes and tanks were heavily isolated by glass wool to prevent any heat losses to environment. In order to measure the temperatures of fluids, four RTDs were mounted at inlets and outlets of plate heat exchanger. Likewise, two pressure transmitters were mounted at both
inlet/outlet ports of nano-fluid loop to measure the pressure drop inside the heat exchanger. More details on type and precision of instruments have been presented in Table 1. The main test section is the plate heat exchanger, a copper-made chevron type, which is working at the counter-current flow condition. Detailed specifications of the plate heat exchanger can be seen in Fig. 2.
Fig. 1. A schema of experimental loop
Fig. 2. Detailed scheme of the plate heat exchanger used in this work.
Table 1. Specifications and accuracy of measurement instruments Instrument/parameter
Model
Accuracy
Flow meter
Flownetix series 100x
RTDs
PT-100 resistance sensor
Bolt heater
1200W, 100mm 10mm(L D)
Pump
Centrifugal, stainless steel impeller
Pressure transmitter (pressure drop)
Sensys pressure transmitter
1% of readings*
PID controller
Autonics T3H
1% of readings0
1% of readings0
1 K* 2% of readings* Pumping head: 34meter
Thermo-physical properties: Viscosity
Brookfield
Density
Anton paar, DMA500
Thermal conductivity
Decagon KD2-Pro
Specific heat
TA 2920
2.5% of readings0 3% of readings0 2.5% of readings0 4% of readings0
Calculated parameters: Reynolds number
Kline-McClintock
±7.8%
Prandtl number
Kline-McClintock
±9.4%
Overall heat transfer coefficient
Kline-McClintock
±14.2%
Nusselt number
Kline-McClintock
±15.1%
*Based on the calibration process 0
Based on manufacturer claim
Table 2 presents the geometrical properties of plate heat exchanger. Table 2. Detailed specification and geometrical properties of plate heat exchanger Parameters
Value
Unit
A:12.5 B:75 Dimensions:
C:400
mm
D:450 E:125 Length of plate
375
mm
Width of plate
120
mm
Thickness and depth of plate
4
mm
Mean channel spacing
2.2
mm
Number of plates
36
---
Total heat transfer area and heat transfer area per plate*
3.23, 0.095
m2
Average channel spacing
3.3
mm
Fabrication material
Copper
---
Corrugation pitch
4
mm
Corrugation angle
27.5
degree
*Total heat transfer area: (number of plates -2) Heat transfer area per plate
2.2.Nano-fluid preparation and characterization: For preparing the nano-fluids, multi-walled carbon nano-tubes were purchased from USNANO and dispersed into the deionized water as a base fluid. Nonylphenolethoxilate was utilized as a nonionic dispersant. In order to prevent any changes in thermo-physical properties of nanofluids, dispersant was added at only 0.2% of general volume of nano-fluids. Nano-fluids were prepared at desired volumetric concentrations of 0.5, 1 and 1.5%. For characterizing the nanofluids, transmission electron microscopic image, (TEM) and x-ray diffraction, (XRD) were provided to ensure about morphology, nanotube size, and purity of nanotubes. As can be seen in Fig. 3, transmission electron microscope images for all three concentrations show that nanotubes are identical in terms of size and morphology. In addition, rare agglomeration and clusters can be seen inside the images, which is a sign of stabilized nanofluids. About 14 days of stability can be guaranteed according to the settlement-experiments. Likewise, as shown in Fig. 4, the x ray diffraction analysis demonstrated that MWCNTs have pure multi-walled structure and there is no
impurity in structure of nanotubes. Thus, material is reliable and can be used for the experiments. Since impurity may change the thermal conductivity of nano-fluid.
a) Vol.%=0.5
b) Vol.%=1 Fig. 3. TEM image of CNT/water
c) Vol.%=1.5
Fig. 4. XRD pattern of CNT/water
2.3.Thermo-physical properties of nano-fluids: The procedure for measuring the thermo-physical properties of nano-fluids summarily included: Initially, stability of nano-fluids was carefully analyzed using time-settlement experiments. Results demonstrated a stable behavior for all nano-fluids up to successive 14 days. Then experiments were established to measure the thermo-physical properties of nanofluids as follows: For measuring the thermal conductivity of nano-fluids, KD2 Decagon instrument with KS1 sensor was utilized. Measurements were performed at different temperature ranged from 50-70°C near the operating temperature of heat exchanger. For maintaining
the
temperature
of
nano-fluids,
cooling/heating
bath
(thermostat
refrigerant/boiling water/ethylene glycol cycle) was used as well. A PID controller was implemented to control the heating/cooling bath with accuracy of ±0.5K. For specific heat, TA 2920 instrument was used. Specific heat data were also measured at the similar conditions. For viscosity, Brookfield viscometer was implemented at temperatures ranged from 50-70°C. The obtained data were measured for three times to ensure about the reproducibility and repeatability of data. For density, Anton-Paar DMA500 was implemented (with the accuracy of 0.0002gr/cm3). Table 3 shows the summary of stabilizing process for the nano-fluids. Table. 3. Stabilization process for MWCNT nano-fluids Vol.%
nanoparticle
sonication time, (min)
0.5 MWCNT 1 MWCNT 1.5 MWCNT *Nonylphenolethoxilates, NPE general volume of nano-fluids.
Sonication Frequency, (kHz)
Stirring (Min)
Adjusted pH
Stability (days)
80 24 80 8.9 Stable, 14 days 80 46 100 9.1 Stable, 14 days 85 46 120 9.4 Stable, 14 days as dispersant was added into nano-fluids at quantity of approximately 0.2% of
2.4.Data reduction, heat loss analysis and uncertainty: For the heating loop, the heat transfer coefficient can be estimated using following correlation:
Q hot nf mnf . C p ,nf (Tin,nf Tout,nf ) For cooling loop, the heat transfer coefficient can be estimated using following correlation:
(1)
Q cold w mw . C p ,w (Tin,w Tout,w )
(2)
In eq. (3), Q hotnf is the rate of heat transfer belonging to hot nano-fluid, mnf is quantity of mass flow the hot nano-fluid. In eq. (4), Q hotnf is the rate of heat transfer belonging to cooling loop and mw is quantity of mass flow of water. The heat transfer rate is defined as follows:
Qave.
Q hot nf Q cold w 2
(3)
Where, Qave is the heat transfer rate between the heating and cooling loops. To calculate the overall heat transfer coefficient of nano-fluid, U, following equation is utilized as: U
Qave A. TLMTD
(4)
In Eq. (4), TLMTD is the log mean temperature difference, which can be calculated by the following equation: TLMTD
(Tout, nf Tin, w ) (Tin , nf Tout, w ) (Tout, nf Tin , w ) ln( (Tin, nf Tout, w )
(5)
Having the plate depth (equivalent to 4mm) and surface enhancement parameters (equivalent to 1.19), the hydraulic diameter of a corrugated plate heat exchanger can be estimated by using Eq. (6) as follows:
Dhydraulic
2 plate depth surface enhancemen t parameter
(6)
Nusselt and Reynolds and Prantdl numbers as a criterion for heat transfer and fluid flow velocity can be calculated as:
Nu
hnf . Dhydraulic
Re nf
k nf
nf .u nf .Dhydraulic nf
(7) (8)
Pr
C p ,nf . nf
(9)
k nf
Noticeably, the heat transfer coefficient belonging to the hot and cold loop should be calculated separately. For cold loop (water), correlation introduced by Huang et al. [4] was used for estimating the heat transfer coefficient in PHE, which comes as follows:
hw .Dhydraulic kw
0.2302 Re 0.745 . Pr 0.4
(10)
For estimating the heat transfer coefficient in hot loop (nano-fluid), Eq. (11) should be employed as:
1 1 1 U hnf k copper hw
(11)
is the thickness of plate, k is the thermal conductivity of copper since the heat exchanger has been fabricated from copper, hnf and hw are convective heat transfer coefficient of MWCNT/water nano-fluid and cold water (estimated by Eq. 10) respectively. Although the heat exchanger was heavily isolated by glass wool, however, there is possibility for heat loss to the environment, which should be carefully considered in calculation steps. Thus, for estimating the heat loss, an energy balance was employed between the quantity of heat applied to heat exchanger and that of removed by cold water as:
Qnf Qw Qheatloss
(12)
Where Qnf and Qw can be estimated using Eq.s (1-2). Results of energy balance revealed a 9.5% of heat loss during the experiments. In addition, reproducibility of experimental data was accurately checked with three runs of experiments. Deviation of 4.3% was reported for reproducibility and repeatability of data. Likewise, using Kline-McKlintock [26], uncertainty value for heat transfer coefficient was found to be 14.2%.
3. Results and discussions: Presence of carbon nanotubes inside the water can have two possible impacts on the heat transfer and pressure drop value as well. Accordingly, results are presented in two sub-sections. 3.1.Heat transfer characteristics:
In order to investigate the influence of carbon nanotubes inside the water, experiments were established such that the heat transfer coefficient of nano-fluids was measured and compared to that of the base fluid. Also, influence of operating parameters such as: flow rate, concentration and inlet temperature on heat transfer coefficient of nano-fluids and water was experimentally investigated. Fig. 5 represents the influence of flow rate inside the PHE. As can be seen, in terms of flow regime, experimental results can be divided into three significant regions namely laminar transient and turbulent flow regions. According to the results, flow rate has a strong impact on heat transfer coefficient of nano-fluid. With increasing the fluid flow rate, The Nusselt number (as a true criterion for heat transfer coefficient of nano-fluid, hnf) considerably increases, which is in a good agreement with those of previously reported by Kannadasan et al. [27], Dixit-Ghosh [28] and Kabeel et al. [29].
Fig. 5. Influence of flow rate on Nusselt number for MWCNT/water nano-fluid at vol. %=0.5. Integrity of experimental data in terms of reproducibility within three successive runs. Repeating the experiments with other concentration of nano-fluids demonstrated that heat transfer coefficient of nano-fluids can be intensified, when concentration of nano-fluid is increased. However, there is an optimum point for the enhancement. Noticeably, results showed that when concentration of nano-fluid increases, influence of viscosity might outweigh the thermal conductivity of nano-fluid, which consequently decreases the heat transfer coefficient.
Fig. 6 comparatively demonstrates the influence of concentration of nano-fluids on Nusselt number. As can be seen, at volumetric concentration of 0.5 % and 1%, Nusselt number can approximately be enhanced up to 7 and 14% respectively, while for vol.%=1.5, Nusselt number suddenly decreases in comparison with vol.% of 0.5 and 1. However, even in this case, values of Nusselt number are still higher than that of the base fluid.
Fig. 6. Comparison between Nusselt number of nano-fluid and base fluid at different concentrations For better understanding regarding to the influence of concentration, an enhancement parameter is utilized as:
Nu nf
(13)
Nu w
In this parameter, Nunf is the measured Nusselt number for nano-fluid and Nuw is the measured Nusselt number of water flowing inside the PHE. Fig. 7 shows the enhancement parameter for different concentration of nano-fluids and Reynolds number.
Fig. 7. Influence of concentration of nano-fluid on enhancement parameter (Re=4500) As can be seen, with increasing the volumetric concentration of nano-fluid, the enhancement value initially increases up to vol. %=1 and suddenly decreases for vol. %=1.5. Therefore, an optimal concentration for our experiments seems to be at vol. %=1, although more experiments are needed in order to find the exact optimal point for concentration of nano-fluid. For interpreting the phenomenon, it can be stated that lower concentrations, presence of carbon nanotube leads the convective heat transfer coefficient to be enhanced due to the Brownian motion intensified by carbon nanotubes and enhancement of thermal conductivity with a very small penalty in pressure drop. At higher volumetric concentrations, due to the CNT loading inside the bulk of nano-fluid, viscosity and density of nano-fluid is enhanced, followed by augmentation in friction factor and pressure drop. Therefore, enhancement parameter decreases at higher concentrations. Noticeably, enhancement in viscosity of nano-fluids has no impacts on the heat transfer coefficient, while by increasing the concentration of nano-fluids more than the optimal concentration, expenses due to the pumping power, consumption of nanoparticles will also be increased which is undesired. In other words, at concentrations higher than optimal concentration, effect of viscosity on the heat transfer coefficient is more significant in comparison with effect of thermal conductivity on the heat transfer coefficient. Briefly speaking, an efficient nano-fluid has both considerably enhanced thermal conductivity and also a low-value
viscosity. Likewise, increase in concentration of nano-fluids can lead the sedimentation rate of carbon nanotube on the wall surface of heat exchanger to be increased and stability of MWCNT nanofluids to be decreased over the time. The other effective parameter on the heat transfer coefficient is the inlet temperature of nano-fluid. In order to investigate the influence of inlet temperature, experiments were performed at three different inlet temperatures of 50, 60 and 70oC. As shown in Fig. 8, results demonstrated that Nusselt number slightly increases with increasing the inlet temperature, which may be due to the changes in thermo-physical properties of nano-fluid. For better understanding, thermal conductivity of nano-fluids at different volumetric fractions and temperature was measured using KD2 Decagon device and represented in Fig. 9. As can be seen, with increasing the vol. % of nano-fluids, thermal conductivity of nano-fluid increased. In addition, temperature was found to have a very slight influence on enhancement of thermal conductivity, which is in accordance with results collected by Morshed et al. [30]. Since thermal conductivity is the key thermo-physical property, in Fig. 9, the experimental results have been presented as a function of time and concentration of nano-fluids.
Fig. 8. Influence of inlet temperature on Nusselt number of MWCNT/water nano-fluid at volumetric concentration of 1%.
Fig. 9. Thermal conductivity of carbon nanotube nano-fluids as a function of volumetric concentration and temperature According to Fig. 9, it can be stated that for MWCNT/water nano-fluid, thermal conductivity is a slight function of temperature. Therefore, it can be concluded that inlet temperature can slightly enhance the heat transfer coefficient, which is due to the increase in thermal conductivity of nano-fluid. However, this enhancement is not significant, when comparing to the concentration effect. For the concentration effect, it can be seen that thermal conductivity can be enhanced up to 21%, 41% and 68% for vol.% of 0.5, 1 and 1.5% respectively. In order to validate the results, comparisons between well-known correlations and experimental results were made. Figs. 10(a-c) comparatively demonstrate the thermal conductivity of carbon nanotube in comparison with well-known predicting correlations [30]. As can be seen, results of comparison between experimental data and Hamilton-Crosser (Fig.10a), Xue et al. (Fig.10b) and Maxwell (Fig.10c) show that experimental results are in a satisfying agreement with those of estimated by the
correlations. As can also be seen, results are in a fair agreement with well-known predicting correlations. Table 4 presents the well-known correlations used for comparing the thermal conductivity. Table 4. Predicting correlations for thermal conductivity of nano-fluids Author
Correlation
Maxwell
k p 1 2 2 k f 1 k eff k f k p 1 k f 2
Hamilton-Crosser
k eff
Xue et al.
1 2 k p k p k f ln k p k f 2 k f k eff k f 1 2 k f k p k f ln k p k f 2 k f
k p 1 5 5 k f 1 kf k p 1 k f 5
Results of comparison between thermal conductivity and well-known predicting correlations can be seen in Figs. 10(A-C).
(a)
(b)
(c) Fig. 10. Rough comparisons between experimental thermal conductivity with well-known predicting correlations 3.2.Pressure drop characteristics: Experiments showed that using the MWCNT/water nano-fluids could slightly enhance the pressure drop in comparison with base fluid. In fact, presence of nanotubes inside the water can enhance the viscosity and as a result, higher values of pressure drop are reported for the nanofluids. Likewise, pressure drop can be intensified with increasing the flow rate and concentration of nano-fluids as well. Fig. 11 shows the pressure drop of MWCNT/water nano-fluid inside the PHE at different volumetric concentrations and flow rates. Conveniently, Reynolds number is implemented as a suitable criterion for the fluid hydrodynamic velocity. As can be seen, pressure drop is significantly intensified with increasing the flow rate, while it is gradually improved with increasing the concentration of CNTs.
Fig. 11. Influence of flow rate and concentration on the pressure drop of MWCNT/water nanofluids in PHE. Investigation on the friction factor demonstrated that this parameter agrees well with that of base fluid, although a small penalty (undesired enhancement) is reported for nano-fluid due to the presence of carbon nanotubes inside the heat exchanger. Noticeably, viscosity of nano-fluid strongly depends on the concentration of dispersed nanotubes. Thus, by increasing the nanotube concentration, viscosity of nano-fluid enhances and results in enhancement of friction factor and pressure drop as well. Fig. 12 represents the influence of flow rate and concentration of nanotubes on friction factor parameter. In order to validate the results, a comparison between well-known predicting correlation for friction factor and experimental data is made and represented in Fig. 13. As can be seen, Chen and Darcy equations can predict the friction factor only in lower Reynolds number; however, Haaland and Blasius correlations can accurately predict the friction factor of MWCNT/water at higher fluid flows. Note that, these correlations are only defined for fluid flow inside the pipes, therefore, such deviations seems to be reasonable. Moreover, this comparison demonstrates that results are in a fair agreement with
those of theoretically obtained by the correlations. This comparison can also be considered as second validation for the experimental friction factor.
Fig. 12. Influence of concentration and flow rate on friction factor
Fig. 13. Comparison between experimental friction factor and well-known correlations of generic Darcy law [31], Haaland [32] and Chen equation and Blasius [33]
3.3.Overall thermal performance: For evaluating the exact overall thermal performance of MWCNT/water, pressure drop and heat transfer characteristics should simultaneously be considered. For this purpose, thermal performance index is defined as [21]:
.1 / 3
(14)
Where:
f nf
(15)
fw
and, can be obtained using Eq. (13). In fact, ratio of the Nusselt number ratio to the friction factor ratio at the same pumping power is defined as real thermal performance index [21]. Fig. 14 comparatively represents the thermal performance of MWCNT/water nano-fluids at different concentrations. As can be seen, with increasing the volumetric concentration, thermal performance initially increased and suddenly suppressed. The reason for this phenomenon can be referred to this point that: presence of nanotubes into the water at higher CNT concentrations has stronger influence on pressure drop rather than thermal conductivity, while for lower concentrations, impact of nanotubes is more seen on thermal conductivity. Therefore, adding the carbon nanotubes can limit the application and thermal properties of MWCNT/water nano-fluids. Consequently, an optimum point for concentration of nanotubes should be considered, which is out of goals of this work.
Fig. 14. Thermal performance of MWCNT/water nano-fluids at different Reynolds number and concentrations for temperature equivalent to 60°C As can be seen in Fig. 14, inlet temperature can only enhance the thermal performance index up to 4.2%, while for concentration effect; there is an optimum vol.%, which should be carefully considered. 3.4.Fouling factor: When a particulate working fluid flows in a heat-exchanging medium, fouling formation is a matter of concern. When it comes to the MWCNT nano-fluids, nanotube aggregation and clustering can intensify the fouling formation. In order to investigate the fouling formation of MWCNT/water, fouling thermal resistance parameter is defined as:
Rf
1 1 U f Uc
(16)
In Eq. (16), Uc is the overall heat transfer coefficient of PHE, when it is clean (at the initial moment of experiment) and Uf, is the heat transfer coefficient when fouling is formed inside the heat exchanger. Rf is fouling thermal resistance. One sign for fouling formation is that the overall heat transfer coefficient is deteriorated over the extended time. In fact, fouling can create a significant thermal resistance, which reduces the rate of heat transfer. In this work, the heat
exchanger let to be continuously operated for about 720 working hours. During the operation, fouling resistance of heat exchanger was constantly monitored. As can be seen in Fig. 15, fouling resistance parameter has a rectilinear behavior with different line slope and this slope value can be intensified with time of operation and increasing the concentration of MWCNT/water. In terms of order of magnitude, in a rough comparison with fouling resistance of metal oxides [3440], it can be seen that metal oxides inside the heat exchangers have fouling resistance about 10fold larger than MWCNT/water. In addition, fouling resistance recorded for metal oxide has asymptotic behavior versus time.
Fig. 15. Fouling resistance of MWCNT/water nano-fluids inside the PHE Noticeably, for fouling resistance at other Reynolds numbers and nano-fluid inlet temperatures, similar trends were seen. It is noteworthy to state that such fouling formation can be important in higher working hours, for instance, at operating times lower than 300hrs, fouling resistance is ranged from 0.001-0.003, while for operating time between 600 and 720hrs, fouling resistance is between 0.1 and 0.3, which is significant. Nevertheless, as mentioned, in comparison with metal oxide nano-fluids, MWCNT/water nano-fluids have smaller fouling resistance, which put them as a promising alternative for long lasting cooling performance. For better understanding the role of fouling on the heat transfer coefficient, thermal fouling resistance parameter and heat transfer coefficient have been plotted in one figure and for three
volumetric concentrations of nano-fluids. As can be seen in Fig. 16, while fouling resistance has a none-linear behavior, rate of decrease for the heat transfer coefficient is none-linear too.
a) Fouling resistance and the heat transfer coefficient for vol.%=0.5
b) Fouling resistance and the heat transfer coefficient for vol.%=0.1
c) Fouling resistance and the heat transfer coefficient for vol.%=0.5 Fig. 16. Influence of the fouling resistance parameter on the heat transfer coefficient for MWCNT/water nanofluids at Reynolds 4500.
4. Conclusions: Experimental investigation on the forced convection heat transfer to multi-walled carbon nanotube nanofluids inside a chevron-type heat exchanger was performed. This work demonstrated that multi-walled carbon nanotube nano-fluid could be a promising option for convective cooling systems. Following conclusions were also made:
Presence of multi-walled carbon nanotubes inside the water can enhance the thermal conductivity coefficient up to 68% in comparison with base fluid (water). Concentration and temperature were found to enhance the thermal conductivity value.
MWCNT/water nano-fluids have higher forced convective heat transfer coefficient in comparison with water (As a conventional coolant), however, presence of CNTs caused a small penalty for friction factor and pressure drop, which can be ignored.
In terms of thermal performance index, results demonstrated that in some concentrations, enhancement in viscosity of nano-fluid outweigh the thermal conductivity enhancement, which limits the cooling application of MWCNT/water nano-fluid. Thus, it can be stated that for MWCNT/water nano-fluid, there is an optimum concentration, which should be carefully determined. In this work, we only examined three concentrations (0.5, 1 and 1.5% by volume).Results demonstrated that the best thermal performance amongst these concentrations can be obtained at vol.%=1. However, more studies are still required to determine the exact optimum concentration for carbon nanotube nanofluids.
Acknowledgements: Authors of this work dedicate the article to Imam Mahdi and appreciate Semnan University for their financial supports. Nomenclatures: A Cp D f h k L Nu P
area, m2 heat capacity, J.kg-1.oC-1 hydraulic diameter, m fanning friction factor convective heat transfer coefficient, W.m-2. oC-1 thermal conductivity, W.m-1.oC-1 length, m Nusselt number pressure, kPa
Pr prantdl Number Q heat, W Re Reynolds number Rf fouling resistance, m2. K/kW T temperature, oC u fluid velocity, m.s-1 U overall heat transfer coefficient, W/m2. K Subscripts-Superscripts ave average b bulk bs base fluid hot heating loop nf nano-fluid cold cooling loop in inlet out outlet n number of data points m mean m mass flow, kg.s-1 w water Greek symbols thermal diffusion, m2.s-1 density, kg.m-3 viscosity, kg.m-1.s-1 volume fraction or particle loading
plate thickness, m
difference
thermal performance index
Enhancement parameter Abbreviation HTC
heat transfer coefficient
LMTD log mean temperature difference PHE
plate heat exchanger
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Highlights
Thermal performance, pressure drop analysis inside a plate heat exchanger is studied MWCNT/water nano-fluid is used as a working medium Enhancement in overall heat transfer coefficient is reported Smaller fouling resistance is reported in comparison with other nano-fluids
S0894-1777(15)00319-2 http://dx.doi.org/10.1016/j.expthermflusci.2015.11.004 ETF 8627
To appear in:
Experimental Thermal and Fluid Science
Received Date: Revised Date: Accepted Date:
29 June 2015 28 October 2015 1 November 2015
Please cite this article as: M.M. Sarafraz, F. Hormozi, Heat transfer, pressure drop and fouling studies of multiwalled carbon nanotube nano-fluids inside a plate heat exchanger, Experimental Thermal and Fluid Science (2015), doi: http://dx.doi.org/10.1016/j.expthermflusci.2015.11.004
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Heat transfer, pressure drop and fouling studies of multi-walled carbon nanotube nano-fluids inside a plate heat exchanger M.M. Sarafraz1*, a, F. Hormozi1, b 1
Faculty of Chemical, Petroleum and Gas Engineering, Semnan University, Semnan, Iran
[*corresponding author] a
E-mail: [email protected]
Tel: +989120976870 b
E-mail: [email protected]
Tel: +989123930495
Abstract: This work presents the results of an experimental research on the heat transfer and pressure drop characteristics of multi-walled carbon nanotube, (MWCNT) aqueous nano-fluids inside a plate heat exchanger, (PHE) with the consideration of fouling formation of carbon nanotubes, (CNTs). Influence of operating parameters such as flow rate (700
Keywords: heat transfer, multi-walled carbon nanotube, pressure drop, plate heat exchanger, friction factor, fouling
1. Introduction: With the continuous progress of process intensification in thermal engineering systems and energy developments, demands for high efficient heat exchangers and cooling systems have been dramatically increased. Since heat exchangers have wide applications in industries, special attention has been paid for the intensification of the heat transfer in this media. For this purpose, there are passive and active techniques, which have been utilized by several researchers to obtain the better thermal performance and energy efficiency. Although there are variety types of heat exchangers with the wide applications in refrigerants, automotive, aerospace, cooling systems and micro-electronics, plate heat exchanger, PHE has been one of the major interests of researchers not only because of its anomalous heat transfer area, but also due to its lower size (compactness) in comparison with other types of heat exchangers. In fact, flat plate heat exchangers are cost-effective devices, which can exchange a large quantity of heat in a small space with extensive heat transfer area [1]. Nano-fluids are considered as not only a passive technique, but also a promising way, which open a new window to the future of advanced thermal fluid science. Nano-fluid is comprised of the solid particles with average size of less than 100 nm dispersing in a base fluid with poor thermal conductivity such as water, ethylene glycol or engine oils. Normally, these solid particles are metal oxides or carbon-based nanotubes. Carbon nanotubes, CNTs have also been regarded as wonderful materials with the special mechanical and thermo-physical properties such as anomalous thermal conductivity and heat capacity [2]. Due to the advantages of nano-fluids, many efforts have been made to investigate the potential application of nanofluids inside the heat exchanging media. For instance, in a set of experiments conducted by Sadr et al. [3], thermal performances of nano-fluids in some industrial heat exchangers were evaluated at three mass concentrations of 2%, 4%, and 6% of SiO2–water nano-fluids. These experiments were established to compare the overall heat transfer coefficient and pressure drop of water and nano-fluids in plate and shell-and-tube heat exchangers. Experimental results showed both augmentation and deterioration of heat transfer coefficient for nano-fluids depending on the flow rate and nano-fluid concentration through the heat exchangers. They explained that the reason for
the enhancement was due to the counter effect of the changes in thermo-physical properties of fluids (e.g. viscosity) together with the fouling on the contact surfaces in the heat exchangers. The measured pressure drop revealed an increase in pressure drop in comparison with the base fluid. In another study, Huang et al. [4] investigated the heat transfer and pressure drop characteristics of aqueous alumina and multi-walled carbon nanotube, (MWCNT) water-based nano-fluids in a chevron-type PHE. They found that the heat transfer can be improved by utilizing the nano-fluids, while a little heat transfer enhancement was observed based on a constant flow velocity. The heat transfer deterioration of MWCNT/water nano-fluids was more intensive than alumina/water nano-fluids due to the relatively large viscosity increase of MWCNT/water nano-fluids. In contrast to results published by Huang et al. [4], Goodarzi et al. [5] demonstrated a significant enhancement in convective heat transfer for MWCNT/water nanofluid inside a heat exchanger. Tiwari et al. [6] put an effort to compare the heat transfer performances of various nano-fluids including CeO2, alumina, Titana and SiO2 in a PHE for different volumetric flow rates and concentrations. The optimum concentrations for different nano-fluids were determined, which yielded the maximum heat transfer improvement over the base fluid. They also showed that CeO2/water nano-fluid presented the best thermal performance (maximum performance index enhancement of 16%) with comparatively lower optimum concentration (0.75 vol. %) within the studied nano-fluids. Prasad et al. [7] conducted experiments to study the turbulent forced convection heat transfer and friction of alumina–water nano-fluid flowing through a concentric tube U-bend heat exchanger. The experiments were performed in the Reynolds number ranged from 3000 to 30000 and nano-fluid concentrations of 0.01%, 0.03%. The results indicated that an increase in Reynolds number and Prandtl number yields to an increase in the average Nusselt number, and augmentation of thermal conductivity in the nano-fluid and subsequently an enhancement in the heat transfer coefficient. The empirical correlations for the Nusselt number and friction factor were obtained as functions of the Reynolds number, Prandtl number, volume concentration and aspect ratio of inserts. Sarafraz et al. [8] investigated the thermal performance and efficiency of a heat exchanger working with biological silver/water nano-fluids. Results demonstrated that the forced convection heat transfer could be intensified in case of using nano-fluid inside the heat exchanger. Rate of intensification can be improved by increasing the mass particle loading and volumetric flow rate. Similar results for intensification of forced convection heat transfer in plate heat exchangers can be found in the
literature [9-17]. In above-explained researches, most of authors have presented the positive influence of nano-fluids on the forced convective heat transfer and approved potential application of nano-fluids in heat exchangers. For carbon nanotubes, there are few studies in the literature, which imply the outstanding thermal conductivity of this nano-fluid and their outstanding convective coefficient [18-25]. Therefore, it can be expected that by using the carbon nanotube nano-fluid as a working fluid inside a highefficient heat exchanger such as chevron-type plate heat exchanger, considerable enhancement of heat transfer may be achieved. In this work, a set of experiments has been performed to quantify the overall heat transfer coefficient of multi-walled carbon nanotube nano-fluid and the pressure drop inside a PHE. For this purpose, experiments were conducted at different flow rates, volumetric concentrations and inlet temperatures of nano-fluids. Fouling of nano-fluid inside the heat exchanger is also measured by defining a parameter so-called fouling thermal resistance.
2. Experimental: 2.1.Test rig: Fig. 1 schematically represents the test facility, which consists of three main sections: 1) Two fluid circulation units including nano-fluid loop (red: hot line) and water loop (blue: cold line). 2) Measurement instruments including the inlet and outlet RTDs, Pressure transmitters and tank thermocouples. 3) Main test section (a counter-current chevron-type plate heat exchanger). Each of circulation units has been equipped with a reservoir tank, which holds the working fluid. Water is the working fluid for water loop and MWCNT/water is the working fluid for nano-fluid loop. In addition, for each loop, a centrifugal pump has been utilized to circulate the fluid inside the system. In order to measure the fluid flow, an ultrasonic flow meter (manufactured by Flownetix) has been installed in the loop of each fluid. In order to apply the heat into the system, an AC bolt heater (manufactured by Cetal Co.) has been mounted inside the nano-fluid tank. For water loop, a refrigerant cooler (working with R-134a) is installed inside the tank for chilling the warm water. Temperature of both of tanks was controlled by a PID controller (manufactured by Autonics). Pipes and tanks were heavily isolated by glass wool to prevent any heat losses to environment. In order to measure the temperatures of fluids, four RTDs were mounted at inlets and outlets of plate heat exchanger. Likewise, two pressure transmitters were mounted at both
inlet/outlet ports of nano-fluid loop to measure the pressure drop inside the heat exchanger. More details on type and precision of instruments have been presented in Table 1. The main test section is the plate heat exchanger, a copper-made chevron type, which is working at the counter-current flow condition. Detailed specifications of the plate heat exchanger can be seen in Fig. 2.
Fig. 1. A schema of experimental loop
Fig. 2. Detailed scheme of the plate heat exchanger used in this work.
Table 1. Specifications and accuracy of measurement instruments Instrument/parameter
Model
Accuracy
Flow meter
Flownetix series 100x
RTDs
PT-100 resistance sensor
Bolt heater
1200W, 100mm 10mm(L D)
Pump
Centrifugal, stainless steel impeller
Pressure transmitter (pressure drop)
Sensys pressure transmitter
1% of readings*
PID controller
Autonics T3H
1% of readings0
1% of readings0
1 K* 2% of readings* Pumping head: 34meter
Thermo-physical properties: Viscosity
Brookfield
Density
Anton paar, DMA500
Thermal conductivity
Decagon KD2-Pro
Specific heat
TA 2920
2.5% of readings0 3% of readings0 2.5% of readings0 4% of readings0
Calculated parameters: Reynolds number
Kline-McClintock
±7.8%
Prandtl number
Kline-McClintock
±9.4%
Overall heat transfer coefficient
Kline-McClintock
±14.2%
Nusselt number
Kline-McClintock
±15.1%
*Based on the calibration process 0
Based on manufacturer claim
Table 2 presents the geometrical properties of plate heat exchanger. Table 2. Detailed specification and geometrical properties of plate heat exchanger Parameters
Value
Unit
A:12.5 B:75 Dimensions:
C:400
mm
D:450 E:125 Length of plate
375
mm
Width of plate
120
mm
Thickness and depth of plate
4
mm
Mean channel spacing
2.2
mm
Number of plates
36
---
Total heat transfer area and heat transfer area per plate*
3.23, 0.095
m2
Average channel spacing
3.3
mm
Fabrication material
Copper
---
Corrugation pitch
4
mm
Corrugation angle
27.5
degree
*Total heat transfer area: (number of plates -2) Heat transfer area per plate
2.2.Nano-fluid preparation and characterization: For preparing the nano-fluids, multi-walled carbon nano-tubes were purchased from USNANO and dispersed into the deionized water as a base fluid. Nonylphenolethoxilate was utilized as a nonionic dispersant. In order to prevent any changes in thermo-physical properties of nanofluids, dispersant was added at only 0.2% of general volume of nano-fluids. Nano-fluids were prepared at desired volumetric concentrations of 0.5, 1 and 1.5%. For characterizing the nanofluids, transmission electron microscopic image, (TEM) and x-ray diffraction, (XRD) were provided to ensure about morphology, nanotube size, and purity of nanotubes. As can be seen in Fig. 3, transmission electron microscope images for all three concentrations show that nanotubes are identical in terms of size and morphology. In addition, rare agglomeration and clusters can be seen inside the images, which is a sign of stabilized nanofluids. About 14 days of stability can be guaranteed according to the settlement-experiments. Likewise, as shown in Fig. 4, the x ray diffraction analysis demonstrated that MWCNTs have pure multi-walled structure and there is no
impurity in structure of nanotubes. Thus, material is reliable and can be used for the experiments. Since impurity may change the thermal conductivity of nano-fluid.
a) Vol.%=0.5
b) Vol.%=1 Fig. 3. TEM image of CNT/water
c) Vol.%=1.5
Fig. 4. XRD pattern of CNT/water
2.3.Thermo-physical properties of nano-fluids: The procedure for measuring the thermo-physical properties of nano-fluids summarily included: Initially, stability of nano-fluids was carefully analyzed using time-settlement experiments. Results demonstrated a stable behavior for all nano-fluids up to successive 14 days. Then experiments were established to measure the thermo-physical properties of nanofluids as follows: For measuring the thermal conductivity of nano-fluids, KD2 Decagon instrument with KS1 sensor was utilized. Measurements were performed at different temperature ranged from 50-70°C near the operating temperature of heat exchanger. For maintaining
the
temperature
of
nano-fluids,
cooling/heating
bath
(thermostat
refrigerant/boiling water/ethylene glycol cycle) was used as well. A PID controller was implemented to control the heating/cooling bath with accuracy of ±0.5K. For specific heat, TA 2920 instrument was used. Specific heat data were also measured at the similar conditions. For viscosity, Brookfield viscometer was implemented at temperatures ranged from 50-70°C. The obtained data were measured for three times to ensure about the reproducibility and repeatability of data. For density, Anton-Paar DMA500 was implemented (with the accuracy of 0.0002gr/cm3). Table 3 shows the summary of stabilizing process for the nano-fluids. Table. 3. Stabilization process for MWCNT nano-fluids Vol.%
nanoparticle
sonication time, (min)
0.5 MWCNT 1 MWCNT 1.5 MWCNT *Nonylphenolethoxilates, NPE general volume of nano-fluids.
Sonication Frequency, (kHz)
Stirring (Min)
Adjusted pH
Stability (days)
80 24 80 8.9 Stable, 14 days 80 46 100 9.1 Stable, 14 days 85 46 120 9.4 Stable, 14 days as dispersant was added into nano-fluids at quantity of approximately 0.2% of
2.4.Data reduction, heat loss analysis and uncertainty: For the heating loop, the heat transfer coefficient can be estimated using following correlation:
Q hot nf mnf . C p ,nf (Tin,nf Tout,nf ) For cooling loop, the heat transfer coefficient can be estimated using following correlation:
(1)
Q cold w mw . C p ,w (Tin,w Tout,w )
(2)
In eq. (3), Q hotnf is the rate of heat transfer belonging to hot nano-fluid, mnf is quantity of mass flow the hot nano-fluid. In eq. (4), Q hotnf is the rate of heat transfer belonging to cooling loop and mw is quantity of mass flow of water. The heat transfer rate is defined as follows:
Qave.
Q hot nf Q cold w 2
(3)
Where, Qave is the heat transfer rate between the heating and cooling loops. To calculate the overall heat transfer coefficient of nano-fluid, U, following equation is utilized as: U
Qave A. TLMTD
(4)
In Eq. (4), TLMTD is the log mean temperature difference, which can be calculated by the following equation: TLMTD
(Tout, nf Tin, w ) (Tin , nf Tout, w ) (Tout, nf Tin , w ) ln( (Tin, nf Tout, w )
(5)
Having the plate depth (equivalent to 4mm) and surface enhancement parameters (equivalent to 1.19), the hydraulic diameter of a corrugated plate heat exchanger can be estimated by using Eq. (6) as follows:
Dhydraulic
2 plate depth surface enhancemen t parameter
(6)
Nusselt and Reynolds and Prantdl numbers as a criterion for heat transfer and fluid flow velocity can be calculated as:
Nu
hnf . Dhydraulic
Re nf
k nf
nf .u nf .Dhydraulic nf
(7) (8)
Pr
C p ,nf . nf
(9)
k nf
Noticeably, the heat transfer coefficient belonging to the hot and cold loop should be calculated separately. For cold loop (water), correlation introduced by Huang et al. [4] was used for estimating the heat transfer coefficient in PHE, which comes as follows:
hw .Dhydraulic kw
0.2302 Re 0.745 . Pr 0.4
(10)
For estimating the heat transfer coefficient in hot loop (nano-fluid), Eq. (11) should be employed as:
1 1 1 U hnf k copper hw
(11)
is the thickness of plate, k is the thermal conductivity of copper since the heat exchanger has been fabricated from copper, hnf and hw are convective heat transfer coefficient of MWCNT/water nano-fluid and cold water (estimated by Eq. 10) respectively. Although the heat exchanger was heavily isolated by glass wool, however, there is possibility for heat loss to the environment, which should be carefully considered in calculation steps. Thus, for estimating the heat loss, an energy balance was employed between the quantity of heat applied to heat exchanger and that of removed by cold water as:
Qnf Qw Qheatloss
(12)
Where Qnf and Qw can be estimated using Eq.s (1-2). Results of energy balance revealed a 9.5% of heat loss during the experiments. In addition, reproducibility of experimental data was accurately checked with three runs of experiments. Deviation of 4.3% was reported for reproducibility and repeatability of data. Likewise, using Kline-McKlintock [26], uncertainty value for heat transfer coefficient was found to be 14.2%.
3. Results and discussions: Presence of carbon nanotubes inside the water can have two possible impacts on the heat transfer and pressure drop value as well. Accordingly, results are presented in two sub-sections. 3.1.Heat transfer characteristics:
In order to investigate the influence of carbon nanotubes inside the water, experiments were established such that the heat transfer coefficient of nano-fluids was measured and compared to that of the base fluid. Also, influence of operating parameters such as: flow rate, concentration and inlet temperature on heat transfer coefficient of nano-fluids and water was experimentally investigated. Fig. 5 represents the influence of flow rate inside the PHE. As can be seen, in terms of flow regime, experimental results can be divided into three significant regions namely laminar transient and turbulent flow regions. According to the results, flow rate has a strong impact on heat transfer coefficient of nano-fluid. With increasing the fluid flow rate, The Nusselt number (as a true criterion for heat transfer coefficient of nano-fluid, hnf) considerably increases, which is in a good agreement with those of previously reported by Kannadasan et al. [27], Dixit-Ghosh [28] and Kabeel et al. [29].
Fig. 5. Influence of flow rate on Nusselt number for MWCNT/water nano-fluid at vol. %=0.5. Integrity of experimental data in terms of reproducibility within three successive runs. Repeating the experiments with other concentration of nano-fluids demonstrated that heat transfer coefficient of nano-fluids can be intensified, when concentration of nano-fluid is increased. However, there is an optimum point for the enhancement. Noticeably, results showed that when concentration of nano-fluid increases, influence of viscosity might outweigh the thermal conductivity of nano-fluid, which consequently decreases the heat transfer coefficient.
Fig. 6 comparatively demonstrates the influence of concentration of nano-fluids on Nusselt number. As can be seen, at volumetric concentration of 0.5 % and 1%, Nusselt number can approximately be enhanced up to 7 and 14% respectively, while for vol.%=1.5, Nusselt number suddenly decreases in comparison with vol.% of 0.5 and 1. However, even in this case, values of Nusselt number are still higher than that of the base fluid.
Fig. 6. Comparison between Nusselt number of nano-fluid and base fluid at different concentrations For better understanding regarding to the influence of concentration, an enhancement parameter is utilized as:
Nu nf
(13)
Nu w
In this parameter, Nunf is the measured Nusselt number for nano-fluid and Nuw is the measured Nusselt number of water flowing inside the PHE. Fig. 7 shows the enhancement parameter for different concentration of nano-fluids and Reynolds number.
Fig. 7. Influence of concentration of nano-fluid on enhancement parameter (Re=4500) As can be seen, with increasing the volumetric concentration of nano-fluid, the enhancement value initially increases up to vol. %=1 and suddenly decreases for vol. %=1.5. Therefore, an optimal concentration for our experiments seems to be at vol. %=1, although more experiments are needed in order to find the exact optimal point for concentration of nano-fluid. For interpreting the phenomenon, it can be stated that lower concentrations, presence of carbon nanotube leads the convective heat transfer coefficient to be enhanced due to the Brownian motion intensified by carbon nanotubes and enhancement of thermal conductivity with a very small penalty in pressure drop. At higher volumetric concentrations, due to the CNT loading inside the bulk of nano-fluid, viscosity and density of nano-fluid is enhanced, followed by augmentation in friction factor and pressure drop. Therefore, enhancement parameter decreases at higher concentrations. Noticeably, enhancement in viscosity of nano-fluids has no impacts on the heat transfer coefficient, while by increasing the concentration of nano-fluids more than the optimal concentration, expenses due to the pumping power, consumption of nanoparticles will also be increased which is undesired. In other words, at concentrations higher than optimal concentration, effect of viscosity on the heat transfer coefficient is more significant in comparison with effect of thermal conductivity on the heat transfer coefficient. Briefly speaking, an efficient nano-fluid has both considerably enhanced thermal conductivity and also a low-value
viscosity. Likewise, increase in concentration of nano-fluids can lead the sedimentation rate of carbon nanotube on the wall surface of heat exchanger to be increased and stability of MWCNT nanofluids to be decreased over the time. The other effective parameter on the heat transfer coefficient is the inlet temperature of nano-fluid. In order to investigate the influence of inlet temperature, experiments were performed at three different inlet temperatures of 50, 60 and 70oC. As shown in Fig. 8, results demonstrated that Nusselt number slightly increases with increasing the inlet temperature, which may be due to the changes in thermo-physical properties of nano-fluid. For better understanding, thermal conductivity of nano-fluids at different volumetric fractions and temperature was measured using KD2 Decagon device and represented in Fig. 9. As can be seen, with increasing the vol. % of nano-fluids, thermal conductivity of nano-fluid increased. In addition, temperature was found to have a very slight influence on enhancement of thermal conductivity, which is in accordance with results collected by Morshed et al. [30]. Since thermal conductivity is the key thermo-physical property, in Fig. 9, the experimental results have been presented as a function of time and concentration of nano-fluids.
Fig. 8. Influence of inlet temperature on Nusselt number of MWCNT/water nano-fluid at volumetric concentration of 1%.
Fig. 9. Thermal conductivity of carbon nanotube nano-fluids as a function of volumetric concentration and temperature According to Fig. 9, it can be stated that for MWCNT/water nano-fluid, thermal conductivity is a slight function of temperature. Therefore, it can be concluded that inlet temperature can slightly enhance the heat transfer coefficient, which is due to the increase in thermal conductivity of nano-fluid. However, this enhancement is not significant, when comparing to the concentration effect. For the concentration effect, it can be seen that thermal conductivity can be enhanced up to 21%, 41% and 68% for vol.% of 0.5, 1 and 1.5% respectively. In order to validate the results, comparisons between well-known correlations and experimental results were made. Figs. 10(a-c) comparatively demonstrate the thermal conductivity of carbon nanotube in comparison with well-known predicting correlations [30]. As can be seen, results of comparison between experimental data and Hamilton-Crosser (Fig.10a), Xue et al. (Fig.10b) and Maxwell (Fig.10c) show that experimental results are in a satisfying agreement with those of estimated by the
correlations. As can also be seen, results are in a fair agreement with well-known predicting correlations. Table 4 presents the well-known correlations used for comparing the thermal conductivity. Table 4. Predicting correlations for thermal conductivity of nano-fluids Author
Correlation
Maxwell
k p 1 2 2 k f 1 k eff k f k p 1 k f 2
Hamilton-Crosser
k eff
Xue et al.
1 2 k p k p k f ln k p k f 2 k f k eff k f 1 2 k f k p k f ln k p k f 2 k f
k p 1 5 5 k f 1 kf k p 1 k f 5
Results of comparison between thermal conductivity and well-known predicting correlations can be seen in Figs. 10(A-C).
(a)
(b)
(c) Fig. 10. Rough comparisons between experimental thermal conductivity with well-known predicting correlations 3.2.Pressure drop characteristics: Experiments showed that using the MWCNT/water nano-fluids could slightly enhance the pressure drop in comparison with base fluid. In fact, presence of nanotubes inside the water can enhance the viscosity and as a result, higher values of pressure drop are reported for the nanofluids. Likewise, pressure drop can be intensified with increasing the flow rate and concentration of nano-fluids as well. Fig. 11 shows the pressure drop of MWCNT/water nano-fluid inside the PHE at different volumetric concentrations and flow rates. Conveniently, Reynolds number is implemented as a suitable criterion for the fluid hydrodynamic velocity. As can be seen, pressure drop is significantly intensified with increasing the flow rate, while it is gradually improved with increasing the concentration of CNTs.
Fig. 11. Influence of flow rate and concentration on the pressure drop of MWCNT/water nanofluids in PHE. Investigation on the friction factor demonstrated that this parameter agrees well with that of base fluid, although a small penalty (undesired enhancement) is reported for nano-fluid due to the presence of carbon nanotubes inside the heat exchanger. Noticeably, viscosity of nano-fluid strongly depends on the concentration of dispersed nanotubes. Thus, by increasing the nanotube concentration, viscosity of nano-fluid enhances and results in enhancement of friction factor and pressure drop as well. Fig. 12 represents the influence of flow rate and concentration of nanotubes on friction factor parameter. In order to validate the results, a comparison between well-known predicting correlation for friction factor and experimental data is made and represented in Fig. 13. As can be seen, Chen and Darcy equations can predict the friction factor only in lower Reynolds number; however, Haaland and Blasius correlations can accurately predict the friction factor of MWCNT/water at higher fluid flows. Note that, these correlations are only defined for fluid flow inside the pipes, therefore, such deviations seems to be reasonable. Moreover, this comparison demonstrates that results are in a fair agreement with
those of theoretically obtained by the correlations. This comparison can also be considered as second validation for the experimental friction factor.
Fig. 12. Influence of concentration and flow rate on friction factor
Fig. 13. Comparison between experimental friction factor and well-known correlations of generic Darcy law [31], Haaland [32] and Chen equation and Blasius [33]
3.3.Overall thermal performance: For evaluating the exact overall thermal performance of MWCNT/water, pressure drop and heat transfer characteristics should simultaneously be considered. For this purpose, thermal performance index is defined as [21]:
.1 / 3
(14)
Where:
f nf
(15)
fw
and, can be obtained using Eq. (13). In fact, ratio of the Nusselt number ratio to the friction factor ratio at the same pumping power is defined as real thermal performance index [21]. Fig. 14 comparatively represents the thermal performance of MWCNT/water nano-fluids at different concentrations. As can be seen, with increasing the volumetric concentration, thermal performance initially increased and suddenly suppressed. The reason for this phenomenon can be referred to this point that: presence of nanotubes into the water at higher CNT concentrations has stronger influence on pressure drop rather than thermal conductivity, while for lower concentrations, impact of nanotubes is more seen on thermal conductivity. Therefore, adding the carbon nanotubes can limit the application and thermal properties of MWCNT/water nano-fluids. Consequently, an optimum point for concentration of nanotubes should be considered, which is out of goals of this work.
Fig. 14. Thermal performance of MWCNT/water nano-fluids at different Reynolds number and concentrations for temperature equivalent to 60°C As can be seen in Fig. 14, inlet temperature can only enhance the thermal performance index up to 4.2%, while for concentration effect; there is an optimum vol.%, which should be carefully considered. 3.4.Fouling factor: When a particulate working fluid flows in a heat-exchanging medium, fouling formation is a matter of concern. When it comes to the MWCNT nano-fluids, nanotube aggregation and clustering can intensify the fouling formation. In order to investigate the fouling formation of MWCNT/water, fouling thermal resistance parameter is defined as:
Rf
1 1 U f Uc
(16)
In Eq. (16), Uc is the overall heat transfer coefficient of PHE, when it is clean (at the initial moment of experiment) and Uf, is the heat transfer coefficient when fouling is formed inside the heat exchanger. Rf is fouling thermal resistance. One sign for fouling formation is that the overall heat transfer coefficient is deteriorated over the extended time. In fact, fouling can create a significant thermal resistance, which reduces the rate of heat transfer. In this work, the heat
exchanger let to be continuously operated for about 720 working hours. During the operation, fouling resistance of heat exchanger was constantly monitored. As can be seen in Fig. 15, fouling resistance parameter has a rectilinear behavior with different line slope and this slope value can be intensified with time of operation and increasing the concentration of MWCNT/water. In terms of order of magnitude, in a rough comparison with fouling resistance of metal oxides [3440], it can be seen that metal oxides inside the heat exchangers have fouling resistance about 10fold larger than MWCNT/water. In addition, fouling resistance recorded for metal oxide has asymptotic behavior versus time.
Fig. 15. Fouling resistance of MWCNT/water nano-fluids inside the PHE Noticeably, for fouling resistance at other Reynolds numbers and nano-fluid inlet temperatures, similar trends were seen. It is noteworthy to state that such fouling formation can be important in higher working hours, for instance, at operating times lower than 300hrs, fouling resistance is ranged from 0.001-0.003, while for operating time between 600 and 720hrs, fouling resistance is between 0.1 and 0.3, which is significant. Nevertheless, as mentioned, in comparison with metal oxide nano-fluids, MWCNT/water nano-fluids have smaller fouling resistance, which put them as a promising alternative for long lasting cooling performance. For better understanding the role of fouling on the heat transfer coefficient, thermal fouling resistance parameter and heat transfer coefficient have been plotted in one figure and for three
volumetric concentrations of nano-fluids. As can be seen in Fig. 16, while fouling resistance has a none-linear behavior, rate of decrease for the heat transfer coefficient is none-linear too.
a) Fouling resistance and the heat transfer coefficient for vol.%=0.5
b) Fouling resistance and the heat transfer coefficient for vol.%=0.1
c) Fouling resistance and the heat transfer coefficient for vol.%=0.5 Fig. 16. Influence of the fouling resistance parameter on the heat transfer coefficient for MWCNT/water nanofluids at Reynolds 4500.
4. Conclusions: Experimental investigation on the forced convection heat transfer to multi-walled carbon nanotube nanofluids inside a chevron-type heat exchanger was performed. This work demonstrated that multi-walled carbon nanotube nano-fluid could be a promising option for convective cooling systems. Following conclusions were also made:
Presence of multi-walled carbon nanotubes inside the water can enhance the thermal conductivity coefficient up to 68% in comparison with base fluid (water). Concentration and temperature were found to enhance the thermal conductivity value.
MWCNT/water nano-fluids have higher forced convective heat transfer coefficient in comparison with water (As a conventional coolant), however, presence of CNTs caused a small penalty for friction factor and pressure drop, which can be ignored.
In terms of thermal performance index, results demonstrated that in some concentrations, enhancement in viscosity of nano-fluid outweigh the thermal conductivity enhancement, which limits the cooling application of MWCNT/water nano-fluid. Thus, it can be stated that for MWCNT/water nano-fluid, there is an optimum concentration, which should be carefully determined. In this work, we only examined three concentrations (0.5, 1 and 1.5% by volume).Results demonstrated that the best thermal performance amongst these concentrations can be obtained at vol.%=1. However, more studies are still required to determine the exact optimum concentration for carbon nanotube nanofluids.
Acknowledgements: Authors of this work dedicate the article to Imam Mahdi and appreciate Semnan University for their financial supports. Nomenclatures: A Cp D f h k L Nu P
area, m2 heat capacity, J.kg-1.oC-1 hydraulic diameter, m fanning friction factor convective heat transfer coefficient, W.m-2. oC-1 thermal conductivity, W.m-1.oC-1 length, m Nusselt number pressure, kPa
Pr prantdl Number Q heat, W Re Reynolds number Rf fouling resistance, m2. K/kW T temperature, oC u fluid velocity, m.s-1 U overall heat transfer coefficient, W/m2. K Subscripts-Superscripts ave average b bulk bs base fluid hot heating loop nf nano-fluid cold cooling loop in inlet out outlet n number of data points m mean m mass flow, kg.s-1 w water Greek symbols thermal diffusion, m2.s-1 density, kg.m-3 viscosity, kg.m-1.s-1 volume fraction or particle loading
plate thickness, m
difference
thermal performance index
Enhancement parameter Abbreviation HTC
heat transfer coefficient
LMTD log mean temperature difference PHE
plate heat exchanger
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Highlights
Thermal performance, pressure drop analysis inside a plate heat exchanger is studied MWCNT/water nano-fluid is used as a working medium Enhancement in overall heat transfer coefficient is reported Smaller fouling resistance is reported in comparison with other nano-fluids