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Particle ratio optimization of Al2O3-MWCNT hybrid nanofluid in minichannel heat sink for best hydrothermal performance
Applied Thermal Engineering 165 (2020) 114546
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Particle ratio optimization of Al2O3-MWCNT hybrid nanofluid in minichannel heat sink for best hydrothermal performance Vivek Kumar, Jahar Sarkar
T
⁎
Department of Mechanical Engineering, Indian Institute of Technology (B.H.U.), Varanasi, UP 221005, India
H I GH L IG H T S
hydrothermal behaviour of Al O -CNT hybrid nanofluid in minichannel. • Experimental of flow rate (Rynolds number), temperature and mixture ratio are studied. • Effects of particle mixture ratio on h/Δp and performance evaluation criteria is studied. • Effect • Optimum particle mixture ratio is found as 3:2 for best hydrothermal performance. 2
3
A R T I C LE I N FO
A B S T R A C T
Keywords: Minichannel heat sink Hybrid nanofluid Nanoparticle mixture ratio Heat transfer Pressure drop Optimization
Interesting hydrothermal behavior of hybrid nanofluid containing combination of dissimilar particles (completely different in terms of shape, size and properties) by changing their ratio is unexplored yet, which is analyzed experimentally for minichannel heat sink. Nine parallel rectangular minichannels are used in the heat sink each of which has 1 mm width, 3 mm depth and 30 mm length. Al2O3–MWCNT/water hybrid nanofluid of 0.01% volume concentration with various nanoparticle mixture ratios is used as coolant to analyze the effect of Reynolds number and working fluid inlet temperature. Nanoparticles mixing ratios (5:0, 4:1, 3:2, 2:3, 1:4 and 0:5) have been taken as volume ratios. Convective heat transfer coefficient, Nusselt number, pressure drop and friction factor increase with an increase in fraction of MWCNT in hybrid nanofluid. A maximum enhancement of 44.02% has been observed for the convective heat transfer coefficient with MWCNT (5:0) hybrid nanofluid as compared to water. Maximum pressure drop has been increased by 51.2% over base fluid at an inlet temperature of 20 °C for the MWCNT (5:0) hybrid nanofluid. Inlet fluid temperature has negative effect on pressure drop and positive effect on heat transfer. The observed optimum volumetric mixing ratio of Al2O3 and MWCNT is around 3:2, for which hybrid nanofluid yields maximum heat transfer coefficient to pressure drop ratio. Performance evaluation criteria (PEC) has been evaluated greater than 1 for all the nanofluids, which concludes that nanofluids are better option as compared to conventional fluid (DI water). Based on the experimental data, correlations for Nusselt number and friction factor have been introduced with R2 values of 98.8% and 96.3%, respectively, and compared with available correlations.
1. Introduction Due to miniaturization and increasing power density of advanced electronic devices, an effective cooling system is needed for their optimal operation. The use of micro/mini channel heat sinks (MCHS) is one of effective ways for high heat flux removal [1,2]. The miniaturized heat exchanger (i.e., MCHS) has gained a lot of attention due to materials preserving, space constraints and demand of higher energy density, which can transmit more heat per unit volume than conventional heat exchangers [3]. The heat transfer capability can be further
⁎
enhanced by using nanofluids as well as hybrid nanofluids due to their better heat transfer characteristics [4]. With proper hybridization and trade-off between advantages and disadvantages of individual suspension, hybrid nanofluid is supposed to yield better thermal properties and hence attracted as cooling media for various applications [5]. Alumina, a most common metal oxide nanoparticle has been used by many researchers due to its low cost, availability, chemical stability and higher heat transfer improvement to pumping power increase ratio [6]. On the other hand, multi-walled carbon nanotube (MWCNT) has attracted many researchers due to its higher thermal conductivity and
Corresponding author. E-mail address: [email protected] (J. Sarkar).
https://doi.org/10.1016/j.applthermaleng.2019.114546 Received 9 April 2019; Received in revised form 21 August 2019; Accepted 16 October 2019 Available online 18 October 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
Applied Thermal Engineering 165 (2020) 114546
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Nomenclature A cp dh f h hch k V̇ Nu Δp Pr Q̇ Re T u vol wch
Lch PEC
effective heat transfer area (m2) specific heat (J/kg K) hydraulic diameter (mm) friction factor heat transfer coefficient (W/m2 K) channel height (mm) thermal conductivity (W/m K) volume flow rate (LPM) Nusselt number pressure drop (Pa) Prandtl number heat transfer rate (W) Reynolds number temperature (°C) velocity (m/s) volume channel width (mm)
channel length (mm) performance evaluation criteria
Greek symbols µ ρ Φ
dynamic viscosity (Pa s) density (kg/m3) volume concentration
Subscripts bf np hnf ch in out s m
base fluid nanoparticle hybrid nanofluid channel inlet outlet surface mean
Al2O3-TiO2 hybrid nanofluid in minichannel and showed improvement over base fluid. Nanoparticle mixture ratio influences the heat transfer characteristics of hybrid nanofluids; although the studies are limited on this important issue. Charab et al. [26] proposed the model of thermal conductivity for 1.0 v% Al2O3-TiO2 hybrid nanofluid with different particle ratios and reported maximum heat transfer enhancement up to 35.3% for 2:3 ratio. Moldoveanu et al. [27] experimentally investigated the thermal conductivity of Al2O3–SiO2/Water hybrid nanofluids and noticed that the hybrid nanofluid has higher thermal conductivity compared to alumina nanofluid. Hamid et al. [28] performed an experimental investigation on thermal conductivity and viscosity of hybrid nanofluids with different particle ratio. They observed maximum thermal conductivity enhancement up to 16% for 1:4 ratio of TiO2-SiO2 and highest dynamic viscosity for the ratio of 5:5. They [29] also performed an experimental study on the heat transfer performance of cooling equipment for the same pair of nanoparticles and found maximum heat transfer enhancement up to 35.3% for 2:3 ratio. Dalkılıç et al. [30] experimentally investigated the viscosity of Graphite-SiO2 hybrid nanofluid at different volume concentrations and different weight ratios and showed a viscosity increase of 0.65–36.32% with an increase in volumetric concentration. Siddiqui et al. [31] studied the stability and thermophysical properties of Cu-Al2O3 hybrid nanofluid for different mixing ratios and showed that 5:5 was the optimum mixing ratio because of its better thermal conductivity and good stability to achieve overall hydrothermal properties. Zawawi et al. [32] investigated the thermophysical properties of Al2O3–SiO2/PAG nanolubricants for various particle mixture ratios and found that 60:40 was the optimum mixture ratio with the lowest property enhancement ratio. Esfe et al. [33] experimentally investigated the optimized ratio of MWCNT (30 vol%)-Al2O3 (70 vol%) in 5 W50 oil with total volume concentrations of 0.05% to 1% and found maximum viscosity enhancement of 24%. Kumar and Sarkar [25] experimentally studied Al2O3-TiO2 hybrid nanofluid in minichannel heat sink with different mixture ratio. As shown, similar shaped nanoparticle combination has been only considered in the investigation. With the best of the authors' knowledge, no similar study on minichannel heat sink using dissimilar nanoparticles combination is available which can equally affect both the hydrodynamic and thermal behaviours. In the previous study [25], we have seen the negligible effect of mixture ratio for similar particles (oxide-oxide) and hence we will experimentally analyze the effect of mixture ratio for dissimilar particles (oxide-carbon nanotube) on the performance of minichannel heat sink using Al2O3-MWCNT/water hybrid nanofluids in this study. The present
very high aspect ratio for application in nanofluids [7]. Hence, alumina and MWCNT can be the best combination in hybrid nanofluid for MCHS. Many experimental studies have been conducted on hydrothermal characteristics of nanofluid in MCHS [8–11]. However, few studies have been conducted on MCHS using hybrid nanofluids. Selvakumar and Suresh [12] investigated hydrothermal characteristics of hybrid nanofluids in minichannel and observed 24.35% enhancement in the convective heat transfer coefficient and 12.61% increase in pumping power. An experimental study carried out by Ho at al. [13,14] using hybrid water-based suspensions of Al2O3 and MEPCM particles in minichannel heat sink and showed that Al2O3–water nanofluid is effective over hybrid nanofluids at higher Reynolds number. Ahammed et al. [15] experimentally investigated the characteristics of graphene–alumina hybrid nanofluids in a minichannel and reported convective heat transfer coefficient enhancement of 63.13%. An experimental study has been done by Nimmagadda and Venkatasubbaiah [16] for Al2O3 + Ag hybrid nanofluid in microchannel showed that Nusselt number enhances with volume concentration. Bahiraei et al. [17] investigated the irreversibilities of CNT/Fe3O4 hybrid nanofluid in the minichannel heat exchanger and showed that at low Fe3O4 concentration, an optimum point found for the total entropy generation. Hussien et al. [18] experimentally investigated the performance of MWCNT/ GNP hybrid nanofluids in microtubes in the Re range of 200–500 and found that average heat transfer coefficient enhances 58% for MWCNT/ GNP hybrid nanofluid. They also numerically examined Al2O3/graphene hybrid nanofluid inside minitube and found maximum enhancement in heat transfer coefficient about 13.7% over Al2O3/water nanofluid [19]. The performance of different microchannel heat sink using graphene-Ag hybrid nanofluid has been numerically investigated in laminar regime [20,21] and revealed that nanofluid is better cooling option in comparison to water. Hydrothermal feature of chaotic twisted minichannel has been studied by Bahiraei and Mazaheri [22] using graphene nanoplatelets decorated with platinum hybrid nanofluid and found figure of merit always more than 1.5. Shahsavar et al. [23] numerically evaluated the hydrothermal performance of Fe3O4/CNT/ water hybrid nanofluid in double pipe minichannel by considering both non-Newtonian and Newtonian fluids and found superior performances of non-Newtonian hybrid nanofluid. A numerical investigation has been done by Uysal et al. [24] in rectangular minichannel using DiamondFe3O4/Water hybrid nanofluid and showed that hybrid nanofluid has higher convective heat transfer coefficient and Nusselt number over the mono particles nanofluids. Kumar and Sarkar [25] numerically and experimentally investigated the heat transfer and pressure drop of 2
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then using Al2O3–MWCNT hybrid nanofluids. Working fluids at different inlet temperatures (20 °C, 30 °C and 40 °C) were supplied into the system from the reservoir by using a pump. The heat input was supplied to the system when the maximum Reynolds number was achieved. Volume flow rates were chosen to yield moderate Reynolds number range, for which, no study is available in the literature for present hybrid nanofluids. Five surface thermocouples were used to calculate the surface temperature. Four thermocouples were inserted at all the four corners of minichannel and one thermocouple was inserted at the centre of the heat sink to the bottom for measure the surface temperature (Ts). The surface temperature has been taken by averaging of all the five readings by thermocouples. All the readings were taken under steady-state condition, i.e., thermal equilibrium, which was achieved in 40–50 min of the time interval. Reliability and verification of the experimental setup have been justified by the published research work based on the present setup [25]. Experimental Nusselt number for the conventional fluid (DI water) was compared from existing correlation given by different authors and found similar trend and close prediction.
experimental work is focused only on the different volume mixing ratio of two dissimilar particles (oxide-carbon nanotube) with fixed volume concentration of 0.01%. At higher volume concentration, heat transfer as well as pressure drop characteristics may be improved; however may lead to minichannel clogging problem due to the presence of MWCNT (its length is in micro level). Effects of Reynolds number, nanoparticle mixture ratio and inlet temperature have been investigated on the convective heat transfer coefficient, Nusselt number, pressure drop and friction factor. Heat transfer coefficient to pressure drop ratio and performance evaluation criteria (PEC) have been discussed as well to find optimum mixture ratio. Also, correlations have been developed for Nusselt number and friction factor and compared with the similar correlations available in the literature. 2. Methodology 2.1. Test facility The layout and photograph of the experimental setup are given in Figs. 1 and 2, respectively. The detailed description of the experimental setup has been reported earlier [25]. For heating, a cartridge heater having the power of 50 W, with cross-section of 10 mm × 30 mm is placed into the bottom of MCHS. Dimension, temperature, volume flow rate, pressure drop and heating power have been measured by vernier calliper, thermocouple, rotameter, pressure transducer and wattmeter, having uncertainties of ± 0.02 mm, ± 0.1 °C, ± 0.5%, ± 0.25% and ± 0.25%, respectively. Photo image and schematic of the minichannel heat sink with dimensions are shown in Fig. 3. Minichannel heat sink made of aluminium consists of 9 parallel rectangular-shaped minichannels each of which has a width of 1 mm, a depth of 3 mm, a length of 30 mm and hydraulic diameter of 1.5 mm. Side fines of minichannel are of 0.7 mm and all others fins are of thickness 1.2 mm. Pitch of minichannel has been shown in Fig. 3 with side pitch of 1.95 mm and all others pitch of 2.20 mm. The experiments were first performed by using distilled water and
2.2. Preparation of hybrid nanofluids A two-step method was used to prepare the Al2O3–MWCNT/DI water hybrid nanofluids. Alumina nanoparticles (45 nm diameter) manufactured by Alfa Aesar, USA and multiwall carbon nanotubes (diameter: 20 nm, Length: 2 µm) manufactured by Otto Chemie Pvt. Ltd. were dispersed in the DI water. Acid-treated MWCNT used in this investigation to make it hydrophilic to DI water. Al2O3 nanoparticle and MWCNT have been taken in the different volume mixing ratios are 5:0, 4:1, 3:2, 2:3, 1:4 and 0:5, respectively to prepare hybrid nanofluid of 0.01 vol% concentration. An electronic balance (SHIMADZU, ATX224, Japan) was used to measure the calculated amounts of Al2O3 and MWCNT nanoparticles and then they were mixed with DI water. Ultrasonicator (Labman Scientific Instruments, India) was used to have cell disruption and homogenization of the colloid for 6–8 h without
Fig. 1. Layout of the experimental setup [25]. 3
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V. Kumar and J. Sarkar
Fig. 2. Photograph of experimental setup [25].
Volumetric heat capacity ( ρcp ) value was measured by Hot Disk TPS500 analyzer and then specific heat cp was obtained by dividing ρcp with ρ . LVDV-II + Pro Brookfield digital viscometer was used to measure the viscosity of samples. Thermal conductivity of the prepared nanofluid samples was measured by Hot Disk TPS-500 analyzer.
surfactant. Scanning electron microscopy (SEM) image of Al2O3MWCNT hybrid nanofluid of 3:2 mixture ratio is shown in Fig. 4, which predicts a uniform distribution of both nanoparticle types and average particle size is of Al2O3 nanoparticles is < 50 nm. The nanofluid sample has been observed stable more than 3 days and experiments were performed using homogenized hybrid nanofluids. The pH value obtained in the range of 7.71–7.74 for the small sample of synthesized hybrid nanofluids taken from the four different places of the container. The pH value obtained is far away from the isoelectric points (IEP) which ensure the homogeneity and stability of the hybrid nanofluids [34]. Better homogeneity and stability is achieved when the pH value of the sample is away from the IEP value due to the large repulsion among the nanoparticles. The density and viscosity of synthesized nanofluids were determined for the same 4 different samples which have been used for the determination of pH values. No appreciable change was measured in the density for all the samples. The Viscosity values are obtained in the range of 1.19 cP to 1.24 cP; which was measured at 100 RPM and room temperature. Scanning electron microscope (SEM) is used to show the distribution of the initial nanoparticles in the hybrid nanofluids. From the SEM image (shown in Fig. 4), it can be observed that particles are uniformly distributed in the synthesized hybrid nanofluid. Similar homogeneity of particles distribution also observed from the SEM images of the other samples. The degree of homogeneity of prepared hybrid nanofluids was found approximately 98% based on the measured properties.
2.4. Data analysis By measuring flow rate, fluid inlet and outlet temperatures, the heat transfer rate has been predicted by,
Q̇ = V̇ ρcp (Tout − Tin )
(1)
Then, the average convection heat transfer coefficient was calculated by,
h=
Q̇ A (Ts − Tm )
(2)
where Tm is mean fluid temperature and Ts is average wall temperature (average of five thermocouples reading, inserted at different locations to bottom of the heat sink). The effective heat transfer area (A) has been calculated as,
A = N (wch + 2ηfin hch) Lch
(3)
where N is the number of channels, ηfin is fin efficiency, Lch, wch and hch are length, width and height of channel, respectively. Fin efficiency can be found out using Eqs. (4) and (5) by iteration [35].
2.3. Thermophysical properties of nanofluids Thermophysical properties for DI water and Alumina-MWCNT hybrid nanofluids for different proportion at 30 °C have been shown in Table 1. Various apparatuses were used to experimentally measure the effective thermophysical properties of prepared hybrid nanofluids. Before collecting data, all the instruments were calibrated and measurements have been repeated three times. The density was obtained by measuring mass (using electronic balance machine) of specified volume (using measuring biker) and then dividing mass by volume ( ρ = m V ).
ηfin =
tanh(mhch ) mhch
(4)
m=
2h (km wfin )
(5)
where km is the heat sink material thermal conductivity and wfin is the width of the fin. Hydraulic diameter (dh) of minichannel is defined as, 4
Applied Thermal Engineering 165 (2020) 114546
V. Kumar and J. Sarkar
Fig. 3. Minichannel heat sink with schematic diagram.
dh =
2wch hch wch + hch
f=
(6)
2dh Δp Lch ρum2
(10)
Thus, the Nusselt number has been predicted as below:
Nu =
hdh k
(7)
2.5. Uncertainty analysis
um is mean fluid velocity and it is calculated based on volumetric flow rate, it is defined as
um =
V̇ Nwch hch
Uncertainty analysis of the convection heat transfer coefficient, Reynolds number, Nusselt number and friction factor have been done by Kline and McClintock [36] equation. During experiments with MCHS, the temperatures, flow rates and pressure loss were measured with appropriate instruments. The values of uncertainty (W) obtained are given in Table 2.
(8)
The Reynolds number is given by,
Re =
ρum dh μ
2
2 1 2
2
⎞⎤ ⎛ ∂R ⎛ ∂R ⎞ ⎛ ∂R ⎞ W=⎡ ⎢ ∂X1 W1 + ∂X2 W2 + ⋯ ∂Xn Wn ⎥ ⎝ ⎠ ⎝ ⎠ ⎠⎦ ⎝ ⎣
(9)
⎜
By measuring the flow rate and pressure drop, the friction factor is defined as, 5
⎟
⎜
⎟
⎜
⎟
(11)
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number can be easily observed by the figures. This may be due to thinning of the thermal boundary layer. Enhancement in heat transfer coefficient is observed with the addition of nanoparticles as compared to DI water, which can result to many reasons including an increase in thermal conductivity, various slip mechanisms and the nano-fin effects [6]. The nanoparticles carrying the heat along with them also contributed to these results along with results from the fluid movement around the nanoparticles causing micro convection [37]. The viscosity decreases with the increase in temperature, which may be the cause of thinning of the boundary layer. For the same temperature difference, heat transfer coefficient increases due to thinning of the thermal boundary layer. The large surface area of particles, micro convection may increase the heat dispersion in the fluid at a faster rate. As MWCNT nanoparticles fraction increasing in the nanofluid, the heat transfer coefficient is increased. The maximum value of the calculated heat transfer coefficient is 4997.5 W/m2 K for MWCNT/DI water nanofluid. The maximum enhancement of the heat transfer coefficient has been predicted by nearly 44.02% for 0.01 vol% MWCNT/DI water nanofluid compared to DI water. It is mainly due to the very high thermal conductivity of MWCNT nanoparticles. Another reason may be a higher specific surface area due to the higher aspect ratio of the carbon nanotube. It shows from the figures that there is an increment in the heat transfer coefficient with an inlet temperature of coolants. The heat transfer coefficient is increased for MWCNT/DI water nanofluid by 12.12% when the temperature of working fluid is increased from 20 °C to 40 °C. The heat transfer enhancement was considerably improved at higher working temperature because of the improvement of thermal properties. The increment of thermal conductivity of nanofluids is more meaningfully at higher operating temperature. Enhancement in thermal conductivity which is dependent on temperature may be another possible advantage for using the nanofluids as the high-temperature coolants. Fig. 8 shows the variation of Nusselt number with Reynolds number for different working fluids at the inlet temperatures of 30 °C. Increase in flow rate causes an increase in Reynolds number and Nusselt number due to an increase in heat transfer coefficient. The addition of nanoparticles results into increase in Nusselt number as shown in figure due to the increase in heat transfer coefficient as discussed above. The Nusselt number for hybrid nanofluids also shows dependency on the mixture ratio of nanoparticles, although the variation is similar to heat transfer coefficient. As the MWCNT fraction increases in the hybrid nanofluid, Nusselt number increases. The maximum increment of 41.04% is found for MWCNT/DI water nanofluid compared to DI water. This is due to increment in the heat transfer coefficient supported by
Fig. 4. SEM image of Al2O3-MWCNT hybrid nanofluid at 100 k magnification. Table 1 Thermo-physical properties of DI water and Alumina-MWCNT hybrid nanofluid (0.01v%) for different proportion at the temperature of 30 °C. Fluids
DI water Al2O3 + CNT Al2O3 + CNT Al2O3 + CNT Al2O3 + CNT Al2O3 + CNT Al2O3 + CNT
(5:0) (4:1) (3:2) (2:3) (1:4) (0:5)
Thermal conductivity (W/ m K)
Specific heat (J/kg K)
Density (kg/m3)
Viscosity (Pa s)
0.6170 0.6175 0.6177 0.6181 0.6188 0.6195 0.6207
4182.4 4181.0 4180.8 4180.4 4180.3 4180.1 7179.9
997.3 998.8 998.7 998.2 998.0 997.9 997.7
0.00091 0.00093 0.00094 0.00095 0.00095 0.00097 0.00099
Table 2 The uncertainties during the measurements of the experimental parameters. Variable
Uncertainty value (%)
Heat transfer rate, Q̇ (W) Effective area, A (m2) Hydraulic diameter, dh (m) Velocity, um (m/s) Thermal conductivity, k (W/m K) Viscosity, µ (Pa/s) Density, ρ (kg/m3) Specific heat, cp (J/kg K) Heat transfer coefficient, h (W/m2 K) Friction factor, f Nusselt number, Nu Reynolds number, Re h/Δp PEC
± 1.2 ± 1.02 ± 0.78 ± 2.0 ± 2.0 ± 2.0 ± 2.0 ± 3.1 ± 2.76 ± 3.83 ± 3.1 ± 3.12 ± 5.47
± 2.87
3. Results and discussion 3.1. Heat transfer characteristics The variations of calculated heat transfer coefficient with Reynolds number for different coolants at the inlet temperatures of 20, 30 and 40 °C has been depicted in Figs. 5–7. Due to the wide range of atmospheric temperature globally, the present investigation is done on different inlet temperature to claim the sustainability and flexibility of the current study. Constant temperature bath having synchronized heating and cooling provisions before flowing into the channel is employed for maintaining these temperatures. In the figures, analysis has been displayed for various coolants synthesized by changing the fraction of MWCNT and Al2O3 nanoparticles for a total of 0.01% volume concentration. Heat transfer coefficient increases with the Reynolds
Fig. 5. Variation of temperature = 20 °C). 6
heat
transfer
coefficient
with
Re
(Inlet
Applied Thermal Engineering 165 (2020) 114546
V. Kumar and J. Sarkar
Fig. 6. Variation of temperature = 30 °C).
heat
transfer
coefficient
with
Re
(Inlet
Fig. 7. Variation of temperature = 40 °C).
heat
transfer
coefficient
with
Re
(Inlet
Fig. 9. Variation of Pressure drop with Re (Inlet temperature = 20 °C).
Fig. 10. Variation of Pressure drop with Re (Inlet temperature = 30 °C).
Fig. 11. Variation of Pressure drop with Re (Inlet temperature = 40 °C). Fig. 8. Variation of temperature = 30 °C).
Nusselt
number
with
Reynolds
number
(Inlet
7
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significantly higher than that at 0:5. Hence, it may be concluded that the optimum mixture ratio is around 3:2 for the Al2O3-MWCNT (spherical-cylindrical) nanoparticle combination in minichannel heat sink. It is also observed that the optimum mixing ratio is not changing with the flow rate (Reynolds number) and hence may be applicable for other Reynolds number range also.
equation (7). Improvement in Nusselt number has been observed while increase in fluid inlet temperature. 3.2. Pressure drop characteristics The variation of pressure drop with Reynolds number at the inlet temperatures of 20 °C, 30 °C and 40 °C are predicted in Figs. 9–11, respectively. There is an increment in pressure drop with an increase in Reynolds number. With the addition of nanoparticles in the base fluid, pressure drop increases due to dual effects increasing viscosity and density. As the MWCNT fraction in the hybrid nanofluid increases, the pressure drop increases with a faster rate. Maximum pressure drop is found 404.8 N/m2 at Reynolds number of 447 and inlet temperature of 20 °C for MWCNT/DI water working fluid. The enhancement of 51.2% has been observed for MWCNT/DI water nanofluid when compared to DI water. Such enhancement in pressure drop for MWCNT/DI water may be due to its high surface area. Inlet temperature has a negative effect on pressure drop due to a decrease in viscosity with temperature. The reason for the decrease in viscosity with the temperature is weakening of intermolecular force between molecules. Pressure drop decreases with an inlet temperature of coolant from 20 °C to 40 °C. It is a well-known fact that pressure drop increases with Reynolds number for the same cross-section, but it is revealed from the Figs. 9–11 that the points diverged at higher Reynolds number. It is understood by the fact that pressure drop directly proportional to mass flux and viscosity, and mass flux has dominance over viscosity at high Reynolds number. Variation of friction factor with Reynold number is shown in Fig. 12 for inlet temperatures of 30 °C. As predicted, the friction factor decreases with increase in Reynold number is due to dual effects of boundary layer thinning and mass velocity increase. Increase in viscosity and slip mechanism may be the reasons to increment in friction factor with the addition of nanoparticles. Friction factor increases for the hybrid nanofluids over base fluid. It has a higher value as the concentration of MWCNT increases in the hybrid nanofluid due to its high surface area. At Re = 149, the maximum value of friction factor for MWCNT/DI water nanofluid is approximately 1.08. It is observed from the figure that the deviation of friction factor for different working fluids is significant at lower Reynolds number. This may be because of at lower Reynolds number viscosity is predominant and it is attributed by the fact that the viscosity increases with increase in MWCNT concentration due to particle surface area and interparticle attraction. Another reason may be rupturing of the boundary layer is predominant for MWCNT compared to Al2O3 nanoparticles. The friction factor decreases with the increase in temperature for the same Reynold number due to a decrease in viscosity. As predicted, there is a significant change of friction factor with the variation of nanoparticle mixture ratio for hybrid nanofluids of same total particle concentration. Hence, it can be concluded that the hybrid nanofluid containing the mixture of dissimilar nanoparticles of different shape and size will give different flow characteristics.
3.4. Performance evaluation criteria (PEC) The heat transfer and pressure drop results do not provide the correct conclusion individually that the usage of nanofluids is a good option and vice-versa as both the heat transfer as well as pressure drop increase by using nanofluids. Therefore, apart from h/pressure drop, another comprehensive assessment factor called PEC (Performance evaluation criteria) has been introduced. PEC is defined as,
PEC =
Nuhnf Nubf (fhnf fbf )1
3
(12)
From Fig. 14, it is concluded that in all the cases PEC is above one. So, it is interpreted that usage of nanofluid is effective as a coolant compared to distilled water in minichannel heat sink. It can be concluded enhancement in pressure drop is less remarkable compared to improvement in heat transfer coefficient. Among all the nanofluids, Al2O3 + CNT (0:5) has maximum PEC value. The maximum value of PEC is 1.26 at higher Reynolds number. As CNT fraction is increasing in the nanofluids, PEC value increases. 3.5. Proposed correlations The proposed correlation for Nusselt number and friction factor are derived based on the experimental results as follows, (150 ≤ Re ≤ 460, 6.84 ≤ Pr ≤ 7, Φ = 0.01%)
Nu = 0.3035Re 0.4407 Pr 0.36 (1 + R)0.317
(13)
f = 43.162Re−0.84807 (1 + R)0.65775
(14)
where R is defind as the ratio of CNT fraction (Φ2) to total Al2O3 + CNT fraction (i.e. R = Φ2/(Φ1 + Φ2)). The range of R varies from 0 to 1. Developed correlation for Nusselt number and friction factor has Rsquare values of 98.8% and 96.3%, respectively. It can be illustrative from Fig. 15 that proposed correlation and experimental values for Nu are in good agreement. Correlation predicted all the experimental data within the error range of +3% to −4.3%. The plot between predicted friction factor and experimental friction factor is shown in Fig. 16. The
3.3. Effect of mixture ratio on h/Δp The ratio of heat transfer coefficient and pressure drop with a mixture ratio for different volume flow rates at an inlet temperature of 30 °C has been shown in Fig. 13. It is found that for the lower volume flow rate, h/Δp ratio is maximum. As the MWCNT concentration increases in the hybrid nanofluid, initially h/Δp ratio increases, after a mixture ratio it starts decreasing. The optimum mixture ratio is found to be around 3:2 after which the h/Δp ratio starts decreasing at a higher rate. It is due to the fact that the pressure drop increases at a higher rate as the concentration of MWCNT increases in the hybrid nanofluids. The reason for the increase in pressure drop is because of its high specific surface area due to the higher aspect ratio for MWCNT. After incorporation of error bar in the graph, it is observed that the value of h/ pressure drop at 3:2 is slightly higher than that at 4:1 or 2:3 and
Fig. 12. Variation of Temperature = 30 °C). 8
Friction
factor
with
Reynolds
number
(Inlet
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Fig. 16. Variation of Predicted friction factor with Experimental friction factor. Fig. 13. Ratio of heat transfer coefficient to pressure drop (h/Δp) with the mixture ratio.
Fig. 17. Deviation of proposed correlation with existing correlation for Nu. Fig. 14. PEC with Re of hybrid nanofluids for different mixing ratio.
maximum error between the proposed correlation and experimental data is lies between +12.5% and −12.5%. The experimental data of Nusselt number obtained for Al2O3/DI water nanofluid is compared with the proposed correlation (taking R = 0) and existing correlations available in the literature as shown in Fig. 17. The proposed correlation has same trends as existing correlations [38,39] without good agreement. Experimental values have best match with proposed correlation. The reason to support this deviation between proposed correlation and existing correlations is that existing correlations have developed for minichannel with dissimilar geometry and heat flux condition. 4. Concluding remarks In this paper, the hydrothermal characteristics of Al2O3-MWCNT (completely different in nature) hybrid nanofluid in minichannel heat sink have been experimentally investigated for volume concentration of 0.01% at different particle ratios (5:0 to 0:5), Reynolds number (140 < Re < 4 6 0) and the inlet temperatures (20 °C, 30 °C and 40 °C). From the results and discussion, the following conclusions can be made: Fig. 15. Variation of Predicted Nu with Experimental Nu.
• With the increase in inlet temperature from 20 °C to 40 °C and na-
noparticle dispersion in the base fluid, convective heat transfer
9
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•
• • • • • • • •
Appendix B. Supplementary material
coefficient and Nusselt number increase due to enhancement of thermal properties due to temperature increases. With the Reynolds number and nanoparticle dispersion in the base fluid, pressure drop and friction factor increase due to dual effects increasing viscosity and density, whereas decreases with inlet temperature because of decrease in viscosity. Viscosity decreases due to cohesion force between molecules decreases as temperature increases. Maximum pressure drop increment is 51.2% for MWCNT/ DI water nanofluid compared to DI water. Among all the working fluids, Al2O3 (5:0) is less effective and MWCNT (0:5) is the most effective hybrid nanofluid in terms of heat transfer coefficient. With the increase in MWCNT concentration in the particle mixture of hybrid nanofluid, the heat transfer coefficient, Nusselt number, pressure drop and friction factor increase. Maximum 44% enhancement of heat transfer coefficient has been observed for MWCNT (5:0) hybrid nanofluid. Relative effect on heat transfer coefficient and pressure drop is analyzed and found that the optimum particle mixture ratio is around 3:2. For hybrid nanofluid with dissimilar nanoparticle concentration, mixture ratio has a significant effect on heat transfer and pressure drop characteristics. Al2O3 and MWCNT at 3:2 mixture ratio shows the maximum value for h/Δp. This research reveals that around 3:2 mixture ratio for Al2O3MWCNT based hybrid nanofluids is the optimum mixture ratio when considering hydrothermal characteristics in minichannel heat sink. Usage of nanofluid is effective as a coolant compared to distilled water. The maximum value of PEC is 1.26 for Al2O3-CNT (0:5) hybrid nanofluid. Developed correlations for Nusselt number and friction factor are in good agreements with experimental data.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114546. References [1] S. Ndao, Y. Peles, M.K. Jensen, Multi-objective thermal design optimization and comparative analysis of electronics cooling technologies, Int. J. Heat Mass Transf. 52 (2009) 4317–4326. [2] A. Falahat, R. Bahoosh, A. Noghrehabadi, M.M. Rashidi, Experimental study of heat transfer enhancement in a novel cylindrical heat sink with helical minichannels, Appl. Therm. Eng. 154 (2019) 585–592. [3] I.A. Ghani, N.A.C. Sidik, N. Kamaruzaman, Hydrothermal performance of microchannel heat sink: the effect of channel design, Int. J. Heat Mass Transf. 107 (2017) 21–44. [4] J. Sarkar, P. Ghosh, A. Adil, A review on hybrid nanofluids: recent research, development and applications, Renew. Sust. Energ. Rev. 43 (2015) 164–177. [5] H. Babar, H.M. Ali, Towards hybrid nanofluids: preparation, thermophysical properties, applications, and challenges, J. Mol. Liq. 281 (2019) 598–633. [6] A.K. Tiwari, P. Ghosh, J. Sarkar, Particle concentration levels of various nanofluids in plate heat exchanger for best performance, Int. J. Heat Mass Transf. 89 (2015) 1110–1118. [7] Z. Nikkhah, A. Karimipour, M.R. Safaei, P.F. Tehrani, M. Goodarzib, M. Dahari, S. Wongwises, Forced convective heat transfer of water/functionalized multi-walled carbon nanotube nanofluids in a microchannel with oscillating heat flux and slip boundary condition, Int. Commun. Heat Mass Transfer 68 (2015) 69–77. [8] J. Zhang, Y. Diao, Y. Zhao, Y. Zhang, Thermal-hydraulic performance of SiC-Water and Al2O3-water nanofluids in the minichannel, J. Heat Transf. 138 (2016) 021705. [9] L. Nakharintr, P. Naphon, S. Wiriyasart, Effect of jet-plate spacing to jet diameter ratios on nanofluids heat transfer in a mini-channel heat sink, Int. J. Heat Mass Transf. 116 (2018) 352–361. [10] C.J. Ho, J.C. Liao, C.H. Li, W.M. Yan, Mohammad Amani, Experimental study of cooling performance of water-based alumina nanofluid in a minichannel heat sink with MEPCM layer embedded in its ceiling, Int. Commun. Heat Mass Transfer 103 (2019) 1–6. [11] O. Yıldız, Ö. Açıkgöz, G. Yıldız, M. Bayrak, A.S. Dalkılıç, S. Wongwises, Single phase flow of nanofluid including graphite and water in a microchannel, Heat Mass Transfer. (2019), https://doi.org/10.1007/s00231-019-02663-5. [12] P. Selvakumar, S. Suresh, Use of Al2O3–Cu/water hybrid nanofluid in an electronic heat sink, IEEE Trans. Compon. Packag. Manuf. Technol. 2 (2012) 1600–1607. [13] C.J. Ho, W.-C. Chen, W.-M. Yan, Experiment on thermal performance of waterbased suspensions of Al 2O3 nanoparticles and MEPCM particles in a minichannel heat sink, Int. J. Heat Mass Transf. 69 (2014) 276–284. [14] C.J. Ho, W.C. Chen, W.M. Yan, P. Amani, Contribution of hybrid Al2O3-water nanofluid and PCM suspension to augment thermal performance of coolant in a minichannel heat sink, Int. J. Heat Mass Transf. 122 (2018) 651–659. [15] N. Ahammed, L.G. Asirvatham, S. Wongwises, Entropy generation analysis of graphene–alumina hybrid nanofluid in multiport minichannel heat exchanger coupled with thermoelectric cooler, Int. J. Heat Mass Transf. 103 (2016) 1084–1097. [16] R. Nimmagadda, K. Venkatasubbaiah, Experimental and multiphase analysis of nanofluids on the conjugate performance of micro-channel at low Reynolds numbers, Heat Mass Transf. 53 (2017) 2099–2115. [17] M. Bahiraei, M. Berahmand, A. Shahsavar, Irreversibility analysis for flow of a nonNewtonian hybrid nanofluid containing coated CNT/Fe3O4 nanoparticles in a minichannel heat exchanger, Appl. Therm. Eng. 125 (2017) 1083–1093. [18] A.A. Hussien, M.Z.N.M. Yusop, W. Al-Kouz, E. Mahmoudi, M. Mehrali, Heat transfer and entropy generation abilities of MWCNTs/GNPs hybrid nanofluids in microtubes, Entropy 21 (480) (2019) 1–17, https://doi.org/10.3390/e21050480. [19] A.A. Hussien, N.M. Yusop, M.A. Al–Nimr, M.Z. Abdullah, A.A. Janvekar, M.H. Elnaggar, Numerical study of heat transfer enhancement using al2o3–graphene/water hybrid nanofuid flow in mini tubes, Iran. J. Sci. Technol. A (2019), https://doi.org/10.1007/s40995-018-0670-1. [20] M. Bahiraei, S. Heshmatian, Thermal performance and second law characteristics of two new microchannel heat sinks operated with hybrid nanofluid containing graphene–silver nanoparticles, Ener. Conv. Manage. 168 (2018) 357–370. [21] M. Goodarzi, I. Tlili, Z. Tian, M. Safaei, Efficiency assessment of using graphene nanoplatelets-silver/water nanofluids in microchannel heat sinks with different crosssections for electronics cooling, Int. J. Numer. Method. H. (2019), https://doi. org/10.1108/HFF-12-2018-0730. [22] M. Bahiraei, N. Mazaheri, Application of a novel hybrid nanofluid containing graphene–platinum nanoparticles in a chaotic twisted geometry for utilization in miniature devices: thermal and energy efficiency considerations, Int. J. Mech. Sci. 138–139 (2018) 337–349. [23] A. Shahsavar, A. Godini, P.T. Sardari, D. Toghraie, H. Salehipour, Impact of variable fluid properties on forced convection of Fe3O4/CNT/water hybrid nanofluid in a double-pipe mini-channel heat exchanger, J. Therm. Anal. Calorim. 137 (2019) 1031–1043. [24] C. Uysal, E. Gedik, A.J. Chamkha, A numerical analysis of laminar forced convection and entropy generation of a diamond-Fe3O4/water hybrid nanofluid in a rectangular minichannel, J. Appl. Fluid Mech. 12 (2019) 391–402. [25] V. Kumar, J. Sarkar, Numerical and experimental investigations on heat transfer and pressure drop characteristics of Al2O3-TiO2 hybrid nanofluid in minichannel heat sink with different mixture ratio, Powder Technol. 345 (2019) 717–727.
Appendix A. Uncertainty analysis Equations to find out the uncertainty for different dependent parameters have been included in this section, Heat transfer rate, Q̇ : ΔQ̇ Q̇
Δcp 2
Δρ 2 ρ
ΔV̇ 2 V̇
( ) +( ) +( ) +(
=
cp
Δ(Tout − Tin) 2 (Tout − Tin)
)
= ± 2.87%
Hydraulic diameter, dh: Δdh dh
Δwch 2 wch
Δhch 2 hch
( ) +( ) +(
=
Δ(wch + hch) 2 (wch + hch)
)
= ± 1.02%
Mean velocity, um: Δum um
ΔV̇ 2 V̇
Δwch 2 wch
Δhch 2 hch
( ) +( ) +( )
=
= ± 0.78%
Effective area, A: ΔA A
(
=
Δ(wch + 2hch) 2 (wch + 2hch)
ΔLch 2 Lch
) +( )
= ± 1.2%
Reynolds number, Re: Δ Re Re
Δρ 2 ρ
Δμ 2 μ
Δdh 2 dh
Δum 2 um
( ) +( ) +( ) +( )
=
= ± 3.10%
Heat transfer coefficient, h: Δh h
ΔQ̇ 2 Q̇
ΔA 2 A
( ) +( ) +(
=
Δ(Ts − Tm) 2 (Ts − Tm)
)
= ± 3.11%
Nusselt number, Nu: ΔNu Nu
Δdh 2 dh
Δh 2 h
Δk 2 k
( ) +( ) +( )
=
= ± 3.83%
Friction factor, f: Δf f
=
Δ(Δp) 2 Δp
Δdh 2 dh
ΔLch 2 Lch
Δρ 2 ρ
2Δum 2 um
( ) +( ) +( ) +( ) +( )
= ± 2.76%
Performance evaluation criteria, PEC: ΔPEC PEC
=
1 3
×
(
3ΔNhnf Nuhnf
2
) +(
3ΔNubf Nubf
2
) + ⎛⎝
Δfhnf fhnf
2
⎞ +⎛ ⎠ ⎝
Δfbf fbf
2
⎞ = ± 5.47% ⎠
10
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different nanoparticle ratios, J. Therm. Anal. Calorim. 135 (2019) 1243–1255. [33] M.H. Esfe, A.T.K. Abad, M. Fouladi, Effect of suspending optimized ratio of nanoadditives MWCNT-Al2O3 on viscosity behavior of 5W50, J. Mol. Liq. 285 (2019) 572–585. [34] H. Maddah, R. Aghayari, M. Mirzaee, M.H. Ahmadi, M. Sadeghzadeh, A.J. Chamkha, Factorial experimental design for the thermal performance of a double pipe heat exchanger using Al2O3-TiO2 hybrid nanofluid, Int. Commun. Heat Mass Transfer 97 (2018) 92–102. [35] C.J. Ho, W.C. Chen, An experimental study on thermal performance of Al2O3/water nanofluid in a minichannel heat sink, Appl. Therm. Eng. 50 (2013) 516–522. [36] S.J. Kline, F.A. McClintock, Describing uncertainties in single-sample experiments, Mech. Eng. 75 (1953) 3–8. [37] A. Bhattad, J. Sarkar, P. Ghosh, Discrete phase numerical model and experimental study of hybrid nanofluid heat transfer and pressure drop in plate heat exchanger, Int. Commun. Heat Mass Transfer 91 (2018) 262–273. [38] A. Dominic, J. Sarangan, S. Suresh, V.S. Devah Dhanush, An experimental investigation of wavy and straight minichannel heat sinks using water and nanofluids, J. Ther. Sci. Eng. Appl. 7 (2015) 031012. [39] C.J. Ho, Wei-Chen Chen, Wei-Mon Yan, Correlations of heat transfer effectiveness in a minichannel heat sink with water-based suspensions of Al2O3 nanoparticles and/ or MEPCM particles, Int. J. Heat Mass Transf. 69 (2014) 293–299.
[26] A.A. Charab, S. Movahedirad, R. Norouzbeigi, Thermal conductivity of Al2O3+ TiO2/water nanofluid: model development and experimental validation, Appl. Therm. Eng. 119 (2017) 42–51. [27] G.M. Moldoveanu, G. Huminic, A.A. Minea, A. Huminic, Experimental study on thermal conductivity of stabilized Al2O3 and SiO2 nanofluids and their hybrid, Int. J. Heat Mass Transf. 127 (2018) 450–457. [28] K.A. Hamid, W.H. Azmi, M.F. Nabil, R. Mamat, K.V. Sharma, Experimental investigation of thermal conductivity and dynamic viscosity on nanoparticle mixture ratios of TiO2-SiO2 nanofluids, Int. J. Heat Mass Transf. 116 (2018) 1143–1152. [29] K.A. Hamid, W.H. Azmi, M.F. Nabil, R. Mamat, Experimental investigation of nanoparticle mixture ratios on TiO2-SiO2 nanofluids heat transfer performance under turbulent flow, Int. J. Heat Mass Transf. 118 (2018) 617–627. [30] A.S. Dalkılıç, Ö. Açıkgöz, B.O. Küçükyıldırım, A.A. Eker, B. Lüleci, C. Jumpholkul, S. Wongwises, Experimental investigation on the viscosity characteristics of water based SiO2-graphite hybrid nanofluids, Int. Commun. Heat Mass Transfer 97 (2018) 30–38. [31] F.R. Siddiqui, C.Y. Tso, K.C. Chan, S.C. Fu, C.Y.H. Chao, On trade-off for dispersion stability and thermal transport of Cu-Al2O3 hybrid nanofluid for various mixing ratios, Int. J. Heat Mass Transf. 132 (2019) 1200–1216. [32] N.N.M. Zawawi, W.H. Azmi, M.Z. Sharif, G. Najafi, Experimental investigation on stability and thermo-physical properties of Al2O3–SiO2/PAG nanolubricants with
11
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Particle ratio optimization of Al2O3-MWCNT hybrid nanofluid in minichannel heat sink for best hydrothermal performance Vivek Kumar, Jahar Sarkar
T
⁎
Department of Mechanical Engineering, Indian Institute of Technology (B.H.U.), Varanasi, UP 221005, India
H I GH L IG H T S
hydrothermal behaviour of Al O -CNT hybrid nanofluid in minichannel. • Experimental of flow rate (Rynolds number), temperature and mixture ratio are studied. • Effects of particle mixture ratio on h/Δp and performance evaluation criteria is studied. • Effect • Optimum particle mixture ratio is found as 3:2 for best hydrothermal performance. 2
3
A R T I C LE I N FO
A B S T R A C T
Keywords: Minichannel heat sink Hybrid nanofluid Nanoparticle mixture ratio Heat transfer Pressure drop Optimization
Interesting hydrothermal behavior of hybrid nanofluid containing combination of dissimilar particles (completely different in terms of shape, size and properties) by changing their ratio is unexplored yet, which is analyzed experimentally for minichannel heat sink. Nine parallel rectangular minichannels are used in the heat sink each of which has 1 mm width, 3 mm depth and 30 mm length. Al2O3–MWCNT/water hybrid nanofluid of 0.01% volume concentration with various nanoparticle mixture ratios is used as coolant to analyze the effect of Reynolds number and working fluid inlet temperature. Nanoparticles mixing ratios (5:0, 4:1, 3:2, 2:3, 1:4 and 0:5) have been taken as volume ratios. Convective heat transfer coefficient, Nusselt number, pressure drop and friction factor increase with an increase in fraction of MWCNT in hybrid nanofluid. A maximum enhancement of 44.02% has been observed for the convective heat transfer coefficient with MWCNT (5:0) hybrid nanofluid as compared to water. Maximum pressure drop has been increased by 51.2% over base fluid at an inlet temperature of 20 °C for the MWCNT (5:0) hybrid nanofluid. Inlet fluid temperature has negative effect on pressure drop and positive effect on heat transfer. The observed optimum volumetric mixing ratio of Al2O3 and MWCNT is around 3:2, for which hybrid nanofluid yields maximum heat transfer coefficient to pressure drop ratio. Performance evaluation criteria (PEC) has been evaluated greater than 1 for all the nanofluids, which concludes that nanofluids are better option as compared to conventional fluid (DI water). Based on the experimental data, correlations for Nusselt number and friction factor have been introduced with R2 values of 98.8% and 96.3%, respectively, and compared with available correlations.
1. Introduction Due to miniaturization and increasing power density of advanced electronic devices, an effective cooling system is needed for their optimal operation. The use of micro/mini channel heat sinks (MCHS) is one of effective ways for high heat flux removal [1,2]. The miniaturized heat exchanger (i.e., MCHS) has gained a lot of attention due to materials preserving, space constraints and demand of higher energy density, which can transmit more heat per unit volume than conventional heat exchangers [3]. The heat transfer capability can be further
⁎
enhanced by using nanofluids as well as hybrid nanofluids due to their better heat transfer characteristics [4]. With proper hybridization and trade-off between advantages and disadvantages of individual suspension, hybrid nanofluid is supposed to yield better thermal properties and hence attracted as cooling media for various applications [5]. Alumina, a most common metal oxide nanoparticle has been used by many researchers due to its low cost, availability, chemical stability and higher heat transfer improvement to pumping power increase ratio [6]. On the other hand, multi-walled carbon nanotube (MWCNT) has attracted many researchers due to its higher thermal conductivity and
Corresponding author. E-mail address: [email protected] (J. Sarkar).
https://doi.org/10.1016/j.applthermaleng.2019.114546 Received 9 April 2019; Received in revised form 21 August 2019; Accepted 16 October 2019 Available online 18 October 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
Applied Thermal Engineering 165 (2020) 114546
V. Kumar and J. Sarkar
Nomenclature A cp dh f h hch k V̇ Nu Δp Pr Q̇ Re T u vol wch
Lch PEC
effective heat transfer area (m2) specific heat (J/kg K) hydraulic diameter (mm) friction factor heat transfer coefficient (W/m2 K) channel height (mm) thermal conductivity (W/m K) volume flow rate (LPM) Nusselt number pressure drop (Pa) Prandtl number heat transfer rate (W) Reynolds number temperature (°C) velocity (m/s) volume channel width (mm)
channel length (mm) performance evaluation criteria
Greek symbols µ ρ Φ
dynamic viscosity (Pa s) density (kg/m3) volume concentration
Subscripts bf np hnf ch in out s m
base fluid nanoparticle hybrid nanofluid channel inlet outlet surface mean
Al2O3-TiO2 hybrid nanofluid in minichannel and showed improvement over base fluid. Nanoparticle mixture ratio influences the heat transfer characteristics of hybrid nanofluids; although the studies are limited on this important issue. Charab et al. [26] proposed the model of thermal conductivity for 1.0 v% Al2O3-TiO2 hybrid nanofluid with different particle ratios and reported maximum heat transfer enhancement up to 35.3% for 2:3 ratio. Moldoveanu et al. [27] experimentally investigated the thermal conductivity of Al2O3–SiO2/Water hybrid nanofluids and noticed that the hybrid nanofluid has higher thermal conductivity compared to alumina nanofluid. Hamid et al. [28] performed an experimental investigation on thermal conductivity and viscosity of hybrid nanofluids with different particle ratio. They observed maximum thermal conductivity enhancement up to 16% for 1:4 ratio of TiO2-SiO2 and highest dynamic viscosity for the ratio of 5:5. They [29] also performed an experimental study on the heat transfer performance of cooling equipment for the same pair of nanoparticles and found maximum heat transfer enhancement up to 35.3% for 2:3 ratio. Dalkılıç et al. [30] experimentally investigated the viscosity of Graphite-SiO2 hybrid nanofluid at different volume concentrations and different weight ratios and showed a viscosity increase of 0.65–36.32% with an increase in volumetric concentration. Siddiqui et al. [31] studied the stability and thermophysical properties of Cu-Al2O3 hybrid nanofluid for different mixing ratios and showed that 5:5 was the optimum mixing ratio because of its better thermal conductivity and good stability to achieve overall hydrothermal properties. Zawawi et al. [32] investigated the thermophysical properties of Al2O3–SiO2/PAG nanolubricants for various particle mixture ratios and found that 60:40 was the optimum mixture ratio with the lowest property enhancement ratio. Esfe et al. [33] experimentally investigated the optimized ratio of MWCNT (30 vol%)-Al2O3 (70 vol%) in 5 W50 oil with total volume concentrations of 0.05% to 1% and found maximum viscosity enhancement of 24%. Kumar and Sarkar [25] experimentally studied Al2O3-TiO2 hybrid nanofluid in minichannel heat sink with different mixture ratio. As shown, similar shaped nanoparticle combination has been only considered in the investigation. With the best of the authors' knowledge, no similar study on minichannel heat sink using dissimilar nanoparticles combination is available which can equally affect both the hydrodynamic and thermal behaviours. In the previous study [25], we have seen the negligible effect of mixture ratio for similar particles (oxide-oxide) and hence we will experimentally analyze the effect of mixture ratio for dissimilar particles (oxide-carbon nanotube) on the performance of minichannel heat sink using Al2O3-MWCNT/water hybrid nanofluids in this study. The present
very high aspect ratio for application in nanofluids [7]. Hence, alumina and MWCNT can be the best combination in hybrid nanofluid for MCHS. Many experimental studies have been conducted on hydrothermal characteristics of nanofluid in MCHS [8–11]. However, few studies have been conducted on MCHS using hybrid nanofluids. Selvakumar and Suresh [12] investigated hydrothermal characteristics of hybrid nanofluids in minichannel and observed 24.35% enhancement in the convective heat transfer coefficient and 12.61% increase in pumping power. An experimental study carried out by Ho at al. [13,14] using hybrid water-based suspensions of Al2O3 and MEPCM particles in minichannel heat sink and showed that Al2O3–water nanofluid is effective over hybrid nanofluids at higher Reynolds number. Ahammed et al. [15] experimentally investigated the characteristics of graphene–alumina hybrid nanofluids in a minichannel and reported convective heat transfer coefficient enhancement of 63.13%. An experimental study has been done by Nimmagadda and Venkatasubbaiah [16] for Al2O3 + Ag hybrid nanofluid in microchannel showed that Nusselt number enhances with volume concentration. Bahiraei et al. [17] investigated the irreversibilities of CNT/Fe3O4 hybrid nanofluid in the minichannel heat exchanger and showed that at low Fe3O4 concentration, an optimum point found for the total entropy generation. Hussien et al. [18] experimentally investigated the performance of MWCNT/ GNP hybrid nanofluids in microtubes in the Re range of 200–500 and found that average heat transfer coefficient enhances 58% for MWCNT/ GNP hybrid nanofluid. They also numerically examined Al2O3/graphene hybrid nanofluid inside minitube and found maximum enhancement in heat transfer coefficient about 13.7% over Al2O3/water nanofluid [19]. The performance of different microchannel heat sink using graphene-Ag hybrid nanofluid has been numerically investigated in laminar regime [20,21] and revealed that nanofluid is better cooling option in comparison to water. Hydrothermal feature of chaotic twisted minichannel has been studied by Bahiraei and Mazaheri [22] using graphene nanoplatelets decorated with platinum hybrid nanofluid and found figure of merit always more than 1.5. Shahsavar et al. [23] numerically evaluated the hydrothermal performance of Fe3O4/CNT/ water hybrid nanofluid in double pipe minichannel by considering both non-Newtonian and Newtonian fluids and found superior performances of non-Newtonian hybrid nanofluid. A numerical investigation has been done by Uysal et al. [24] in rectangular minichannel using DiamondFe3O4/Water hybrid nanofluid and showed that hybrid nanofluid has higher convective heat transfer coefficient and Nusselt number over the mono particles nanofluids. Kumar and Sarkar [25] numerically and experimentally investigated the heat transfer and pressure drop of 2
Applied Thermal Engineering 165 (2020) 114546
V. Kumar and J. Sarkar
then using Al2O3–MWCNT hybrid nanofluids. Working fluids at different inlet temperatures (20 °C, 30 °C and 40 °C) were supplied into the system from the reservoir by using a pump. The heat input was supplied to the system when the maximum Reynolds number was achieved. Volume flow rates were chosen to yield moderate Reynolds number range, for which, no study is available in the literature for present hybrid nanofluids. Five surface thermocouples were used to calculate the surface temperature. Four thermocouples were inserted at all the four corners of minichannel and one thermocouple was inserted at the centre of the heat sink to the bottom for measure the surface temperature (Ts). The surface temperature has been taken by averaging of all the five readings by thermocouples. All the readings were taken under steady-state condition, i.e., thermal equilibrium, which was achieved in 40–50 min of the time interval. Reliability and verification of the experimental setup have been justified by the published research work based on the present setup [25]. Experimental Nusselt number for the conventional fluid (DI water) was compared from existing correlation given by different authors and found similar trend and close prediction.
experimental work is focused only on the different volume mixing ratio of two dissimilar particles (oxide-carbon nanotube) with fixed volume concentration of 0.01%. At higher volume concentration, heat transfer as well as pressure drop characteristics may be improved; however may lead to minichannel clogging problem due to the presence of MWCNT (its length is in micro level). Effects of Reynolds number, nanoparticle mixture ratio and inlet temperature have been investigated on the convective heat transfer coefficient, Nusselt number, pressure drop and friction factor. Heat transfer coefficient to pressure drop ratio and performance evaluation criteria (PEC) have been discussed as well to find optimum mixture ratio. Also, correlations have been developed for Nusselt number and friction factor and compared with the similar correlations available in the literature. 2. Methodology 2.1. Test facility The layout and photograph of the experimental setup are given in Figs. 1 and 2, respectively. The detailed description of the experimental setup has been reported earlier [25]. For heating, a cartridge heater having the power of 50 W, with cross-section of 10 mm × 30 mm is placed into the bottom of MCHS. Dimension, temperature, volume flow rate, pressure drop and heating power have been measured by vernier calliper, thermocouple, rotameter, pressure transducer and wattmeter, having uncertainties of ± 0.02 mm, ± 0.1 °C, ± 0.5%, ± 0.25% and ± 0.25%, respectively. Photo image and schematic of the minichannel heat sink with dimensions are shown in Fig. 3. Minichannel heat sink made of aluminium consists of 9 parallel rectangular-shaped minichannels each of which has a width of 1 mm, a depth of 3 mm, a length of 30 mm and hydraulic diameter of 1.5 mm. Side fines of minichannel are of 0.7 mm and all others fins are of thickness 1.2 mm. Pitch of minichannel has been shown in Fig. 3 with side pitch of 1.95 mm and all others pitch of 2.20 mm. The experiments were first performed by using distilled water and
2.2. Preparation of hybrid nanofluids A two-step method was used to prepare the Al2O3–MWCNT/DI water hybrid nanofluids. Alumina nanoparticles (45 nm diameter) manufactured by Alfa Aesar, USA and multiwall carbon nanotubes (diameter: 20 nm, Length: 2 µm) manufactured by Otto Chemie Pvt. Ltd. were dispersed in the DI water. Acid-treated MWCNT used in this investigation to make it hydrophilic to DI water. Al2O3 nanoparticle and MWCNT have been taken in the different volume mixing ratios are 5:0, 4:1, 3:2, 2:3, 1:4 and 0:5, respectively to prepare hybrid nanofluid of 0.01 vol% concentration. An electronic balance (SHIMADZU, ATX224, Japan) was used to measure the calculated amounts of Al2O3 and MWCNT nanoparticles and then they were mixed with DI water. Ultrasonicator (Labman Scientific Instruments, India) was used to have cell disruption and homogenization of the colloid for 6–8 h without
Fig. 1. Layout of the experimental setup [25]. 3
Applied Thermal Engineering 165 (2020) 114546
V. Kumar and J. Sarkar
Fig. 2. Photograph of experimental setup [25].
Volumetric heat capacity ( ρcp ) value was measured by Hot Disk TPS500 analyzer and then specific heat cp was obtained by dividing ρcp with ρ . LVDV-II + Pro Brookfield digital viscometer was used to measure the viscosity of samples. Thermal conductivity of the prepared nanofluid samples was measured by Hot Disk TPS-500 analyzer.
surfactant. Scanning electron microscopy (SEM) image of Al2O3MWCNT hybrid nanofluid of 3:2 mixture ratio is shown in Fig. 4, which predicts a uniform distribution of both nanoparticle types and average particle size is of Al2O3 nanoparticles is < 50 nm. The nanofluid sample has been observed stable more than 3 days and experiments were performed using homogenized hybrid nanofluids. The pH value obtained in the range of 7.71–7.74 for the small sample of synthesized hybrid nanofluids taken from the four different places of the container. The pH value obtained is far away from the isoelectric points (IEP) which ensure the homogeneity and stability of the hybrid nanofluids [34]. Better homogeneity and stability is achieved when the pH value of the sample is away from the IEP value due to the large repulsion among the nanoparticles. The density and viscosity of synthesized nanofluids were determined for the same 4 different samples which have been used for the determination of pH values. No appreciable change was measured in the density for all the samples. The Viscosity values are obtained in the range of 1.19 cP to 1.24 cP; which was measured at 100 RPM and room temperature. Scanning electron microscope (SEM) is used to show the distribution of the initial nanoparticles in the hybrid nanofluids. From the SEM image (shown in Fig. 4), it can be observed that particles are uniformly distributed in the synthesized hybrid nanofluid. Similar homogeneity of particles distribution also observed from the SEM images of the other samples. The degree of homogeneity of prepared hybrid nanofluids was found approximately 98% based on the measured properties.
2.4. Data analysis By measuring flow rate, fluid inlet and outlet temperatures, the heat transfer rate has been predicted by,
Q̇ = V̇ ρcp (Tout − Tin )
(1)
Then, the average convection heat transfer coefficient was calculated by,
h=
Q̇ A (Ts − Tm )
(2)
where Tm is mean fluid temperature and Ts is average wall temperature (average of five thermocouples reading, inserted at different locations to bottom of the heat sink). The effective heat transfer area (A) has been calculated as,
A = N (wch + 2ηfin hch) Lch
(3)
where N is the number of channels, ηfin is fin efficiency, Lch, wch and hch are length, width and height of channel, respectively. Fin efficiency can be found out using Eqs. (4) and (5) by iteration [35].
2.3. Thermophysical properties of nanofluids Thermophysical properties for DI water and Alumina-MWCNT hybrid nanofluids for different proportion at 30 °C have been shown in Table 1. Various apparatuses were used to experimentally measure the effective thermophysical properties of prepared hybrid nanofluids. Before collecting data, all the instruments were calibrated and measurements have been repeated three times. The density was obtained by measuring mass (using electronic balance machine) of specified volume (using measuring biker) and then dividing mass by volume ( ρ = m V ).
ηfin =
tanh(mhch ) mhch
(4)
m=
2h (km wfin )
(5)
where km is the heat sink material thermal conductivity and wfin is the width of the fin. Hydraulic diameter (dh) of minichannel is defined as, 4
Applied Thermal Engineering 165 (2020) 114546
V. Kumar and J. Sarkar
Fig. 3. Minichannel heat sink with schematic diagram.
dh =
2wch hch wch + hch
f=
(6)
2dh Δp Lch ρum2
(10)
Thus, the Nusselt number has been predicted as below:
Nu =
hdh k
(7)
2.5. Uncertainty analysis
um is mean fluid velocity and it is calculated based on volumetric flow rate, it is defined as
um =
V̇ Nwch hch
Uncertainty analysis of the convection heat transfer coefficient, Reynolds number, Nusselt number and friction factor have been done by Kline and McClintock [36] equation. During experiments with MCHS, the temperatures, flow rates and pressure loss were measured with appropriate instruments. The values of uncertainty (W) obtained are given in Table 2.
(8)
The Reynolds number is given by,
Re =
ρum dh μ
2
2 1 2
2
⎞⎤ ⎛ ∂R ⎛ ∂R ⎞ ⎛ ∂R ⎞ W=⎡ ⎢ ∂X1 W1 + ∂X2 W2 + ⋯ ∂Xn Wn ⎥ ⎝ ⎠ ⎝ ⎠ ⎠⎦ ⎝ ⎣
(9)
⎜
By measuring the flow rate and pressure drop, the friction factor is defined as, 5
⎟
⎜
⎟
⎜
⎟
(11)
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V. Kumar and J. Sarkar
number can be easily observed by the figures. This may be due to thinning of the thermal boundary layer. Enhancement in heat transfer coefficient is observed with the addition of nanoparticles as compared to DI water, which can result to many reasons including an increase in thermal conductivity, various slip mechanisms and the nano-fin effects [6]. The nanoparticles carrying the heat along with them also contributed to these results along with results from the fluid movement around the nanoparticles causing micro convection [37]. The viscosity decreases with the increase in temperature, which may be the cause of thinning of the boundary layer. For the same temperature difference, heat transfer coefficient increases due to thinning of the thermal boundary layer. The large surface area of particles, micro convection may increase the heat dispersion in the fluid at a faster rate. As MWCNT nanoparticles fraction increasing in the nanofluid, the heat transfer coefficient is increased. The maximum value of the calculated heat transfer coefficient is 4997.5 W/m2 K for MWCNT/DI water nanofluid. The maximum enhancement of the heat transfer coefficient has been predicted by nearly 44.02% for 0.01 vol% MWCNT/DI water nanofluid compared to DI water. It is mainly due to the very high thermal conductivity of MWCNT nanoparticles. Another reason may be a higher specific surface area due to the higher aspect ratio of the carbon nanotube. It shows from the figures that there is an increment in the heat transfer coefficient with an inlet temperature of coolants. The heat transfer coefficient is increased for MWCNT/DI water nanofluid by 12.12% when the temperature of working fluid is increased from 20 °C to 40 °C. The heat transfer enhancement was considerably improved at higher working temperature because of the improvement of thermal properties. The increment of thermal conductivity of nanofluids is more meaningfully at higher operating temperature. Enhancement in thermal conductivity which is dependent on temperature may be another possible advantage for using the nanofluids as the high-temperature coolants. Fig. 8 shows the variation of Nusselt number with Reynolds number for different working fluids at the inlet temperatures of 30 °C. Increase in flow rate causes an increase in Reynolds number and Nusselt number due to an increase in heat transfer coefficient. The addition of nanoparticles results into increase in Nusselt number as shown in figure due to the increase in heat transfer coefficient as discussed above. The Nusselt number for hybrid nanofluids also shows dependency on the mixture ratio of nanoparticles, although the variation is similar to heat transfer coefficient. As the MWCNT fraction increases in the hybrid nanofluid, Nusselt number increases. The maximum increment of 41.04% is found for MWCNT/DI water nanofluid compared to DI water. This is due to increment in the heat transfer coefficient supported by
Fig. 4. SEM image of Al2O3-MWCNT hybrid nanofluid at 100 k magnification. Table 1 Thermo-physical properties of DI water and Alumina-MWCNT hybrid nanofluid (0.01v%) for different proportion at the temperature of 30 °C. Fluids
DI water Al2O3 + CNT Al2O3 + CNT Al2O3 + CNT Al2O3 + CNT Al2O3 + CNT Al2O3 + CNT
(5:0) (4:1) (3:2) (2:3) (1:4) (0:5)
Thermal conductivity (W/ m K)
Specific heat (J/kg K)
Density (kg/m3)
Viscosity (Pa s)
0.6170 0.6175 0.6177 0.6181 0.6188 0.6195 0.6207
4182.4 4181.0 4180.8 4180.4 4180.3 4180.1 7179.9
997.3 998.8 998.7 998.2 998.0 997.9 997.7
0.00091 0.00093 0.00094 0.00095 0.00095 0.00097 0.00099
Table 2 The uncertainties during the measurements of the experimental parameters. Variable
Uncertainty value (%)
Heat transfer rate, Q̇ (W) Effective area, A (m2) Hydraulic diameter, dh (m) Velocity, um (m/s) Thermal conductivity, k (W/m K) Viscosity, µ (Pa/s) Density, ρ (kg/m3) Specific heat, cp (J/kg K) Heat transfer coefficient, h (W/m2 K) Friction factor, f Nusselt number, Nu Reynolds number, Re h/Δp PEC
± 1.2 ± 1.02 ± 0.78 ± 2.0 ± 2.0 ± 2.0 ± 2.0 ± 3.1 ± 2.76 ± 3.83 ± 3.1 ± 3.12 ± 5.47
± 2.87
3. Results and discussion 3.1. Heat transfer characteristics The variations of calculated heat transfer coefficient with Reynolds number for different coolants at the inlet temperatures of 20, 30 and 40 °C has been depicted in Figs. 5–7. Due to the wide range of atmospheric temperature globally, the present investigation is done on different inlet temperature to claim the sustainability and flexibility of the current study. Constant temperature bath having synchronized heating and cooling provisions before flowing into the channel is employed for maintaining these temperatures. In the figures, analysis has been displayed for various coolants synthesized by changing the fraction of MWCNT and Al2O3 nanoparticles for a total of 0.01% volume concentration. Heat transfer coefficient increases with the Reynolds
Fig. 5. Variation of temperature = 20 °C). 6
heat
transfer
coefficient
with
Re
(Inlet
Applied Thermal Engineering 165 (2020) 114546
V. Kumar and J. Sarkar
Fig. 6. Variation of temperature = 30 °C).
heat
transfer
coefficient
with
Re
(Inlet
Fig. 7. Variation of temperature = 40 °C).
heat
transfer
coefficient
with
Re
(Inlet
Fig. 9. Variation of Pressure drop with Re (Inlet temperature = 20 °C).
Fig. 10. Variation of Pressure drop with Re (Inlet temperature = 30 °C).
Fig. 11. Variation of Pressure drop with Re (Inlet temperature = 40 °C). Fig. 8. Variation of temperature = 30 °C).
Nusselt
number
with
Reynolds
number
(Inlet
7
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V. Kumar and J. Sarkar
significantly higher than that at 0:5. Hence, it may be concluded that the optimum mixture ratio is around 3:2 for the Al2O3-MWCNT (spherical-cylindrical) nanoparticle combination in minichannel heat sink. It is also observed that the optimum mixing ratio is not changing with the flow rate (Reynolds number) and hence may be applicable for other Reynolds number range also.
equation (7). Improvement in Nusselt number has been observed while increase in fluid inlet temperature. 3.2. Pressure drop characteristics The variation of pressure drop with Reynolds number at the inlet temperatures of 20 °C, 30 °C and 40 °C are predicted in Figs. 9–11, respectively. There is an increment in pressure drop with an increase in Reynolds number. With the addition of nanoparticles in the base fluid, pressure drop increases due to dual effects increasing viscosity and density. As the MWCNT fraction in the hybrid nanofluid increases, the pressure drop increases with a faster rate. Maximum pressure drop is found 404.8 N/m2 at Reynolds number of 447 and inlet temperature of 20 °C for MWCNT/DI water working fluid. The enhancement of 51.2% has been observed for MWCNT/DI water nanofluid when compared to DI water. Such enhancement in pressure drop for MWCNT/DI water may be due to its high surface area. Inlet temperature has a negative effect on pressure drop due to a decrease in viscosity with temperature. The reason for the decrease in viscosity with the temperature is weakening of intermolecular force between molecules. Pressure drop decreases with an inlet temperature of coolant from 20 °C to 40 °C. It is a well-known fact that pressure drop increases with Reynolds number for the same cross-section, but it is revealed from the Figs. 9–11 that the points diverged at higher Reynolds number. It is understood by the fact that pressure drop directly proportional to mass flux and viscosity, and mass flux has dominance over viscosity at high Reynolds number. Variation of friction factor with Reynold number is shown in Fig. 12 for inlet temperatures of 30 °C. As predicted, the friction factor decreases with increase in Reynold number is due to dual effects of boundary layer thinning and mass velocity increase. Increase in viscosity and slip mechanism may be the reasons to increment in friction factor with the addition of nanoparticles. Friction factor increases for the hybrid nanofluids over base fluid. It has a higher value as the concentration of MWCNT increases in the hybrid nanofluid due to its high surface area. At Re = 149, the maximum value of friction factor for MWCNT/DI water nanofluid is approximately 1.08. It is observed from the figure that the deviation of friction factor for different working fluids is significant at lower Reynolds number. This may be because of at lower Reynolds number viscosity is predominant and it is attributed by the fact that the viscosity increases with increase in MWCNT concentration due to particle surface area and interparticle attraction. Another reason may be rupturing of the boundary layer is predominant for MWCNT compared to Al2O3 nanoparticles. The friction factor decreases with the increase in temperature for the same Reynold number due to a decrease in viscosity. As predicted, there is a significant change of friction factor with the variation of nanoparticle mixture ratio for hybrid nanofluids of same total particle concentration. Hence, it can be concluded that the hybrid nanofluid containing the mixture of dissimilar nanoparticles of different shape and size will give different flow characteristics.
3.4. Performance evaluation criteria (PEC) The heat transfer and pressure drop results do not provide the correct conclusion individually that the usage of nanofluids is a good option and vice-versa as both the heat transfer as well as pressure drop increase by using nanofluids. Therefore, apart from h/pressure drop, another comprehensive assessment factor called PEC (Performance evaluation criteria) has been introduced. PEC is defined as,
PEC =
Nuhnf Nubf (fhnf fbf )1
3
(12)
From Fig. 14, it is concluded that in all the cases PEC is above one. So, it is interpreted that usage of nanofluid is effective as a coolant compared to distilled water in minichannel heat sink. It can be concluded enhancement in pressure drop is less remarkable compared to improvement in heat transfer coefficient. Among all the nanofluids, Al2O3 + CNT (0:5) has maximum PEC value. The maximum value of PEC is 1.26 at higher Reynolds number. As CNT fraction is increasing in the nanofluids, PEC value increases. 3.5. Proposed correlations The proposed correlation for Nusselt number and friction factor are derived based on the experimental results as follows, (150 ≤ Re ≤ 460, 6.84 ≤ Pr ≤ 7, Φ = 0.01%)
Nu = 0.3035Re 0.4407 Pr 0.36 (1 + R)0.317
(13)
f = 43.162Re−0.84807 (1 + R)0.65775
(14)
where R is defind as the ratio of CNT fraction (Φ2) to total Al2O3 + CNT fraction (i.e. R = Φ2/(Φ1 + Φ2)). The range of R varies from 0 to 1. Developed correlation for Nusselt number and friction factor has Rsquare values of 98.8% and 96.3%, respectively. It can be illustrative from Fig. 15 that proposed correlation and experimental values for Nu are in good agreement. Correlation predicted all the experimental data within the error range of +3% to −4.3%. The plot between predicted friction factor and experimental friction factor is shown in Fig. 16. The
3.3. Effect of mixture ratio on h/Δp The ratio of heat transfer coefficient and pressure drop with a mixture ratio for different volume flow rates at an inlet temperature of 30 °C has been shown in Fig. 13. It is found that for the lower volume flow rate, h/Δp ratio is maximum. As the MWCNT concentration increases in the hybrid nanofluid, initially h/Δp ratio increases, after a mixture ratio it starts decreasing. The optimum mixture ratio is found to be around 3:2 after which the h/Δp ratio starts decreasing at a higher rate. It is due to the fact that the pressure drop increases at a higher rate as the concentration of MWCNT increases in the hybrid nanofluids. The reason for the increase in pressure drop is because of its high specific surface area due to the higher aspect ratio for MWCNT. After incorporation of error bar in the graph, it is observed that the value of h/ pressure drop at 3:2 is slightly higher than that at 4:1 or 2:3 and
Fig. 12. Variation of Temperature = 30 °C). 8
Friction
factor
with
Reynolds
number
(Inlet
Applied Thermal Engineering 165 (2020) 114546
V. Kumar and J. Sarkar
Fig. 16. Variation of Predicted friction factor with Experimental friction factor. Fig. 13. Ratio of heat transfer coefficient to pressure drop (h/Δp) with the mixture ratio.
Fig. 17. Deviation of proposed correlation with existing correlation for Nu. Fig. 14. PEC with Re of hybrid nanofluids for different mixing ratio.
maximum error between the proposed correlation and experimental data is lies between +12.5% and −12.5%. The experimental data of Nusselt number obtained for Al2O3/DI water nanofluid is compared with the proposed correlation (taking R = 0) and existing correlations available in the literature as shown in Fig. 17. The proposed correlation has same trends as existing correlations [38,39] without good agreement. Experimental values have best match with proposed correlation. The reason to support this deviation between proposed correlation and existing correlations is that existing correlations have developed for minichannel with dissimilar geometry and heat flux condition. 4. Concluding remarks In this paper, the hydrothermal characteristics of Al2O3-MWCNT (completely different in nature) hybrid nanofluid in minichannel heat sink have been experimentally investigated for volume concentration of 0.01% at different particle ratios (5:0 to 0:5), Reynolds number (140 < Re < 4 6 0) and the inlet temperatures (20 °C, 30 °C and 40 °C). From the results and discussion, the following conclusions can be made: Fig. 15. Variation of Predicted Nu with Experimental Nu.
• With the increase in inlet temperature from 20 °C to 40 °C and na-
noparticle dispersion in the base fluid, convective heat transfer
9
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V. Kumar and J. Sarkar
•
• • • • • • • •
Appendix B. Supplementary material
coefficient and Nusselt number increase due to enhancement of thermal properties due to temperature increases. With the Reynolds number and nanoparticle dispersion in the base fluid, pressure drop and friction factor increase due to dual effects increasing viscosity and density, whereas decreases with inlet temperature because of decrease in viscosity. Viscosity decreases due to cohesion force between molecules decreases as temperature increases. Maximum pressure drop increment is 51.2% for MWCNT/ DI water nanofluid compared to DI water. Among all the working fluids, Al2O3 (5:0) is less effective and MWCNT (0:5) is the most effective hybrid nanofluid in terms of heat transfer coefficient. With the increase in MWCNT concentration in the particle mixture of hybrid nanofluid, the heat transfer coefficient, Nusselt number, pressure drop and friction factor increase. Maximum 44% enhancement of heat transfer coefficient has been observed for MWCNT (5:0) hybrid nanofluid. Relative effect on heat transfer coefficient and pressure drop is analyzed and found that the optimum particle mixture ratio is around 3:2. For hybrid nanofluid with dissimilar nanoparticle concentration, mixture ratio has a significant effect on heat transfer and pressure drop characteristics. Al2O3 and MWCNT at 3:2 mixture ratio shows the maximum value for h/Δp. This research reveals that around 3:2 mixture ratio for Al2O3MWCNT based hybrid nanofluids is the optimum mixture ratio when considering hydrothermal characteristics in minichannel heat sink. Usage of nanofluid is effective as a coolant compared to distilled water. The maximum value of PEC is 1.26 for Al2O3-CNT (0:5) hybrid nanofluid. Developed correlations for Nusselt number and friction factor are in good agreements with experimental data.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114546. References [1] S. Ndao, Y. Peles, M.K. Jensen, Multi-objective thermal design optimization and comparative analysis of electronics cooling technologies, Int. J. Heat Mass Transf. 52 (2009) 4317–4326. [2] A. Falahat, R. Bahoosh, A. Noghrehabadi, M.M. Rashidi, Experimental study of heat transfer enhancement in a novel cylindrical heat sink with helical minichannels, Appl. Therm. Eng. 154 (2019) 585–592. [3] I.A. Ghani, N.A.C. Sidik, N. Kamaruzaman, Hydrothermal performance of microchannel heat sink: the effect of channel design, Int. J. Heat Mass Transf. 107 (2017) 21–44. [4] J. Sarkar, P. Ghosh, A. Adil, A review on hybrid nanofluids: recent research, development and applications, Renew. Sust. Energ. Rev. 43 (2015) 164–177. [5] H. Babar, H.M. Ali, Towards hybrid nanofluids: preparation, thermophysical properties, applications, and challenges, J. Mol. Liq. 281 (2019) 598–633. [6] A.K. Tiwari, P. Ghosh, J. Sarkar, Particle concentration levels of various nanofluids in plate heat exchanger for best performance, Int. J. Heat Mass Transf. 89 (2015) 1110–1118. [7] Z. Nikkhah, A. Karimipour, M.R. Safaei, P.F. Tehrani, M. Goodarzib, M. Dahari, S. Wongwises, Forced convective heat transfer of water/functionalized multi-walled carbon nanotube nanofluids in a microchannel with oscillating heat flux and slip boundary condition, Int. Commun. Heat Mass Transfer 68 (2015) 69–77. [8] J. Zhang, Y. Diao, Y. Zhao, Y. Zhang, Thermal-hydraulic performance of SiC-Water and Al2O3-water nanofluids in the minichannel, J. Heat Transf. 138 (2016) 021705. [9] L. Nakharintr, P. Naphon, S. Wiriyasart, Effect of jet-plate spacing to jet diameter ratios on nanofluids heat transfer in a mini-channel heat sink, Int. J. Heat Mass Transf. 116 (2018) 352–361. [10] C.J. Ho, J.C. Liao, C.H. Li, W.M. Yan, Mohammad Amani, Experimental study of cooling performance of water-based alumina nanofluid in a minichannel heat sink with MEPCM layer embedded in its ceiling, Int. Commun. Heat Mass Transfer 103 (2019) 1–6. [11] O. Yıldız, Ö. Açıkgöz, G. Yıldız, M. Bayrak, A.S. Dalkılıç, S. Wongwises, Single phase flow of nanofluid including graphite and water in a microchannel, Heat Mass Transfer. (2019), https://doi.org/10.1007/s00231-019-02663-5. [12] P. Selvakumar, S. Suresh, Use of Al2O3–Cu/water hybrid nanofluid in an electronic heat sink, IEEE Trans. Compon. Packag. Manuf. Technol. 2 (2012) 1600–1607. [13] C.J. Ho, W.-C. Chen, W.-M. Yan, Experiment on thermal performance of waterbased suspensions of Al 2O3 nanoparticles and MEPCM particles in a minichannel heat sink, Int. J. Heat Mass Transf. 69 (2014) 276–284. [14] C.J. Ho, W.C. Chen, W.M. Yan, P. Amani, Contribution of hybrid Al2O3-water nanofluid and PCM suspension to augment thermal performance of coolant in a minichannel heat sink, Int. J. Heat Mass Transf. 122 (2018) 651–659. [15] N. Ahammed, L.G. Asirvatham, S. Wongwises, Entropy generation analysis of graphene–alumina hybrid nanofluid in multiport minichannel heat exchanger coupled with thermoelectric cooler, Int. J. Heat Mass Transf. 103 (2016) 1084–1097. [16] R. Nimmagadda, K. Venkatasubbaiah, Experimental and multiphase analysis of nanofluids on the conjugate performance of micro-channel at low Reynolds numbers, Heat Mass Transf. 53 (2017) 2099–2115. [17] M. Bahiraei, M. Berahmand, A. Shahsavar, Irreversibility analysis for flow of a nonNewtonian hybrid nanofluid containing coated CNT/Fe3O4 nanoparticles in a minichannel heat exchanger, Appl. Therm. Eng. 125 (2017) 1083–1093. [18] A.A. Hussien, M.Z.N.M. Yusop, W. Al-Kouz, E. Mahmoudi, M. Mehrali, Heat transfer and entropy generation abilities of MWCNTs/GNPs hybrid nanofluids in microtubes, Entropy 21 (480) (2019) 1–17, https://doi.org/10.3390/e21050480. [19] A.A. Hussien, N.M. Yusop, M.A. Al–Nimr, M.Z. Abdullah, A.A. Janvekar, M.H. Elnaggar, Numerical study of heat transfer enhancement using al2o3–graphene/water hybrid nanofuid flow in mini tubes, Iran. J. Sci. Technol. A (2019), https://doi.org/10.1007/s40995-018-0670-1. [20] M. Bahiraei, S. Heshmatian, Thermal performance and second law characteristics of two new microchannel heat sinks operated with hybrid nanofluid containing graphene–silver nanoparticles, Ener. Conv. Manage. 168 (2018) 357–370. [21] M. Goodarzi, I. Tlili, Z. Tian, M. Safaei, Efficiency assessment of using graphene nanoplatelets-silver/water nanofluids in microchannel heat sinks with different crosssections for electronics cooling, Int. J. Numer. Method. H. (2019), https://doi. org/10.1108/HFF-12-2018-0730. [22] M. Bahiraei, N. Mazaheri, Application of a novel hybrid nanofluid containing graphene–platinum nanoparticles in a chaotic twisted geometry for utilization in miniature devices: thermal and energy efficiency considerations, Int. J. Mech. Sci. 138–139 (2018) 337–349. [23] A. Shahsavar, A. Godini, P.T. Sardari, D. Toghraie, H. Salehipour, Impact of variable fluid properties on forced convection of Fe3O4/CNT/water hybrid nanofluid in a double-pipe mini-channel heat exchanger, J. Therm. Anal. Calorim. 137 (2019) 1031–1043. [24] C. Uysal, E. Gedik, A.J. Chamkha, A numerical analysis of laminar forced convection and entropy generation of a diamond-Fe3O4/water hybrid nanofluid in a rectangular minichannel, J. Appl. Fluid Mech. 12 (2019) 391–402. [25] V. Kumar, J. Sarkar, Numerical and experimental investigations on heat transfer and pressure drop characteristics of Al2O3-TiO2 hybrid nanofluid in minichannel heat sink with different mixture ratio, Powder Technol. 345 (2019) 717–727.
Appendix A. Uncertainty analysis Equations to find out the uncertainty for different dependent parameters have been included in this section, Heat transfer rate, Q̇ : ΔQ̇ Q̇
Δcp 2
Δρ 2 ρ
ΔV̇ 2 V̇
( ) +( ) +( ) +(
=
cp
Δ(Tout − Tin) 2 (Tout − Tin)
)
= ± 2.87%
Hydraulic diameter, dh: Δdh dh
Δwch 2 wch
Δhch 2 hch
( ) +( ) +(
=
Δ(wch + hch) 2 (wch + hch)
)
= ± 1.02%
Mean velocity, um: Δum um
ΔV̇ 2 V̇
Δwch 2 wch
Δhch 2 hch
( ) +( ) +( )
=
= ± 0.78%
Effective area, A: ΔA A
(
=
Δ(wch + 2hch) 2 (wch + 2hch)
ΔLch 2 Lch
) +( )
= ± 1.2%
Reynolds number, Re: Δ Re Re
Δρ 2 ρ
Δμ 2 μ
Δdh 2 dh
Δum 2 um
( ) +( ) +( ) +( )
=
= ± 3.10%
Heat transfer coefficient, h: Δh h
ΔQ̇ 2 Q̇
ΔA 2 A
( ) +( ) +(
=
Δ(Ts − Tm) 2 (Ts − Tm)
)
= ± 3.11%
Nusselt number, Nu: ΔNu Nu
Δdh 2 dh
Δh 2 h
Δk 2 k
( ) +( ) +( )
=
= ± 3.83%
Friction factor, f: Δf f
=
Δ(Δp) 2 Δp
Δdh 2 dh
ΔLch 2 Lch
Δρ 2 ρ
2Δum 2 um
( ) +( ) +( ) +( ) +( )
= ± 2.76%
Performance evaluation criteria, PEC: ΔPEC PEC
=
1 3
×
(
3ΔNhnf Nuhnf
2
) +(
3ΔNubf Nubf
2
) + ⎛⎝
Δfhnf fhnf
2
⎞ +⎛ ⎠ ⎝
Δfbf fbf
2
⎞ = ± 5.47% ⎠
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different nanoparticle ratios, J. Therm. Anal. Calorim. 135 (2019) 1243–1255. [33] M.H. Esfe, A.T.K. Abad, M. Fouladi, Effect of suspending optimized ratio of nanoadditives MWCNT-Al2O3 on viscosity behavior of 5W50, J. Mol. Liq. 285 (2019) 572–585. [34] H. Maddah, R. Aghayari, M. Mirzaee, M.H. Ahmadi, M. Sadeghzadeh, A.J. Chamkha, Factorial experimental design for the thermal performance of a double pipe heat exchanger using Al2O3-TiO2 hybrid nanofluid, Int. Commun. Heat Mass Transfer 97 (2018) 92–102. [35] C.J. Ho, W.C. Chen, An experimental study on thermal performance of Al2O3/water nanofluid in a minichannel heat sink, Appl. Therm. Eng. 50 (2013) 516–522. [36] S.J. Kline, F.A. McClintock, Describing uncertainties in single-sample experiments, Mech. Eng. 75 (1953) 3–8. [37] A. Bhattad, J. Sarkar, P. Ghosh, Discrete phase numerical model and experimental study of hybrid nanofluid heat transfer and pressure drop in plate heat exchanger, Int. Commun. Heat Mass Transfer 91 (2018) 262–273. [38] A. Dominic, J. Sarangan, S. Suresh, V.S. Devah Dhanush, An experimental investigation of wavy and straight minichannel heat sinks using water and nanofluids, J. Ther. Sci. Eng. Appl. 7 (2015) 031012. [39] C.J. Ho, Wei-Chen Chen, Wei-Mon Yan, Correlations of heat transfer effectiveness in a minichannel heat sink with water-based suspensions of Al2O3 nanoparticles and/ or MEPCM particles, Int. J. Heat Mass Transf. 69 (2014) 293–299.
[26] A.A. Charab, S. Movahedirad, R. Norouzbeigi, Thermal conductivity of Al2O3+ TiO2/water nanofluid: model development and experimental validation, Appl. Therm. Eng. 119 (2017) 42–51. [27] G.M. Moldoveanu, G. Huminic, A.A. Minea, A. Huminic, Experimental study on thermal conductivity of stabilized Al2O3 and SiO2 nanofluids and their hybrid, Int. J. Heat Mass Transf. 127 (2018) 450–457. [28] K.A. Hamid, W.H. Azmi, M.F. Nabil, R. Mamat, K.V. Sharma, Experimental investigation of thermal conductivity and dynamic viscosity on nanoparticle mixture ratios of TiO2-SiO2 nanofluids, Int. J. Heat Mass Transf. 116 (2018) 1143–1152. [29] K.A. Hamid, W.H. Azmi, M.F. Nabil, R. Mamat, Experimental investigation of nanoparticle mixture ratios on TiO2-SiO2 nanofluids heat transfer performance under turbulent flow, Int. J. Heat Mass Transf. 118 (2018) 617–627. [30] A.S. Dalkılıç, Ö. Açıkgöz, B.O. Küçükyıldırım, A.A. Eker, B. Lüleci, C. Jumpholkul, S. Wongwises, Experimental investigation on the viscosity characteristics of water based SiO2-graphite hybrid nanofluids, Int. Commun. Heat Mass Transfer 97 (2018) 30–38. [31] F.R. Siddiqui, C.Y. Tso, K.C. Chan, S.C. Fu, C.Y.H. Chao, On trade-off for dispersion stability and thermal transport of Cu-Al2O3 hybrid nanofluid for various mixing ratios, Int. J. Heat Mass Transf. 132 (2019) 1200–1216. [32] N.N.M. Zawawi, W.H. Azmi, M.Z. Sharif, G. Najafi, Experimental investigation on stability and thermo-physical properties of Al2O3–SiO2/PAG nanolubricants with
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