- Email: [email protected]
water nanofluid in an electronic heat sink
Experimental Thermal and Fluid Science 40 (2012) 57–63
Contents lists available at SciVerse ScienceDirect
Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Convective performance of CuO/water nanofluid in an electronic heat sink P. Selvakumar, S. Suresh ⇑ Department of Mechanical Engineering, National Institute of Technology, Tiruchirappalli 620 015, India
a r t i c l e
i n f o
Article history: Received 7 October 2011 Received in revised form 30 January 2012 Accepted 30 January 2012 Available online 8 February 2012 Keywords: Electronic cooling CuO/water nanofluid Interface temperature Convective heat transfer coefficient Pumping power
a b s t r a c t Heat dissipation in the electronic components is being a critical issue due to the faster increase in the components’ heat flux and increasing demand for the miniature in features’ size. In the present work CuO/water nanofluids of volume fractions 0.1% and 0.2% are prepared by dispersing the nanoparticles in deionised water. A thin channelled copper water block of overall dimension 55 55 19 mm is used for the study. The interface temperature of the water block is measured and a maximum reduction of 1.15 °C is observed when nanofluid of 0.2% volume fraction is used as the working fluid compared to deionised water. Convective heat transfer coefficient of water block is found to increase with the volume flow rate and nanoparticle volume fraction and the maximum rise in convective heat transfer coefficient is observed as 29.63% for the 0.2% volume fraction compared to deionised water. Pumping power for the deionised water and nanofluids are calculated based on the pressure drop in the water block and the average increase in pumping power is 15.11% for the nanofluid volume fraction of 0.2% compared to deionised water. A correlation is proposed for Nusselt number which fits the experimental Nusselt number with in ±7.5%. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction There is a need for the development of efficient and high performance heat transfer fluids which are employed for heat transfer in the engineering industries. Power generation, transportation, chemical processing plants and electronics are some of the industrial sectors in which heat transfer plays essential role. One of the most focussing areas of heat transfer is electronic components cooling. Thermal management of electronic components is attracted widely due to the drastically increasing transistor density, decreasing feature size and increasing computational speeds which lead to very high heat flux [1]. Since overheating of the electronic components degrades the components’ performance, reliability and cause failure of the components, heat dissipation from these components must be efficiently handled for developing a successful high performance electronic system. The heat flux of the electronic chips may exceed 150 W/cm2 [2] in order to meet the demand for high performance electronic components. Air cooled heat sinks show very low potential in removing heat from electronic components due to its very low thermal conductivity and heat carrying capacity. Liquids possess very high thermal conductivity than air which can make the electronic cooling system a high efficient and compact [3]. Improving the thermal properties of the conventional heat transfer fluids will further increase the efficiency of the heat exchanger system and reduce its size. ⇑ Corresponding author. Tel.: +91 431 2503422; fax: +91 431 2500133. E-mail addresses: [email protected] (P. Selvakumar), [email protected] (S. Suresh). 0894-1777/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2012.01.033
Nanofluids are the emerging composites consist of nanoscale solid particles dispersed in the conventional heat transfer base fluids. The inclusion of solid particles having higher thermal conductivity values than the base fluids such as water or ethylene glycol increase the effective thermal conductivity of the nanofluids. Nanofluid was first introduced by Choi of Argonne National laboratory, USA [4]. Nanoparticles dispersed in the base fluids have overcome the limitations of micron sized particles. The main demerits of micron sized particles are (i) Settlement of particles which affect the dispersion of the particles in the base fluids and thermal conductivity. (ii) Clogging in narrow channels. (iii) Erosion of flow passages. (iv) Rapid increase in viscosity with increase in particle volume fraction which causes increase in pumping power. Eastman et al. [5] dispersed copper nanoparticles in ethylene glycol and studied the rise in thermal conductivity of the nanofluids. They observed from their results that there was a rise in thermal conductivity up to 40% in the 0.3% volume fraction copper/ethylene glycol nanofluid. The mean diameter of the nanoparticles used in their study was less than 10 nm. Namburu et al. [6] prepared copper oxide nanofluids by dispersing the solid nanoparticles in the mixture of ethylene glycol and water and investigated the rheological properties. They conducted experiments with nanoparticle volume concentration ranging from 0% to 6.12% in the temperature span of 35 °C to 50 °C. From their experimental measurement of viscosity they observed that viscosity of the nanofluid increases with increasing volume concentration at a particular temperature and decreases with increase in temperature.
58
P. Selvakumar, S. Suresh / Experimental Thermal and Fluid Science 40 (2012) 57–63
Nomenclature A Cp D h k m Nu Pr q Q Re T V_
surface area (m2) specific heat (J/kg K) orifice diameter (m) convective heat transfer coefficient (W/m2 K) thermal conductivity (W/m K) mass flow rate (kg/s) Nusselt number (hD/k) Prandtl number (lcp/k) actual heat flux (W/m2) heat input (W) Reynolds number (4 m/pdl) temperature (°C) volume flow rate (m3/s)
Greek symbols u volume concentration (%) DP pressure drop (Pa) l dynamic viscosity (N s/m2)
Saeedinia et al. [7] prepared oil based copper oxide nanofluids with volume concentration ranging from 0.2% to 2% and the heat transfer and rheological properties like thermal conductivity, specific heat and viscosity were measured and the effect of nanoparticle concentration on the properties at different temperatures were studied. Also they studied the heat transfer characteristics of the prepared CuO/oil nanofluids in the laminar flow regime under constant heat flux boundary condition. From their experimental results it was observed that the thermal conductivity and viscosity increase with the increase in nanoparticle volume concentration. But the increase in viscosity with respect to weight concentration is more at low temperature compared to high temperature. The maximum increase in convective heat transfer coefficient was observed as 12.7% for the particle weight fraction of 2% at the maximum Reynolds number of the experiment compared to pure oil. Suresh et al. [8] synthesised CuO nanoparticles using sol–gel method and prepared water based nanofluids of volume concentration 0.1%, 0.2% and 0.3%. They experimentally investigated the effect of surface modification in the plain tube (dimples) and the nanofluid on the convective heat transfer coefficient compared to plain tube and distilled water. They showed from their experimental results that the increase in Nusselt number under the turbulent flow regime in the dimpled tube were 19%, 27% and 39% respectively for the volume concentrations of 0.1%, 0.2% and 0.3% compared to plain tube and water. Also they observed a small rise in pressure drop of 2–10% compared to plain tube and water. Wongcharee and Eiamsa-ard [9] studied heat transfer and friction factor characteristics of CuO/water nanofluid in a circular tube fitted with twisted tape with alternate axis. They assessed the performance of nanofluid and alternate axis twisted tape using thermal performance factor. They obtained a maximum thermal performance factor of 5.53 when nanofluid is used as the working fluid with twisted tape. Kulkarni et al. [10] investigated experimentally the rheological behaviour of copper oxide nanoparticles dispersed in a mixture containing water and propylene glycol in the ratio of 60:40. They conducted experiments with particle volume concentration ranging between 0% and 6% in the temperature range of 35 °C to 50 °C and it was observed that the nanofluids possess Newtonian behaviour. Saeedinia et al. [11] studied experimentally the heat transfer and rheological characteristics of CuO/oil nanofluids of weight fractions 0.2% to 2% in a smooth tube under constant heat flux conditions. They observed that the increase in dynamic viscosity of the
q
density (kg/m3)
Subscripts f fluid fm fluid mean i interface in inlet nf nanofluid out outlet s solid phase w water block Abbreviations APS average particle size MW molecular weight SA surface area TIM Thermal Interface Material
nanofluid is high compared to pure oil at low temperatures and the viscosity increases with increasing weight fraction of the nanoparticles. Nanofluids showed Newtonian behaviour for the entire range of weight fractions considered in their experimental work. The maximum increase in convective heat transfer coefficient was 12.7% at a particular particle weight fraction of 2%. Sidy Ndao et al. [12] conducted experiments on jet impingement heat transfer in a micro pin finned plate using water and R134a as heat transfer fluids. They compared the effect of micro pin fins on a surface with a plain surface without micro pin fins. The observed enhancement in convective heat transfer coefficient for the circular micro pin finned surface having the pin fin diameter 125 lm and height 230 lm was up to 200% compared with the plain surface. They also observed that the correlations for impingement zone Nusselt numbers underpredict compared to their experimental data. Reyes et al. [13] conducted experiments to study the heat transfer and pressure drop characteristics of water in a micro channel heat sink. They used a copper heat sink machined with square channels of size 500 lm and overall heat size of 15 mm 15 mm. They studied the effect of tip clearance on the heat transfer and pressure drop compared to the configuration without tip clearance. They found from their experiments that the reduction in heat transfer due to tip clearance is low compared to the reduction in pumping power which is large. Weilin Qu et al. [14] studied the heat transfer and pressure drop characteristics of micro channel heat sink of channel dimensions 231 lm wide and 713 lm deep with two heat fluxes of 100 W/cm2 and 200 W/cm2 experimentally and numerically. They analysed the performance of micro channels in the Reynolds number range of 139–1672 and observed that higher Reynolds number flows are useful in reducing the fluid outlet temperature and heat sink temperature but this happens at a higher pressure drop. They compared their results obtained from the numerical analysis with the experimental data and found good agreement between them. Kou et al. [15] analysed the flow in the micro channel heat sink using a commercial CFD code CFD-ACE+. They analysed the fully developed flow and optimised for the minimum thermal resistance and channel width using simulated annealing technique. They found that the optimum channel width is independent of the channel height at constant flow power. Vafai and Zhu [16] studied the heat transfer and pressure drop characteristics of two layered micro channel heat sink numerically and found that the stream wise temperature rise at the surface of the heat sink is smaller
P. Selvakumar, S. Suresh / Experimental Thermal and Fluid Science 40 (2012) 57–63
compared to the normal heat sink. Also they observed that the pressure drop for the counter flow arrangement in the two layered rectangular micro channel heat sink is less than the single layered heat sink. Tiselj et al. [17] studied the heat transfer characteristics of triangular section silicon micro channels experimentally and numerically using water as the cooling fluid in the Reynolds number range of 3.2–64. They observed non monotonous behaviour of the cooling fluid and the wall temperatures due to high values of heat flux in the axial direction. Li et al. [18] analysed the performance of the CuO/water nanofluid of 1% and 4% volume concentrations in trapezoidal micro channels and compared the results with pure water. They modelled thermal conductivity of nanofluids based on Brownian motion induced micro convection and modelled the heat transfer and pressure drop characteristics using a commercial CFD code CFX. They found from their work that nanofluids significantly increase the thermal performance with small increase in pumping power. Nguyen et al. [19] studied the convective heat transfer performance of Al2O3/water nanofluid in a water block (make: swiftech, USA) with different nanoparticle size and volume concentrations. They observed an increase in convective heat transfer coefficient for nanofluids in the water block with a maximum rise of 40% compared to base fluid at the volume concentration of 6.8%. They did not present the pressure drop values associated with the use of nanofluids in the water block. Gherasim et al. [20] conducted experiments in a radial flow heat sink using Al2O3/water nanofluid. They obtained the increasing Nusselt numbers with increase in volume concentration and Reynolds numbers. But upon decreasing space between the heated plate and the disc the Nusselt numbers were found to decrease. Roberts and Walker [21] conducted experiments in a plain tube to verify the results shown by Lai et al. [22] using Al2O3/water nanofluid. They used constant heat flux as boundary condition for their study. After verifying the results in the plain tube they conducted experiments in a commercial water block to study the convective heat transfer performance and pressure drop of Al2O3/ water nanofluid. From their experimental results they showed an enhancement of 20% in thermal conductance for the nanofluid containing alumina nanoparticles of size 20–30 nm. The convective heat transfer performance of silica/water nanofluids for the cooling of electronic components was evaluated by Escher et al. [23]. They determined Nusselt number using their experimental results and compared with theoretical predictions with considering the change in bulk fluid properties. They concluded that the rise in thermal conductivity should be more than the rise in viscosity of the nanofluids due to inclusion of nanoparticles. Also they reported
59
that increase in volumetric heat capacity will be better than increasing thermal conductivity. In this work, CuO/water nanofluid is formulated with a volume concentration of 0.1% and 0.2% by dispersing specified amount of CuO nanoparticles in deionised water to investigate the effect of flow rate on the interface temperature and geometry of the water block. The effect of particle volume concentration on convective heat transfer coefficient and pressure drop is also presented. 2. Experimental setup The general layout of the experimental setup with major components is shown in Fig. 1. Heated aluminium block, water block (Make: EK-Supreme) high density cartridge heater, air cooled cross flow heat exchanger and peristaltic pump are the major components of the setup. An aluminium block of size 55 55 75 mm is used as the heated block, which is used to simulate the heat generated by any electronic equipment. The capacity of the high density cartridge heater which is inserted into the precisely machined hole of the aluminium block is 150 W. The heater is fitted into the hole in such a way that there is no air gap between the heater and the hole which would cause damaging of the heater. The top surface of the aluminium heated block and the bottom surface of the copper water block are machined and polished to have the surface flatness of ±0.7 l. K type thermocouples of accuracy ±0.2 °C are placed at four different locations in the aluminium block to measure the interface temperature of the heated and water block (Fig. 2). Two more type k thermocouples are used to measure the inlet and outlet fluid temperature. One more thermocouple is used to measure the temperature at the extreme surface of insulation provided. All the temperature readings were stored using a data acquisition system (data taker, Australia). To improve the thermal conductance at the interface of the heated and water blocks, Thermal Interface Material (TIM, Make: Artic silver) is applied between the two surfaces which are in physical contact. The water block is set over the heated aluminium block and positioned firmly using screws and spring loaded nut at the four corners. It applies pressure at the water and a heated block interface (up to a maximum of 7 kg at the full compression of the spring) and helps to decrease the thermal resistance of the interface junction. The inlet and outlet diameters of the orifices are 8 mm. A peristaltic pump (capacity: 5 l/min, Make: Ravel Tech. India) was used to circulate the fluid through the circuit. The electric current to the high density cartridge heater was supplied using an auto transformer at any required voltage. The voltage and current supplied to the heater was
Fig. 1. Schematic of experimental set up.
60
P. Selvakumar, S. Suresh / Experimental Thermal and Fluid Science 40 (2012) 57–63 Table 1 Experimental conditions.
Fig. 2. Location of thermocouple and heater in the heated block.
monitored by using the calibrated digital voltmeter and ammeter respectively. The whole heated and water block assembly is placed inside a wooden casing of size 160 160 160 mm. Glass wool insulation of thickness 50 mm is provided all around the surfaces of the heated – water block assembly to minimise the thermal losses to the surroundings. U tube manometer is used to measure the pressure drop between the inlet and outlet. A plastic vessel of 3 l capacity is used as the reservoir. The hot fluid comes out from the water block is cooled by using a air cooled heat exchanger whose performance is adjusted by varying speed of the fan installed in the cooling system. By varying the speed of the high speed fan the temperature of the fluid comes out from the heat exchange is maintained constant. Separate experiments were conducted to obtain the reduction in inlet fluid temperature with respect to the speed of the air thrown thorough the fins of the heat exchanger. 2.1. Details of the water block The water block is consisting of two parts. A copper base of size 55 55 12.6 mm and acrylic top. The inlet and outlet for the water block are machined at the acrylic top (Fig. 3b). 49 thin channels of length 28 mm and, channel width 0.3 mm and channel height 2 mm are machined at the centre of the copper water block. The inlet flow is directed through the thin channels using a jet plate which is kept over the thin channels as shown in Fig. 3b. The water block’s base is very thin having the thickness of 1 mm to minimise the conductive resistance at the bottom. 3. Experimental procedure Initially the setup is run with water and without any heat flux for 1 h to ensure the leak proof circuit. Constant heat flux is applied to
Heat input
Flow rate
Reynolds number
Volume concentration
102 W
0.79–2.45 LPM
2985–9360
0.1%, 0.2%
aluminium block by supplying specified voltage to cartridge heater using an auto transformer. Interface temperatures, fluid inlet and outlet temperatures, surface temperatures are recorded periodically and readings are saved once steady state is reached. The pressure drop readings are also noted at isothermal conditions. For the first reading the steady state is reached at around one hour and for the subsequent readings it takes around 20 min. The flow in the circuit is measured using a measuring jar and a stop watch. The heat loss from the heated and water block assembly to the surrounding is found to be around 4%. Deionised water and CuO/water nanofluids of volume concentrations 0.1% and 0.2% are used to conduct experiments. Experimental conditions of this study are given in Table 1. Uncertainties associated with Nusselt number, Reynolds number and pressure drop are calculated based on the probable error in the measured quantities. Coleman and Steele method [24] is used to determine the uncertainty considering the measurement errors. The uncertainties involved in the Nusselt number, Reynolds number and pressure drop are respectively ±3.59%, ±0.74% and ±0.125%. Whereas the uncertainties associated with input heat and volume flow rate are ±3.7% and ±1.7% respectively. The uncertainties in the inlet, outlet and interface temperature measurements are ±0.2 °C. 4. Nanofluid preparation and property evaluation Copper oxide nanoparticles purchased from Alfa Aesar (copper (II) Oxide (CuO), 97.5%, MW = 79.54, Nano Arc, SA = 25–40 m2/g, APS = 27–37 nm) was used for the preparation of nanofluid. CuO/ water nanofluids with volume concentrations of 0.1% and 0.2% were prepared by dispersing specified amount of CuO nanoparticles in deionised water by using an ultrasonic vibrator (Power = 180 W, Operating frequency = 25 kHz; Lark, India). To obtain a homogeneous dispersion of the nanoparticles in the DI water, ultrasonication was done continuously for 6 h [25]. Ensuring the stability of the prepared nanofluid is vital for retaining the improved thermal properties of the nanofluids. Normally the particles suspended in the base fluids are stable when the pH of the nanofluid is far from the isoelectric point at which the zeta potential is zero. pH of the prepared nanofluids was measured using a digital pH metre (Deep vision: 111, India) and the values are found to be around 4.8 which is far away from the isoelectric point. Even after a week, the prepared nanofluids were observed to be stable except very little sedimentation.
Fig. 3. (a) Photograph of water block (b) thin channelled copper base with jet plate.
61
P. Selvakumar, S. Suresh / Experimental Thermal and Fluid Science 40 (2012) 57–63
The thermo physical properties of the prepared CuO/water nanofluid of two volume concentrations are determined at the fluids’ bulk mean temperature by using the correlations widely used in the literature. Also thermal conductivity and viscosity of the prepared nanofluids were measured experimentally and the measured properties have been taken for the heat transfer calculations which deviate from the theoretical values determined using standard equations which lack in including the mechanism for improvement of properties. The density of the nanofluid is determined using Pak and Cho’s equation [26]:
of viscosity of DI water and glycerin are 0.9 and 10.9 cP respectively which agrees with the available literature values with the accuracy of ±5%. All the viscosity measurements were done at room temperature of 32 °C with ±2 °C accuracy. The measured values of viscosity are higher than the values obtained from the Einstein model by 17.1% and 21.4% respectively for 0.1% and 0.2% volume concentrations.
qnf ¼ uqs þ ð1 uÞq
Q ¼ Q in Q loss
ð1Þ
4.2. Data processing
ð5Þ
The effective thermal conductivity is calculated using Maxwell’s model [27] for nanofluids of volume concentration less than one.
where Qin is the input heat supplied by cartridge heater, Qloss is the heat loss from the heated and water blocks assembly
knf ks þ 2k þ 2uðks kÞ ¼ ks þ 2k uðks kÞ k
Heat flux; q00 ¼
ð2Þ
The specific heat of the nanofluid is calculated using Xuan and Roetzel’s equation [28]
C pnf 1 þ kc u ¼ Cp 1 þ kp u
ð3Þ
lnf ¼ lð1 þ 2:5uÞ
ð4Þ
qC where kc ¼ qS C pps 1 ; kp ¼ qqs 1 The viscosity of the nanofluid is calculated using the viscosity correlation proposed by Einstein [29]
4.1.1. Thermal conductivity Thermal conductivity of the prepared CuO/water nanofluids of 0.1% and 0.2% volume concentrations were measured using KD2 Pro Thermal property analyser (Decagon Devices, Inc., USA). This instrument consists of a micro controller and a sensor needle. The sensor needle consists of a line heat source and a temperature sensor. The sensor needle is connected to a micro controller by means of a data cable. There are three different types of sensors available with this instrument for different materials and KS-1 sensor was used for the present measurement of thermal conductivity for nanofluids. This sensor needle can be used for measuring thermal conductivity of liquids in the range of 0.2–2 W/mK with an accuracy of ±5%. Each measurement cycle consists of 90 s. During the first 30 s, the instrument will equilibrate which is then followed by heating and cooling of sensor needle for 30 s each. The measured thermal conductivity values for 0.1% and 0.2% volume concentrations of CuO/water nanofluids are 2.9% and 4.3% higher than the values given by the Maxwell’s model [27] of thermal conductivity for two liquid–solid mixtures. 4.1.2. Viscosity The viscosity of the prepared CuO/water nanofluids of 0.1% and 0.2% volume concentrations were measured using Brookfield cone and plate viscometer (LVDV-I PRIME C/P). It is equipped with a 2.4 cm 0.8° cone. The cone of the viscometer is connected with a spindle CPE-40 which can be used for the viscosity measurement of fluid samples in the range of 0.3 to 1.028 cP. 0.013 mm gap is maintained between the cone and plate of the viscometer between which the fluids viscosity to be measured is placed by the electronic adjusting feature of the instrument. When the spindle is rotated the viscous drag of the fluid against the spindle rotation is measured by a calibrated spring. The instrument was benchmarked by measuring viscosity of DI water and glycerin at room temperature. The measured values
ð6Þ
A is the exposed heat transfer area of the water block.
A ¼ A1 þ A2
ð7Þ
A1 is the total surface area of the thin channels, A2 is the inside surface area of the block other than channel area. The average of the measured interface temperatures (Ti), bulk mean temperature of the inlet and outlet fluids and heat flux are used to calculate the heat transfer coefficient of the water block as given in Eq. (8).
hw ¼ 4.1. Thermo physical properties measurement
Q A
q00 ðT i T fm Þ
ð8Þ
where Tfm is the bulk mean temperature of the fluid. Reynolds number is calculated using the following equation.
Re ¼
4m
plDi
ð9Þ
The Nusselt number is then calculated as,
Nu ¼
hw D i k
_ Dp Pumping power ¼ ðVÞ
ð10Þ ð11Þ
5. Results and discussions 5.1. Interface temperature The interface temperature measured between the heated and water block is an indication of performance of the cooling system. The effect of CuO/water nanofluids of volume concentrations 0.1% and 0.2% on interface temperature at different volume flow rate of DI water and nanofluids is shown in Fig. 4. It is clear from the figure that the interface temperature reduces with increasing volume flow rate of the DI water and CuO/water nanofluids. The volume flow rate of the water and nanofluids of two different concentrations are varied from 0.79 LPM to 2.45 LPM. At the minimum volume flow rate of 0.79 LPM the temperature gain obtained for 0.1% and 0.2% volume concentration CuO/water nanofluids compared to deionised water are 0.65 °C and 1.35 °C respectively. At a maximum volume flow rate of 2.45 LPM, the interface temperature obtained for 0.2% volume concentration CuO/water nanofluid is 43.15 °C which is 1.15 °C lower than the temperature obtained for deionised water. It can be understood from the values of interface temperature that the nanofluids remove more heat from the heated block compared to deionised water and keep the interface temperature minimum.
62
P. Selvakumar, S. Suresh / Experimental Thermal and Fluid Science 40 (2012) 57–63
Fig. 4. Effect of volume flow rate on interface temperature.
Fig. 6. Variation of water block pressure drop for water and nanofluid.
5.2. Convective heat transfer coefficient The effect of nanofluid and the volume flow rate on the convective heat transfer coefficient is shown in Fig. 5. The heat transfer coefficients are calculated from the measured average value of the interface temperature, the bulk mean temperature of the fluid and the actual heat flux supplied to the fluid. The experimental results reveal that the copper oxide nanoparticles suspended in the deionised water increases the convective heat transfer coefficient of the water block. It is clear from the figure that the convective heat transfer coefficient increases with increase in volume flow rate of deionised water and nanoparticle volume concentration. At a particular volume flow rate of 2.44 LPM the increase in convective heat transfer coefficient for 0.1% and 0.2% volume concentration nanofluids compared to deionised water are 18.27% and 29.63% respectively. The reason for such enhancement in the convective heat transfer is due to the enhanced transport properties of nanofluids due to the inclusion of solid nanoparticles, Brownian motion of the suspended particles, particle shape and more chances of particles’ direct contact with solid surfaces in thin channels. 5.3. Pressure drop studies To have a nanofluid as a successful heat transfer fluid in practical applications such as compact electronic applications, it must possess enhanced heat transfer characteristics without much pen-
Fig. 5. Effect of volume flow rate and nanofluid volume concentration on convective heat transfer coefficient.
alty in pumping power. To ensure the benefit of nanofluid in cooling the electronic components, pressure drop of CuO/water nanofluids of 0.1% and 0.2% volume concentrations in the water block of the electronic cooling system is measured experimentally at all flow rates. Fig. 6 shows the pressure drop characteristics of CuO/water nanofluids and deionised water in the water block. It is clear from the figure that there is no much rise in pressure drop values for the nanofluids of two different volume concentrations compared to deionised water. Fig. 7 shows the pumping power requirements of the deionised water and CuO/water nanofluids at different volume flow rates. The average increase in pumping power for 0.2% volume concentration nanofluid is 15.11% compared to deionised water which shows a encouraging sign to use nanofluid as the heat transfer fluid for the practical applications such as electronic cooling system. 5.4. Correlation for Nusselt number The Nusselt number obtained from experimental results has been correlated as follows.
Nu ¼ 0:346ðReÞ0:3143 ðPrÞ0:638 ð1 þ /Þ115:341 nf nf
ð12Þ
The above equation is fitting with the present experimental data within ±7.5% for Nusselt number as shown in Fig. 8.
Fig. 7. Variation of pumping power for water and nanofluid.
P. Selvakumar, S. Suresh / Experimental Thermal and Fluid Science 40 (2012) 57–63
Fig. 8. Comparison of experimental and predicted Nusselt number.
6. Conclusions The effect of CuO/water nanofluids in a thin channelled copper heat sink under constant heat flux condition is studied with nanofluids volume fractions 0.1% and 0.2%. The volume flow rate of the deionised water and nanofluids considered for the study is in the range of 0.79 LPM to 2.45 LPM. Significant reduction in interface temperature of the heated and water blocks is observed when CuO/water nanofluids are used compared to deionised water. A maximum increase in convective heat transfer coefficient is determined as 29.63% for the nanofluid volume fraction of 0.2% compared to deionised water for the same volume flow rate. The pressure drop characteristics of CuO/water nanofluids is also studied and rise in pressure drop associated with the inclusion of nanoparticles in deionised water is not much compared to the rise in convective heat transfer coefficient. A correlation for Nusselt number is proposed which fits the experimental results within ±7.5%. Acknowledgements The authors wish to thank Council of Scientific and Industrial Research (CSIR Sanction letter No. 22(0484)/09/EMR-II dated 10th November 2009.), Government of India for its financial support to this work. References [1] Xingcun Colin Tong, Advanced Materials for Thermal management of Electronic Packaging, Springer New York Heidelberg Dordrecht, London, 2011. [2] Avram Bar Cohen, Mehmet Arik, Michael Ohad, Direct liquid cooling of high flux micro and nanoelectronic components, in: Proceedings of the IEE, vol. 94, no. 8, August 2006. [3] F. Incropera, Liquid Cooling of Electronic Devices by Single-Phase Convection, John Wiley & Sons, New York, 1999. [4] S.U.S. Choi, Enhancing thermal conductivity of fluids with nanoparticles, developments and applications of non-newtonian flows, in: D.A. Siginer, H.P. Wang (Eds.), ASME, New York, 1995, pp. 99–105. FED-231/MD-66.
63
[5] J.A. Eastman, S.U.S. Choi, S. Li, W. Yu, L.J. Thomson, Anomalously increased effective thermal conductivities of ethylene glycol based nanofluids containing copper nanoparticles, Appl. Phys. Lett. 78 (2001) 718–720. [6] Praveen K. Namburu, Devdatta P. Kulkarni, Debasmita Misra, Debendra K. Das, Viscosity of copper oxide nanoparticles dispersed in ethylene glycol and water mixture, Exp. Therm. Fluid Sci. 32 (2007) 397–402. [7] M. Saeedinia, M.A. Akhavan-Behabadi, P. Razi, Thermal and rheological characteristics of CuO–Base oil nanofluid flow inside a circular tube, Int. Commun. Heat Mass Transfer (2011), doi:10.1016/ j.icheatmasstransfer.2011.08.001. [8] S. Suresh, M. Chandrasekar, S. Chandra Sekhar, Experimental studies on heat transfer and friction factor characteristics of CuO/water nanofluid under turbulent flow in a helically dimpled tube, Exp. Therm. Fluid Sci. 35 (2011) 542–549. [9] Khwanchit. Wongcharee, Smith. Eiamsa-ard, Enhancement of heat transfer using CuO/water nanofluid and twisted tape with alternate axis, Int. Commun. Heat Mass Transfer 38 (2011) 742–774. [10] D.P. Kulkarni, D.K. Das, S.L. Partil, Effect of temperature on rheological properties of copper oxide nanoparticles dispersed in propylene glycol and water mixture, J. Nano Sci. Nano Technol. L7 (2) (2007) 1–5. [11] M. Saeedinia, M.A. Akhavan-Behabadi, P. Razi, Thermal and rheological characteristics of CuO–Base oil nanofluid flow inside a circular tube, Int. Commun. Heat Mass Transfer (2011), doi:10.1016/ j.icheatmasstransfer.2011.08.001. [12] Sidy Ndao, Hee Joon Lee, Yoav Peles, Michael K. Jensen, Heat transfer enhancement from micro pin fins subjected to an impinging jet, Int. J. Heat Mass Transfer 55 (2012) 413–421. [13] M. Reyes, J.R. Arias, A. Velazquez, J.M. Vega, Experimental study of heat transfer and pressure drop in micro-channel based heat sinks with tip clearance, Appl. Therm. Eng. 31 (2011) 887–893. [14] Qu Weilin, Issam Mudawar, Experimental and numerical study of pressure drop and heat transfer in a single-phase micro-channel heat sink, Int. J. Heat Mass Transfer 45 (2002) 2549–2565. [15] Hong-Sen Kou, Ji-Jen Lee, Chih-Wei Chen, Optimum thermal performance of microchannel heat sink, by adjusting channel width and height, Int. Commun. Heat Mass Transfer 35 (2008) 577–582. [16] Kambiz Vafai, Lu Zhu, Analysis of two layered micro channel heat sink concept in electronic cooling, Int. J. Heat Mass Transfer 42 (1999) 2287–2297. [17] Tiselj, G. Hetsroni, B. Mavko, A. Mosyak, E. Pogrebnyak, Z. Segal, Effect of axial conduction on the heat transfer in micro-channels, Int. J. Heat Mass Transfer 47 (2004) 2551–2565. [18] Jie Li, Clement Kleinstreuer, Thermal performance of nanofluid flow in microchannels, Int. J. Heat Fluid Flow 29 (2008) 1221–1232. [19] T. Nguyen, G. Roy, C. Gauthier, N. Galanis, Heat transfer enhancement using Al2O3–water nanofluid for an electronic liquid cooling system, App. Therm. Eng. 27 (2007) 1501–1506. [20] I. Gherasim, G. Roy, T. Nguyen, D. Vo-Ngoc, Experimental investigation of nanofluids in confined laminar radial flows, Int. J. Therm. Sci. 48 (2009) 1486– 1493. [21] N.A. Roberts, D.G. Walker, Convective performance of nanofluids in commercial electronics cooling systems, Appl. Therm. Eng. 30 (2010) 2499– 2504. [22] W.Y. Lai, S. Vinod, P.E. Phelan, R. Prasher, Convective heat transfer for water based alumina nanofluids in a single 1.02 mm tube, J. Heat Transfer 131 (2009) 112401. [23] W. Escher, T. Brunschwiler, N. Shalkevich, A. Shalkevich, T. Burgi, B. Michel, D. Poulikakos, On the cooling of electronics with nanofluids, J. Heat Transfer 133 (2011) 051401. [24] H.W. Coleman, W.G. Steele, Experimental and Uncertainty Analysis for Engineers, Wiley, New York, 1989. [25] J.H. Lee, K.S. Hwang, S.P. Jang, B.H. Lee, J.H. Kim, S.U.S. Choi, C.J. Choi, Effective viscosities and thermal conductivities of aqueous nanofluids containing low volume concentrations of Al2O3 nanoparticles, Int. J. Heat Mass Transfer 51 (2008) 2651–2656. [26] B.C. Pak, Y. Cho, Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particle, Exp. Heat Transfer 11 (1998) 151–170. [27] J.C. Maxwell, Treatise on Electricity and Magnetism, Dover, New York, 1954. [28] Y. Xuan, W. Roetzel, Conceptions for heat transfer correlation of nanofluids, Int. J. Heat Mass Transfer 43 (2000) 3701–3707. [29] A. Einstein, Investigation on the Theory of Brownian motion, Dover, New York, 1956.
Contents lists available at SciVerse ScienceDirect
Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Convective performance of CuO/water nanofluid in an electronic heat sink P. Selvakumar, S. Suresh ⇑ Department of Mechanical Engineering, National Institute of Technology, Tiruchirappalli 620 015, India
a r t i c l e
i n f o
Article history: Received 7 October 2011 Received in revised form 30 January 2012 Accepted 30 January 2012 Available online 8 February 2012 Keywords: Electronic cooling CuO/water nanofluid Interface temperature Convective heat transfer coefficient Pumping power
a b s t r a c t Heat dissipation in the electronic components is being a critical issue due to the faster increase in the components’ heat flux and increasing demand for the miniature in features’ size. In the present work CuO/water nanofluids of volume fractions 0.1% and 0.2% are prepared by dispersing the nanoparticles in deionised water. A thin channelled copper water block of overall dimension 55 55 19 mm is used for the study. The interface temperature of the water block is measured and a maximum reduction of 1.15 °C is observed when nanofluid of 0.2% volume fraction is used as the working fluid compared to deionised water. Convective heat transfer coefficient of water block is found to increase with the volume flow rate and nanoparticle volume fraction and the maximum rise in convective heat transfer coefficient is observed as 29.63% for the 0.2% volume fraction compared to deionised water. Pumping power for the deionised water and nanofluids are calculated based on the pressure drop in the water block and the average increase in pumping power is 15.11% for the nanofluid volume fraction of 0.2% compared to deionised water. A correlation is proposed for Nusselt number which fits the experimental Nusselt number with in ±7.5%. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction There is a need for the development of efficient and high performance heat transfer fluids which are employed for heat transfer in the engineering industries. Power generation, transportation, chemical processing plants and electronics are some of the industrial sectors in which heat transfer plays essential role. One of the most focussing areas of heat transfer is electronic components cooling. Thermal management of electronic components is attracted widely due to the drastically increasing transistor density, decreasing feature size and increasing computational speeds which lead to very high heat flux [1]. Since overheating of the electronic components degrades the components’ performance, reliability and cause failure of the components, heat dissipation from these components must be efficiently handled for developing a successful high performance electronic system. The heat flux of the electronic chips may exceed 150 W/cm2 [2] in order to meet the demand for high performance electronic components. Air cooled heat sinks show very low potential in removing heat from electronic components due to its very low thermal conductivity and heat carrying capacity. Liquids possess very high thermal conductivity than air which can make the electronic cooling system a high efficient and compact [3]. Improving the thermal properties of the conventional heat transfer fluids will further increase the efficiency of the heat exchanger system and reduce its size. ⇑ Corresponding author. Tel.: +91 431 2503422; fax: +91 431 2500133. E-mail addresses: [email protected] (P. Selvakumar), [email protected] (S. Suresh). 0894-1777/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2012.01.033
Nanofluids are the emerging composites consist of nanoscale solid particles dispersed in the conventional heat transfer base fluids. The inclusion of solid particles having higher thermal conductivity values than the base fluids such as water or ethylene glycol increase the effective thermal conductivity of the nanofluids. Nanofluid was first introduced by Choi of Argonne National laboratory, USA [4]. Nanoparticles dispersed in the base fluids have overcome the limitations of micron sized particles. The main demerits of micron sized particles are (i) Settlement of particles which affect the dispersion of the particles in the base fluids and thermal conductivity. (ii) Clogging in narrow channels. (iii) Erosion of flow passages. (iv) Rapid increase in viscosity with increase in particle volume fraction which causes increase in pumping power. Eastman et al. [5] dispersed copper nanoparticles in ethylene glycol and studied the rise in thermal conductivity of the nanofluids. They observed from their results that there was a rise in thermal conductivity up to 40% in the 0.3% volume fraction copper/ethylene glycol nanofluid. The mean diameter of the nanoparticles used in their study was less than 10 nm. Namburu et al. [6] prepared copper oxide nanofluids by dispersing the solid nanoparticles in the mixture of ethylene glycol and water and investigated the rheological properties. They conducted experiments with nanoparticle volume concentration ranging from 0% to 6.12% in the temperature span of 35 °C to 50 °C. From their experimental measurement of viscosity they observed that viscosity of the nanofluid increases with increasing volume concentration at a particular temperature and decreases with increase in temperature.
58
P. Selvakumar, S. Suresh / Experimental Thermal and Fluid Science 40 (2012) 57–63
Nomenclature A Cp D h k m Nu Pr q Q Re T V_
surface area (m2) specific heat (J/kg K) orifice diameter (m) convective heat transfer coefficient (W/m2 K) thermal conductivity (W/m K) mass flow rate (kg/s) Nusselt number (hD/k) Prandtl number (lcp/k) actual heat flux (W/m2) heat input (W) Reynolds number (4 m/pdl) temperature (°C) volume flow rate (m3/s)
Greek symbols u volume concentration (%) DP pressure drop (Pa) l dynamic viscosity (N s/m2)
Saeedinia et al. [7] prepared oil based copper oxide nanofluids with volume concentration ranging from 0.2% to 2% and the heat transfer and rheological properties like thermal conductivity, specific heat and viscosity were measured and the effect of nanoparticle concentration on the properties at different temperatures were studied. Also they studied the heat transfer characteristics of the prepared CuO/oil nanofluids in the laminar flow regime under constant heat flux boundary condition. From their experimental results it was observed that the thermal conductivity and viscosity increase with the increase in nanoparticle volume concentration. But the increase in viscosity with respect to weight concentration is more at low temperature compared to high temperature. The maximum increase in convective heat transfer coefficient was observed as 12.7% for the particle weight fraction of 2% at the maximum Reynolds number of the experiment compared to pure oil. Suresh et al. [8] synthesised CuO nanoparticles using sol–gel method and prepared water based nanofluids of volume concentration 0.1%, 0.2% and 0.3%. They experimentally investigated the effect of surface modification in the plain tube (dimples) and the nanofluid on the convective heat transfer coefficient compared to plain tube and distilled water. They showed from their experimental results that the increase in Nusselt number under the turbulent flow regime in the dimpled tube were 19%, 27% and 39% respectively for the volume concentrations of 0.1%, 0.2% and 0.3% compared to plain tube and water. Also they observed a small rise in pressure drop of 2–10% compared to plain tube and water. Wongcharee and Eiamsa-ard [9] studied heat transfer and friction factor characteristics of CuO/water nanofluid in a circular tube fitted with twisted tape with alternate axis. They assessed the performance of nanofluid and alternate axis twisted tape using thermal performance factor. They obtained a maximum thermal performance factor of 5.53 when nanofluid is used as the working fluid with twisted tape. Kulkarni et al. [10] investigated experimentally the rheological behaviour of copper oxide nanoparticles dispersed in a mixture containing water and propylene glycol in the ratio of 60:40. They conducted experiments with particle volume concentration ranging between 0% and 6% in the temperature range of 35 °C to 50 °C and it was observed that the nanofluids possess Newtonian behaviour. Saeedinia et al. [11] studied experimentally the heat transfer and rheological characteristics of CuO/oil nanofluids of weight fractions 0.2% to 2% in a smooth tube under constant heat flux conditions. They observed that the increase in dynamic viscosity of the
q
density (kg/m3)
Subscripts f fluid fm fluid mean i interface in inlet nf nanofluid out outlet s solid phase w water block Abbreviations APS average particle size MW molecular weight SA surface area TIM Thermal Interface Material
nanofluid is high compared to pure oil at low temperatures and the viscosity increases with increasing weight fraction of the nanoparticles. Nanofluids showed Newtonian behaviour for the entire range of weight fractions considered in their experimental work. The maximum increase in convective heat transfer coefficient was 12.7% at a particular particle weight fraction of 2%. Sidy Ndao et al. [12] conducted experiments on jet impingement heat transfer in a micro pin finned plate using water and R134a as heat transfer fluids. They compared the effect of micro pin fins on a surface with a plain surface without micro pin fins. The observed enhancement in convective heat transfer coefficient for the circular micro pin finned surface having the pin fin diameter 125 lm and height 230 lm was up to 200% compared with the plain surface. They also observed that the correlations for impingement zone Nusselt numbers underpredict compared to their experimental data. Reyes et al. [13] conducted experiments to study the heat transfer and pressure drop characteristics of water in a micro channel heat sink. They used a copper heat sink machined with square channels of size 500 lm and overall heat size of 15 mm 15 mm. They studied the effect of tip clearance on the heat transfer and pressure drop compared to the configuration without tip clearance. They found from their experiments that the reduction in heat transfer due to tip clearance is low compared to the reduction in pumping power which is large. Weilin Qu et al. [14] studied the heat transfer and pressure drop characteristics of micro channel heat sink of channel dimensions 231 lm wide and 713 lm deep with two heat fluxes of 100 W/cm2 and 200 W/cm2 experimentally and numerically. They analysed the performance of micro channels in the Reynolds number range of 139–1672 and observed that higher Reynolds number flows are useful in reducing the fluid outlet temperature and heat sink temperature but this happens at a higher pressure drop. They compared their results obtained from the numerical analysis with the experimental data and found good agreement between them. Kou et al. [15] analysed the flow in the micro channel heat sink using a commercial CFD code CFD-ACE+. They analysed the fully developed flow and optimised for the minimum thermal resistance and channel width using simulated annealing technique. They found that the optimum channel width is independent of the channel height at constant flow power. Vafai and Zhu [16] studied the heat transfer and pressure drop characteristics of two layered micro channel heat sink numerically and found that the stream wise temperature rise at the surface of the heat sink is smaller
P. Selvakumar, S. Suresh / Experimental Thermal and Fluid Science 40 (2012) 57–63
compared to the normal heat sink. Also they observed that the pressure drop for the counter flow arrangement in the two layered rectangular micro channel heat sink is less than the single layered heat sink. Tiselj et al. [17] studied the heat transfer characteristics of triangular section silicon micro channels experimentally and numerically using water as the cooling fluid in the Reynolds number range of 3.2–64. They observed non monotonous behaviour of the cooling fluid and the wall temperatures due to high values of heat flux in the axial direction. Li et al. [18] analysed the performance of the CuO/water nanofluid of 1% and 4% volume concentrations in trapezoidal micro channels and compared the results with pure water. They modelled thermal conductivity of nanofluids based on Brownian motion induced micro convection and modelled the heat transfer and pressure drop characteristics using a commercial CFD code CFX. They found from their work that nanofluids significantly increase the thermal performance with small increase in pumping power. Nguyen et al. [19] studied the convective heat transfer performance of Al2O3/water nanofluid in a water block (make: swiftech, USA) with different nanoparticle size and volume concentrations. They observed an increase in convective heat transfer coefficient for nanofluids in the water block with a maximum rise of 40% compared to base fluid at the volume concentration of 6.8%. They did not present the pressure drop values associated with the use of nanofluids in the water block. Gherasim et al. [20] conducted experiments in a radial flow heat sink using Al2O3/water nanofluid. They obtained the increasing Nusselt numbers with increase in volume concentration and Reynolds numbers. But upon decreasing space between the heated plate and the disc the Nusselt numbers were found to decrease. Roberts and Walker [21] conducted experiments in a plain tube to verify the results shown by Lai et al. [22] using Al2O3/water nanofluid. They used constant heat flux as boundary condition for their study. After verifying the results in the plain tube they conducted experiments in a commercial water block to study the convective heat transfer performance and pressure drop of Al2O3/ water nanofluid. From their experimental results they showed an enhancement of 20% in thermal conductance for the nanofluid containing alumina nanoparticles of size 20–30 nm. The convective heat transfer performance of silica/water nanofluids for the cooling of electronic components was evaluated by Escher et al. [23]. They determined Nusselt number using their experimental results and compared with theoretical predictions with considering the change in bulk fluid properties. They concluded that the rise in thermal conductivity should be more than the rise in viscosity of the nanofluids due to inclusion of nanoparticles. Also they reported
59
that increase in volumetric heat capacity will be better than increasing thermal conductivity. In this work, CuO/water nanofluid is formulated with a volume concentration of 0.1% and 0.2% by dispersing specified amount of CuO nanoparticles in deionised water to investigate the effect of flow rate on the interface temperature and geometry of the water block. The effect of particle volume concentration on convective heat transfer coefficient and pressure drop is also presented. 2. Experimental setup The general layout of the experimental setup with major components is shown in Fig. 1. Heated aluminium block, water block (Make: EK-Supreme) high density cartridge heater, air cooled cross flow heat exchanger and peristaltic pump are the major components of the setup. An aluminium block of size 55 55 75 mm is used as the heated block, which is used to simulate the heat generated by any electronic equipment. The capacity of the high density cartridge heater which is inserted into the precisely machined hole of the aluminium block is 150 W. The heater is fitted into the hole in such a way that there is no air gap between the heater and the hole which would cause damaging of the heater. The top surface of the aluminium heated block and the bottom surface of the copper water block are machined and polished to have the surface flatness of ±0.7 l. K type thermocouples of accuracy ±0.2 °C are placed at four different locations in the aluminium block to measure the interface temperature of the heated and water block (Fig. 2). Two more type k thermocouples are used to measure the inlet and outlet fluid temperature. One more thermocouple is used to measure the temperature at the extreme surface of insulation provided. All the temperature readings were stored using a data acquisition system (data taker, Australia). To improve the thermal conductance at the interface of the heated and water blocks, Thermal Interface Material (TIM, Make: Artic silver) is applied between the two surfaces which are in physical contact. The water block is set over the heated aluminium block and positioned firmly using screws and spring loaded nut at the four corners. It applies pressure at the water and a heated block interface (up to a maximum of 7 kg at the full compression of the spring) and helps to decrease the thermal resistance of the interface junction. The inlet and outlet diameters of the orifices are 8 mm. A peristaltic pump (capacity: 5 l/min, Make: Ravel Tech. India) was used to circulate the fluid through the circuit. The electric current to the high density cartridge heater was supplied using an auto transformer at any required voltage. The voltage and current supplied to the heater was
Fig. 1. Schematic of experimental set up.
60
P. Selvakumar, S. Suresh / Experimental Thermal and Fluid Science 40 (2012) 57–63 Table 1 Experimental conditions.
Fig. 2. Location of thermocouple and heater in the heated block.
monitored by using the calibrated digital voltmeter and ammeter respectively. The whole heated and water block assembly is placed inside a wooden casing of size 160 160 160 mm. Glass wool insulation of thickness 50 mm is provided all around the surfaces of the heated – water block assembly to minimise the thermal losses to the surroundings. U tube manometer is used to measure the pressure drop between the inlet and outlet. A plastic vessel of 3 l capacity is used as the reservoir. The hot fluid comes out from the water block is cooled by using a air cooled heat exchanger whose performance is adjusted by varying speed of the fan installed in the cooling system. By varying the speed of the high speed fan the temperature of the fluid comes out from the heat exchange is maintained constant. Separate experiments were conducted to obtain the reduction in inlet fluid temperature with respect to the speed of the air thrown thorough the fins of the heat exchanger. 2.1. Details of the water block The water block is consisting of two parts. A copper base of size 55 55 12.6 mm and acrylic top. The inlet and outlet for the water block are machined at the acrylic top (Fig. 3b). 49 thin channels of length 28 mm and, channel width 0.3 mm and channel height 2 mm are machined at the centre of the copper water block. The inlet flow is directed through the thin channels using a jet plate which is kept over the thin channels as shown in Fig. 3b. The water block’s base is very thin having the thickness of 1 mm to minimise the conductive resistance at the bottom. 3. Experimental procedure Initially the setup is run with water and without any heat flux for 1 h to ensure the leak proof circuit. Constant heat flux is applied to
Heat input
Flow rate
Reynolds number
Volume concentration
102 W
0.79–2.45 LPM
2985–9360
0.1%, 0.2%
aluminium block by supplying specified voltage to cartridge heater using an auto transformer. Interface temperatures, fluid inlet and outlet temperatures, surface temperatures are recorded periodically and readings are saved once steady state is reached. The pressure drop readings are also noted at isothermal conditions. For the first reading the steady state is reached at around one hour and for the subsequent readings it takes around 20 min. The flow in the circuit is measured using a measuring jar and a stop watch. The heat loss from the heated and water block assembly to the surrounding is found to be around 4%. Deionised water and CuO/water nanofluids of volume concentrations 0.1% and 0.2% are used to conduct experiments. Experimental conditions of this study are given in Table 1. Uncertainties associated with Nusselt number, Reynolds number and pressure drop are calculated based on the probable error in the measured quantities. Coleman and Steele method [24] is used to determine the uncertainty considering the measurement errors. The uncertainties involved in the Nusselt number, Reynolds number and pressure drop are respectively ±3.59%, ±0.74% and ±0.125%. Whereas the uncertainties associated with input heat and volume flow rate are ±3.7% and ±1.7% respectively. The uncertainties in the inlet, outlet and interface temperature measurements are ±0.2 °C. 4. Nanofluid preparation and property evaluation Copper oxide nanoparticles purchased from Alfa Aesar (copper (II) Oxide (CuO), 97.5%, MW = 79.54, Nano Arc, SA = 25–40 m2/g, APS = 27–37 nm) was used for the preparation of nanofluid. CuO/ water nanofluids with volume concentrations of 0.1% and 0.2% were prepared by dispersing specified amount of CuO nanoparticles in deionised water by using an ultrasonic vibrator (Power = 180 W, Operating frequency = 25 kHz; Lark, India). To obtain a homogeneous dispersion of the nanoparticles in the DI water, ultrasonication was done continuously for 6 h [25]. Ensuring the stability of the prepared nanofluid is vital for retaining the improved thermal properties of the nanofluids. Normally the particles suspended in the base fluids are stable when the pH of the nanofluid is far from the isoelectric point at which the zeta potential is zero. pH of the prepared nanofluids was measured using a digital pH metre (Deep vision: 111, India) and the values are found to be around 4.8 which is far away from the isoelectric point. Even after a week, the prepared nanofluids were observed to be stable except very little sedimentation.
Fig. 3. (a) Photograph of water block (b) thin channelled copper base with jet plate.
61
P. Selvakumar, S. Suresh / Experimental Thermal and Fluid Science 40 (2012) 57–63
The thermo physical properties of the prepared CuO/water nanofluid of two volume concentrations are determined at the fluids’ bulk mean temperature by using the correlations widely used in the literature. Also thermal conductivity and viscosity of the prepared nanofluids were measured experimentally and the measured properties have been taken for the heat transfer calculations which deviate from the theoretical values determined using standard equations which lack in including the mechanism for improvement of properties. The density of the nanofluid is determined using Pak and Cho’s equation [26]:
of viscosity of DI water and glycerin are 0.9 and 10.9 cP respectively which agrees with the available literature values with the accuracy of ±5%. All the viscosity measurements were done at room temperature of 32 °C with ±2 °C accuracy. The measured values of viscosity are higher than the values obtained from the Einstein model by 17.1% and 21.4% respectively for 0.1% and 0.2% volume concentrations.
qnf ¼ uqs þ ð1 uÞq
Q ¼ Q in Q loss
ð1Þ
4.2. Data processing
ð5Þ
The effective thermal conductivity is calculated using Maxwell’s model [27] for nanofluids of volume concentration less than one.
where Qin is the input heat supplied by cartridge heater, Qloss is the heat loss from the heated and water blocks assembly
knf ks þ 2k þ 2uðks kÞ ¼ ks þ 2k uðks kÞ k
Heat flux; q00 ¼
ð2Þ
The specific heat of the nanofluid is calculated using Xuan and Roetzel’s equation [28]
C pnf 1 þ kc u ¼ Cp 1 þ kp u
ð3Þ
lnf ¼ lð1 þ 2:5uÞ
ð4Þ
qC where kc ¼ qS C pps 1 ; kp ¼ qqs 1 The viscosity of the nanofluid is calculated using the viscosity correlation proposed by Einstein [29]
4.1.1. Thermal conductivity Thermal conductivity of the prepared CuO/water nanofluids of 0.1% and 0.2% volume concentrations were measured using KD2 Pro Thermal property analyser (Decagon Devices, Inc., USA). This instrument consists of a micro controller and a sensor needle. The sensor needle consists of a line heat source and a temperature sensor. The sensor needle is connected to a micro controller by means of a data cable. There are three different types of sensors available with this instrument for different materials and KS-1 sensor was used for the present measurement of thermal conductivity for nanofluids. This sensor needle can be used for measuring thermal conductivity of liquids in the range of 0.2–2 W/mK with an accuracy of ±5%. Each measurement cycle consists of 90 s. During the first 30 s, the instrument will equilibrate which is then followed by heating and cooling of sensor needle for 30 s each. The measured thermal conductivity values for 0.1% and 0.2% volume concentrations of CuO/water nanofluids are 2.9% and 4.3% higher than the values given by the Maxwell’s model [27] of thermal conductivity for two liquid–solid mixtures. 4.1.2. Viscosity The viscosity of the prepared CuO/water nanofluids of 0.1% and 0.2% volume concentrations were measured using Brookfield cone and plate viscometer (LVDV-I PRIME C/P). It is equipped with a 2.4 cm 0.8° cone. The cone of the viscometer is connected with a spindle CPE-40 which can be used for the viscosity measurement of fluid samples in the range of 0.3 to 1.028 cP. 0.013 mm gap is maintained between the cone and plate of the viscometer between which the fluids viscosity to be measured is placed by the electronic adjusting feature of the instrument. When the spindle is rotated the viscous drag of the fluid against the spindle rotation is measured by a calibrated spring. The instrument was benchmarked by measuring viscosity of DI water and glycerin at room temperature. The measured values
ð6Þ
A is the exposed heat transfer area of the water block.
A ¼ A1 þ A2
ð7Þ
A1 is the total surface area of the thin channels, A2 is the inside surface area of the block other than channel area. The average of the measured interface temperatures (Ti), bulk mean temperature of the inlet and outlet fluids and heat flux are used to calculate the heat transfer coefficient of the water block as given in Eq. (8).
hw ¼ 4.1. Thermo physical properties measurement
Q A
q00 ðT i T fm Þ
ð8Þ
where Tfm is the bulk mean temperature of the fluid. Reynolds number is calculated using the following equation.
Re ¼
4m
plDi
ð9Þ
The Nusselt number is then calculated as,
Nu ¼
hw D i k
_ Dp Pumping power ¼ ðVÞ
ð10Þ ð11Þ
5. Results and discussions 5.1. Interface temperature The interface temperature measured between the heated and water block is an indication of performance of the cooling system. The effect of CuO/water nanofluids of volume concentrations 0.1% and 0.2% on interface temperature at different volume flow rate of DI water and nanofluids is shown in Fig. 4. It is clear from the figure that the interface temperature reduces with increasing volume flow rate of the DI water and CuO/water nanofluids. The volume flow rate of the water and nanofluids of two different concentrations are varied from 0.79 LPM to 2.45 LPM. At the minimum volume flow rate of 0.79 LPM the temperature gain obtained for 0.1% and 0.2% volume concentration CuO/water nanofluids compared to deionised water are 0.65 °C and 1.35 °C respectively. At a maximum volume flow rate of 2.45 LPM, the interface temperature obtained for 0.2% volume concentration CuO/water nanofluid is 43.15 °C which is 1.15 °C lower than the temperature obtained for deionised water. It can be understood from the values of interface temperature that the nanofluids remove more heat from the heated block compared to deionised water and keep the interface temperature minimum.
62
P. Selvakumar, S. Suresh / Experimental Thermal and Fluid Science 40 (2012) 57–63
Fig. 4. Effect of volume flow rate on interface temperature.
Fig. 6. Variation of water block pressure drop for water and nanofluid.
5.2. Convective heat transfer coefficient The effect of nanofluid and the volume flow rate on the convective heat transfer coefficient is shown in Fig. 5. The heat transfer coefficients are calculated from the measured average value of the interface temperature, the bulk mean temperature of the fluid and the actual heat flux supplied to the fluid. The experimental results reveal that the copper oxide nanoparticles suspended in the deionised water increases the convective heat transfer coefficient of the water block. It is clear from the figure that the convective heat transfer coefficient increases with increase in volume flow rate of deionised water and nanoparticle volume concentration. At a particular volume flow rate of 2.44 LPM the increase in convective heat transfer coefficient for 0.1% and 0.2% volume concentration nanofluids compared to deionised water are 18.27% and 29.63% respectively. The reason for such enhancement in the convective heat transfer is due to the enhanced transport properties of nanofluids due to the inclusion of solid nanoparticles, Brownian motion of the suspended particles, particle shape and more chances of particles’ direct contact with solid surfaces in thin channels. 5.3. Pressure drop studies To have a nanofluid as a successful heat transfer fluid in practical applications such as compact electronic applications, it must possess enhanced heat transfer characteristics without much pen-
Fig. 5. Effect of volume flow rate and nanofluid volume concentration on convective heat transfer coefficient.
alty in pumping power. To ensure the benefit of nanofluid in cooling the electronic components, pressure drop of CuO/water nanofluids of 0.1% and 0.2% volume concentrations in the water block of the electronic cooling system is measured experimentally at all flow rates. Fig. 6 shows the pressure drop characteristics of CuO/water nanofluids and deionised water in the water block. It is clear from the figure that there is no much rise in pressure drop values for the nanofluids of two different volume concentrations compared to deionised water. Fig. 7 shows the pumping power requirements of the deionised water and CuO/water nanofluids at different volume flow rates. The average increase in pumping power for 0.2% volume concentration nanofluid is 15.11% compared to deionised water which shows a encouraging sign to use nanofluid as the heat transfer fluid for the practical applications such as electronic cooling system. 5.4. Correlation for Nusselt number The Nusselt number obtained from experimental results has been correlated as follows.
Nu ¼ 0:346ðReÞ0:3143 ðPrÞ0:638 ð1 þ /Þ115:341 nf nf
ð12Þ
The above equation is fitting with the present experimental data within ±7.5% for Nusselt number as shown in Fig. 8.
Fig. 7. Variation of pumping power for water and nanofluid.
P. Selvakumar, S. Suresh / Experimental Thermal and Fluid Science 40 (2012) 57–63
Fig. 8. Comparison of experimental and predicted Nusselt number.
6. Conclusions The effect of CuO/water nanofluids in a thin channelled copper heat sink under constant heat flux condition is studied with nanofluids volume fractions 0.1% and 0.2%. The volume flow rate of the deionised water and nanofluids considered for the study is in the range of 0.79 LPM to 2.45 LPM. Significant reduction in interface temperature of the heated and water blocks is observed when CuO/water nanofluids are used compared to deionised water. A maximum increase in convective heat transfer coefficient is determined as 29.63% for the nanofluid volume fraction of 0.2% compared to deionised water for the same volume flow rate. The pressure drop characteristics of CuO/water nanofluids is also studied and rise in pressure drop associated with the inclusion of nanoparticles in deionised water is not much compared to the rise in convective heat transfer coefficient. A correlation for Nusselt number is proposed which fits the experimental results within ±7.5%. Acknowledgements The authors wish to thank Council of Scientific and Industrial Research (CSIR Sanction letter No. 22(0484)/09/EMR-II dated 10th November 2009.), Government of India for its financial support to this work. References [1] Xingcun Colin Tong, Advanced Materials for Thermal management of Electronic Packaging, Springer New York Heidelberg Dordrecht, London, 2011. [2] Avram Bar Cohen, Mehmet Arik, Michael Ohad, Direct liquid cooling of high flux micro and nanoelectronic components, in: Proceedings of the IEE, vol. 94, no. 8, August 2006. [3] F. Incropera, Liquid Cooling of Electronic Devices by Single-Phase Convection, John Wiley & Sons, New York, 1999. [4] S.U.S. Choi, Enhancing thermal conductivity of fluids with nanoparticles, developments and applications of non-newtonian flows, in: D.A. Siginer, H.P. Wang (Eds.), ASME, New York, 1995, pp. 99–105. FED-231/MD-66.
63
[5] J.A. Eastman, S.U.S. Choi, S. Li, W. Yu, L.J. Thomson, Anomalously increased effective thermal conductivities of ethylene glycol based nanofluids containing copper nanoparticles, Appl. Phys. Lett. 78 (2001) 718–720. [6] Praveen K. Namburu, Devdatta P. Kulkarni, Debasmita Misra, Debendra K. Das, Viscosity of copper oxide nanoparticles dispersed in ethylene glycol and water mixture, Exp. Therm. Fluid Sci. 32 (2007) 397–402. [7] M. Saeedinia, M.A. Akhavan-Behabadi, P. Razi, Thermal and rheological characteristics of CuO–Base oil nanofluid flow inside a circular tube, Int. Commun. Heat Mass Transfer (2011), doi:10.1016/ j.icheatmasstransfer.2011.08.001. [8] S. Suresh, M. Chandrasekar, S. Chandra Sekhar, Experimental studies on heat transfer and friction factor characteristics of CuO/water nanofluid under turbulent flow in a helically dimpled tube, Exp. Therm. Fluid Sci. 35 (2011) 542–549. [9] Khwanchit. Wongcharee, Smith. Eiamsa-ard, Enhancement of heat transfer using CuO/water nanofluid and twisted tape with alternate axis, Int. Commun. Heat Mass Transfer 38 (2011) 742–774. [10] D.P. Kulkarni, D.K. Das, S.L. Partil, Effect of temperature on rheological properties of copper oxide nanoparticles dispersed in propylene glycol and water mixture, J. Nano Sci. Nano Technol. L7 (2) (2007) 1–5. [11] M. Saeedinia, M.A. Akhavan-Behabadi, P. Razi, Thermal and rheological characteristics of CuO–Base oil nanofluid flow inside a circular tube, Int. Commun. Heat Mass Transfer (2011), doi:10.1016/ j.icheatmasstransfer.2011.08.001. [12] Sidy Ndao, Hee Joon Lee, Yoav Peles, Michael K. Jensen, Heat transfer enhancement from micro pin fins subjected to an impinging jet, Int. J. Heat Mass Transfer 55 (2012) 413–421. [13] M. Reyes, J.R. Arias, A. Velazquez, J.M. Vega, Experimental study of heat transfer and pressure drop in micro-channel based heat sinks with tip clearance, Appl. Therm. Eng. 31 (2011) 887–893. [14] Qu Weilin, Issam Mudawar, Experimental and numerical study of pressure drop and heat transfer in a single-phase micro-channel heat sink, Int. J. Heat Mass Transfer 45 (2002) 2549–2565. [15] Hong-Sen Kou, Ji-Jen Lee, Chih-Wei Chen, Optimum thermal performance of microchannel heat sink, by adjusting channel width and height, Int. Commun. Heat Mass Transfer 35 (2008) 577–582. [16] Kambiz Vafai, Lu Zhu, Analysis of two layered micro channel heat sink concept in electronic cooling, Int. J. Heat Mass Transfer 42 (1999) 2287–2297. [17] Tiselj, G. Hetsroni, B. Mavko, A. Mosyak, E. Pogrebnyak, Z. Segal, Effect of axial conduction on the heat transfer in micro-channels, Int. J. Heat Mass Transfer 47 (2004) 2551–2565. [18] Jie Li, Clement Kleinstreuer, Thermal performance of nanofluid flow in microchannels, Int. J. Heat Fluid Flow 29 (2008) 1221–1232. [19] T. Nguyen, G. Roy, C. Gauthier, N. Galanis, Heat transfer enhancement using Al2O3–water nanofluid for an electronic liquid cooling system, App. Therm. Eng. 27 (2007) 1501–1506. [20] I. Gherasim, G. Roy, T. Nguyen, D. Vo-Ngoc, Experimental investigation of nanofluids in confined laminar radial flows, Int. J. Therm. Sci. 48 (2009) 1486– 1493. [21] N.A. Roberts, D.G. Walker, Convective performance of nanofluids in commercial electronics cooling systems, Appl. Therm. Eng. 30 (2010) 2499– 2504. [22] W.Y. Lai, S. Vinod, P.E. Phelan, R. Prasher, Convective heat transfer for water based alumina nanofluids in a single 1.02 mm tube, J. Heat Transfer 131 (2009) 112401. [23] W. Escher, T. Brunschwiler, N. Shalkevich, A. Shalkevich, T. Burgi, B. Michel, D. Poulikakos, On the cooling of electronics with nanofluids, J. Heat Transfer 133 (2011) 051401. [24] H.W. Coleman, W.G. Steele, Experimental and Uncertainty Analysis for Engineers, Wiley, New York, 1989. [25] J.H. Lee, K.S. Hwang, S.P. Jang, B.H. Lee, J.H. Kim, S.U.S. Choi, C.J. Choi, Effective viscosities and thermal conductivities of aqueous nanofluids containing low volume concentrations of Al2O3 nanoparticles, Int. J. Heat Mass Transfer 51 (2008) 2651–2656. [26] B.C. Pak, Y. Cho, Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particle, Exp. Heat Transfer 11 (1998) 151–170. [27] J.C. Maxwell, Treatise on Electricity and Magnetism, Dover, New York, 1954. [28] Y. Xuan, W. Roetzel, Conceptions for heat transfer correlation of nanofluids, Int. J. Heat Mass Transfer 43 (2000) 3701–3707. [29] A. Einstein, Investigation on the Theory of Brownian motion, Dover, New York, 1956.