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ethylene glycol based nanofluids as a new coolant for car radiators
International Communications in Heat and Mass Transfer 38 (2011) 1283–1290
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International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
Experimental study of heat transfer enhancement using water/ethylene glycol based nanofluids as a new coolant for car radiators☆ S.M. Peyghambarzadeh a, S.H. Hashemabadi b,⁎, S.M. Hoseini a, M. Seifi Jamnani a a b
Department of Chemical Engineering, Mahshahr branch, Islamic Azad University, Mahshahr, Iran CFD Research Laboratory, School of Chemical Engineering, Iran University of Science and Technology, Narmak, Tehran, 16846, Iran
a r t i c l e
i n f o
Available online 16 July 2011 Keywords: Water/Al2O3 nanofluid Ethylene glycol/Al2O3 nanofluid Heat transfer enhancement Car radiator Cooling performance Experimental study
a b s t r a c t Traditionally forced convection heat transfer in a car radiator is performed to cool circulating fluid which consisted of water or a mixture of water and anti-freezing materials like ethylene glycol (EG). In this paper, the heat transfer performance of pure water and pure EG has been compared with their binary mixtures. Furthermore, different amounts of Al2O3 nanoparticle have been added into these base fluids and its effects on the heat transfer performance of the car radiator have been determined experimentally. Liquid flow rate has been changed in the range of 2–6 l per minute and the fluid inlet temperature has been changed for all the experiments. The results demonstrate that nanofluids clearly enhance heat transfer compared to their own base fluid. In the best conditions, the heat transfer enhancement of about 40% compared to the base fluids has been recorded. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction After the publication of our previous paper [1] about the application of water/Al2O3 nanofluids instead of pure water in the car radiator and recording the interesting heat transfer enhancement of about 45%, we want to investigate the application of nanoparticle in the mixture of water and anti-freeze materials (as the base fluid) which is conventionally used in the cars' radiators. It is common in the area of cold or hot weathers that some additives are added to the water in the automotive radiator which decrease freezing point and elevate boiling point of water. It keeps the radiator fluid from freezing when it is very cold and keeps the car from overheating on very hot days. Almost all of these additives are from glycol family specially ethylene glycol (EG). The major use of EG is as a medium for convective heat transfer in, for example car radiators, liquid cooled computers, chilled water air conditioning systems, and the like. Because water is a much better engine coolant, the mixture of water and EG has been used. The trouble with water is that it freezes or boils at extreme temperatures. Anti-freezing agents like EG can withstand much greater temperature extremes, so by adding it to water we can make a compromise. Most of the good cooling abilities of water are retained but the ability to withstand extreme temperatures comes from the anti-freeze. As can be seen in Fig. 1, a mixture of 60% EG and 40% water does not freeze to temperatures below − 45 °C. EG disrupts hydrogen bonding when dissolved in water. Pure EG freezes at about −12 °C, but when intermixed with water, the freezing point of the ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (S.H. Hashemabadi). 0735-1933/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2011.07.001
mixture is depressed significantly. The minimum freezing point is observed when the EG percent in water is about 70%, as shown in Fig. 1. However, the boiling point for aqueous EG increases monotonically with increasing EG percentage. Thus, the use of EG not only declines the freezing point but also elevates the boiling point such that the operating range for the heat transfer fluid is broadened on both ends of the temperature scale [2]. It has been proved that conventional fluids, such as water and EG have poor convective heat transfer performance and therefore high compactness and effectiveness of heat transfer systems are necessary to achieve the required heat transfer. Among the efforts for enhancement of heat transfer the application of nanoparticle additives to liquids is more noticeable and currently a large number of investigations are devoted to this subject [3–8]. Nanofluids are formed by suspending metallic or non-metallic oxide nanoparticles (that are significantly smaller than 100 nm) in traditional heat transfer fluids. These so-called nanofluids display good thermal properties compared with fluids conventionally used for heat transfer and fluids containing particles on the micrometer scale. These fluids are a new window which has been opened recently and it was confirmed by several authors that these working fluids can enhance heat transfer performance [9,10]. In the car radiators, the coolant media is pumped through the flat tubes while the air is drawn over the fins by forced convection, thereby heat exchanges between the hot circulating fluid and air. The application of nanofluids in these finned tube radiators may result in several potential benefits including increased heating output for equal liquid flow. These performance impacts, in turn, may be translated into a reduction in total required heat transfer area. Superior heat transfer properties of nanofluids may also result in lower liquid flow
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Nomenclature A Cp d dp f h k L m˙ Nu P Pe Pr Q Re S T VB z
Peripheral area (m 2) Specific heat capacity (J/kg K) Hydraulic diameter of the tube (m) Diameter of the nanoparticles (m) Friction factor Heat transfer coefficient (W/m 2K) Thermal conductivity (W/m K) Length of the tube (m) Mass flow rate (kg/s) Nusselt number = (hdhy/k) Tube periphery (m) Peclet number = Re.Pr Prandtl number = (Cpμ/k) Heat transfer rate (W) Reynolds number = (ρudhy/μ) Cross sectional area of the tube (m 2) Temperature (K) Brownian velocity Axial distance from the tube inlet (m)
Greek letters α Thermal diffusivity (m 2/s) ρ Density (kg/m 3) δ Distance between the centers of the particles (m) μ Viscosity (kg/m.s) φ Nanoparticle volume fraction (%) Φ Shape factor ψ Particle sphericity
Subscripts ave Average b Bulk bf Base fluid exp Experimental in Input nf Nanofluid out Output p Particle w Wall
rate for a given rate of heat transfer, yielding a reduction in the liquid pumping power consumed compared to the base fluid. In order to have more understanding about the application of nanofluids in various heat exchangers, a brief literature survey is performed in this paper. Pak and Cho [11] presented an experimental investigation of the convective turbulent heat transfer characteristics of nanofluids (Al2O3–water) with 1 to 3 vol.%. Their results show that Nusselt number for the nanofluids enhances with increasing of volume concentration and Reynolds number. Heris et al. [12] examined and proved the enhancement of in-tube laminar flow heat transfer of nanofluids (water–Al2O3) in a constant wall temperature boundary condition. In other work, Heris et al. [13] presented an investigation of the laminar flow convective heat transfer of Al2O3–water under constant wall temperature with 0.2 to 2.5 vol.% of nanoparticle for Reynolds number varying between 700 and 2050. They presented again the Nusselt number for the nanofluid which is greater than the base fluid. Lai et al. [14] studied the flow behavior of nanofluids (20 nm Al2O3 nanoparticle in water) in a millimeter-sized stainless steel test tube, subjected to constant wall heat flux and a low Reynolds
Fig. 1. Boiling and freezing points of water/EG mixtures [2].
number (Re b 270). The maximum promotion of Nusselt number for 1 vol.% nanofluid was 8%. Jung et al. [15] conducted convective heat transfer experiments for a nanofluid (Al2O3–water) in a rectangular micro-channel under laminar flow conditions. Their results show the heat transfer coefficient increases by more than 32% for 1.8 vol.% nanoparticle. Sharma et al. [16] implemented 1 to 2.5 vol.% Al2O3 in water in horizontal tube geometry and concluded while the Peclet number is between 3500 and 6000, up to 41% promotion in heat transfer coefficient compared to pure water may have occurred. Ho et al. [17] conducted an experiment for cooling in horizontal tube in laminar flow of Al2O3–water at 1 and 2 vol.% concentrations and concluded the interesting enhancement of 51% in heat transfer coefficient. Nguyen et al. [18] performed their experiments in a microprocessor type cooling heat exchanger and at 6.8 vol.% Al2O3 in water obtained 40% growing in heat transfer coefficient. Xie et al. [19] reported the convective heat transfer enhancement of nanofluids as coolants in laminar flows inside a circular copper tube with constant wall temperature. Different nanofluids consisting of Al2O3, ZnO, TiO2, and MgO nanoparticles were prepared with a mixture of 55 vol.% distilled water and 45 vol.% EG as base fluid. MgO, Al2O3, and ZnO nanofluids exhibited superior enhancements of heat transfer coefficient, with the highest enhancement up to 252% at a Reynolds number of 1000 for MgO nanofluid. The performance of finned tube heating units with nanofluids has been compared mathematically with a conventional heat transfer fluid which comprised of 60% EG and 40% water by Strandberg and Das [20]. Their model predicted an 11.6% increase in finned tube heating output under certain conditions with the 4% Al2O3/60% EG nanofluid and an 8.7% increase with the 4% CuO/60% EG nanofluid compared to heating output with the base fluid. Application of EG based copper nanofluids in an automotive cooling system has been studied by Leong et al. [21]. Relevant input data, nanofluid properties and empirical correlations were obtained from literatures to investigate the heat transfer enhancement of an automotive car radiator operated with nanofluid-based coolants. It is observed that, about 3.8% of heat transfer enhancement could be achieved with the addition of 2% copper nanoparticles in a base fluid at the Reynolds number of 6000 and 5000 for air and coolant respectively. Some extensive reviews in the nanofluid heat transfer have also been published by Godson et al. [22], Kakaç et al. [23] and Wang et al. [24]. The interested reader can refer to them for complete reviewing of the previous studies performed. It should be emphasized that almost no document can be found to describe experimental evaluation of nanofluid performance in the car radiator. In this paper, experimental comparisons have been accomplished between the heat transfer performance of pure water and pure EG and some concentrations of their mixtures in the car radiator.
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How do anti-freeze materials like EG affect the heat transfer performance of the radiator? What happens when you increase the EG concentration? Furthermore, when small amounts of alumina nanoparticle are added to water or EG or their mixtures, does the rate of heat transfer change compared with the base fluids? What will be the effects of operating parameters like nanoparticle concentration, flow rate, and temperature of circulating fluid on the heat transfer performance? These are the main questions which have been answered along this paper. 2. Experimental 2.1. Experimental rig and procedure In order to measure the liquid side heat transfer coefficients in the car radiator, a flow loop shown in Fig. 2(A) has been used. This experimental rig includes a storage tank, a heater, a pump, a flow meter, a forced draft fan, a cross flow finned tube heat exchanger (car radiator), and flow lines. The test fluid flows through the five layer insulated tubes (0.75 inch diameter) from the feed tank to the radiator by a centrifugal pump with constant flow rate of 10 l per
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minute. A recycle line included a globe valve was prepared to obtain the predetermined flow rate. Storage tank (height of 35 cm and diameter of 30 cm) has volume of 30 l and the working liquid would fill 25%. Consequently, the total volume of the circulating liquid is constant in all the experiments. A flow meter (Technical Group LZM15Z Type) was used to control and manipulate the flow rate with the precision of 0.1 l per minute. For heating the working fluid, an electrical heater and a controller were used to maintain the temperature between 40 and 80 °C. Two RTDs (Pt-100 Ω) were implemented on the flow line to record radiator fluid inlet and outlet temperatures. Two other J-type thermocouples were also used for radiator wall temperature measurement. These thermocouples were installed at the center of the radiator surfaces (both sides). Due to very small thickness and very high thermal conductivity of the flat tubes, it is reasonable to equate the inside temperature of the tube with the outside one. The measured temperatures from these thermocouples and RTDs have been shown on two digital monitors with the accuracy of 0.1 °C. All used thermocouples and RTDs were thoroughly calibrated by using a constant temperature water bath, and their accuracy was estimated to be ±0.2 °C.
Fig. 2. A) Schematic of experimental set up. B) Schematic drawing of the applied louvered fin and flat tube radiator and their dimensions.
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Car radiator configuration is louvered fin and flat tube as shown in Fig. 2(B). This air cooler includes 34 vertical aluminum tubes with elliptical cross section. The distances between the tube rows have been filled with thin perpendicular aluminum fins. For cooling the liquid, an axial forced fan (Techno Pars 1400 rpm) was installed close on axis line of the radiator and consequently air and water have indirect cross flow contact. Gamma alumina nanoparticles (0.1, 0.3, 0.5, 0.7, and 1 vol.%) have been added to different base fluids including pure water, pure EG and their mixtures (5, 10, 20 vol.% EG). The mean grain size of this gamma alumina is 20 nm and some of its other properties are shown in Table 1. There was no dispersant or stabilizer added to the nanofluid. This is due to the fact that the addition of any agent may change the fluid properties [11] and the authors were interested to simulate the easiest actual condition encountered in the car radiator. Additionally, creating highly turbulent flow condition in the radiator tubes and connecting pipes can improve the stabilization of the nanoparticle in water. Table 2 shows the ranges of all the experimental parameters.
Table 2 Range of experimental operational conditions. Parameters
Water based nanofluids
EG based nanofluids
Nanoparticle type Nanoparticle concentration (vol.%) Flow rate (l/min) Reynolds number Flow type Inlet temperature
γ-Al2O3 0 to 1
γ-Al2O3 0 to 1
2 to 5 9000 to 23,000 Turbulent 35 to 50 °C
3 to 6 1200 to 2500 Laminar 45 to 60 °C
sphere with volume equal to that of the particle, to the surface area of the particle, and in this paper Φ is considered to be 3. For calculation of water based nanofluid viscosities, the following correlation has been applied [26–28]: 2 μ nf = μ bf 123φ + 7:3φ + 1
ð4Þ
2.2. Uncertainty analysis Uncertainty analysis is carried out by calculating the error of the measurements. The uncertainty range of Reynolds number comes from the errors in the measurement of volume flow rate and hydraulic diameter of the tubes and the uncertainty of Nusselt number refers to the errors in the measurements of volume flow rate, hydraulic diameter, and all the temperatures. According to uncertainty analysis described by Moffat [25], the measurement errors of the main parameters are summarized in Table 3. Furthermore, to check the reproducibility of the experiments, some runs were repeated later which proved to be excellent. 3. Estimation of nanofluid physical properties
ρnf = φρp + ð1−φÞρ bf
ð1Þ
= φ ρCp + ð1−φÞ ρCp ρCp
knf
p
μ nf = μ bf +
ð2Þ
bf
ρp VB d2p 72Cδ
ð5Þ
Where the second term is the apparent viscosity arising from the effects of nanoparticles in the fluid. The distance between the centers of the nanoparticles, δ and correction factor (C) are calculated respectively:
δ=
By assuming that the nanoparticles are well dispersed within the base fluid, i.e. the particle concentration can be considered uniform throughout the system; the effective thermophysical properties of the mixtures can be evaluated using some classical formulas as usually used for two phase flow. The following correlations have been used to predict nanofluid density, specific heat, and thermal conductivity respectively at different temperatures and concentrations [26–28]:
nf
For EG based and mixture based nanofluids, the correlation proposed by Masoumi et al. [29] has been used:
rffiffiffiffiffiffiffi π 3 d 6φ p
ð6Þ
−1
C = μ bf ðaφ + bÞ
ð7Þ
Where a and b are experimental parameters, which for the engine coolant–Al2O 3 nanofluids were estimated to be 0.00004 and 7.1274×10 − 7, respectively [30]. Various correlations were proposed for temperature dependence of the nanofluid viscosity. Kole and Dey [30] showed only that the following correlation proposed by Namburu et al. [31] can give an acceptable agreement to the temperature dependence of viscosity of the Al2O3/conventional coolant nanofluids. log μ nf = M expð NT Þ
kp + ðΦ−1Þkbf −φðΦ−1Þ kbf −kp kbf = kp + ðΦ−1Þkbf + φ kbf −kp
ð8Þ
ð3Þ
Where Φ is empirical shape factor given by Φ = 3/ψ, and ψ is the particle sphericity, that is defined as the ratio of the surface area of a
Table 1 Some characteristics of alumina nanoparticle. Specification
Value
Appearance Purity Grain size (nm) Specific surface area (m2/g) Silicon (Si) (ppm) Calcium (Ca) (ppm) Iron (Fe) (ppm) Cobalt (Co) (ppm)
White powder + 99% 20 200 3.5 1.6 0.2 0.8
Where two parameters (M and N) are functions of nanoparticle concentration [30]. Table 4 depicts pure water and pure EG physical properties. As can be seen in Table 4, these two base fluids show significant differences in the physical properties. The mentioned water/EG mixture properties in Table 4 were used by Dai et al. [35] to develop computational models for the calculations of water/EG physical properties. Their findings were also used whenever no experimental data existed.
Table 3 The uncertainty of the measured parameters. Parameter
Value
Uncertainty
d (mm) Re Nu
6.53 1200 to 23,000 24 to 120
1.6% 5.2% 18%
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Table 4 Physical properties of pure water, pure EG and mixture of water–EG at 40 °C. Physical properties
Water
EG
ρ (kg/m3) μ (kg/m.s) k (W/m °C) Cp (J/kg.°C) α (m2/s) Pr
992 0.00065 0.633 4174 1.5 × 10− 7 4.3
1101 0.0095 0.256 2382 9.8 × 10− 8 93
Water 95%
Water 90%
Water 80%
EG 5%
EG 10%
EG 20%
995a 0.00101b 0.615b 4157c 1.49 × 10− 7 6.8
1002 0.00165 0.6 4090 1.46 × 10− 7 11.2
1008 0.0019 0.58 4020 1.43 × 10− 7 13.1
a
The density of water/EG mixtures have been obtained from [32]. The experimental thermal conductivities and viscosities of water/EG mixtures have been obtained from [33]. c The heat capacities of water/EG mixtures have been extracted from [34]. b
4. Calculation of heat transfer coefficient The heat transfer coefficient and corresponding Nusselt number can be derived as follows [1]: Nu =
˙ p ðTin −Tout Þ mC h:d = k AðTb −Tw Þ
ð9Þ
where m˙ is mass flow rate which is the product of density and volume flow rate of the fluid, Tb is bulk temperature which is assumed to be the average values of inlet and outlet temperatures of the fluid moving through the radiator, and Tw is tube wall temperature which is the mean value measured by two surface thermocouples. In Eq. (9), k is fluid thermal conductivity and d is hydraulic diameter of the flat tube. It should also be mentioned that all the physical properties were calculated at the fluid bulk temperature. 5. Results and discussions
Fig. 3. The results for: A) pure water (Tin = 50 °C) in comparison with Dittus–Boelter correlation [36]. B) Pure EG (Tin = 40 °C) in comparison with the Vajjha et al. correlations [37].
5.1. Heat transfer to pure water and pure EG Before running the experiments on the nanofluids as a coolant for car radiator, some tests with pure water and pure EG were done in order to check the reliability and accuracy of the experimental setup. Fig. 3(A) shows the experimental results for water flow through the radiator at constant inlet temperature of 50 °C. It is shown that the higher Reynolds number increases the heat transfer coefficient of pure water. The experimental data has been compared with following empirical correlation suggested by Dittus–Boelter [36] in turbulent flow: 0:8
Nu = 0:0236Re
0:3
Pr
ð10Þ
Two correlations were developed by Vajjha et al. [37] for nanofluids from the numerical analysis of the flat tube geometry which were shown in Eq. (11): 8 Dh 0:3 d > > RePr ≥33:33 > < Nu = 1:9421 × RePr z z > > D d > : Nu = 6:1 + 0:003675 × RePr h RePr ≤33:33 z z
ð11Þ
The average Nusselt number can be calculated as follow: L
Nuave: =
1 ∫ Nu: dz L0
ð12Þ
Fig. 3(B) represents the comparison of experimental results for pure EG entering the radiator at constant temperature of 40 °C with the prediction of Vajjha et al. [37] correlation in laminar flow. As can be seen in Fig. 3(A) and 3(B), the experimental results show good agreements
with these empirical correlations over the Reynolds number range used in this study. The experimental data in different water temperatures at the radiator inlet including 35, 45, and 50 °C have 10% absolute average error with respect to Dittus–Boelter [36] relation and for all the different EG temperatures at the radiator inlet including 40, 45, and 50 °C, Vajjha et al.'s [37] relation has 6% absolute average error.
5.2. Heat transfer to water based and EG based nanofluids The nanofluid is implemented in different Al2O3 concentrations, i.e. 0.1, 0.3, 0.5, 0.7, and 1 vol.% and at different flow rates of 2, 3, 4, 5, and 6 l per minute. In order to consider the effect of temperature on thermal performance of the radiator, different inlet temperatures have been applied for each concentration. The inlet temperatures include 35, 45, and 50 °C for the water based nanofluids and 45, 50, and 60 °C for EG based nanofluids. Fig. 4(A) and (B) shows the heat transfer enhancement due to the replacement of water as a conventional coolant with water and EG based nanofluids respectively. As can be seen, the ratio of the nanofluid Nusselt number to the base fluid Nusselt number (Nunf/Nubf) has increased by enhancement in the concentrations of nanoparticle at constant Reynolds number for both nanofluids. By the addition of only 1 vol.% of Al2O3 nanoparticle into the water or EG, an increase of about 40% in comparison with the pure water and pure EG Nusselt number was recorded. The Nusselt number increases monotonously with Reynolds number, but Nunf/Nubf presents different variation tendencies with Reynolds number for these two nanofluids. For the water based nanofluid it is obvious from Fig. 4(A) that Nunf/Nuw increases with Reynolds number and in higher concentrations of nanoparticle the effect of Reynolds number becomes pronounced. This result is in
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that an increase in the fluid inlet temperature (in the range of our experiments) slightly improves the heat transfer performance. Inspecting the results reveals that increasing the inlet temperature of water based nanofluids from 35 °C to 50 °C can enhance Nusselt number up to 16%. For EG based nanofluids, the temperature elevation from 45 to 60 °C creates maximum enhancement of 7%. This variation in Nusselt number may be attributed to the effect of temperature on the physical properties and also to the increased effect of test liquid radiation to the internal wall of the tubes. 5.4. Heat transfer to water/EG binary mixture nanofluids A conventional fluid usually used in the car radiator is the mixture of water/EG in different concentrations depending on the regional weather. In order to have more insights to the influence of nanoparticle addition in the radiator coolant, three different concentrations of water/EG binary mixtures which include 5, 10, and 20 vol.% EG were prepared as the base fluids. Four different values of Al2O3 nanoparticle (0, 0.05, 0.15, and 0.3 vol.%) were added to each concentration of water/EG mixtures and finally the effect of flow rate on the heat transfer performance was studied for each case. All the obtained experimental data are summarized in Table 5. The results obtained for pure water and pure EG based nanofluids (which were shown previously) were also presented in this table for more comfortable comparison of the results. Furthermore, due to the large variations in the physical properties of the base fluids, it is not possible to imply Nusselt number as a function of Reynolds number. Reynolds number greatly reduced when EG concentration enhances. At extreme conditions, Reynolds number changes between 9000 and 23,000 for water based nanofluids while it changes between 1200 and
Fig. 4. Variations of dimensionless Nusselt number at different Reynolds numbers as a function of nanoparticle concentration (Tin = 45 °C). A) Water based nanofluids. B) EG based nanofluids.
contradiction to the EG based nanofluids in which Nunf/NuEG exhibited irregular trends against Reynolds number, as can be seen in Fig. 4(B). It was shown that when small amounts of Al2O3 nanoparticles are added to the base fluids, the density and the thermal conductivity increase and the specific heat decreases slightly while the viscosity increases more markedly compared to the base fluid [1]. These variations, however, are too small (of about 4%) to explain heat transfer enhancement of up to 40% gained in this study. Heris et al. [12] have done experiments with Al2O3 nanoparticles in water under laminar flow up to turbulence. They found the heat transfer enhancement as high as 40% with Al2O3 particles while the thermal conductivity enhancement was less than 15%. Many researchers have suggested that in fact Brownian motion is one of the most important factors in the enhancement of heat transfer. The presence of nanoparticles and their random motion within the base fluid cause the thickness of thermal boundary layer to reduce and it has important contribution to such heat transfer improvement [38]. This random motion of ultra-fine particles would create a slip velocity between the solid particles and the fluid medium [23]. Xuan and Roetzel [26] also suggested the role of small perturbations in the temperature and velocity formulation to account for the Brownian motion. 5.3. Effect of temperature on heat transfer coefficient of nanofluids Fig. 5(A) and (B) compares the nanofluid Nusselt numbers at different inlet temperatures in order to analyze the effect of temperature variation on the heat transfer performance of the car radiator. It is clear
Fig. 5. Effect of inlet temperature on the Nusselt numbers. A) Water based nanofluid at the nanoparticle concentration of 1 vol.%. B) EG based nanofluid at the nanoparticle concentration of 0.7 vol.%.
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2500 for EG based nanofluids. It should be mentioned that all the data in Table 5 have been obtained at the constant liquid inlet temperature to the radiator namely 45 °C. Data in Table 5 obviously indicate that higher Nusselt numbers have been obtained for the pure water compared to pure EG at constant flow rate (more than twice). This is basically due to the sharp differences in the physical properties of the two pure liquids. For the sake of clarity, some of the important physical properties of water and EG were compared in Table 2 at the temperature of 45 °C. The Prandtl number (Pr) as an important parameter affecting the heat transfer coefficient differs greatly in these two liquids and it can be the main reason for the lower Nusselt number in EG compared to water. Furthermore, it is obvious from Table 5 that the addition of EG into water decreases the Nusselt numbers. It is the reason for mixing of EG with water; water does a much better job at the cooling of the engine. In all the experimental data shown in Table 5, it is proven that the addition of nanoparticle to each base fluid enhances the heat transfer coefficient. This effect manifests itself at water/EG binary mixtures with lower concentrations of EG. Different correlations were introduced for the prediction of nanofluid forced convection heat transfer in the literature up to now. Xuan and Li [39] suggested two empirical correlations for laminar and turbulent flow of the nanofluids in tube: 0:218 0:754 0:333 0:4 Nu = 0:4328 1 + 11:285φ Re Pe Pr 0:001 0:6886 0:9238 Re Pe Nu = 0:0059 1 + 7:6286φ
Re b 2100 ð13Þ Re b 2100
ð14Þ
The experimental data obtained in the present investigation have been compared with these two correlations. Fig. 6(A) and (B) compare the experimental results for the Nusselt number of Al2O3/ water and Al2O3/EG nanofluids respectively with the prediction of Xuan and Li correlations. Very good agreement can be seen in these two figures. Calculating the absolute average errors reveals that the prediction error for water based nanofluids is 7% and for EG based nanofluids is 12.5%. As can be seen in Fig. 6, Xuan and Li correlations almost over-predict Nusselt number for these nanofluids. The overprediction of these empirical correlations has been mentioned before by other investigators in the case of other nanofluids [40]. 6. Conclusion In this paper, the convective heat transfer enhancement of water and EG based nanofluids as the coolants inside flat aluminum tubes of the car radiator has been investigated. Significant increases of the
Table 5 Experimental Nusselt numbers measured for different concentrations of EG/water mixtures as the base fluid (inlet temperature 45 °C). φ (%) Flow rate (l/min)
EG 5% water 95% 0
0.05
0.15
0.3
EG 10% water 90% 0
0.05
0.15
0.3
3 4 5 6
63.93 79.26 89.15 101.87
66.58 81.56 92.59 104.95
71.03 86.50 97.19 109.31
76.67 92.24 103.16 114.56
63.28 79.45 91.27 99.99
66.22 81.98 93.93 108.07
71.94 87.53 99.99 111.55
77.50 92.54 104.38 114.32
Fig. 6. Comparison of measured Nusselt numbers with those predicted from Xuan and Li correlation [39]. A) Water based nanofluid. B) EG based nanofluid.
total heat transfer rates have been observed with the nanoparticle addition. A highest Nusselt number enhancement up to 40% was obtained at the best conditions for both nanofluids. The experimental results have demonstrated that the heat transfer behaviors of the nanofluids were highly depended on the particle concentration and the flow conditions and weakly dependent on the temperature. Our results indicate that nanofluids have great potential for heat transfer enhancement and are highly suited to apply in practical heat transfer processes. This provides promising ways for engineers to develop highly compact and effective radiators for cars. These higher heat transfer coefficients obtained by using nanofluid instead of water allow the working fluid in the car radiator to be cooler. The addition of nanoparticles to the coolant has the potential to improve automotive and heavy-duty engine cooling rates or equally causes to remove the engine heat with a reduced-size cooling system. Smaller cooling systems result in smaller and lighter radiators, which in turn benefit almost every aspect of vehicle performance and lead to increased fuel economy. References
φ (%) Flow rate (l/min)
EG 20% water 80%
Pure EG
Pure water
0
0.05
0.15
0.3
0
0.3
0
0.3
3 4 5 6
61.59 79.35 90.48 90.77
63.45 82.43 92.55 101.38
68.78 87.29 98.83 107.23
71.78 90.81 102.40 110.55
24.69 30.23 33.67 37.47
27.01 32.63 36.00 40.20
67.76 83.72 96.46 -
76.77 99.28 115.69 -
[1] S.M. Peyghambarzadeh, S.H. Hashemabadi, M. Seifi Jamnani, S.M. Hoseini, Improving the cooling performance of automobile radiator with Al2O3/water nanofluid, Applied Thermal Engineering 31 (2011) 1833–1838. [2] S. Rebsdat, D. Mayer, “Ethylene Glycol” in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2002. [3] S.M. Fotukian, M. Nasr Esfahany, Experimental study of turbulent convective heat transfer and pressure drop of dilute CuO/water nanofluid inside a circular tube, International Communications in Heat and Mass Transfer 37 (2010) 214–219.
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[4] W. Duangthongsuk, S. Wongwises, An experimental study on the heat transfer performance and pressure drop of TiO2–water nanofluids flowing under a turbulent flow regime, International Journal of Heat and Mass Transfer 53 (2010) 334–344. [5] M. Emami Meibodi, M. Vafaie-Sefti, A.M. Rashidi, A. Amrollahi, M. Tabasi, H. Sid Kalal, The role of different parameters on the stability and thermal conductivity of carbon nanotube/water nanofluids, International Communications in Heat and Mass Transfer 37 (2010) 319–323. [6] P. Naphon, P. Assadamongkol, T. Borirak, Experimental investigation of titanium nanofluids on the heat pipe thermal efficiency, International Communications in Heat and Mass Transfer 35 (2008) 1316–1319. [7] H. Demir, A.S. Dalkilic, N.A. Kürekci, W. Duangthongsuk, S. Wongwises, Numerical investigation on the single phase forced convection heat transfer characteristics of TiO2 nanofluids in a double-tube counter flow heat exchanger, International Communications in Heat and Mass Transfer 38 (2) (2011) 218–228. [8] W. Duangthongsuk, S. Wongwises, Heat transfer enhancement and pressure drop characteristics of TiO2–water nanofluid in a double-tube counter flow heat exchanger, International Journal of Heat and Mass Transfer 52 (2009) 2059–2067. [9] D. Wen, Y. Ding, Experimental investigation into convective heat transfer of nanofluids at the entrance region under laminar flow conditions, International Journal of Heat and Mass Transfer 47 (2004) 5181–5188. [10] M.S. Liu, M.C.C. Lin, I.T. Huang, C.C. Wang, Enhancement of thermal conductivity with CuO for nanofluids, Chemical Engineering and Technology 29 (1) (2006) 72–77. [11] B.C. Pak, I.Y. Cho, Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles, Experimental Heat Transfer 11 (1998) 151–170. [12] S.Z. Heris, S.Gh. Etemad, M. Nasr Esfahany, Experimental investigation of oxide nanofluids laminar flow convective heat transfer, International Communications in Heat and Mass Transfer 33 (4) (2006) 529–535. [13] S.Z. Heris, M. Nasr Esfahany, S.Gh. Etemad, Experimental investigation of convective heat transfer of Al2O3/water nanofluid in circular tube, International Journal of Heat and Fluid Flow 28 (2) (2007) 203–210. [14] W.Y. Lai, B. Duculescu, P.E. Phelan, R.S. Prasher, Convective heat transfer with nanofluids in a single 1.02-mm tube, Proceedings of ASME International Mechanical Engineering Congress and Exposition (IMECE 2006), 2006. [15] J.Y. Jung, H.S. Oh, H.Y. Kwak, Forced convective heat transfer of nanofluids in microchannels, Proceeding of ASME International Mechanical Engineering Congress and Exposition (IMECE 2006), 2006. [16] K.V. Sharma, L. Syam Sundar, P.K. Sarma, Estimation of heat transfer coefficient and friction factor in the transition flow with low volume concentration of Al2O3 nanofluid flowing in a circular tube and with twisted tape insert, International Communications in Heat and Mass Transfer 36 (2009) 503–507. [17] C.J. Ho, L.C. Wei, Z.W. Li, An experimental investigation of forced convective cooling performance of a microchannel heat sink with Al2O3/water nanofluid, Applied Thermal Engineering 30 (2009) 96–103. [18] C.T. Nguyen, G. Roy, C. Gauthier, N. Galanis, Heat transfer enhancement using Al2O3–water nanofluid for an electronic liquid cooling system, Applied Thermal Engineering 27 (2007) 1501–1506. [19] H. Xie, Y. Li, W. Yu, Intriguingly high convective heat transfer enhancement of nanofluid coolants in laminar flows, Physics Letters A 374 (2010) 2566–2568. [20] R. Strandberg, D.K. Das, Finned tube performance evaluation with nanofluids and conventional heat transfer fluids, International Journal of Thermal Sciences 49 (2010) 580–588.
[21] K.Y. Leong, R. Saidur, S.N. Kazi, A.H. Mamun, Performance investigation of an automotive car radiator operated with nanofluid-based coolants (nanofluid as a coolant in a radiator), Applied Thermal Engineering 30 (2010) 2685–2692. [22] L. Godson, B. Raja, D. Mohan La, S. Wongwises, Enhancement of heat transfer using nanofluids—an overview, Renewable and Sustainable Energy Reviews 14 (2010) 629–641. [23] S. Kakaç, A. Pramuanjaroenkij, Review of convective heat transfer enhancement with nanofluids, International Journal of Heat and Mass Transfer 52 (2009) 3187–3196. [24] X.Q. Wang, A.S. Mujumdar, A review on nanofluids — Part II: experiments and applications, Brazilian Journal of Chemical Engineering 25 (4) (2008) 631–648. [25] R.J. Moffat, Describing the uncertainties in experimental results, Experimental Thermal and Fluid Science 1 (1988) 3–17. [26] Y. Xuan, W. Roetzel, Conceptions for heat transfer correlation of nanofluids, International Journal of Heat and Mass Transfer 43 (2000) 3701–3708. [27] X. Wang, X. Xu, S.U.S. Choi, Thermal conductivity of nanoparticles–fluid mixture, Journal of Thermophysics and Heat Transfer 13 (4) (1999) 474–480. [28] W. Duangthongsuk, S. Wongwises, Effect of thermophysical properties models on the predicting of the convective heat transfer coefficient for low concentration nanofluid, International Communications in Heat and Mass Transfer 35 (2008) 1320–1326. [29] N. Masoumi, N. Sohrabi, A. Behzadmehr, A new model for calculating the effective viscosity of nanofluids, Journal of Physics D: Applied Physics 42 (2009) 055501. [30] M. Kole, T.K. Dey, Viscosity of alumina nanoparticles dispersed in car engine coolant, Experimental Thermal and Fluid Science 34 (2010) 677–683. [31] P.K. Namburu, D.P. Kulkarni, D. Misra, D.K. Das, Viscosity of copper oxide nanoparticles dispersed in ethylene glycol and water mixture, Experimental Thermal and Fluid Science 32 (2007) 397–402. [32] J.Y. Huot, E. Battistel, R. Lumry, G. Villeneuve, J.F. Lavallee, A. Anusiem, C. Jolicoeur, A comprehensive thermodynamic investigation of water-ethylene glycol mixtures at 5, 25, and 45 °C, Journal of Solution Chemistry 17 (7) (1988) 601–636. [33] T. Sun, A.S. Teja, Density, viscosity, and thermal conductivity of aqueous ethylene, diethylene, and triethylene glycol mixtures between 290 K and 450 K, Journal of Chemical Engineering Data 48 (2003) 198–202. [34] Z. Nan, B. Liu, Z. Tan, Calorimetric investigation of excess molar heat capacities for water + ethylene glycol from T = 273.15 to T = 373.15 K, The Journal of Chemical Thermodynamics 34 (6) (2002) 915–926. [35] J. Dai, L. Wang, Y. Sun, L. Wang, H. Sun, Prediction of thermodynamic, transport and vapor–liquid equilibrium properties of binary mixtures of ethylene glycol and water, Fluid Phase Equilibria 301 (2011) 137–144. [36] F.W. Dittus, L.M.K. Boelter, Heat Transfer in Automobile Radiators of Tubular Type, University of California Press, Berkeley, CA, 1930, pp. 13–18. [37] R.S. Vajjha, D.K. Das, P.K. Namburu, Numerical study of fluid dynamic and heat transfer performance of Al2O3 and CuO nanofluids in the flat tubes of a radiator, International Journal of Heat and Fluid Flow 31 (2010) 613–621. [38] S.E.B. Maiga, S.J. Palm, C.T. Nguyen, G. Roy, N. Galanis, Heat transfer enhancement by using nanofluids in forced convection flows, International Journal of Heat and Fluid Flow 26 (2005) 530–546. [39] Y. Xuan, Q. Li, Investigation on convective heat transfer and flow features of nanofluids, Journal of Heat Transfer 125 (2003) 151–155. [40] B. Farajollahi, S.Gh. Etemad, M. Hojjat, Heat transfer of nanofluids in a shell and tube heat exchanger, International Journal of Heat and Mass Transfer 53 (2010) 12–17.
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
Experimental study of heat transfer enhancement using water/ethylene glycol based nanofluids as a new coolant for car radiators☆ S.M. Peyghambarzadeh a, S.H. Hashemabadi b,⁎, S.M. Hoseini a, M. Seifi Jamnani a a b
Department of Chemical Engineering, Mahshahr branch, Islamic Azad University, Mahshahr, Iran CFD Research Laboratory, School of Chemical Engineering, Iran University of Science and Technology, Narmak, Tehran, 16846, Iran
a r t i c l e
i n f o
Available online 16 July 2011 Keywords: Water/Al2O3 nanofluid Ethylene glycol/Al2O3 nanofluid Heat transfer enhancement Car radiator Cooling performance Experimental study
a b s t r a c t Traditionally forced convection heat transfer in a car radiator is performed to cool circulating fluid which consisted of water or a mixture of water and anti-freezing materials like ethylene glycol (EG). In this paper, the heat transfer performance of pure water and pure EG has been compared with their binary mixtures. Furthermore, different amounts of Al2O3 nanoparticle have been added into these base fluids and its effects on the heat transfer performance of the car radiator have been determined experimentally. Liquid flow rate has been changed in the range of 2–6 l per minute and the fluid inlet temperature has been changed for all the experiments. The results demonstrate that nanofluids clearly enhance heat transfer compared to their own base fluid. In the best conditions, the heat transfer enhancement of about 40% compared to the base fluids has been recorded. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction After the publication of our previous paper [1] about the application of water/Al2O3 nanofluids instead of pure water in the car radiator and recording the interesting heat transfer enhancement of about 45%, we want to investigate the application of nanoparticle in the mixture of water and anti-freeze materials (as the base fluid) which is conventionally used in the cars' radiators. It is common in the area of cold or hot weathers that some additives are added to the water in the automotive radiator which decrease freezing point and elevate boiling point of water. It keeps the radiator fluid from freezing when it is very cold and keeps the car from overheating on very hot days. Almost all of these additives are from glycol family specially ethylene glycol (EG). The major use of EG is as a medium for convective heat transfer in, for example car radiators, liquid cooled computers, chilled water air conditioning systems, and the like. Because water is a much better engine coolant, the mixture of water and EG has been used. The trouble with water is that it freezes or boils at extreme temperatures. Anti-freezing agents like EG can withstand much greater temperature extremes, so by adding it to water we can make a compromise. Most of the good cooling abilities of water are retained but the ability to withstand extreme temperatures comes from the anti-freeze. As can be seen in Fig. 1, a mixture of 60% EG and 40% water does not freeze to temperatures below − 45 °C. EG disrupts hydrogen bonding when dissolved in water. Pure EG freezes at about −12 °C, but when intermixed with water, the freezing point of the ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (S.H. Hashemabadi). 0735-1933/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2011.07.001
mixture is depressed significantly. The minimum freezing point is observed when the EG percent in water is about 70%, as shown in Fig. 1. However, the boiling point for aqueous EG increases monotonically with increasing EG percentage. Thus, the use of EG not only declines the freezing point but also elevates the boiling point such that the operating range for the heat transfer fluid is broadened on both ends of the temperature scale [2]. It has been proved that conventional fluids, such as water and EG have poor convective heat transfer performance and therefore high compactness and effectiveness of heat transfer systems are necessary to achieve the required heat transfer. Among the efforts for enhancement of heat transfer the application of nanoparticle additives to liquids is more noticeable and currently a large number of investigations are devoted to this subject [3–8]. Nanofluids are formed by suspending metallic or non-metallic oxide nanoparticles (that are significantly smaller than 100 nm) in traditional heat transfer fluids. These so-called nanofluids display good thermal properties compared with fluids conventionally used for heat transfer and fluids containing particles on the micrometer scale. These fluids are a new window which has been opened recently and it was confirmed by several authors that these working fluids can enhance heat transfer performance [9,10]. In the car radiators, the coolant media is pumped through the flat tubes while the air is drawn over the fins by forced convection, thereby heat exchanges between the hot circulating fluid and air. The application of nanofluids in these finned tube radiators may result in several potential benefits including increased heating output for equal liquid flow. These performance impacts, in turn, may be translated into a reduction in total required heat transfer area. Superior heat transfer properties of nanofluids may also result in lower liquid flow
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Nomenclature A Cp d dp f h k L m˙ Nu P Pe Pr Q Re S T VB z
Peripheral area (m 2) Specific heat capacity (J/kg K) Hydraulic diameter of the tube (m) Diameter of the nanoparticles (m) Friction factor Heat transfer coefficient (W/m 2K) Thermal conductivity (W/m K) Length of the tube (m) Mass flow rate (kg/s) Nusselt number = (hdhy/k) Tube periphery (m) Peclet number = Re.Pr Prandtl number = (Cpμ/k) Heat transfer rate (W) Reynolds number = (ρudhy/μ) Cross sectional area of the tube (m 2) Temperature (K) Brownian velocity Axial distance from the tube inlet (m)
Greek letters α Thermal diffusivity (m 2/s) ρ Density (kg/m 3) δ Distance between the centers of the particles (m) μ Viscosity (kg/m.s) φ Nanoparticle volume fraction (%) Φ Shape factor ψ Particle sphericity
Subscripts ave Average b Bulk bf Base fluid exp Experimental in Input nf Nanofluid out Output p Particle w Wall
rate for a given rate of heat transfer, yielding a reduction in the liquid pumping power consumed compared to the base fluid. In order to have more understanding about the application of nanofluids in various heat exchangers, a brief literature survey is performed in this paper. Pak and Cho [11] presented an experimental investigation of the convective turbulent heat transfer characteristics of nanofluids (Al2O3–water) with 1 to 3 vol.%. Their results show that Nusselt number for the nanofluids enhances with increasing of volume concentration and Reynolds number. Heris et al. [12] examined and proved the enhancement of in-tube laminar flow heat transfer of nanofluids (water–Al2O3) in a constant wall temperature boundary condition. In other work, Heris et al. [13] presented an investigation of the laminar flow convective heat transfer of Al2O3–water under constant wall temperature with 0.2 to 2.5 vol.% of nanoparticle for Reynolds number varying between 700 and 2050. They presented again the Nusselt number for the nanofluid which is greater than the base fluid. Lai et al. [14] studied the flow behavior of nanofluids (20 nm Al2O3 nanoparticle in water) in a millimeter-sized stainless steel test tube, subjected to constant wall heat flux and a low Reynolds
Fig. 1. Boiling and freezing points of water/EG mixtures [2].
number (Re b 270). The maximum promotion of Nusselt number for 1 vol.% nanofluid was 8%. Jung et al. [15] conducted convective heat transfer experiments for a nanofluid (Al2O3–water) in a rectangular micro-channel under laminar flow conditions. Their results show the heat transfer coefficient increases by more than 32% for 1.8 vol.% nanoparticle. Sharma et al. [16] implemented 1 to 2.5 vol.% Al2O3 in water in horizontal tube geometry and concluded while the Peclet number is between 3500 and 6000, up to 41% promotion in heat transfer coefficient compared to pure water may have occurred. Ho et al. [17] conducted an experiment for cooling in horizontal tube in laminar flow of Al2O3–water at 1 and 2 vol.% concentrations and concluded the interesting enhancement of 51% in heat transfer coefficient. Nguyen et al. [18] performed their experiments in a microprocessor type cooling heat exchanger and at 6.8 vol.% Al2O3 in water obtained 40% growing in heat transfer coefficient. Xie et al. [19] reported the convective heat transfer enhancement of nanofluids as coolants in laminar flows inside a circular copper tube with constant wall temperature. Different nanofluids consisting of Al2O3, ZnO, TiO2, and MgO nanoparticles were prepared with a mixture of 55 vol.% distilled water and 45 vol.% EG as base fluid. MgO, Al2O3, and ZnO nanofluids exhibited superior enhancements of heat transfer coefficient, with the highest enhancement up to 252% at a Reynolds number of 1000 for MgO nanofluid. The performance of finned tube heating units with nanofluids has been compared mathematically with a conventional heat transfer fluid which comprised of 60% EG and 40% water by Strandberg and Das [20]. Their model predicted an 11.6% increase in finned tube heating output under certain conditions with the 4% Al2O3/60% EG nanofluid and an 8.7% increase with the 4% CuO/60% EG nanofluid compared to heating output with the base fluid. Application of EG based copper nanofluids in an automotive cooling system has been studied by Leong et al. [21]. Relevant input data, nanofluid properties and empirical correlations were obtained from literatures to investigate the heat transfer enhancement of an automotive car radiator operated with nanofluid-based coolants. It is observed that, about 3.8% of heat transfer enhancement could be achieved with the addition of 2% copper nanoparticles in a base fluid at the Reynolds number of 6000 and 5000 for air and coolant respectively. Some extensive reviews in the nanofluid heat transfer have also been published by Godson et al. [22], Kakaç et al. [23] and Wang et al. [24]. The interested reader can refer to them for complete reviewing of the previous studies performed. It should be emphasized that almost no document can be found to describe experimental evaluation of nanofluid performance in the car radiator. In this paper, experimental comparisons have been accomplished between the heat transfer performance of pure water and pure EG and some concentrations of their mixtures in the car radiator.
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How do anti-freeze materials like EG affect the heat transfer performance of the radiator? What happens when you increase the EG concentration? Furthermore, when small amounts of alumina nanoparticle are added to water or EG or their mixtures, does the rate of heat transfer change compared with the base fluids? What will be the effects of operating parameters like nanoparticle concentration, flow rate, and temperature of circulating fluid on the heat transfer performance? These are the main questions which have been answered along this paper. 2. Experimental 2.1. Experimental rig and procedure In order to measure the liquid side heat transfer coefficients in the car radiator, a flow loop shown in Fig. 2(A) has been used. This experimental rig includes a storage tank, a heater, a pump, a flow meter, a forced draft fan, a cross flow finned tube heat exchanger (car radiator), and flow lines. The test fluid flows through the five layer insulated tubes (0.75 inch diameter) from the feed tank to the radiator by a centrifugal pump with constant flow rate of 10 l per
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minute. A recycle line included a globe valve was prepared to obtain the predetermined flow rate. Storage tank (height of 35 cm and diameter of 30 cm) has volume of 30 l and the working liquid would fill 25%. Consequently, the total volume of the circulating liquid is constant in all the experiments. A flow meter (Technical Group LZM15Z Type) was used to control and manipulate the flow rate with the precision of 0.1 l per minute. For heating the working fluid, an electrical heater and a controller were used to maintain the temperature between 40 and 80 °C. Two RTDs (Pt-100 Ω) were implemented on the flow line to record radiator fluid inlet and outlet temperatures. Two other J-type thermocouples were also used for radiator wall temperature measurement. These thermocouples were installed at the center of the radiator surfaces (both sides). Due to very small thickness and very high thermal conductivity of the flat tubes, it is reasonable to equate the inside temperature of the tube with the outside one. The measured temperatures from these thermocouples and RTDs have been shown on two digital monitors with the accuracy of 0.1 °C. All used thermocouples and RTDs were thoroughly calibrated by using a constant temperature water bath, and their accuracy was estimated to be ±0.2 °C.
Fig. 2. A) Schematic of experimental set up. B) Schematic drawing of the applied louvered fin and flat tube radiator and their dimensions.
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Car radiator configuration is louvered fin and flat tube as shown in Fig. 2(B). This air cooler includes 34 vertical aluminum tubes with elliptical cross section. The distances between the tube rows have been filled with thin perpendicular aluminum fins. For cooling the liquid, an axial forced fan (Techno Pars 1400 rpm) was installed close on axis line of the radiator and consequently air and water have indirect cross flow contact. Gamma alumina nanoparticles (0.1, 0.3, 0.5, 0.7, and 1 vol.%) have been added to different base fluids including pure water, pure EG and their mixtures (5, 10, 20 vol.% EG). The mean grain size of this gamma alumina is 20 nm and some of its other properties are shown in Table 1. There was no dispersant or stabilizer added to the nanofluid. This is due to the fact that the addition of any agent may change the fluid properties [11] and the authors were interested to simulate the easiest actual condition encountered in the car radiator. Additionally, creating highly turbulent flow condition in the radiator tubes and connecting pipes can improve the stabilization of the nanoparticle in water. Table 2 shows the ranges of all the experimental parameters.
Table 2 Range of experimental operational conditions. Parameters
Water based nanofluids
EG based nanofluids
Nanoparticle type Nanoparticle concentration (vol.%) Flow rate (l/min) Reynolds number Flow type Inlet temperature
γ-Al2O3 0 to 1
γ-Al2O3 0 to 1
2 to 5 9000 to 23,000 Turbulent 35 to 50 °C
3 to 6 1200 to 2500 Laminar 45 to 60 °C
sphere with volume equal to that of the particle, to the surface area of the particle, and in this paper Φ is considered to be 3. For calculation of water based nanofluid viscosities, the following correlation has been applied [26–28]: 2 μ nf = μ bf 123φ + 7:3φ + 1
ð4Þ
2.2. Uncertainty analysis Uncertainty analysis is carried out by calculating the error of the measurements. The uncertainty range of Reynolds number comes from the errors in the measurement of volume flow rate and hydraulic diameter of the tubes and the uncertainty of Nusselt number refers to the errors in the measurements of volume flow rate, hydraulic diameter, and all the temperatures. According to uncertainty analysis described by Moffat [25], the measurement errors of the main parameters are summarized in Table 3. Furthermore, to check the reproducibility of the experiments, some runs were repeated later which proved to be excellent. 3. Estimation of nanofluid physical properties
ρnf = φρp + ð1−φÞρ bf
ð1Þ
= φ ρCp + ð1−φÞ ρCp ρCp
knf
p
μ nf = μ bf +
ð2Þ
bf
ρp VB d2p 72Cδ
ð5Þ
Where the second term is the apparent viscosity arising from the effects of nanoparticles in the fluid. The distance between the centers of the nanoparticles, δ and correction factor (C) are calculated respectively:
δ=
By assuming that the nanoparticles are well dispersed within the base fluid, i.e. the particle concentration can be considered uniform throughout the system; the effective thermophysical properties of the mixtures can be evaluated using some classical formulas as usually used for two phase flow. The following correlations have been used to predict nanofluid density, specific heat, and thermal conductivity respectively at different temperatures and concentrations [26–28]:
nf
For EG based and mixture based nanofluids, the correlation proposed by Masoumi et al. [29] has been used:
rffiffiffiffiffiffiffi π 3 d 6φ p
ð6Þ
−1
C = μ bf ðaφ + bÞ
ð7Þ
Where a and b are experimental parameters, which for the engine coolant–Al2O 3 nanofluids were estimated to be 0.00004 and 7.1274×10 − 7, respectively [30]. Various correlations were proposed for temperature dependence of the nanofluid viscosity. Kole and Dey [30] showed only that the following correlation proposed by Namburu et al. [31] can give an acceptable agreement to the temperature dependence of viscosity of the Al2O3/conventional coolant nanofluids. log μ nf = M expð NT Þ
kp + ðΦ−1Þkbf −φðΦ−1Þ kbf −kp kbf = kp + ðΦ−1Þkbf + φ kbf −kp
ð8Þ
ð3Þ
Where Φ is empirical shape factor given by Φ = 3/ψ, and ψ is the particle sphericity, that is defined as the ratio of the surface area of a
Table 1 Some characteristics of alumina nanoparticle. Specification
Value
Appearance Purity Grain size (nm) Specific surface area (m2/g) Silicon (Si) (ppm) Calcium (Ca) (ppm) Iron (Fe) (ppm) Cobalt (Co) (ppm)
White powder + 99% 20 200 3.5 1.6 0.2 0.8
Where two parameters (M and N) are functions of nanoparticle concentration [30]. Table 4 depicts pure water and pure EG physical properties. As can be seen in Table 4, these two base fluids show significant differences in the physical properties. The mentioned water/EG mixture properties in Table 4 were used by Dai et al. [35] to develop computational models for the calculations of water/EG physical properties. Their findings were also used whenever no experimental data existed.
Table 3 The uncertainty of the measured parameters. Parameter
Value
Uncertainty
d (mm) Re Nu
6.53 1200 to 23,000 24 to 120
1.6% 5.2% 18%
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Table 4 Physical properties of pure water, pure EG and mixture of water–EG at 40 °C. Physical properties
Water
EG
ρ (kg/m3) μ (kg/m.s) k (W/m °C) Cp (J/kg.°C) α (m2/s) Pr
992 0.00065 0.633 4174 1.5 × 10− 7 4.3
1101 0.0095 0.256 2382 9.8 × 10− 8 93
Water 95%
Water 90%
Water 80%
EG 5%
EG 10%
EG 20%
995a 0.00101b 0.615b 4157c 1.49 × 10− 7 6.8
1002 0.00165 0.6 4090 1.46 × 10− 7 11.2
1008 0.0019 0.58 4020 1.43 × 10− 7 13.1
a
The density of water/EG mixtures have been obtained from [32]. The experimental thermal conductivities and viscosities of water/EG mixtures have been obtained from [33]. c The heat capacities of water/EG mixtures have been extracted from [34]. b
4. Calculation of heat transfer coefficient The heat transfer coefficient and corresponding Nusselt number can be derived as follows [1]: Nu =
˙ p ðTin −Tout Þ mC h:d = k AðTb −Tw Þ
ð9Þ
where m˙ is mass flow rate which is the product of density and volume flow rate of the fluid, Tb is bulk temperature which is assumed to be the average values of inlet and outlet temperatures of the fluid moving through the radiator, and Tw is tube wall temperature which is the mean value measured by two surface thermocouples. In Eq. (9), k is fluid thermal conductivity and d is hydraulic diameter of the flat tube. It should also be mentioned that all the physical properties were calculated at the fluid bulk temperature. 5. Results and discussions
Fig. 3. The results for: A) pure water (Tin = 50 °C) in comparison with Dittus–Boelter correlation [36]. B) Pure EG (Tin = 40 °C) in comparison with the Vajjha et al. correlations [37].
5.1. Heat transfer to pure water and pure EG Before running the experiments on the nanofluids as a coolant for car radiator, some tests with pure water and pure EG were done in order to check the reliability and accuracy of the experimental setup. Fig. 3(A) shows the experimental results for water flow through the radiator at constant inlet temperature of 50 °C. It is shown that the higher Reynolds number increases the heat transfer coefficient of pure water. The experimental data has been compared with following empirical correlation suggested by Dittus–Boelter [36] in turbulent flow: 0:8
Nu = 0:0236Re
0:3
Pr
ð10Þ
Two correlations were developed by Vajjha et al. [37] for nanofluids from the numerical analysis of the flat tube geometry which were shown in Eq. (11): 8 Dh 0:3 d > > RePr ≥33:33 > < Nu = 1:9421 × RePr z z > > D d > : Nu = 6:1 + 0:003675 × RePr h RePr ≤33:33 z z
ð11Þ
The average Nusselt number can be calculated as follow: L
Nuave: =
1 ∫ Nu: dz L0
ð12Þ
Fig. 3(B) represents the comparison of experimental results for pure EG entering the radiator at constant temperature of 40 °C with the prediction of Vajjha et al. [37] correlation in laminar flow. As can be seen in Fig. 3(A) and 3(B), the experimental results show good agreements
with these empirical correlations over the Reynolds number range used in this study. The experimental data in different water temperatures at the radiator inlet including 35, 45, and 50 °C have 10% absolute average error with respect to Dittus–Boelter [36] relation and for all the different EG temperatures at the radiator inlet including 40, 45, and 50 °C, Vajjha et al.'s [37] relation has 6% absolute average error.
5.2. Heat transfer to water based and EG based nanofluids The nanofluid is implemented in different Al2O3 concentrations, i.e. 0.1, 0.3, 0.5, 0.7, and 1 vol.% and at different flow rates of 2, 3, 4, 5, and 6 l per minute. In order to consider the effect of temperature on thermal performance of the radiator, different inlet temperatures have been applied for each concentration. The inlet temperatures include 35, 45, and 50 °C for the water based nanofluids and 45, 50, and 60 °C for EG based nanofluids. Fig. 4(A) and (B) shows the heat transfer enhancement due to the replacement of water as a conventional coolant with water and EG based nanofluids respectively. As can be seen, the ratio of the nanofluid Nusselt number to the base fluid Nusselt number (Nunf/Nubf) has increased by enhancement in the concentrations of nanoparticle at constant Reynolds number for both nanofluids. By the addition of only 1 vol.% of Al2O3 nanoparticle into the water or EG, an increase of about 40% in comparison with the pure water and pure EG Nusselt number was recorded. The Nusselt number increases monotonously with Reynolds number, but Nunf/Nubf presents different variation tendencies with Reynolds number for these two nanofluids. For the water based nanofluid it is obvious from Fig. 4(A) that Nunf/Nuw increases with Reynolds number and in higher concentrations of nanoparticle the effect of Reynolds number becomes pronounced. This result is in
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that an increase in the fluid inlet temperature (in the range of our experiments) slightly improves the heat transfer performance. Inspecting the results reveals that increasing the inlet temperature of water based nanofluids from 35 °C to 50 °C can enhance Nusselt number up to 16%. For EG based nanofluids, the temperature elevation from 45 to 60 °C creates maximum enhancement of 7%. This variation in Nusselt number may be attributed to the effect of temperature on the physical properties and also to the increased effect of test liquid radiation to the internal wall of the tubes. 5.4. Heat transfer to water/EG binary mixture nanofluids A conventional fluid usually used in the car radiator is the mixture of water/EG in different concentrations depending on the regional weather. In order to have more insights to the influence of nanoparticle addition in the radiator coolant, three different concentrations of water/EG binary mixtures which include 5, 10, and 20 vol.% EG were prepared as the base fluids. Four different values of Al2O3 nanoparticle (0, 0.05, 0.15, and 0.3 vol.%) were added to each concentration of water/EG mixtures and finally the effect of flow rate on the heat transfer performance was studied for each case. All the obtained experimental data are summarized in Table 5. The results obtained for pure water and pure EG based nanofluids (which were shown previously) were also presented in this table for more comfortable comparison of the results. Furthermore, due to the large variations in the physical properties of the base fluids, it is not possible to imply Nusselt number as a function of Reynolds number. Reynolds number greatly reduced when EG concentration enhances. At extreme conditions, Reynolds number changes between 9000 and 23,000 for water based nanofluids while it changes between 1200 and
Fig. 4. Variations of dimensionless Nusselt number at different Reynolds numbers as a function of nanoparticle concentration (Tin = 45 °C). A) Water based nanofluids. B) EG based nanofluids.
contradiction to the EG based nanofluids in which Nunf/NuEG exhibited irregular trends against Reynolds number, as can be seen in Fig. 4(B). It was shown that when small amounts of Al2O3 nanoparticles are added to the base fluids, the density and the thermal conductivity increase and the specific heat decreases slightly while the viscosity increases more markedly compared to the base fluid [1]. These variations, however, are too small (of about 4%) to explain heat transfer enhancement of up to 40% gained in this study. Heris et al. [12] have done experiments with Al2O3 nanoparticles in water under laminar flow up to turbulence. They found the heat transfer enhancement as high as 40% with Al2O3 particles while the thermal conductivity enhancement was less than 15%. Many researchers have suggested that in fact Brownian motion is one of the most important factors in the enhancement of heat transfer. The presence of nanoparticles and their random motion within the base fluid cause the thickness of thermal boundary layer to reduce and it has important contribution to such heat transfer improvement [38]. This random motion of ultra-fine particles would create a slip velocity between the solid particles and the fluid medium [23]. Xuan and Roetzel [26] also suggested the role of small perturbations in the temperature and velocity formulation to account for the Brownian motion. 5.3. Effect of temperature on heat transfer coefficient of nanofluids Fig. 5(A) and (B) compares the nanofluid Nusselt numbers at different inlet temperatures in order to analyze the effect of temperature variation on the heat transfer performance of the car radiator. It is clear
Fig. 5. Effect of inlet temperature on the Nusselt numbers. A) Water based nanofluid at the nanoparticle concentration of 1 vol.%. B) EG based nanofluid at the nanoparticle concentration of 0.7 vol.%.
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2500 for EG based nanofluids. It should be mentioned that all the data in Table 5 have been obtained at the constant liquid inlet temperature to the radiator namely 45 °C. Data in Table 5 obviously indicate that higher Nusselt numbers have been obtained for the pure water compared to pure EG at constant flow rate (more than twice). This is basically due to the sharp differences in the physical properties of the two pure liquids. For the sake of clarity, some of the important physical properties of water and EG were compared in Table 2 at the temperature of 45 °C. The Prandtl number (Pr) as an important parameter affecting the heat transfer coefficient differs greatly in these two liquids and it can be the main reason for the lower Nusselt number in EG compared to water. Furthermore, it is obvious from Table 5 that the addition of EG into water decreases the Nusselt numbers. It is the reason for mixing of EG with water; water does a much better job at the cooling of the engine. In all the experimental data shown in Table 5, it is proven that the addition of nanoparticle to each base fluid enhances the heat transfer coefficient. This effect manifests itself at water/EG binary mixtures with lower concentrations of EG. Different correlations were introduced for the prediction of nanofluid forced convection heat transfer in the literature up to now. Xuan and Li [39] suggested two empirical correlations for laminar and turbulent flow of the nanofluids in tube: 0:218 0:754 0:333 0:4 Nu = 0:4328 1 + 11:285φ Re Pe Pr 0:001 0:6886 0:9238 Re Pe Nu = 0:0059 1 + 7:6286φ
Re b 2100 ð13Þ Re b 2100
ð14Þ
The experimental data obtained in the present investigation have been compared with these two correlations. Fig. 6(A) and (B) compare the experimental results for the Nusselt number of Al2O3/ water and Al2O3/EG nanofluids respectively with the prediction of Xuan and Li correlations. Very good agreement can be seen in these two figures. Calculating the absolute average errors reveals that the prediction error for water based nanofluids is 7% and for EG based nanofluids is 12.5%. As can be seen in Fig. 6, Xuan and Li correlations almost over-predict Nusselt number for these nanofluids. The overprediction of these empirical correlations has been mentioned before by other investigators in the case of other nanofluids [40]. 6. Conclusion In this paper, the convective heat transfer enhancement of water and EG based nanofluids as the coolants inside flat aluminum tubes of the car radiator has been investigated. Significant increases of the
Table 5 Experimental Nusselt numbers measured for different concentrations of EG/water mixtures as the base fluid (inlet temperature 45 °C). φ (%) Flow rate (l/min)
EG 5% water 95% 0
0.05
0.15
0.3
EG 10% water 90% 0
0.05
0.15
0.3
3 4 5 6
63.93 79.26 89.15 101.87
66.58 81.56 92.59 104.95
71.03 86.50 97.19 109.31
76.67 92.24 103.16 114.56
63.28 79.45 91.27 99.99
66.22 81.98 93.93 108.07
71.94 87.53 99.99 111.55
77.50 92.54 104.38 114.32
Fig. 6. Comparison of measured Nusselt numbers with those predicted from Xuan and Li correlation [39]. A) Water based nanofluid. B) EG based nanofluid.
total heat transfer rates have been observed with the nanoparticle addition. A highest Nusselt number enhancement up to 40% was obtained at the best conditions for both nanofluids. The experimental results have demonstrated that the heat transfer behaviors of the nanofluids were highly depended on the particle concentration and the flow conditions and weakly dependent on the temperature. Our results indicate that nanofluids have great potential for heat transfer enhancement and are highly suited to apply in practical heat transfer processes. This provides promising ways for engineers to develop highly compact and effective radiators for cars. These higher heat transfer coefficients obtained by using nanofluid instead of water allow the working fluid in the car radiator to be cooler. The addition of nanoparticles to the coolant has the potential to improve automotive and heavy-duty engine cooling rates or equally causes to remove the engine heat with a reduced-size cooling system. Smaller cooling systems result in smaller and lighter radiators, which in turn benefit almost every aspect of vehicle performance and lead to increased fuel economy. References
φ (%) Flow rate (l/min)
EG 20% water 80%
Pure EG
Pure water
0
0.05
0.15
0.3
0
0.3
0
0.3
3 4 5 6
61.59 79.35 90.48 90.77
63.45 82.43 92.55 101.38
68.78 87.29 98.83 107.23
71.78 90.81 102.40 110.55
24.69 30.23 33.67 37.47
27.01 32.63 36.00 40.20
67.76 83.72 96.46 -
76.77 99.28 115.69 -
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