Experimental study of heat transfer enhancement using water/ethylene glycol based nanofluids as a new coolant for car radiators

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Abstract

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.

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 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], [4], [5], [6], [7], [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 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 number (Re < 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. 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.

Section snippets

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 minute. A recycle line

Estimation of nanofluid physical properties

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], [27], [28]:ρnf=φρp+1φρbfρC

Calculation of heat transfer coefficient

The heat transfer coefficient and corresponding Nusselt number can be derived as follows [1]:Nu=h.dk=m˙CpTinToutA(TbTw)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

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

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 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

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    Communicated by W.J. Minkowycz

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