Improving the cooling performance of automobile radiator with Al₂O₃/water nanofluid

Abstract

In this paper, forced convective heat transfer in a water based nanofluid has experimentally been compared to that of pure water in an automobile radiator. Five different concentrations of nanofluids in the range of 0.1–1 vol.% have been prepared by the addition of Al₂O₃ nanoparticles into the water. The test liquid flows through the radiator consisted of 34 vertical tubes with elliptical cross section and air makes a cross flow inside the tube bank with constant speed. Liquid flow rate has been changed in the range of 2–5 l/min to have the fully turbulent regime (9 × 10³ < Re < 2.3 × 10⁴). Additionally, the effect of fluid inlet temperature to the radiator on heat transfer coefficient has also been analyzed by varying the temperature in the range of 37–49 °C. Results demonstrate that increasing the fluid circulating rate can improve the heat transfer performance while the fluid inlet temperature to the radiator has trivial effects. Meanwhile, application of nanofluid with low concentrations can enhance heat transfer efficiency up to 45% in comparison with pure water.

Introduction

A reduction in energy consumption is possible by improving the performance of heat exchange systems and introducing various heat transfer enhancement techniques. Since the middle of the 1950s, some efforts have been done on the variation in geometry of heat exchanger apparatus using different fin types or various tube inserts or rough surface and the like [1], [2], [3], [4], [5], [6], [7]. Some of the published investigations have focused on electric or magnetic field application or vibration techniques [8], [9], [10], [11]. Even though an improvement in energy efficiency is possible from the topological and configuration points of view, much more is needed from the perspective of the heat transfer fluid. Further enhancement in heat transfer is always in demand, as the operational speed of these devices depends on the cooling rate. New technology and advanced fluids with greater potential to improve the flow and thermal characteristics are two options to enhance the heat transfer rate and the present article deals with the latter option.

Conventional fluids, such as refrigerants, water, engine oil, ethylene glycol, etc. have poor 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 additives to liquids is more noticeable. Recent advances in nanotechnology have allowed development of a new category of fluids termed nanofluids. Such fluids are liquid suspensions containing particles that are significantly smaller than 100 nm, and have a bulk solids thermal conductivity higher than the base liquids [12]. Nanofluids are formed by suspending metallic or non-metallic oxide nanoparticles 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 [13]. Nanofluids are the new window which was opened recently and it was confirmed by several authors that these working fluid can enhance heat transfer performance.

Pak and Cho [14] presented an experimental investigation of the convective turbulent heat transfer characteristics of nanofluids (Al₂O₃–water) with 1–3 vol.%. The Nusselt number for the nanofluids increases with the increase of volume concentration and Reynolds number.

Wen and Ding [12] assessed the convective heat transfer of nanofluids in the entrance region under laminar flow conditions. Aqueous based nanofluids containing Al₂O₃ nanoparticles (27–56 nm; 0.6–1.6 vol.%) with sodium dodecyl benzene sulfonate (SDBS) as the dispersant, were tested under a constant heat flux boundary condition. For nanofluids containing 1.6 vol.%, the local heat transfer coefficient in the entrance region was found to be 41% higher than that of the base fluid at the same flow rate. Heris et al. [15] 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. [16] presented an investigation of the laminar flow convective heat transfer of Al₂O₃–water under constant wall temperature with 0.22.5 vol.% of nanoparticle for Reynolds number varying between 700 and 2050. The Nusselt number for the nanofluid was found to be greater than that of the base fluid; and the heat transfer coefficient increased with an increase in particle concentration. The ratio of the measured heat transfer coefficients increases with the Peclet number as well as nanoparticle concentrations.

Lai et al. [17] studied the flow behavior of nanofluids (Al₂O₃–water; 20 nm) in a millimeter-sized stainless steel test tube, subjected to constant wall heat flux and a low Reynolds number (Re < 270). The maximum Nusselt number enhancement of the nanofluid of 8% at the concentration of 1 vol.% was recorded. Jung et al. [18] conducted convective heat transfer experiments for a nanofluid (Al₂O₃–water) in a rectangular microchannel under laminar flow conditions. The convective heat transfer coefficient increased by more than 32% for 1.8 vol.% nanoparticle in the base fluids. The Nusselt number increased with an increasing Reynolds number in the laminar flow regime (5 < Re < 300) and a new convective heat transfer correlation for nanofluids in microchannels was also proposed.

Sharma et al. [19] implemented 12.5 vol.% Al₂O₃ in water in a horizontal tube geometry and concluded that at Pe number of 3500 and 6000 up to 41% promotion in heat transfer coefficient compared to pure water may be occurred. Ho et al. [20] conducted an experiment for cooling in horizontal tube in laminar flow of Al₂O₃–water at 1 and 2 vol.% concentration and concluded the interesting enhancement of 51% in heat transfer coefficient. Nguyen et al. [21] performed their experiments in the radiator type heat exchanger and at 6.8 vol.% Al₂O3 in water obtained 40% increase in heat transfer coefficient.

Some extensive reviews in nanofluid heat transfer have also been published by Godson et al. [22], Kakaç and Pramuanjaroenkij [23] and Wang and Mujumdar [24]. The interested reader can refer to them for complete reviewing of the previous studies performed.

In this paper, forced convection heat transfer coefficients are reported for pure water and water/alumina nanopowder mixtures under fully turbulent conditions. The test section is made up with a typical automobile radiator, and the effects of the operating conditions on its heat transfer performance are analyzed.