Heat transfer enhancement by using nanofluid in the automotive cooling system

Abstract The increasing demand of nanofluids for the industrial applications has been led to focus on it from many researchers in the last decade. This thesis includes both experimental and numerical study to improve heat transfer with slightly pressure drop in the automotive cooling system. The friction factor and heat transfer enhancement using different types of nanofluids are studied. The TiO2 and SiO2 nanopowders suspended to four different base fluids (pure water, EG, 10%EG+90%W and 20%EG+80%W) are prepared experimentally. The thermophysical properties of both nanofluids and base fluids are measured and validated with the standard and the experimental data available. The test section is setup including car radiator and the effects under the operating conditions on the heat transfer enhancement analyzed under laminar flow condition. The volume flowrate, inlet temperature and nanofluid volume concentrations are in the range of (1-5LPM) for pure water and (3-12LPM) for other base fluids, (60-80 oC) and (1- 4%) respectively. On the other side, the CFD analysis for the nanofluids flow inside the flat tube of a car radiator under laminar flow is carried out. A simulation study is conducted by using the finite volume technical to solve the continuity, momentum and energy equations. The processes of the geometry meshing of problem and describing the boundary conditions are performed in the GAMBIT then achieving of FLUENT software to find the friction factor and heat transfer coefficient. The experimental results show the friction factor decreases with the increase of the volume flowrate and increases with the nanofluid volume fraction but slightly decreases with the increase of the inlet temperature. Furthermore, the simulation results show good agreement with the experimental data with deviation, not more than 4%. The experimental results show the heat transfer coefficient increases with the increase of the volume flowrate, the nanofluid volume fraction and the inlet temperature. Likewise, the simulation results show good agreement with the experimental data with deviation not more than 6%. In additions, the SiO2 nanofluid appears high values of the friction factor and heat transfer coefficient than TiO2 nanofluid. Also, the base fluid (20%EG+80%W) gives high values of the heat transfer coefficient and proper values of friction factor than other base fluids. It seems that the SiO2 nanoparticles dispersed to (20%EG+80%W) base fluid is a significant enhancement of the thermal properties than others. It observed, the SiO2 nanoparticles dispersed to (20%EG+80%W) base fluid is significant augmentation of heat transfer in the automobile radiator. The regression equations among input (Reynolds number, Prandtl number and nanofluid volume concentration) and response (friction factor and Nusselt number) are found. The results of the analysis indicated that a significant input parameters to enhance heat transfer with the automotive cooling system. The comparison between experimental results and other researchers’ data are conducted, and there is a good agreement with a maximum deviation approximately 10%.

1      Introduction

The general definition of convection is the energy transfer between the surface and fluid due to the temperature difference and this energy transfer by either forced (external, internal flow) or natural convection (Kays et al., 1993). Forced convection is a mechanism, or type of transport in which fluid motion is generated by an external source like (a fan, a pump, a suction device, etc.). It should be considered as one of the main methods of useful heat transfer as significant amounts of heat energy can be transported very efficiently, and this mechanism is found very commonly in everyday life, including air conditioning, central heating, steam turbines and in many other machines. Forced convection is often encountered by engineers designing or analyzing heat exchangers, flow over a flat plate and pipe flow at a different temperature than the stream (Incropera, 2001). The increasing demand for more efficient heat transfer fluids in many applications has been led to enhance heat transfer to meet the cooling challenge necessary such as the photonics, transportation, electronics and energy supply industries (Das et al., 2007). Conventional fluids now a day are inherently poor heat transfer fluids and with the increasing demand of industries micro-sized heat generating systems, are unable to provide adequate heat transfer. One of the possible solutions to this limited capability can be achieved by integrating the high heat transfer capability of solid metals into a flowing heat transfer fluid. In the past, attempts have been conducted to add micro sized metal particles into conventional liquids, which ended to significant results as well as large disadvantages. Flow characteristics such as viscosity will change which led to the need to higher pumping power due to add the micro particles,. There is also the concern of agglomeration over time as well as corrosively of the system which can both result in high maintenance demands. The concept of nanofluids refers to a new kind of heat transfer fluid formed by dispersing nano-scaled metallic or nonmetallic particles in base fluids (water, ethylene glycol and oil). Energy transport of the nanofluid is affected by the properties and dimension of nanoparticles as well as a solid volume fraction. Some experimental investigations have revealed that the nanofluids have remarkably higher thermal conductivities than those of conventional pure fluids and have great potential for heat transfer enhancement. The addition of nano-sized particles is very proper to augment heat transfer as compared with the adding millimeter or micrometer sized to liquids with little penalty in pressure drop. A possible effective method for heat enhancement is to include high thermal conductivity particles in the liquid. Some general examples of applications that can benefit from this technology include home heating and cooling appliances, automotive radiator systems, power plant cooling systems, computer processing cooling equipment and more examples including heat are transferred from one medium to another. The use of high conductivity heat transfer materials will lead to benefit fully the available energy of a system which will reduce the environmental footprint of companies as well as their operating costs. It is believed that the most important reasons for enhance heat transfer of the nanofluids may be from intensification of a turbulence eddy, repression or interruption of the boundary layer as well as nanoparticles suspension. Therefore, the convective heat transfer coefficient of nanofluids is a function of properties, volume fraction of suspended nanoparticles, dimension as well as the flow velocity. Take advantage of the nanoparticles in the liquid causes the particles to stay in the solution for a long time. Another feature is that these particles have large surface area for thermal conductivity than ordinary liquids. From an engineering point of view, forced convection utilizing liquid coolants in laminar or turbulent flow regimes are always a key heat transfer solution (Manglik, 2004). The better convective heat transfer performance means that higher values of heat transfer coefficient. There are a number of techniques to enhance heat transfer like modified heat transfer surface roughness, fins (extended surfaces), injection, and so on. However, these techniques have been led to higher pressure drop and hence lift pumping power requirement. Also, with low thermal conductivity and high viscosity of conventional heat transfer fluids as water, ethylene glycol, oil and ammonia the convective thermal performance created barriers in designing small heat rejecting devices. Therefore, an innovative coolant with improving heat transfer properties is required. The solid particles, usually exhibit high thermal conductivity than liquids, one approach for enhance thermal conductivity of liquids is by using suspensions, which contain dispersed particles into base fluids. One of the pioneering researchers of stationary, dilute, dispersions of solid spheres has been studied by (Maxwell, 1891). Ahuja A.S. (1975) performed number of tests on thermal conductivity and heat transfer coefficients of 40-100 μm-sized polystyrene-water based solutions with 1-mm inside tube. Furthermore, the effective thermal conductivity of the suspension increases with the increasing of Reynolds number and nanofluid volume fraction. Because of shortage available technology in those years the particles size was large (in micro- scale). So this size has been led to two penalties the first on are not stable enough, and other are the larger particles can easily cause erosion to flow loop components.

M.M.Noor