Studies on Heat Transfer and Pressure Drop Characteristics of Nanofluids in Flat Vertical Tubes

Abstract

Cooling is one of the critical problems being faced by the modern industry due to the technological development such as microelectronic devices, high power engines, and ultrahigh heat-flux optical devices. Conventional heat transfer fluids such as water and ethylene glycol have been used for cooling purposes in automobile radiators; however, the heat transfer performance of these liquids is limited owing to their poor thermo-physical properties. An enhancement in the heat transfer performance of these fluids can result in improved performance of the automobile engines. Nanofluids can be used in heat exchangers and automobile cooling systems due to their better features than micrometer and millimeter sized particles i.e., high specific surface area, less erosion of components, low pumping power, etc. In this work two types of nanoparticles such as aluminum oxide (Al2O3) and copper oxide (CuO) were used to prepare the nanofluids. The objective of the work is estimation of thermo physical properties of different concentrations of Al2O3 and CuO nanoparticles at different temperatures. The various thermophysical properties of prepared nanofluids such as thermal conductivity, density, specific heat and viscosity were measured experimentally using KD2 Pro, specific gravity bottle, differential scanning calorimeter and viscometer, respectively. The stability of nanofluids is checked by Zeta potential, measuring the absorbance using UV-vis spectrophotometer and thermal conductivity. The aluminum oxide nanofluids remain more stable than copper oxide nanofluids. The thermal conductivity enhanced significantly with increase in particle concentration and temperature. The thermal conductivity of nanofluids was more sensitive to temperature than that of the base fluid. The density and viscosity increased with increasing the particle concentration, while they both decreased with increase in temperature. The heat capacity of nanofluids increases with temperature but diminishes with increase in particle volume concentration. The experimental study has been reported so far on the heat transfer performance of nanofluids in flat tubes of a radiator under turbulent flow conditions at which most of the automobile radiators are operated. Therefore, the main objective of the present work is to study the heat transfer and pressure drop performance of alumina-water and copper–water nanofluids in a flat vertical tubes under forced turbulent convection. -vi- The nanoparticles plays a vital role in enhancing the Nusselt number and suspending the nanoparticles in base fluid leads to an increase in heat transfer coefficient for flat vertical tubes of radiator. The heat transfer rate increased with increase in fluid inlet temperature, particle concentration, Reynolds number as well as air inlet velocity. The pressure drop increased with increasing the Reynolds number and particle volume concentration, while it slightly decreased with increase in fluid inlet temperature because density and viscosity decrease with increase in temperature. The friction factor (f) of nanofluid in the flat tube increased with increase in Al2O3 and CuO nanoparticle concentration. However, it decreased with increase in Reynolds number and the lowest friction factor value was obtained for the base fluid. The pumping power needed by nanofluids was considerably higher than that needed for the base fluid. The heat transfer performance of compact heat exchanger evaluated at various fluid inlet temperature such as 40 oC, 50 oC, 60 oC, 70 oC and 80 oC and at Reynolds number range from 5000 to 14000 and water based nanofluids in the range of 0.25% v/v to 1% v/v concentration of alumina oxide and copper oxide nanoparticles. The heat transfer rate increased with increase in fluid inlet temperature, particle concentration, Reynolds number as well as air inlet velocity. Heat transfer enhancement was more than the enhancement in the thermal conductivity of nanofluid at same temperature and concentration. This indicates that besides thermal conductivity, other factors such as fluid inlet temperature, Reynolds number and air velocity also affect heat transfer coefficient. This study proves that the size of automotive cooling system can decreased with the use of nanofluids in place of conventional cooling fluids because the heat transfer is the primary concern for cooling systems.