Investigation of the effect of TiO2 based nanofluid usage on heat transfer performance in automobile radiator

In the context of this thesis, effects of using nanofluids on heat transfer performance in automobile radiators was investigated. 4 different types of nanoparticles were prepared to be used in nanofluids; (i) pure TiO_2, (ii) TiO_2 doped with 0.1% Ag, (iii) TiO_2 doped with 0.3% Ag, (iv) TiO_2 doped with 0.1% Cu. These nanoparticles added into base-fluid (50:50 ethylene glycol – water mixture by volume) with ratios of 0.3, 0.5, 1 and 2%. The experiments were carried out for different flow rates (17, 19, 21, 23 and 25 l/min) in an experimental setup where a real automobile radiator was used and specifically designed to simulate the real – environment as best as it could. With the given flow rates, Reynolds number (Re) were calculated for base-fluid and nanofluids and found to be varying from 337 to 496 (base fluid) and from 370 to 830 (nanofluids). Air side of the radiator had constant air flow rate (V ̇=3500 m^3/h,Re=930). To compare heat transfer performance of each nanofluid under same conditions, all experiments were conducted with a constant power heat source (8.6 kW). The main goal of this thesis is to substitute the traditional cooling fluids that are used in automobile radiators with nanofluids to increase thermal performance. The benefit of this study is to contribute to the development of an automobile with lesser weight and thus lesser fuel consumption due to the downsizing of its cooling system. In a broader scope, outcomes of this thesis will aid to transportation industry. To achieve this goal, several innovative approaches were taken. It is known that metal oxides doped with another metal show completely different and sometimes better characteristics when compared to their pure forms. However, literature is in absence of studies that are focused on preparation of nanofluids based on TiO_2 doped with metals with high heat transfer coefficient such as Cu and Ag and usage of these nanofluids in cooling systems. Applications of new types of nanofluids in model engine cooling system (automobile radiator) where even the usage of nanofluids are innovative, investigation of relationship between particle features and engine cooling system parameters ensures this thesis to be innovative, unique and comprehensive. Each nanofluid is different than the other one. This is can be due to their nanoparticle composition or their chosen preparation method. Thus, it is difficult to calculate their thermophysical properties with empirical equations and this often leads to serious errors. Due to this threat, thermophysical properties of nanofluids and nanoparticles that prepared and used in context of this thesis are calculated from experimental observations while considering changes in the temperature and used for precise calculations and presented to the scientific community. In the process of preparation of nanofluids, no additional chemicals are used to increase stability. As any chemical that is used adds a new feature to the mixture departs nanofluids from simplicity. With all these aspects, this thesis is different than the majority of the literature. Results of this study evaluated as both experimentally and theoretically. According to experimental results a net change was observed in thermal conductivity of 0.3% Ag doped nanofluids. Increase in nanoparticle concentration of nanofluids decreased the log mean temperature difference and total heat transfer coefficient is increased. The highest increase for nanofluids with concentrations of 1% and 2% was found to be 5.615% and 11.094%, respectively. According to the results of performance analysis where increase in heat transfer and pressure losses are considered together, same nanofluids also fulfill η>1 condition (i.e. performance factor) experimentally. Theoretical analyses were conducted to deduce the maximum potential of nanofluids in the absence of operational problems. According to Mouromtseff number where nanofluids are compared on thermophysical property basis, all 13 nanofluids are determined to be better than the base-fluid. This potential can be order ascendingly from pure TiO_2 to 0.1% doped nanofluids and to 0.3% doped nanofluids. It is safe to say that doping with Ag improves heat transfer features of nanoparticles, and this improvement is consistent with increase in doping amount. Measured thermal conductivity of nanofluids also support these results. According to the results of the theoretical analysis, increase in convective heat transfer coefficient were determined to be 26.15% and 27.72% for 0.3% Ag doped nanofluids with concentration of 1% and 2%, respectively.