Experimental and Numerical Investigation of Subcooled Flow Boiling Heat Transfer in Conventional Channels

Abstract

Electric vehicles (EVs) and hybrid electric vehicles (HEVs) have quickly become a technological focus to reduce fossil fuel usage and limit negative environmental impacts such as climate change, regional smog, and ground level ozone over the past decade. The battery is a crucial component in the development of EVs and HEVs due to its specific energy, cycling life, and high power. It is crucial to dissipate heat from the battery module and maintain its temperature within the required range, which otherwise, could lead to battery life degradation and reduction in performance. To manage high heat fluxes effectively, it is important to have an efficient thermal management system. One method that shows promise is liquid cooling, which extracts heat from the cold plate. The cold plates will have conventional channels for the coolant to flow through. Using mixtures as a cooling medium have several advantages over using pure fluids as a coolant. These include better adjustment with the intended thermal load, improved system coefficient of performance, and more environmentally friendly and safer fluids. The amount of heat extraction from the cold plate is greater in two phase flow boiling heat transfer than in the single phase convective heat transfer. Hence two phase heat transfer using binary mixtures has attracted wide attention as they provide a wider range of boiling temperature at a given pressure. Further, by geometrical modification of the channels, enhancement in the heat dissipation rate can be achieved. In view of the above facts, the heat extraction from the conventional channels under two phase flow boiling condition with ethanol-water mixture as coolant is investigated in this research work. The flow boiling heat transfer characteristics of the water, ethanol and ethanol-water mixture (25%/75% by volume) were experimentally investigated in conventional rectangular channels. Test sections of channel aspect ratio (AR=w/d) 0.2 and 1.25 were examined for the thermal performance under horizontal flow boiling conditions. The effect of ribs in the flow channels of AR=1.25 on the heat transfer characteristics were studied. Experiments were conducted for different values of mass flux, heat flux and subcooled inlet temperature. The investigation was carried out at atmospheric pressure. i Flow patterns were recorded by employing a Promon-501 high-speed camera, and stages in the bubble cycle were studied. The results show that the AR has a dominant effect on the heat transfer coefficient (HTC). At low heat flux values, higher HTC was observed for the channel of higher AR (AR= 1.25) whereas, at high heat flux conditions, the HTC is higher for the channel of lower AR (AR= 0.2). For all the working fluids, high HTC was observed for the AR=1.25 channel in the forced convective region, whereas in the subcooled boiling region, high HTC was observed for the AR=0.2 channel. The average subcooled HTC obtained for the ethanol-water mixture was 15.75% and 38.85% higher than that of water and ethanol respectively for the AR=0.2 channel. However, it was 18.11% and 41.2% higher than that for water and ethanol for the AR=1.25 channel. With the aid of visualization results, it was found that the bubble waiting and the growth period were minimum for the mixture and maximum for the ethanol, resulting in a higher HTC for the mixture and lower HTC for ethanol in both channels. With an increase in inlet subcooled temperature, the HTC decreased for both channels due to increased thermal boundary layer thickness and reduced bubble formation. Furthermore, the channel of AR=1.25 with ribs performed better than the smooth channel due to the high bubble nucleation rate. Along with the experimentation, numerical investigation was also carried out by using ANSYS Fluent 2022R1 commercial software. The numerical simulation was performed by selecting the Rensselaer Polytechnic Institute (RPI) wall heat flux partitioning approach by employing the Eulerian-Eulerian two-phase model. A three-dimensional computational domain was used for simulation to understand the fluid boiling inside the conventional channel under steady state conditions. The focus of the numerical investigation was to determine the vapor fraction which could not be measured experimentally. In addition to the two test sections considered for experimentation, for numerical work, another two test sections, (AR=0.5 and AR=1) were considered. The simulations were performed for a constant mass flux of 150.46 kg/m2-s with the heat flux value ranging from 10-100 kW/m2 and at the inlet subcooled temperatures of 303K, 313K and 323K. ii The channel surface temperature and the HTC obtained numerically were compared with the experimental results and it was found that the results are in good agreement. The volume of vapor fraction increased with the increase in heat flux for all values of inlet subcooled temperature considered in this study for all the test sections. At low inlet subcooled temperature, the volume of vapor fraction decreased with an increase in AR at all heat fluxes. However, there was no observable trend at higher heat flux and at high inlet subcooled temperature.