Experimental study on heat transfer characteristics of the phase change chambers for heat flux enhancement

One promising way to increase the output power and the thermoelectric conversion efficiency of an automotive exhaust thermoelectric generator (TEG) is to add a phase change chamber between the exhaust channel and the thermoelectric (TE) modules with the chamber surface contacting with the exhaust channel where boiling occurs being larger than that with the module surfaces where the condensation occurs. The power is increased because a phase change chamber significantly increases the heat flux through the modules; thus, increasing the temperature difference between the module hot and cold sides. Although there is a significant body of literature on phase change chambers, most prior research has focused on chambers with larger condensation areas than boiling areas since they act as heat spreaders, while little is on chambers with condensation areas smaller than boiling areas for heat flux enhancement. These chambers have great potential to improve TEG power generation by enhancing the heat flux, therefore, more studies are needed to understand the chamber heat transfer characteristics. This research experimentally investigated the heat transfer characteristics of three chambers with condensation to boiling area ratios of 1/5, 2/5, and 3/5 for boiling heat fluxes from 4.50 to 22.70 kW/m2 and deionized water filling ratios from 33 to 50%. The results show that the boiling heat transfer coefficient increases with increasing heat flux or decreasing area ratio and is weakly influenced by the filling ratio. The condensation heat transfer characteristics depend on the distribution of droplets which is strongly affected by both the heat flux and the area ratio. The influence of the filling ratio on the condensation heat transfer coefficient increases with increasing area ratio. The thermal resistance of the chamber decreases with increasing heat flux and the lowest thermal resistance is obtained at 1/5 area ratio at lower heat fluxes and at 2/5 or 3/5 area ratio at higher heat fluxes.