Investigation of a two-phase flow natural circulation loop with divergent microchannel evaporator

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.The development of microelectronics is toward high performance, high efficiency and yet small size. Thermal management of microelectronics is of critical concern and significant interest. Microchannel boiling is an advanced cooling technology for high heat flux devices. The present study explores heat removal capability of a two-phase natural circulation loop with divergent microchannel evaporator. Our previous studies revealed that a diverging cross section design significantly could stabilize and enhance the heat transfer of flow boiling. The temperatures at the inlet and outlet of both evaporator and condensing units are measured to evaluate the heat removal capability of the loop. Moreover, the pressure changes through the downcomer and lower horizontal tube are both measured to deduce the flow rate through the loop based on the relationship between flow rate and pressure drop. This study uses the high speed video camera to capture the flow patterns in the evaporator and riser. The working fluid employed in the present study is ethanol, as its boiling temperature at atmospheric pressure is 78.4 ℃, which is below the temperature limit of the most microelectronic materials. The results show that the loop mass flow rate increases monotonically with increasing the heating power of the evaporator after boiling incipience. The current experimental results indicate that the highest base heat flux could achieve is about 105 kWm-2 with no sign of dry-out and it has great potential to reach a higher heat flux. Moreover, it is found that the loop instability appears at low heating powers after boiling begins, while it can be suppressed if the input power is higher than 20W. Indeed, the present two-phase natural circulation loop with divergent microchannel evaporator demonstrates stable circulation with high heat transfer capability.