Heat transfer enhancement due to surface modification in the close-loop R410A flash evaporation spray cooling

Flash spray cooling has been considered as one of the most promising technologies of heat dissipation for high power electronic devices because of its high cooling capacity at low surface temperature. In this study, experiments were conducted to study the heat transfer enhancement due to surface modification in the close-loop R410A flash spray cooling system. The test surfaces including the macro-structured surfaces with pyramid and square fins with two orders of roughness, and the nano-porous surfaces with different pore diameters were examined. The experimental results indicated that the surface with macro fins could tremendously enhance the heat transfer due to the increase of wetted area. However the pyramid fins with less increase in wetted area showed a better heat transfer performance than square fins, indicating fin structure played a more important role than the increase in wetted area. Higher roughness could further improve the cooling performance of macro-structured surface. The maximum CHF of 330 W/cm² and heat transfer coefficient of 300 kW/(m² K) were achieved by the surface with rough pyramid fins, corresponding to 60% enhancement and 5 times respectively over smooth flat surface, while the wall temperature was maintained below 10 °C. The nano-porous surface could also lead to better heat transfer performance by increasing the number of nucleation sites and improving the wettability to working fluid. The result of pore size effect showed that the CHF value at first declined and then increased with the increasing pore size. It was postulated that this phenomenon was related to the transition of dominating heat transfer mechanism from “evaporation-limited region” to “viscosity-limited regime”. These findings helped to guide further investigations of enhanced surface aiming at enhancing heat transfer performance in flash evaporation spray cooling.