Heat transfer enhancement of spray cooling with ammonia by microcavity surfaces

In the present work, spray cooling heat transfer performances with ammonia as coolant were experimentally investigated on three self-manufactured microcavity surfaces and the enhancement of heat transfer over that of flat surface was also examined. The experimental results showed that almost the same heat transfer performance was obtained at low surface superheats for different heat transfer surfaces due to the fact that the single phase convection dominated the heat transfer process. The microcavity surfaces exhibited uniform temperature distribution and higher heat transfer coefficient than that on the flat surface at high surface superheats once the heat transfer was dominated by the nucleate boiling. This was because that the capillary effect induced by the microcavity structure results in dramatic reduction in heat transfer resistance and then enhancement of the nucleate boiling. It was also found that the microcavity surface with the lowest Bo number of 0.1004 yielded the maximum heat transfer coefficient of 148,245 W/m²·K at the heat flux of 451 W/cm² as a result of the strongest capillary effect. In the meantime, low surface temperature of below 0 °C and uniform temperature distribution with deviation below ±1.5 °C at the heat flux of 420 W/cm² was simultaneously achieved.