Pass Into The Air When The Interfacial Vaporization Heat Sink Effect. Vertical Annulus

Heat transfer of nucleate boiling of water and n-hexane in a vertical cylindrical annulus with inner side heated has been experimentally investigated.Experiments of boiling of water with and without air introduction are conducted and the results reveal that boiling heat transfer is significantly enhanced by air introduction. It is also noticed that the heat transfer enhancement caused by introduction of unsaturated air is higher than that caused by introduction of air saturated with water. The heat transfer enhancement mechanism is analyzed by the idea that both the effect of increased turbulence and that of the "interfacial vaporization heat sink" caused by air bubbles contribute significantly to the total heat transfer enhancement. The effect of air humidity on heat transfer enhancement is also explained by such an idea. The boiling heat transfer enhancement due to air introduction is also related to air superficial velocity. The experimental result shows that when the heat flux remains constant, the heat transfer enhancement by air introduction increases with increasing air velocity at the beginning. However, when the air velocity reaches a certain value and is considerably high, a further increase in air velocity will cause the heat transfer enhancement to decrease. Such a result is explained by the flow pattern transition of the two-phase flow inside the annulus and the consequent formation of large slugs.When air is introduced into the liquid with a temperature below boiling point, an enhancement of heat transfer is also detected, which is even higher than that in the case of nucleate boiling. The enhancement is attributed to the increased effect of interfacial vaporization heat sink due to air introduction.To discuss the potential possibility of the application of boiling with air introduction to the cooling of microelectronic elements, nucleate boiling heat transfer and the temperature overshoot at the incipience of nucleate boiling are examined for n-hexane. It is found that the heat transfer coefficient is increased and the wall temperature is lowered by introduction of air bubbles. The air bubbles also suppress wall temperature overshoot at the incipience of nucleate boiling.The range of the experimental conditions is as follows: heat flux is 16Kwm-2 ~122Kwm-2; air superficial velocity is 1.8mm/s ~13.3mm/s...