竖直环隙内通入空气时的界面汽化热阱效应

【作者】 苗君；

【导师】 沈自求； 王军；

【作者基本信息】 大连理工大学 ， 化学工程， 2002， 硕士

【摘要】 以水和正己烷作为介质，考察了通入空气对内管加热的竖直圆柱形环隙内的沸腾传热性能的影响。 实验考察了环隙内通入空气与不通空气时的沸腾传热情况。结果表明，通空气可使环隙内沸腾传热得以显著增强，且空气的湿度对传热的增强作用有一定影响，湿度小的空气引起的传热增强更为显著。本文以扰动增强和“界面汽化热阱效应”增强传热的观点阐述了这一沸腾传热强化机理，解释了空气湿度对传热增强作用的影响。另外，通空气对沸腾传热的增强作用还与空气的表观气速有关。在恒定的热流密度下，空气表观流速较小时，通气对传热的增强作用随气速的增大而增大，而当气速增大到一定值时，气速的进一步增大会使通气对传热的增强作用减小。本文用两相流流型转变的观点对这一实验现象进行了分析。 实验结果还表明，通入空气对液温低于沸点时的对流传热亦有增强作用，且这种增强作用比沸腾传热时的增强作用更为显著。分析这一增强作用，可以认为通入空气非但使对流传热体系的扰动得以增强，而且产生了“界面汽化热阱”效应。 为了探讨这种通气增强沸腾传热的方法在发热微电子元件冷却中的应用可行性，本文还以正己烷作为沸腾介质，考察了通空气对沸腾传热以及沸腾起始时的壁温过冲的影响。结果表明通空气仍可使饱和沸腾阶段的传热系数增大、壁温降低，并且对核沸腾起始时的壁温过冲有一定的抑制作用。 上述实验的范围是：加热功率密度为16Kwm^-2～122Kwm^-2； 空气表观流速为1.8mm／s～13.3mm／s。更多还原

【Abstract】 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更多还原