AN INVESTIGATION OF MICROBUBBLE EMISSION BOILING AND APPLICATION TO ULTRA-HIGH HEAT FLUX COOLING TECHNOLOGY FOR HIGH POWERED ELECTRONIC DEVICES

在高过冷沸腾过程中，受热面上的聚结气泡释放出许多微气泡，热流密度高于过渡沸腾时的一般临界热流密度。这种沸腾状态被称为微泡发射沸腾，简称MEB。在高度为17mm，宽度为14mm的水平矩形通道中，蒸馏水在过冷流动沸腾中发生了显著的MEB，获得的最大热流密度为10MW/m^2，通道底表面为10mm×10mm方形受热面。根据气泡的行为和压力波动情况，将MEB分为两种类型，即暴力MEB和沉默MEB。在剧烈MEB中，随着受热面温度的升高，压力波动幅度增大，热流密度急剧增大。在剧烈MEB中观察到周期性的MEB。在周期型MEB中，经历了气泡破裂、供液、气泡生成和气泡生长的一系列过程，并在通道中观察到周期性的压力波。热流密度随压力波动的频率成比例地增加。压力频率被认为是液体进入受热面的频率。当MEB达到最大热流密度点后，受热面被一层薄薄的蒸汽膜覆盖，表面像铣床表面一样变亮，然后表面温度迅速上升，热流密度下降。这是MEB的终末阶段。水的末端表面温度约为200℃，与非MEB相比，温度很高。然后沸腾迅速转变为薄膜沸腾。研究了直径为2.5mm、5mm、10mm和16mm的水平圆形通道的MEB。在铜制成的圆形加热块中心制造通道，加热块与直圆管相连。受热面为通道的一部分，通道长度为10mm。30个筒式加热器与加热块中的通道平行组装。MEB发生在过渡沸腾中，热流密度高于普通临界热流密度。MEB内的热流密度随液体流速的增大而增大。例如，在直径为10mm的通道中，液体过冷30K时，MEB的最大热流分别为0.5m/s时5MW/m^2, 1.0m/s时6MW/m^2, 1.5m/s时9MW/m^2和2.5m/s时10MW/m^2。液体速度与液体过冷度一样是MEB的重要影响因素之一。对于不同直径的通道，0.25m/s流速较低时，直径较大的通道MEB的热流密度增加，而在1.0m/s流速下，通道之间的热流密度没有差异。无论液体过冷度、液体流速和通道直径如何，圆形通道中也存在周期性MEB，热流密度随压力频率的增加而增加。本研究的实验结果将发展为大功率电子器件的超高热流密度冷却技术。少

In highly subcooled boiling, many microbubbles are emitted from coalesced bubbles on the heating surface and the heat flux increases higher than the ordinary critical heat flux in transition boiling. The boiling regime has been called Microbubble Emission Boiling, shortened MEB. MEB occurred remarkably in subcooled flow boiling and the maximum heat flux obtained was 10MW/m^2 for distilled water in the horizontal rectangular channel of 17mm height and 14mm width with square heating surface of 10mm×10mm placed on the bottom surface of the channel. According to the bubble behaviors and the pressure fluctuations, MEB was categorized into two type, they were violent MEB and silent MEB. In the violent MEB, the pressure fluctuations rose high and the heat flux increased steeply with the temperature rise of heating surface. A periodic type of MEB was observed in the violent MEB.In the periodic type of MEB, a series of bubble collapse, liquid supply, bubble generation and bubble growth was cond … More ucted periodically and the periodic pressure waves were observed in the channel. The heat flux increased proportionally to the frequency of pressure fluctuations. The pressure frequency is considered to be the frequency of liquid supply into the heating surface.After MEB reached the maximum heat flux point, the heating surface was covered with a thin vapor film and it brightened like a miller surface, then the surface temperature rose rapidly and the heat flux decreased. This is a terminal stage of MEB. The surface temperature at terminal stage was about 200℃ for water and it was very high compared with the case of non MEB. Then the boiling turned rapidly to film boiling.MEB was investigated for horizontal circular channels of 2.5mm, 5mm, 10mm and 16mm in diameter. The channel was manufactured in the center of circular heating block made of copper and straight circular tubes were connected with the heating block. The heating surface was a part of the channel and the length was 10mm for the channels. Thirty cartridge heaters were assembled parallel to the channel in the heating block. MEB occurred in transition boiling and the heat flux was higher than the ordinary critical heat flux. The heat fluxes in MEB increased with increasing liquid flow velocity. For example, the maximum heat fluxes in MEB were 5MW/m^2 at 0.5m/s, 6MW/m^2 at 1.0m/s, 9MW/m^2 at 1.5m/s and 10MW/m^2 at 2.5m/s at 30K of liquid subcooling in the channel of 10mm diameter. The liquid velocity is one of the strong factors in MEB as same as liquid subcooling. For the various channels with different diameters, the heat flux in MEB increased for the channel with the larger diameter 0.25m/s of low liquid velocity, however, no differences of heat fluxes between the channels were observed at 1.0m/s of liquid velocity. A periodic MEB also occurred in the circular channels and the heat flux increases with the pressure frequency regardless of liquid subcooling, liquid velocity and channel diameter.The experimental results obtained in the present study will be developed to an ultra-high heat flux cooling technology for high powered electronic devices. Less