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

在过冷沸腾过程中，受热面上的气泡会产生大量的微气泡，并使沸腾热流密度增加到高于过渡沸腾时的临界热流密度。沸腾状态被称为微泡排放沸腾，简称MEB。在高17 mm、宽14 mm、底面为10 mm × 10 mm方形加热面的水平矩形通道中，蒸馏水的过冷流动沸腾发生了明显的微电子束沸腾，最大热流密度可达10 MW/m^2。根据气泡行为和压力波动的不同，将MEB分为两种类型，即剧烈MEB和沉默MEB。在剧烈的MEB中，随着受热面温度的升高，压力波动增大，热流密度急剧增加。在剧烈的MEB中观察到一种周期性的MEB，在周期性的MEB中，存在一系列的气泡溃灭、液体供给、气泡生成和气泡生长过程， 关于我们 周期性地引导，并且在通道中观察到周期性压力波。热通量的增加与压力波动的频率成比例。压力频率被认为是液体供应到加热表面的频率，当MEB达到最大热流密度点后，加热表面被一层薄的蒸汽膜覆盖，并像米勒表面一样变亮，然后表面温度迅速上升，热流密度下降。这是MEB的终末阶段。水的终末表面温度约为200℃，与无微生物床的情况相比非常高。实验研究了直径为2.5mm、5 mm、10 mm和16 mm的水平圆通道内的膜沸腾。通道制作在铜制圆形加热块的中心，直圆管与加热块连接。加热表面是通道的一部分，通道的长度为10 mm。平行于加热块中的通道组装30个筒式加热器。临界沸腾发生在过渡沸腾阶段，其热流密度高于正常临界热流密度。随着液体流速的增加，MEB中的热通量增加。例如，在直径为10 mm的通道中，在30 K的液体过冷度下，MEB中的最大热流密度在0.5m/s时为5 MW/m^2，在1.0m/s时为6 MW/m^2，在1.5m/s时为9 MW/m^2，在2.5m/s时为10 MW/m^2。与过冷度一样，液体流速也是影响MEB的重要因素之一。对于不同直径的通道，在低液速下，直径为0.25m/s的通道内的热流密度增加，而在1.0m/s的液速下，通道间的热流密度无明显差异。循环通道中也出现了周期性的MEB现象，且热流密度随压力频率的增加而增加，与液体过冷度、液体流速和通道直径无关。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