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

在深度过冷沸腾中，受热面上的聚结气泡会释放出大量微气泡，其热流密度高于一般过渡沸腾的临界热流密度。沸腾过程被称为微泡发射沸腾，简称MEB。在过冷流沸腾过程中，蒸馏水的最大热流密度为10 mW/m~2，通道底部放置10 mm×10 mm的方形加热面，通道高度为17 mm，宽度为14 mm的水平矩形通道内的最大热流密度为10 mW/m~2。根据气泡行为和压力波动的不同，将其分为两种类型，即猛烈的和无声的。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。在猛烈的大气泡中观察到了一种周期性的气泡。在周期性的气泡破裂、液体供应、气泡的产生和气泡的生长过程中，…在流道中观察到较多的周期性压力波和周期性压力波。热流密度与压力波动的频率成正比。压力频率是液体进入加热面的频率，当MEB达到最大热流点后，加热面被一层薄薄的蒸汽膜覆盖，表面变亮，表面温度迅速上升，热流密度下降。这是MEB的终极阶段。对于水，末端阶段的表面温度约为2 0 0℃，与非微循环相比，这一温度很高。对直径分别为2.5 mm、5 mm、10 mm和16 mm的水平圆形流道进行了膜沸腾实验。该通道位于铜制圆形加热块的中心，并与加热块连接直圆管。受热面为槽道的一部分，槽道长度为10 mm。30个盒式加热器平行于加热块中的通道组装。在过渡沸腾过程中出现了MEB，且热流密度高于一般的临界热流密度。随着液体流速的增加，MEB内的热流密度增大。例如，在直径为10 mm的通道内，液体过冷度为30K时，MEB内的最大热流密度分别为0.5m/S、1.0m/S、9 mW/m~2和10 mW/m~2，分别为0.5m/S、6 mW/m~2、1.5 m/m~2和10 mW/m~2。与液体过冷度一样，液体速度也是影响MEB性能的重要因素之一。对于不同直径的通道，低流速下通道直径为0.25m/S的通道内热流密度增大，而流速为1.0m/S时通道间的热流密度无明显差异。在圆形通道中也出现了周期性的MEB，且热流密度随压力频率的增加而增大，与液体过冷度、液体速度和通道直径无关。本研究的实验结果将发展为一种适用于高功率电子器件的超高热流密度冷却技术。较少

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