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On the Occurrence, Stability and Heat Transfer Enhancement of Microbubble Emission Boiling

微气泡发射沸腾的发生、稳定性及强化传热研究

基本信息

批准号：
13650206
负责人：
KUMAGAI Satoshi
金额：
$ 2.24万
依托单位：
SHINSHU UNIVERSITY (2003)Tohoku University (2001-2002)
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2001
资助国家：
日本
起止时间：
2001 至 2003
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-13650206/
关键词：
Boiling Heat Transfer Microbubble Emission Phenomena Liquid Subcooling Bubble Motion Conditional Sampling Bubble Collapsing Velocity Power Spectrum Surface Erosion 伝熱沸騰気泡微細化気泡崩壊周期固液接触ボイド信号局所温度分布気泡崩壊

项目摘要

In subcooled boiling under certain conditions of subcooling and velocity of the liquid, the heat flux does not decrease with increasing of the surface temperature in transition boiling regime, but increases steeply. In this phenomenon a lot of minute bubbles are emitted from the surface accompanied by extraordinary loud sound. Therefore, we call this phenomenon microbubble emission boiling, abbreviated as MEB. The high heat flux in MEB far exceeding CHF should be considered a result of violent growing and collapsing behavior of coalescent bubbles in that region, which strongly introduces subcooled liquid to the heated surface.In this study, the liquid temperature profiles were measured at the process of growing and collapsing of bubbles separately with conditional sampling method, by using a thermocouple set at a suitable position above the center of the surface as a detecting prove of bubble motion. In the period of bubble collapse, the high temperature appeared only on the central pa … More rt of the surface. This high temperature region indicates the position of high provability for vapor phase to have existed, suggesting that the bubbles shrink and collapse toward the center on the surface. Here, we tried to get an information about the moving velocity of liquid-vapor interface in the bubble shrinking stage near the moment of collapse, which was named as the collapsing velocity, here, with a twin electrode void probe. The velocity was very high in MEB comparing with that of nucleate boiling. In MEB, the collapsing velocity increased with increasing of the heat flux, to be as much as 20 m/s at 20 MW/sq.m in the heat flux.The bubble motion can be monitored directly by an electrode void probe. The frequency of the fluctuation of the void signal indicates the frequency of the chance of liquid supply onto the surface. Therefore, the peak frequency of the power spectrum of the void signal fluctuation should be an index of heat transfer, and heat flux must be a function of the frequency. The relation between the peak frequency and heat flux was obtained for various condition of liquid subcooling and velocity. Though the data showed a wide scattering, a trend of higher peak frequency is distinguished in lower liquid subcooling. It was rearranged for excess heat flux from CHF, to get a correlation function. Less

在一定的过冷度和流速条件下，过冷沸腾传热通量在过渡沸腾区不随表面温度的升高而降低，而是急剧增加。在这种现象中，许多微小的气泡从表面发出，伴随着非常大的声音。因此，我们称这种现象为微泡喷射沸腾，简称MEB。本文采用条件采样法分别测量了气泡生长和溃灭过程中的液体温度分布，并对气泡的生长和溃灭过程进行了分析，得到了气泡生长和溃灭过程中的液体温度分布，并对气泡的生长和溃灭过程中的液体温度分布进行了分析。利用置于液面中心上方适当位置的热电偶作为气泡运动的检测点。在汽泡溃灭阶段，高温只出现在中心区域 ...更多信息的表面。该高温区域指示汽相存在的高可证明性的位置，这表明气泡在表面上向中心收缩和塌陷。本文采用双电极空隙探针，测量了汽泡收缩阶段汽液界面在溃灭时刻附近的运动速度，称之为溃灭速度。与核态沸腾相比，微泡沸腾的速度很高。在MEB中，随着热流密度的增大，气泡的溃灭速度增大，当热流密度为20 MW/m ~ 2时，溃灭速度可达20 m/s。空隙信号的波动频率指示液体供应到表面上的机会的频率。因此，空隙率信号波动功率谱的峰值频率应该是传热的一个指标，并且热流必须是频率的函数。得到了不同过冷度和流速条件下的峰值频率与热流密度的关系。虽然数据显示出广泛的分散，较高的峰值频率的趋势是在较低的液体过冷度区分。对CHF产生的过剩热流进行了重新排列，得到了关联函数。少