Clarification of Nucleate Boiling Heat Transfer by the Observation of Bubble Base Area through the Transparent Heating Surface and by the Development of a Sensor for the Measurement of Thin Liquid Film Thickness

通过透明加热表面观察气泡底部面积并开发用于测量薄液膜厚度的传感器来澄清核沸腾传热

• 批准号: 12650207
• 负责人: OHTA Haruhiko
• 金额: $ 2.37万
• 依托单位: Kyushu University
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (C)
• 财政年份: 2000
• 资助国家: 日本
• 起止时间: 2000 至 2001
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-12650207/
• 关键词: boiling; two phase flow; macrolayer; microlayer; sensor

In nucleate boiling at high heat flux, both trends of the heat transfer enhancement and the boiling crisis due to burnout are coexisted and the mechanisms of both heat transfer and the transition to burnout are to be clarified for the safe operation of heat exchangers utilizing boiling phenomena.In the present research, detailed examination on the methods for the measurement of liquid film thickness underneath bubbles by the improvement of the existing conductance method, which will give data directly related to the heat transfer mechanisms, i) By the analysis of voltage distribution around electrodes, the upper limit of film thickness to be measured was related to the distance between electrodes for current supply, and the distribution of electric current around electrodes was clarified, ii) The sensitivity or accuracy of measured thickness was checked in the relation of the distance between electrodes for voltage pick up. The results are to be utilized for the reduction of sensor siz … More e and for the optimal arrangement of sensor electrodes. iii) The time period required for the release of electric charge accumulated in the electric double layer was examined experimentally and the minimum interval for the measurement of liquid film thickness was determined.To relate the measured heat transfer data to the observed liquid-vapor behaviors in the nucleate boiling in narrow gaps where evaporation of thin liquid layer underneath flattened bubbles dominates the heat transfer, a test section of narrow gaps between transparent tubes was developed. The thin metal film was coated on a inner tube wall and it was used as a heater by direct application of electric current and as also resistance thermometer. The thickness of the metal film is thin so that the liquid-vapor behaviors underneath flattened bubbles are directly observed through it. The followings were clarified from the experiments of flow boiling in a vertical channel. i) Heat transfer is enhanced with the reduction of gap size under a constant mass velocity. ii) At small gap sizes, the rate of heat transfer enhancement is more emphasized at lower mass velocity because of increase in the size of flattened bubbles, iii) In the microlayer at the base of flattened bubbles, dry patches are extended and the coexistence in the trends of heat transfer enhancement and deterioration is confirmed experimentally. Less

在高热流密度核态沸腾中，强化传热和烧坏沸腾危机两种趋势并存，为了利用沸腾现象换热器的安全运行，有待明确传热和烧坏转变的机理。 .在本研究中，通过改进现有的电导方法，对气泡下方液膜厚度的测量方法进行了详细研究，这将给出与传热机制直接相关的数据，i）通过分析周围的电压分布电极，要测量的薄膜厚度的上限与供电的电极之间的距离有关，并且明确了电极周围电流的分布，ii）根据距离的关系检查测量厚度的灵敏度或准确性电极之间的电压拾取结果将用于减小传感器尺寸以及传感器电极的最佳布置 iii) 释放双电层中积累的电荷所需的时间为。通过实验检验和最小间隔为了将测得的传热数据与狭窄间隙中核态沸腾中观察到的液-汽行为联系起来，其中扁平气泡下方的薄液体层的蒸发主导传热，窄间隙的测试部分在内管壁上涂有薄金属膜，通过直接施加电流用作加热器，也可用作电阻温度计。金属膜的厚度很薄，因此液体温度计。蒸气行为通过垂直通道中的流动沸腾实验可以直接观察到下面的现象： i) 在恒定质量速度下，传热随着间隙尺寸的减小而增强。由于扁平气泡尺寸的增加，在较低质量速度下更强调传热强化速率，iii）在扁平气泡底部的微层中，干斑扩展并且传热强化和强化传热的趋势并存。劣化是通过实验证实的。