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

在高热流密度下的核态沸腾中，传热强化和燃尽引起的沸腾危机并存，为了利用沸腾现象的换热器的安全运行，需要阐明传热和向燃尽过渡的机理。本研究通过对现有电导法的改进，对测量气泡下液膜厚度的方法进行了详细的考察，给出了与传热机理直接相关的数据：1)通过对电极周围电压分布的分析，被测膜厚的上限与供电电极之间的距离有关，并阐明了电流在电极周围的分布情况；ii)根据拾压电极间距的关系，检验了测量厚度的灵敏度或准确性。结果将被用于减小传感器尺寸…更多的是为了传感器电极的最佳布置。3)通过实验考察了双电层中电荷释放所需的时间，确定了测量液膜厚度的最小间隔时间。为了将所测得的传热数据与窄缝隙中观察到的汽液两相沸腾行为相关联，在窄缝隙中，扁平气泡下的薄液层蒸发是主要的换热方式，为此，设计了一段透明管间窄缝隙实验。这种金属薄膜被涂在管子的内壁上，既可以直接作为加热器使用，也可以作为电阻温度计使用。金属膜的厚度很薄，因此可以通过它直接观察到扁平气泡下的汽液行为。通过对竖直通道内流动沸腾的实验研究，阐明了以下几点。I)在恒定质量速度下，随着间隙尺寸的减小，换热得到加强。Ii)在小间隙尺寸下，由于扁平气泡尺寸的增大，在较低的质量流速下，强化换热的速率更加突出；iii)在扁平气泡底部的微层中，干斑延伸，并从实验上证实了强化与劣化并存的趋势。较少

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