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过程中，传热强化与沸腾燃尽危机并存，传热机理和沸腾燃尽过渡机理的研究对于利用沸腾现象的换热器的安全运行具有重要意义。通过对现有电导法的改进，对气泡下液膜厚度的测量方法进行了详细的考察，其将给出与传热机理直接相关的数据，i）通过对电极周围电压分布的分析，将待测膜厚的上限与用于供电的电极之间的距离联系起来，并且阐明了电极周围的电流分布，ii）在用于电压拾取的电极之间的距离的关系中检查测量厚度的灵敏度或准确度。研究结果可用于减小传感器尺寸 ...更多信息 e和传感器电极的最佳布置。（3）实验研究了双电层中积累的电荷释放所需的时间，并确定了测量液膜厚度的最小时间间隔。为了将测量的传热数据与观察到的窄间隙内核态沸腾中的液-汽行为联系起来，其中扁平气泡下的薄液层蒸发主导传热，开发了透明管之间窄间隙的试验段。金属薄膜被涂敷在内管壁上，它被用作直接施加电流的加热器，也可用作电阻温度计。由于金属膜的厚度很薄，可以直接观察扁平气泡下的汽液两相流动。i）在质量流速一定的情况下，随着间隙尺寸的减小，换热得到了增强。ii）在小的间隙尺寸，传热增强的速率更强调在较低的质量速度，因为在扁平气泡的尺寸增加，iii）在扁平气泡的底部的微层中，干补丁扩展和传热增强和恶化的趋势的共存实验证实。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