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

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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