EFFECT OF SURFACE ROUGHNESS ON HYSTERESIS PHENOMENON OF CONDENSATION CURVE

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This report is consist of three parts. The abstract is written in the next.[Part I] Dropwise and filmwise condensation heat transfer on vertical rough surfaces scratched horizontally, obliquely and vertically by various grades of sand paper was measured over a wide range of subcooled temperature DELTAt.By the results, the transient and filmwise condensation which appear in the higher range of DELTAt occurred easily compared with that on the smooth surface. The heat flux values became very much lower and showed different values under the condition that DELTAt was increased and decreased. One could easily reproduce this hysteresis phenomenon on the rough surface.[Part II] Two methods of estimating the heat flux (q_r) during dropwise condensation on rough surfaces is proposed. Namely, by assuming the ratio of the departure drop radius on the rough and smooth surfaces was equal to those of the statics (R_r/R_s) without condensation, q_r was formulated as a function of q_s, R_r/R_s and the … More ratio of sweeping cycle. But this method could not represent the geometical characterisities of the surfaces directly. Then, another estimation method was proposed by replacing R_r/R_s and the ratio of sweeping cycle with the friction factors in the region of fully developed roughness flow.As a result, these two kinds of estimated qr agreed well with experimental value.[Part III] In the transition regime of condensation curve, the surface temperature to calculate DELTAt changes of a large scale wheather it's measureing position is covered with condensate film or drop. Then, the transition regime of condensation curves on rough surfaces was modified by the average surface temperature of those which were calculated by using the measured heat flux in filmwise condensation area, the measured covering ratio of condensate film and the calculated heat flux in dropwise condensation area.By the results, it was found that the modified condensation curves in transition condensation regime shift to right side in the relatively lower DELTAt region and shift to left side in the higher DELTAt region out of the previous curves. Less