本研究採用暫態數值模擬探討低溫垂直板面膜式冷凝行為。數值方法採用FLUENT商業軟體之 VOF(void of fluid)數值模型，液膜表面張力採用連續表面力(continuum surface force)，兩相熱質傳部分採用能量跳躍法，透過UDF (user-defined function)進行計算。本研究選用R134a流體，採不同網格尺寸計算溫度梯度與液膜厚度，與不同條件下之Nusselt解析解作比較。特別針對垂直板前沿與中後段冷凝液膜驗證其準確性，結果顯示前沿冷凝液膜受垂直液膜方向網格大小、汽液密度比影響甚大，而中後段液膜厚之準確度較與網格大小、汽液密度比的關係不顯著，顯示介面網格位於壁邊界處時，因薄液膜中溫度梯度之計算誤差，須採用較小網格。隨後進一步提升垂直板長度，使液膜涵蓋層流與波動(wavy film)兩區域，以模擬自然對流下波動液膜冷凝情形。本計算中如未加入外在刺激，在液膜雷諾數遠大於臨界值20~30時液膜仍未產生波動。但當液膜波動因外在因素而形成後，其平均熱對流係數較無波動時增加45%。

In this study, the surface film condensation behavior on the low-temperature vertical plate is investigated by transient numerical simulation. The numerical methods including the VOF (void of fluid) model, continuum surface force for the surface tension, and the energy jump method through UDF (user-defined function) for the two-phase heat and mass transfer are adopted in the commercial software, FLUENT. R134a fluid is selected as the working fluid. The liquid film thickness is compared with the Nusselt analytical solution under different mesh sizes, temperature gradients, and liquid film thicknesses.
The accuracy of the condensate film thickness in the different sections of the vertical plate is examined separately. The results show that the leading-edge of the condensate film was greatly affected by the grid size and the vapor-liquid density ratio in the direction perpendicular to the cooling wall, while the accuracy of the liquid film thickness away from the leading-edge is insensitive to the above factors. The reason is due to the inaccurate calculation of the temperature gradient when the interface grids are located next to the wall boundary, which can be improved using finer grids .
To simulate the fluctuating liquid film under natural convection, the length of the vertical plate is further increased, so that the liquid film covers the laminar flow and wavy film regions. When no external oscillation is imposed, no wave appears for a film Reynolds number much larger than the critical Reynolds number of 20~30. Once surface wave in activated by external flow fluctuations, the average heat convection coefficient is enhanced by 45%.

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