應用DSMC法模擬曳引式分子幫浦之壓縮特性

Abstract

本計畫使用直接模擬蒙地卡羅法(DSMC)，分析鼓式曳引分子幫浦(MDP)的簡化模型，以得到它的壓縮特性及傳輸機率。文中主要在探討在不同的上板速度(正比於轉子速度)、轉子凹槽的長寬比與背壓值等條件下，對整個幫浦的抽氣效率之影響，並與Iguchi等人[1]實驗所得的抽氣速率曲線作趨勢性的比較。以往在預測最大壓縮比的公式中，多假設與背壓的大小無關，或在模擬時假設進出口的平均速度為零，本文將針對上述的假設做改進，使模擬更接近真實的情形。最後，所得的結果分別與Lee等人[2]的模擬結果作比較，希望對MDP的性能有更佳的預測結果。預測結果顯示：在分子流中當板速超過0.25VMP時，在流體速度向量場中，於出口端呈現渦流的情形，相關的CFD數值模擬也有類似的現象產生；在自由分子流域中，順著流體方向的傳輸機率，隨著上板的速度增加而漸增，最後到達一個定值，其值略小於1；當轉子上凹槽的長寬比漸增，傳輸機率並沒有明顯地改變，而幾近一定值；此外，在過渡流域中，MDP所呈現的傳輸機率，於不同的背壓下呈現一定值；隨著操作壓力的範圍不同，最大壓縮比也有明顯地改變。最後，用改進的模型分別與Lee等人的數值和Iguchi等人的實驗曲線作比較，在趨勢上可得到不錯的一致性。

This project simulates the performance of transmission probability and compression characteristics for a simple drum type of molecular drag pump (MDP) model by using DSMC method. The modification of the present model is that the inlet and outlet bulk velocities are no longer null, but obtained by meeting the requirement of mass balance. The work aims at exploring the pumping efficiency with various operating parameters, including the wall velocity, channel aspect ratio and backpressure. The predicted results are compared with the experimental data by Iguchi et al. [1] to verify them in qualities. In the past studies, the maximum compression ratio (Ko) is not affected by backpressure, or the mean bulk velocity at the MDP inlet and outlet are assumed zero. Therefore, the present work is motivated to relax these limitations to achieve a better prediction for MDP performances. Finally, the computed results are compared with those by Lee et al. [2]. The predicted results show that according to the velocity vector fields, a vortex at outlet is found as the wall velocity is greater than 0.25VMP at very rarefied condition which the similar phenomenon was observed by the related CFD numerical simulation. In free molecular regime, transmission probability in flow direction (Tr1) increases with an increase in wall speed (uwall), and it approaches a constant value, smaller than unity. The change in transmission probability (Tr1) is strongly dependent on the wall speed, but weakly dependent on the channel length. In the transition regime, the transmission probability can be thought to have nearly constant value irrespective of the backpressure (P2). The maximum compression ratio (Ko) is found to change significantly according to Kundsen number in the flow field. Finally, the predicted results by this model show a good agreement with the ones by Lee et al. and the experimental data by Iguchi et al..