沉浸邊界法應用於高壓氣艙之球閥移動洩氣暫態過程研究

Abstract

本文以三維可壓縮流計算數值程式探討且高壓氣艙球閥移動釋氣之流場狀態，將諸多數值方法結合貫通並以C++作為演算法使用之平台，流場使用有限體積法(Finite Volume Method)，將網格形變與沉浸邊界技術混合使用，兩種方法各有優缺點，而混合使用兩方法將可達到相互相補的效果。最後使用平行化計算技術OpenMP與CUDA加速流場計算速度，期以達到快速分析球閥移動釋氣之流體現象。 經由本研究之結果，當氣艙內外壓力比值超過臨界壓力比值時(約為1.89)，釋出氣體之速度會將超過音速，而研究中分別探討了超音速與次音速之流場結果，氣體釋出至外部空間之後，即會產生正負壓力之結構並往下游推移，另一方面，釋出氣體會產生推力，而氣艙內外之壓差也會對氣缸產生反作用力，經由實驗結果得知，作用力會小於反作用力，其原因為釋出氣體部分能量會產生亂度與渦流，導至作用力與反作用力不平衡現象。此外，關於閥門開關過程、式氣之質量流率的計算結果，本研究與修改過之孔洞釋氣經驗公式做對照，計算模擬結果與經驗公式有良好的一致性。

The aim of this thesis is to investigate gases discharged from a high pressure vessel numerically. To simulate this subject more realistically, the viscosity and compressibility of the gas are taken into consideration simultaneously. The methods of the Roe scheme, preconditioning and dual time stepping matching the LUSGS method are adopted to solve compressible flow problems during gaseous discharge processes. The non-reflecting boundary condition is used to prevent flow fields from being polluted by the reflection of the pressure wave induced by the compressible flow on the boundary. Computing procedures are performed on the Compute Unified Device Architecture (CUDA) computation platform which was recently developed and a highly effective technology for accelerating computational speed. The pressure ratio, which means the ratio of the pressure in the vessel to the pressure of outside, is larger than 1.89. Thus both subsonic and supersonic speeds of the discharged gas are investigated. Results show that the mass flow rate of this work is consistent with the existing experimental work. Due to a sudden expansion at a small opening, the phenomena of an alternating variation of the pressures of gases, rapid decrements of the temperature of gases and a quick acceleration of the velocities of gases are clearly observed in the mainstream direction. The ratio of the thrust caused by the gases released to the reaction force is less than 1 because of the dissipation of entropy generation. Also, a modified equation for predicting transient mass flow rates is derived. Results obtained by the equation have good agreements with results that calculated by the numerical method developed bt this work.