Abstract
本文主要內容主要為用數值方法探討一同心圓管機構內所建立之環形噴流及其中心引致流場之結構。在分析過程中，流體假設為不可壓縮牛頓流體，流場為等溫軸對稱的穩態層流流場，不考慮三維的渦旋運動，為了處理不規則形狀的邊界，引入了曲線座標系統，並使用一種橢圓型網格產生法來建立計算用網格，可以精確地控制邊界上的格點分佈，所有的偏微分方程組均使用權重函數法 (weighting function scheme)來差分化，差分方程式所構成的矩陣則以 SIS 解出，流場計算使用NAPPLE 計算法則，在非交錯網格系統上直接求出速度及壓力分佈。研究重點放在不同的管長及不同的雷諾數對流場結構之影響。結果顯示，在較高的雷諾數或較短的管長下，由於流體突張引起的壓力降，使得迴流區附近壓力低於外界大氣壓，而使中心管可以將外界流體吸入。這股被吸入的流體會和迴流區相衝，形成一個停滯點，而環狀噴流在經過突張步階後及主迴流區後，逐漸會合，會形成類似背向式步階突張流reattachment的現象。反之，在較低的雷諾數或較長的管長下，中心管則有逆流發生，迴流區一部份的流體直接沿中心管被逆向的壓力梯度推出管外。另外，本研究也發現，不論在何種情況下，內管管壁形成的步階所引發的迴流區對管心的吸入效應均有不利的影響。The main purpose of the present thesis is to investigate theflow structure of an annular jet and its induced flow along thecenter line in two concentric circular flow channels. In orderto deal with the irregular-shaped solid boundaries, acurvilinear coordinate system associated with a Poisson typegrid generation technique are employed to construct the gridsystem needed in the computation. All of the partialdifferential equation are of elliptic type and are discretizedbased on the weighting function scheme. The resulting matrixequations are solved by using the SIS (Strongly ImplicitSolver) solver. The velocity and the pressure field aredirectly obtained by following the NAPPLE algorithm on a non-staggered grid system. The effects of Reynolds numbers and pipelengths on the flow structure are emphasized. According to thenumerical results, At higher Reynolds number or shorter pipelengths, due to the significant pressure loss at the suddenexpansion, the pressure level of the main recirculation regionis lower than that of the outer environment. Hence the fluid isdrawn into the central pipe by the annular jet. On the otherhand, at lower Reynolds number or longer pipe lengths, thefluid in the central pipe tends to be blowed out by therelatively high pressure near the main recirculation region.The step formed by the wall thickness of the central pipe isfound not to have any advantage to the suction effect.