具外罩同步旋轉圓盤內流場之量測與分析

Abstract

本文旨在研究具外罩同步旋轉圓盤內流場的流體行為及其形成機制。首先以流場可視化方法，觀察流場結構，並偵測其隨參數的變化。針對高轉速的典型流場，則分別利用LDV及PIV作速度量測，以定量重建流場結構。再轉換LDV時序列速度為能量頻譜，以分析流場的週期性，同時作為PIV連續量測的週期參考依據，達成具相位解析效果的PIV全域量測。 可視化實驗發現，改變雷諾數（ ）及盤間距（ ）時，流場會隨參數變化呈現三種不同的型態：低雷諾數且小盤間距時，近外罩處會出現依盤中間平面對稱的切向渦旋對；高雷諾數且大盤間距時，渦旋對則發生對稱破裂。在兩種流型之間的過渡區，渦旋對會出現shift-and-reflect對稱；綜整實驗結果所建立的流變圖，涵蓋了上述可能的型態，較前人實驗更為完整。 高速典型流場（ ）的PIV相位解析量測結果顯示，軸罩中點兩側的瞬時切向速度呈現相反趨勢的週期性震盪，不同位置上的震盪幅度與頻譜能量值的徑向分佈對應，此為定量表現近轉軸流區切向速度週期震盪的首例。 分析時序列全域渦度場發現，切向渦旋對的對稱破裂，會引發徑向擾動，且在以平均切向速度極大處為半徑的圓周上，生成正、負交互相間的週期擾動渦度，並分別引發逆、順時針方向的週期擾動渦流。匯集了流體經過外罩邊界層所生成的軸向渦度後，擾動渦流得以維持其自轉行為，並引發環繞其中心的切（徑）速度徑（切）向梯度。於旋轉參考座標下，週期擾動渦流所引發的速度分佈形式，則形成軸向大尺度渦流結構。全域的週期雷諾應力場，亦反應了週期擾動渦流的自轉行為，並凸顯了流場的週期性，可作為數值計算的參考依據。

This paper presents the measurements of the flow in the space between a pair of corotating disks. The present effort seeks to investigate the flow behavior and the mechanism responsible for it. Flow visualization techniques were employed to observe the flow structure, and detect its variations with parameters. Moreover, the typical high-speed flow was quantitatively measured using Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV). LDV gave the time history of velocity for power spectral analysis, whose results were used to understand the periodicity of flow. Based on the time period information, the phase resolved PIV measurements were conducted. Flow visualization experiments conducted in the range of Reynolds numbers from to and of the disk spacing from to showed that, the flow may present three different regimes in accordance with the change of the value of (Re, S) pair. With low and small , a pair of circumferentially aligned vortex pair showing symmetry with respect the inter-disk mid-plane appears near the shroud. As increases or enlarges, the symmetry of this vortex pair breaks. Within the transition between the two foresaid regimes, the vortex pair shows shift-and-reflect symmetry. Phase-resolved PIV measurements on the typical high-speed flow ( ) revealed that the instantaneous circumferential velocity oscillates periodically on both radial sides of the mid-point between the hub and the shroud, but in opposite trends. The oscillating amplitudes at different radial locations correspond to the radial profile of power spectral intensity. This is the first case which quantitatively presents the velocity variations in the flow region near the hub. The analysis on the time-series vorticity field deduced that the symmetry breaking of circumferentially aligned vortex pair leads to the disturbances in the radial direction. As a result, a chain of alternating negative and positive periodic fluctuating vorticity foci occur around a circle, where the mean circumferential velocity achieves the maximum, and cause the clockwise and counter-clockwise fluctuating vortices respectively. The fluctuating vortices maintain their rotations by accumulating the axially aligned vorticity acquired as the fluids pass through the shroud boundary layer, and induce the radial (circumferential) gradient of the circumferential (radial) velocity around their centers. In the rotating reference frame, the resulting velocity patterns form the axially aligned vortical structures. The periodic Reynolds stress fields are responsible for the self-rotation of the fluctuating vortices, and emphasize the periodicity of the flow. The whole-field stress patterns may serve as the reference for numerical simulation.