micro-PIV diagnostics on unsteady flow in a microdiffuser under oscillating pressure actuation

• Main Authors: Hung-Chien Lee, 李鴻健
• Other Authors: Chen-Li Sun
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/04861210450887853560

碩士 === 國立臺灣科技大學 === 機械工程系 === 97 === In this study, the evolution of flow field in a microdiffuser under sinusoidal pressure actuation is investigated by using 贡PIV (micro particle image velocimetry). Three parameters are explored : half-angle, pressure amplitude and excitation frequency. From the micro-PIV results, the influence of excitation frequency is neglected. Flow separations are observed in large-half-angle microdiffuser. For Cam40, recirculations appear in microdiffusers with half angle > 35°. Increasing the pressure amplitude to Cam50 or Cam60 leads to flow separation in microdiffusers with 针half angle < 25°. When the magnitude of positive pressure increase, the backflow region grows. In contrast, the backflow region shrinks when the magnitude of positive pressure decreases. In general, best flow rectification occurs when on flow separation or very small recirculation appears in a microdiffuser. According to the vortex evolution, flow separation appears at a earlier time and disappears at a later time as the half-angle increases, resulting in a larger vortex duration for large-half-angle microdiffuser. Moreover, vortex duration grows with the preesure head prominently for 针 40°. For the half angle汹> 40°, the influence of pressure head in vortex duration is weak. Besides the primary vorticity zone in circulation, there exists a pair of secondary vorticity peaks near the wall. When the magnitude of positive pressure head increases, all vorticity regions grow. In general, maximum vorticity in a microdiffuser coincides with maximum positive pressure. While the half angle increases, velocity gradient in a microdiffuser decreases and so does the average vorticity. From topological analysis, separation and reattachment points are classified as negative/positive bifurcation lines, and recirculations are identified as foci points. Three-dimensional fluid motion is observed near the back flow region. While fluid flows toward the center of the back flow, the fluid is also pulled toward the top or bottom of microdiffuser, resulting in a helical motion. Then the fluid is flushed away by the though flow in the expansion direction