在超音速流場中達成燃燒反應的困難在於燃料與氣流在燃燒室停留時間極短，難以進行燃燒反應。而凹槽駐焰器能提供一低速迴流區增加燃料與空氣停留時間並進行燃燒反應，燃燒反應的熱釋放與生成物能進一步加速點燃自由流中的燃料。與其他形式的駐焰器相比，凹槽駐焰器有著阻力小以及總壓損失少的優勢，因此有許多團隊使用凹槽駐焰器進行地面與飛行測試。凹槽駐焰器的流場中存在著超音速與次音速的交互作用、次音速迴流區，以及壓縮波傳遞造成的不穩定現象，這些現象會影響燃料與空氣在凹槽中混合以及燃燒的情形，並且是傳統的紋影法或陰影法無法觀測，因此需要使用粒子影像測速儀進行二維的流場定量量測。
本實驗在運作時間為0.8 ms的震波風洞中以流體化床將粒徑250 nm的示蹤粒子投入自由流流速為馬赫2的測試段中，透過邏輯閘進行兩道雷射與高速攝影機的時序控制，並以此套系統量測L/D＝3的凹槽於馬赫2測試條件產生的流場，成功拍攝到間隔約為500 ns的粒子影像。實驗將流體化床全壓固定為90 psia，並調整流化速度為15 cm/s，觀察不同噴注孔對粒子分布的影響，結果顯示凹槽上游噴注的粒子能被捲入迴流區之內，並且此流化速度所產生的粒子量過多，使得流場計算結果無法呈現迴流區的流場。後續調降流化速度至10 cm/s，此參數產生的粒子量能進行PIV運算，計算結果成功顯示出迴流區、剪切層以及氣流於後壁面轉向產生的斜震波，流場結構與先前紋影法的觀測結果對比後相符，但由於粒子慣性較氣體分子高，在經由斜震波減速後較遠的距離才會達到流場速率，因此計算結果呈現的斜震波位置相對紋影法會更靠近斜壁面。為了避免迴流區的粒子在500 ns的時間內位移量太少造成程式誤判，因此保持流體化床參數並將雷射間隔時間延長至1 μs。結果顯示過長的時間造成粒子的減速行為被平均速度所掩蓋，速度變化極大的區域如剪切層與斜震波其特徵皆消失。然而在500 ns與1 μs的實驗結果皆在凹槽底部出現速度高達自由流40%~50%的逆向流場，此現象於國外的矩形凹槽研究中亦有發現，不過本研究中此現象的詳細成因以及對凹槽內部燃料與空氣混合的影響值得後續持續研究。
以PIV計算結果與紋影法觀測的影像對比來看，本實驗所架設的超音速PIV系統能成功量測出凹槽駐焰器內部的流場結構，後續可替換不同構型的凹槽進行量測，以流場量測結果優化燃料噴注位置及角度，再進行連管風洞的燃燒實驗。後續可以在連管風洞另架設一套PIV系統，能觀測凹槽內流場發展的情形，並搭配燃燒實驗觀察燃料噴注與燃燒反應對流場的影響。

The subject of this study is developing a PIV system in a shock tunnel for investigating the supersonic cavity flow. The shock tunnel provides Mach 2 freestream with 0.8 ms test time in the test section. 250 nm SiO2 particle is fluidized first in a fluidized bed with total pressure 90 psia and fluidized speed 10 cm/s, then starts to inject into the test section through the holes on the cavity flame holder(L/D=3) before the shock tunnel operating. Two Nd:YAG pulse lasers are used in this study to create two laser sheets. The particle image pair is recorded by Phantom v711 high speed camera, which is triggered by a signal from the pressure sensor on the shock tunnel wall. The camera trigger signal and shutter signal are performed AND operation to indicate the start time of the first frame. Thus, the time of the two laser sheets can be counted from this time point and the particle images are exposed on the two-consecutive frame with 500 ns separation time.
The PIV results show recirculation zone、 shear layer、 oblique shock wave within the cavity flame holder. The shear layer turns into the cavity and reattaches to the bottom wall. The freestream turns into the cavity by an expansion fan, accelerates to over 1600 m/s and turns out along the rear wall by an oblique shock at rear wall. These flow structures are also observed in previous schlieren images. The PIV results also reveal that there is a high speed reversed flow existing in the bottom of cavity. However, further research is needed to determine the cause of this phenomenon.

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