在超音速燃燒衝壓引擎中，燃料在燃燒室內的滯留時間非常短暫，需要駐焰機構以增加燃料停留時間與燃燒穩定性，而在設計上必須考慮壓力的損失，因此凹槽駐焰特性被廣泛的探討。一般來說，在燃料的噴注位置的選擇上有兩種，一種在凹槽上游進行噴注，可獲得大範圍的燃燒區域；另一種是在凹槽內進行噴注，有利於凹槽內局部穩定燃燒，但無法得到大範圍的燃燒。本研究於凹槽前壁面進行液態燃料噴注，透過改變凹槽幾何形狀與燃料噴注方式，以觀察流場與燃料霧化的現象。實驗結果顯示，斜向噴注時，在剪切層上有較多燃料剝離霧化，而在水平噴注時，燃料集中於凹槽底部在迴流區中混合，因此可藉由噴注角度的改變，調整燃料進入迴流區的比例。在凹槽的幾何形狀上，直角凹槽有較大的迴流區域使燃料可以在迴流區內混合，但是氣流不易夾帶燃料至主流場中；而斜面凹槽迴流區域較小，但是氣流將燃料夾帶至後壁面時，燃料可在此區域進行二次氣化混合。

In a very short residence time, a proper mixing between liquid fuel and supersonic air flow must be achieved, following by stable combustion which also highly relies on the resulted of the mixing effects. During the last few years, cavity in flow path has gained the attention of the supersonic combustion community as a promising flame-holding device. Experimental studies of the effects of cavity in Mach 2 airflow have been conducted in a reflected shock tunnel at National Cheng Kung University. The objective of this thesis research is to observe the mixing behavior of cavity base-injected fuel spray in supersonic airflow. Two open-type cavities (90 and 22.5 rear-wall angles) are tested in this research, and Schlieren photography coupled with high-speed camera are adopted for flow and spray visualization.
Experiments are performed with various injected fuel mass flux (m ̇) and injection angle (δ^*). Results show a portion of fuel entry the shear layer and mixed with the high speed airflow, while the rest of the fuel circulates inside the recirculation zone and mix with air and then escapes from the cavity through the near-wall flow path. More fuel enters the shear layer with the higher angle injection (δ^*=30° , relative to the horizontal wall) than that of the lower angle ( δ^*=30° ) one, and 90 rear-wall cavity provides larger recirculation zone than that of the 22.5 one however less air entrainment rate to the bulk flow. For the 22.5 rear wall cavity, secondary mixing zone is shown besides the mixing occurring in the recirculation zone. By adjusting the fuel injection angle, one can meter the mixing ratio distribution at the downstream for the purposes of near-wall ignition and possibly flame holding.

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