因燃料於超燃衝壓引擎燃燒室的滯留時間極為短暫，故需要於燃燒室設置駐焰機構，使燃料得以滯留霧化並與空氣混和。凹槽做為一非侵入式駐焰機構，能夠創造一個低速的迴流區以利點火及穩焰，且相對於其他駐焰機構具有較小的壓力損失。
凹槽構型的不同會影響到凹槽內的迴流區、混和特性及空氣質量交換率，也會影響到後續點火駐焰的條件。本研究將以L/D=3、後壁面22.5度基本構型為基礎，延伸出L/D=4凹槽構型、後壁面18度凹槽購型及L/D=3加大構型，共四個凹槽構型於馬赫2連管風洞進行實驗。風洞自由流全壓~122psi、全溫~760K，測試段靜壓~1bar，而空氣總質量流率約為3.36kg/s。
冷流實驗研究以槽內4.5g/s燃油流量進行觀察不同凹槽構型之迴流區與燃油混和霧化效果，點火實驗則以調整槽內噴油量，找出各個凹槽能夠穩定駐焰的最佳條件，並與冷流結果進行比對。
實驗結果顯示L/D=3基本構型於槽內噴注~4g/s燃油時可點火駐焰，當後斜壁角度減少，會造成迴流區域縮小，使得大量燃油直接匯入主流場且混和效果較差；點火燃燒時，槽內需要較大量的燃油(~5g/s)才得以駐焰。冷流觀察顯示在凹槽深度固定的情況下加大凹槽長深比(L/D)會使得混和效果較好；於凹槽長深比固定下，加大凹槽容積會造成剪切層略向槽內推動的狀況且混和效益較佳。雖已知各凹槽混和特性，但燃油迴流槽內與匯入自由流的比例仍未控制，點火實驗顯示L/D=3基本構型、L/D=4凹槽構型、L/D=3加大構型的駐焰條件皆為槽內~4g/s的燃油流量。在各型凹槽駐焰條件下，槽外上游8g/s燃油噴注狀態，均成功引火燃燒。

In a scramjet engine, since the residence time of the fuel in the combustion chamber is extremely short, it is necessary to install a flame-holding mechanism in the combustion chamber so that the fuel can be retained, atomized, and mixed. The cavity-based is a non-intrusive flame-holding mechanism that can create a low-speed recirculation zone to facilitate ignition and flame stabilization and has a smaller pressure loss than other flame-holding mechanisms.
The difference in cavity-based geometry will affect the recirculation zone, mixing characteristics, and air exchange rate within the cavity, as well as the conditions for subsequent ignition and flame anchoring. This research will be based on the basic geometry of L/D=3, the rear wall 18-degree, and then extend the L/D=4 geometry, the rear wall 18-degree geometry, and the enlarged L/D=3 geometry, there are four cavity-based geometry in total.
Cold flow experiments involve observing the recirculation zones and fuel mixing and atomization effects for different cavity-based geometry using an internal fuel flow rate of 4.5g/s within the cavity. Ignition experiments, on the other hand, involve adjusting the fuel injection rate within the cavity to determine the optimal conditions for stable flame anchoring for each cavity-based and comparing these results with the observations from the cold flow experiments.
The experimental results indicate that the basic geometry of L/D=3 can achieve flame ignition and anchoring with an internal fuel injection of around 4g/s. When the rear wall angle decreases, it results in a smaller recirculation zone, causing a significant amount of fuel to directly enter the mainstream flow with poorer mixing efficiency. During ignition combustion, a larger amount of fuel (~5g/s) within the cavity is required to achieve successful flame anchoring. Cold flow observations demonstrate that increasing the cavity-based length-to-depth ratio (L/D) under fixed cavity depth conditions improves mixing efficiency. Under fixed L/D conditions, enlarging the cavity volume causes a slight inward push of the shear layer into the cavity, resulting in better mixing efficiency. Although the mixing characteristics of each cavity-based are known, the proportion of fuel recirculating within the cavity versus flowing into the freestream remains uncontrolled. Ignition experiments reveal that the optimal flame anchoring conditions for the basic geometry of L/D=3, L/D=4 geometry, and enlarged L/D=3 geometry all correspond to an internal fuel flow rate of around 4g/s inside the cavity.

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