超音速燃燒室噴注氫氣與鋁顆粒加入多孔性圓柱燃燒器之數值模擬與分析

Abstract

本研究由免費開源CFD軟體OpenFOAM進行超音速燃燒衝壓引擎添加鋁顆粒燃料。鋁是體積能量密度極高的燃料，然而鋁顆粒的點火需要較長時間且高溫的受熱環境，在文獻上大多以火箭藥裝或二次燃燒之方式來燃燒鋁顆粒，且為了增加其停留時間，會以凹腔或增長燃燒室來使其完全燃燒，但這樣的方式容易腐蝕燃燒室壁面使材料耗損，少有人探討以支架式的超音速燃燒室的鋁顆粒燃燒，因此本文對相關物理模型進行開發與驗證，參考了前人文獻之算法、物理模型來對此問題進行研究。 而通過裝入多孔性圓柱燃燒器於超音速流場中，能使原本因減少氫氣而無法燃燒的鋁/氫燃料能有效燃燒，這是因為多孔性圓柱燃燒器的加入改變了流場許多物理上的限制，對於氫氣來說，加入多孔性圓柱燃燒器使其在圓柱內或表層預混，降低其燃燒極限門檻，使點火延遲簡短，進一步使鋁顆粒能提早進行預熱，並且在支架與圓柱燃燒器間的高溫回流區進行多次碰撞彈跳，鋁顆粒在下游將能更有效、更快地進行化學反應。 由研究結果表明，加入多孔性圓柱燃燒器能在添加鋁顆粒減少氫氣使用量的情況下讓燃燒室產生推力，並提升鋁的消耗速度來提升燃燒效率；同時也發現由支架後的燃料注入口控制較多氫氣質量流率而不是多孔性圓柱燃燒器，能夠使鋁有較好的燃燒效率；在加入多孔性圓柱燃燒器的情況下，以支架後方注入燃料能比支架側向注入燃料有更好的鋁顆粒燃燒效率；在壁面加入凹腔能夠讓圓柱燃燒器後方的回流區進行橫向擴張，使氫氣燃燒效率提升。

This study utilizes the open-source CFD software OpenFOAM to investigate the addition of aluminum particles as fuel in a supersonic combustion ramjet engine. Aluminum has a very high volumetric energy density, but igniting aluminum particles requires a prolonged period and a high-temperature heating environment. In the literature, aluminum particles are often burned using rocket propellants or secondary combustion methods. To increase their residence time, cavities or extended combustion chambers are employed to ensure complete combustion. However, such methods tend to corrode the combustion chamber walls, leading to material loss. There is limited research on the combustion of aluminum particles in a strut-based supersonic combustor. Therefore, this study develops and validates relevant physical models, referencing previous literature on algorithms and physical models to explore this issue. By installing a porous cylindrical burner in the supersonic flow field, aluminum/hydrogen fuel, which could not be burned due to reduced hydrogen, can be effectively combusted. The introduction of the porous cylindrical burner changes many physical constraints of the flow field. For hydrogen, the porous cylindrical burner allows premixing inside or on the surface of the cylinder, lowering the combustion limit threshold, shortening the ignition delay, and further enabling the aluminum particles to preheat earlier. Additionally, in the high-temperature recirculation zone between the strut and the cylindrical burner, multiple collisions and rebounds occur, allowing the aluminum particles to react more effectively and quickly downstream. The results indicate that the addition of a porous cylindrical burner enables the combustor to generate thrust even with reduced hydrogen usage and increases the aluminum consumption rate to improve combustion efficiency. It was also found that controlling a higher hydrogen mass flow rate from the fuel injection port behind the strut, rather than the porous cylindrical burner, results in better combustion efficiency for aluminum. With the addition of a porous cylindrical burner, injecting fuel behind the strut achieves better aluminum particle combustion efficiency compared to lateral injection. Adding a cavity to the wall can laterally expand the recirculation zone behind the cylindrical burner, improving hydrogen combustion efficiency.