Numerical investigation of the effect of flame holder geometry on air-fuel mixture in scramjet engines

Scramjet engines are jet engines designed to compress air to supersonic speeds and then perform combustion at that speed. Unlike traditional jet engines, scramjets do not have a rotating fan or compressor. Instead, the air is accelerated and compressed by fixed parts and the combustion process is carried out by spraying fuel. Research has shown that the most suitable fuel for scramjet engines is hydrogen in terms of the energy it contains. In literature studies, different methods have been tried to ensure that high-speed air and hydrogen are mixed and burned more efficiently in scramjet engines. The use of flame holders with different geometries, different hydrogen injection methods, and shock wave generation mechanisms are among these methods. In this thesis, the objective is to realize a hybrid geometric flame holder design composed of the combination of basic geometric shapes, with the aim of expanding the recirculation zones to enhance the air-fuel mixture. To achieve this, combinations of trapezoidal, rectangular, triangular, and semicircular geometries have been utilized. Parameters such as aft wall angle and flame holder depth have been defined as variable parameters for the created flame holder geometries. In this study, the effects of flame holder geometry with different depths (H=14 mm, 22 mm, 30 mm, and 35 mm) and rear angles (α=50°, 70°, and 90°) on air-hydrogen mixture, temperature, and pressure were investigated numerically. The increase in flame holder depth has ensured a more homogeneous mixture of air-hydrogen by generating larger-sized and uniformly distributed vortex flows within the flame holder.The increase in the rear angle caused the air-hydrogen mixture to concentrate in the exit region of the flame holder. It was understood that at a low rear angle (α=50°), air and hydrogen mixed more homogeneously. For the same depth and rear wall angle values, a more uniform and higher concentration distribution of hydrogen has been achieved in the trapezoidal+triangle geometry. On the other hand, for the rectangular flame holder, a decrease in pressure on the flame holder walls has been observed at low depth values, while the variation in depth and rear wall angle in other hybrid geometries has had a negligible effect on pressure. In trapezoidal and trapezoidal+triangle geometries, a uniform temperature distribution can be obtained within the flame holder depending on the depth and rear wall angle, whereas the rectangular flame holder geometry exhibited a non-uniform temperature distribution for all parameter values. Taking all findings into consideration, it is concluded that the base surface geometry of the flame holder has a directing effect on the flow. Furthermore, optimization of both the flame holder depth and rear wall angle is crucial for efficient combustion and thrust performance depending on design criteria such as size and material type.