Abstract:
The supersonic combustion ramjet (Scramjet) engine is an engineering marvel for next-generation hypersonic flights. It typically operates at speeds greater than Mach 5 and utilizes the forward momentum of the flight to compress the incoming air. Even after compression, the air stream remains supersonic, and fuel-air mixing and combustion occur at the supersonic speed. Due to high streamwise momentum and short residence time, efficient mixing of fuel jets with incoming air has been a long concern for the development of scramjet-powered vehicles.
In this thesis, the mixing of steady and partially modulated pulsating fuel jets in supersonic airflow is numerically investigated under different jet oscillating conditions. Gaseous hydrogen is injected as fuel and the pulsation is achieved by sinusoidally perturbing the injection pressure profile. The frequency and amplitude of pressure perturbation are varied in such a way that the jets are never off and hence can be categorized as partially modulated jets. The base pressure of oscillation is set as that of the equivalent steady jet to produce the same cycle-averaged mass flow rate of fuel for the direct comparison of two injection schemes. Injection frequencies of 4, 8, 16, 32, and 64 kHz are studied with amplitudes of 0.25, 0.50, and 0.75 times the base pressure. The flow fields are mathematically modeled using the Reynolds-Averaged Navier-Stokes (RANS) equations with additional equations for species conservation. Turbulence closure is obtained by applying the SST k − ω model. The results show complicated shock structures inside the combustor due to the fuel injection in supersonic airflow. Vorticity is produced at the jet shear layer and at the interface where the shock wave impinges the mixing plume. The shock waves become unsteady in response to the pulsation in fuel injection pressure. The concentration of fuel increases during the increasing phases of the pulse and quickly decays due to the back-and-forth oscillation of the bow shock, indicating unsteady shock-induced mixing inside the combustor. Both the penetration and mixing for the pulsed jets are higher than the equivalent steady jet irrespective of the injection frequency and amplitude. For the fuel injection frequency of 32 kHz with an amplitude of 0.75 times the base pressure, about 39% fuel-air mixing is obtained at the combustor exit which is 5% higher than the equivalent steady jet. The time-averaged results also reveal that the pulsed jets introduce no additional penalty in total pressure and thus unveil a minimally intrusive fuel delivery technique in supersonic airflow.