Fuel Injector Design of a Hypersonic Jet Engine Using Computational Fluid Dynamics

The development of hypersonic airbreathing engines, such as a supersonic combustion ramjet, or scramjet, are implemented for flight Mach numbers over 5 where combustion must occur in supersonic conditions. The advancement of scramjet propulsion has led to favored usage over rocket propulsion systems for in atmosphere applications due to their lighter weight, higher specific impulse, and greater maneuverability [1]. The combustor section of a scramjet engine houses the fuel injectors. Fuel is injected into the supersonic flow with the main objective of achieving rapid and thorough fuel-air mixing because the residence time in the combustion chamber has a timescale of about 1 millisecond [1]. Therefore, fuel injection mixing and combustion must occur on a timescale of that order. Taking into account complications such excess heating and air dissociation is critical while still reducing pressure and momentum losses in the combustor [1]. This research study focuses on the influence of a cavity flameholder and the addition of a second fuel injector to the German Aerospace Centre (DLR) scramjet engine. These were used to determine the effects in combustion efficiency and total pressure loss compared to the standard model. A numerical study using the Reynolds Averaged Navier Stokes (RANS) equations with the shear stress transport (SST) k-omega turbulence model was utilized. To model combustion, the finite-rate/eddy dissipation model with a single-step hydrogen-air reaction mechanism were utilized. The walls of the combustor were assumed to be no-slip and adiabatic. The DLR model with the cavity flameholder enhanced the recirculation between the fuel and air. For the DLR model with the cavity flameholder, it was determined that a 0.4% decrease in combustion efficiency and 0.5% decrease in total pressure loss occurred when compared to the standard DLR model. The DLR model with the two fuel injectors increased the amount of fuel added to the flow, created two mixing zones for the fuel, and enhanced the fuel and air mixing from the increased shock waves having two struts produced. A bow shock was created by the parallel fuel injectors. For the DLR model with the two fuel injectors, it was determined that a 1.4% increase in combustion efficiency and 49.5% increase in total pressure loss occurred when compared to the standard DLR model. Modifying the fuel injector in various forms can extend the study to determine a combustor design that produces high combustion efficiency with low total pressure loss across the combustor. A standard cavity flameholder was used in the present study and by changing the dimensions or adding more steps in the cavity flameholder can greatly enhance the recirculation between the fuel and air. As well as, designing the cavity flameholder to be closer to the fuel injector for more enhanced recirculation can be studied. Three-dimensional studies of the DLR combustor is important to comprehensively observe the behaviors of the flow