Optimising Porthole Geometry for Maximum Jet Strength

Research into the use of scramjet engines has become more prevalent due to the fact that scramjet-based launch systems are seen as a promising way to access space in terms of safety, reliability and cost. Currently, a critical issue concerning scramjet operation is optimising the fuel and air mixing efficiency of scramjet engines. Therefore, to improve fuelling and to optimise fuel and air mixing, non-circular porthole jets are currently being investigated.This thesis has studied the effects of sonic fuel injection via injectors on fuel and air mixing for a crossflow of air at Mach 5, assuming airflow after the initial compression in a scramjet engine operating at Mach 10 at an altitude of 26 km. Injector configurations with different orifice geometries have been selected and examined in this investigation, including square, rectangular (aspect ratio of 4:1), circular and diamond injectors. The cross sectional area of each geometry has been kept constant to ensure a constant mass flow injection.Modelling has been performed using the Eilmer3 CFD Code, which solves the Reynolds-averaged Navier-Stokes equations for steady flowfields in the threedimensional domain, to examine invicous interactions created by the various injector geometries. A mesh was designed with the aim of encompassing the entire flowfield, while enabling the user to easily change the injector size, plane orientations and block sizes through the use of automated python scripts. Three objectives, namely, mixing efficiency, fuel penetration height and streamwise circulation, were used to characterise the performance of each injector orifice.Flowfields produced by the different injectors were created and analysed with respect to parameters such as Mach number, pressure, density and streamwise vortical formations. The results of the investigation found that the square injector was superior in fuel penetration height, due to its wide fore opening and stronger bow shock formation. Although all injector geometries were determined to exhibit very similar mixing efficiencies, the 4:1 rectangular injector was determined to be slightly superior in mixing efficiency, benefiting from a so-called buffering effect.