Numerical Simulation of SCRAMSPACE I Flight Experiment

Abstract

The results for the internal flowpath at zero degrees angle of attack for the 'SCRAMSPACE I' axially-symmetric, free-flying, supersonic combustion ramjet (scramjet) hypersonic flight experiment are presented. The results demonstrate the performance of the intake, combustion chamber and contoured nozzle for fuel-on and fuel-off conditions at 27km altitude. The simulations investigate the effect of three popular turbulence models on the flowfield and compare them to laminar flow assumptions. The choice of turbulence model is shown to have a strong influence on the internal flowfield, with significant implications for the engine starting characteristics. The results show that engine unstart occurs if laminar flow is assumed. The unstart is caused by a boundary layer separation propagating upstream and into the scramjet inlet, which originates near the end of the constant area (circular) cross-section combustion chamber. This process can be suppressed by applying the shear stress transport (SST) turbulence model using a 1% freestream turbulence intensity (Fig 1.), but use of the Spalart-Allmaras (SA) model with 1% freestream turbulence instensity proves insufficient to resist the upstream propagation of the separation region. The physical basis for this behaviour is discussed and means to mitigate the risk posed by this phenomenon proposed. Premixed combustion simulations using the ES2 combustion model show that small regions of separated flow within the combustion chamber can play a significant role in the combustion process through the scramjet engine. he SCRAMSPACE I hypersonic flight experiment seeks to measure the reduction in drag when Hydrogen fuel is pulsed on and off for a free-flying, Mach 8, radical farming, axially symmetric scramjet (supersonic combustion ramjet) as it descends from 32km to 27km altitude, using a typical HyShot trajectory. The flight experiment requires a close coupling between many interdependent disciplines to ensure that the SCRAMSPACE I vehicle can be deployed, reach the experiment point and perform within specification. Two of the major factors determining the success of the flight are the static stability margin, defined as the distance between the centre of gravity and pressure and expressed in percent of total vehicle length, and the requirement for the scramjet engine to remain started throughout the flight envelope over a range of +/- 2-degrees angle of attack. This work will focus on the aerothermodynamic aspects of the vehicle and will present the results from a number of computational fluid dynamics (CFD) analyses for the external and internal flowpaths for the vehicle. The CFD results are built from the confidence gained from the results of an experimental campaign conducted within the University of Queensland T4 shock tunnel. The experiment demonstrated that the scramjet remains started throughout the experiment and over the specified angle of attack range. It was further found that significant heat release was produced by the combustion process when a fuel equivalence ratio of 0.5 is used. The experimental work did not directly measure the thrust produced by the engine, but did generate excellent pressure data with which to validate CFD simulations for fuel-on and fuel-off cases. The experimental and accompanying 3D fuel injection CFD results will not be presented here, as they form the PhD thesis work of others, rather we will focus on the fueloff condition and the important factors that affect stability and engine starting. The external shape of the vehicle has not been optimized to produce a net thrust therefore the vehicle is expected to produce a net drag force. The goal of the flight experiment is therefore to measure the change in drag when the fuel is pulsed on and off and measure the pressure and heat flux through the inlet and combustion chamber.