Modeling Scramjet Supersonic Combustion Via Eddy Dissipation Model

Scramjet technology has gained considerable interest in multi-stage to orbit design concepts due to its reusability and high specific impulse at high-Mach regimes. The aim of the present work is to introduce Reynolds Averaged Navier-Stokes CFD calculations in the design phase of scramjet vehicles and increase the fidelity of engine performance assessment. The turbulence-chemistry interaction is described by the Eddy Dissipation Model (EDM) introduced by Magnussen and Hjertager, which assumes that turbulent motions and not chemistry is the main driver in the rate of combustion. The use of the EDM is explored by application to three hydrogen fueled scramjet test cases. The model requires constants to be prescribed, which have found to be case dependent. Optimal values for the cases simulated are discussed along with appropriateness of the model for general design simulations. The advantage in computational cost is demonstrated by comparison with a no-model finite-rate chemistry approach

Effect of Vortex-injection Interaction on Wall Heat Transfer in a Flat Plate and Fin Corner Geometry

More flexible and economical access to space is achievable using hypersonic air-breathing propulsion. One of the main challenges for hypersonic air-breathing propulsion is reaching high combustion efficiency within the short residence time of the flow in the engine. Lengthening the combustor is not a viable option due to its many drawbacks, and the use of hypermixers or strut injectors increases mixing efficiency at the cost of increasing losses and heat load. On the contrary, inlet-generated vortices are an intrinsic feature of many scramjet inlets, and can be used to enhance mixing, incurring minimal losses and heat load increase. A previous computational study used a canonical geometry consisting of a flat plate with a fin at different deflection angles to investigate the ability of inlet-generated vortices to enhance the mixing rate. Significant increases in mixing rate were obtained due to the vortex-fuel plume interaction. The flow conditions were equivalent to those found in a rectangular-to-elliptical shape transition scramjet inlet at a Mach 12, 50 kPa constant dynamic pressure trajectory. Despite the minimal heat load increase of this approach, characterization of the vortex-fuel plume interaction effect on the wall heat transfer is required. In this work, the previous study is extended, describing the effect of the vortex-fuel plume interaction on wall heat transfer. Heat flux in the vicinity of the porthole injector reaches 200 % compared to the baseline case with no vortex interaction. Moreover, the injection bow shock affects the corner region, creating pockets of heat flux up to 75 % larger than the unaffected region. Additionally, the evolution of the fuel plume downstream of the injector location is investigated, describing the relationship between local maxima and minima of heat flux, and the location of the fuel on the wall surface. This relationship can be exploited in experimental data acquisition to obtain the fuel location from heat flux data. The viability of this experimental approach is explored using computational data, confirming that through careful sensor placement, position measurements with an accuracy higher than ±5 mm can be achieved

Numerical study of the effect of wall temperature profiles on the premixed methane–air flame dynamics in a narrow channel

Time-accurate simulations of premixed CH/air flame in a narrow, heated channel are performed using the DRM-19 reaction mechanism. The effect of different wall temperature profiles on the flame dynamics is investigated for three different inflow velocity conditions. At a low inflow velocity of 0.2 m s, the flame shows instabilities in the form of spatial oscillations and even flame extinction. With the increase of the inflow velocity, flames are prone to showing more stability at a medium inflow velocity of 0.4 m s, and eventually show flame stabilisation at a high inflow velocity condition of 0.8 m s for all the wall temperature profiles examined. The total chemical heat release rate and total gas-solid heat exchange rate are found to have a combined effect on the flame propagation speed that determines flame behaviours. Since the flame behaviours in terms of the oscillation frequency and amplitude for spatially oscillating flames, or the stream-wise stabilisation location for steady-state flames, are very sensitive to the chosen wall temperature profile, a "real" conjugate heat transfer model is recommended in order to capture all of the relevant combustion physics accurately

On the influence of modelling choices on combustion in narrow channels

This paper examines the effect of modelling choices on the numerical simulation of premixed methane/air combustion in narrow channels. Knowledge on standard and well-accepted numerical methods in literature are collected in a cohesive document. The less well-established modelling choices have been thoroughly evaluated and discussed. A systematic method of computing the grid convergence index (GCI) has been presented for refining the computational grid. Two types of inflow boundary conditions have been tested and compared in terms of their wave-damping characteristics. The effect of different reaction schemes on simulation results have been examined and an appropriate mechanism (DRM-19) has been selected. Various types of ignition strategies to initiate the flame have been tested and compared. The transient ignition process which has not been discussed extensively in existing literature has been quantitatively described in this paper

Turbulence chemistry interaction via eddy dissipation model for scramjet analysis and design

This paper considers the Eddy Dissipation Model to address the combustion process inside scramjet engines designed to operate at high flight Mach numbers. The aim is to demonstrate the most appropriate use of the model for design purposes. To this end, two hydrogen-fueled experimental scramjet configurations with different fuel injection approaches are studied numerically. In the case of parallel fuel injection, it is demonstrated that relying on estimates of ignition delay from a one-dimensional kinetics program can greatly improve the use of the EDM. In the second case, the transverse injection of hydrogen resulted in an overall good agreement of the model with experimental pressure traces except in the vicinity of the injection location. Overall, the EDM appears to be a suitable tool for scramjet combustor design incorporating a parallel or transverse fuel injection mechanism