Large Eddy simulation of turbulent hydrogen-fuelled supersonic combustion in an air cross-flow

Abstract

The main aim of this article is to provide a theoretical understanding of the physics of supersonic mixing and combustion. Research in advanced air-breathing propulsion systems able to push vehicles well beyond is of interest around the world. In a scramjet, the air stream flow captured by the inlet is decelerated but still maintains supersonic conditions. As the residence time is very short , the study of an efficient mixing and combustion is a key issue in the ongoing research on compressible flows. Due to experimental difficulties in measuring complex high-speed unsteady flowfields, the most convenient way to understand unsteady features of supersonic mixing and combustion is to use computational fluid dynamics. This work investigates supersonic combustion physics in the Hyshot II combustion chamber within the Large Eddy simulation framework. The resolution of this turbulent compressible reacting flow requires: (1) highly accurate non-dissipative numerical schemes to properly simulate strong gradients near shock waves and turbulent structures away from these discontinuities; (2) proper modelling of the small subgrid scales for supersonic combustion, including effects from compressibility on mixing and combustion; (3) highly detailed kinetic mechanisms (the Warnatz scheme including 9 species and 38 reactions is adopted) accounting for the formation and recombination of radicals to properly predict flame anchoring. Numerical results reveal the complex topology of the flow under investigation. The importance of baroclinic and dilatational effects on mixing and flame anchoring is evidenced. Moreover, their effects on turbulence-scale generation and the scaling law are analysed

Some Key Issues in Hypersonic Propulsion

“…This range coincides with that of the air at the combustor entrance, and information from DNS of astrophysical high-Mach turbulence seems to confirm the energy cascade model of K41 [11]. This said, preliminary analyses of compressibility effects on turbulence [12] indicate they may play a role in supersonic combustion, for instance, by creating turbulent KE through the baroclinic and dilatation effects and by increasing kinetic rates. This is a frontier area in turbulence theory.…”

“…Fast mixing depends on 'turbulence', that is, on sufficient vorticity, ω. In crossflow injection, normal or angled, penetration is a function of J, and both air and fuel KE contribute to mixing: vorticity is formed by stretching but also by baroclinic torque and dilatation, as shown by the Equation ( 7) reported below [12]. These additional terms (the second and third on the RHS of Equation ( 7)) are associated to compressibility and should not be ignored by modelers:…”

Large eddy simulations of the HIFiRE scramjet using a compressible flamelet/progress variable approach

“…Such approaches require the closure of the chemical source term in the species transport equations due to nonlinear interactions with the turbulent flow field. Although many types of closure models have been investigated [3][4][5][6][7][8][9][10], research is ongoing to construct models that describe fundamental physics, reduce computational expense, and increase accuracy and robustness of simulation tools.…”

The flow physics of inlet-fueled, low-compression scramjets

“…Although it should be noted that for Figure 7.7c the slope appears to be marginally steeper than suggested by the −5/3 power law. In fact several studies by Ingenito et al [93,95,96] imply that the turbulence kinetic energy decays faster due to compressibility effects. According to Ingenito et al [93], for isotropic, compressible turbulence additional vortex transport mechanisms aside from vortex stretching, such as dilatation, compression and baroclinic torque, become relevant for the turbulence kinetic energy decay, which led to the conclusion of an increases exponent of −8/3 for the turbulence kinetic energy power law.…”

“…Experimental studies indicate as well an increased exponent of −11/3 for the turbulence kinetic energy power law [16,164]. In subsequent investigations of the HyShot combustor by Ingenito et al [95,96] inconclusive data is presented, showing spectral data matching a range of power laws. In summary, the physics of compressible turbulence, in particular with regard to scramjet flows, are still subject to ongoing research.…”