Great efforts have been dedicated during the last decades to the research and development of hypersonic aircrafts that can fly at several times the speed of sound. These aerospace vehicles have revolutionary applications in national security as advanced hypersonic weapons, in space exploration as reusable stages for access to low Earth orbit, and in commercial aviation as fast long-range methods for air transportation of passengers around the globe. This review addresses the topic of supersonic combustion, which represents the central physical process that enables scramjet hypersonic propulsion systems to accelerate aircrafts to ultra-high speeds. The description focuses on recent experimental flights and ground-based research programs and highlights associated fundamental flow physics, subgrid-scale model development, and full-system numerical simulations.

Supersonic Combustion in Air-Breathing Propulsion Systems for Hypersonic Flight

Modeling of turbulent dilute spray combustion

Computational Models for Turbulent Reacting Flows: References

Flame propagation and stabilization in dual-mode scramjet combustors: A survey

Cavities are widely used as flameholders in supersonic combustors due to their outstanding potential to stabilize combustion without excessive total pressure loss. A review of cavity-stabilized combustion for scramjet applications is provided in this article. The topics cover the fundamental problems and recent advances regarding cavity-organized combustion in high-speed flows, including combustion stabilization modes and mechanisms, flame stability analyses and correlations, combustion oscillations, and other related issues. Remarkable questions such as cavity-coupled fuel injection, flow and combustion coupling, optimal cavity geometry and scale, auto-ignition and flame propagation interactions, and unsteady effects are discussed. Then, an attempt is made to provide some guidelines for the future research of cavity flameholders.

Review of cavity-stabilized combustion for scramjet applications:

The joint-scalar probability density function (PDF) approach provides a comprehensive framework for large eddy simulation (LES) based combustion modeling. However, currently available stochastic approaches for solving the high-dimensional PDF transport equation can be error prone and numerically unstable in highly compressible shock-containing flows. In this work, a novel Eulerian approach called the direct quadrature method of moments (DQMOM) is developed for evolving the PDF-based supersonic combustion model. The DQMOM technique uses a set of scalar transport equations with specific source terms to recover the PDF. The new technique is coupled to a compressible LES solver through the energy equation. The DQMOM approach is then used to simulate two practical flow configurations: a supersonic reacting jet and a cavity-stabilized supersonic combustor. Comparisons with experimental data demonstrate the predictive accuracy of the method.

A quadrature-based LES/transported probability density function approach for modeling supersonic combustion

The isolator is an important flow section in a dual-mode scramjet engine that provides stable compressed flow to the combustor. However, if the combustor-induced backpressure exceeds a limiting value, the compression structure inside the isolator could be disgorged, leading to inlet unstart. Numerical tools that can predict unstart would be valuable in the design of robust scramjet engines. Here, the predictive capability of the large-eddy-simulation methodology is assessed by validating against experimental studies of isolator unstart. A conservative finitedifference-based large-eddy-simulation approach incorporating immersed-boundary methodology has been developed for this purpose. Using a variety of numerical schemes, subfilter models, and computational grids, it is demonstrated that large-eddy simulation is able to predict fully started flow quite accurately. Further simulations of unstart configurations indicate that large-eddy simulation is able to capture the large-scale features of the unstart process remarkably well, exhibiting unsteady flow structures nearly identical to the experiment. Quantitatively, large-eddy simulation overpredicts boundary-layer separation that leads to faster shock propagation as compared to experiments.

Large-eddy simulation of a supersonic inlet-isolator

A multivariate quadrature based moment method for LES based modeling of supersonic combustion

Flow inside the combustor of a scramjet engine is likely to be in thermal nonequilibrium due to upstream shock-based compression. Given the short flow-through timescales in such engines, the flow will not reach equilibrium in the combustor. Since the distribution of energy in the internal modes affects chemical reactions, nonequilibrium could have significant impact on ignition and flame stabilization inside scramjet combustors. In this study, direct numerical simulation (DNS) is used to understand the impact of thermal nonequilibrium on flame structure in a supersonic flow. The operating conditions correspond to the HyShot II scramjet experiment. A two-temperature model along with detailed chemical kinetics is used to model supersonic combustion. It is observed that when vibrational temperature is lower than translational temperature, there is significant delay in flame ignition. In addition, combustion reactions are weaker leading to reduced heat release. The vibrational temperature exhibits a broad dissipation structure compared to a conserved scalar, but contains thin dissipation elements near the shear layer between the fuel and oxidizer. There is a small improvement in turbulent mixing with nonequilibrium, primarily due to reduced viscosity arising from lower translational temperature in the domain. Overall, thermal equilibrium occurs over time scales longer than the integral scale, indicating that nonequilibrium will play a crucial role in the internal structure of the flame.

Direct numerical simulation of supersonic combustion with thermal nonequilibrium

A probability density function (PDF) approach to account for turbulence–chemistry interaction in the context of large eddy simulation (LES) based simulation of scramjets is developed. To solve the high-dimensional joint-composition PDF transport equation robustly, the semi-discrete quadrature method of moments (SeQMOM) is employed. The SeQMOM approach addresses key numerical issues in LES related to the inaccuracies in computing filter-scale gradients, enabling an efficient and numerically consistent solution of the PDF transport equation. The computational tool is used to simulate a cavity-stabilized Mach 2.2 supersonic combustor. The LES–SeQMOM approach captures the pressure profiles qualitatively, in spite of the large uncertainties in the boundary conditions. Further, this flow configuration is found to exhibit low-frequency dynamics that lead to oscillatory motions of the shock-system inside the combustor. Detailed analyses of the LES data are used to describe the physical mechanism leading to these oscillations.

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Heeseok Koo

Papers

A quadrature-based LES/transported probability density function approach for modeling supersonic combustion

Large-eddy simulation of a supersonic inlet-isolator

A multivariate quadrature based moment method for LES based modeling of supersonic combustion

Direct numerical simulation of supersonic combustion with thermal nonequilibrium

LES-based Eulerian PDF approach for the simulation of scramjet combustors