Computational fluid dynamics (CFD) has proven to be an invaluable tool for the design and analysis of highspeed propulsion devices. Massively parallel computing, together with the maturation of robust CFD codes, has made it possible to perform simulations of complete engine flowpaths. Steady-state Reynolds-Averaged Navier-Stokes simulations are now routinely used in the scramjet engine development cycle to determine optimal fuel injector arrangements, investigate trends noted during testing, and extract various measures of engine efficiency. Unfortunately, the turbulence and combustion models used in these codes have not changed significantly over the past decade. Hence, the CFD practitioner must often rely heavily on existing measurements (at similar flow conditions) to calibrate model coefficients on a caseby-case basis. This paper provides an overview of the modeled equations typically employed by commercialquality CFD codes for high-speed combustion applications. Careful attention is given to the approximations employed for each of the unclosed terms in the averaged equation set. The salient features (and shortcomings) of common models used to close these terms are covered in detail, and several academic efforts aimed at addressing these shortcomings are discussed.

https://ntrs.nasa.gov/api/citations/20040035566/downloads/20040035566.pdf

Modeling of high speed reacting flows: established practices and future challenges

Challenges in Fuel Injection for High-Speed Propulsion Systems

: The VULCAN CFD code integrated with a reduced chemical kinetic mechanism was applied to simulate cavity-stabilized ethylene-air flames and to predict flame stability limits in supersonic flows based on an experimental study. A 15-step reduced kinetic mechanism for ethylene was systematically developed through skeletal reduction with a directed relation graph and time scale reduction based on quasi-steady state assumptions. The accuracy of the reduced kinetic mechanism and its implementation in the VULCAN code were demonstrated in an auto-ignition problem with a range of parameters. 3D simulations were then carried out for cavity-stabilized flames at different fuel flow rates and turbulent Schmidt numbers. For comparison with the performance of the present reduced mechanism, a 3- and a 10-step global kinetic model were applied to simulate the same cavity combustor, and the results show that the 15-step reduced model predicts experimental results much better than the 3- and 10-step models. The importance of including accurate chemical kinetics in CFD simulations is therefore demonstrated.

Simulations of cavity-stabilized flames in supersonic flows using reduced chemical kinetic mechanisms

Modeling of the SCHOLAR experiment, where the hydrogen fuel is injected at 30 deg angle to the vitiated air flow in the main combustion duct is considered. The turbulent model employed calculates the turbulent Schmidt and Prandtl numbers as part of the solution and models the turbulence/chemistry interactions. Two chemical kinetic models are employed: a seven species / seven reaction model, and a nine species/nineteen reaction model, where the reaction rates are pressure and temperature dependent. Computed results lie within the scatter of the experimental data. However, predicted ignition location is upstream of measured location. Because of the many factors that affects ignition location, it has not been possible to pinpoint the cause of the discrepancy.

Simulation of Supersonic Combustion Using Variable Turbulent Prandtl/Schmidt Number Formulation

The multi-scale turbulence approach is useful in predicting mean flows in problems containing complex turbulent structures that are otherwise unattainable using standard Reynolds-averaged Navier-Stokes models. In crossflow simulations using the multi-scale turbulence approach, turbulent mass diffusion in the resolved field showed variations not correlated with the eddy viscosity. This work is aimed at modifying the multi-scale turbulence approach to allow the resolved field to adaptively influence the value of turbulent Schmidt number in the Reynolds- averaged Navier-Stokes sub-filter model. The proposed model estimates a time-averaged resolved turbulent Schmidt number that is used in place of the constant value common to standard Reynolds-averaged Navier-Stokes approaches. This approach is assessed by grid refinement study in which different amounts of turbulence are resolved. Fuel concentration predictions show an improvement when compared with experimental measurements versus the multi-scale model without the adaptive approach.

/pdf/adaptive-turbulent-schmidt-number-approach-for-multi-scale-1ee472jmt4.pdf

Yanhu Guo

Papers

The development of a variable Schmidt number model for jet-in-crossflows using genetic algorithms