Full-system simulation of the HyShot II scramjet

The renewed interest in high-speed flight has recently demonstrated the need for the development of hypersonic air-breathing propulsion systems, i.e., in which the ambient air is used as oxidizer. Because the internal flow in a scramjet is supersonic, it has a very short residence time during which air and fuel must mix on a molecular level, and chemical reactions have to be completed before leaving the engine. Moreover, the kinetic energy of the free stream of air entering the propulsion engine is of the same order of magnitude as the combustion heat re- lease. This results in a small net thrust compared to the aerodynamic drag. On the other hand, an excessive heat release can lead to the unstart phenomenon, i.e., the choking of the engine, that causes a sudden drop in thrust and large mechanical loads on the structure. Performance optimization requires thus maximizing thrust without crossing the unstart bound. To achieve predictive computations, the quantification of margins and uncertainties (QMU) with respect to this unstart bound is critical.
Recent advances within the PSAAP program at Stanford University to develop computational tools to study the unstart phenomenon will be presented. In the first part, an overview of the program will be given. The second part will focus on the combustion model. Since heat release is at the heart of a scramjet operation and the main contributor to unstart, it is also the major source of uncertainties. A novel model for supersonic combustion based on a flamelet/progress variable approach has been developed. This approach allows the use of com- plex chemistry with only 2 or 3 additional scalar transport equations. The model is applied in a RANS computation of the hydrogen fueled HyShot II scramjet and simulation results are compared with experimental data. LES results for a jet in a supersonic crossflow will also be presented.