ENGINEERING AND CFD

A scramjet is an air breathing jet engine for hypersonic flight velocities of about ten to twenty times the speed of sound. Flight at such velocities is beyond the reach of turbo-jet engines, because they require subsonic airflow to operate and a deceleration of the incoming flow would result in a tremendous temperature increase and total-pressure loss of the flow. Scramjets do not need a turbo compressor but rather directly exploit the aircraft’s forward motion to forcefully compress the incoming airflow through oblique shock waves.

To operate efficiently, the airflow is never decelerated to subsonic speed. Hence, also combustion takes place at supersonic flow velocity. Fast mixing of fuel (usually hydrogen) and the compressed airflow is thus essential to enable combustion during the very short residence time of the reactants in the combustion chamber. Otherwise fuel would be ejected unburned through the nozzle. Because the penetration depth of fuel injection perpendicular to the main flow direction is very small, strut injectors that are positioned directly within the supersonic core flow are usually used in large chambers.

Within this project a team of researchers performed large-eddy simulations for a generic strut-injector geometry that is based on a experimental setup at Technische Universität München. The main objective of this project was the analysis of flame stabilization through a pilot injection of hydrogen and air at the base of the strut injector.

The figure provides an overview of the flow field: A primary shock is generated by the sharp leading edge of the injector. This shock causes separation of the boundary layer at the top wall and a separation shock, which is reflected towards the recirculation region following the strut's base. There the shock is again reflected as an expansion. Hydrogen and air is injected through six injectors at the base of the strut.

A second, horizontal, recirculation is created by the injection jets, which are oriented in such a way that the three jets of one side cross in a single point.

Following the expansion, the recirculation collapses, creating a re-compression shock. In the wake of the injector a turbulent shear- and mixing-zone is formed.

It was found that the chosen six-hole pilot injector leads to a large stoichiometric surface and a highly turbulent mixing zone. The simulation results also showed a stable subsonic recirculation region behind the injector base that has a higher temperature than the surrounding supersonic flow and is positively affected by oblique shock waves, which are generated by the injector’s leading edge and then reflected back from the combustion chamber walls. This indicates that the chosen design is a rather good solution for enabling ignition and for flame stabilization.

Acknowledgements

The simulations have been performed with the flow solver INCA on 2.048 processing units on HPC system Hermit of the High Performance Computing Center Stuttgart (HLRS).

Research Team & Contact Information

Stefan Hickel & Sebastian Eberhardt
Technische Universität München
Fakultät für Maschinenwesen
Boltzmannstr. 15, 85748 Garching
E-mail: sh@tum.de