Thermoacoustic instability in future hybrid rocket engines

Reaction Engines Ltd is currently developing the SABRE engine, an air-breathing rocket engine designed to power the SKYLON spaceplane up to Mach 5 and transition into rocket mode to reach low-earth orbit. The main novel component of the engine is the pre-cooler, which cools the incoming hot air, allowing the energy extracted to power the engine turbomachinery. At low speeds, the absence of intake shock waves means an extra heat source is required to supply this energy. This is the role of a component known as the pre-burner; a hydrogen flame heats incoming air to provide extra heat through a heat exchanger.
The objective of this project is to develop computational tools to study the thermoacoustic stability of the pre-burner. Thermoacoustic instability is caused by a positive feedback between acoustic waves and unsteady heat release rate and/or unsteady heat transfer. It is undesirable as it leads to high amplitude pressure oscillations. Analytical models for the acoustic impedance of heat exchanger tubes in co-flow will be developed. Unsteady CFD simulations of the flame and heat exchanger tubes will be performed, allowing models for the unsteady response of these components to be extracted. The constituent models will be coupled into a unified computational tool that will be able to offer design guidance for instability avoidance.