I. Introduction

Most practical combustion systems, such as gas-turbine engines, internal combustion engines, ramjets, and rocket motors are confined systems in which operational design and size and weight constraints define the scale of the device. Confined combustion systems can have dynamical features that are not apparent in unconfined systems. For example, many of these devices have choked outflow, and passage of vortical structures or hot spots through choked nozzles can result in acoustic wave generation that can propagate upstream and interact with the incoming flow and the flame zone. Geometric features such as acoustic liner cavities, secondary injectors and complex ducts can all affect coupling among vortex flow, acoustic motion, and unsteady heat release in these devices.

Gas-turbine combustors, which are the focus of discussion in this chapter, have other unique features. The inlet to the combustor typically contains a complex swirl-vane structure that induces a swirl to the hot air from the compressor. The airflow may be split into multiple streams and each stream swirled independently in either the counterdirection or the codirection.1 Fuel (liquid or gas) is injected before, through or after these swirl vanes, and fuel-air mixing occurs in a highly turbulent, swirling flow. Many propulsion gas-turbine combustors have both primary and secondary combustion zones, and fuel is also split between them depending on operation conditions.1 ^,2 Additional complexity...