Forced Combustion Experiments on Aero Combustors

In the world of gas turbine combustion there is always the spectre of thermo-acoustic instability. Over the past few decades there has been significant effort afforded to researching the phenomenon of thermo-acoustics. The results of the research have produced numerous mathematical models and at system level these models have been used to predict and postdict where noise is likely to occur in a given system. The models also allow the combustion system to be numerically tested through the flight or operational envelope to identify areas where instability may occur before testing is carried out, thus reducing the risk of unexpected noise occurring. The weakness of many of these models is that they require, what is known as a flame transfer function. The flame transfer function is normally measured after the combustor has been fully designed and at a high TRL (Technology Readiness Level) so significant investment in time and money are already baked into the design. Remedial action if required can result in a significant loss of time and money in the development of the combustor. This paper describes the design and use of a test rig that allows combustion systems to be tested at much lower TRL. A ‘siren’ rig has been developed and used to identify what particular design changes in either combustor flow field or fuel delivery systems have effects on the thermo-acoustics. The exit boundary of the unit has a representative choked end point. This end point has the ability to be modulated in time, thus forcing the whole system. How the system reacts to the forcing is measured over a range of frequencies. The rig has been successfully used to influence design changes required to avoid combustion driven oscillations within the next generation of aero gas turbine combustors. The rig is not a representation of a complete 360 degree annular combustor system, but of a smaller sector. The objective is to isolate the Fuel Spray Nozzle (FSN) and corresponding combustor sector from acoustic resonances and derive functions expressing the relationship between unsteady heat release rate and unsteady aerodynamics for a range of operating conditions by controlling the modulation of air mass flow rate. Such functions can be used in conjunction with acoustic linear theory to predict wave modes and growth rates in combustor geometries.