Lean-limit combustion instabilities of a lean premixed prevaporized gas turbine combustor

Abstract

One type of low-frequency combustion instability was studied in a lean premixed prevaporized (LPP) gas turbine combustor experiment that was operated at elevated pressures using liquid Jet-A fuel. This intense low-frequency (15 Hz) instability is called a “lean-limit” instability, while in other papers it has been called a “cold tone”, an “incipient blowout” instability or an “entropy wave”. High-speed movies identified the physical mechanism. At first the flame enters the shear layer, is locally extinguished and lifts off. The extinguished region fills with unburned reactants. Finally the premixed flame flashes back into the region of unburned reactants, leading to a large pressure rise which triggers the next cycle. The observations are in general agreement with LES computations published by Huang and Yang. Measurements quantify the time lag between the flame luminosity buildup and subsequent pressure rise as well as the correlation between these two quantities and their spectra. The frequency of this “lean-limit” instability is found to be related to the flow velocity, flame velocity, and the combustor length, as is predicted for an entropy wave. A discussion is presented to qualitatively explain the measured frequencies, which increase as the fuel equivalence ratio is raised. The experiment includes the following realistic conditions: Jet-A liquid fuel, preheated air, elevated pressure and realistic sources of instabilities, including a commercial fuel injector/mixer that creates flame–flame interactions between a non-premixed Pilot and premixed Main flame that are necessary for aircraft applications.