Experimental Investigation of Self Sustained Lean Premixed Prevaporized Combustion Instabilities by Phase Averaged Laser Diagnostic Techniques.

Abstract

Interest in NOX pollution reduction has led to novel combustion techniques including Lean Premixed Prevaporized (LPP) combustion. However, LPP combustion is inherently prone to instabilities which can lead to gas turbine engine blowout and engine component failure. While these instabilities previously have been studied in simple laboratory burners, there is a need to show the connection between these lab scales and the instabilities that occur when a realistic, commercial LPP multi-swirl fuel injector is operated with Jet-A fuel at elevated pressures, temperatures and mass flow rates (such as in the present study). In order to investigate the combustion instabilities in an LPP combustor, a realistic test rig was modified to test two similar commercial LPP fuel injectors using liquid Jet-A fuel at high pressures and high pre-heat temperatures. Three groups of instabilities were identified based on the frequency of the pressure oscillations in the combustion chamber: high frequency (>250 Hz), low frequency (50-200 Hz), and very low frequency (<50 Hz). The low frequency and very low frequency oscillations were further investigated through the use of simultaneous high speed video and pressure data. Additionally, the flow field during the low frequency instability was measured through phase averaged particle image velocimetry (PIV). These results show that the flame has a large scale response at both types of frequencies. The low frequency instability was determined to be a Helmholtz bulk mode due to equivalence ratio oscillations from the oscillating air flow through the injector. The very low frequency instability was determined to be a flashback / blowout phenomenon near the lean blowout limit. A mathematical model was proposed which predicts the frequency of the very low frequency instability. The findings of this thesis with regard to the source of combustion instabilities provide data which can guide future studies in this area and designs of advanced, low polluting gas turbines.