An Experimental Study of the Propagation of Equivalence Ratio Oscillations on their Effect on Combustion Instabilities

Abstract

Lean premixed combustors have become a promising method in gas turbine engines to operate more efficiently and within the regulatory standards for emissions. However, this lean premixed operation is susceptible to combustion instabilities within the combustor. These instabilities are a coupling between the heat release and pressure oscillations which is impacted by many factors, one of which is the fuel oscillations. This study analyzed the way in which fuel oscillations propagate through the mixing chamber and combustor utilizing optical diagnostics. A swirl combustor was designed and fabricated to be the test bed for these optical diagnostics to measure the equivalence ratio both before and in the flame. The first was the development and implementation of a helium-neon laser probe to measure equivalence ratio in the mixing chamber of the combustor. The second was a photomultiplier tube system that was utilized to measure chemiluminescence data that has shown to correlate well with equivalence ratio within the flame. These two optical diagnostics provided information about the fuel mixture before the flame and in the flame itself. The results showed that acoustic forcing increased the mixing both axially and radially. Furthermore, there was a drop in equivalence ratio oscillations across the swirler under acoustic forcing and under controlled oscillating conditions. Finally, the photomultiplier tube system proved accurate under lean steady state conditions but needs further verification under controlled oscillating conditions and acoustic forcing conditions.