Large eddy simulation of CH4-air and C2H4-air combustion in a model annular gas turbine combustor

Abstract

Combustion instabilities are one of the major challenges in developing and operating propulsion and power generating gas-turbine engines. More specifically, techniques for managing the increasingly stringent emissions regulations and efficiency demands have often given rise to thermo-acoustic instabilities, particularly for annular combustors operating in a lean premixed mode. In this paper, we combine experimental and computational methods to examine unsteady gas turbine combustion in a full annular model gas turbine combustor installed at NTNU, operating both methane- and ethylene-air blends. The experimental data consists of flame images, high-speed OH* chemiluminescence images, as well as pressure and heat-release time-series at discrete locations for the ethylene-air case. The computational set-up consists of the 18 inlet tubes and swirlers, and the full annular combustor placed in a large external domain. The computational model consists of a compressible finite rate chemistry LES model using skeletal methane-air and ethylene-air combustion chemistry. The combustor is simulated in its self-excited state, without external forcing. From the experiments and simulations the methane and ethylene cases are found to behave differently: The ethylene-air flames are much smaller than the methane-air flames, resulting in different interaction between adjacent flames. The LES predictions show good qualitative agreement with the measurements in terms of instantaneous and time-averaged flame structure. Comparing measured and predicted time-series of pressure and heat-release also shows good quantitative agreement with respect to the dynamics and structure for the ethylene-air case. Investigating the predicted combustion dynamics using Proper Orthogonal Decomposition (POD) confirms the importance of the self-excited azimuthal mode on the behavior of the flame: the presence of nodes and anti-nodes of pressure induced fluctuations of the swirler mass-flow, which then, in turn, influence the heat-release. These events occur shifted in time.