Thermoacoustic Modeling and Transfer Functions Determination for a Matrix Burner Using Unsteady CFD

Abstract

Turbulent combustion of a lean premixed methane-air mixture is simulated numerically using unsteady CFD. The configuration is a matrix burner suitable for stationary gas turbine applications. The geometry consists of the following: seven slots which constitute the flame holder for stabilizing the flame, a diffuser which serves the purpose of lowering the pressure loss across the burner and a combustion chamber. A contraction ensures that a recirculation zone is created close to the exit of the flame holder for anchoring and stabilizing the flame. Fuel is injected in 112 holes, 8 along each end of the 7 slots. The injected fuel meets the in-coming high velocity air stream for mixing to begin in the premixed ducts before finally entering the combustion chamber. This paper validates the cold flow velocity field and the steady flame results from CFD with measurements and investigates combustion instability in a matrix burner, the onset of which can be attributed to changes in flow variables using URANS. Particularly, the effect of the mixture strength variation caused by fluctuations in the velocity field on the unsteady heat rate inside the combustor is investigated. The fuel inlet is assumed to be choked due to the high pressure drop across it. The time lag between the time fuel is injected and the time it reaches the flame front is estimated. Quantifying this time delay (or “flight time”) helps to characterize the burner with respect to thermo-acoustic instabilities. The flame frequency response to a white noise forcing at the air inlet is determined. This is followed by the determination of the acoustic transfer matrix linking the pressure and velocity downstream and upstream of the burner/flame. This is done by using system identification that is common in control theory. The determined flame frequency response and the time lag are used in a 1D acoustic network code for determining the longitudinal eigenmodes of the combustor of the matrix burner.