An Experimental Investigation of the Nonlinear Response of an Atmospheric Swirl-Stabilized Premixed Flame

Abstract

Due to stringent emission restrictions, modern gas turbines mostly rely on lean premixed combustion. Since this combustion mode is susceptible to thermoacoustic instabilities, there is a need for modeling tools with predictive capabilities. Linear network models are able to predict the occurrence of thermoacoustic instabilities but yield no information on the oscillation amplitude. The prediction of the pulsation levels and hence an estimation whether a certain operating condition has to be avoided is only possible if information on the nonlinear flame response is available. Typically, the flame response shows saturation at high forcing amplitudes. A newly constructed atmospheric test rig, specifically designed for the realization of high excitation amplitudes over a broad frequency range, is used to generate extremely high acoustic forcing power with velocity fluctuations of up to 100% of the mean flow. The test rig consists of a generic combustor with a premixed swirl-stabilized natural gas flame, where the upstream part has a variable length to generate adaptive resonances of the acoustic field. The OH* chemiluminescence response, with respect to velocity fluctuations at the burner is measured for various excitation frequencies and amplitudes. From these measurements, an amplitude dependent flame transfer function is obtained. Phase-averaged OH* pictures are used to identify changes in the flame shape related to saturation mechanisms. For different frequency regimes, different saturation mechanisms are identified.

Combining a Helmholtz solver with the flame describing function to assess combustion instability in a premixed swirled combustor

“…The output of this quasi-analytical approach are the growth rate of the acoustic modes, their frequency of oscillation and spatial structure as a function of the perturbation amplitude level. For a given frequency and for increasing values of the acoustic velocity amplitude, the gain and phase of the FDF evolve (Balachandran et al, 2005;Bellows et al, 2007;Durox et al, 2009;Schimek et al, 2011;Kim and Hochgreb, 2011). Depending on û , a mode may be found stable or unstable; when the growth rate ω i exceeds damping at small amplitude the mode is unstable and the limit cycle corresponds to the particular value of û for which the growth rate tends to the damping rate.…”

Response of flames with different degrees of premixedness to acoustic oscillations

“…In the past, experimental studies have focused mainly on the response of lean, fully premixed in space and in time, gas-fueled turbulent flames to acoustic oscillations Bellows et al, 2007;Kim et al, 2010a;Schimek et al, 2011). It was shown that the interaction of the flame with vortices caused by the oscillating flow constitutes an important mechanism of nonlinear response.…”

“…Acoustically forced fully premixed flames experience only velocity oscillation Bellows et al, 2007;Kim et al, 2010a;Schimek et al, 2011) because the mixture is always and everywhere the same. If the fuel injection system is such that the fuel flow rate is constant and its location is far enough upstream to produce uniform equivalence ratio across the mixing duct, a slowly time-varying velocity field will result also in a slowly varying equivalence ratio associated with the constant fuel flow rate and the unsteady air flow rate Balachandran et al, 2011).…”

Prediction of the Nonlinear Dynamics of a Multiple Flame Combustor by Coupling the Describing Function Methodology With a Helmholtz Solver

“…While nonlinear acoustics can be of significant importance in the case of rocket engines [17], it was shown that the flame is often the main nonlinear component of the system [9,[18][19][20][21]. Recently, new methods have been developed to take into account this nonlinearity.…”