Flame Transfer Functions and Dynamics of a Closely Confined Premixed Bluff Body Stabilized Flame With Swirl

Håkon T Nygård,Nicholas A Worth

doi:10.1115/1.4049513

Abstract

Abstract The flame transfer function (FTF) and flame dynamics of a highly swirled, closely confined, premixed flame is studied over a wide range of equivalence ratios and bulk velocities at a fixed perturbation level at the dump plane. The operating conditions are varied to examine the ratio of flame height to velocity in scaling the FTF. The enclosure geometry is kept constant, resulting in strong flame-wall interactions for some operating conditions due to varying flame height. The resulting effect on the FTF due to changes in the “effective flame confinement” can therefore be studied. For sufficiently high equivalence ratio, and the resulting sufficiently small effective confinement, modulations of the FTF are observed due to interference of the perturbations created at the swirler and at the dump plane. The small length scales and high velocities result in modulations centered at high frequencies and spanning a wide range of frequencies compared to previous studies of similar phenomena. A critical point was reached for increasing effective confinement, where the modulations are suppressed. This is linked to a temporal shift in the heat release rate where the flame impinges on the combustion chamber walls. The shift reduced the expected level of interference, demonstrating effective confinement is important for the FTF response. Additionally, a distributed time lag (DTL) model with two time lags is successfully applied to the FTFs, providing a simple method to capture the two dominant time scales in the problem, recreate the FTF, and examine the effect of effective confinement.

Highlights

The occurrence of thermoacoustic instabilities is an issue which may restrict fuel and operational flexibility in gas turbine engines, hindering the development of low emission systems
The flame structure resembles that of a Vflame, but with some flame elements stabilized in the outer shear layer
The flame transfer functions (FTF) are shown to exhibit characteristic low-pass behavior and collapse reasonably well when plotted nondimensionally against a Strouhal number based on the flame height and bulk flow velocity

Summary

Introduction

The occurrence of thermoacoustic instabilities is an issue which may restrict fuel and operational flexibility in gas turbine engines, hindering the development of low emission systems. A common framework to predict such instabilities during the design phase relies on accurate knowledge of the magnitude and delay of a flame’s heat release rate oscillations, in response to reference input oscillations over a range of frequencies and amplitudes. Such response functions are denoted as flame transfer functions (FTF) which are valid for low and moderate oscillation amplitudes, where the response is assumed to be linear, or flame defining functions if these capture high amplitude and the nonlinear response of the flame. The potential utility of this approach, in reducing the significant complexity associated with the reacting flow to a simple response function, has resulted in considerable effort in understanding the behavior, scaling, and generality of such functions [6–11]

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