Heat release response of premixed flames to equivalence ratio fluctuations: Comparison between DNS and Reduced order modelling

摘要

Combustion instability events in lean premixed combustion systems can cause spatio-temporal variations in unburnt mixture fuel/air ratio. These fuel/air ratio variations couple with the flame result in heat-release oscillations. A key component needed to predict of combustor stability is a model for the response of the heat-release rate of the flame to fuel-air ratio fluctuations. Predictions of the heat release rate and burning area response describing functions for a two dimensional slot stabilized methane-air flame, subject to harmonic upstream equivalence ratio oscillations are determined using a Reduced Order Model (ROM) based on the level-set method. A quantitative comparison is performed between these results and corresponding results from detailed, fully compressible reacting flow computations of the same configuration. This comparison shows that the predictions are sensitive to small geometric differences in the shape of the nominally steady flame used in the two computational approaches. Accounting for this influence shows that describing function magnitudes are well predicted for frequencies lesser than and greater than a lower and upper cutoff respectively. The amplification of flame surface wrinkling by the Darrieus-Landau instability causes the agreement to degrade in between these two cutoffs. However, as the ROM recovers the transit time of flame surface perturbations through the flame in good agreement with detailed computations at all frequencies, excellent agreement is seen between the corresponding describing function phase predictions at all frequencies. Next, good agreement is seen for both magnitude and phase of the flame response, for large forcing amplitudes, at frequencies where the DL instability has a minimal influence. Thus, the present ROM can predict flame response as long as the DL instability, caused by gas expansion at the flame front, does not significantly alter the amplitudes of flame front perturbations as they traverse the flame.