Background noise effects on combustor stability

This paper considers the effects of background turbulent fluctuations upon a combustor's stability boundaries. Inherent turbulent fluctuations act as both additive and parametric excitation sources to acoustic waves in combustors. Although additive noise sources exert primarily quantitative effects upon combustor oscillations, parametric noise sources can exert qualitative impacts upon its dynamics; particularly of interest here is their ability to destabilize a system that is stable in the absence of these noise sources. The significance of these parametric noise sources increases with increased background noise levels and, thus, can play more of a role in realistic, high-Reynolds-number systems than experiments on simplified, lab-scale combustors might suggest. The objective of this paper is to determine whether and/or when these effects might be significant. The analysis considers the effects of fluctuations in damping rate, frequency, and combustion response. It is found that the effects of noisy damping and frequency upon the combustor's stability limits is relatively small, at least for the fluctuation intensities estimated here. The effects of a noisy combustion response, particularly of a fluctuating time delay between flow and heat-release perturbations, can be quite significant, however, in some cases for turbulence intensities as low as u'(rms)/(u) over bar similar to 5-10%. These results suggest that deterministic stability models calibrated on low turbulence intensity, ab-scale combustors might not adequately describe the stability limits of realistic, highly turbulent combustors.