Effects of Flame Stretch on the Localized Heat Release Rate from an Oscillating Bunsen Flame

摘要

Accurate modeling of thermoacoustic instabilities has been hampered by the difficulty of deriving a generalized transfer function that describes the response of the flame to an acoustic disturbance. A common simplification often made in modeling unstable combustion problems is to assume a homogeneous heat release rate across the surface of the flame. Negating the effects of a variable equivalence ratio, this assumption requires that the flame speed be constant and most often, equal to the laminar flame speed. This is a rather limiting approximation in that it ignores the impact of flame stretch on the flame speed and thus the localized heat release rate. Through the use of time-resolved particle image velocimetry (PIV) a series of experiments were conducted to investigate the local heat release rate from a laminar Bunsen flame subject to an acoustic disturbance by quantifying the dilatation rate field in the pre- and post-flame regions of the flow. Dilatation indicates the rate of volume expansion of a fluid, which can be a direct indicator of the heat release rate. If alterations of the local heat release rate from significant regions of the flame surface are in phase with acoustic perturbations, a driving of the instability could ensue that may not be predicted from models that only consider a variation in the global heat release rate from the flame.