Empirical Mapping of Fluctuating Heat Release in Vortex Driven Combustion Instability

Abstract

Phase-locked measurements of CH* chemiluminescence associated with a twodimensional dump combustor were used to map local heat release rate from selfexcited combustion instability as a function of space and instability phase. The naturally unstable condition, targeted for detailed measurements, occurred at an inlet velocity of 45 m/s and an equivalence ratio of 0.67, and was dominated by periodic motion of large vortical structures at 150 Hz. The models for fluctuating heat release rate were empirically established by assuming a separation of variables and a sinusoidal temporal dependence and calibrating the coefficients using either the local CH* chemiluminescence or the broadband light emission measurements in the visible spectrum. Model reconstruction of the fluctuating heat release and the flame structure resulted in a good agreement with the measured data at various instability phases both qualitatively and quantitatively. In particular, both the local and global Rayleigh index calculations yielded excellent agreements with the experimentally determined values, suggesting the empirically constructed models can provide good insight into the driving and damping mechanisms as well as an accurate prediction of combustor stability. The results showed a good correlation between the local chemiluminescence and the periodic vortex motion, opening up a possibility of building a physics-based heat release fluctuation model by simply utilizing the understanding of the vortex dynamics. It was also noted that, for this particular operating condition tested, the broadband emission measurements can be used to build an approximate model of fluctuating heat release that can accurately predict the stability characteristics of the combustor.