Effect of preheating of the reactants on the transition to thermoacoustic instability in a bluff-body stabilized dump combustor

In the present study, we investigate the effect of preheating of the reactants on the dynamics of a turbulent bluff-body-stabilized dump combustor during the onset of thermoacoustic instability. As we decrease the equivalence ratio towards a low fuel-lean value, we observe the dynamical transition of the combustor from the state of combustion noise to thermoacoustic instability via intermittency. Such an intermittency route to thermoacoustic instability is preserved for the range of preheat temperatures considered in this study. Further, we notice that an increase in the temperature of the reactants advances the point of the onset of thermoacoustic instability in the system. During the state of combustion noise, high temperature preheating of the reactants shifts the anchoring point of the flame from the shaft to the tip of the bluff-body and suppresses the oscillatory behavior of the global heat release rate in the system. We also observe a change in the multifractal characteristics of the heat release rate fluctuations such that the signal properties change from short-range to long-range correlations. On the other hand, preheating of the reactants to high temperatures during the state of thermoacoustic instability engenders a shift in the frequency of the acoustic field from the third unstable mode to the first unstable mode. Moreover, such preheating expanses the flame surface from one half of the combustor (lower) to both halves and, hence, increases the Pearson's correlation between the local heat release rate fluctuations measured on both halves of the combustor. Thus, we report that the preheating of the reactants significantly alters the dynamical characteristics of a turbulent combustor during the transition to thermoacoustic instability from that observed in the absence of preheating.