Investigation of temporal variation of combustion instability intensity in a back step combustor using LES

Abstract

Lean combustion is susceptible to combustion instabilities, especially pressure oscillation, and this severely damages the combustor. To prevent this, many studies have attempted to elucidate this phenomenon. However, the physical understanding of the mechanism is still incomplete owing to its complexity. In this study, combustion instability in turbulent spray combustion in lean fuel condition is investigated using Large Eddy Simulation (LES). The combustion chamber has a backward facing step, and kerosene fuel droplets are injected vertically just upstream from the step. The dynamic thickened flame model is used as the turbulent combustion model, and a two-step global reaction model is used for kerosene-air flames. The influence of pressure oscillation on fuel atomization behavior is considered using a model, which predicts the Sauter Mean Diameter of injected the fuel droplets with time. The equivalence ratio is set as 0.6, 0.8, and 1.0. The results show that combustion instability is observed in all cases, and the intensity of combustion instability decreases with a decrease in the equivalence ratio; however a unique phenomenon is observed for the lowest equivalence ratio of 0.6. The unique phenomenon here is the time variation of amplitude of pressure oscillation, whose behavior has been also observed in some other studies. At this condition, while the frequency of pressure oscillation is temporally constant, the frequency of heat release rate varies with time, which is investigated with newly proposed index Time Gap. In addition, the spatial distribution of heat release rate temporally changes with varying frequency. They cause the time variations of correlation between pressure and heat release rate, and finally the amplitude of pressure oscillation temporally varies.

Keywords

• combustion instability
• spray combustion
• heat release rate
• back step combustor
• les