The influence of reactant temperature on the dynamics of bluff body stabilized premixed flames

Abstract

The impact of increased reactant temperature on the dynamics of bluff-body stabilized premixed flows is investigated using numerical simulation. A two-dimensional triangular bluff body is considered. Flow compressibility is assumed to exist at the low Mach number limit and combustion is fast and robust such that a flamesheet representation is assumed to apply. In this formulation, reactant temperature variations are represented via corresponding temperature ratio and flame speed variations. The Lagrangian, Transport Element Method is used to provide the numerical solutions. Results indicate that as reactant temperature increases, the fluid dynamics transition from a low amplitude, broadband, coarsely symmetric (about the bluff-body centerline) behavior, to a high amplitude, tonal and asymmetric one that bears similarities to the corresponding non-reacting flow. The reasons for this are that as the reactant temperature increases, (i) the temperature ratio across the flame is reduced, thus reducing combustion exothermicity, and (ii) the flame speed increases causing the flame to propagate away from the bluff-body wake. In both cases the ability of the two main combustion-driven fluid dynamical processes, namely volumetric expansion and baroclinic generation to impact the bluff body generated vorticity is reduced. Reduction in baroclinic generation enables to wake to survive futher downsteam and makes the flow susceptible to the wake instability. As reactant temperature is increased the location of the onset of the instability moves upstream. At very high reactant temperatures even the near field symmetrizing effect of volumetric expansion is overwhelmed and asymmetric vortex shedding is witnessed at the bluff body. Even in this regime, the flow differs from the non-reacting flow in that it is susceptible to bifurcations in vortex shedding behavior that are linked to local flame-vortex interactions. Results also show that in the general case, knowledge of the fluid dynamics alone is not sufficient to characterize the flame dynamics, as the flame position in relation to the vorticity field is critical to the unsteady flame response. Specifically, the flame exhibits a similar transition from a broadband to a tonal response but the amplitude is not monotonically increasing. Rather it experiences a regime of decreasing response for intermediate reactant temperatures where the flame propagation away from the wake appears to dominate the increase in fluid dynamical induced oscillations due to the enhancement of the asymmetric mode.