Parametric effects on combustion instability in a lean premixed dump combustor

Two-dimensional numerical simulations of an axi- symmetric lean premixed dump combustor were con¬ducted to study the effects of key flow, thermo¬chemical, and geometric parameters on combus¬tion instability. The unsteady compressible Navier- Stokes equations, together with an equation for a reaction progress variable, were numerically inte¬grated using a fourth-order Runge-Kutta method for temporal discretization and a sixth-order com¬pact finite-diflference method for spatial discretiza¬tion. The combustion model employed a single-step, irreversible, global exothermic chemical reaction of the type F + rO → (1 + r)P with Arrhenius kinetics. Characteristic-based non-reflecting boundary condi¬tions were used together with an exit buffer-zone. Grid resolution was sufficient to capture the complex flame dynamics without resorting to a subgrid-scale model. The two-dimensional assumption precludes turbulence. The axisymmetric combustor consists of a plenum chamber, designed to deliver premixed re¬actants at nearly constant pressure, upstream of an inlet duct followed by a combustion chamber and an outlet duct. Two additional combustion cham¬ber geometries were studied, including cases with a cavity and an orifice, both located midway in the combustion chamber. The effect of inlet equivalence ratio, reactant temperature, and swirl were also stud¬ied for the baseline cylindrical combustion chamber. Pressure spectra peaks were associated with reson¬ ance modes considering the combustion chamber as a half-wave resonance tube. The results indicated the shape of the combustor had a small effect on both the amplitude and frequency of the pressure oscilla¬tions within the combustor. Compared to the base¬line case, the presence of the orifice increased the am¬plitude of the pressure oscillations at all frequencies, whereas the cavity increased the amplitude of the low frequency pressure oscillations but decreased the am¬plitude of the high frequency pressure oscillations. A small increase in inlet temperature caused a marked decrease in the amplitude of the low frequency pres¬sure oscillations. Small increases or decreases in the inlet equivalence ratio increased or considerably de¬creased pressure oscillation amplitudes, respectively. Mild swirl decreased the amplitude of the pressure os¬cillations at all frequencies. Preliminary results from three-dimensional large eddy simulations of swirling non-reacting flow revealed more complex flows fea¬turing small scale turbulent structures.