Simulations and Analysis of Vortex Driven Combustion Instability

Abstract

Large solid fuel combustion chambers are likely to experience surface, obstacle or corner vortex shedding. Vortex dynamics may excite flow acoustics and generates pressure fluctuation. These oscillations may generate critical dynamic loads, generate severe structural vibrations, and can eventually cause chamber failure. Vortex driven flow instability is one of the most persistent and complicated challenge and cannot be computed through conventional stability methods. A cold flow test case having obstacle vortex shedding is numerically studied in present study. Test case consist of cylindrical combustion chamber, inhibitor ring, and nozzle. Vortex shed from inhibitor ring travel with flow and strike at nozzle wall. Unsteady Reynolds-Averaged Navier-Stokes (URANS) computations are performed to compute vortex governed flow features. Pressure time histories at locations before and after obstacle have been recorded. Fast Fourier Transform (FFT) of pressure data has been performed. Computed oscillation frequency and amplitude are compared with available experimental data. Flow phenomenon in the present test case depends on the large-scale vortex dynamics therefore results of URANS differ from available experimental data. URANS model assumes small as well as large scale eddies as universal and isotropic. However, large eddies are more problem dependent and are required to be resolved for present test case. Therefore, Large Eddy Simulation (LES) has also been performed for computation. Oscillation frequency computed through LES is very much close to experimental data however amplitude is over predicted. This may be due to the two dimensional axi-symmetry assumption which is not truly applicable in turbulent vortex flows. Real turbulent flows are always transient and three dimensional. Results confirmed that scale resolving simulations are essential for capturing flow physics of vortex shed, dynamics, and impingement.