Stability of a spontaneous vortex structure in flame under gas oscillations

The stability of a spontaneous vortex structure formed during combustion of a gas injected through a round hole in a flat horizontal plate onto its lower surface was studied. The unsteady velocity, vorticity, temperature, and pressure fields in the vortex structure were studied experimentally and numerically. It is established that for Reynolds numbers $\mathrm{Re}\approx10-75$ and Rayleigh numbers $\mathrm{Ra}\approx10^3-10^4$, the vortex structure is stable against single velocity perturbations and changes under acoustic oscillations with a frequency of $100$–$150$ Hz. Small cells whose dimensions are comparable with the flame front thickness are destroyed, and the velocity of the vortex gas flow in large cells changes with the frequency of forced oscillations. The amplitude of linear gas velocity oscillations in the vortex is larger than that of gas velocity oscillations in the burner nozzle. It is assumed that the observed increase in the amplitude of gas velocity oscillations in the vortex is due to both acceleration of the combustion products in the gravity field and their thermal expansion. The gas velocity oscillations in the vortex structure lag behind the gas flow rate oscillations by a quarter of the period.