Experimental investigation of velocity coupling in combustion instability

An investigation has been conducted of the velocity coupling phenomenon reported in acoustically unstable solid-propellant rocket motors. An innovative simulation facility has been built using solid carbon dioxide as the simulated propellant. Acoustic disturbances are introduced over the dry-ice surface by means of a mechanically driven piston. Experiments have been conducted with dry ice located near both a velocity antinode and a velocity node. Mass flow rate and acoustic pressure measurements indicate the existence of a coupling mechanism, strongly dependent on the acoustic velocity amplitude, between the acoustic disturbance and the dry-ice sublimation process. Flow visualization and hot-film anemometry both show that the flow is turbulent near resonance. Transition to turbulence near a velocity node appears to occur at a smaller critical acoustic velocity amplitude than that near a velocity antinode. Some preturbulent instability has also been observed. Acoustically induced, turbulent forced convection is believed to be responsible for the increase in the sublimation rate of the dry ice (simulated burning of the propellant). Turbulence is believed to be one of the principal mechanisms in the velocity coupling phenomenon. An empirical correlation was developed which appeared to apply to the real propellant cases.