Effects of periodic atomization on combustion instability in liquid-fueled propulsion systems

A detailed understanding of the mechanism by which combustion instability occurs In liquid rocket engines does not exist, This paper examines the specific role that atomization may play in combustion instability. The effects of mean drop size, drop size distribution, and atomization periodicity are examined explicitly with a combustion response model, whose results indicate that all of these effects are important, It is shown that periodic atomization results in large variations in the magnitude of the pressure response when the atomization frequency is within a factor of 10 of the acoustic oscillation frequency. These results are consistent with an explanation of an empirical stability correlation, whereby the nonuniform and unsteady production of drops is a controlling factor in the growth of combustion instabilities. Experimental results from a subscale rocket combustor that support the importance of periodic atomization are presented. Atomization was electromechanically forced at frequencies from 2000 to 8000 Hz to accentuate the natural tendency for periodic atomization associated with impinging jet injectors. High-amplitude pressure oscillations at frequencies corresponding to the forced atomization frequencies substantiate the importance of periodic atomization, Pressure oscillation amplitudes approaching 10% of mean chamber pressure were measured when the drivers were operating at near optimal conditions. A conceptual model that is consistent with both these results and the empirical correlation is provided.