Effects of periodic atomization on combustion instability in liquid-fueledpropulsion systems

A detailed understanding of the mechanism by which combustion instability o ccurs In liquid rocket engines does not exist, This paper examines the spec ific 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 indicat e that all of these effects are important, It is shown that periodic atomiz ation results in large variations in the magnitude of the pressure response when the atomization frequency is within a factor of 10 of the acoustic os cillation frequency. These results are consistent with an explanation of an empirical stability correlation, whereby the nonuniform and unsteady produ ction of drops is a controlling factor in the growth of combustion instabil ities. Experimental results from a subscale rocket combustor that support t he importance of periodic atomization are presented. Atomization was electr omechanically forced at frequencies from 2000 to 8000 Hz to accentuate the natural tendency for periodic atomization associated with impinging jet inj ectors. High-amplitude pressure oscillations at frequencies corresponding t o the forced atomization frequencies substantiate the importance of periodi c atomization, Pressure oscillation amplitudes approaching 10% of mean cham ber pressure were measured when the drivers were operating at near optimal conditions. A conceptual model that is consistent with both these results a nd the empirical correlation is provided.