Combustion instabilities in a small MMH/NTO liquid rocket engine used for satellite attitude and course control are numerically\ninvestigated. A three-dimensional Navier-Stokes code is developed to simulate two-phase spray combustion for cases with five\ndifferent droplet Sauter Mean Diameters. As the droplet size increases from 30 microns to 80 microns, pressure oscillations are\nstronger with larger amplitudes. But an increase of the droplet size in the range of 80 microns to 140 microns indicates a\nreduction in the amplitudes of pressure oscillations. This trend is the same as the Hewitt criterion. The first tangential (1T)\nmode and the first longitudinal (1L) mode self-excited combustion instabilities are captured in the 60-micron and 80-micron\ncases. Abrupt spikes occur in the mass fraction of MMH and coincide with abrupt spikes in the mass fraction of NTO at the\ndownstream regions just adjacent to the impinging points. Thus, local combustible high-dense mixtures are formed, which\nresult in quasiconstant volume combustion and abrupt pressure spikes. The propagation and reflection of pressure waves in\nthe chamber stimulate the combustion instability. When the droplet size is too small or too large, it is difficult to form local\nhigh-dense premixtures and combustion is stable in the chamber.
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