Nonlinear Response of Plain-Orifice Injectors to Nonacoustic Pressure Oscillations

Abstract

The response of plain-orifice injectors to finite amplitude nonacoustic pressure pulsations within the downstream chamber was addressed with a series of unsteady axisymmetric Navier-Stokes calculations. Linear theories are reviewed and it is shown that results can be collapsed to a single curve when characterizing the response in terms of a frequency related to the fluid transit time within the orifice passage. Computational results were obtained for a wide range of frequencies, perturbation amplitudes, flow conditions, and orifice designs to quantify the effects of nonlinear fluid mechanics. In general, the simulations all show an unforced response (or self-pulsation) due to instabilities of the vena contracta region lying downstream of the inlet lip. For conditions pertinent to rocket injectors, the nonlinear response is generally bounded by the linear theory (except at high frequencies, in which self-pulsations not accounted for in linear theory remain present). Although a local resonance condition exists when the driving frequency is near the natural/unforced oscillation frequency of the injector, the overall response in this region is still bounded by the linear theory. The effects of orifice rounding and orifice length are also studied parametrically.