An experimental study on stability rating of impinging-jet injectors using air injection in a subscale chamber

Abstract

Combustion instability rating is investigated experimentally in a subscale chamber with impinging-jet injectors. This study is focused on jet flow of both propellants of fuel and oxidizer without considering a chemical reaction. Air-injection tests using scaling techniques are proposed to predict acoustic instability in an actual full-scale combustion chamber of a rocket engine. In the present approach, a subscale chamber has been designed to have the same natural frequencies as those in a full-scale chamber, and air is injected into the subscale chamber instead of fuel and oxidizer and their flow rates are maintained. From the tests, damping factors are obtained as a function of a hydraulic parameter of jet flow. Based on the data, the instability map is drawn, where three acoustically unstable regions are presented. It is found that they coincide fundamentally with the results from hot-fire tests. Accordingly, as the first approximation, the proposed approach can be applied cost-effectively to the stability rating of jet injectors without losing essential features of acoustic instability when mixing of fuel and oxidizer jets is the dominant process in instability triggering.