Numerical study of acoustic resonance in a LOX injector induced by orifice flow

Abstract

High-frequency combustion instability was observed under certain conditions in a multiinjector research thrust chamber operated with hydrogen and liquid oxygen (H2/LOX). The instability frequency was matched with the 2nd longitudinal acoustic mode of the injector LOX post and the whistling phenomenon through the LOX orifice was suspected as an excitation source of acoustic resonance. In this study, large eddy simulation (LES) was used to determine whether the flow through the orifice could excite acoustic resonance in the BKD injectors and thereby play a role in the combustion instability. A single LOX injector was modeled, including the simplified orifice. A WMLES with coarse unstructured mesh was able to recreate self-sustained flow oscillation. A spectral analysis with dynamic mode decomposition revealed flow patterns representing the whistling phenomenon at a Strouhal number of 0.38. The mechanism of the flow induced oscillation was periodical detachment and re-attachment of the boundary layer at the orifice wall. The same dynamic mode displayed the longitudinal acoustic mode of the downstream LOX post in resonance. Thus the unsteady orifice flow was shown to excite acoustic resonance in the LOX post without external driving sources. Furthermore, the whistling phenomenon occurs in a specific Strouhal number range accompanied by higher harmonics which also can excite higher order post resonance modes when their frequencies are matched. The results of this study illustrate the role that orifice whistling can play in injector-coupled combustion instability in liquid rocket engines with shear coaxial injectors.