Non-linear interactions in liquid-propellant rocket engine injectors

Abstract

Experimental results and theoretical explanations of such non-linear phenomena as self-pulsation and non-periodical unsteadiness due to hydraulic instability in liquid propellant flow both in jet and swirl injectors as well as interaction of liquid and gaseous propellant flows in gas-liquid injectors are described and discussed. It is shown that additionally to the abilities of Liquid-Propellant Rocket Engine (LRE) injector to serve as a sensor, amplifier, phase shifter and actuator in LRE as a dynamic system, in some circumstances it can also be a generator of oscillations that can influence both secondary heat release and combustion chamber pressure oscillations, and change stationary characteristics of mixing zone that cause the dynamic characteristics of combustion. Self-pulsation motion was found in liquid injectors: jet injectors with enlargements in their flow path, swirl injectors when using compressible or boiling liquid or having excess radial component of liquid velocity inside the vortex chamber. The latter is typical for jet engine fuel injectors and can lead to cavitation erosion of interior injector passages. Most powerful self-pulsation occurs due to interaction of liquid and gaseous propellant flows in gas-liquid injectors. Such self-pulsation motion was discovered in all types of injectors used in LRE: shear/coaxial, shear/swirl with central LOX and exterior hydrogen stages, and both swirl/swirl coaxial injector with central liquid flow. Dependence of boundaries of self-pulsatio n regime on both operational parameters and principal design features are presented and showed strong influence on real operational conditions. Influence of self-pulsatio n motion on atomization and mixing is presented. Positive and negative consequences of self-pulsation are observed. The phenomena of non-periodica l unsteadiness of injection process in LRE is discovered and studied. It presents temporal changes of flow angle, thickness of spray at the injector exit and cause so called vibro-activity of combustion, that can cause bifurcations of the operational process in LRE. So, designers of LRE injectors must be informed not only about the linear dynamic characteristics of propellant injectors they use hi LRE but also about the non-linear properties that strongly depends on real parameters of the combustion chamber and hardly can be modelled in ambient conditions of cold flow tests.