Modeling of Transient Flow Mixing of Streams Injected into a Mixing Chamber

The transient mixing between gases of dissimilar molecular weight is simulated numerically and compared with companion experiments. The two -dimensional geometry of interest is composed of two helium channe ls on either side of a central nitrogen channel feeding into a mixing chamber. The mixing process starts by first establishing a steady flow of nitrogen and then injecting the helium streams into this nitrogen -filled chamber in transient fashion. Simulat ions of the steady nitrogen flow indicate a strong tendency for the jet to attach to the chamber walls. As the helium enters the chamber the attachment point of this established nitrogen jet moves farther downstream but the asymmetric flow pattern still p ersists. The helium flow fills the smaller recirculation region on the attachment side in 50 to 100 ms and eventually fills the larger recirculation region. Comparisons between two - dimensional simulations and quantitative PLIF images indicate reasonable qualitative agreement. Representative three -dimensional solutions show non -planar effects have a significant effect on the mixing process . I. Introduction Ignition is recognized as one the critical drivers in the reliability of multiple -start rocket engin es. Residual combustion products from previous engine operation can condense on valves and related structures thereby creating difficulties for subsequent starting procedures. Alternative ignition methods that require fewer valves can mitigate the valve reliability problem, but require improved understanding of the spatial and temporal propellant distribution in the pre -ignition chamber. Current design tools based mainly on one -dimensional analysis and empirical models cannot predict local details of the injection and ignition processes. The goal of this work is to evaluate the capability of the modern computational fluid dynamics (CFD) tools in predicting the transient flow mixing in pre -ignition environment by comparing the results with the experimental data. This study is a part of a program to improve analytical methods and methodologies to analyze reliability and durability of combustion devices. In the present paper we describe a series of detailed computational simulations of the unsteady mixing events as the cold propellants are first introduced into the chamber as a first step in providing this necessary environmental description. The present computational modeling represents a complement to companion experimental simulations 1,2 and includes co mparisons with experimental results from those efforts. A large number of rocket engine ignition studies have been previously reported. Here we limit our discussion to the works discussed in Refs. 3, 4 and 5 which are both similar to and different from t he present approach.