High thrust-to-weight ratio and consistent performance over a range of operating conditions make cryogenic rocket engines one of the best options for space propulsion. However, future space explorations and missions to moon, mars and beyond require improvements in our present knowledge of the rocket engine combustion technology. In order to help improve the performance and reliability of current rocket engine combustors, several key issues need to be considered. Injector performance is one such issue related to the development of a new generation of rocket engine combustors.

Previous research has suggested that coaxial injectors are most preferable for injection of cryogenic propellants inside the combustion chamber because of their simple design, low losses and high combustion stability. An experimental facility was designed and fabricated to simulate a single element shear coaxial injector. Gases of different densities were injected through the annulus between the two injector tubes over a large range of velocities, while liquid nitrogen flows through the inner tube. In this research, liquid nitrogen was used to simulate liquid oxygen because it is very similar to liquid oxygen, chemically inert, easy and safe to install in laboratory testing. High speed cinematography and Schlieren imaging have been used to examine the evolutionary flow behavior and global features of the liquid nitrogen jet, while PIV imaging was used to characterize the gaseous flow.

This research has analyzed the transient behavior and unfolds the detailed evolutionary characteristics of both the cryogenic liquid and gaseous phase evolving from the shear coaxial injector for the first time. The effect of density ratio, velocity ratio and momentum ratio on the behavior of steady-state liquid nitrogen jet from a coaxial injector at atmospheric pressure has also been examined in detail. The impact of these parameters on primary instability of liquid core, the shear/spreading angle and its potential core length have been examined. Furthermore, the impact of some of the important non-dimensional numbers such as, Reynolds number, Weber number and Prandtl number, have been examined to develop scaling laws for the prediction of cryogenic potential core lengths. New correlations have been provided that describes the cryogenic jet behavior under simulated rocket injector operating conditions.