Numerical Investigation on Three-dimensional Effects of Over-expanded Rocket Nozzle

Abstract

In the early stage of liquid rocket engine startup with an over-expanded state, internal flow begins to develop in the form of FSS (Free Shock Separation) which the separated plume stays away from the wall. However, depending on the type of the nozzle shape, a drastic and unpredictable transition occurs to the form of RSS (Restricted Shock Separation) that the separated plume is reattached to the wall surface. In this research, viscous unsteady two-dimensional axisymmetric and three-dimensional nozzle internal flow analysis using CFD simulations were conducted with a structured-unstructured mixed grid system. Especially, the numerical three-dimensional effects when a transition from FSS to RSS occurs were analyzed changing Nozzle Pressure Ratio (NPR). As a result, it was identified that the transition occurs more slowly in the three-dimensional flow analysis than the axisymmetric two-dimensional case as the NPR increases. And the nozzle wall surface temperature asymmetry phenomenon was identified, and the cause for it was analyzed. Also, the magnitude of the side load and the moment acting on the neck of the nozzle generated by the asymmetric pressure distribution on a nozzle wall was measured.