Parametric Study of Separation in Outlet Diffuser of Rocket Nozzle Cooling Channel Rig : The Effect of Heat Flux and Angle of Outlet Diffuser for Rectangular-to-Circular Cross Section Transitions

Abstract: The use of natural gas with high methane content as rocket fuel has gained substantial industrial attention over the past number of years. Several actors including SpaceX and Blue Origin are developing natural gas powered rocket engines. Attention is also shown from GKN Aerospace, a Sweden-based aerospace engine development company, who together with KTH Royal Institute of Technology has initiated the MERiT project. The project is intended to investigate different aspects of methane powered engines. The project is centered around a physical test rig of a rocket nozzle cooling channel along with an ANSYS CFX simulationmodel of the same rig to investigate the operation. The rig is heated from one side to simulate the boundary conditions of a real rocket nozzle. This report is a follow-up to the previous work by Pettersson (2019), which determined rig design points for two channel geometries (channel 3, channel 4) and studied the behaviours and limitations in regards to overheating, cooling and coking. The channels feature outlet diffusers transitioning from rectangular to circular cross sections. The inputs investigated were mass flow, inlet temperature, outlet pressure and heat flux. Following the discovery of flow separation occurring in the rig at certain design points, it was suggested that a parametric study of the outlet diffuser angle could investigate the effect on separation in the outlet diffuser of the rig channel geometry. This is the task at hand in this thesis, and a complementary investigation on the effect heat flux has on separation is also performed for single selected diffuser angles. To achieve this, the full rig model geometry is first reduced to its core components to reduce simulation run time, and the parametric diffuser is implemented for both channel geometries. The mesh and the model definition is then adjusted to accommodate the changes made, by for example replacing the full rig heater block with a constant heat flux boundary condition. After this, a total of 40 test cases of different diffuser angle and heat flux combinations are used to establish trends in the behaviour of the separation. The results show that separation occurs more easily for the channel 3 configuration, which sees separation occur for lower diffuser angles and heat flux settings. The separation grows diminishinly as the heat flux and diffuser angle is increased. The separation onset location is found to consistently be in the corners of the outlet diffuser, after which it expands and rotates into the symmetry plane further downstream. The channel 4 solution convergence is found to be increasingly poor for higher diffuser angles, which suggests the solutions may be transient in nature. The Reynolds and Mach number is found to be correlated to the heat flux applied but no conclusion can be made about their link to separation for the cases studied.