The 'Vortex Reloaded' project: experimental investigation on fully tangential vortex injection in N2O - paraffin hybrid motors

摘要

This paper deals with an experimental and numerical project intended to study fully tangential vortex injection in a hybrid motor of 1kN class. Due to the knowledge of the CISAS Hybrid Team, the choice for oxidizer has been pressurized nitrous oxide, while paraffin wax has been used as fuel. This investigation follows a previous project where a mixed axial/vortex device has been tested: also if an increase in performance has been observed, a non-uniform grain consumption showed an issue of such device. The work starts with a mission scenario that gives the design drivers for the subsequent preliminary design, performed with an iterative, transient and OD numerical code. A successive step is the detailed design of the combustion chamber and test bed. The aim of the study was to investigate three objectives: vortex injection and a comparison with axial; throttling behavior at fixed mass flows (reduction of 75% and 50% of oxidizer flow); combustion chamber performance changing its configurations, in particular using inside a mixer. Performance parameters taken into account were chamber pressure oscillations, combustion efficiency and regression rate. In the preliminary phase two different issues have been discovered and solved: the first regards a chamber pressure behavior variation, linked to a too long postchamber; the second is referred to pressure pikes in the ignition phase, solved using a longer prechamber and a different ignition configuration. It has been shown that vortex injection lowers the chamber pressure oscillations respect to axial case from more than 7% down to 4%. Moreover, regression rate has been increased of 41%, and the a coefficient of its law up to 67% from axial. This last value, indeed, shows a constant behavior throttling down the oxidizer mass flow. The increase is due to the higher wall heat flux in the grain surface, given by the higher velocity and thermal gradient of the fluid. Combustion efficiency has been increased with vortex injection given by the higher turbulence flow that enhances the mixing of the reactants. Axial case showed a value of 76% of this last parameter, while vortex case went up to 90%. Coupling of injection and mixer (a diaphragm-like device inside combustion chamber) increases this value up to 96%. A cost analysis has been performed for this project, showing that hybrid propulsion is a low cost technology.