Thrust chamber modelling for the analysis of liquid rocket engine transients

The analysis of a Liquid Rocket Engine (LRE) can be performed either focusing on a single component of the complete system or studying the interaction of more parts. Modelling the subsystems of an LRE with the aim to investigate the influence of each part on the overall performance is a challenging topic. The simulation of the entire system requires the employment of proper simplified sub-models able to get a reasonable computational time with a satisfactory level of reliability. EcosimPro is an object oriented simulation tool focused on the analysis of complete systems that spans different engineering fields. Thanks to the object oriented philosophy the single components can be connected to each other in order to analyse systems at different complexity levels. Propulsion systems can be investigated by the European Space Propulsion System Simulation (ESPSS) library, in which the typical components of a liquid rocket engine (tanks, turbo machinery, feeding lines, valves, gas generator, combustion chamber, etc.) are modelled to study both steady states and transients. The available system modelling tools for combustion chamber are based on an unsteady component in chemical equilibrium for the first part of the chamber including the converging section of the nozzle, while the nozzle diverging section is modelled with a quasi-steady component both in frozen and equilibrium conditions. The present work aims to improve the capabilities of the combustion chamber modelling by means of a new library. To preserve the lightweight philosophy of the tool, a reduced model to analyse inviscid flows with more than one species in chemical non-equilibrium has been introduced directly in the EcosimPro environment. To overcome the limits related to the use of a simple centred scheme, such as the one presently coded, an approximate Roe's Riemann solver for multispecies flows has been implemented. The correct upwinding of the convective fluxes guarantees a suitable representation of the wave propagation phenomena, while the non-equilibrium analysis, conducted by means of finite-rate chemistry models, leads to a more accurate flow description. This is especially true in the simulation of transients. The new sub-models are validated through a comparison with the most common literature results in order to demonstrate the effectiveness of the implemented scheme. The validated component is then connected with the entire system and a complete engine analysis is conducted in order to investigate the typical transient phases (start-up and shut-down) of a liquid rocket engine.