EST HF-10 test case: Simulation of combustion instabilities induced by flow rate modulations with diffuse interface modelling
Creators: name:  A. NICOLE 
name:   L. DOREY 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: PROPULSION PHYSICS
LIQUID ROCKET ENGINE
COMBUSTION INSTABILITIES

description: Submitted for the proposed special session 4th Modelling Workshop of the Rocket Engine Stability iniTiative Since 2000, REST group (Rocket Engine Stability iniTiative), a FrenchGerman research cooperation, has worked on one of the major issues of liquid rocket engines development, that is to say the high-frequency (HF) combustion instabilities. The groups objective is to provide models and data for predictive engineering tools to assist in the design of stable combustion chambers and to assess engine stability margins. Confidence in numerical tools has greatly increased over the last 20 years and allows computational fluid dynamics (CFD) studies to be carried out in order to better understand some phenomena that cannot yet be explored on experimental test benches. In this framework, a modelling test case was proposed to study the HF combustion instabilities driven by the propellants inflow rate oscillations. Therefore, different numerical approaches and codes of all the REST research teams can be be compared in a representative application for rocket combustion dynamics.The computational domain represents a portion of an engine chamber consisting of a hexagonal domain with periodic boundary conditions and a Lox/Methane coaxial injection element. Conditions are similar to combustion chambers ones: the chamber pressure is equal to 100 bar, the injection is transcritical and the mixture ratio is nearby stoichiometry (MR = 3.4). ONERA has computed this case with an unsteady Reynolds-averaged NavierStokes approach (U-RANS), using CEDRE, the ONERA in-house CFD tool [1]. A diffuse interface model [2] was used successfully to simulate the transition from subcritical to supercritical conditions for the oxygen side. Oxygen will therefore be considered in two forms: a cold dense form for the liquid injection part and a hot light form that will burn with the methane. A pseudo-evaporation reaction will modelled mass transfer between the dense and light forms. Concerning the thermodynamic, two types are used: the light type which corresponds to ideal gases and the dense type which uses a real gas cubic state law. A steady-state computation was first performed. We found the typical shape of cryogenic flames at supercritical pressure with the sudden expansion of the flame angle [3]. Then three modulated cases were carried out by imposing directly a sinusoidal excitation of the mass flow rate on the boundary inlet condition: one on the oxygen side at 5 kHz, an other at 1kHz and one for the methane inflow at 5 kHz. The dense core is deformed as well as the flame front. The effect is greater at 1 kHz than at 5 kHz. The modulation on the oxygen mass inflow has more influence on the flame and the dense core then the one of the methane. The length core is decreased of 25% with the O2 modulation at 1 kHz. The enhancement of the heat release is moderate. These simulations showed the effectiveness of the CEDRE code in simulating cryogenic flames in supercritical conditions. The effects of inflow rate modulations on the flame have been quantified separately from combustion instabilities in the chamber. This numerical study opens the door to a more detailed understanding of the phenomena involved in combustion instabilities. [1] Refloch A., Courbet B., Murrone A., Villedieu P., Laurent C., Gilbank P., Troyes J., Tesse L., Chaineray G., Dargaud J.B., Quemerais E. Vuillot F. (2011). CEDRE Software. Aerospace Lab, vol. 2. [2] Gaillard, P., Giovangigli, V., and Matuszewski, L. 2016. A diffuse interface Lox/hydrogen transcritical flame model. Combustion Theory and Modelling 20: 486 520 [3] Juniper, M. Structure et stabilisation des flammes cryotechniques, Ph.D. thesis, Ecole Centrale de Paris, 2001.