REST HF-10 test case: Flame response to forced excitation of a methane driven rocket combustion chamber
Creators: name:  A. LECHTENBERG 
name:   P. GERLINGER 
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: Designated for the pre-arranged REST modelling workshop session In rocket engines an enormous amount of energy is converted. Thus, small disturbances can grow exponentially. Such combustion chamber instabilities are still a major problem in the development of new propulsion systems. The consequences reach from irregular thrust to total failure of the combustor. In the history of rocket engine development, every innovation had to be tested experimentally, with the risk of losing the test object. Nowadays, accurate Computational Fluid Dynamics (CFD) simulation can reduce these risks in advance design. Kerosene (RP-1) and hydrogen are the most frequently used fuels combined with Liquid Oxygen (LOx) as oxidizer in liquid-propellant rockets. Recently, methane has come into focus as a promising rocket fuel. Kerosene is easier to handle and has a higher volumetric energy density than hydrogen. In contrast, hydrogen has a higher gravimetric energy density and a higher specific impulse. Methane is not as powerful as hydrogen but more easy to operate. Further, it can be extracted from biogas as sustainable fuel and perhaps synthesized on Mars, what makes return missions from Mars to Earth possible. Current rocket engines in development powered by liquid methane/ liquid oxygen are for example the Raptor engine by SpaceX and the M10 rocket engine by Avio. From a numerical point of view, highly accurate simulations of Kerosene combustors are extremely difficult. Due to the large number of species that has to be considered (about 80 for highly reduced reaction mechanisms) even highperformance computing (HPC) systems have not enough computational resources for time accurate simulations of highly resolved rocket combustors with detailed chemistry. The combustion mechanisms of hydrogen are significantly smaller (less than ten species), so that such simulations are possible for model combustors. The model combustor presented in this paper is driven by LOx-CH4. The number of species required is approximately twice the size of the hydrogen combustion. This makes efficient solution algorithms indispensable. Here, the improved Delayed Detached Eddy Simulation (iDDES) method is chosen. In contrast to a wall resolving Large-Eddy Simulation (LES), this method is characterized by a hybrid URANS/ LES approach, in which large proportions of turbulence in the core flow are resolved by LES. By simulating the near wall regime with Unsteady Reynolds-Averaged Navier- Stokes (URANS) the mesh resolution close to the wall is kept reasonable to reduce the computational effort. To further reduce the amount of grid volumes and the computational costs, a face value reconstruction of up to 5th order is used. Chemical processes are modeled in detail by Finite-Rate Chemistry (FRC) and an assumed Probability Density Function (aPDF) method for Turbulence/ Chemistry Interaction (TCI). The combustor consists of a single coaxial injection element and a wallless combustion chamber. The cross section of the chamber is a hexagon, where the opposing edges are periodically connected. This creates a virtual multiinjector combustor with a honeycomb pattern. Using a subsonic outflow boundary condition, the mean combustion pressure is kept at 100 bar. Because of the low injection temperatures real gas thermodynamics has to be considered. Two test cases are investigated with the same mean mass flow. For the second test case, the oxidizer mass flow is sinusoidal excited. Aim is the investigation of combustion chamber oscillations and the flame response due to the forced excitation.