Comparisons between different combustion models for Highpressure LOX/ CH4 jet flames

This paper presents an investigation of LOX/CH4 Liquid Rocket Engines injector flames to investigate the impact of real gas effects and the combustion models on the predictions. In liquid-propellant rocket engines the combustion occurs at operating conditions well above of the thermodynamic critical points of the fluid where reactants properties show liquid-like densities, gas-like diffusivities, and pressure-dependent solubility. So using real gas properties as accurately as possible is a key issue in the preliminary design of LRE injectors and combustion chambers. In the numerical study of LOX/CH4 jet flames a critical aspect is the choice of the combustion model, that should be accurate and a well compromise between phenomena description and computational costs. Complex, simplified and reduced kinetics scheme could be implemented in the CFD modeling of cryogenic spray and to make a well choice of the modeling approach it is necessary to estimate the mixing and kinetic time scales in the case of study. In the rockets it’s generally possible to assume that the chemistry is infinitely fast and that burnt gas conditions can be approximated like similar to the chemical equilibrium condition. Under these conditions a simple finite rate combustion model is less realistic; otherwise an Eddy Dissipation Approach or Flamelet model could more realistically model phenomena. In this work LOX/CH4 jet flames at high pressure have been simulated by implementing different kinetic mechanisms usually presented in literature for the methane/air flame, starting from the simple one step mechanism up to the detailed Skeletal model derived from the Grimech 3.0. Then results obtained using some simplified mechanisms, as the Modified Jones-Lindstedt kinetics model, more accurate for combustion in presence of pure oxygen have been compared with the previous ones. EDC and Non- Premixed Combustion Model, including flamelet approaches are implemented in the CFD Fluent v.13.0 code, like in the present work, it is not possible to take into account real gas effetcs with the non-premixed model. Real gas effects have been also considered in the case of EDC combustion model. In a liquid rocket engine at high pressures real gas effect needs to be modelled by using a Real Gas Equation of State (RG EOS). In the current work the real gas effects have been modelled by the Soave-Redlich-Kwong (SRK) real gas model.