Large-eddy simulations of high-pressure shear coaxial flows relevant for H2/O2 rocket engines

Combustion;Supercritical;Scaling;LES;LRE;CFD

Masquelet, Matthieu Marc ; Aerospace Engineering

University:Georgia Institute of Technology

Department:Aerospace Engineering

关键词: Combustion;    Supercritical;    Scaling;    LES;    LRE;    CFD;   

Others  :  https://smartech.gatech.edu/bitstream/1853/47522/1/masquelet_matthieu_m_201305_phd.pdf

美国|英语

来源: SMARTech Repository

【 摘 要 】

The understanding and prediction of transient phenomena inside Liquid Rocket Engines(LREs) have been very difficult because of the many challenges posed by theconditions inside the combustion chamber. This is especially true for injectors involvingliquid oxygen LOX and gaseous hydrogen GH₂. A wide range of length scalesneeds to be captured from high-pressure flame thicknesses of a few microns to the lengthof the chamber of the order of a meter. A wide range of time scales needs to be captured,again from the very small timescales involved in hydrogen chemistry to low-frequencylongitudinal acoustics in the chamber. A wide range of densities needs to be captured,from the cryogenic liquid oxygen to the very hot and light combustion products. A widerange of flow speeds needs to be captured, from the incompressible liquid oxygen jet tothe supersonic nozzle. Whether one desires to study these issues numerically orexperimentally, they combine to make simulations and measurements very difficult whereasreliable and accurate data are required to understand the complex physics at stake. Thisthesis focuses on the numerical simulations of flows relevant to LRE applicationsusing Large Eddy Simulations (LES). It identifies the required features to tacklesuch complex flows, implements and develops state-of-the-art solutionsand apply them to a variety of increasingly difficult problems.More precisely, a multi-species real gas framework is developed inside a conservative,compressible solver that uses a state-of-the-art hybrid scheme to capture at the same timethe large density gradients and the turbulent structures that can be found in ahigh-pressure liquid rocket engine.Particular care is applied to theimplementation of the real gas framework with detailed derivations of thermodynamicproperties, a modular implementation of select equations of state in the solver.and a new efficient iterative method.Several verification cases are performed to evaluate this implementation and theconservative properties of the solver. It is then validated against laboratory-scaledflows relevant to rocket engines, from a gas-gas reacting injector to a liquid-gasinjector under non-reacting and reacting conditions. All the injectors considered containa single shear coaxial element and the reacting cases only deal with H₂-O₂ systems.A gaseous oyxgen-gaseous hydrogen (GOX-GH₂) shear coaxial injector, typicalof a staged combustion engine, is first investigated. Available experimental data islimited to the wall heat flux but extensive comparisons are conducted betweenthree-dimensional and axisymmetric solutions generated by this solver as well as by otherstate-of-the-art solvers through a NASA validation campaign. It is found that the unsteadyand three-dimensional character of LES is critical in capturing physical flow features,even on a relatively coarse grid and using a 7-step mechanism instead of a 21-stepmechanism. The predictions of the wall heat flux, the only available data, are not very good andhighlight the importance of grid resolution and near-wall models for LES.To perform more quantitative comparisons, a new experimental setup is investigated underboth non-reacting and reacting conditions. The main difference with the previous setup,and in fact with most of the other laboratory rigs from the literature, is the presence ofa strong co-flow to mimic the surrounding flow of other injecting elements. For thenon-reacting case, agreement with the experimental high-speed visualization is very good,both qualitatively and quantitatively but for the reacting case, only poor agreement isobtained, with the numerical flame significantly shorter than the observed one. In bothcases, the role of the co-flow and inlet conditions are investigated and highlighted.A validated LES solver should be able to go beyond some experimentalconstraints and help define thenext direction of investigation. For the non-reacting case, a new scaling law is suggested after areview of the existing literature and a new numerical experiment agrees with theprediction of this scaling law.A slightly modified version of this non-reacting setup isalso used to investigate and validate the Linear-Eddy Model (LEM), an advanced sub-grid closuremodel, in real gas flows for the first time.Finally, the structure of the trans-criticalflame observed in the reacting case hints at the need for such more advancedturbulent combustion model for this class of flow.