Swirling fluid mixing and combustion dynamics at supercritical conditions
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A unified theoretical and numerical framework is established to study supercritical mixing and combustion over the entire range of fluid thermodynamic states of concern. Turbulence closure is achieved using a large-eddy-simulation (LES) technique. A steady laminar flamelet approach is implemented to model turbulence/chemistry interactions. Three-dimensional flow dynamics of a liquid oxygen (LOX) swirl injector at supercritical pressure is studied for the first time. Various mechanisms governing the flow evolution, including hydrodynamic instabilities, acoustic waves, and their interactions are explored using spectral analysis and proper orthogonal decomposition. Then, the mixing and combustion characteristics of LOX/kerosene bi-swirl injectors are investigated under conventional rocket engine operating conditions. Emphasis is placed on the near-field flow and flame development downstream of the inner swirler. The flame is stabilized by two counter-rotating vortices in the wake region of the LOX post which is covered by the kerosene-rich mixture. The influence of important injector design attributes, including the recess length, LOX post thickness, and kerosene annulus width, upon mixing and combustion characteristics is examined. The results provide critical information for future injector designs. In addition, counterflow diffusion flames of general fluids are investigated in a wide range of pressures and flow strain rates. An improved two-point flame-controlling continuation method is employed to solve the singularity problem at the turning points of the flame-response curve (the S-curve). General similarities are developed in terms of flame temperature, species concentrations, flame thickness, and heat-release rate for all pressures of concern. This can be utilized to improve computational efficiency for turbulent combustion models using tabulated chemistry.