LARGE-EDDY SIMULATION OF SUPERCRITICAL FLUID FLOW AND COMBUSTION

Open Access
Author:
Huo, Hongfa
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 13, 2010
Committee Members:
  • Vigor Yang, Dissertation Advisor
  • Daniel Connell Haworth, Committee Chair
  • Vigor Yang, Committee Chair
  • Andre Louis Boehman, Committee Member
  • Robert John Santoro, Committee Member
  • Richard A Yetter, Committee Member
Keywords:
  • Turbulent combustion
  • Supercritical combustion
  • swirl coaxial injector
  • LOX/Methane combustion
  • Supercritical mixing
  • shear coaxial injector
Abstract:
In liquid-propellant rocket engines, the chamber pressure and temperature are well above supercritical conditions, and the propellants undergo a series of physicochemical processes that are dramatically different from the low-pressure processes. Although extensive experimental studies and many numerical simulations have been performed on propellant mixing and combustion in the context of liquid rocket engines, the current understanding of the flow and combustion dynamics under supercritical conditions is not sufficient to support the engine design and optimization. The present study focuses on the modeling and simulation of injection, mixing, and combustion of real fluids at supercritical conditions. The objectives of the study are: (1) to establish a unified theoretical framework that can be used to study the turbulent combustion of real fluids; (2) to implement the theoretical framework and conduct numerical studies with the aim of improving the understanding of the flow and combustion dynamics at conditions representative of contemporary liquid-propellant rocket engine operation; (3) to identify the key design parameters and the flow variables which dictate the dynamics characteristics of swirl- and shear- coaxial injectors. The resulting theoretical and numerical framework accommodates the full conservation laws and includes real-fluid thermodynamics and transport phenomena over the entire temperature and pressure regimes of concern. Thermodynamic properties, such as enthalpy, internal energy, and heat capacity, are directly calculated from fundamental thermodynamics theories and a modified Soave-Redlich-Kwong (SRK) equation of state. The transport properties, such as viscosity and thermal conductivity, are estimated using Chung’s method. Mass diffusivity is obtained using the Takahashi method, which is calibrated for high-pressure conditions. Turbulence closure is achieved using a Large-Eddy-Simulation (LES) technique, in which the large-scale structures are resolved and the effects of unresolved small-scale motions are modeled. The static and dynamic Smagorinsky models are incorporated to model the effect of the sub-grid scale motions. The steady flamelet and the extended flamelet/progress-variable models are used to handle turbulence/chemistry interactions. The resulting set of equations is solved numerically using a preconditioned, density-based finite volume method along with a dual-time stepping technique. Rigorous effort is made to ensure mass conservation and higher-order numerical accuracy. The robustness and the effectiveness of the resulting framework has also been validated. The framework utilizes a parallel computation scheme that involves the Message-Passing Interface (MPI) library and multi-block treatment. The theoretical and numerical framework is validated by simulating the Sandia Flame D. The calculated axial and radial profiles of velocity, temperature, and mass fractions of major species are in reasonably good agreement with the experimental measurements. The conditionally averaged mass fraction profiles agree very well with the experimental results at different axial locations. The validated model is first employed to examine the flow dynamics of liquid oxygen in a pressure swirl injector at supercritical conditions. Emphasis is placed on analyzing the effects of external excitations on the dynamic response of the injector. The high-frequency fluctuations do not significantly affect the flow field as they are dissipated shortly after being introduced into the flow. However, the lower-frequency fluctuations are amplified by the flow. As a result, the film thickness and the spreading angle at the nozzle exit fluctuate strongly for low-frequency external excitations. The combustion of gaseous oxygen/gaseous hydrogen in a high-pressure combustion chamber for a shear coaxial injector is simulated to assess the accuracy and the credibility of the computer program when applied to a sub-scale model of a combustor. The predicted heat flux profile is compared with the experimental and numerical studies. The predicted heat flux profile agrees very well with the experimental data. The steady flamelet model and the flamelet/progress-variable have been used to study the LOX/methane flame stabilized by a splitter plate. Results show that the flame is always anchored in the recirculation zone that is immediately after the splitter plate. Turbulence is not strong enough to extinguish the non-premixed flame. The flame stabilization is found to be achieved through the recirculation zone and the vortex shedding processes in the near field of the splitter plate. The flamelet-progress-variable case further confirms that the artificially quenched flame can be re-established as long as the quenching distance is within the mean recirculation zone.