매연생성 기체화염장과 초임계 실제유체 화염장에 대한 모델링

Abstract

The present study has been mainly motivated to develop the comprehensive combustion model to predict the laminar and turbulent non-premixed gaseous sooting flames as well as the real-fluid chemically reacting flows encountered in the supercritical-pressure environment. First, The Interactive Transient Flamelet (ITF) model has been devised to realistically simulate slow processes such as soot formation and radiation in a laminar and turbulent non-premixed flame. In context with the ITF model, soot formation in the laminar methane/air non-premixed jet flame is modeled by the two-equation soot model and the ethylene/air by the Method of Moments Interpolative Closure (MOMIC) model. The ITF model accounts for radiative cooling induced by gaseous species and soot particles in the same mixture fraction space simultaneously while it treats the unphysical diffusion of soot to be practically zero. Moreover, the ITF procedure can maintain consistency to extend the transient flamelet model to the simulation of turbulent non-premixed sooting flames. To validate the ITF approach with the two-equation and MOMIC soot model, in terms of soot volume fraction, number density, temperature, species mass fractions, as well as reaction rates for soot, numerical results are compared with those obtained by the Full Transport Equation approach. Furthermore, to assess the applicability of the ITF approach about smoking or non-smoking characteristic and to evaluate the effects of gaseous differential diffusion on the soot formation processes, numerical results obtained by the ITF model are also compared with experimental data. And the pressurized laminar and turbulent non-premixed flames are also applied to validate the extendibility of ITF model. Based on these numerical results, the detailed discussion has been made for the capability and limitations of the ITF approach to predict the precise flame structure and soot formation characteristics in the laminar and turbulent non-premixed flames. Second, the real-fluid flamelet model has been implemented to realistically simulate turbulent mixing and non-premixed flame of non-ideal fluid. In the present work, first of all, the flamelet equations in the mixture fraction space are extended to treat the flame field of general fluids over transcritical and supercritical states. To validate this real-fluid model, flamelet computations are carried out for gaseous hydrogen and cryogenic liquid oxygen flames under a wide range of thermodynamic conditions. Based on numerical results, the detailed discussions are made for the effects of real fluid, pressure, and differential diffusion on the local flame structure and the characteristics encountered in liquid propellant rocket engines. Then, it will be discussed about numerical modeling of the mixing processes of cryogenic liquids. In the present approach, turbulence is represented by an extended k-ε turbulence model. A conserved scalar approach together with a presumed probability density function approach is utilized to account for scalar fluctuation effects on the turbulent mixing processes of real-fluid over transcritical and supercritical states. The two real-fluid equations of state (EOS) and dense-fluid correction schemes incorporated into our real-fluid code are validated for thermodynamic and transport properties over a wide range of pressures and temperatures. In this state, computations are made for four cryogen nitrogen jets at near-critical and supercritical pressures. Numerical results indicate that the present real-fluid model has the predicative capabilities to simulate the essential features of the cryogenic liquid nitrogen jets which is called ‘pseudo-boiling effect’. Finally, for the primary goal of this real-fluid model, the transcritical mixing and reacting flow processes are numerically modeled and validated. In order to realistically represent turbulence-chemistry interactions, detailed chemical kinetics, and non-ideal thermodynamic behaviors related to the liquid rocket combustion at supercritical pressures, the flamelet approach is coupled with real-fluid modeling based on the Soave-Redlich-Kwong (SRK) equation of state. To validate the real-fluid flamelet model, a gaseous hydrogen/cryogenic liquid oxygen and gaseous methane/cryogenic liquid oxygen coaxial jet flame at supercritical pressure has been chosen as benchmark cases. Numerical results are compared with experimental data obtained for the OH radical and temperature distribution. It was found that weak for GH2/LOx and strong for GCH4/LOx flow recirculation are induced by the sudden expansion of cold core cryogenic oxygen associated with the pseudo-boiling process. These recirculation zones substantially influences the fundamental characteristics of liquid propellant reacting flows at supercritical pressures in terms of the spreading and the flame length. Numerical results suggest that the real-fluid based flamelet model is capable of realistically predicting the overall characteristics of a turbulent non-premixed GH2/LOx and GCH4/LOx flame at supercritical pressures. And, for GCH4/LOx flame, numerical experiments about the effects of pressure and LOx inlet temperature on turbulent flame structure are made a comparative study. Numerical results indicate that the transcritical GCH4/LOx coaxial flame fields are greatly influenced by varying inlet LOx temperature and chamber pressure. Increase in chamber pressure and LOx inlet temperature results in the weaker expansion-induced flow reversal, the narrower flame spreading, and the longer flame length. Based on numerical results, the detailed discussions are made for the real-fluid effects and the precise structure of the gaseous methane/cryogen liquid oxygen coaxial jet flames.