MILD 연소조건에서 층류 및 난류 화염장 수치해석

Abstract

This study is ultimately aimed at developing the comprehensive combustion model to realistically predict the laminar and turbulent partially premixed flames under moderate or intense low-oxy dilution (MILD) combustion conditions. In the present work, under the MILD combustion conditions established by internal exhaust-gas recirculation or the supply of combustion products through a nozzle, numerical simulations are carried out for the laminar flameless combustion and the sequence of turbulent flameless combustion validation cases. Prior to the modeling for the turbulent flames under the MILD combustion conditions, to understand the precise flame structure under MILD combustion, full-transport equations were solved for laminar jet flames for MILD combustion condition as well as conventional non-premixed combustion conditions. On the other hand, to simulate the turbulent flames under flameless combustion conditions, the present study has adopted the multi-environment probability density function (MEPDF) approach. In the present MEPDF approach, in order to circumvent excessive computational burden, the direct quadrature method of moments (DQMOM) has been adopted as an alternative approach to solve the transported Probability Density Function (PDF) equation. The MEPDF approach has the form of a conventional Eulerian scheme and retains the desirable property of a particle-based method. In this study, the joint-composition PDF is approximated using the MEPDF, expressed via the combination of weights and abscissas on the composition and physical space. Micro-mixing can be represented via the interaction by exchange with mean (IEM) model, and the chemistry is based on the detailed chemical mechanism. The validation cases for the turbulent flameless combustion include the turbulent CH4/H2 Flames in the jet-in-hot-coflow (JHC) burner, turbulent CH4 flames with recirculated flue gases, and the MILD oxy-combustion flame encountered in the non-catalytic partial oxidation (POX) gasifier. In case of the turbulent CH4/H2 Flames in the JHC burner, the predicted profiles are in reasonably good agreement with experimental data in terms of the unconditional means and conditional statistics for temperature and mass fractions of species and pollutants. Numerical results suggest that the present MEPDF approach is able to realistically predict the effects of inflow oxygen and hydrogen levels on the flame lift-off, auto-ignition, flame structure, and NOx formation characteristics in turbulent CH4/H2 jet flames issuing from the JHC burner. Moreover, numerical investigations for turbulent CH4/H2 flames under MILD combustion conditions was carried out according to H2 ratio of fuel. In the turbulent CH4 flameless flames with recirculated flue gases, special emphasis is given to the effects of the micro-mixing model constants on the structure and characteristics of recirculated MILD combustion processes. In terms of the integrated NO production rate for the MILD combustion condition, the N2O path yields the highest level, followed by the prompt, reburn, NO2, thermal, and NNH mechanisms in that order. The detailed discussions are also made for the flame stabilization and auto-ignition processes in terms of recirculation rate, distribution of Damköhler number, scalar dissipation rate (SDR) and H2CO mass fraction. For the MILD oxy-combustion flame in the non-catalytic POX gasifier, in terms of the mean temperature, the present MEPDF approach yields the overall agreement with the measurements in the highly fuel-rich MILD oxy-combustion situation with the strong flue gas recirculation even if there exist the certain discrepancies in the upstream region. The present study has systematically analyzed the effects of the fuel/oxygen injection velocity and O2/CH4 ratio on the characteristics of the strongly recirculated MILD oxy-combustion processes. Depending on fuel/oxygen injection velocity or O2/CH4 ratio, the present MEPDF approach well reproduces the qualitative flame transition characteristics from MILD combustion to conventional combustion. The higher fuel/oxygen injection velocity leads to the much longer jet penetration and the much higher SDR level which makes the ignition to occur at further downstream region. The relatively lower O2/CH4 ratios maintain the basic characteristics of the MILD combustion while the highest O2/CH4 ratio locally creates the oxy-flame like structure rather than the non-visible flame field. Base on numerical results obtained in this study, the detailed discussions are made for the essential characteristics of the laminar and turbulent flames under the MILD combustion conditions as well as the capability and limitations of the present MEPDF approach.