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The present study has numerically investigated the lifted flame field frequently encountered in the actual fire situations. This study employs the physical submodels including the detailed chemical kinetics, the variable transport properties, and the radiation model. A lifted laminar CH4/air flame with a central diluted fuel jet and a surrounding fuel-lean coflow is chosen as a validation case. Computations are done for three models which include the detailed chemistry model considering the radiative heat loss, the detailed chemistry model neglecting radiation, and the single-step irreversible chemistry considering radiation. Among three models, the detailed chemistry model considering the radiative heat transfer yields the best conformity with the measured velocity distribution. Numerical results indicate the present approach well captures the detailed structure and stabilization mechanism of the partially premixed triple flame.

SUMMARYThe present study focuses on numerically investigating the flame structure, flame liftoff, and stabilization in a lifted turbulent H 2 /N 2 jet flame with a vitiated coflow. To realistically represent the turbulent partially premixed nature in the flow region between nozzle exit and flame base, the level-set approach coupled with the conserved scalar flamelet model has been applied. The unstructured-grid level-set approach has been developed to allow the geometric flexibility and computational efficiency for the solution of the physically and geometrically complex reacting flows. The pressure-velocity coupling is handled by the multiple pressure-correction method. The predicted flame pattern is in good conformity with the measured one. In terms of the liftoff height, the agreement between prediction and experiment is quite good. Even if there are noticeable deviations in a certain region, the predicted profiles for the overall flame structure agree reasonably well with the experimental data. These numerical results indicate that the present levelset-based flamelet approach in conjunction with the unstructured-grid finite-volume method is capable of realistically predicting the essential features and precise structure of the turbulent-lifted jet flame with computational efficiency.

The present study has focused on numerical investigation on the flame structure, flame lift-off and stabilization in the partially premixed turbulent lifted jet flames. Since the lifted jet flames have the partially premixed nature in the flow region between nozzle exit and flame base, level set approach is applied to simulate the partially premixed turbulent lifted jet flames for various fuel jet velocities and co-flow velocities. The flame stabilization mechanism and the flame structure near flame base are presented in detail. The predicted lift-off heights are compared with the measured ones.

The Representative Interactive Flamelet (RIF) concept has been applied to numerically simulate the combustion processes and pollutant formation in the direct injection diesel engine. Due to the ability for interactively describing the transient behaviors of local flame structures with CFD solver, the RIF concept has the capabilities to predict the auto-ignition and subsequent flame propagation in the diesel engine combustion chamber as well as to effectively account for the detailed mechanisms of soot formation, NOx formation including thermal NO path, prompt and nitrous NOx formation, and reburning process. Special emphasis is given to the turbulent combustion model which properly accounts for vaporization effects on the mixture fraction fluctuations and the pdf model. The results of numerical modeling using the RIF concept are compared with experimental data and with numerical results of the commonly applied procedure which the low-temperature and high-temperature oxidation processes are represented by the Shell ignition model and the eddy dissipation model, respectively. Numerical results indicate that the RIF approach including the vaporization effect on turbulent spray combustion process successfully predicts the ignition delay time and location as well as the pollutant formation.