Yongmo Kim

Comparisons of Diesel PCCI Combustion Simulations Using a Representative Interactive Flamelet Model and Direct Integration of CFD With Detailed Chemistry

Diesel engine simulation results using two different combustion models are presented in this study, namely the representative interactive flamelet (RIF) model and the direct integration of computational fluid dynamics and CHEMKIN. Both models have been implemented into an improved version of the KIVA code. The KIVA/RIF model uses a single flamelet approach and also considers the effects of vaporization on turbulence-chemistry interactions. The KIVA/CHEMKIN model uses a direct integration approach that solves for the chemical reactions in each computational cell. The above two models are applied to simulate combustion and emissions in diesel engines with comparable results. Detailed comparisons of predicted heat release data and in-cylinder flows also indicate that both models predict very similar combustion characteristics. This is likely due to the fact that after ignition, combustion rates are mixing controlled rather than chemistry controlled under the diesel conditions studied.

FLAMELET MODELING FOR COMBUSTION PROCESSES AND NOx FORMATION IN THE TURBULENT NONPREMIXED CO/H2/N2 JET FLAMES

Two turbulent nonpremixed jet flames having the same jet Reynolds number but different nozzle diameters have been numerically investigated by using the steady and unsteady flamelet models. These validation cases have been chosen to critically evaluate the predicative capability of the present turbulent combustion models whether the model is able to predict correctly both the similarity of temperature and major species by scaling with nozzle diameter, sensitivity of minor species to fluid-dynamic scaling and distribution of radicals and NO. In order to accurately resolve the physically and geometrically complex reacting flow fields, the present study adopts the unstructured grid finite-volume method with the cell-centered scheme and the edge-based storage. In terms of unconditional mean scalar structures, the steady flamelet model with the additional treatment for the slow processes such as radiation and NOx formation is able to yield the reasonable results. On the other hand, without additional ad-hoc procedure, the unsteady flamelet model based on a Lagrangian approach can correctly predict the experimentally observed streamwise evolutions of conditional mean scalar structure and unconditional means as well as the full NOx chemistry. Furthermore, in the context of unsteady flamelet model, the detailed discussions have been made for the potential error of the optically thin radiation model, the full NO chemistry and the effect of differential diffusion, which the steady flamelet approach hardly addresses.

Multidimensional effects on structure and extinction process of counterflow nonpremixed hydrogen : Air flames

Abstract The axisymmetric Navier-Stokes model together with detailed chemical kinetics and variable transport properties has been applied to analyze the effects of the multidimensional flow on the flame characteristics in the nitrogen-diluted hydrogen counterflow nonpremixed flame. Computations are carried out for the laminar counterflow hydrogen-air flames with two fuel dilutions. In order to investigate the effects of the jet exit velocity profiles on the hydrogen-air diffusion flame structure, computations are made for two boundary conditions simulating the plug-flow and parabolic-flow velocity profiles at nozzle exit. In case of the highly diluted hydrogen flames, the near-extinction flame structure and extinction flame process are numerically studied for two nozzle exit area-averaged velocities based on the plug-flow profile. Numerical results indicate that the jet exit profiles significantly influence the flame structure in terms of strain rate, flame thickness, peak temperature, overlap of fuel and...

Comparisons of Combustion Simulations Using a Representative Interactive Flamelet Model and Direct Integration of CFD With Detailed Chemistry

Diesel engine simulation results using two different combustion models are presented in this study, namely the Representative Interactive Flamelet (RIF) model and the direct integration of CFD and CHEMKIN. Both models have been implemented into an improved version of the KIVA code. The KIVA/RIF model uses a single flamelet approach and also considers the effects of vaporization on turbulence-chemistry interactions. The KIVA/CHEMKIN model uses a direct integration approach that solves for the chemical reactions in each computational cell. The above two models are applied to simulate combustion and emissions in diesel engines with comparable results. Detailed comparisons of predicted heat release data and in-cylinder flows also indicate that both models predict very similar combustion characteristics. This is likely due to the fact that after ignition, combustion rates are mixing controlled rather than chemistry controlled under the diesel conditions studied.Copyright

Prediction of Detailed Structure and NOx Formation Characteristics in Turbulent Nonpremixed Hydrogen Jet Flames

The present study numerically investigates the turbulent nonpremixed hydrogen jet flames. The turbulent combustion processes are represented by the reaction progress variable model coupled with the presumed joint probability density function. The reaction progress variables are derived assuming the radicals O, H, and OH to be in partial equilibrium and additional species HO2 and H2O2 in steady state. The turbulent combustion model is extended to nonadiabatic flame by introducing additional variable for the transport equation of enthalpy and radiative heat loss is calculated using a local, geometry independent model. The predictive capability of the RPV combustion model has been validated against the detailed experimental data involving the distribution of temperature, major species, radicals, and NO. Effects of the HO2/H2O2 chemistry and radiative heat loss on the thermal NO formation are discussed in detail. In order to examine the validity of the optically thin radiation model in conjunction with the improved RPV model, the present numerical results for the radiant fraction have been compared with measurements and other computational results. Furthermore the capability of the present RPV model reproducing the NOx emission index (EINOx) is critically evaluated.

Combustion Characteristics for Varying Flow Velocity on Methane/Oxygen Diffusion Flames

The combustion characteristics of methane oxygen diffusion flames have been investigated to give basic information for designing industrial oxyfuel combustors. NOx reduction has become one of the most determining factors in the combustor design since the small amount of nitrogen is included from the current low cost oxygen production process. Flame lengths decreased with increasing fuel or oxygen velocity because of the enhancement of mixing effect. Correlation equation between flame length and turbulent kinetic energy was proposed. NOx concentration was reduced with increasing fuel or oxygen velocity because of the enhanced entrainment of the product gas into flame zone as well as the reduction of residence time in combustion zone.

Numerical modeling of combustion processes and pollutant formations in direct-injection diesel engines

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.

NUMERICAL MODELING FOR AUTO-IGNITION AND COMBUSTION PROCESSES OF FUEL SPRAYS IN HIGH-PRESSURE ENVIRONMENT

The present study is mainly motivated to investigate the vaporization, auto-ignition and combustion processes in high-pressure engine conditions. The high-pressure vaporization model is utilized to realistically simulate the spray dynamics and vaporization characteristics in high-pressure and high-temperature environment. The interaction between chemistry and lurbulencc is treated by employing the Representative Interactive Flamelet (RIF) Model. The detailed chemistry of 114 elementary steps and 44 chemical species is adopted for the n-heptane/air reaction. In order to account for the spatial inhomogeneity of the scalar dissipation rate, the multiple RIFs are introduced. Special emphasis is given lo vaporization effects on the mixture fraction fluctuations and the pdf model. Numerical results indicate that the RIF approach, together with the high-pressure vaporization model, successfully predicts the essential features of ignition and spray combustion processes.

Numerical modeling of turbulent nonpremixed lifted flames

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.

An Experimental Study on the Performances of a Coupled Reactor with Catalytic Combustion and Steam Reforming for SOFC and MCFC

>> The performances of a coupled reactor in which a steam reformer and a catalytic combustor were mounted simultaneously had been investigated and compared. The combustible offgas exhausted from the anode of SOFC and MCFC were utilized as heat sources for the endothermic steam methane reforming. The catalytic combustion was used in order to burn the combustible offgas. Thermal energy released by the catalytic combustion is directly transferred to the reformer surrounding the combustor. The various operational conditions such as fuel utilization rate, steam to carbon ratio, amount of catalysts, fuel cell loads were changed. And operating variables were comprehensively identified by sensitivity analysis. The fundamental results from this experimental study show the potential abilities of the coupled reactor. Therefore the results will be of help to design and manufacture the more better coupled reactor in the future.