Song-Charng Kong

Numerical study of premixed HCCI engine combustion and its sensitivity to computational mesh and model uncertainties

This study used a numerical model to investigate the combustion process in a premixed iso-octane homogeneous charge compression ignition (HCCI) engine. The engine was a supercharged Cummins C engine operated under HCCI conditions. The CHEMKIN code was implemented into an updated KIVA-3V code so that the combustion could be modelled using detailed chemistry in the context of engine CFD simulations. The model was able to accurately simulate the ignition timing and combustion phasing for various engine conditions. The unburned hydrocarbon emissions were also well predicted while the carbon monoxide emissions were under predicted. Model results showed that the majority of unburned hydrocarbon is located in the piston-ring crevice region and the carbon monoxide resides in the vicinity of the cylinder walls. A sensitivity study of the computational grid resolution indicated that the combustion predictions were relatively insensitive to the grid density. However, the piston-ring crevice region needed to be simulated with high resolution to obtain accurate emissions predictions. The model results also indicated that HCCI combustion and emissions are very sensitive to the initial mixture temperature. The computations also show that the carbon monoxide emissions prediction can be significantly improved by modifying a key oxidation reaction rate constant.

Application of detailed chemistry and CFD for predicting direct injection HCCI engine combustion and emissions

The present study applied a detailed chemical kinetic mechanism to simulate the combustion and emissions in a direct-injection homogeneous charge compression ignition (HCCI) engine. The engine is a heavy-duty diesel engine equipped with a pressure-swirl injector using gasoline as the fuel. The intake air was heated to help fuel vaporization to achieve HCCI conditions. The CHEMKIN code was implemented into KIVA-3V so that the chemistry is solved in the context of the engine computational fluid dynamics simulation. The effects of turbulent mixing on the reaction rate were considered. The computations started from intake valve closure with the initial conditions provided by a one-dimensional cycle simulation code that can simulate the gas-exchange process. Good levels of agreement in combustion and emissions were obtained using the present model. Predicted cylinder pressures and heat release rates agreed well with measurements. The computational results showed that a lean and fairly homogeneous mixture was obtained under the current engine configurations. Relatively low gas temperature (with peak value only about 2000 K) was observed in the present HCCI combustion that produced low NOx emissions. The model also predicted the correct trends in unburned hydrocarbon and carbon monoxide emissions as the start-of-injection timing and engine load were varied. It was also found that the unburned hydrocarbons and carbon monoxide emissions increased drastically if the overall equivalence ratio was less than a certain limit, for example, 0.15, due to poor combustion.

Use of Detailed Chemical Kinetics to Study HCCI Engine Combustion With Consideration of Turbulent Mixing Effects

Detailed chemical kinetics was used in an engine CFD code to study the combustion process in HCCI engines. The CHEMKIN code was implemented in KIVA such that the chemistry and flow solutions were coupled. The reaction mechanism consists of hundreds of reactions and species and is derived from fundamental flame chemistry. Effects of turbulent mixing on the reaction rates were also considered. The results show that the present KIVA/CHEMKIN model is able to simulate the ignition and combustion process in three different HCCI engines including a CFR engine and two modified heavy-duty diesel engines. Ignition timings were predicted correctly over a wide range of engine conditions without the need to adjust any kinetic constants. However, it was found that the use of chemical kinetics alone was not sufficient to accurately simulate the overall combustion rate. The effects of turbulent mixing on the reaction rates need to be considered to correctly simulate the combustion and heat release rates.

Modeling and Experiments of Dual-Fuel Engine Combustion and Emissions

The combustion and emissions of a diesel/natural gas dual-fuel engine are studied. Available engine experimental data demonstrates that the dual-fuel configuration provides a potential alternative to diesel engine operation for reducing emissions. The experiments are compared to multi-dimensional model results. The computer code used is based on the KIVA-3V code and consists of updated sub-models to simulate more accurately the fuel spray atomization, auto-ignition, combustion and emissions processes. The model results show that dual-fuel engine combustion and emissions are well predicted by the present multi-dimensional model. Significant reduction in NO x emissions is observed in both the experiments and simulations when natural gas is substituted for diesel fuel. The HC emissions are under predicted by numerical model as the natural gas substitution is increased. The capabilities and limitations of the combustion model to simulate premixed combustion of air and natural gas were identified. It was found that the combustion model previously developed for diesel combustion provides adequately accuracy when extended to model the present dual-fuel cases. However, the accuracy of the predictions deteriorates for small pilot quantities. A brief discussion is given of a new combustion modeling approach that is applicable to very low pilot diesel fuel cases.

Modeling Diesel Spray Flame Lift-Off, Sooting Tendency and NOx Emissions Using Detailed Chemistry With Phenomenological Soot Model

A detailed chemistry-based CFD model was developed to simulate the diesel spray combustion and emission process. A reaction mechanism of n-heptane is coupled with a reduced NOx mechanism to simulate diesel fuel oxidation and NOx formation. The soot emission process is simulated by a phenomenological soot model that uses a competing formation and oxidation rate formulation. The model is applied to predict the diesel spray lift-off length and its sooting tendency under high temperature and pressure conditions with good agreement with experiments of Sandia. Various nozzle diameters and chamber conditions were investigated. The model successfully predicts that the sooting tendency is reduced as the nozzle diameter is reduced and/or the initial chamber gas temperature is decreased, as observed by the experiments. The model is also applied to simulate diesel engine combustion under PCCI-like conditions. Trends of heat release rate, NOx and soot emissions with respect to EGR levels and start-of-injection timings are also well predicted. Both experiments and models reveal that soot emissions peak when the start of injection occurs close to TDC. The model indicates that low soot emission at early SOI is due to better oxidation while low soot emission at late SOI is due to less formation. Since NOx emissions decrease monotonically with injection retardation, a late injection scheme can be utilized for simultaneous soot and NOx reduction for the engine conditions investigated in this study.Copyright

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.

Development of a Flame Propagation Model for Dual-Fuel Partially Premixed Compression Ignition Engines

Abstract The limitations of an existing characteristic-time combustion (CTC) model are explored and a new combustion model is developed and applied to simulate combustion in dual-fuel engines in which the premixed natural gas is ignited by the combustion flame initiated by a diesel spray. The model consists of a diesel auto-ignition model and a flame propagation model. A G-equation-based model previously developed to simulate SI engine combustion was incorporated with an auto-ignition model to simulate the flame propagation process in partially premixed environments. The computer code is based on the KIVA-3V code and consists of updated sub-models to simulate more accurately the fuel spray atomization, auto-ignition, combustion, and emissions processes. Modifications were made to implement the level set G-equation approach and to track the location of the flame as a function of the turbulent flame speed, flame curvature, flow velocity, and the movement of the computational mesh in the engine environment. Good agreement with engine experiments was obtained by using the present model.

Modeling the Effects of Geometry Generated Turbulence on HCCI Engine Combustion

The present study uses a numerical model to investigate the effects of flow turbulence on premixed iso-octane HCCI engine combustion. Different levels of in-cylinder turbulence are generated by using different piston geometries, namely a disc-shape versus a square-shape bowl. The numerical model is based on the KIVA code which is modified to use CHEMKIN as the chemistry solver. A detailed reaction mechanism is used to simulate the fuel chemistry. It is found that turbulence has significant effects on HCCI combustion. In the current engine setup, the main effect of turbulence is to affect the wall heat transfer, and hence to change the mixture temperature which, in turn, influences the ignition timing and combustion duration. The model also predicts that the combustion duration in the square bowl case is longer than that in the disc piston case which agrees with the measurements. The results imply that it is preferable to incorporate detailed chemistry in CFD codes for HCCI combustion simulations so that the effect of turbulence on wall heat transfer can be better simulated. On the other hand, it is also found that the onset of combustion is also very sensitive to the initial conditions so that an accurate estimate of initial mixture conditions is essential for combustion simulations.

Diesel combustion modelling using LES turbulence model with detailed chemistry

Diesel spray combustion and emissions are modelled in this study using large eddy simulation (LES) turbulence models coupled with spray breakup and detailed chemistry models. The objective of this study is to develop numerical models that can be used to predict the diesel spray combustion process. The LES filtering procedure yields unknown subgrid scale terms that need to be modelled. A one-equation, non-viscosity dynamic structure model is used to model the subgrid stress tensor and the gradient method is used to model the subgrid scalar flux. The model uses a tensor coefficient determined by a dynamic procedure and the sub-grid kinetic energy that is to enforce a budget on the energy flow between the resolved and unresolved scales. A skeletal n-heptane reaction mechanism is used to simulate the diesel fuel chemistry. A reduced NO x mechanism is incorporated into the n-heptane mechanism to simulate NO x formation, and the soot emission process is simulated by a phenomenological model that uses a competing formation and oxidation rate formulation. The present models were validated by comparing predicted results against experimental data of a diesel engine. The predicted and measured cylinder pressure history and heat release rate data are in good agreement. Trends of NO x and soot emissions are also captured with respect to different injection timings and exhaust gas recirculation (EGR) levels. Results indicated that the present models can capture the overall combustion process and can be further developed into a useful tool for engine combustion simulations.

Characterizing Effects of the Shape of Screw Conveyors in Gas–Solid Fluidized Beds Using Advanced Numerical Models

A numerical study of the effects of the shape of an enclosed screw conveyor on the mixing and heat transfer in a horizontal gas–solid fluidized bed was conducted using computational fluid dynamics (CFD). A two-fluid model (TFM) was employed to model the gas and solid phases as continua through mass, momentum, and energy conservations. The motion of the screw conveyor was simulated by using a rotating reference frame (RRF) such that the computational mesh was free from dynamic reconstruction. The diameters of the screw flight and shaft, the pitch, and the blade thickness were varied in the parametric study. Under the operating conditions studied, it was found that the increase in the diameter of the screw flight results in the enhancement of the solid mixing and conveyance. The increase in the diameters of the screw shaft and the screw blade thickness lead to the enhanced solid mixing but reduced conveyance. The variation in the screw pitch gives rise to rather complex behaviors in the solid mixing and conveyance. As the screw pitch is decreased, the solid mixing increases initially but then decreases before it increases eventually. The solid conveyance capability was found to first increase and then decrease. Explanations to the effects of the shape of the screw conveyor were discussed in this work.