Study On The Development Of A Biodiesel Chemical Mechanism Model And Its Application To Engine Combustion Analysis

Biodiesel fuel is considered as a promising alternative fuel for diesel engines dueto its special physical and chemical properties. In order to cleanly and efficientlyutilize biodiesel, it is essential to deeply understand the combustion process in anengine using biodiesel fuels. Recently, the advanced simulation technology ofcomputational fluid dynamics (CFD) coupled with chemical kinetics has became aneffective method for understanding the combustion processes of an engine. Thismethod is also the mainstream technology for three-dimensional (3D) modeling ofbiodiesel combustion. Because the time comsumption of CFD coupled with detailedchemical reaction mechanisms is unacceptable for industry applications, a robust andaccurate reduced mechanism is required. The reduced chemical mechanisms ofbiodiesel are yet developed in recent years. There are still many uncertainties to beresolved. There are two major challenges in the development of reduced biodieselmechanisms based on the detailed mechanisms. One is that it is difficult to reduce thedetailed mechanisms because of their tremendous sizes. The other one is that thereduced mechanisms should be updated when the detailed mechanisms are updated.Therefore, the present study focus on adapting engine CFD coupled with chemicalkinetics, improving the mechanism reduction method, developing a code for reductionof complex detailed mechanisms, developing techniques for generating a reducedmechanism, and creating a new reduced kinetics mechanism for biodiesel with itsapplication to the3D modeling of low temperature combustion in a diesel engine. Themain results and conclusions of this dissertation are listed as follows:(1) The CHEMKIN-Ⅲ chemistry solver was integrated into the KIVA-3V R2code forming a KIVA-CHEMKIN model. After the KIVA-CHEMKIN model wasvalidated, it was applied to the combustion simulations of diesel engines fueled withdiesel and biodiesel. Then an advanced commercial software, CONVERGE, wasintroduced in this study. The CONVERGE model was compared with theKIVA-CHEMKIN model. Based on these two modeling tools, the principles of3Dmodeling of enigne in-cylinder combustion with chemical kinetics were described indetails. This paved the way for the development of the mechanism reduction code andreduced surrogate mechanism for biodiesel in this study.(2) A method named herein Group of Species Elimination (GSE) was proposedfor eliminating the unimportant species. As a coefficient of brute-force sensitivity analysis (BFSA) for ignition delay time may have a negative or positive sign, a groupof species can be simultaneously removed from an intial mechanism. The specieswhich have the coefficients with opposite signs will give an error cancellation. Byselecting a reasonable speices group, the GSE can efficiently remove unimportantspecies out of the mechanism and control the induced error due to the specieselimination. Compared to other method of species elimination based on BFSA, theGSE can reduce the time consumption during the mechanism reduction, because itdose not need to delete unimportant species one by one and assess the induced errorevery time. The GSE can also avoid prematurely end the reduction procedure.(3) A so-called Mechanism Reduction Code (MRC) was also generated using theFORTRAN language to reduce detailed mechanisms. The MRC consists of twomodules, one is based on the algrithm of direct relation graph with error propagation(DRGEP) and other one is based on the BFSA and GSE. The MRC was first applied tothe reduction calculation of a detailed iso-octane mechanism. The results show thatthe MRC is an effective tool for mechanism reduction and can be applied to thereduction of other hydrocarbon fuel mechanisms. The reduction process of theiso-octane mechanism also show that for a fuel having a negative temperaturecoefficient (NTC) behaviour, the reactions and species have the most complicatedcoupling relationship in NTC region. By setting the initial conditions in the NTCregion, a reduced mechanism with comprehensive performance can be obtainedthrough the iterative DRGEP reduction. This mechanism can be used as a basis tobuild the final DRGEP-out mechanism for the next reduction stage. In the secondreduction stage, only few representative cases selected to perform the iterative BFSAand GSE to form a final reduced mechanism with high accuracy. These representativecases may be the NTC region, high temperature region, and low temperature region.(4) A new reduced surrogate mechanism for biodiesel was generated byconsidering n-heptane (NHP) blended with methyl decanoate (MD) as the surrogatefor biodiesel. According to the guideline obtained during the mechanism reduction ofiso-octane, the detailed mechanisms of NHP and MD developed by U.S. LawrenceLivermore National Laboatory (LLNL) were reduced with the MRC. As a result, thedetailed NHP mechanism including654species and2827reactions was reduced into asimple one consisting of60species and216reactions. The detailed MD mechanismincluding2878species and8555reactions was reduced into a simple one consistingof87species and219reactions. Then the reduced mechanisms of NHP and MD weremerged into a final surrogate mechanism, named Bio111mechanism hereby, which consists of111species and310reactions. Finally, the Bio111was validated againstthe comprehensive experimental data obtaining from shock tubes, jet stirred reactors,and a diesel engine. The validations show that the Bio111has a good predictiveability.(5) The study showed that the Bio111predicted well the combustion heat releasecharacteristics when it was applied to3D modeling of the low temperaturecombustion of soybean biodiesel in a diesel engine with two fuel injections. Thepremixed charge compression ignition mode with early fuel injection showed anapparent cool flame phenomenon. During the simulations the Bio111predicted thiscool flame well. The kinetics flow-rate analysis of engine in-cylinder combustionshowed that the reaction paths of low temperature branching, propagation, anddecompositions of NHP radical and MD radical simultaneously occurred undered theNTC conditions. These reactions resulted in a cool flame. It is concluded that theBio111mechanism can form a basis to integrate advanced chemical kinetics emissionmodels for biodiesel combustion in the future research.