Modeling Study Of Ethylene And Ethylene/Ethanol Coflow Diffusion Flame

Diffusion flame is the dominant flame geometry in practical combustion devices, and is easer to produce polycyclic aromatic hydrocarbons comparing to premixed flame, thus the practical combustion devices that contain diffusion flames are the major source of soot emissions. Axisymmetric coflow diffusion flame has a relatively simple2D geometry, which makes it easy to study with coupled computational fluid dynamics (CFD) and the chemical reaction dynamics. In this dissertation, numerical simulations of coflow diffusion ethylene and ethylene/ethanol flames were performed with LaminarSMOKE code combined with detailed kinetic models, and the formation of aromatic hydrocarbons was discussed from a kinetic point of view.Ethylene is an important component in practical fuels as well as a significant intermediate that appears in the combustion of most large hydrocarbons. Because of its simple structure and containing an unsaturated bond, ethylene is a typical fuel used in the study of the formation of polycyclic aromatic hydrocarbons (PAHs) and soot. In this dissertation, a detailed kinetic model of ethylene combustion with formation submechanism of aromatic hydrocarbons was developed based on the previous aromatic hydrocarbons combustion model and butene isomers pyrolysis model, and then numerical simulation of a previous coflow diffusion ethylene flame was performed with LaminarSMOKE code. The present model is able to reproduce the fuel decomposition and the formation of aromatic hydrocarbons in ethylene diffusion flame. Rate of production analysis was performed, and reveals that ethylene is mainly consumed by the H-abstraction reactions to produce vinyl radical and unimolecular decomposition reaction to produce H2CC and H2. Reactions from ethylene can produce propargyl radical which is the major precursor of aromatic hydrocarbons in the coflow diffusion ethylene flame, along the reaction sequence:C2H4â†'1,3-C4H6â†'1,2-C4H6â†'C3H3â†'C6H6.Phenyl radical is the major decomposition product of benzene, and the H-Abstraction-Carbon-Addition (HACA) reaction sequence from phenyl radical is the dominant formation pathway of naphthalene which is the most important bicyclic aromatic hydrocarbon:C6H6â†'C6H5â†'C6H5C2Hâ†'C6H4C2Hâ†'C6H4(C2H)2â†'C10H7â†'C10H8.Ethanol is one of the most used biofuel, it can be used as the surrogate fuel of gasoline or the fuel additive. But the addition of ethanol in ethylene flame may increase the PAHs and soot’s emissions in some experimental studies of ethylene/ethanol diffusion flames. In this dissertation, a detailed ethylene/ethanol mixed fuel combustion model was developed by adding an ethanol submechanism to the former ethylene combustion model, and numerical simulation of a coflow diffusion ethylene/ethanol flame was performed with LaminarSMOKE code. The excellent agreements between computational and experimental results indicate that the model is able to accurately predict the effects of adding ethanol to ethylene diffusion flame. When the mixture parameter β=QC2H6O/QC2H4=0.1, the temperature field and velocity field remain unchanged, which indicates that the variations of flame intermediates’concentrations are mainly caused by the chemical effect of ethanol addition. Rate of production analysis was performed, and reveals that ethanol is mainly consumed alone two paths, one is by dehydration reaction to produce ethylene and another is decomposed to produce methyl and CO. More methyl leads to more propyne and allene (C3H4), and further alters the main production rout of propargyl radical, from C2H4â†'1,3-C4H6â†'1,2-C4H6â†'C3H3to C2H4â†'C2H2â†'C3H4â†'C3H3. The main production and consumption path of aromatic hydrocarbons remain unchanged. The increase of propargyl produces more benzene through the C3+C3route. The acetylene just has a very small decrease, which has a limited impact on the HACA route. So the production of phenylacetylene and naphthalene are enhanced through HACA route. Thus in ethylene/ethanol diffusion flame, the increase of naphthalene mainly due to the increase of methyl, C2H5OHâ†'CH3â†'C3H4â†'C3H3â†'C6H6â†'C6H5â†'C6HSC2Hâ†'C6H4C2Hâ†'C6H4(C2H)2â†'C10H7â†'C10H8.