Analysis Of Main Reaction Characteristics Of Alkane Fuel And Its Sensitive Parameters

Alkanes are widely studied as important components and intermediate products of gasoline.With the evolution of the oil industry and the development of new combustion mode,understanding the chemical reaction mechanism of alkanes and mastering the ignition characteristics,its sensitive parameters are helpful to realize the control of the combustion process and provide a theoretical basis for optimizing the performance of engine.Based on the basic parameters of constant volume combustion bomb test platform,a closed homogeneous reactor model is established by CHEMKIN-PRO software.This model is coupled with chemical reaction mechanism of n-heptane,n-pentane and iso-octane,which studies the chemical kinetics and ignition characteristics of alkanes.The main study contents include three parts:The first part,the reliability validation of simulation model is completed by n-heptane ignition test data in constant volume combustion bomb.The sensitivity of alkane OH under different temperature,pressure and equivalence ratio is studied through single factor research method.Determing the main components and reaction types of alkanes based on sensitivity analysis and elaborating the reasons for the insensitivity of the ignition temperature of n-heptane under different conditions.The results show that:?1?Alkanes ignition delay are very sensitive to temperature,The main reaction types to promote ignition at low temperature include R16,H-atom abstraction from alkanes,alkylperoxy isomerization,decomposition of ketohydroperoxides,etc.The main reaction types to promote ignition at medium temperature increase the reaction of generating ketohydroperoxides and OH on the basis of above low temperature reactions.This temperature also occurs?-scission,which causes negative temperature coefficient.The main reaction types to promote ignition at high temperature include R1,R16 and alkyl cracking into small molecules.?2?The effect of initial pressure and equivalent ratio on alkanes ignition delay is far less significant than that of temperature.The reaction types remain the same in general but change the rate of each reaction.The greater initial pressure,the greater the absolute value of reaction sensitivity coefficient,and the deeper degree of promoting/inhibiting ignition.The larger equivalent ratio,the shorter the ignition delay.?3?Under different conditions,various reaction paths,multiple reactions under the same reaction path and very low energy barriers make the ignition temperature of n-heptane insensitive to the external conditions.?4?Ignition sensitivity of CC₈H₁₇7 produced by iso-octane H-atom abstraction is different from that of A-,B-,and D-alkyl.H-atom abstraction and alkylperoxy isomerization reaction at low temperature inhibit ignition.This is due to the difference in the reaction path between CC₈H₁₇7 and the other three octane alkyl,which produces this specificity.The second part,based on sensitivity analysis,determines the main components and reaction types of alkanes.By analyzing the yield of each component,the detailed chemical reaction path of alkanes is constructed at different temperatures,and the oxidation process of the fuel is clearly defined.The results show that:?1?At 900K initial temperature,alkanes main reaction path include low temperature reactions and medium temperature multi group?-scissions.The formation of ketohydroperoxides reaction and its decomposition reaction are the two most important reactions at low temperature,which generate large amounts of OH and introduce chain branching reactions,indicating the ignition of n-alkanes.At medium temperature multi group?-scissions include:alkylperoxy crackingproducesconjugatedalkene+HO₂,hydroperoxy-alkylcrackingproduces aldehydes+alkenes+OH or cyclic ethers+OH.?-scissions is chain propagation reaction,which competes intermediate product from chain branch at low temperature,so ignition delay extended.At 1250K high temperature,the low and medium temperature reaction path of n-alkanes are strongly inhibited.Alkylperoxies directly crack into small molecules of alkyl and olefin.?2?Most A-,C-,D-alkyl of iso-octane have low temperature reactions at 725K initial temperature.Only 17.58%CC₈H₁₇7 has low temperature reactions,and the rest has cracking reaction.At 900K initial temperature,4.1%iso-octane cracks directly,B-,D-alkyl hardly react low temperature reactions,A-alkyl only has medium temperature?-scission and high temperature cracking reaction,all C-alkyl has high temperature cracking reaction.At 1250K initial temperature,the proportion of iso-octane cracking reaction increases to 25.6%.A-,D-alkyl have?-scission and high temperature cracking reaction.All B-alky has?-scission reaction,and C-alky is all cracked at high temperature.Under the same condition,CC₈H₁₇reaction type is different from the other three alkyl of iso-octane.The third part,ignition characteristics of alkanes are studied.First,use variance analysis method to find the influence weight on ignition delay of temperature,pressure and equivalent ratio.Secondly,the influence of the above parameters on the ignition delay and the combustion temperature is analyzed.Finally,the reasons for different ignition delay are explained from carbon chain difference.The results show that:?1?The influence of the temperature on ignition delay is far more than the pressure and equivalent ratio.The ignition delay of alkanes can be divided into three stages:low,medium and high temperature.The ignition delay shorten with the temperature increase in low and high temperature.However,the ignition delay extend with temperature increase in medium temperature,that negative temperature coefficient appears.The ignition delay of alkanes shorten with the increase of the initial pressure and equivalent ratio.?2?The highest combustion temperature in reactor increases with the increase of initial temperature,which is not related to the initial pressure.The maximum combustion temperature is the highest when equivalent ratio is 1,and the maximum combustion temperature of rich mixture?equivalent ratio is 1.5?is higher than that of lean mixture?equivalent ratio is 0.5?.?3?The reaction rate of H-atom abstraction from iso-octane is related to the type of carbon atom connected by hydrogen atom and the position of carbon atom?a,d?.The C-H bond energy of iso-octane:primary carbon of position a>primary carbon of position d>secondary carbon>tertiary carbon,Stability of its H-atom abstraction product:AC₈H₁₇>DC₈H₁₇>BC₈H₁₇>CC₈H₁₇.The low stability of CC₈H₁₇7 leads to reaction path different fro m the other three alkyl of iso-octane,and cracking reaction is easy to occur.At low tem perature,CC₈H₁₇7 does not need a longer low temperature reaction path but direct crackin g,which leads to the maximum reaction rate.?4?Ignition delay of alkanes is always:iso-octane>n-pentane>n-heptane.The isomerization reaction of alkylperoxy to hydroperoxy-alkyl and the reaction of generating ketohydroperoxides and OH at low temperature both happen the formation of transition ring and hydrogen atom transfer,which cause the difference between ignition characteristics of alkanes.Iso-octane ignition delay is the longest,because it has too many primary carbon to happen the formation of transition ring and hydrogen atom transfer.The branched chain structure not only increases ring stress when forming transition ring,but also increases the steric hindrance and reduces the probability of O-H combination.The more carbon atoms of n-alkanes,the more fragile the interatomic chain connection,the shorter the ignition delay.