Experimental And Modeling Investigations On The Combustion Kinetics Of Acetaldehyde Under A Wide Range Conditions

The kinetic models for small fuels containing less than four carbon atoms,i.e.,C0-C4 mechanisms,serve as the foundation for the combustion models for fossil fuels and biofuels.Lots of experimental and modeling efforts have been spent on the combustion of hydrocarbons or alcohols,however,less investigations have been conducted on small aldehydes.Acetaldehyde is a key intermediate and toxic emission in the combustion of fuels,especially for biofuels.Its combustion model is an important part of core mechainms,but few researches about acetaldehyde combustion have been reported,most of which focused on the global combustion properties,such as ignition delay times.Previously,the observations of cool flames and oscillatory ignition during acetaldehyde oxidation showed that the low-temperature oxidation of acetaldehyde should exhibt the negative temperature coefficient（NTC）behavior,but until now no direct experimental evidence has been provided.The second O₂-addition pathway,which is the key to explain the NTC behavior in the low-temperature oxidation of common fuel,proved to be impossible in the oxidation of acetaldehyde.To better understand acetaldehyde combustion characteristics,the chemical structures of premixed flame and counterflow flame fueled by acetaldehyde have been measured by employing synchrotron vacuum ultra-violet photoionization molecular-beam mass spectrometer（SVUV-PI-MBMS）,with 38 intermediates indentified and quantified,including the fuel radicals（C₂H₃O）and small hydrocarbons.The oxidation of acetaldehyde was also studied at low-temperatures（528 to 946 K）in a jet-stirred reactor（JSR）using the same experimental method.The mole fractions of 28 species,including the peroxide products of CH₃CO radical（C₂H₃O₃ and C₂H₄O₃）were measured as a function of temperature.In addition,ignition delay times at 700-1100 K were measured in a rapid compression machine（RCM）.The NTC phenomenon was directly observed in both JSR and RCM experiments results,and a two-stage ignition were also observed in the RCM experiments when temperatures were lower than 800 K.The detailed concentration profiles,as well as the measured ignition delay times,provided valuable information for the validation and development of kinetic model.A detailed chemical kinetic model,containing 498 species and 2762 reactions,has been developed based on AramcoMech 2.0.In the present model,the hydrogen abstractions on CH₃CHO by various radicals were updated for a more reasonable branching ratio for the two types of H-abstractions forming CH₃CO and CH₂CHO radicals,separately.The O₂-addition pathway for CH₃CO radical was suppleted,which is important to the low-temperature oxidation of acetaldehyde.The sub-mechanisms of some important intermediates to the the combustion and oxidation of acetaldehyde,for example ketene（CH₂CO）,were also updated or added.The present kinetic model has been comprehensively validated against the present experimental results and various literature data,covering wide ranges of temperatures（300-2300 K）,pressures（0.02-10 atm）and equivalence ratios（0.5-∞）.Based on the results of the present experimental and modeling work,the explanations for the observed NTC phenomenon in both JSR and RCM experiments are explored.The O₂-addition pathway to CH₃CO radical is found to be responsible for the high reactivity of acetaldehyde at low temperatures in both cases.The observed NTC behavior in the JSR experiment is explained.by the competition between the O₂-addition to and the decomposition of the CH₃CO radical.While,the NTC behavior in the RCM experiments is believed to be caused by the competition between multiple oxidation pathways for the methyl radicals and their self-recombination forming ethane,a relatively stable specie at temperatures below 1000 K.