An Experimental Determination Of Reaction Rate In Combustion Using Rapid Compression Machine

Energy crisis and environment pollution are two main issues threatening our world.High-efficiency low-emission combustion technologies are developed to address these issues.Detailed combustion kinetics model is crucial for developing engines equipped with these technologies.These kinetics models typically consist of thousands of reactions,in which the rate constants are often determined by analogy,theory,and experiment.Among the three methods,experiment is the most decisive method and always being used as a benchmark of other methods.However,current experiment facilities are unable to cover the intermediate temperature range,which is a major concern of the advanced engine technologies mentioned above.Rapid compression machine?RCM?is a facility that already being widely used in the study of some important phenomena happening in the intermediate temperature range,such as negative temperature coefficient?NTC?regime,knock,and auto-ignition.Therefore,the aim of this work is exploring the possibility of using RCM in measuring reaction rate.A RCM operating at 650-1100 K and 5-100 bar was built in this work,featuring with a large bore and a large clearance height.Additionally,it was equipped with a heating system to elevate the initial temperature up to 200?,and a fast sampling system coupled with a gas chromatograph to quantify intermediate species in pyrolysis or combustion studies.With the newly developed RCM,the NTC regime of the first-stage ignition was observed in the study of iso-octane and methyl-cyclohexane.Five reaction classes competing with the low temperature branching channel were identified of importance for this NTC regime.The first-stage NTC was regard as one of the two main reasons for the NTC of total ignition.With the RCM and the fast sampling system,a new method measuring reaction rate was proposed in this work.The main idea is that when the uncertainty of the concentration of a species is dominated by a single reaction,the rate of this reaction can be derived by measuring the concentration profile of this species.As a validation of the proposed method,the rate of the reaction CH₃OCHO?methyl formate,MF?=>CH₃OH+CO was determined by measuring the CO concentration in the MF pyrolysis in 948-1112 K,yielding a reaction rate consistent with previous experimental and theoretical studies.A unimolecular reaction,which is CH₃OCOOCH₃?dimethyl carbonate,DMC?=>CH₃OCH₃?dimethyl ether,DME?+CO₂,was studied by measuring DME concentration in the DMC pyrolysis in 994-1068 K.The optimized reaction rate agrees well with the RRKM/Master Equation calculation based on a high level quantum chemical potential energy surface,which casts more doubts on the previous calculation,which claimed a reaction rates of 4-5 times faster than the current study.Additionally,the reaction rate of DMC=>CH₃COO+CH₃ was derived by matching the measured ethane profile,resulting in a reaction rate consistent with experimental and theoretical studies in literature.A bimolecular reaction,which is 1,5-hexadiene+allyl radical=>hexa-2,5-dien-1-yl+propene,was studied by measuring propene concentration in the 1,5-hexadiene pyrolysis in 893-1007 K in RCM.Using the similar approach,flow reactor?FR?measurement at 776-833 K and jet stirred reactor?JSR?measurement at 800-875 K were adopted from literature to constrain the rate coefficients of this reaction as additional information.We finally generalize the rate coefficients measured by the RCM,FR and JSR to obtain an Arrhenius expression valid for 776-1007 K.Compared with the previous theoretical prediction computed at a moderate theoretical level,our measured reaction rates are⁵0%slower but are still located within the stated uncertainties.