Study On Construction,Optimization,and Assessment Of Reduced Chemical Reaction Mechanisms Of Macromolecular Fuels

With the development of fundamental combustion experiments and quantum chemistry,the study on the reaction kinetic models of liquid fuels becomes more and more thorough.Due to the limitation of computational power,reduced reaction mechanisms are still the best choices for the three-dimensional numerical simulation for engines.Therefore,a reduced model with compact structure,reliable performance,and high fidelity of chemical characteristics is greatly significant for engineering applications.Since the application range and the prediction accuracy of reaction mechanisms depend on the inherent kinetic characteristics,a novel quantitative assessment method for the reduced models was proposed,which is significant for the construction of reduced models with reliable performance and high fidelity of chemical characteristics.Besides,at present,a reduced model is mostly constructed from a single-component fuel.A construction method for reduced mechanisms of the same series of fuels was proposed,which expands the mechanism reduction method.Finally,to explore the reduction limit of a detailed model with a large size,a target-oriented construction strategy for building extremely reduced models was investigated.The above work positively promotes the construction of reduced mechanisms with compact size,reliable performance,and high fidelity of chemical characteristics.Firstly,by coupling the CHEMKIN-? package and the genetic algorithm(GA),the efficient and automatic optimization of the reduced mechanisms was realized in this paper.A skeletal mechanism of primary reference fuel(PRF)composed of n-heptane and iso-octane constructed using the decoupling methodology is regarded as the initial model for mechanism optimization.The ignition delay times(IDTs)in shock tubes(ST)and major species concentrations in jet-stirred reactors(JSR)are chosen as the optimization targets,and objective functions are built.The influence of the parameters in GA,the selection of experimental data,and the temperature weighting factors on the optimized results are investigated.Finally,extensive validations using the optimized mechanism are conducted in various fundamental reactors and a homogeneous charge compression ignition(HCCI)engine.The results indicate that coupling the GA program with the CHEMKIN-? package is an effective way to realize mechanism optimization.Secondly,by using uncertainty analysis,global sensitivity analysis(GSA),and path sensitivity analysis(PSA),a quantitative assessment method of reduced mechanisms based on the inherent chemical kinetic characteristics was proposed.The detailed mechanism of n-heptane and two reduced mechanisms derived from it are taken as examples to illustrate the assessment method.The overall chemical kinetic structure and the kinetic characteristics of the fuel-specific sub-mechanism in the detailed model and the reduced ones are analyzed based on sub-mechanisms and reaction classes,respectively.By utilizing the similarity coefficients,the quantitative assessment of the reduced models based on the detailed one is realized.The results show that the inherent kinetic characteristics of the chemical reaction mechanisms are mainly related to the temperature regions,involving low-temperature,negative temperature coefficient(NTC),and high-temperature ones,and are less dependent on the specific pressures and equivalence ratios.The C0-C4 and the fuel-specific sub-mechanisms are quite important for maintaining the overall chemical characteristics of the detailed model.When necessary to adjust the reaction rate constants,it should be conducted within their uncertainty ranges.Based on the above conclusions,a novel method to construct the reduced mechanism was proposed in this paper:GSA and PSA based on reaction classes are adopted to reduce the fuel-specific sub-mechanism,and the directed relation graph with error propagation and sensitivity analysis method(DRGEPSA)is used to reduce the C0-C4 core sub-mechanisms.Thirdly,according to the decoupling methodology and the reaction rate rule,the construction method of reduced mechanisms for the same series of fuels was proposed.The construction method is investigated based on polyoxymethylene dimethyl ether(PODEn).According to the detailed mechanism,the reduced models of PODE1-2 are constructed using reaction class-based GSA and PSA and the decoupling methodology.Through the analysis of the fuel-specific reaction classes in the detailed model,it is found that the distribution of path sensitivity coefficients,global sensitivity coefficients,and variance coefficients of the fuel-related reaction classes in PODE1-4 are quite consistent.Based on the reduced mechanism of PODE2,the fuel-related reduced pathways of PODE3-6 can be obtained using the analogy method.Moreover,the rate constants of the fuel-related reactions of PODE3-4 are calculated using the modified linear lumping method.It is found that there is an approximately linear relationship between the rate constants of fuel-related reactions in the reduced models of PODE2₄ and the number of the-CH2O-radicals contained in their molecule structures.For PODE5-6,which has no detailed model as a reference,the linear extrapolation method is used to estimate the rate constants of macromolecular reactions in the reduced models.Besides,the THERM program is used to calculate the thermodynamic properties of macromolecular species in the reduced models of PODE5₆.Finally,the reduced mechanism of PODE1-6 is achieved by combing the reduced sub-mechanisms of PODE1-2 and the reduced fuel-related sub-mechanisms of PODE3-6.The reduced mechanism is widely validated in various fundamental reactors and HCCI engines.The result proves that the reduced mechanism can capture the oxidation characteristics of PODE1-6 well.Finally,this paper explored the reduction limitation of a detailed mechanism of PRF with a large size,and an application-oriented construction method of the extremely reduced mechanism was proposed.The reduction process can be divided into two parts:the fuel-specific sub-mechanism and the C0-C4 core sub-mechanisms.For the fuel-specific sub-mechanism,the following reduction strategies are employed:GSA and PSA are used to identify key reaction classes;important isomers are identified with the ROP analysis;the reduced mechanism is optimized using GA.The following reduction strategies are used for the C0-C4 sub-mechanisms:the C0-C4 sub-mechanisms are efficiently reduced using the DRGEPSA method;based on the three objectives,namely IDTs in ST,the major species concentrations in JSR,and laminar flame speeds in premixed laminar flame,the binary-coded GA is used to automatically reduce the small-molecule sub-mechanisms;Quasi-steady-state analysis is finally utilized to lump reactions.Finally,three extremely reduced mechanisms are achieved.The reliable prediction abilities of them are validated by comparing them with the predictions using the detailed model and amounts of experimental data.Finally,through the local sensitivity analysis and the pathway analysis,the relationship and the difference of small-molecule reactions in the three extremely reduced models are investigated.It is found that the difference of the small-molecule reaction pathways between the extremely reduced mechanism based on IDTs in ST and the one based on species concentrations in JSR is mainly caused by the difference in the variation of temperatures and pressures in the two reactors.Besides,IDTs are closely related to the production and consumption of hydroxyl radicals,while flame propagation is directly related to the accumulation and the consumption of hydrogen radicals.Overall,a systematical study,involving construction,optimization,and assessment of the reduced mechanisms,was conducted in this paper.This work involves the following aspects:the efficient optimization method of the reduced mechanisms,the quantitative assessment method of the reduced mechanisms based on the inherent kinetic characteristics,the construction method of the reduced mechanisms for the same series of fuels.This paper also explored the reduction limitation of the large-scale detailed model of PRF,and the construction method of the extremely reduced mechanisms oriented towards the specific applications was proposed.This work provides a valuable reference for the construction and the assessment of reduction mechanisms.It also has a positive significance for the construction of the reduction mechanisms for other fuels and provides the necessary support for the structure design and the control strategies of the advanced engines.