Study On Reduction Methods For Reaction Kinetic Mechanisms Of Hydrocarbon Fuels Under Uncertainty

Combustion numerical calculation plays an important role in engine design,and combustion kinetic mechanism largely determines whether the numerical calculation can accurately control the heat release law of fu el,structure of the flow field and the reaction path of pollutants.However,it is still a challenge to numerically solve the kinetics system which composed of large-scale combustion mechanism.Moreover,the uncertainties in the reaction rates bring furth er challenges to the combustion numerical calculation,which has a huge impact on the important path and the active reactions in the fuel combustion process.Therefore,the development of a mechanisms reduction method considering the uncertainties in the reaction rates(Global reduction method)can effectively improve the application and guidance of combustion numerical calculation in practice.In order to address the problem that the traditional simplification method is affected by the uncertainties in the reaction rates,the influence of the uncertainty of rate coefficient on the minimum size of simplification mechanism is studied.The results show that the uncertainties in the reaction rates seriously affect the resulting of the size of the reduced mechanism obtained by graphical and sensitivity methods.Under the impact of the uncertainties in the reaction rates,the maximum difference of the reduced mechanism sizes of n-decane combustion kinetic mechanism obtained by the graphic method under uncertainty is nearly 40 species,and the maximum difference of the reduced mechanism sizes of n-heptane combustion kinetic mechanism size obtained by the local sensitivity method is 15 species.Considering the computational cost of a reduction of the reaction kinetic mechanism of hydrocarbon fuel with large molecule under the uncertainties in the rates,this paper proposes a method to deal with such problem by combining the traditional reduction and the global reduction methods.In view of the current situation that the traditional reduction method can not effectively obtain the reduced mechanism,this paper proposes the weight and the improved graphic methods.Under a wide initial operating conditions,the weight method can obtain reduced mechanism with only 35 species when reducing ethylene kinetic mechanism,and obtain reduced mechanism with only 38 species when reducing n-heptane combustion mechanism,obtain the reduced mechanism with 145 species when reducing the aviation kerosene mechanism including low temperatur e effects.To simplify the kinetic mechanism of hydrocarbon combustion under the uncertainties in the reaction rates,a global species sensitivity method is developed.The reduction results show that such method can accurately obtain the rank of the important species under uncertainty,and can obtain a very compact global reduced mechanism.The global reduction method obtains the global reduced mechanism with 31 species for ethylene combustion,and 37 species for n-heptane combustion.Moreover,the global reduced mechanism can not only give the error range of reduced mechanism,but also accurately predict combustion characteristic parameters under standard reaction rates.In addition,it also obtain the global reduced mechanism with 124 species for the aviation kerosene with low temperature effects.In order to analyze the kinetic characteristics of the reduced mechanism,a similarity method based on sensitivity coefficients is proposed to analyze the discrepancy in the kinetic characteristics between reduced and detailed mechanisms.The results show that the traditional criteria for evaluating the simplified mechanism may lead to the bias of the kinetic characteristics of the reduced mechanism,which makes the reduced mechanism appear incomplete or lack of se rious kinetic characteristics,and seriously affects the applicability and characterization of the reduced mechanism in numerical calculation.Therefore,this paper puts forward an evaluation method based on similarity to analyze the reduced mechanism,and selects the reliability reduced mechanism for practical application.