| Ethanol is a common liquid hydrocarbon fuel.In recent years,advances in the production of second-generation biofuels from lignocellulosic crops have made ethanol more promising as a fuel.At the same time,the construction of an accurate and efficient combustion reaction kinetic model using ethanol as fuel is the basis for the coupling calculation of ethanol combustion reaction kinetics model and computational fluid dynamics(CFD)software on a micro scale.For the combustion process,it is of great significance to design and develop an efficient and environmentally friendly ethanol micro power combustion device.Due to the relatively large scale of the detailed mechanism of ethanol,it will lead to too long calculation time and calculation stiffness problems,and it is difficult to directly apply to the coupled calculation of complex combustion.For this reason,this paper simplified and optimized the detailed combustion reaction kinetic model of ethanol,and analyzed the influence law of ethanol’s micro-scale combustion characteristics based on the coupling calculation of the optimized and improved mechanism and CFD software.The main work is as follows:(1)The simplification effects of different simplification methods on the detailed mechanism of ethanol are compared and analyzed.Taking the detailed mechanism of ethanol(57 species,383 elementary reactions)developed by Lawrence Livermore National Laboratory(LLNL)as the research object,at 18 different initial temperatures,pressures and equivalent ratios under the conditions,use Directed Relation Graph with Error Propagation(DRGEP),Sensitivity Analysis after Directed Relation Graph with Error Propagation(DRGEPSA)and Full Species Sensitivity Analysis(FSSA)three methods are simplified,and the simplified mechanism and detailed mechanism are compared and analyzed for the predicted value of ignition delay time and laminar flame velocity.The results show that the simplified mechanism scales obtained by the three methods are the DRGEP mechanism(44 species,286 elementary reactions),the DRGEPSA mechanism(33 species,178 elementary reactions),and the FSSA mechanism(26 species,130 elementary reactions);compared with the detailed mechanism,the maximum relative errors of the ignition delay predicted by the three simplified mechanisms are 7.48%,9.81%,and 16.09%,respectively,and the maximum relative errors of the predicted laminar flame velocity are 15.08%,17.73% and 33.61%,respectively.The simplified mechanism obtained by the DRGEPSA method has better comprehensive prediction performance.This is mainly due to the fact that the DRGEPSA mechanism is smaller than the DRGEP mechanism and the prediction performance is better when the laminar flame velocity mixture concentration is dilute(φ<1.0),and the comprehensive effect is the best.(2)Based on the idea of hierarchical optimization,the simplification mechanism of ethanol is optimized by the pre-factor disturbance method with different disturbance amplitudes.Use sensitivity analysis and Rate of Production(ROP)analysis to determine the key element response,use the pre-factor perturbation method to give the pre-exponential factor of the key element reaction rate constant a primary perturbation of V1 to V6;combine the results of primary disturbance,sensitivity analysis and ROP analysis,we conducted targeted secondary disturbances,and finally sorted out 13 adjusted pre-index factor changes and integrated them into the final optimization mechanism.The detailed mechanism,simplified mechanism and optimized mechanism are compared and analyzed for the prediction values of ignition delay time,laminar flame velocity and main species concentration.The results show that the optimization results under V4 disturbance have initially achieved the expected results.The prediction errors of the three are12.07%,6.44%,and 7.92%～17.18% respectively;the prediction errors of the three detailed mechanisms are respectively 11.07%～13.57%,2.81%～5.06%,4.70%～15.25%;simplified mechanisms are between 12.74%～15.16%,5.27%～10.18%,and 4.71%～17.62%;the final optimization mechanisms are between 11.57%～13.65%,3.31% ～5.46% and 4.71%～16.07%.The optimization mechanism is better than the simplified mechanism and is closer to the prediction effect of the detailed mechanism.This is mainly because the optimization mechanism improves the prediction ability by modifying the key element reaction rate constant.(3)A numerical simulation of the combustion characteristics of ethanol on a small scale was carried out.Using a micro-scale diffusion flame combustion test device,the flame shape and flame temperature under different ethanol flow rates/ air flow rates/ inner diameter of the quartz tube of the burner were measured.The optimized ethanol mechanism was calculated by coupling with Fluent software,and a calculation model of ethanol micro-scale combustion was established.The experimental results and the calculation results of the model were compared and analyzed;the influence of the inner diameter of the quartz tube of different burners on the main species concentration,the reaction rate of key elements,the temperature field and the velocity field and other small-scale combustion characteristics of ethanol were discussed.The results show that the maximum relative error of flame height and flame width is less than 10%,and the maximum relative error of flame temperature is less than 5%,which verifies the accuracy of the calculation model;as the distance between the central axis and the nozzle increases,the width of ethanol continues to increase,and the concentration Gradually decrease;with the increase of pipe diameter on the central axis,the maximum flame temperature is reduced by 12%,the peak concentration of the product is reduced by 8%～18%,and the chemical reaction rate is reduced by 27%～56%;with the increase of pipe diameter in x-axis,the peak value of the main intermediate products increased by 30%～50%;with the increase of pipe diameter,the velocity of fuel flow and air flow caused by heat convection increased by about 50%,which was mainly due to the smaller the inner diameter.The more the micro-scale effect is manifested,the better the performance of the combustion characteristics of ethanol at the micro-scale. |
PDF Full Text Request