Study On Mechanism Reduction Of Coal Based Diesel Surrogate Fuel

The current general energy feature in china is "rich in coal,lack of oil and own a little gas." The usage of coal-based alternative fuels can effectively alleviate China’s energy shortage and imbalances,help our country to change its direction of energy development and improve energy efficiency,it is of great significance for alleviating China’s shortage of oil resources,ensure national energy security and achieve economic health and sustainable development.F-T diesel fuel synthesized by indirect coal liquefaction technology is an important coal-based alternative fuel with high cetane number,high combustion efficiency and clean emissions.The experimental characteristics and production methods of coal-based diesel have been extensively studied.However,there are few researches on the kinetic model of coal-based diesel.The main reason is that the composition of coal-based diesel fuel is very complex and contains many heavy hydrocarbons with complex structure.Therefore,the chemical reaction mechanism is very large.When its detailed mechanism is applied to the numerical simulation of flow-combustion coupling,it will produce great rigidity problems and often require huge computational costs.Therefore,it is essential to develop the reduced mechanism of coal-based diesel with compact structure,reliable performance and when it concerns to the multi-dimensional computational fluid dynamics in the engine simulation.In this paper,n-dodecane and iso-cetane were selected as the surrogate fuel components for coal-based diesel according to their physical and chemical parameters such as C/H mass ratio,cetane number(CN)and low heat value(LHV),then to develop the reduced mechanism for the two componets respectively.In the process of reduction,the shock tube,the jet sttired reactor model and the PREMIX were chosen as reactor models to research the combustion mechanism.The ignition delay time,the change of mole fraction of important intermediate species and the laminar flow flame speed were chosen as the objects of investigation,while the prediction of ignition delay time of the mechanisms was focused.(1)In this paper,DRGEP,ROP and SA were used to simplify the reaction mechanism of iso-cetane and n-dodecane.Firstly,DRGEP is used to eliminate the redundant reaction from the coal-based alternative fuel synthesis mechanism,and the effect of eliminating redundant operation on the ignition delay time was mainly investigated.Then,the main components of the reaction process were analyzed by ROP and the combustion oxidation reaction paths were determined,delete the secondary elementary reactions to further reduce the number of mechanisms’ steps;sensitivity analysis of the ignition delay time and premixed flame propagation speed after the the two-step reduced operation which talked above,using SA method and custom threshold to delete elementary reations of the smaller sensitivity coefficient.The actual error of the premixed flame velocity was investigated,and the partial sensitivity of the elementary reaction was supplemented to correct the error of the mechanism.The changes of the concentration of the important intermediate species were compared with the original mechanism and the experimental data,to analyze the damage of the correlation between the delete operations to each elementary reaction,and then to correct the error according to the correlation.(2)Particle swarm optimization(PSO)algorithm was used to optimize the reduced mechanism obtained by DRGEP,ROP or SA methods to reduce the computational error caused by the reduced method and to compensate the accuracy loss of iso-cetane.(3)The n-dodecane and iso-cetane reduced mechanism obtained by the methods above were combined as the coal-based diesel model fuel mechanism,and the mechanisms of the two components were used in the shock tube,jet-stirred reactor and PREMIX models to calculate,all the results are reasonable.Then according to the physical and chemical parameters,the reasonable ratio between the two components in the fuel mechanism was used to calculate the CFD three-dimensional CVCC model in Converge software.The results were quite well and the numerical stiffness problem has got solved and greatly saved the computation consumption.