Numerical Study On The Fuel Spray Combustion Based On Large-Eddy Simulation And Detailed Reaction Mechanism

During the normal operation of diesel engine,complex multiscale turbulent structures exist in the cylinder and can have strong interactions with fuel spray and combustion processes.Thus,understanding the fundamental mechanisms of in-cylinder turbulence,spray,and combustion and their interactions is crucial for improving the overall performance and reducing the pullutant emissions of diesel engine.The present dissertation is concerned with the Large-Eddy Simulation(LES)of diesel spray and combustion process.Research was first conducted to obtain methods for evaluating the interactions between turbulence and liquid fuel spray at subgrid scale in the framework of Very large-Eddy Simulation(VLES).By further implementing a detailed combustion reaction chemistry,multidimensional simulation of diesel spray and combustion with high level of overall accuracy can be realized.In this way,the reliability of the multidimensional engine simulation can be significantly improved,and it can play a more important role in improving the in-cylinder combustion of diesel engine.Based on the KIVA program for spray and combustion simulation,LES models were first implemented into the program to replace the existing Reynolds Average Navier Stokes(RANS)models.On this basis,the theoretical research work of the present dissertation was carried out in the following two aspects.Firstly,in the theoretical study of the interaction between turbulence and fuel spray,impact of the high-speed motion of fuel droplets on the gas-phase turbulence intensity at subgrid scale was first evaluated.Mathematical formula of the impact was derived,resulting in an additional source term for the transport equation of the subgrid turbulence kinetic energy.The model was used to simulate the non-evaporative spray process in a constant-volume combustion chamber.Compared to existing simple LES models and RANS models,improved predictions of spray shape and penetration length were found.The grid density requirement of the present LES was also found to be significantly reduced.On the other hand,considering the complex composition of petroleum fuels,a multicomponent vaporization model based on continuous thermodynamics was developed by using a probability density function to describe the distribution of fuel components in the composition.Filtered LES transport equations of the first two moments of the distribution was also derived.By solving the filtered transport equations,the impact of turbulence on the drop vaporization process was considered.The multicomponent model was integrated with the LES to replace the original single-component model and used in the simulation of diesel spray combustion process under engine operating conditions.Compared to the RANS with single-component drop vaporization model,enhanced simulation performance was obtained and results were in good agreement with the corresponding experimental data.Secondly,the present dissertation aims at further improving the overall performance of turbulent spray combustion simulation of diesel engine,by constructing more detailed combustion reaction chemistry for a more accurate combustion simulation.From the complex biodiesel reaction mechanism by the Lawrence National Laboratory(including 3299 components and 10806 elementary reactions),a skeletal chemical mechanism with 160 components and 601 elementary reactions was obtained using the Path Flux Analysis approach.By calculating reactions in a closed homogeneous reactor,a well stirred reactor and a one-dimensional laminar premixed flame under different conditions,the accuracy of the present skeletal reaction mechanism was verified,and it was found to be capable of reproducing the combustion characteristics of the original complex mechanism.The present skeletal mechanism was implemented into the KIVA program with CHEMKIN reaction solver and was used in the three-dimensional simulation of biodiesel spray combustion process under diesel engine operating conditions.By comparing the simulation results with the corresponding experimental data,it was found that improved overall accuracy of the simulation was achieved by using the present skeletal mechanism.