Applications Of Large Eddy Simulation To Fuel Sprays And Porous Media Engines

An important precondition for accurate simulating the combustion process as well as working and emissions characteristics of the internal combustion (IC) engine is correctly predicting the fuel-air mixture formation and spray structure in the engine. As an advanced method for turbulence modeling, the large eddy simulation (LES) has been applied to various disciplinary and engineering fields recently. However, little has been reported on the application of LES to the in-cylinder flow and combustion of IC engines, especially rarely in China. As a preliminary study on the LES approach, in this paper a LES model is applied to the numerical computation of fuel sprays in a constant volume vessel and a diesel engine. Meanwhile, computations and analysis of the turbulent flow and combustion process in porous media (PM) engines are performed with a multi-dimensional model. Research work completed in this thesis is as the following.(1) The LES approach was implemented in the general engine CFD software KIVA-3V, which was employed for numerical computation of in-cylinder flows and fuel sprays in a constant volume vessel and in a Caterpillar 3400 series diesel engine. Computational results are compared with those obtained by a RANS (RNG k-ε) model as well as with experimental data. The influence of injection pressure was researched on relevant parameters, i.e. the spray penetration, the droplet SMD, the velocity and turbulent kinetic energy of the gas phase and the fuel evaporation rate. Computational results show that three dimensional, complex and stochastic turbulent eddy structures in the spray field can be captured by using the LES method, and that higher injection pressure can enhance prominently the spray penetration, reduce droplet sizes and accelerate fuel evaporation.(2) Sub-grid turbulence model is a key factor affecting LES computation results. Besides the one equation subgrid model (K-equation model), the applicability of three common algebraic subgrid models (Smagorinsky model, dynamic Smagorinsky model and WALE model) to fuel spray in IC engines were analyzed. Three different experiments on the evolution process of fuel sprays in constant volume vessels were used to validate the numerical simulations. The results show that the predicted evolution of the liquid and vapor phases in the spray by the three sub-grid models are generally in agreement with the experiment data, and the predicted penetrations are close to the measurement. (3) Based on the K-equation sub-grid turbulent kinetic energy model, the LES approach was employed for numerical computation of fuel sprays and combustion in a Caterpillar 3400 series diesel engine. Computational results are compared with those obtained by a RANS (RNG k-ε) model as well as with experimental data. The sensitivity of the LES results to mesh resolution is also discussed. The results show that LES generally provides flow and spray characteristics in better agreement with experimental data than RANS; and that small-scale random vortical structures of the in-cylinder turbulent spray field can be captured by LES. Furthermore, the penetrations of fuel droplets and vapors calculated by LES are larger than the RANS result, and the sub-grid turbulent kinetic energy and sub-grid turbulent viscosity provided by the LES model are evidently less than those calculated by the RANS model. Furthermore, it is found that the initial swirl significantly affects the spray penetration and the distribution of fuel vapor within the combustion chamber.(4) The LES approach and different breakup models were used to analyze sub-grid turbulent kinetic energy, especially the source term induced by the fuel spray, on the droplet movement and spray characteristics. Furthermore, the effects of the subgrid kinetic energy on the turbulent dispersion of droplets as well as on the fuel spray are examined. Computational results show that the KH-RT breakup model is superior to the MTAB model, and both the subgrid turbulent energy source term induced by the spray and the turbulent dispersion of the droplets have important effects on the fuel spray, resulting in decreased spray penetration. The effect of the turbulent dispersion is more prominent.(5) Since the porous media (PM) combustion technology is a promising and advanced combustion technique, in this thesis, a preliminary exploration of the combustion mechanism and working process in PM engine was carried out for different configurations and work parameters by a multi-dimensional simulation method.The characteristics of combustion and emission of the regenerative PM engine proposed by Ferrenberg were computed and analyzed using two dimension models under different equivalence ratios and porosities. Results show that the regenerative engine has advantages in both combustion efficiency and pollution emissions over conventional engines, and that using lower equivalence ratios can reduce emissions significantly, while the effect of the porosity is dependent on the equivalence ratio used. The regenerative PM engine is an effective alternative for realizing lean homogeneous combustion.A two-dimensional model for combustion and emission characteristics in the PM engine with a PM insert mounted onto the piston head is presented. Comparisons with the traditional engine without the PM insert were conducted. Temperature evolution of the PM and its effects are discussed with emphasis. Relevant influencing factors on combustion and emission of the PM engine, such as porosity, the initial PM temperature, engine speed and equivalence ratio, are analyzed.A preliminary LES Analysis of the the combustion characteristics of the PM engine was conducted. First, the gas fuel injection process in a constant volume vessel was computed by a PM model taking into account the random structure of porous media, and compared with that in the same vessel but without PM. It is demonstrated that the small-scale structure of turbulent eddies is enhanced and the fuel distribution is modified due to the presence of the porous media. Compared to the RANS model, the flow field due to gas injection obtained by LES is more irregular. Finally, a preliminary calculation and analysis of the combustion process in two PM engines with different configurations was performed by using the LES model.