Study On Combustion Process And Flame Stability Of Low Calorific Value Gases Fueled SI Engine

With the increasing prominence of energy and environment problems recently, the gas fuel has become the research focus around the world because of its cleanliness and sustainability. Therefore, the research and development of the gas engine is necessary. When the gas engine is fueled with the low calorific value gas (LCV gas), the combustion instability phenomenon become more and more serious. Therefore, the systematic study on the micro-evolution of the flame structure during the ignition and combustion process of a single engine cycle is necessary for the stability of the gas engine. Through the modeling of the vortex-flame interactions and flame intrinsic instability during the in-cylinder gas fuel’s premixed turbulent combustion, the unstable propagation mechanism of the flame can be elucidated and the theoretical basis for the development of the clean and efficient engine combustion system can be provided, which has high academic and engineering application value.Darrieus-Landau instability (D-L instability) phenomenon existed in turbulence disturbed plane flame propagation is studied. The Michelson-Sivashinsky equation forced with external turbulence (T-M-S equation) is solved. The fitting equation for the velocity increment of the plane flame propagation is formulated. Instability function is introduced into the chemical reaction source term of the species equation. The PaSR-LES combustion model with D-L instability is established. The relationships among the characteristic parameters such as the turbulence fractal dimension, the eddy turn over time, and the viscous cutoff scale are investigated. An improved expression for the turbulent micro-mixing time scale is proposed. Based on the arc and flame kernel tracking-Euler (AKTIM-Euler) method, the large eddy simulation model of the spark ignition is set up. The ignition energy distribution of the weight of the combustion process variable values is described. The three-dimensional numerical simulation platform for the low calorific value (LCV) gas fueled engine combustion is established. In order to deal with the engine dynamic mesh, a solution method is proposed to couple polyhedron vertex movement algorithm and Mesquite algorithm. This method can avoid the large skewness and the negative volume of the mesh cells. The experimental research on the constant volume vessel and the engine fueled with the LCV gas is conduced. The turbulent combustion model and the numerical simulation platform for the engine combustion in this paper are validated.The change rule of the plane flame front position and shape with respect to different turbulent intensity and dimensionless Makstein length is studied for the weak turbulent flame. Furthermore, the factors which influence the flame propagation increment are considered. Through the large eddy simulation on the working process of LCV gas fueled engine, the evolution of the large-scale coherent structures during the intake and compression stages is investigated. In addition, the generation, development, and dissipation of the coherent structure under different engine speed are compared. The development process of the flame kernel formation and the characteristic regime area of the flame kernel-vortex pair interactions under different engine speed are analyzed. The effect of the vortex-pair movement on the flame surface morphology is researched. The in-cylinder evolution of the turbulent flame surface with the D-L instability effect is studied. The effect of baroclinic torque on increasing the flame wrinkles is analyzed. It is shown from the simulation results that:1. The higher strength vortex mostly appears in the flame free propagation area. The vortex on the flame front is helpful to the increase of flame wrinkles and turbulent flame speed. The vortex pair produces entrainment and stretching effects on the flame surface, resulting in the formation of the wrinkles.2. With the increase of the volume fraction of inert species in LCV gases, the flame wrinkles decrease. A certain amount of hydrogen addition in LCV gases improves the flame propagation speed, promotes the flame-vortex pair interactions, increases the flame wrinkles.3. D-L instability generates the baroclinic torque which incrases the flame wrinkles. The flame strain rate which has the same symbol with the flame surface curvature is influenced by the D-L instability as the flame front passes through the vortex pairs. The vortex in the burnt area has the opposite symbols with the one in the unburnt area.