| As the regulations of emissions and fuel consumption are becoming more strigent,internal combustion engines continue self-improvement,moving towards more clean,efficient and energy-saving.The fuel supply and injection system,which injection pressure could reach about 250 MPa or even higher is required to fulfill the increasingly strict emission regulations.Consequently,the common rail technique has become the mainstream in diesel engines because of its unique technical advantages,where the injector,as the final actuator connecting upstream high pressure common rail system and downstream fuel injection,is critical.The needle valve couple,being composed of needle valve and needle valve body,not only bears the role of fuel lubrication,sealing and circulation,but also acts as the direct execution components of fuel injection,determining the fuel injection and spray patterns.Hence,the subsequent combustion process,power performance and emissions will be affected.As the injection pressure going higher,the fuel flowing within the nozzle(is consist of needle valve pair,SAC,et al.)is not only accompanied by the instantaneous turbulence with high Reynolds number,but also the cavitation caused by vena-contraction and vortex flow.As a result,the high pressure fuel within the nozzle becomes the complex vapour-liquid two-phase flow,affects the atomization of the spray jet at the nozzle outlet.Based on the above,the impact of needle transient moving in terms of the regular motion(non-eccentric)and irregular motion(eccentric)on the internal flow and injection characteristics of each nozzle hole were the main focus of the paper.Besides,a vapour-liquid mixture model considering the compressibility of the fuel was developed to predict the cavitation within the nozzle.Last but not the least,the influence factors of needle valve elastic deformation and eccentric motion was explored,which provides a theoretical basis for optimizing the design and manufacture of the injector.The main research contents of the thesis are as follows:(1)A measurement system for transient injection characteristics of each nozzle hole was designed and built based on the momentum conservation and Bernoulli's theorem.The measurement system could acquire the instantaneous injection rates of each nozzle holes in the multi-hole injector simultaneously,and realize the cyclic acquisition of multiple injection durations in the meantime.Finally,the accuracy and reliability of the test system are validated.(2)The code solving the compressible vapour-liquid two-phase flow in the nozzle was developed and implemented based on the rho Central Foam in Open FOAM?,an open source C++platform.The solver utilizes the homogeneous equilibrium model to solve the multiphase flow within the nozzle,while the phase change is predicted by the barotropic equation of state.Since the standard RNG k-?turbulence model is not accurate enough for solving the compressible flow,especially for capturing the vortex shedding,a density function related to cavitation state is introduced to replace the constant density,and thus the turbulence model is modified.In addition,due to the strong hyperbolic mathematical characteristics of Euler equation for compressible flow,there is the Riemann problem which usually has discontinuous solutions.In this thesis,the approximate Riemann solver is used to deal with the numerical flux of the grid to capture the shock wave in the compressible flow.The established CFD model was validated by the measured injection rates of each nozzle holes and used to analyze the influence of transient needle movement on the internal cavitating flow with a double-layered hole nozzle.The numerical results indicated that the model could capture the periodic vortex shedding and turbulent coherent structure within the nozzle accurately,and the cavitation development in the upper layered hole is more acute than that of the lower layered hole.(3)The cavitation flow within the nozzle has a critical impact on the atomization of spray jet at the exit of the nozzle hole.The coupling model of the Euler-Euler two-fluid simulation method for the cavitation flow in the nozzle and the Lagrangian discrete droplet spray simulation method at the nozzle outlet of the multi-hole injector is established.This model indirectly coupled the two completely different numerical methods for solving continuous phase and discrete phase respectively,which could analyze the correlation between internal flow and fuel atomization effectively.The established CFD model was validated through the measurement system of the injection characteristics for each hole and the spray test bench based on the schlieren method.To be specific,the injection characteristics such as injection rates,cycle fuel injection quantities and et al.,were measured together with the macroscopic spray patterns in terms of spray penetrations of each orifices.Afterwards the results between the experiment and simulation were compared to validate the model.The results show the good consistency between the experimental and numerical results.In addition,the injection rates,cycle fuel injection quantities and spray tip penetration of the lower layer nozzle holes were larger than those of the upper layer nozzle holes.The acuteness of the upper layer nozzle holes contributed to the formation of a higher degree of cavitation development in them as compared to the less acute lower layer nozzle holes.(4)The mathematical model of elastic deformation and eccentric movement of the needle within the nozzle is established based on the theory of material mechanics,elastic mechanics and linear elasticity.The model successfully built the relationship between the deformation of the needle valve couple in the upstream and the eccentricity of the needle tip in the downstream.By means of the theory of flow in the annular clearance and fluid-structure interaction(FSI)numerical method,the impact factors of needle eccentric motion were analyzed.The study found that the eccentricity of needle valve changed linearly with the ratio of needle valve length to needle valve guide length.In addition,as the fuel pressure increases to 100MPa,especially more than 200MPa,the total eccentricity increases remarkably,and the changes rate of the eccentricity increases with the growth of fuel pressure.(5)Based on the mathematical model of the needle eccentric moving mentioned above,the calculated eccentric magnitude was used as a boundary condition to analyze the effects of needle eccentricity together with elastic deformation on the internal flow as well as injection characteristics through the three-dimensional CFD model.The numerical results showed that the eccentric motion of the needle valve is one of the most important reasons for the discrepancies in the flow and injection characteristics among each nozzle hole.When needle is oriented at one orifice,the injection rate and cycle fuel injection quantity are the largest among the same layer compared with other orifices.In addition,when needle orients at one nozzle hole,the cavitation is primary developed at the lower surface of the orifice,and gradually moved to the upper surface as the needle valve lifts. |
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