科创中国

Supercavitation in the diesel nozzle increases the instability of droplets in part due to the two-phase mixture,while the effect of cavitation bubbles on the instability of drops is still unclear.In order to investigate the breakup of cavitation bubbles within the diesel droplet,a new mathematical model describing the disturbance growth rate of the diesel bubble instability is developed.The new mathematical model is applied to predict the effects of fluids viscosity on the stability of cavitation bubbles.The predicted values reveal that the comprehensive effect of fluids viscosity makes cavitation bubbles more stable.Compared with the viscosities of air and cavitation bubble,the diesel droplet's viscosity plays a dominant role on the stability of cavitation bubbles.Furthermore,based on the modified bubble breakup criterion,the effects of bubble growth speed,sound speed,droplet viscosity,droplet density,and bubble-droplet radius ratio on the breakup time and the breakup radius of cavitation bubbles are studied respectively.It is found that a bubble with large bubble-droplet radius ratio has the initial condition for breaking easily.For a given bubble-droplet radius ratio(0.2),as the bubble growth speed increases(from 2 m/s to 60 m/s),the bubble breakup time decreases(from 3.59μs to 0.17μs)rapidly.Both the greater diesel droplet viscosity and the greater diesel droplet density result in the increase of the breakup time.With increasing initial bubble-droplet radius ratio(from 0.2 to 0.8),the bubble breakup radius decreases(from 8.86μm to 6.23μm).There is a limited breakup radius for a bubble with a certain initial bubble-droplet radius ratio.The mathematical model and the modified bubble breakup criterion are helpful to improve the study on the breakup mechanism of the secondary diesel droplet under the condition of supercavitation.

The T-junction model of engine exhaust manifolds significantly influences the simulation precision of the pressure wave and mass flow rate in the intake and exhaust manifolds of diesel engines. Current studies have focused on constant pressure models, constant static pressure models and pressure loss models. However, low model precision is a common disadvantage when simulating engine exhaust manifolds, particularly for turbocharged systems. To study the performance of junction flow, a cold wind tunnel experiment with high velocities at the junction of a diesel exhaust manifold is performed, and the variation in the pressure loss in the T-junction under different flow conditions is obtained. Despite the trend of the calculated total pressure loss coefficient, which is obtained by using the original pressure loss model and is the same as that obtained from the experimental results, large differences exist between the calculated and experimental values. Furthermore, the deviation becomes larger as the flow velocity increases. By improving the Vazsonyi formula considering the flow velocity and introducing the distribution function, a modified pressure loss model is established, which is suitable for a higher velocity range. Then, the new model is adopted to solve one-dimensional, unsteady flow in a D6114 turbocharged diesel engine. The calculated values are compared with the measured data, and the result shows that the simulation accuracy of the pressure wave before the turbine is improved by 4.3% with the modified pressure loss model because gas compressibility is considered when the flow velocities are high. The research results provide valuable information for further junction flow research, particularly the correction of the boundary condition in one-dimensional simulation models.