Study On Heat Transfer Enhancement Characteristics Of Backflow Vortex In Shear Layer Of Self-excited Oscillating Pulse Jet

The use of vortex to heat transfer has become one of the hot spots in the field of heat exchange.However,there are some problems in the existing vortex heat transfer technology,such as extra energy consumption and instability of vortex.It can produce a continuous and efficient backflow vortex in the heat flow channel without external energy input due to the special structure chamber with self-excited oscillation,so it has a high research value.In this paper,the advantages of vortex heat transfer technology and self-excited oscillation effect were combined.In order to solve the bottleneck problem of heat exchange research and application research by using the backflow vortex generated by special structure of self-excited oscillation,the self-excited oscillation instantaneous vorticity hot runner backflow vortex disturbance effect was proposed to realize the scientific explanation of the heat transfer mechanism and the design method of the hot runner structure.The main work and valuable conclusions were as follows:（1）Heat transfer mechanism of self-excited oscillation pulsation effect was proposed.Based on the analysis of the research status and influencing factors of vortex heat transfer,the generation and effective feedback conditions of pulsating flow in self-excited oscillating hot runner were studied.Because the fluid system is very similar to the circuit system,the circuit system was used to analyze the natural frequency of the self-excited oscillation chamber.（2）The influence of boundary layer and pulsating jet on heat transfer in hot runner was studied.The heat transfer in the boundary layer mainly depends on the heat conduction.Pulsating jet can accelerate the mixing of fluids,destroy the boundary layer,and improve the heat transfer efficiency.Under the condition of increasing heat transfer and pressure drop at the same time,the comprehensive evaluation factor PI was proposed.（3）Based on the formation mechanism of the backflow vortex near the wall of the self-excited oscillation hot runner,the physical model and turbulence model of the self-excited oscillation chamber are established.Large eddy simulation was used to study the evolution of self-excited oscillation vortex,wall shear force and Nu number.The backflow vortex near the wall of the hot runner in the self-excited oscillation chamber presents"wavy"flow.The backflow vortex has a strong disturbance effect on the fluid near the wall of the hot runner,which can accelerate the heat exchange between the fluid and near the wall.When d₂/d₁=1.5,the maximum PI is 1.24,and the heat transfer efficiency can be increased by 24%.（4）The influence of different structural parameters of self-excited oscillation was on the performance of backflow vortex and pulsation in hot runner.Under certain conditions,backflow vortices can be formed in the hot runner of the self-excited oscillation chamber.Large eddy simulation was used to study the vorticity distribution and velocity vector field distribution characteristics of the hot runner under different structural parameters.At the same time,the comprehensive evaluation factor PI was used to evaluate the comprehensive performance of different hot runner structures.When the structure parameters were L/d₁=3.6-3.8,D/d₁=7.2-8,α₂=120-140°,the pulsation performance,backflow vortex disturbance effect and heat transfer performance were the best.The maximum comprehensive evaluation factor was 1.17,and the heat transfer efficiency was increased by 17%.