Investigation On The Energy Separation Mechanism And Optimization Criteria Of Ranque-Hilsch Vortex Tube With Analyses On The Flow Strcuture

Ranque-Hilsch vortex tube has important application prospect in refrigeration,natural gas and aerospace industries because of its unique energy separation phenomenon.Due to the complex fluid motion and heat and mass transfer processes inside the vortex tube,the energy separation mechanism and optimization design of it have not been solved,which has become an important problem restricting its development.Begin with the analyses on the flow field structure and the oscillation characteristics of the unsteady flow field in vortex tube,the thesis adopts numerical simulations and visualization experiments to study the influence on the mass transfer and energy transfer process with large-scale vortex structure in the strong swirling flow,the flow separation mechanism and energy separation mechanism of vortex tube are revealed,and a new optimization design method is put forward,the specific research contents and results are listed as follows:Firstly,the difference between the flow structure of the secondary circulation flow calculated by the steady solver and the precessing vortex core calculated by the unsteady solver in vortex tube is analyzed,the generation mechanism and flow characteristics of the two different flow structures are clarified,and it is found that the strong swirling flow field is controlled by the unsteady characteristics.Through solving the flow structure with unsteady numerical simulation,the vortex breakdown phenomenon in the strong swirl flow field of the vortex tube is found.According to the vortex breakdown theory,the vortex core scale is coupled with the axial pressure gradient,and the flow structure with the reverse flow boundary as a significant feature was obtained under different cold mass fractions,and it is found that the shape of the reverse flow boundary is closely related to the energy separation performance.By analyzing the frequency spectrum of the velocity time series of the monitoring points,the flow structure presents periodic oscillation characteristics is revealed.Also it is found that the precessing vortex core structure had a significant influence on its surrounding flow field and made it exhibit the oscillation characteristic at the same frequency.In addition,changing the cold mass fraction has little impact on the particle oscillation in the flow field,while the performance improvement brings by increasing inlet pressure can be attributed to the increase of the oscillation characteristic frequency in the flow field.Secondly,the non-contact velocity measuring devices of PIV and LDV are adopted to conduct the transient and time-averaged tests on the flow field in vortex tube,and the axial and radial velocity field distributions under different main tube lengths,cold mass fractions and inlet pressures are obtained.The vortex breakdown phenomenon and the large-scale vortex structure of the precessing vortex core located at the reverse flow boundary are captured.It is revealed that the reverse flow boundary presents two opposite flow patterns with increasing cold mass fraction in the over-short and over-long tube,respectively,and the variations of reverse flow boundary correspond to two different deterioration mechanisms of the energy separation performance.It is found that the axial stagnation point is not a basic flow structure as well,but only occurs in during suboptimal performance.Besides,there are similarities of the flow structures under different inlet pressures are also discovered,and changing the inlet pressure has little influence on the reverse flow boundary distribution.Besides,the accuracy of the numerical simulation is verified by the velocity fields obtained by the LDV measurement.Again,through studying the influence on the energy transfer between the inner and the outer layers with the oscillation characteristics of the flow field,it is revealed that the large-scale vortex structure of precessing vortex core determined the flow separation and energy separation processes in the vortex tube.Based on the energy transfer process under inverse temperature gradient through the infinitesimal particle periodic oscillations,a heat pump-like energy separation model is established.It is pointed out that the work for the heat pump cycle is offered by the precessing vortex core concussion,and the reverse flow boundary shape and the flow field oscillation characteristic frequency determines energy separation performance.Finally,the main tube optimization criteria of the vortex tube are proposed through combining the external conditions,large-scale flow structures,flow separation and energy separation processes.Based on the analysis of the vortex structure,the Navier-Stokes equation is solved and the optimal design procedure of the main tube key parameters is given.