| "Energy separation" refers to the spontaneous generation of a part of the high temperature region and the other part of the low temperature region by a fluid without external work or heat exchange.The vortex tube is an energy separation device.The gas is compressed and expanded in the nozzle,and enters the vortex tube to rotate at high speed,then separates into a part of high temperature region and the other part of low temperature region.Energy separation can also occur in jet,vortex,and blunt body flow,but it is not the same as the energy separation mechanism in vortex tube.At present,domestic researches of energy separation mainly focus on vortex tube and the vortex board,and energy separation in the fluid domain is rarely involved.However,abroad have done a lot of experimental and theoretical research in this field.So far,the energy separation mechanism in the fluid field has not been explained in exact theory.Based on the successful foreign experience of experimental and numerical studies and the dimensionless turbulence control equation,the formation mechanism of free jet energy separation is explored on the basis of making full use of previous theories and experimental methods.This paper introduces the free air jet and simulate the energy separation of a free jet with subsonic inlet velocity,considering the velocity(Reynolds number),pressure,acoustic excitation factors of energy separation,and makes a series of conclusions.The specific research contents are as follows:(1)Through FLUENT software,the UDF function is used to numerically simulate the free jet flow field with Reynolds number of 410000 at the entrance of free jet flow,and the distribution of energy separation factor,total pressure distribution,and vorticity distribution in the free jet flow field were given.The distribution of energy separation factor at different axial locations has been analyzed.The numerical simulation results clearly show the process of energy separation generation,increase,decrease and disappear of free jet flow,and prove the existence of energy separation of free jet flow under subsonic conditions.Excluding viscous dissipation and heat conduction,the energy separation of free jets is closely related to the pressure fluctuation associated with the motion of vortices.(2)The vorticity distribution and total pressure distribution of free jet flow field under different acoustic excitation frequencies are numerically simulated,and the variation curve of energy separation factor in flow field with axial direction is given.It is found that acoustic excitation can significantly change the structure of free jet vortex,enhance the process of vortex pairing and fusion,enhance the pressure fluctuation of free jet flow field,and significantly enhance the energy separation of free jet flow.(3)The energy separation phenomenon of free jet flow field under different Reynolds number is simulated numerically with viscosity and subsonic range,and the curve of energy separation factor with different axial positions is given.It is concluded that in a certain range(Re:96,000 ~ 116,000),increasing the Reynolds number can enhance the energy separation effect in the free jet flow field.When the Reynolds number exceeds 116,000,the inlet temperature of free jet increases with the increase of the Reynolds number,fluid viscosity increases,energy dissipation and heat loss increase,and energy separation effect decreases.(4)Under the condition of subsonic range and non-viscous flow,the energy separation phenomenon of free jet flow field under different Reynolds number is simulated numerically,and the curve of energy separation factor with different axial positions is given.The free jet energy separation caused by pressure fluctuation is stronger than the free jet energy separation caused by viscous action.With the increase of Reynolds number,the pressure fluctuation generated by the vortex motion increases and the energy separation factor increases.When the inlet velocity of free jet flow is 0.9 Ma and the number of Reynolds number is 410,000,the proportion of pressure fluctuation can reach 70.1%.(5)The velocity frequency density spectra and total temperature frequency density spectra of the free jet flow field with different inlet velocities under non-viscous conditions are given by means of spectral analysis.The energy separation mechanism of free jet flow is further explained. |
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