| Pressure swirl nozzle is widely used in horizontal tube falling film evaporator of low temperature multi-effect desalination unit.The spray density and uniformity of the nozzle will directly affect the heat transfer effect outside the tube bundle.Improving the atomization effect of the nozzle can improve the working efficiency of the system.At present,the research on nozzles at home and abroad focuses on the size and shape of droplet atomization,lacking in-depth study on the overall spraying effect of nozzles,and most of them are suitable for the jet problem of structurally symmetrical nozzles under high pressure conditions.The research on the one-sided tangential inlet pressure nozzles used in desalination units under low pressure conditions is still in its infancy.In this paper,FLUENT software is used to calculate and analyze the internal flow field and external atomization flow field of pressure swirl nozzle.The main research contents include velocity,pressure,turbulent flow energy,air column,inlet and outlet pressure drop,effective spray density and circumferential spray density inhomogeneity in external flow field of pressure swirl nozzle.Finally,the optimization direction of nozzle structure is predicted.According to the actual size of the nozzle,a three-dimensional geometric model is established by PROE.The whole flow field is divided into structured grids by ICEM.The flow characteristics of the nozzle are described by VOF multiphase flow model,surface tension CSF model and Realizable k-e turbulence model.At the same inlet flow rate,the numerical results and experimental results are verified qualitatively and quantitatively,which proves the reliability of the model.By changing the inlet velocity conditions,it is found that the pressure difference is the cause of the formation of the air column,but it does not affect the position distribution of the gas-liquid two-phase in the swirl chamber.The velocity of liquid phase in swirl chamber is much higher than that of gas phase.Water flow plays a dominant role in the swirl motion of gas-liquid phase.The distribution of air column is affected by the gradient distribution of liquid phase velocity.The inlet velocity affects the size and stability of air column,and the instability of air column in turn aggravates the turbulence of liquid phase flow field.At a certain spray height,increasing the inlet velocity can increase the effective spray density of the external flow field,but it can not improve the circumferential uniformity of the spray density.The influence of nozzle geometric parameters such as inlet diameter,swirl chamber diameter and swirl chamber convergence angle on spray characteristics was further studied.Under the same inlet velocity and spray height,the flow energy consumption increases with the increase of nozzle inlet diameter and the severe disturbance of liquid on the air column.With the increase of circumferential effective spraying density,the uniformity first increases and then decreases.Increasing the diameter of swirl chamber provides conditions for the full development of liquid.The impact of swirl chamber on air column is buffer space,which improves the stability of air column.The increase of wall-fluid contact resistance leads to the increase of energy consumption,while the circumferential effective spraying density decreases gradually,the uniformity is improved.Convergence angle can play a transitional role,reduce the friction resistance between the wall and the fluid,restrain the tangential velocity,increase the axial velocity can accelerate the downward movement of water,the effective spray area radius gradually decreases,but improve the uniformity of circumferential spray density.There is a swirl chamber diameter,entrance diameter and convergence angle of the swirl chamber,which makes the distribution of effective spray density of the nozzle the best. |
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