Numerical Study On The Separation Performance Of A Steam Water Separator Used In Nuclear Power Plant

Cyclone separator is widely used in material separation because of its small footprint and high efficiency.The steam water separator of high temperature gas cooled reactor(MHTR)used in Shidaowan nuclear power plant,drawing on the separation principle of cyclone separator,utilizes centrifugal force to separate liquid droplets from steam by using a pair of axisymmetric inclined tangential inlet tubes.Based on ANSYS Fluent 15.0 commercial software,this paper investigates the motion state of the two phase flow,the separation principle,and the separation efficiency of the MHTR steam water separator,providing theoretic support for its stable and high effective operation.Computational model is verified by the experimental results of a micro steam water cyclone separator.For the flow field of continuous phase,the pressure drop values calculated by RNG k-? and RSM turbulence model used second order accurate difference format show good agreement with the experimental results.The motion of discrete phase is calculated by Discrete Phase Model(DPM).Compared with the experimental results,the separation efficiency results calculated by two-way coupling with wall surface boundary condition of “wall film” or “Eulerian wall films” are more accurate than that of one-way coupling with wall surface boundary condition of “trap”,and show a more real changing trend with the change of inlet condition.By two-way coupling,the separation efficiency results calculated by RNG k-? turbulence model are closer to the experimental results than that of RSM turbulence model,in which “wall film” model behaves better in modeling the reentrainment of the liquid droplets.The continuous phase flow field of the MHTR water steam separator is calculated by RNG k-? turbulence model used second order accurate difference format,while the motion of discrete phase is calculated by DPM.Both of the inner forced vortex and the outer free vortex inside the flow field have a good symmetry.With inlet mass flow constant,the total pressure drop of outlet and static pressure drop of drain port increase rapidly with the increasing of inlet dryness.For one-way coupling,the boundary condition of wall surfaces is set as “trap”.Without turbulence dispersion: the lift tracks of droplets are all close to wall surface,resulting in that only the small droplets can escape from the outlet;when the inlet dryness is constant,the separation efficiency increases with the increasing of droplet diameter,and tends to be 100% when the droplet diameter is larger than 10 ?m;when the droplet diameter is constant,the separation efficiency increases with the increasing of inlet dryness.With turbulence dispersion: affecting by the fluctuating velocity of continuous phase,the lift tracks of droplets passing by the inlet divide into two regions,the near wall region and the central region;the residence time of droplets is longer,while larger droplets are able to escape from the outlet;the increasing trend of separation efficiency with the increasing of droplet diameter is weakened.For two-way coupling,the wall surface boundary condition is set as “reflect” with inelastic collision or “wall film”.Because droplet collision is taken into consideration in two-way coupling,the lower the dryness is,the higher the volume fraction of the droplets is,and hence,the droplets are more likely to collide.In this study,the droplets are more liable to coalesce to form larger droplets after colliding.With “reflect” wall surface boundary condition: the separation efficiency decreases with the increasing of inlet dryness;when the diameter size constant is larger than 50 ?m,a inlet dryness higher than 60% can insure that the outlet dryness is higher than 85%.With “wall film” wall surface boundary condition: most of the droplets escaping from the outlet are smaller than the smallest droplets of inlet due to the reentrainment and the break up of colliding droplets;most of the droplets trapped by wall surfaces are larger than the largest droplets of inlet due to the coalescence of colliding droplets.