Numerical Simulation On Heat Transfer Of Supercritical Pressure Water In The Triangular Sub-channel

The Supercritical Water Cooled Reactor(SCWR)is one of the s six Generation IV nuclear reactors,which has broad prospects for the development.Therefore,it is very significant to investigate the thermal-hydraulic behavior of supercritical water in rod bundles.At present,the experimental data can't meet the needs of the development and design of SCWR because there is few experimental study on the supercritical water in rod bundles due to the measurement technique and cost.In this thesis,the numerical simulation method is accepted to investigate the flow and heat transfer characteristics of supercritical water in the triangular sub-channel based on the experiment.The sub-channel of rod bundles with triangular distribution is chose to establish the fluid solid coupling numerical analysis model.The fuel rod diameter is 8 mm,pitch-to-diameter P/D is 1.4,the wall thickness is 2.5 mm.The computational fluid dynamics code CFX 14.0 is used to study the influences of different turbulence models,system parameters and standard spacer grid on fluid temperature field,flow field and wall temperature in the channel.The calculation results show that the axial distribution of wall temperature calculated by SSG model is the highest,the axial distribution of wall temperature calculated by RNG ?-? model is the lowest.The circumferential distributions of wall temperature,fluid temperature,secondary flow velocity and turbulent kinetic energy are similar,which is the largest in the intersection of the arc edge and the straight edge and the smallest at the midpoint of the arc edge,This phenomenon indicates that the circumferential heat transfer is not uniform in the sub-channel.The circumferential distribution of outer wall temperature is more uniform than internal wall temperature.Compared with other turbulence models,wall temperature calculated by SSG model is most uniform.The turbulence model has little effect on the axial distribution of average fluid temperature.Fluid temperature,secondary flow velocity and turbulent kinetic energy at the cross section gradually increase from the center to the wall in the radial direction,the radial distribution law of mainstream velocity is just the opposite.The axial distribution of average mainstream velocity calculated by different turbulence models has little difference.Mainstream velocity gradually decreases from the center to the wall in the radial direction.The circumferential distribution of mainstream velocity near the wall is not uniform,which is the smallest in the intersection of the arc edge and the straight edge,the largest at the midpoint of the straight edge.At q=0°feature line,the mainstream velocity calculated by SSG model changes fastest.At q=33.7°feature line,the mainstream velocity calculated by SSG model changes slowest.At q=60°feature line,the mainstream velocity calculated by RNG ?-? model changes fastest.The secondary flow velocity calculated by SSG model is much larger than other models.The turbulent kinetic energy calculated by RNG ?-? model is largest.In summary,SSG model has the best prediction of supercritical water heat transfer characteristics and the most uniform circumferential temperature and flow field.The effects of mass flux,pressure and heat flux on heat transfer characteristics of supercritical water are as follows: with the increase of mass flux,the wall temperature decreases and the heat transfer coefficient(HTC)goes up when the pressure and heat flux are constant,which indicates that the increase of mass flux could enhance heat transfer in the sub-channel.Besides,the heat transfer capability of supercritical water is better than supercritical gas.In low enthalpy region,wall temperature and HTC stay the same at different pressures.The increase of wall temperature near pseudo critical pressure is slower than that in low enthalpy region or high enthalpy region.In high enthalpy region,the difference of wall temperatures under different pressure gradually decreases with the increase of bulk enthalpy.The influence of pressure on HTC in the great specific heat region(GSHR)is largest,the HTC decreases with the increase of pressure.The greater heat flux is,the higher wall temperature is.In low enthalpy region,heat flux has little effect on HTC.However,the HTC in the GSHR or high enthalpy region would decrease when heat flux becomes higher.The axial distribution of wall temperature with the grid spacer is a little lower,the difference is particularly evident in the position of grid spacer,which proves that grid spacer can strengthen the heat transfer of fluid in the channel.The circumferential distribution of wall temperature at the same cross section is consistent within the range of q from 0 degree to 60 degree,which indicates that the grid spacer has little influence on circumferential heat transfer.The wall temperature difference at the same angle would decrease along the flow direction,which shows that heat transfer enhancement produced by the grid spacer would gradually disappear with the increase of axial distance.The axial distribution of average fluid temperature in the channel with grid spacer is slightly higher in general,but the average fluid temperature firstly rises and then decline in the grid spacer section.The existence of grid spacer would make the circumferential distribution of the fluid temperature,mass flux,mainstream velocity,secondary flow velocity and turbulent kinetic energy become non uniform.The average mass flux,mainstream velocity,secondary flow velocity and turbulent kinetic energy have the same axial distribution,they all firstly increase sharply and then decrease rapidly in the grid spacer section,which is caused by the sudden change of flow area.In addition,the grid spacer could significantly improve the secondary flow velocity.