Research Onthe Effect Of Turbulent Mixing Through Rod Bundle Supported By Spacer Grid With Mixing Vane

At present,the sub-channel model is mainly used for the refined analysis of fuel assemblies,while the interaction models between channels still need to be improved in subchannel model.The mixing effects of the coolant in the sub-channels mainly include cross-flow,turbulent-mixing and void effects.Among the three effects,the turbulent-mixing effect defines mixing of coolant due to turbulent flow between subchannels.To this degree,the momentum and energy exchange of the coolant is achieved without net coolant mass exchange between adjacent channels,which contribute to flatten the distribution of velocity and enthalpy in the channels.However,due to the complexity and uncertainty of the turbulent flow,the subchannel programs can't quantitatively reflect the relevant phenomena only by the codes based on coarse models.By contrast,the Computational Fluid Dynamics(CFD)codes based on more refined models can provide better flow field analysis,which may help to analyze the turbulent-mixing effect.Therefore,in this paper,flow field analysis software FLUENT 15.0 is employed to analyze the coolant flow in core fuel bundle with mixing vanes and space grid.At the same time,a new calculation method of the turbulence mixing velocity is proposed by theoretical analysis.And the empirical correlation of the turbulence mixing coefficient under the influence of the mixing vanes is obtained by using the dimensional analysis method and the control variables method,which is based on massive data.As to the requirements for validation and verification of CFD results,this paper takes geometric parameters and flow field conditions released by Karoutas as the object,using the simplest 2×3 rod bundle channel,combined with inlet velocity,outlet pressure and circumferential periodic boundary conditions for simulation calculations.The optimal meshing method,boundary layer division method and turbulence model were determined by comparing with the experimental data.The distribution of turbulence mixing velocity along the flow direction of coolant on different Reynolds numbers(9630 ~ 95537)and P/D(1.25 ~ 1.42)can be obtained by changing the inlet velocity and geometric condition.The results show that the hybrid grid method and the SST k-? model with the enhanced wall function treatment method can get the best results of the flow field under the above-mentioned computational geometry and flow conditions.At the same time,the turbulence mixing coefficient and the amplitude is the function of the average Reynolds number of the adjacent channel.The results are in good agreement with the experimental results.At the same time,the P/D ratio is the key factor,which has a significant effect on the amplitude,the decay constant and the vibration cycle parameters of the mixing coefficient,and the amplitude reaches the maximum when the pitch-to-diameter ratio is about 1.35,which can guide the design of core fuel assemblies.Then according to the characteristics of turbulent mixing flow,the correlation of turbulent mixing is utilized for the advanced subchannel calculating function for THEATRe by modifying source items of momentum and energy in the corresponding equations.Calculation modules for cross flow and pressure drop calculation are also included to show the influence by spacer grid with mixing vane.In order to show the reliability and correctness of the developed subchannel code,a benchmark based on GE 3×3 experiment data has been modeled and calculated.The result shows an irreplaceable status of turbulent mixing effect due to its contribution to temperature distribution.The subchannel code also shows a reasonable result in transient calculation process.It can be effectively shown that the calculating precision has been improved by using this modeling method in thermal hydraulic field anlysis.