Experimental Study On The Structure Of The Boundary Layer In Rod Bundle Channel Under Unsteady State Conditions

Subject to the influence of viscosity,the coolant produces a boundary layer with a large velocity or temperature gradient near the fuel rod wall.In the boundary layer,especially in the viscous bottom layer,fluid momentum and energy transport are weak.The boundary layer forms the main thermal resistance for heat transfer between the coolant and the fuel rod.Therefore,the structure of the boundary layer and flow and heat transfer characteristics are the key to determining the thermal and hydraulic performance of the fuel element.The unsteady state flow and the strong perturbation of the spacer grid are important factors that affect the flow and heat transfer mechanism of the boundary layer.This paper separates the two factors of the strong perturbation of the spacer grid and the unsteady state flow.Based on the research on the structural characteristics of the boundary layer of the rod bundle channel in the fully developed region.This paper studies the structural characteristics of the boundary layer in the rod bundle channel under unsteady state conditions.Firstly,in the present study,Particle Image Velocimetry(PIV)and Laser-Induced Fluorescence(LIF)techniques are used to measure and reconstruct the structures of the velocity and thermal boundary layers under steady-state conditions.Using a high-speed camera with a telephoto lens increases the spatial resolution to 10 microns.The dimensionless velocity distribution of the fuel assembly is obtained by fitting the experimental data to the Spalding formula.The structure of the boundary layer and flow and heat transfer characteristics under different flow and heat transfer conditions are analyzed and compared quantitatively.The experimental results indicate that with the increase of the Reynolds number(Re),the range of the inner layer of the boundary layer decreases gradually.In the inner layer of the boundary layer,with the increase of the Reynolds number,the proportion of the viscous bottom layer in comparison to the inner layer decreases,and the proportion of the logarithmic law layer in comparison to the inner layer increases,which enhances the flow and heat transfer performance in the channel.The increase in buoyancy reduces the turbulent fluctuation in the channel,decreasing the proportion of the logarithmic law layer in comparison to the inner layer.As a result,the temperature gradient near the rod surface decreases with the increase of buoyancy.This weakens the convective heat transfer in the channel.Next,PIV technology is used to measure the structure of the boundary layer at four typical locations downstream of the spacer grid.To explore the influence of the strong perturbation of the spacer grid on the structure of the boundary layer.The experimental results indicate that the strong perturbation of the spacer grid can reduce the range of the inner layer of the boundary layer,affect the development of the boundary layer,and cause the fluid to enter the turbulent core region in advance.As the height of the distance spacer grid increases,the strong perturbation gradually decreases,and the range of the inner layer of the boundary layer gradually increases.The strong perturbation of the spacer grid enhances the turbulent fluctuation in the channel and weakens the influence of buoyancy,which enhances the flow and heat transfer performance in the channel.Subsequently,TR-PIV technology is used to measure the distribution of the boundary layer of the rod bundle channel under unsteady state conditions.The influence of the key parameters of unsteady flow on the structure of the boundary layer is studied by controlling variables.Based on the research on the structural characteristics of the boundary layer of the rod bundle channel in the fully developed region.The effects of the unsteady state conditions and the strong perturbation coupling on the structure of the boundary layer are comprehensively analyzed.Finally,master the structure of the boundary layer and flow and heat transfer characteristics of the rod bundle channels under unsteady state conditions.The experimental results indicate that the perturbation introduced by the unsteady flow can disrupt the development of the boundary layer and significantly reduce the thickness of the inner layer of the boundary layer.The larger the amplitude and the smaller the period,the greater the perturbation introduced by the unsteady flow.At 1 hydraulic diameter(D_h)downstream of the spacer grid,compared to the perturbation introduced by unsteady state conditions,the strong perturbation of the spacer grid is dominated.Superimposed strong perturbation of the spacer grid and unsteady state conditions further weakens the impact of buoyancy,which enhances the flow and heat transfer performance in the channel.