Experimental Study on the Structure of the Velocity Boundary Layer in a 3×3 Rod Bundle Channel

Velocity boundary layer exists near the fuel rod surface in the fuel assembly due to the viscous effects. The boundary layer forms the main thermal resistance for heat transfer between the rod and the coolant since the momentum and energy transport is weak. In order to investigate the flow characteristics between the rod and the coolant in the rod bundle channel, this study directly measures velocity distribution in the boundary layer using the Particle Image Velocimetry (PIV) technique. The wall friction velocity is obtained by fitting the experimental data to the Spalding formula, choosing 0.436 as the Von Kármán constant, then the dimensionless velocity distribution of the boundary layer is obtained. The dimensionless velocity distribution under different Reynolds numbers is quantitatively studied. In addition, the effect of the gap size on the structure of the velocity boundary layer is investigated. The experimental results indicate that with the increase of the Reynolds number, the range of the inner layer of the boundary layer decreases gradually. In the inner layer of the boundary layer, 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. Reducing the gap size restricts the development of the velocity profile and increases the proportion of the inner layer of the boundary layer in comparison to the channel gap. A smaller gap size weakens the turbulence fluctuation, thus reducing the transverse mixing in the channel gap