Experimental study of transient friction characteristics and velocity distribution of pulsatile flow in rod bundles

Abstract

Rod bundles are widely used in various devices and energy engineering including heat exchangers, nuclear fuel assembly and so on. The transient fluctuation of flow rate in the thermal devices is a common phenomenon under accident scenarios or ocean condition, the study of pulsatile flows has significant practical relevance for the design and safety of thermal devices. In order to determine the flow characteristics of pulsatile flows in rod bundles, the experimental investigations for transient friction loss and evolution of flow field of pulsatile flows of a 5 × 5 rod bundle (P/D = 1.326) are performed. The experiments were conducted under the ranges of time-averaged Reynolds number Reta = 2000–7000, pulsatile amplitude Au = 0.2–0.6 and pulsatile period T = 10 s–100 s. A comparison of friction factors under steady flows and pulsatile flows is performed to determine the influence of pulsatile flows on frictional characteristics. The results show that the transient friction factors versus flow rate under pulsatile flow presents an “oval” shape, and the “oval” shape has a variation with pulsatile parameters changing. To further reveal the details of pulsatile flow fields, the evolution of flow pattern in the pulsatile flow of Reta = 5200, Au = 0.6, T = 10 s is visualized in rod bundles by Particle Image Velocimetry (PIV). The velocity distribution in different stages of pulsatile flows are displayed and discussed. The influence of velocity gradient and turbulent shear stress in the whole pulsatile period for frictional loss is analyzed, and the variation of transient friction factors under different pulsatile parameters is revealed. The results describe transient behaviors to contribute a better understanding for transient flows in fuel assemblies.