Experimental investigation on effect of rod bowing on subchannel void fraction in 5 x 5 rod bundles

During operation of a nuclear power plant, the bending deformation of fuel rods usually occurs and has a substantial effect on the heat transfer and flow behavior of coolant. To assess the influence of rod bowing on thermal-hydraulic phenomena in a nuclear reactor, it is crucial to consider the void fraction characteristics in rod bundles. In this study, experiments were conducted using an irregular sub-channel impedance void meter and high-speed camera in 5 x 5 rod bundle channels with three different bow deformation ratios, as well as a normal straight rod bundle channel. The results show that when the bowing deformation of rods occurs in a rod bundle channel, bubble coalescence enhances. Bubbles which are far away from the bowing rod bundle subchannel tend to move laterally towards the region near the bowing rod bundle subchannel, leading to an increase in void fraction compared to a normal straight subchannel. Besides, as the liquid flow rate increases, the influence of the rod bowing on void fraction within the subchannels increases. Moreover, the axial development characteristics of the void fraction are increasingly affected by the bowing rod with the increase of bowing deformation ratio.