The effect of functional spacers on the liquid film thickness and dryout in a BWR fuel bundle model

For the BWRs, the dryout margin is one of the core design limitation factors. Today’s industry standard is to use a large margin to dryout and functional spacer grids with vanes to enhance the heat transfer and to reduce the fraction of entrained droplets. Difficulties for precise measurements under reactor conditions lead to a lack of knowledge on the exact effects of the spacers on the flow and suggest the use of scaled experiments. For this experiment, the goal is to provide high-resolution data for CFD code validation as well as visualizing the effects of functional spacers and the liquid film and potentially the dryout front. The Dryout Tomography Experiment (DoToX) facility at ETH Zürich is a closed loop experiment for two-phase flow investigations in a fuel bundle model using a modelling fluid. Key aspects are a single undisturbed subchannel and the surrounding four heating rods containing a liquid heating system. This setup allows for a steady state dryout without endangering the structural integrity of the facility and for the 3D reconstruction of the time averaged void distribution within the flow channel by means of an X-Ray and cold neutron Computer Tomography (CT). In this study we pay special attention to the annular flow in the upper half of the sub channel. We investigate the first results delivered by the facility. Prototypical spacer designs available in the open literature were used. We present the Liquid Film Thickness (LFT) distributions on the walls of the heating rods. Improvements towards the dryout performance as well as drawbacks of the specified spacer design are highlighted.