Experimental droplet deposition distribution downstream of functional spacers in an adiabatic BWR mock-up model

Functional spacers play an important role for the annular steam-water flow in boiling water reactors. They are designed to enhance droplet deposition and therefore delay dryout by sustaining the liquid film. To measure droplet deposition distributions downstream of functional spacers, experiments have been conducted in an adiabatic BWR mock-up model of a double subchannel. The experiments were conducted with six different spacer types and three different gas densities. They are designed to produce high-resolved experimental data, both in time in space, by measuring the conductivity distribution in the liquid film, which increases when salt-labeled droplets deposit. The results show, that all studied spacer types enhance the droplet deposition compared to the experiments without spacer. The droplet deposition distribution is very distinct for each spacer shape. The spacer shape, especially the blockage ratio, plays an important role in regard to deposition enhancement. The blockage ratio alone is however not enough to predict deposition. The droplet size distribution seems to shift towards smaller droplets when passing through the spacer. The experiments are also modeled with a Lagrangian-Euler CFD model, which contains turbulence effects on the droplets by means of a random walk model. The model is for some cases in excellent, for other cases in decent agreement with the experiments. The model shows, that the direct droplet-spacer impact and the deflection of streamlines are mainly responsible for the droplet deposition enhancement. For the chosen flow conditions the turbulence plays a minor role for droplet deposition. This might change for BWR conditions. (author)