Free-surface flows are a key multiphase regime in light water reactors. In particular, studying flowing films thickness and velocity – both experimentally and numerically - is central for safety thermal analysis, since these parameters determine the heat transfer coefficient of film flows ensuring a proper cooling of several critical components ranging from passive safety systems to fuel rods in BWRs. Particularly, measuring velocity profiles in thin liquid films is crucial for validation of CFD and best-estimate system codes used in safety analysis. However, common acoustic or optical velocity measurement techniques have intrinsic limitations with thin films. The proof of principle of an alternative technique for velocity profile reconstruction and thickness measurement in thin water films is proposed and validated. Water films are investigated using a flush wall film conductivity sensor, and a high-speed camera. The sensor measures film thickness over a 96 × 26 spatial matrix with 2 mm pitch. A conductive tracer is injected into the film, with the film sensor providing both film thickness and tracer concentration. The tracer concentration after injection serves as the initial condition for 2D advection-diffusion simulations of each liquid layer. These are averaged to match the integrated sensor measurements. An optimization algorithm determines the unknown advection velocity for each layer by minimizing the difference between simulated and measured average concentrations. The velocity profile across the film is reconstructed using the top layer velocity, measured by the camera, as a boundary condition. Different mass flow rates and slopes are tested. Moreover, non-trivial VOF simulations using Ansys CFX 2020R2 are run to further validate the presented methodology. CFD results exhibit a behavior similar to that observed experimentally, and both simulation and experiments are cross-validated also using correlations from the literature as a reference. • Focus on thickness and velocity of thin water films falling down inclined planes. • Measurement of thin falling films using conductivity film sensor and camera. • Description of implementation of VOF numerical simulation of thin films. • Cross validation of experiments and simulations to correlations from the literature. • Proof of concept of new method for velocity profile reconstruction in flowing films.
Grasso et al. (Thu,) studied this question.