Microscopic molecular tagging velocimetry within PWR bundles for wall shear stress measurements

Wall shear stress and near wall velocity profile strongly affect the thermal and hydraulic performances of nuclear fuel assemblies. There exist very few datasets that have attempted to measure the wall shear stress directly.  The Reynolds number in most reactor make the numerical measurement with direct simulations out of reach for any high-performance computers, still today. In this work we implement microscopic Molecular Tagging Velocimetry (μ-MTV) for direct measurement of wall shear stress and near wall velocity profile around fuel bundle rods in a geometry typical of pressurized water reactors (PWR). MTV is a non-intrusive time-of-flight technique that relies on fluorescent molecules as tracers. One key benefit of MTV is the ability to write lines or patterns within the fluid (without need for solid particles).To obtain high resolution and accurate data, the laser light and the images need to be delivered and collected locally with custom optics, optical fibers and borsescopic imaging. In the paper a demonstration design with typical data available from this technique will be illustrated with the goal of proving the capabilities of this technique.