The ways to achieve a fundamental understanding of the two-phase flow inside the absorber tube with measuring technique procedure are limited. One interesting way is to observe the flow with a wire-mesh sensor. The sensor, which is situated at the cross-section of the absorber tube, delivers high definition data of the distribution of steam and water in time and space. This information can be used to validate flow pattern profiles, which characterizes the water-steam boundary layer as e.g. "stratified", "wavy", "slug" or "plug". Furthermore, the average steam mass fraction in the cross section can be determined based on the set of measuring points . The different flow patterns result in different heat transfer properties on the inside of the tube and thereby determine the "cooling" of the pipe.
The wire-mesh sensor, developed by the HZDR, is based on a matrix-like arrangement of the measuring points. It comprises two planes that are installed perpendicular to the flow direction with a small gap between the planes. On each plane, wire electrodes are arranged in parallel and the planes are perpendicular to each other. This gives a grid of electrodes in the axial perspective, which covers the entire cross-section. The transmitter electrodes are sequentially activated while all receiver electrodes are parallel sampled, in such a way, that the electrical conductivity of the fluid is evaluated in each crossing point. The measured current depends on the local instantaneous conductivity of the fluid at the virtual crossing point. In case of water, a current is measured at a crossing point between transmitter and receiver electrode due to the electrical conductivity of water. A current cannot be measured when a steam bubble encloses a crossing point. The receiver currents are converted to a voltage and subsequently to a digital signal, which is transferred to the measurement PC. Here, the instantaneous void fraction is determined for each crossing point. All local instantaneous void fractions in the cross-section of the pipe are collected in a frame. The wire-mesh sensor achieves a maximum temporal resolution of 10000 frames per second.
Experiments measuring conductivity with a wire-mesh sensor were conducted at the direct steam parabolic through plant at Plataforma Solar de Álmería in 2016. The sensor was exclusively designed and delivered by HZDR for the tube dimension at site. The experiments were carried out in collaboration with DLR, HZDR, and Spanish CIEMAT.
Two exemplifying videos from the measuring sequence are shown. The first video shows the event of a water slug at the wire-mesh sensor. The water surge flows towards the observer and fills for a short while the whole tube. The second video shows a wavy flow. The crooked wave fronts propagate periodically along the tube axis, just like waves rolling on the ocean towards a beach.
The procedure can be utilized to develop the basic understanding of the two-phase flow, but also to validate the function and confirm made assumptions when working with prototypes. The application areas stretch from direct steam solar fields with operating pressures of 100 bar or more down to process heat units with low operating pressures.