Bubble paths in two-phase flows through open-porous foams: Imaging measurements by X-ray and neutron radiography

In water electrolysis, the porous transport layer (PTL) is an essential component of both proton (PEM) as well as anion exchange membrane (AEM) electrolysers. Besides establishing an electrical contact, the PTL enables the electrolyte to be transported to the anode. In the opposite direction, the oxygen (O2) formed at the anode must be transported away, resulting in a complex counterflow of liquid and gas through the PTL, thus limiting the mass transport and, consequently, the conversion of electrical energy. The further development of electrolysers faces so far unexplored operating conditions, in particular by increasing the electric current density. This, in turn, affects the formation and transport of gas bubbles in the PTL, which is not yet sufficiently understood.

As the gas-liquid two-phase flow in the PTL is inaccessible for flow measurement by optical methods, we employed time-resolved X-ray and neutron radiography. Using the model experiment sketched in Fig. 1, we aimed for imaging measurements of the gas transport through open-porous foam by mapping the gas fraction distribution over time. In previous experimental studies, we have used X-ray and neutron radiography for flow visualisation in optically opaque fluids such as liquid metal [1] and aqueous foam [2]. Similar to the approach of radiographic measurements of the liquid fraction in aqueous foam [3], this conference contribution showcases the detection and tracking of bubbles based on their gas fraction in X-ray or neutron images. As exemplarily illustrated in Fig. 2, we observed preferred paths of the bubbles moving upwards through the open-porous foam samples. Moreover, we found that bubbles smaller than the pore size are significantly slowed down, even in the case of a hydrophilic surface character of the foam. In summary, the measurement results and conclusions from our experimental parameter study are available for comparison with computational fluid dynamics.