Mapping the gas fraction distribution in bubble flows through open-porous foams by radiographic imaging

The cost-efficient production of green hydrogen using renewable energies requires next-generation proton exchange membrane (PEM) electrolysers to be operated at higher current density. Under this new operating condition, the elevated temperature of the ultra-pure water and its supersaturation with oxygen on the anode side have strong effects on the formation and transport of gas bubbles. The resulting gas-liquid two-phase flow through the porous transport layer at the membrane electrode assembly is characterised by up to 50 % gas fraction, which is exceptionally high. Such a foam-like flow within the porous medium is not accessible by optical measurements. Instead, we performed imaging flow measurements by means of time-resolved radiography using polychromatic X-rays as well as thermal neutrons at 100 frames per second imaging frame rate. On a laboratory scale, we aimed to study the bubble transport by mapping the local gas fraction distribution over time. This conference contribution presents two model experiments with open-porous metal and polymer foams, namely made of nickel and polyurethane, showcasing the advantages but also limitations of X-ray and neutron radiography for investigating bubble transport phenomena within such foam structures. In both experiments, foam samples of approximately 70 mm x 70 mm in width and height were sandwiched between the X-ray- or neutron-transparent front and back windows of a vessel filled with deionised water. As neutrons are strongly attenuated in water, the thickness of the water-filled vessel and the foam sample were set to 5 mm along the beam direction in all measurements. Bubbles were generated continuously by injecting compressed air at different but constant volumetric flow rates through a single hollow needle releasing the bubbles directly into the water-soaked foam. Based on calibration radiographs acquired both in the absence and presence of water, quantitative image analysis yielded a pixelwise mapping of the gas fraction at approximately 0.06 mm image pixel size without binning. While X-ray radiography visualised the pulsating transport of bubble plumes through a nickel foam of 1.2 mm pore size, neutron radiography gave insights into the jumping motion of single bubbles through a polyurethane foam of approximately 3 mm pore size. In conclusion, we characterised the gas transport depending on the volumetric gas flow rate, the bubble size in relation to the foam pore size and the wettability of the inner foam surface. Further radiographic studies will consider bubble flows through open-porous materials with different pore geometry or functionalised surface wettability.