Effects of wettability and flow direction on gas retention and flow resistance of water flowing through carbon felts with thermally induced gas evolutions

Abstract

Owing to superior properties and low cost, carbon felts are widely used in flow batteries as versatile porous electrodes, through which liquid electrolyte is usually pumped to transport reactants/products. In operation, electrochemically/physically induced gas evolution occurring inside the carbon felt is almost inevitable which, even though seems trivial in most cases, may lead to significant reduction in liquid saturation and permeability. To study the impact of the carbon felt wettability and the flow direction on this in-situ generated liquid-gas two-phase flow, we experimentally visualize gas distributions and measure average gas saturations and liquid permeabilities in hydrophobic/hydrophilic carbon felts and in upward/downward/horizontal flows, which is a continuation of our recent work exploring the thermally induced evolution of dissolved air in water flowing through a carbon felt sample. After being heated at 500°Cfor 7 hours in air, the carbon felt becomes rather hydrophilic, as indicated by its capability of water-droplet sucking. As expected, the gas saturation measured in this hydrophilic felt is lower and the liquid relative permeability is higher than those measured in the un-treated hydrophobic felt. It is also found that the buoyancy force is non-negligible in this flow, which is confirmed by the fact that both the liquid phase saturation and the permeability are considerably lower in the down-ward flow compared with those in the upward and horizontal flows.