Combined Two-phase Co-flow and Counter-flow in a Gas Channel/Porous Transport Layer Assembly


Combined Two-phase Co-flow and Counter-flow in a Gas Channel/Porous Transport Layer Assembly

Beale, S. B.; Andersson, M.; Weber, N.; Marschall, H.; Lehnert, W.

Polymer electrolyte fuel cells and electrolysers are low temperature devices whereby both gases and liquids intermingle within the porous transport layers and open channels. The flow of the liquid and gas is of paramount importance to the functioning of the unit. This motion is poorly understood. Cell-level models typically employ volume-averaging techniques to describe the motion of the flowing reactants and products. Until recently, detailed analysis of the two-phase liquid gas mixture, employing front-tracking methods has proved to be computationally prohibitive. Previous work has considered the motion of liquid drops in gas channels, it being assumed the drops are formed at specific nucleation sites on the sides of the channels. The present work considers a detailed numerical analysis of combined liquid-gas co-flow in a gas channel with liquid-gas counter-flow in a porous transport layer, (PTL). The geometry considered is in the form of a ‘T-shape’ with the porous transport layer of a thin rectangular prism of dimensions 0.5×0.5×0.1 mm3 located at the base of the ‘T’, and the gas flowing across the top in channel. The PTL is reproduced by digital reconstruction of nano-computer tomography images of a Freudenberg H2315 PTL as a sterolithography file. From this, the domain is tessellated with an unstructured castellated, or octree, type mesh. Liquid water is introduced at an electrode at the base of the PTL and gaseous oxygen is simultaneously removed by electrochemical reduction; the resulting liquid-gas counter flow in the porous transport layer results in liquid droplets being entrained in co-flow in the gas channels and convected downstream.

The equations of mass and momentum are solved by means of the open source software library OpenFOAM. A volume-of-fluid approach based on the multidimensional universal limiter for explicit solution was employed within a volume-of-fluid method. At T = 0, the channel is presumed to be filled with gas, and the PTL saturated with liquid water. Gas is introduced at the inlet at a given velocity. Water is added and gas removed at the electrode (counter flow) whereas both water and gas are removed at the outlet (coflow). At the channel and PTL walls as well as on the GDL fibres, the static contact angle was fixed. It can be seen that the location and size of the shed drops varies somewhat in space and time, i.e., there is a stochastic component to the motion of the fluid, due to the spatial distribution of the fibres in the PTL, the transient shedding process, and the merging of liquid streams flowing into the gas channel. Nonetheless, a definite periodicity is observed with drops being injected into the channel at a fairly regular rate. Some relatively minor switching with time is observed within the PTL due to the random packing of fibres, but again these transients are relatively quiescent, as might be expected for porous media flow.

In addition to providing new and important information about flow and pressure losses in channels and PTLs of electrochemical cells as a function of gas and liquid flow rates, i.e. current density and stoichiometry, the present model may be also used to enumerate properties such as relative permeability which can subsequently be employed in cell-scale models.

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Publ.-Id: 31029