The H2 Gas Flow Channel and O2 Gas Flow Channel nodes specify domains with only a Gas Phase node being active.

This node will be available if the Include gas phase diffusion check box or the Use Darcy’s law for momentum transport check box has been selected in the corresponding Gas Mixture section on the interface top node.

If the Include gas phase diffusion check box has been selected, there are no settings on this node. The gas diffusivity defined in the Gas Phase node.

If the Use Darcy’s law for momentum transport check box has been selected, the Gas permeability is defined in this node.

The Gas permeability option of Straight channels can be used to specify the permeability for a straight channel with a uniform cross section, for which analytical correlations exist for expressing the flow rate as proportional to the volumetric flow. For a rectangular cross section, with height H and width W, the corresponding Darcy permeability κg (derived by Boussinesq in 1868) is

with βn being a help variable equal to

In the COMSOL implementation of the above expression, the sum is truncated to the first ten terms. It should be noted that the analytical expression renders a correct average flow rate for a straight channel with no slip conditions on the channel walls. To resolve the local flow field close to a wall, or for curved channels, solving for the full set of the Navier–Stokes equations (as defined by the Laminar Flow, Brinkman Equations, or Free and Porous Media Flow interfaces) is typically required. However for cases were full accuracy of the local flow field is not required, the above approximation may save substantial computational resources in terms of memory and computation time.