The H2 Gas Phase and O2 Gas Phase nodes define the governing domain equations for gas phase transport.

Note that these nodes are added by default, and that you cannot alter the selection of the nodes by settings on the nodes themselves. The H2 Gas Phase and O2 Gas Phase are active on domains selected by the corresponding H2/O2 Gas Diffusion Electrode, Gas Diffusion Layer, Gas-Electrolyte Compartment or Gas Flow Channel nodes.

The H2 Gas Phase and O2 Gas Phase subnodes are only enabled if at least one of the checkboxes Include gas phase diffusion or Use Darcy’s law for momentum transport is selected in the corresponding Gas Mixture section on the interface top node.

This section is only visible if any of the Include species checkboxes has been selected and the Include gas phase diffusion checkbox has not been selected in the physics interface top node.

Use Mixture specification to specify mixture composition in Mole fractions or Humidified mixture for all gas phase domains. The O2 Gas Phase node has an additional option of Humidified air for O2 gas phase domains.

The option Mole fractions is always available and text fields are used to specify the mixture composition for all included species.

Humidified mixture allows for specifying a gas mixture using values for a dry gas mixture, which is subsequently humidified to a specified relative humidity. The setting is available if H2O has been selected in the corresponding Gas Mixture section on the interface top node. Text fields are used to specify the mixture composition for all included species except H2O. Additionally, the Relative humidity, Humidification temperature and Humidification absolute pressure of the humidifier are also specified.

Humidified air is available if H2O and N2 have been selected in the O2 Gas Mixture section on the interface top node. Relative humidity, Humidification temperature, and Humidification absolute pressure of the humidifier are also specified. The dry mole fractions of the included species are set internally as follows: 0.79 (N2), 4e-4 (CO2) and 1e-6(CO).

Given that the humidifier (inlet) temperature (Thum), absolute pressure (pA,hum) and relative humidity (RHhum) of the humidifier (inlet) gases are known, the conversion from a set of dry (containing no water vapor) molar fraction to a corresponding humidified mixture may be calculated as follows. The water mole fraction is defined as

In this section, the gas phase values for the Density and Dynamic viscosity are specified.

Depending on the settings on the interface top node, the mixture property values may or may not be used by the interface itself. However, the interface will define mixture property variables that may be used when coupling to other physics interfaces, either manually or by multiphysics nodes such as Reacting Flow, H2/O2 Gas Phase. Additionally, the interface also defines liquid water properties (density and dynamic viscosity) in the H2/O2 Gas Phase domain nodes that may be used when coupling to other physics interfaces, if H2O has been selected in the corresponding Gas Mixture section on the interface top node.

This section is only visible if at least one of the checkboxes H2O under Include species or Include H2O(l) in reaction stoichiometry under Reactions has been selected in the corresponding Gas Mixture section on the interface top node.

In this section, the values for the Vapor pressure and Heat of vaporization are specified. The Vapor pressure is for instance used when specifying inlet and initial values for the gas composition in terms of relative humidity. The Heat of vaporization is used for automatic calculation of equilibrium and thermoneutral electrode reaction potentials involving liquid water, as well as heat sources added by the Water Condensation–Evaporation nodes and the Water Absorption–Desorption nodes.

This section is only visible if the Use Darcy’s law for momentum transport checkbox has been disabled in the interface top node.

Specify the Velocity field of the gas. The velocity field can be specified explicitly, or using an analytical expression, or coupled from any Fluid Flow interface, or by using an Reacting Flow, H2/O2 Gas Phase multiphysics coupling node.

This section is only visible if the Include gas phase diffusion checkbox has been selected in the interface top node.

Specify the Binary diffusion coefficients for all pairs of species in the gas.

See also Gas Phase Theory.

This section is only visible if Auxiliary species checkbox has been selected in the interface top node.

Specify the properties of the auxiliary species, such as Molar Mass, Molar standard enthalpy of formation, Molar standard entropy, and Molar heat capacity. The Fuller diffusion volume is available if the Include gas phase diffusion checkbox has been selected in the interface top node and if using Built-in Binary diffusion coefficients. The Dynamic viscosity, Thermal conductivity and Boiling point are used for calculating the built-in gas phase dynamic viscosity and thermal conductivity, and are available if using Built-in Dynamic viscosity.