Mass Flow
The Mass flow boundary condition constrains the mass flowing into the domain across an inlet boundary. The mass flow can be specified in a number of ways.
Pointwise Mass Flux
The pointwise mass flux sets the velocity at the boundary to:
where mf is the normal mass flux and ρ is the density.
Mass Flow Rate
The mass flow rate boundary condition sets the total mass flow through the boundary according to:
where dbc (only present in the 2D Cartesian axis system) is the boundary thickness normal to the fluid-flow domain and m is the total mass flow rate.
In addition to the constraint on the total flow across the boundary, the tangential velocity components are set to zero on the boundary
(4-37)
Standard Flow Rate
The standard flow rate boundary condition specifies the mass flow as a standard volumetric flow rate. The mass flow through the boundary is set by the equation:
where dbc (only present in the 2D component Cartesian axis system) is the boundary thickness normal to the fluid-flow domain, ρst is the standard density, and Qsv is the standard flow rate. The standard density is defined by one of the following equations:
where Mn is the mean molar mass of the fluid, Vn is the standard molar volume, pst is the standard pressure, R is the universal molar gas constant, and Tst is the standard temperature.
Equation 4-37 or Equation 4-38 is also enforced for compressible and incompressible flow, respectively, ensuring that the normal component of the viscous stress and the tangential component of the velocity are zero at the boundary.