7.3.12 Outlet Vent Boundary Conditions

Outlet vent boundary conditions are used to model an outlet vent with a specified loss coefficient and ambient (discharge) pressure and temperature.

Inputs at Outlet Vent Boundaries

You will enter the following information for an outlet vent boundary:

• static pressure

• backflow conditions
  • total (stagnation) temperature (for energy calculations)

  • turbulence parameters (for turbulent calculations)

  • chemical species mass or mole fractions (for species calculations)

  • mixture fraction and variance (for non-premixed or partially premixed combustion calculations)

  • progress variable (for premixed or partially premixed combustion calculations)

  • multiphase boundary conditions (for general multiphase calculations)

• radiation parameters (for calculations using the P-1 model, the DTRM, the DO model, or the surface-to-surface model)

• discrete phase boundary conditions (for discrete phase calculations)

• loss coefficient

• open channel flow parameters (for open channel flow calculations using the VOF multiphase model)

All values are entered in the Outlet Vent dialog box (Figure  7.3.13), which is opened from the Boundary Conditions task page (as described in Section  7.1.4).

The first 4 items listed above are specified in the same way that they are specified at pressure outlet boundaries. See Section  7.3.8 for details. Specification of the loss coefficient is described here. Open channel boundary condition inputs are described in Section  24.3.1.

Figure 7.3.13: The Outlet Vent Dialog Box

Specifying the Loss Coefficient

An outlet vent is considered to be infinitely thin, and the pressure drop through the vent is assumed to be proportional to the dynamic head of the fluid, with an empirically determined loss coefficient which you supply. That is, the pressure drop, $\Delta p$, varies with the normal component of velocity through the vent, $v$, as follows:

where $\rho$ is the fluid density, and $k_L$ is the nondimensional loss coefficient.

$\Delta p$ is the pressure drop in the direction of the flow; therefore the vent will appear as a resistance even in the case of backflow.

You can define a constant, polynomial, piecewise-linear, or piecewise-polynomial function for the Loss Coefficient across the vent. The dialog boxes for defining these functions are the same as those used for defining temperature-dependent properties. See Section  8.2 for details.