## 7.11.1 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 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 panel (Figure  7.11.1), which is opened from the Boundary Conditions panel (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  7.8.1 for details. Specification of the loss coefficient is described here. Open channel boundary condition inputs are described in Section  23.10.2.

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, , varies with the normal component of velocity through the vent, , as follows:

 (7.11-1)

where is the fluid density, and is the nondimensional loss coefficient.

 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 panels for defining these functions are the same as those used for defining temperature-dependent properties. See Section  8.2 for details.

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