7.3.13 Exhaust Fan Boundary Conditions

Exhaust fan boundary conditions are used to model an external exhaust fan with a specified pressure jump and ambient (discharge) pressure.

Inputs at Exhaust Fan Boundaries

You will enter the following information for an exhaust fan 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)

  • user-defined scalar boundary conditions (for user-defined scalar 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)

• pressure jump

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

All values are entered in the Exhaust Fan dialog box (Figure  7.3.14), 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 pressure jump is described here. Open channel boundary condition inputs are described in Section  24.3.1.

Specifying the Pressure Jump

An exhaust fan is considered to be infinitely thin, and the discontinuous pressure rise across it is specified as a function of the local fluid velocity normal to the fan. You can define a constant, polynomial, piecewise-linear, or piecewise-polynomial function for the Pressure Jump across the fan. The dialog boxes for defining these functions are the same as those used for defining temperature-dependent properties. See Section  8.2 for details.

Figure 7.3.14: The Exhaust Fan Dialog Box

You must be careful to model the exhaust fan so that a pressure rise occurs for forward flow through the fan. In the case of reversed flow, the fan is treated like an inlet vent with a loss coefficient of unity.