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7.5.1 Inputs at Mass Flow Inlet Boundaries



Summary


You will enter the following information for a mass flow inlet boundary:

All values are entered in the Mass-Flow Inlet panel (Figure  7.5.1), which is opened from the Boundary Conditions panel (as described in Section  7.1.4). Note that open channel boundary condition inputs are described in Section  23.10.2.

Figure 7.5.1: The Mass-Flow Inlet Panel
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Defining the Mass Flow Rate or Mass Flux


You can specify the mass flow rate through the inlet zone and have FLUENT convert this value to mass flux, or specify the mass flux directly. For cases where the mass flux varies across the boundary, you can also specify an average mass flux; see below for more information about this specification method.

If you set the mass flow rate, it will be converted internally to a uniform mass flux over the zone by dividing the flow rate by the area of the zone. You can define the mass flux (but not the mass flow rate) using a boundary profile or a user-defined function.

The inputs for mass flow rate or flux are as follows:

1.   Choose the method you will use to specify the mass flow by selecting Mass Flow Rate, Mass Flux, or Mass Flux with Average Mass Flux in the Mass Flow Specification Method drop-down list.

2.   If you selected Mass Flow Rate (the default), set the prescribed mass flow rate in the Mass Flow-Rate field.

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Note that for axisymmetric problems, this mass flow rate is the flow rate through the entire ( $2\pi$-radian) domain, not through a 1-radian slice.

If you selected Mass Flux, set the prescribed mass flux in the Mass Flux field.

If you selected Mass Flux with Average Mass Flux, set the prescribed mass flux and average mass flux in the Mass Flux and Average Mass Flux fields.

More About Mass Flux and Average Mass Flux

As noted above, you can specify an average mass flux with the mass flux. If, for example, you specify a mass flux profile such that the average mass flux integrated over the zone area is 4.7, but you actually want to have a total mass flux of 5, you can keep the profile unchanged, and specify an average mass flux of 5. FLUENT will maintain the profile shape but adjust the values so that the resulting mass flux across the boundary is 5.

The mass flux with average mass flux specification method is also used by the mixing plane model described in Section  10.3.2. If the mass flow inlet boundary is going to represent one of the mixing planes, then you do not need to specify the mass flux or flow rate; you can keep the default Mass Flow-Rate of 1. When you create the mixing plane later on in the problem setup, FLUENT will automatically select the Mass Flux with Average Mass Flux method in the Mass-Flow Inlet panel and set the Average Mass Flux to the value obtained by integrating the mass flux profile for the upstream zone. This will ensure that mass is conserved between the upstream zone and the downstream (mass flow inlet) zone.



Defining the Total Temperature


Enter the value for the total (stagnation) temperature of the inflow stream in the Total Temperature field in the Mass-Flow Inlet panel.



Defining Static Pressure


The static pressure (termed the Supersonic/Initial Gauge Pressure) must be specified if the inlet flow is supersonic or if you plan to initialize the solution based on the pressure inlet boundary conditions. Solution initialization is discussed in Section  25.14.

The Supersonic/Initial Gauge Pressure is ignored by FLUENT whenever the flow is subsonic. If you choose to initialize the flow based on the mass flow inlet conditions, the Supersonic/Initial Gauge Pressure will be used in conjunction with the specified stagnation quantities to compute initial values according to isentropic relations.

Remember that the static pressure value you enter is relative to the operating pressure set in the Operating Conditions panel. Note the comments in Section  7.3.1 regarding hydrostatic pressure.



Defining the Flow Direction


You can define the flow direction at a mass flow inlet explicitly, or you can define the flow to be normal to the boundary.

You will have the option to specify the flow direction either in the absolute or relative reference frame, when the cell zone adjacent to the mass flow inlet is moving. If the cell zone adjacent to the mass flow inlet is not moving, both formulations are equivalent.

The procedure for defining the flow direction is as follows, referring to Figure  7.5.1:

1.   Choose which method you will use to specify the flow direction by selecting Direction Vector or Normal to Boundary in the Direction Specification Method drop-down list.

2.   If you selected Normal to Boundary, there are no additional inputs for flow direction.

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Note that if you are modeling axisymmetric swirl, the flow direction will be normal to the boundary; i.e., there will be no swirl component at the boundary for axisymmetric swirl.

3.   If the cell zone adjacent to the mass flow inlet is moving, choose Absolute (the default) or Relative in the Reference Frame drop-down list.

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These options are equivalent when the cell zone next to the mass flow inlet is stationary.

4.   If you selected Direction Vector and your geometry is 2D, go to the next step. If your geometry is 3D, you will next choose the coordinate system in which you will define the flow direction components. Choose Cartesian (X, Y, Z), Cylindrical (Radial, Tangential, Axial), or Local Cylindrical (Radial, Tangential, Axial) in the Coordinate System drop-down list. See Section  7.3.1 for information about Cartesian, cylindrical, and local cylindrical coordinate systems.

5.   If you selected Direction Vector, set the vector components as follows:

  • If your geometry is 2D non-axisymmetric, or you chose to use a 3D Cartesian coordinate system, enter the appropriate values for X, Y, and (in 3D) Z-Component of Flow Direction.

  • If your geometry is 2D axisymmetric, or you chose to use a 3D Cylindrical coordinate system, enter the appropriate values for Axial, Radial, and (if you are modeling swirl or using cylindrical coordinates) Tangential-Component of Flow Direction.

  • If you chose to use a 3D Local Cylindrical coordinate system, enter the appropriate values for Axial, Radial, and Tangential-Component of Flow Direction, and then specify the X, Y, and Z-Component of Axis Direction and the X, Y, and Z-Coordinate of Axis Origin.

Figure  7.3.2 shows the vector components for these different coordinate systems.



Defining Turbulence Parameters


For turbulent calculations, there are several ways in which you can define the turbulence parameters. Instructions for deciding which method to use and determining appropriate values for these inputs are provided in Section  7.2.2. Turbulence modeling is described in Chapter  12.



Defining Radiation Parameters


If you are using the P-1 radiation model, the DTRM, the DO model, or the surface-to-surface model, you will set the Internal Emissivity and (optionally) Black Body Temperature. See Section  13.3.15 for details. (The Rosseland radiation model does not require any boundary condition inputs.)



Defining Species Mass Fractions


If you are modeling species transport, you will set the species mass fractions under Species Mass Fractions. For details, see Section  14.1.5.



Defining Non-Premixed Combustion Parameters


If you are using the non-premixed or partially premixed combustion model, you will set the Mean Mixture Fraction and Mixture Fraction Variance (and the Secondary Mean Mixture Fraction and Secondary Mixture Fraction Variance, if you are using two mixture fractions), as described in Section  15.13.



Defining Premixed Combustion Boundary Conditions


If you are using the premixed or partially premixed combustion model, you will set the Progress Variable, as described in Section  16.3.5.



Defining Discrete Phase Boundary Conditions


If you are modeling a discrete phase of particles, you can set the fate of particle trajectories at the mass flow inlet. See Section  22.13 for details.



Defining Open Channel Boundary Conditions


If you are using the VOF model for multiphase flow and modeling open channel flows, you will need to specify the Free Surface Level, Bottom Level, and additional parameters. See Section  23.10.2 for details.


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