Add CFD Boundary

When at surface level you can add a CFD boundary in much the same way as a window or any other opening . Please refer to the instructions for the Draw window tool for details on the process of drawing the CFD boundary.

Note: There is one important difference with CFD boundaries compared with openings such as windows, which is that the edges of supply/extract boundaries must not coincide with parent surface edges. The CFD boundary should be separated from the surface edges by a distance of at least 0.1 m.

After selecting the Add CFD boundary tool, the boundary settings are displayed on the Drawing options data panel. You can select the required boundary type from the Boundary type drop list. The following boundary types are available for surfaces of all orientations:

• 1–Supply (general purpose HVAC supply diffuser)
• 4–Extract (HVAC extract grille)
• 5–Temperature (surface temperature patch). the heat transfer is calculated in one of two ways depending on the Surface heat transfer option on the CFD Calculation options dialog selected. If option 2-User defined is selected, the heat transfer is calculated by combining the specified surface transfer coefficient with the cell surface area and the delta T between the surface and the adjacent cell. If the Surface heat transfer option is set to 1-Calculated, the surface heat transfer coefficient is calculated using a wall function.
• 6–Flux (surface heat flux patch) - the calculation scheme incorporates this flux directly within the cells adjacent to the surface at which the flux is specified. The heat flux is incorporated after multiplying it by the surface area of the cell.

The following additional boundary types for multi-directional diffusers are available for ceiling and other downward-facing surfaces only:

• 2–Four-way (four-way supply diffuser)
• 3–Two-way (two-way supply diffuser)

Various settings are available depending on the boundary type:

1-Supply:

• Boundary temperature - the temperature of the air entering the space.
• Flow rate - the supply volume flow rate.
• X-discharge angle (°) - the discharge angle between the local surface X-axis and an inward facing normal to the surface. At the surface level, taking a normal view from the inside of the zone to the surface, the X-axis discharge angle is positive between the normal and the positive X-axis (i.e. the axis pointing towards the right) and negative between the normal and the negative X-axis (i.e. the axis pointing towards the left). For example to define a discharge angle of 45° pointing towards the left of the centre of a diffuser, looking at it from the inside of the zone, you would enter –45. On the other hand, to define a discharge angle of 45° pointing towards the right of the centre, you would enter 45:

• Y-discharge angle (°) - the discharge angle between the local surface Y-axis and an inward facing normal to the surface. At the surface level, taking a normal view from the inside of the zone to the surface, the Y-axis discharge angle is positive between the normal and the positive Y-axis (i.e. the axis pointing upwards) and negative between the normal and the negative Y-axis (i.e. the axis pointing downwards). For example to define a discharge angle of 45°  pointing downwards from the centre of a diffuser, looking at it from the inside of the zone, you would enter –45. On the other hand, to define a discharge angle of 45°, pointing upwards from the centre, you would enter 45:

• Minimum discharge velocity is the minimum required velocity at the face of the diffuser. If the minimum discharge velocity cannot be achieved by combining the specified flow rate with the face area of the diffuser as drawn, supply diffusers are created using a number of elements, the areas of which are determined by combining the specified volume flow rate together with the minimum discharge velocity. The maximum linear dimension of each component element is currently 'hard-set' at 0.2m. If the minimum discharge velocity is set to a very small value (e.g. 0.01m/s), a single element will be created spanning the entire area of the diffuser as drawn and the actual discharge velocity will be determined from the geometry of the diffuser face and the supply volume (i.e. v=V/A for orthogonal diffusers) - the actual discharge velocity is displayed for diffusers on the CFD data tab at the opening level.
  
  

Note 1: the minimum discharge velocity is an important setting in that the diffuser supply velocity in conjunction with the diffuser discharge angle(s) can determine the flow pattern in a space.

Note 2: DesignBuilder CFD can only guarantee minimum discharge velocities for supply diffusers that are located on orthogonal surfaces, that is surfaces that are parallel to the one of the 3 grid axes.

2–Four-way

• Boundary temperature - temperature of the air entering the space.
• Flow rate - the supply flow rate.
• Multi-way diffuser discharge angle (°) - the discharge angle is the angle between the downward pointing normal and the diffuser jet.
• Minimum discharge velocity - the minimum required velocity at the face of the diffuser. Four-way diffusers are created using four separate elements, one located at each edge of the diffuser, the area of each being determined by combining the specified volume flow rate together with the minimum discharge velocity.

3–Two-way

• Boundary temperature - the temperature of the air entering the space.
• Flow rate - enter the supply flow rate.
• Multi-way diffuser discharge angle (°) - the angle between the downward pointing normal and the diffuser jet (see illustration in ‘Four-way’ section).
• 2-way diffuser discharge direction - the local surface axis along which the diffuser is to discharge.

• Minimum discharge velocity - the minimum required velocity at the face of the diffuser. Two-way diffusers are created using two separate elements, one on either side of the diffuser along the discharge axis, the area of each being determined by combining the specified volume flow rate together with the minimum discharge velocity.

4–Extract

• Flow rate - the extract flow rate.

5–Temperature

• Boundary temperature - temperature for the patch.

6– Flux

• Heat flux - heat flux for the patch.

Note: Generally, you should ensure that there are at least three grid cells between a supply boundary and an obstruction (such as a wall, component or assembly), otherwise, during simulation, the flow can ‘lock’ and you will see the mass residual plot forms a flat line with no convergence