Boundary Conditions for Internal CFD Analyses

Internal boundary conditions can be defined in three ways:

 

  1. Surface temperatures can be defined for all zone surfaces including windows and other openings.
  2. Zone surface boundary conditions including supply diffusers, extract grilles, temperatures and heat fluxes can be added in the form of surface patches using a similar method to that used for adding windows and doors.
  3. Component blocks and component assemblies can be defined as temperature or heat flux boundaries or they can also have boundary condition patches added as described above for Zone surfaces.

 

In all cases the CFD boundary conditions can be edited on the CFD Model data tab.

Note: The easiest way to accurately include the effect of radiation heat transfer in CFD calculations is to run an EnergyPlus simulation first and then import surface temperatures from the EnergyPlus simulation results as CFD boundary conditions using the CFD boundary conditions dialog. The EnergyPlus surface temperatures include the effect of radiation heat transfer, however in order to arrive at the same heat balance in the CFD calculation, it is necessary to include the convective portion of any specified heat gain used in the EnergyPlus simulation as specified boundary conditions in the CFD model (using assembly flux, heat flux surface patches, etc).

Default Wall and Window Temperatures

All zone surfaces within a model have default wall and window boundary temperatures. These boundary temperatures are inherited from the building level down to the zone surface level and may be set at any intermediate level. This mechanism provides a convenient method for defining wall and window temperature boundary conditions throughout a building model in the absence of imported EnergyPlus boundary data.

 

Default inside and outside wall and window boundary temperatures can be accessed under the CFD Boundary header on the CFD tab.

Note: You can include multiple zones in a single domain, but in this case it is important to ensure that all internal partitions and floors/ceilings separating zones in the domain have finite (non-zero) thickness. Any internal surfaces in the domain with zero thickness will be ignored and their boundary conditions will not take effect.

Zone and Component Block Surface Boundaries

Supply diffusers, extract grilles, temperature and heat flux patches can be added to zone and component block surfaces using the Add CFD boundary tool.

 

To add a surface boundary, move down to the surface on which you want to locate the boundary, either by selecting it from the model navigator or by navigating to it in the Edit screen. At the surface level, you can then select the Add CFD boundary tool from the toolbar.

Note: If you do not set a boundary condition for an internal block-level component block, when it comes to calculating the MRT distributions DesignBuilder assumes that the component block surfaces are at the mean room temperature.