The Thermal Environment in Simulation CFD

In order for Simulation CFD to make air or equipment temperature predictions, it requires the input of thermal boundary conditions to define the environment surrounding the simulation model. The air supply temperature, equipment heat load, and U-factors for construction components are all examples of known quantities used as thermal boundary conditions used in Simulation CFD.

Before beginning, it may be helpful to review the following online help topics:

When defining thermal Boundary Conditions, it’s best to start with openings—and inlets in particular—as no thermal boundary conditions are required on outlets.  Outlet temperature should be solved for rather than defined.

The inlet thermal boundary condition should be a known temperature value such as a HVAC supply temperature or an outdoor ambient temperature.  This thermal boundary condition alone will not produce thermal gradients.  Other thermal boundary conditions are required to transfer heat, which is a process related to a temperature differential.

Film Coefficients

Film coefficients are prescribed heat transfer rates.  They are used to define the amount of heat transferred between the simulation model and its surrounding environment, without having to model the surroundings.

Film coefficients in Simulation CFD are the equivalent to U-factors, making them a direct replacement for the geometry of externally facing components such walls, windows, and doors.  Additional information can be found in the Walls and Windows Component Characterization Module.

1: Supply - Velocity BC & Temperature

2: Return - 0 gauge

3: U-factors (All surfaces with green stripe)

Top: Boundary conditions of a small office space.
Bottom: Temperature results based on boundary conditions shown above. All surfaces use a film coefficient (U-factor) for brick while the surface labeled 1 is defined with a window’s U-factor
Note: The human is extremely cold!  This is because the human has no Heat Generation Boundary Condition applied.   

Heat Generation

Accounting for heat generating components in Simulation CFD for AEC applications consists of defining Total Heat Generation Boundary Conditions.

Heat generation is typically used to represent the heat load from occupants, equipment (computers or various types of machinery), and lighting.  The heat load on volumes such as those can be applied directly to the component (if modeled explicitly) or applied to the air space without modeling the component.  An example of this is found in the Occupants Section of the Component Characterization Module.

Temperature results of office space based on inlet temperature, film coefficients (U-factors), and total heat generation boundary conditions for the human and computer.  All surfaces use a film coefficient for brick while the surface labeled as number 1 represents a window.

Thermal Boundary Condition Combinations

Properly constraining a simulation with boundary conditions is also a consideration for thermal inputs.  Typically the temperature at the outlet and the temperature of components within the simulation should be solved for (unknown and undefined quantities).  This is most commonly accomplished by defining an inlet temperature and component heat loads.

Properly Constrained Thermal Boundary Conditions


1.     Temperature - Inlet
2.     Film Coefficient - U Factors on all external surfaces of air domain (only top surface labeled)
3.     Total Heat Generation - Equipment (Computer)
4.     Total Heat Generation (Human)

Properly defined flow conditions are assumed

The most commonly under- and over-constrained thermal boundary combinations are presented below. 

Forgetting to define a temperature at the inlet is the main cause of a thermally under-constrained analysis

Thermally Under-Constrained

Supply

No thermal boundary condition


Return

No thermal boundary condition

Properly defined flow conditions are assumed

Thermally over-constrained analyses typically define the temperature at the outlet or assign too many conditions to a single entity such as defining a film coefficient and a temperature to the same surface.

Thermally Over-Constrained

Supply

Temperature


Return

Temperature

Supply

Temperature


Film coefficients and temperatures applied to the same surface(s) of air volume.

Return

No thermal boundary condition

Properly defined flow conditions are assumed