Air Handling Systems: Characterization for CFD

An air handler is a typical piece of equipment that needs to be represented in many AEC applications. When the intent of the analysis is something other than designing the air handler, strategically reducing geometric complexity reduces run times without sacrificing results fidelity.

Air handlers are used to force conditioned air through the ducting and other components of an HVAC system.  For example, your home would have an air handler in the attic or basement which circulates the heated or cooled air throughout the entire ducting system.

Identify the Goals of Characterization

The key operating characteristics that need to be captured are the flow and heat transfer performance of the air handler.

Defining the Characterization Method

A heat exchanger device, a fan with heat generation or set of boundary conditions can be used to characterize an air handler, depending on the analysis scope.  The flow chart below describes three types of analysis alternatives; a full closed loop model (air handler, ducting, and conditions space), boundary conditions only (no ducting), and modeling the ducting fluid volume separately.

Flow chart to determine characterization method of air handler.

Flow chart to determine characterization method of air handler.

Each of these is designed to accommodate different design insight needs as detailed further below.

Option 1: Closed Loop Model


This office simulation represents a closed loop model where the air handler (blue block) and ducting are included with the office rooms.

In the Closed Loop Model, the air handler and ducting are included with the primary fluid domain to represent the entire system in one simulation.

The advantage of this approach is that it can directly account for pressure losses in the ducting and any flow distribution imbalance (e.g., the 2 supplies flow at different rates) from a less than ideal ducting design.       

The primary disadvantage is that the ducting adds elements that increase solver time.  This makes it the most complex of the 3 characterization options. 

Option 2: Ducting Boundary Conditions

When the focus is on the internal AEC space, the ducting is complex, and the supply flow rates are known, the ducting and air handler can be omitted from the analysis.  This is very typical of HVAC layout analyses, an example can be found in the HVAC Layout Exercise.  The air handling system is substituted with the proper boundary conditions which allow air to move into and exit the simulation space.


In this office space, the air handler and ducting have been removed.  The supplies (2) are represented with velocity of flow rate boundary conditions.  To allow air to exit the room, a pressure opening is applied at the return (1).

 

The advantage of this approach is that it keeps the focus on the AEC space and eliminates the complexity of the ducting system, which speeds up solver times.  As a result, this is the most popular option used when simulating large AEC applications such as buildings and warehouses. The more complex the ducting, the greater the benefit that this alternate approach will have compared to modeling the full closed loop. 

A potential disadvantage is that the boundary conditions representing the inlet flows are either assumed or based on measured values.  If the air handler system is modified or not designed properly, then the flow rates may vary from the actual application, which will impact the flow performance in the space.

Option 3: Ducting Fluid Volume

To minimize energy consumption, the ideal ducting design will supply the correct amount of flow where needed while also minimizing frictional losses.  When the focus is on the ducting design, the model becomes the opposite of the Ducting Boundary Condition alternative and only the ducting is included in the simulation.

The office space ducting system is simulated to optimize the ducting design.  The blue block represents the air handler (a fan material).  Pressure openings are used at the end faces (1) of the ducting runs.

The office space ducting system is simulated to optimize the ducting design.  The blue block represents the air handler (a fan material).  Pressure openings are used at the end faces (1) of the ducting runs.


This approach has the same advantage as the Closed Loop Model in that it accounts for the ducting performance directly; however, it eliminates the added complexity of the simulation space.  This type of model will assist the designer in sizing the air handler and strategically routing the ducts to minimize losses.