Heat Exchangers: Characterization for CFD

Heat exchangers are a common component in the AEC industry. When the intent of an analysis is not to design the heat exchanger itself, characterizing it to only account for its physical impact in a simulation will reduce overall complexity.

In developing a strategy for characterization, the first step is to determine the general type of heat exchanger:

Active heat exchangers contain a momentum source such as a fan or blower which moves the fluid to be conditioned through the unit.

Passive heat exchangers have no momentum source. A passive heat exchanger relies on natural convection currents to move air across the unit.

Active heat exchangers (left) have a fan to move fluid over the fins.  Passive heat exchangers, such as this hydronic baseboard heater (right), depend on buoyancy effects (i.e., warmer air rising) to pull air across the fins.

Active heat exchangers (left) have a fan to move fluid over the fins.  Passive heat exchangers, such as this hydronic baseboard heater (right), depend on buoyancy effects (i.e., warmer air rising) to pull air across the fins. 

Although they both transfer heat, active and passive heat exchangers are different enough to require separate strategies for characterization.

Active Heat Exchangers

The many small fins and tubes of an active heat exchanger contain more detail than is necessary for the majority of AEC applications.  Unless the design objective is to optimize the heat exchanger itself, this detail must be substituted with much simpler geometry to keep the overall simulation practical.  

Identify the Goals of the Characterization

The physical effects that need to be represented are as follows:

  • An active heat exchanger moves existing air in the system with a fan or blower.
  • Thermal energy is added or removed from the fluid going through the heat exchanger.

Defining Characterization Method

In Simulation CFD, there are two primary methods of representing a heat exchanger:

  1. Heat Exchanger device: Options for both flow rate and energy transfer are contained in this single device.
  2. Internal Fan device: A heat generation boundary condition must be added separately to the fan volume to account for energy transfer.

In both of these methods, the complex inner detailsof the actual heat exchanger are replaced with a simple CAD volume.  Detailed information on heat exchangers is available at the following resources:

  • Common AEC components that are typically characterized by heat exchangers are listed here.
  • Heat transfer variation methods are described at the bottom of this page.
  • Modeling guidelines for heat exchangers can be found here.
  • Results visualization of heat exchanger devices.

The choice between using a Heat Exchanger or Internal Fan device will depend on simulation parameters and personal preference.

  • Heat Transfer Variation
    • The Internal Fan option requires the amount of heat transferred (e.g., Watts) to be specified by a heat generation boundary condition.
    • The Heat Exchanger device has additional variation methods which may be more convenient (e.g., the Temperature Change variation option allows the user to directly enter the temperature rise or drop).
  • Geometry
    • The Internal Fan has no special requirements.
    • The Heat Exchanger can only touch the rest of the model at the inlet and outlet faces.  The side faces cannot touch the meshed model, so parts (e.g., housing) in those locations must be suppressed or removed.
  • Quantity
    • Multiple Internal Fan devices, with the same flow rate and direction, can be defined in a single operation.
    • A Heat Exchanger device can only be defined one at a time.  This adds up to a lot of mouse clicks when there are a large number of heat exchangers in the simulation (e.g., factory with dozens of rooftop conditioning units)

To see an example of using both an Internal Fan and a Heat Exchanger in the same simulation, please go to the Exercise in the Data Centers section.

Internal Fan (1) and Heat Exchanger (2) devices are both used in this case to move and heat the incoming air.  Note that results visualization is not available for the Heat Exchanger; it is depicted as a void in the model.

Internal Fan (1) and Heat Exchanger (2) devices are both used in this case to move and heat the incoming air.  Note that results visualization is not available for the Heat Exchanger; it is depicted as a void in the model.

Gathering Necessary Inputs

The manufacturer’s specification sheet should provide the flow and heat transfer information required as inputs.  Otherwise, physical testing will be required to obtain the flow rates and difference between the inlet and outlet temperatures.

Verification

The characterization of the heat exchanger can be validated by reviewing the results and sampling the flow velocity and temperatures leaving the device.

Passive Heat Exchangers

This is a short section of a common radiator that would be mounted on or near a wall to condition a room.

This is a short section of a common radiator that would be mounted on or near a wall to condition a room.

Similar to an active heat exchanger, the fins and tubes of most passive heat exchangers can add too much geometric complexity to keep a simulation practical.  For example, a baseboard heater that extends along the length of wall can have hundreds or even thousands of fins.  

Identify the Goals of the Characterization

The physical effects that need to be represented are as follows:

  • Thermal energy is added or removed to the fluid moving in and around the fins of the heat exchanger.
  • The air is restricted, but not totally blocked, from moving through the heat exchanger.

Defining Characterization Method

To achieve the characterization goals, a resistance material can be used in combination with a heat generation boundary condition to more efficiently represent the passive heat exchanger.

The fully detailed radiator (left) can be represented with a simple volume (right) defined as a resistance material with an applied heat generation.

The fully detailed radiator (left) can be represented with a simple volume (right) defined as a resistance material with an applied heat generation.

Gathering Necessary Inputs

The radiator specification sheet should provide the energy value to input for the boundary condition.  For the resistance region, the geometric flow obstruction needs to be determined.  The most common method is to input the free area ratios for the resistance region by taking actual measurements or performing section area analysis in a CAD tool.

Verification

A simple air space around a radiator and the same model with a resistance block can be compared to see similar velocity and thermal profiles.

The global performance characteristics of this room are essentially the same for a fully-detailed radiator (left) and the characterized version (right).

The global performance characteristics of this room are essentially the same for a fully-detailed radiator (left) and the characterized version (right).

Although the results in the immediate vicinity of the passive heat exchanger are not exact, the overall effect to the domain is the same. 




The mesh on the detailed radiator (up) is very fine to capture the thin plates, while the simpler version (down) only requires a fraction of the mesh.  This can make a substantial difference in complexity for an AEC space that has long passive heat exchangers.