Evaluating CFD results of an HVAC Layout

In this section, the HVAC layout of a cafeteria will be evaluated by using the results visualization and data extraction tools of Simulation CFD.

The air movement and temperature distribution of the space will be quantified with Part results and XY plots, and then visually inspected with Planes and ISO Surfaces to understand performance and guide design decisions.

HVAC Layout Overview


Conditioned air is supplied (1), interacts with the space (2), and returned to the ventilation system (3).

In most HVAC layout designs, properly conditioned air is supplied, interacts with the space, and then returns to the ventilation system.

The HVAC Layout here consists of seven (7) supplies and two (2) returns and is simulated by defining the following:

  • Flow rate at supplies
  • 0 pressure at the returns
  • Heat generation (from occupant, lighting, and solar influence) on the air space volume
  • Film coefficients on the two external windows

Temperatures of the Air Space

The average and maximum temperatures of the air space being conditioned are critical values that are easily extracted in Simulation CFD (as “Parts” results).  Parts results will output these values and can also be added to the Decision Center as a critical value to be compared against other design scenarios.

Parts results display the min, max, and average temperatures of the selected volume (air space).

Parts results display the min, max, and average temperatures of the selected volume (air space).

The air space part is added to the Decision Center to compare with future designs.

The air space part is added to the Decision Center to compare with future designs.

TIP: The parts dialog output value is based on the unit system defined by the simulation.  The Decision Center provides an option to change the part output units.

The temperatures of this space appear warmer than desired; before considering a design revision, the results will be evaluated further to determine why the temperatures are so high and how they can be reduced.

Locating the Hottest Locations with ISO Surfaces

Now that we know what the hottest and coldest temperatures are, we can use ISO surfaces to visualize where they occur in our model.

An ISO surface visualizes all of the air at a certain temperature, and viewing these results in 3D helps us see temperature distribution in the space, and isolate the highest temperatures in the model.

TIP: Stepping through various values of an ISO surface (dragging the slider in the dialog) can help visualize the movement of flow, thermal, and pressure gradients.






Temperature values are isolated at 85, 82, 80, 78, and 75F from top to bottom respectively.  The highest temperatures are found near the two windows and don’t penetrate the space until 78F.  

The ISO surface visually depicts a large volume at 78F, which is in agreement with the Parts result output shown earlier.  The windows in this scenario are exposed to 90F conditions and appear to have a substantial influence on the space.

Using Wall Results to Quantify Window Thermal Energy

Wall results for windows (surface IDs 62 and 83 respectively).

Wall results for windows (surface IDs 62 and 83 respectively).

The Wall Calculator can quantify the heat transfer through the windows.  

The Wall results show an average temperature of 85F (~29C) at the windows, which transfer approximately 1 kW of thermal energy into the space (700W through surface 62 and 325W through surface 83).  The Film Coefficient (U-factor) defined on those faces is confirmed here as 4 W/m2/K and represents a double pane window with poor thermal performance.

Supply Throw Patterns

The 75F iso surface revealed a visualization of the supplies throw pattern worth further investigation.  The diffusers appear to converge their flow toward the center of the room, minimizing coverage throughout the rest of the space. 

Section Views and XY plots

A velocity plane and XY plot can be used to visualize and quantify the supply throw patterns.

Velocity plane used to visualize supply throw pattern

Velocity plane used to visualize supply throw pattern.

XY plot extracting data across the room at one (1) meter above the floor.  The highest velocities occurring in the center of the space (black rectangle) are 2-3 times higher than the rest of the space. This is a potential concern.

XY plot extracting data across the room at one (1) meter above the floor.  The highest velocities occurring in the center of the space (black rectangle) are 2-3 times higher than the rest of the space. This is a potential concern.

The variation in velocity will consequently influence the thermal distribution in the space. The thermal distribution can be visualized and quantified in the same manner as the velocities shown above.


XY plot extracting temperature data across the room at one (1) meter above the floor.  Note that the lowest temperatures are found in the center of the space, where the cool air supplies converge their flow.  A temperature variation of approximately 3F is identified here.

Plan Views

A plane located one (1) meter above and parallel to the floor will help depict the gradients that occupants would experience across the entire space.

Velocity (top) and temperature (bottom) planes are located one (1) meter above the floor.  These planes portray the highest velocities and lowest temperatures occurring in the center of the space.


The 75F ISO surface from earlier is displayed with the previously shown temperature plane located one (1) meter above the floor.  Black arrows have been added to help visualize the flow directions that cause the temperature gradients in the space.

The velocity and temperature variation in the space appears to be dependent on the location and performance of the supplies and returns.  Adding the 75F ISO surface to the temperature plane located one (1) meter above the floor will help visualize this relationship.


HVAC Layout Results Summary

Inspecting the performance of this HVAC layout with Simulation CFD has revealed a number of design deficiencies:

  1. The overall temperature of the space is too high for occupant comfort.
  2. The windows transfer a large amount of thermal energy and are the location for the highest temperatures found.
  3. Large temperature and velocity variations exist across the space.

Addressing the deficiencies identified above would improve the performance of this space.  The results and conclusions shown will inform the following types of modifications:

  • Increasing the amount of air supplied.
  • Window selection.
  • Supply location and diffuser design / selection.

Each view and extracted data set can be compared, in the Decision Center, across multiple design modifications to converge on the most suitable design solution.


A presentation image (CFD superimposed over CAD) compares the velocity distribution of the initial design (left) vs. an HVAC layout with an additional center duct (right).