Exercise: HVAC Layout in Simulation CFD

hvac_layouts4_exercise_image1.jpg

Throw patterns from the HVAC system studied in this exercise.

This hands-on tutorial will guide the user through best practices of setting up an HVAC layout for a cafeteria.

Learning Objectives

  • Set up an HVAC layout analysis efficiently.
  • Interrogate results to validate proper setup.
  • Review HVAC layout results.

Assumptions

The simulation will have several simplifying assumptions to reduce overall complexity and expedite completion of this HVAC Layout tutorial.

  • Interior walls, floor, and ceiling will be considered adiabatic (well insulated).
  • Radiation will be assumed to be negligible.  The room is on the bottom floor and the sun will be directly above the building during heavy usage of the cafeteria.
  • Conduction through ducting will be considered negligible.

 

Cafeteria space showing location of returns (1) and exposed ducting system (2).  The cafeteria is located on the corner of the ground floor for the building.  The two walls in front (3) are window walls; the 2 back walls (4) are inner walls to other space

Cafeteria space showing location of returns (1) and exposed ducting system (2).  The cafeteria is located on the corner of the ground floor for the building.  The two walls in front (3) are window walls; the 2 back walls (4) are inner walls to other spaces in the building.

NOTE:  Setup will begin from a support file and it is assumed that general click and pick skills have already been obtained.  This exercise is intended for a user that has already learned the basics of rotating, hiding, selecting, and applying settings to parts. 

Simulation Parameters

  • Inlets from ducting:
    • 825 L/s total (8.7 air changes per hour)
    • Supply temperature of 65F
  • Energy from occupants in the main room is 5000 watts
  • Window U-factor is 4 W/m^2-K
  • Mixed Convection settings

Simulation Process

  1. Download the Exercise-HVAC-LAYOUT.cfz share file to a local hard drive folder.
  2. Open the share file
    • Open up the Simulation CFD interface and click Open in the upper left ribbon to open the Exercise-HVAC-LAYOUT.cfz.

      NOTE:  Materials were already applied in the share file.  The materials are Air [Variable] for the room and concrete for the solids;  any solid material could be used because the solids will be suppressed from the meshing.
  3. Apply Boundary Conditions

    • Click Boundary Conditions in the ribbon.
    • Click on the Groups:Returns node in the design study bar.

      TIPGroups can be very useful for the setup of analyses with many inlets and outlets.  More information can be found about groups here.
    • Apply 0 pressure at the end of the return extensions.

      Above depicts the two surfaces that will be the outlets.

      Above depicts the two surfaces that will be the outlets.

    • Apply Film Coefficients with 4 W/m2/K with a reference temperature of 90F to the surfaces that are the exterior windows of the building.

      Above shows the two exterior window surfaces of the building that would be subject to outdoor influence.

      Above shows the two exterior window surfaces of the building that would be subject to outdoor influence.



      To account for the heat of occupants in the space, their total heat generation of 5000 Watts (50 occupants times 100 watts each) will be applied to the air volume as a boundary condition. 

      NOTE: The heat load of lighting and solar energy coming through the windows could also be added to the air volume.

    • Apply Total Heat Generation of 5000 W to the main air volume.
      • Click volume selection.
      • Select the main air volume.
      • Click Edit.
      • Enter 5000 W into the Total Heat Generation quantity.
      • Hit Apply.
    • Apply inlet conditions.
      • Hide the main air volume after applying the heat load.
      • Click on the Groups:Supplies node in the design study bar.
      • Apply a temperature of 65 F.
      • Click Select Previous.
      • Click Edit.
      • Change the boundary condition Type to Volume Flow Rate.
      • Click on Reverse Normal.
      • Set Units to l/s.
      • Set value to 125.
      • Click Apply.

        Prior to pushing the reverse normal button the volume flow rate is pulling air into the ducting (shown on the left).  The desired direction of the flow is into the room (shown on right).

        Prior to pushing the reverse normal button the volume flow rate is pulling air into the ducting (shown on the left).  The desired direction of the flow is into the room (shown on right).



        All of the solids will be suppressed in most HVAC layout analyses unless they are generating or transferring heat.  In this case the solids were already suppressed in the support file. 

        NOTE:  Solids that have flow boundary conditions on them must be suppressed or the boundary condition will not be taken into account

  4. Apply Mesh settings and refine the mesh on the supplies and returns.

    • Click on the Mesh Sizing icon in the ribbon.
    • Click Autosize.
    • Click Edit.
    • Click on the Groups:Supplies node in the design study bar.
    • Click on the Use uniform button.
    • Adjust the refinement to 0.5 and click the Enter key.
    • Click Apply changes.
    • Click on the Groups:Returns node in the design study bar.
    • Click on the Use uniform button.
    • Adjust the refinement to 0.5 and click the Enter key.
    • Click Apply changes.
    • Click Apply to close the dialog.

      Mesh Sizing nodal preview before (left) and after (right) modifying the mesh.

      Mesh Sizing nodal preview before (left) and after (right) modifying the mesh.

  5. Solve the model. 

    • Note that the proper solver settings for mixed convection have already been applied; refer to the AEC Solver Settings page for more details.
    • Hit Solve
  6. Validate model setup.

    • Change the global result to Temperature.  Then rotate the model as shown below.

      Global temperatures shown on the main air volume.  Notice that the window surfaces are the warmest temperatures on the volume.

      Global temperatures shown on the main air volume.  Notice that the window surfaces are the warmest temperatures on the volume.  

    • A common setup issue in a large HVAC layout analysis is to have an inlet assigned in the wrong direction.  Global Vectors should be used to verify the flow direction input.
      • Go to the View Tab of the ribbon.
      • Select Surface Blanking.
      • Right click on the top surface and click Hide.
      • Go to the Results Tab of the ribbon.
      • Set the Global Vector: to (1) Velocity Vector.

        Notice that the global vectors point in from the supplies and out at the outlets.

        Notice that the global vectors point in from the supplies and out at the outlets.



        The wall calculator will be used to verify the film coefficient input and quantify the heat added to the room from the exterior environment.

    • Select the Wall Calculator.
    • Click the Checkbox for Heat flux and Film coefficient.
    • Note: Film coefficient can only be selected after the analysis finishes.
    • Select the outer 2 walls that represent the windows.
    • Click Calculate.
      • The total heat flux from the ambient environment should be about 920 W, and the film coefficient should be the 4 W/m^2/K entered as a film coefficient.

        Above shows the wall calculator Selection and Results (left) and Output (right).

        Above shows the wall calculator Selection and Results (left) and Output (right).

  7. Evaluate the performance of the HVAC layout.

    • Add a cut plane at the level of people, and look at the temperature.
      • Go to the View Tab in the ribbon.
      • Click Apply View.
      • Select 1 m Plane.xvs.
      • Click Open.

         Above shows a cut plane located 1 meter off of the floor.  The coolest temperatures are found in the center of the space.

         Above shows a cut plane located 1 meter off of the floor.  The coolest temperatures are found in the center of the space.

    • To quantify the temperature difference we will add an XY Plot.
      • Click on the Results Tab of the ribbon.
      • Click on XY plot.
      • Click the Read from File radio button.
      • Click Browse.
      • Click 1 m Plot.xyp.
      • Click Open.
      • Click Plot.
      • Close the XY Plot dialog box and observe the plot.

        Looking at the XY Plot we can see that there is a 1.5 C (about 2.5 F) temperature difference in the main area for occupants.

        Looking at the XY Plot we can see that there is a 1.5 C (about 2.5 F) temperature difference in the main area for occupants.  



        Understanding the cause of this gradient will drive an informed design modification. 

    • Create an Iso surface to investigate the cold area in the middle of the room.
      • Close the XY-plot.
      • Click on Iso Surfaces.
      • Click Add in the ribbon.
      • Click Edit.
      • Type in 75 and hit Enter (this will show the surface of everywhere that is 75 F).

Here we can see that the throw of cold air goes all the way across the room and is pulled back towards the return.  Notice the amount of cooler air against the window; this will also cause more energy to enter the space in that location from the outdoors.

Here we can see that the throw of cold air goes all the way across the room and is pulled back towards the return.  Notice the amount of cooler air against the window; this will also cause more energy to enter the space in that location from the outdoors.

Conclusion

After verifying that the settings were applied correctly, a potential design flaw was observed.  Occupants would experience a noticeable temperature difference walking across the room.  Another layout design should be considered to reduce the throw of the cold air or provide a more uniform temperature distribution.  This can also affect the energy efficiency of the room as reducing the throw will also remove the buildup of cooler air against the window. 

This can be verified by looking at the Global Wall Heat flux (shown below), the highest rate of heat transfer into the room is the same location where the iso surface was hitting the wall.

The cold air blowing up against the window is the most inefficient area in the model.  A reduction in that effect will affect operating cost of the room.

The cold air blowing up against the window is the most inefficient area in the model.  A reduction in that effect will affect operating cost of the room.