A Simulation CFD Exercise in Transient Simulation

To conserve energy in the winter, the thermostat for an office space is set to 60F during weekends. In this tutorial, a transient simulation will be used to observe the office heating up to normal temperatures for when the occupants arrive.

Learning Objectives

  • Gain experience assessing the validity of results.
  • Compare designs side by side based on simulation results.
  • Determine the effects of air exchange rates on temperature gradients and energy consumption.

NOTE:  This exercise requires the Advanced, Motion or Sim 360 options for Simulation CFD to access the transient solver.

Learning Objectives

  • Gain experience setting up and solving a transient simulation.
  • Determine time required for the office to be heated from 60F to 70F.

Assumptions

  • Empty office space with uniform air temperature of 60F.
  • Air handler is off when thermostat is raised to 70F.
  • Buoyancy effects are not considered; simulation is run in forced convection mode.
  • Overcast sky with minimal solar radiation.
  • Floor and inner three walls are perfectly insulated and do not transfer heat.

Simulation Parameters

  • Total flow rate:158 cfm (~4 air exchanges per hour)
  • Winter conditions
    • Room Inlet Air Temperature: 100F
    • Outside Air Temperature: 32F


Item Location Description U-Factor

(BTU/hr-ft2-R)
1 Window Single-pane, steel frame 1.18
2 Inlets 79 cfm each @ 100F  
3 Outlet P=0 psig  
4 Wall R13, wood studs 0.08
5 Door 1" wood 0.64
6 Ceiling Flat, insulated 0.16

Simulation Process

The focus of this exercise is setting up a transient simulation using the Solve dialog.  The starting point is a share file where other settings, such as boundary conditions, material properties and meshing, have already been defined.  The solver settings use the best practices for forced convection as detailed here in the AEC Setting section of the Solver module.

1. Download the CFD-Exercise 5.cfz share file to a local hard drive folder. 

2. Load share file.

a. Start the Simulation CFD interface and click Open in the upper left ribbon, then navigate to and open the CFD-Exercise 5.cfz share file.

Initial conditions will now be assigned to the air volumes.  By default, the starting temperature of the air volumes will be the lowest temperature of the applied boundary conditions, which in this case is the outside air temperature of 32F.  Initial conditions are used to override the default assumption so that the starting air temperature can be designated 55F.

3.    Assign initial conditions to the air domains:

a.    Click on Initial Conditions button in the top ribbon (or click the Initial Conditions node in the Design Study bar to the left).

b.    In the Selection menu in the ribbon, click Volume to ensure that volumes will be selected (not surfaces).

c.     Click Select All in the selection menu to select all 4 volumes representing the air domain.  By selecting volumes, every node of the mesh in the volumes will be assigned the initial condition.

d.    Click on Edit in the ribbon Initial Conditions menu.

e.    In the Initial Conditions dialog that pops up, click in the column to the right of Type, scroll down the pull-down list and click on Temperature.

f.    Set the Temperature to 60 and click the Apply button. 


Initial Conditions are used to assign the starting temperature of 60F for the air volumes.

TIP: Unlike a boundary condition, which persists for the duration of a transient simulation, an initial condition is only enforced at the starting point of the simulation.

Monitor points are a low impact means of tracking transient results at a specific location over time.  For simplicity, this tutorial will use a single monitor point in the middle of the office.  In actual practice, it is common to have multiple monitor points at strategic locations to have more comprehensive sampling of the space.

4.    Set up a monitor point to track the temperature at a point:

a.    Right click in the graphics window and click on Monitor point …

b.    In the Runtime Monitor Points dialog, adjust the X, Y and Z sliders or type in the coordinate <-75,0,48> to place the point in the middle of the office room.

c.    Click in the Name: field and type in “OFFICE”.

d.    Hit the green “+” button to add the point.

e.    Close the dialog.


The Monitor Point is located in the geometric center of the office section of the space.

5.    Adjust solving parameters to run for a total transient duration of 15 minutes:

a.    Click the Solve button in the ribbon.

b.    In the Control tab of the Solve dialog, set the Solution Mode to Transient.

c.    Decrease the Inner Iterations from 10 to 1.

d.    Set the Time Steps to Run to 900.


Transient settings in the Control tab of the Solve dialog box.

e.    Click the Solution Control button

f.    Uncheck  the Intelligent Solution Control checkbox (this is necessary or the solver will modify the time step;  more information can be found here)

g.    Click the OK button in the Solution Controls dialog

h.    Click the Solve button in the Solve dialog to initiate the simulation.

6.    Results Visualization

Monitor point data is available in the Output bar and can be visualized while the solution is in progress.

a.    If needed, click on the Convergence Plot tab in the Output Bar

b.    In the upper right corner of the convergence plot click on the Global button and use the pulldown menu to select the OFFICE monitor point.

c.    Now click on the next button down and use the pulldown menu to change the results from All to Temperature.


Temperature results at monitor point location over time.

d.    Click on the Table tab of the convergence plot.  The temperature at the monitor point location first hits 70F in just over 9  minutes.


Tabular data for the monitor point temperature plot.  The monitor point first reaches 70F in 562 seconds.  Results may vary slightly due to minor differences in mesh or processor rounding errors.

Summary

A transient simulation was used to evaluate the temperature changes over a short period of time, which is not possible with a steady-state simulation.  This kind of data can be used to assess the energy implications of lowering the thermostat during periods when the space is not being used.

NOTES: 

1.    If the office is filled with dense equipment such as steel filing cabinets, the overall thermal capacitance (i.e. resistance to sudden change in temperature) will increase and the room will take longer to heat up.

2.    Most residential furnaces have a lower limit of 55F or 60F on the thermostat.  If the return air is any colder, condensation may tend to form which can cause corrosion and premature failure of furnace components.