Advanced Tutorial: Thermal Comfort in a Meeting Room

This article provides a step-by-step tutorial for the full thermal comfort assessment in a meeting room using CFD simulation, including models for convective heat transfer, radiation heat transfer, and age of air.

Overview

This tutorial teaches how to:

• Set up and run a convective heat transfer flow simulation.
• Assign boundary conditions, material, and other models to the simulation.
• Mesh the geometry with the SimScale standard meshing algorithm.
• Set up scalar species transport analysis for the Local Mean Age of Air computation.
• Set up Radiation heat transfer.
• Set up Thermal Comfort Parameters computation.

The typical SimScale workflow will be followed:

1. Prepare the CAD model for the simulation.
2. Set up the simulation.
3. Set up the mesh.
4. Run the simulation and analyze the results.

1. Prepare the CAD Model and Select the Analysis Type

First of all, click the button below. It will copy the tutorial project containing the geometry into your own workbench.

The following picture demonstrates what should be visible after importing the tutorial project.

You can notice an Open inner region operation performed on the geometry called Meeting Room. As the input CAD model included the room walls, occupants, and furniture, the operation was used to create geometry for the volume of air, which represents the negative of the room and everything inside.

The original geometry without the flow volume extraction is included for comparison, under the name Meeting Room (Original). If you want to learn the details of the operation, you can refer to the documentation page on Flow Volume Extraction.

1.1 Create the Topological Entity Sets

Topological entity sets are groups of faces created at this point to be used in assignments for boundary conditions and other concepts, during the simulation setup. They can be found at the right panel:

A number of sets are already provided in the project, but one set for the window is still missing. The picture below shows how to add one:

1. Create the entity set by first selecting the window face (highlighted in red),
2. then the ‘+’ icon at the left of Topological Entity Sets in the right panel.

In the pop-up dialog that appears, name the set Window and click ‘Create new set‘.

1.2 Create the Simulation

Now we can start with the simulation setup. Follow the steps presented in the picture below to create a new simulation:

1. Select the ‘Meeting Room’ geometry from the left panel,
2. then click the Create Simulation button of the dialog:

The simulation library window appears:

Here you can select the analysis type you need.

• Choose ‘Convective Heat Transfer’ and click ‘Create Simulation‘.
• Now, you will see a new simulation tree element with it’s default settings dialog open:

Perform the following changes:

• Activate Radiation for the simulation.
• Change turbulence model to k-omega SST
• Select passive species to 1.

Do not forget to click the check-mark button at the top to save the changes.

2. Setup the Simulation

In order to have an overview, the following is a description of the simulation model and conditions:

1. Heat transfer is performed through forced convection, natural convection, and radiation,
2. Conditions for summer with an ambient temperature of 30°C and relative humidity of 65%,
3. The inlet flow rate of 0.1 m3/s and 18°C at the top duct,
4. Output flow at the back duct,
5. The radiation heat load of 200 W/m2 at the window,
6. Conductive walls to an external temperature of 30°C for the external walls and window,
7. Adiabatic walls for the internal walls, ceiling, and floor,
8. Flux power heat source of 58.2 W/m2 for the occupants metabolic rate,
9. Adiabatic walls for the furniture,
10. The age of air is modeled through the Local Mean Age of Air (LMA) model by the integration of a transported passive scalar.

You can explore the corresponding faces for each condition by clicking the topological entity sets at the right panel.

2.1 Global Model

Select the ‘Model’ tree element to specify the scalar transport properties and gravitational acceleration. For the LMA (Local Mean Age of Air) model the following parameters are used:

• Turbulent Schmidt number: 1.0.
• Diffusion coefficient: 1e-9.
• And for gravitational acceleration, a value of 9.81 m/s^2 is used in the negative Z direction.

2.2 Material Model

To define and assign a material, please click on ‘+’ next to ‘Materials’. Doing so, the SimScale material library will pop up:

1. Select ‘Air’ from the materials library and click Apply.
2. As there is only one flow volume, it is automatically assigned.
3. Accept the selection with the check-mark button.

2.3 Boundary Conditions

Now we need to set up the boundary conditions. For the first boundary condition, the detailed procedure is explained and for the rest, only the description and relevant parameters are presented.

a. Flow Inlet

A flow velocity inlet boundary condition is created according to the following picture:

1. After hitting the ‘+’ button next to Boundary conditions a drop-down menu will pop-up where different types of boundary conditions can be chosen.
2. Select Velocity inlet from the list.

Now the setup for the velocity inlet boundary condition will pop up:

Please modify the following:

• Set an inlet volumetric flow rate of 0.1 m^3/s,
• Temperature of 18°C,
• Passive scalar 0 to model fresh air with zero LMA.
• For the assignment, select the inlet surface. You can find it as the predefined Inlet entity set in the geometry tree on the right side of the workbench.

b. Flow Outlet

For the flow outlet boundary condition follow the same procedure, but select a Pressure outlet. Leave all values as default and assign the Outlet Set (or the outlet sufaces), as shown in the picture:

c. Internal Walls

Internal walls are assigned as a wall boundary condition, and as are facing the inside of the building, are considered adiabatic. Create a wall boundary condition just like the former two and assign the Internal wall set. Set up the parameters as shown in the reference picture:

d. Ceiling

The ceiling is also assigned as an adiabatic wall boundary condition. Create a wall boundary condition and assign the Ceiling set. Set up the parameters as shown in the picture:

e. Floor

The floor is also assigned as an adiabatic wall boundary condition. Create a wall boundary condition and assign the Floor set. Set up the parameters as shown in the picture:

f. Window

For the window, a wall boundary condition is also used, but this time with a layer wall thermal model, convection to the exterior temperature, and an external radiation source to model sunlight. Create a Wall boundary condition and assign the Window set. Set up the parameters as shown in the picture:

g. Occupants

For the occupants, a wall boundary condition is also used, this time with a heat flux power source to model the metabolic heat generation rate. Create a Wall boundary condition and assign the Occupants set. Set up the parameters as shown in the picture:

h. Furniture

For the furniture, a wall boundary condition is also used, with adiabatic thermal behavior. Create a Wall boundary condition and assign the Furniture set. Set up the parameters as shown in the picture:

i. External Wall

For the external wall, a wall boundary condition is also used, with a thermal wall model and convection to the exterior. Create a Wall boundary condition and assign the External Wall set. Set up the parameters as shown in the picture:

2.4 Advanced Concepts

Under advanced concepts, you can define further advanced physical conditions to set up your simulation. For this tutorial, you need to define a passive scalar source.

a. Passive Scalar Source

For the LMA aging model, a Volumetric passive scalar source is used, as shown in the picture:

1. Expand Advanced concepts,
2. Click the ‘+’ button next to Passive scalar sources,
3. Select Volumetric passive scalar source

Assign the flow region to the concept and specify a Flux value of 1.

2.5 Numerics and Simulation Control

For the numeric solver parameters, the only change made is the addition of non-orthogonality correctors. This will improve the convergence for the tetrahedral mesh created by the Standard mesher algorithm. Setup the Number of non-orthogonal correctors as shown in the picture:

1. Click Numerics to display the solver parameters.
2. Set the Number of non-orthogonal correctors to 4.

Regarding simulation control, we keep the setup to the default values.

2.6 Result Control Items

Result control items are used to retrieve specific computations from the numerical solver. By using them, we can have a look at specific variables at specific regions by querying the computation and output of our quantities of interest.

a. Local Mean Age of Air

In order to measure the average LMA at the outlet face, a Result control item is used. It is created as shown in the picture:

1. Expand Result control,
2. Click the ‘+’ icon next to Surface data,
3. Select Area average.

Assign the Outlet set and check that the setup coincides with the picture:

b. Thermal Comfort Parameters

Thermal comfort parameters are also queried in the result control items, under Field calculations. Create the concept as shown in the picture:

1. Click the ‘+’ button next to Field calculations,
2. Select Thermal Comfort Parameters.

Setup the parameters as shown in the picture:

• In this case, the clothing coefficient is 0.5,
• the metabolic rate is 1.0
• and the relative air humidity is 65%.

A description of the computed quantities and resulting fields can be found at SimScale’s documentation page on Thermal Comfort Parameters.

3. Mesh

In the mesh setup, all settings are left as default. You do not need to click Generate, as the mesh will be computed as part of the simulation run.

4. Start the Simulation

Now that the simulation setup is complete, a new Simulation Run can be created to perform the computation. In the following picture, the whole tree setup is shown, and the item to create the simulation run is highlighted:

In the pop-up window, press Start to immediately begin he computation run.

The computation takes around one and a half hours to complete. If you can’t wait to see the results, at the end of the article there is a link to the completed version of the project.

5. Post-Processing

The following pictures showing the results were generated using the online post-processor.

The computed mean radiant temperature for the whole room is 22.7°C, with the following temperature distribution over the surfaces:

The temperature range is from 18°C to 43°C, with the highest values confined to small places. The mean radiant temperature of 22.7°C indicates a comfortable temperature in the room.

The predicted mean vote (PMV) thermal comfort index distribution is displayed in the following picture:

The value of the index is clipped to the recommended range of [-3, 3]. This way, we can visualize the areas that are below and above them, with blue and red colored regions respectively.

The predicted percentage of dissatisfied (PPD) thermal comfort index distribution:

The recommended value of the index is [0, 20], and we can see that most of the areas in this room are uncomfortable for occupants.

The mean Local Mean Age (LMA) at the outlet was computed at 360.7 s. The Result Control Item plot shows the convergence of the quantity:

The distribution across the room of the LMA can be visualized using cutting planes:

Blue regions show areas with low age of air and red regions show areas with high age of air. Red areas have poor ventilation reach.

Analyze your results with the SimScale post-processor. Have a look at our post-processing guide to learn how to use the post-processor.

Congratulations! You finished the tutorial!

Last updated: October 14th, 2020