Hello Everyone
In this video, I will show you how to set up a model that includes relative humidity. Let's get started by opening Workbench.
Geometry Setup
In the geometry, you can see the two chimneys inside and on the right, the boundary conditions. Note that both inlets have a given velocity, temperature, and relative humidity.
- The geometry may come from different CAD software.
- SpaceClaim allows you to create the fluid domain and name selections easily.
- The different name selections on the left will be used for the meshing process and the boundary conditions in CFX-PRE.
Meshing Process
In ANSYS Meshing, we create the mesh manually by defining:
- Element size
- Growth rate
- Other parameters
The mesh will have tetrahedral elements. We also define the element size on the chimney walls and the inflation layers. Finally, generate the mesh. It will take a few minutes.
When the mesh is done, check the maximum number of elements and nodes. Next, create a section view to examine how the inflation layers on the walls look.
Model Setup
Begin by adding water vapor to the material list. Then, create a new material including this vapor and dry air. Finally, set up the Ozen interfacing core.
- To create the new CPL Y engineer model, drag and drop a layer and set up the model type as shown in the last top image.
- Set up the domain by adding the new material, gravity vector, and reference density.
- In the fluid models tab, activate the energy equation, the turbulence model, and set the air as constrained.
Boundary Conditions
For the main inlet, set up the air speed, temperature, and the mass fraction of the water vapor. But how do you calculate this value? Let's have a look.
Calculating Relative Humidity
This is the workflow to calculate the relative humidity when you know the mass fraction of the water vapor and its temperature:
- Calculate the mixing ratio.
- Determine the partial pressure and the saturation pressure to get the answer.
In this case, we need to do the opposite. We know the temperature and the relative humidity, and we need the mass fraction of the water vapor. Here's an example:
Assume air at 30°C and relative humidity of 50%. Calculate the saturation and partial pressures to finally get the mixing ratio. Then, calculate the temperature and the ratio iteratively to find the mass fraction. Write this value in CFX-PRE.
Repeat the process for those boundary conditions that define the ambient. Note that the mass fraction for the chimney flows is different.
Simulation and Postprocessing
Using 4 to 8 cores, the model will take some minutes to reach convergence. In CFD-Post, go to the expressions tab and create the equations you saw on the previous slides. Then, use the last expression to create the variable for the relative humidity.
Here you see the contour plots in two parallel planes showing the relative humidity of the air, especially that from the chimneys.
Conclusion
And that's it. Thanks for watching.
Hello everyone. In this video, I will show you how to set up this model, including relative humidity. In the geometry, you can see the two chimneys inside, and on the right, the boundary conditions. Note that both inlets have a given velocity, temperature, and relative humidity.
So, open Workbench, and let's begin. The geometry may come from a different CAD software, but SpaceClaim allows creating the fluid domain and the name selections easily. The different name selections on the left are going to be used for the meshing process and the boundary conditions in CFX-PRE.
In ANSYS Meshing, we create the mesh manually by defining the element size, grow rate, and other parameters. The mesh will have tetrahedral elements. We also define the element size on the chimney walls and the inflation layers. Finally, generate the mesh. It will take a few minutes.
When the mesh is done, check the maximum number of elements and nodes. Next, create a section view to find out how the inflation layers on the walls look like. The model setup begins by adding water vapor to the material list. Then, create a new material including this vapor and the dry air.
Finally, set up the Ozen interfacing core. To create the CPL engineer model, drag and drop a layer and set up the model type as shown in the last top image. Now, set up the domain, adding the new material, the gravity vector, and the reference density.
In the fluid models tab, activate the energy equation, the turbulence model, and set the air as a constraint. For the main inlet, we set up the airspeed, temperature, and the mass fraction of the water vapor. But, how do I calculate this value? Let's have a look.
This is the workflow to calculate the relative humidity when you know the mass fraction of the water vapor and its temperature. Basically, you calculate the mixing ratio, the partial pressure, and the saturation pressure to get the answer. But in this case, we have to do the opposite.
We know the temperature and the relative humidity, and we need the mass fraction of the water vapor. Here's an example. Let's assume air at 30°C and a relative humidity of 50%. We calculate the saturation and partial pressures to finally get the mixing ratio.
Then, we calculate the temperature and temperature of the water vapor. Next, we calculate the temperature, the ratio, and the mass fraction—the last two in an iterative way. Then, write this value on CFX.
Now that you know how to calculate the mass fraction, repeat the process for those boundary conditions that define the ambient. Note that the mass fraction for the chimney flows is different. Using 4 to 8 cores, the model will take some minutes to reach convergence.
In the CFX-POST, go to the expressions tab and create the equations you saw on the previous slides. Then, use the last expression to create the variable for the relative humidity.
Here, you see the contour plots in two parallel planes, showing the relative humidity of the air, especially that from the chimneys. And that's it. Thanks for watching.
