Transient Thermal Modeling of PCR Thermocycling Using ANSYS Fluent

Hello and welcome to this tutorial. I will walk you through how to set up a thermal simulation using ANSYS Fluent. We will start off with the mesh geometry, and I will walk you through what the physics of the problem is.

Here, you see a reactor where we have an inlet indicated by the blue vectors and an outlet on the opposite side with the red arrows. The bottom surface is where we apply the heating. I have named this as a boundary condition, which we can select.

The entire chamber is filled with the PCR mix, which we are modeling as a simple fluid using water properties. As you can see, ANSYS Fluent has a menu on the left. We can walk through setting up the physics by going through this one by one.

It also has a ribbon on the top where the user is guided through setting up the problem. Let's use the outline view to set up the problem. The first thing we want to do is go under General and essentially define the type of simulation we have.

In this case, it is a transient simulation, which means that we need to provide the temperature profile for the simulation. We can do this by reading a file with a certain format. You click on Profile, and the format would need to be either a CSV, PROF, or TAB.

I have already prepared this file for you, called the pre-CR profile. We would click on this and say OK. Once we do this, it would actually read the file, and we can see on the console that it read the file, which contains time and temperature. I want to show you also how this thing looks like.

Here, I have the temperature profile, which I created in Excel. It has two columns: the first one is always time, and the second one is temperature, in Kelvin. The format needs to have a unique name, the number of columns, and the number of data points.

When you save the file as text, you need to overwrite this to change the extension from TXT to TTAB. We successfully uploaded the time and temperature profile. We are ready to move to the next part, which is defining the model. The model is simple; we have a thermal model.

We have checked off Energy, and the other physics are not selected. We have a simple flow, and we're looking at any mixing involved due to heating. We have turned on the laminar and viscous options. Next, we need to define the properties of materials.

In this case, everything is defined as water, so the entire chamber is filled with water, making it a single-phase problem. We copied this from the Fluent database. Next, we need to define what's inside the geometry.

We have selected water liquid as the material that we want the entire geometry to be filled with. The next thing we need to do is apply the boundary conditions. We have an inlet and outlet, but since nothing is flowing, we have left this as default.

The next thing we need to do is define the temperature profile at the bottom. We have named the boundary conditions the bottom of the chamber as bottom. We can double-click on this and select the temperature profile. We will leave everything else as default.

We are assuming heat loss due to natural convection on the other surfaces. We can do this by selecting all the other walls and selecting a heat transfer coefficient. In this case, I've selected 10 and room temperature, which is at 26. 85. We're ready now to move into how to set up the solution.

We can bypass these because they are not related to our simulation. Named expressions, we don't have that. Under solutions, the first thing we got to do is go under methods and take the defaults. We have a number of different solvers; Simple is a good one.

We want to make sure that these are second order upwind. We look at the control panel and take the defaults. We can look at the residuals and set the criteria for convergence. Now, we're ready to do a final step, which is setting up the run calculations.

In this case, the run calculations have a check case, which I highly recommend. It gives you some information and hints if there are any issues with the model.

Since this is a transient simulation, we have to define three different variables: the number of iterations, the total number of steps, and the time.

At every iteration, it's moving at 0. 25. It's basically iterating 40 times and either going to 40 iterations or the convergence criteria, whichever comes first. We're ready to start the simulation. The next thing you got to do is initialize. We recommend using hybrid initialization.

We would click on initialize, and this would essentially start the simulation. We would go under results, surfaces, and define planes. We can define a line and a plane. We can also create a mesh or contour. We can see the maximum and minimum temperatures and the time step.

We can see that the minimum temperature was 25 degrees, which was the room temperature. The maximum was 93.85, which came from our temperature profile. We can see that this is a time step in the simulation. We can also look at a contour on a plane.

We can see that in this particular design, we never reach the set temperature, even during the day. This tells us that this design will not work, and we would need to implement another way of heating. One simple way would be to add a heated top. However, this would not solve the optics problem.

We need to come up with a clever way of addressing this and achieving the temperature uniformities we're looking for. This concludes this tutorial. Thank you.