Modeling Electric Potential and Electrochemistry with Fluent
Hello everyone, welcome to another Ozen Engineering video on Ansys products. Today, we're going to explore something quite different. We'll use Ansys Fluent, our well-known fluid solver, as a non-fluid solver applied to an electronics problem.
Introduction
The objects of interest include:
- Anode zone and material
- Cathode material on the left-hand side
- A fluid in between, marked by green
Fluent contains an electric potential solver integrated with the electrochemical reaction model. Although my expertise is in fluids, not electrochemistry, the Fluent Theory Guide provides extensive information on electrochemical reactions. I'll refer to it during this presentation if you have further questions.
Objective
My goal in this tutorial is to demonstrate how to set up the model in Ansys Fluent. Using the electric potential solver, we can simulate the electric potential field in both fluid and solid zones. Our domain includes both zones.
Limitations
One limitation of the electric potential model is that it cannot be used in simulations with walls that have a shell conduction boundary condition, which we do not have in this case.
Setup Process
Electric Potential Model
We'll start by setting up the electric potential model. This is the main panel we'll focus on. Here, we examine the potential equation of Joule heating in the energy equation. We have three options, and since we're interested in a lithium-ion battery model, we'll select the lithium-ion option, which opens an additional tab below.
Zone Selection
First, we perform the zone selection:
- Negative electrode zone: Solid - an (anode)
- Electrolyte (fluid region): Fluid - el
- Positive electrode: Solid - ca (cathode)
These three zones must be present for the simulation. We will not include electrochemical heating in the energy equation, so we won't solve the energy equation at all in this case.
Electrochemistry Rate Tab
Next, we work through the E-chem (electrochemistry rate) tab. Here, you'll see a set of numbers representing Butler-Volmer rate parameters. We could use a single number for equilibrium potential for anode and cathode, but to improve accuracy, we're using user-defined functions (UDFs) to define equilibrium potential for both the anode and cathode. These files should be placed in the appropriate location in the file structure.
Material Properties
The material properties are as follows:
- Maximum value of lithium concentration in cathode and anode
- Transference number
For further details, refer to the Fluent Theory Guide, as well as the activity term.
Advanced Tab
In the Advanced tab, we have multiple options. We won't use the physics-based aging model in this example, but if needed, information for SEI growth must be provided. Options for lithium plating and SEI film growth model are available but not covered in this tutorial. We will keep all expert control options on by default, including the lithium migration term, which usually helps improve solution convergence. If you encounter convergence issues, you can toggle these options to improve results.
Model Setup
Materials
We'll use air as our electrolyte between the solids. For the anode, the key properties to define are electrical conductivity and lithium diffusivity, which will be derived from our user-defined functions. This also applies to the cathode material.
Boundary Conditions
We'll have symmetry conditions all around our object, meaning a repeating pattern of anode and cathode. When defining theory for atomic flux, it will be defined in a user library.
Residuals and Solution Controls
In the residuals section, we're solving for only two equations: the potential and the lithium equation. Under solution residuals, you can change the convergence criteria. In solution controls, confirm that only these two equations are considered.
Solution Reporting
Once you enable potential electrochemistry, Ansys Fluent automatically generates two report definitions:
- Lithium ion battery SOC
- Lithium ion battery capacity
Ensure you input the correct numbers to obtain these reports.
Running the Simulation
Next, we'll run the simulation. This will be a transient simulation with 700 time steps, each 1 second long, totaling over 10 minutes. We'll have 25 sub-iterations. You may need to adjust the number of time steps and maximum iterations for your specific application. Ensure the total current at the anode and cathode is balanced during each time step.
Finally, hit the calculate button to start the solution. Initially, you'll see the lithium concentration at time zero. Once the solution is available, you can observe variations at different time steps.
Conclusion
This concludes our presentation for today. Thank you for your interest and for watching!
Hello everyone, welcome to another Ozen Engineering video on Ansys products. Today we're going to look at something very different. We're going to use Ansys Fluent, our well-known fluid solver, as a non-fluid solver. We're going to apply it to an electronics problem.
The objects of interest are anode zone, anode material here, and cathode material on the left-hand side, and it is filled with fluid in between, which is marked by green. So Fluent contains an electric potential solver which has been integrated with the electrochemical reaction model.
And frankly I'm not an expert on electrochemistry, I have more of a fluids background, but there's a lot of information on electrochemical reactions in the Fluent Theory Guide. I'll be referring to it a couple of times during this presentation if you have further questions.
My goal in this tutorial is to show you how to set it up in Ansys Fluent. When we use the electric potential solver, we can simulate the electric potential field in both fluid and solid zones. Hence, we have both zones in our domain.
One limitation for electric potential model is that it cannot be used in simulations with walls that have shell conduction boundary condition, which we do not have. I'm going to start talking about setting up electric potential model since we kind of introduced our model.
So this is going to be the main panel we're going to be spending time on. So here we're looking at the potential equation of the Joule heating in the energy equation. We can also use electrochemistry or not, we have three options. Here we are interested in a lithium ion battery model.
Therefore, we are going to select the lithium ion option, which then is going to open up this additional tab below. So the first thing we want to do is we want to do the zone selection and keep in mind that we're not going to include the electrochemical heating in the energy.
So we're not going to be solving energy equation at all in this case. So here our negative electrode zone is going to be our solid - an for anode. Our electrolyte, i.e., fluid region, is our fluid - el.
And then our positive electrode then is, you know, will be called solid - ca for the cathode zone. So these three need to be present in order to do this simulation. The next tab we need to work through is the E-chem, electrochemistry rate tab. And here you will see a set of numbers.
And these are essentially Butler-Volmer rate parameters. These numbers we just put in. We could also use a single number for equilibrium potential for anode and cathode.
In this case, we want to do a better job, so we're using user libraries, UDFs, to define our equilibrium potential for the anode and the cathode. And these files should be placed in the appropriate location in the file structure. The material properties are as follows.
So when we go through this again, these numbers are coming from the fluid theory guide. You know, this is the maximum value of lithium concentration in cathode and anode. Transference number is defined as the following constant.
For further detail, I'm going to refer you to the Fluent Theory Guide, as well as the activity term. In our next tab, we're going to move on to the Advanced tab. And here we have multiple options.
We're not going to use in this example the physics-based aging model, but if it wants to be used, such information needs to be provided for SEI growth. And then you obviously can do lithium plating and SEI film growth model, and we're not going to go into this in this tutorial.
And we're going to keep all the expert control options on. So, by default, we want to include lithium migration term. It usually helps improve solution convergence. And also, you know, the other two gradients, they have to do with convergence.
If you see any convergence issues, you can turn these options on and off to get better convergence. Before I go further on here, I'm going to get out of here and do the setup of the rest of the model. So, we're going to talk about materials.
We're going to have air as our electrolyte in between the solids. So let's talk about our anode first. Here the two key properties we want to define are electrical conductivity and lithium diffusivity. And again, these will be coming from our user-defined functions.
This also applies to our cathode material. And let's move on with our boundary conditions. We're going to have symmetry conditions all around our object. So let me quickly display one of them. So we'll have symmetry all around, which means we have a repeating pattern of anode and cathode.
When defining theory for atomic, the flux will be defined again in a user library. Another key setup is we want to go to residuals. And here we're essentially solving for only two equations, the potential and the lithium equation.
If we go to solution residuals, double click here, we can change the convergence criteria here. Again, if we go under our solution controls, we do not have to touch these. We can go to equations and just confirm only two equations are being considered.
So once that setup is done, let's go back to our potential model, talk a little about solution reporting. So once you turn your potential electrochemistry on, Ansys Fluent is going to automatically generate two report definitions.
Let's look at the lithium ion battery SOC and lithium ion battery capacity. So you can find these definitions if you go under here. Those are the two definitions Fluent is going to add in. So here they are. And what you want to do is to get those reports, you want to put in correct numbers.
So, next step is going to be running the simulation. So, let's go to our run calculation. See, this is obviously going to be a transient simulation. We're going to put 700 time steps with each time step size is 1 second, so it will be over 10 minutes. We're going to have 25 sub-iterations.
So obviously, you may have to play with the number of time steps versus maximum iterations to find the appropriate value for your specific application.
And you can check the total current at the anode and cathode, and the goal is that the two current values must be adequately balanced out during every time step during the simulation. So please keep that in mind. So then we essentially hit the calculate button, and then the solution starts going.
So here what we're looking at is the lithium concentration at time zero. So once the solution is available, you can look at this variation at different time steps. So this concludes our presentation for today. Thank you for your interest. Thank you for watching!
