Figure 1: Flickering candle in RandSim carved Pumpkin
We’re getting into the spirit here at RandSim, and I thought I’d take the opportunity to play around with combustion modelling in Fluent. Though I had worked for many years in the turbomachinery CFD space, I was focused on the aero/thermal side, working either in heat transfer modelling of the cooled blades, or in the compressor/turbine aerodynamics. That is to say – upstream or downstream of the combustor, but never the combustion itself. Others on my team have done combustion modelling, so I figure I’d give it a try!
Once I dove in, I found it followed much of the familiar path of learning something new in ANSYS. I start out intimidated by what I don’t know until I crack open the user guide, put one mouse click in front of the other and soon enough I’m looking back at the end-result wondering why it took me so long to try. I thought I’d chronicle a bit of my path here, not on the specifics of combustion modelling, but more on the journey of learning a new feature in ANSYS.
First, I started where I’m very familiar, geometry modelling and meshing. I found a model file of a pumpkin online, and it imported into SpaceClaim cleanly and passed all geometry checks. It did have a removable lid, but for my purposes I didn’t need it, so I merged the bodies together. Then I cut a hole in the top, and added a cylinder in the middle to represent a candle with a small, indented surface to act as the fuel inlet. Last, I grabbed an image of our RandSIM logo and overlayed it on the pumpkin and traced out a cut in the pumpkin face.
Figure 2: Pumpkin geometry in SpaceClaim using image projection to cut logo
I then created an air volume around it, named all my surfaces and proceeded into Fluent meshing. From there, I used the watertight meshing workflow with Poly-Hexcore selected as the volume fill. I used mostly default settings for the mesh, with the exception of increasing the number of buffer layers to 6 to give a bit more mesh quality around where I expected a dancing flame and airflow around the pumpkin.
Figure 3: Mosaic Mesh of Pumpkin created using Watertight Meshing Workflow
So far, this was all familiar territory, but once I got into Fluent and was preparing to set up the combustion model, I knew I needed to get some material to reference. Luckily ANSYS has some fantastic places to find this information. The very first thing I did was select the Non-Premixed Combustion model within the Species dialog box and click the Help button. This (like the Help button in all dialog boxes) sent me directly to the relevant section in the Fluent User’s Guide, and described the meaning and use for each button or input field on this box. Furthermore, for when I need a bit more information on the mathematical models used, (such as the Diffusion Flamelet Models), I can click on those relevant links to be sent to the Fluent Theory Guide.
Figure 4: Example Snippet from Fluent Theory Guide
The user guide is really the best way to get quick info for specific setup options when working through a problem, but for my case, I was not aware of where to start and needed a bit more of a workflow demonstration. For that, I turned to the Ansys Learning Hub. I navigated to the Fluids Building and found a learning room specifically for Fluent Combustion Modelling with a few instructor-led recordings, and folders of training material with lecture slides and workshops. If I wanted to become more deeply involved in combustion, I would go through all the content there, but for now I just wanted to get a feel for the Non-Premixed combustion model, so I navigated to that folder and brought up both the lecture slides and the workshop slides. The workshop was reasonably close to my scenario (though obviously not of a pumpkin), so I was able to start there and just apply the settings they used to my problem with a few slight modifications, referencing the lecture slides whenever I needed clarification on what a setting meant.
For post processing, I created a few scene views where the flame structure itself was visualized via multiple levels of iso-surfaces around the combustion by-products. And then I just colored those with the “flame” contour type for a good visual. I set the animation to export every timestep and then saved the video file at the end.
In addition to creating a fun Halloween visual, this serves as a great case study in how easy it is to learn something new in Ansys by using the quick access available resources. The user and theory guides provide for quick reference of specific input fields and features, and the learning hub provides deeper descriptions for specific workflows and physics types. It doesn’t take long to learn something new, so feel free to experiment and let us know if you would like further information on anything you find!
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