## General Mechanical

#### Question about Transient Thermal and Ansys Fluent 2-way coupling

• Zhi Wei Ang
Subscriber

Hello everyone,

I am a student who is currently trying to study and simulate heat dissipation and thermal runaway cases of battery cells (specifically 18650 battery cells) inside battery packs.

I have a question regarding the Transient Thermal and Ansys Fluent 2-way coupling by use of System Coupling in Workbench 2021R1. I have briefly read through the document from Ansys in the link "https://forum.ansys.com/uploads/149/HUBQN99F4L5X.pdf" and shown that Ansys Mechanical and Ansys Fluent are able to be coupled to produce results 2-way. However, what was shown inside the tutorial was a connection between Ansys Transient Structural and Ansys Fluent. For purposes of a tutorial, I understand that a tutorial cannot cover everything.

Please note that this is a multi-body problem that has the battery cells connected to a series of solids (bus bars, cell holders, etc...) all the way to heat sink. This may be more relevant information for the Transient Thermal part.

For my purpose of using Transient Thermal and Ansys Fluent with 2-way coupling, it is because of the following Solid to Fluid to Solid situation:

Case 1, normal operations

1. I want to simulate the heating up of battery cells inside a battery pack (bound by cell holders, the usual battery pack things, etc...). The battery cells will be connected to bus bars (or electrical connectors) which will also be in contact with heat sinks that has the top surface with fins exposed to ambient air of 22°C for cooling, this is taken as natural convection in convection boundary condition.

2. The heating up of battery cells will heat up the air around the battery cells, causing convection to occur, which will change the film coefficient and temperature of the air causing the convection, and also that the air will transfer heat to other battery cells. This is a closed system, so the air inside the battery pack does not get exchanged with air outside of the battery pack.
3. This air will also be in contact with the inner wall of the battery pack, and the inner wall of the battery pack will have heat transfer to the outer walls of the battery pack which will be cooled down by means of natural convection (assume charging the battery in a room with ambient air temperature of 22°C), which will slightly cool the air inside the battery pack. The air will also be in contact with other components inside the battery pack and heat transfer will occur between the air and other components.

Case 2, thermal runaway

1. A single cell will continue to heat up until it becomes uncontrollable.
The body of the cell overall climbs from 75°C (max normal operating temperature) to 160°C and slowly climbs to 210°C. Afterwards this boundary condition is deactivated after it hits 210°C by turning off the time steps where the temperature after 210°C is assigned.
The body stops heating up from there, and positive terminal of the battery cell starts to heat up from 210°C to 600°C, dwells for 600°C for a short duration before spiking up to 1100°C (in moment of 3secs).
After the point at 1100°C, all remaining time steps for temperature boundary condition is turned off and the system enters cooling phase.

2. The air eventually also gets heated up by this battery cell and the large change in temperature causes the body of air to be turbulent. This is assumed that it will significantly. The air is especially assumed to have significantly effect as it will have a large change in a short period of time (when temperature suddenly rises).

3. The overall system is left to be cooled down, and the air plays a part in cooling down the system as it transfers heat from parts with higher temperature to the inner walls of the battery pack, which will transfer heat from inner walls to outer walls of battery pack, where the outer walls of the battery pack will be exposed to natural convection like the heat sink.

That is the end of the description for my case, and I would like to ask if there is a way to do it, such as syncing the time steps of both Transient Thermal and Ansys Fluent, what recommended settings are there.

It would be nice if there was a link or document for this kind of simulations to do as tutorial as well!

Thank you so much for reading this post. I hope to receive some nice replies.

Kind regards,

Ang Zhi Wei

• RM
Ansys Employee

Hi,

Please refer to the video to understand the two-way coupling setup using System Coupling. ANSYS System Coupling: Two Way Fluid Structure Interaction - Part 1 - YouTube and ANSYS System Coupling: Two Way Fluid Structure Interaction - Part 2 - YouTube , similarly you can setup your thermal case.

In document System Coupling (ansys.com) you'll find the supported capabilities and limitation, variables availabe for SyC, related settings in mechanical and troubleshooting related information.

Hope this helps!