Enclosure Design for Thermal-Runaway Venting and Fire Barriers

The problem

When a lithium-ion cell enters thermal runaway it releases hot, flammable gas and heat. If the enclosure cannot vent the gas safely and slow heat transfer to neighbouring cells, failure can spread through the pack. Designers must choose vent locations, gas paths and barrier materials, and simulation is one tool to compare options before costly abuse tests.

Objectives

• Define a vent gas release boundary condition from published cell test data.
• Model enclosure internal volume, vent paths and burst vent in Flow Simulation.
• Compare vent configurations for peak pack pressure and gas exit routing.
• Compare barrier materials for heat transfer to neighbouring cells.
• Define the limits of the approach and a validation plan.

What you need

Key concepts

Thermal runaway

A self-heating failure in which exothermic reactions in a cell accelerate, releasing heat and gas. It can spread to neighbours through heat transfer and hot gas.

Thermal propagation

Spread of runaway from one cell to others. Battery standards such as AIS-156 and AIS-038 Rev 2 include related requirements; check the current text.

Conjugate heat transfer

Solving heat transfer in fluids and solids together, so gas heating of barriers and cells is captured. Flow Simulation does this with heat conduction in solids switched on.

Burst vent

A panel or membrane that opens at a set pressure to release gas through a planned path away from occupants.

Step-by-step guide

1. 1

   Frame the research question

   Define the question, for example: for an example 21700-based pack, how do vent location and barrier material affect peak pack pressure and the temperature of cells next to a failing cell? Review literature on runaway gas release and propagation for the chosen chemistry. Identify what Flow Simulation can and cannot model: it handles gas flow and heat transfer, not reaction chemistry, combustion or particle ejecta in a predictive way.

   Checkpoint: A written research question, a literature summary and a list of modelling scope and exclusions.

2. 2

   Derive the release boundary condition

   From published test data, derive gas mass flow rate against time, gas temperature and an equivalent gas mixture for one cell. Create a custom gas in the Engineering Database with molar mass and specific heat representative of the mixture. Plot the release profile in MATLAB or Excel and check that integrated mass equals the reported total. Record each source. Use a range from several sources if they differ.

   Checkpoint: A release profile whose integral matches the published gas mass, with sources.

3. 3

   Build the pack geometry

   Model a section of the pack: a module of cells with the failing cell at the centre, barriers between cells, the module cover, internal gas channels and the enclosure volume up to the vents. Simplify cells as solid cylinders with effective thermal properties. Use Lid features to close the fluid volume where needed and run Check Geometry in Flow Simulation to confirm a closed internal volume.

   Checkpoint: Check Geometry reports a valid fluid volume and the solid volumes are as expected.

4. 4

   Set up the transient study

   Create an internal, transient Flow Simulation project with heat conduction in solids and gravity on. Apply the release profile as an inlet mass flow on the failing cell's vent face, with gas temperature from data. Model vents as environment pressure openings, or as closed until a burst pressure in a sequential study. Set initial temperature and air in the pack. Add goals for pack pressure, vent mass flow and neighbour-cell maximum temperature.

   Checkpoint: The study solves for the release duration and goals converge at each time step.

5. 5

   Check mesh and time step

   Run the baseline with three mesh levels and local refinement around the failing cell and vents. Compare peak pressure and neighbour-cell maximum temperature. Check time step sensitivity by tightening the time step settings. Choose settings where key results change by less than about 5 per cent, and note the run time. Make sure the mesh resolves the narrowest gas channel with several cells across it, since under-resolved channels give wrong pressure losses.

   Checkpoint: A sensitivity table for mesh and time step and the chosen settings.

6. 6

   Verify with a lumped model

   In MATLAB, build a lumped model of the pack as a single gas volume with mass inflow and outflow through an orifice at the vent, using ideal gas law and an orifice equation. Compare its predicted peak pressure with Flow Simulation. Agreement in order of magnitude confirms the setup; large differences point to boundary or geometry errors. Use the same vent area and discharge coefficient in both.

   Checkpoint: Lumped and Flow Simulation peak pressure agree within a reasonable band.

7. 7

   Compare vent configurations

   Use parametric studies or configurations for three vent options: single vent at one end, distributed vents per module and a dedicated vent channel along the pack floor. Record peak pressure, time to peak, the path of hot gas visualised with flow trajectories, and where gas exits. Note which configuration keeps hot gas away from other modules and from the cabin side.

   Checkpoint: A comparison table and flow trajectory plots for three vent options.

8. 8

   Compare barrier materials

   Define candidate barriers in the Engineering Database, such as a mica sheet, a ceramic-fibre paper and an air gap only, with datasheet properties. Run each with the best vent option and record neighbour-cell surface temperature against time. Compare the time for neighbours to reach a temperature threshold taken from literature for the chosen chemistry. Run also a barrier-thickness sweep for the best material, since thickness trades heat protection against pack volume and mass.

   Checkpoint: Neighbour-cell temperature histories for each barrier and the time to threshold.

9. 9

   Check structure under pressure

   Export the peak pressure on the lid and vent panel and apply it in a Static study of the enclosure. Check lid deflection and stress, and whether the burst vent opens before the lid deforms excessively. Iterate burst pressure and lid stiffness so the vent works as intended. Note that a pressure that rises very fast acts dynamically, so a static check may underestimate stress; a margin should be applied. Record the vent opening pressure on the drawing.

   Checkpoint: The lid stays within stress limits at peak pressure and the vent opens below that pressure.

10. 10

    Assess limitations

    List limitations: no combustion or chemistry, assumed gas properties, simplified cell thermal properties, no ejected particles and no electrical effects. Estimate how each might change conclusions. Write a validation plan: a heated-cell or nail-penetration abuse test in an accredited facility, with thermocouples and pressure sensors matching simulation points. Rank limitations by how likely they are to change the ranking of designs, not just absolute values. Note which can be removed with more data.

    Checkpoint: A limitations table and a validation test plan.

11. 11

    Publish the method and results

    Write a paper-style report: research question, literature, release model, CFD setup, verification, vent and barrier comparisons, structural check and limitations. Present results as comparative rankings rather than absolute safety predictions. Provide models and data processing scripts so others can reproduce the study. Include enough detail on boundary conditions, gas properties and mesh for another researcher to reproduce the results. Discuss how the method could support early design reviews before abuse testing, without replacing it.

    Checkpoint: A reproducible report with comparative conclusions and clearly stated scope.

Testing & validation

• Release profile integrated mass matches literature data.
• Mesh and time-step sensitivity reported.
• Lumped-model pressure check.
• Structural check of lid under peak pressure.
• Limitations and an abuse-test validation plan stated.

Expected results

A transient Flow Simulation study of an example pack section showing how vent configuration and barrier material affect peak pressure, gas routing and neighbour-cell temperatures during a single-cell runaway, with verification checks and an honest scope statement.

Take it further

• Couple the study with a lumped thermal-runaway model for neighbour cells to estimate propagation timing.
• Use Flow Simulation's Particle Study to trace particle paths qualitatively from the vent.
• Compare results with a dedicated combustion-capable CFD code.

Viva and review questions

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