The problem
Many light EVs and stationary packs use forced air cooling because it is simple and cheap, but air has low heat capacity, so cells far from the inlet run hotter. Uneven temperature ages cells at different rates and limits power. Students need a method to see where air goes inside a pack and how layout choices change the temperature spread.
Objectives
- Model a simplified module of cylindrical cells with holders, inlet and outlet plenums and a fan.
- Set up an internal conjugate heat transfer study with volume heat sources in the cells.
- Predict maximum cell temperature, cell-to-cell temperature difference and pressure drop.
- Compare at least three layouts (Z-type, U-type and a tapered plenum) and two cell spacings.
What you need
Tools and toolboxes
- SOLIDWORKS Part and Assembly — Cell, holder, enclosure and plenum models, with linear patterns of cells driven by a spacing global variable.
- Configurations / Design Tables — Layout and spacing variants held in one assembly for side-by-side runs.
- SOLIDWORKS Flow Simulation — Internal analysis with Heat Conduction in Solids, volume heat sources, a fan curve from the Engineering Database, lids and volume goals.
- Flow Simulation Parametric Study — Sweeping fan speed or inlet flow and comparing configurations on identical mesh settings.
- Excel — Cell heat generation from current and internal resistance, energy-balance check and comparison tables.
Software
- SOLIDWORKS 2023 or later
- SOLIDWORKS Flow Simulation
- Excel
Extra parts and data
- Cell datasheet — Internal resistance, dimensions and recommended temperature limits set the heat source and acceptance targets.
- Fan datasheet with a pressure–flow curve — A real fan curve makes the operating point realistic instead of assuming a fixed flow rate.
Key concepts
- Conjugate heat transfer
- Heat conduction in the solid cells and holders is solved together with convection in the air, so cell temperatures come out directly rather than from an assumed heat transfer coefficient.
- Cell heat generation
- A first estimate is Joule heating Q = I²·R per cell. At an example 2C discharge the heat per cell can be a few watts, which sets the volume heat source.
- Z-type and U-type plenum
- In a Z-type layout air enters on one side and leaves on the opposite end; in a U-type it leaves on the same side. Each gives a different pressure distribution and therefore different flow through each cell row.
- Fan operating point
- The fan delivers the flow at which its pressure curve meets the system resistance curve of the pack. A layout with more pressure drop gets less air from the same fan.
Step-by-step guide
- 1
Set targets and example load
Choose an example module of 21700 cells in a 4 by 8 arrangement, a continuous discharge of 2C and an ambient of 35 °C to suit Indian summer conditions. From the datasheet resistance, compute heat per cell and total module heat. Set example targets: maximum cell temperature under 45 °C and cell-to-cell difference under 5 °C, both commonly used design aims, and a pressure drop the chosen fan can supply. Write these in a requirements table.
Checkpoint: A requirements table with heat per cell, total heat, ambient and the temperature and pressure targets.
- 2
Model cells and holders
Model one cell as a cylinder with the datasheet diameter and length, then build a plastic holder plate with pockets. Pattern the cell with a Linear Pattern driven by a global variable for pitch so spacing can change in one place. Simplify each cell as a single solid with an effective orthotropic or averaged conductivity; you do not need jelly-roll layers for a module-level study. Remove tabs and busbars at first, and note this simplification.
Checkpoint: A cell pattern that updates correctly when the pitch variable changes from, for example, 23 mm to 25 mm.
- 3
Model the enclosure and plenums
Build the enclosure around the cell block with an inlet plenum on one end and an outlet on the other. Create configurations for Z-type, U-type and a tapered inlet plenum that narrows along the flow direction to even out pressure. Keep wall thickness and external size fixed across variants. Add a fan opening at the inlet and an outlet grille. Close openings with the Flow Simulation Create Lids tool so the internal fluid volume is sealed.
Checkpoint: Flow Simulation Check Geometry reports a single fluid volume for each configuration.
- 4
Set up the internal analysis
Run the Wizard as an Internal analysis with Heat Conduction in Solids enabled and gravity on. Choose air as the fluid. Assign materials: an aluminium or plastic for the holder from the Engineering Database, and a user-defined solid for the cells using effective conductivity and specific heat from literature or the cell maker, clearly flagged as assumptions. Set initial temperatures equal to ambient.
Checkpoint: The Wizard completes and the Solid Materials list shows every component with a material assigned.
- 5
Apply fan, outlet and heat sources
Apply an External Inlet Fan on the inlet lid, using a fan from the Engineering Database or a user-defined fan entered from the datasheet pressure–flow curve. Set the outlet lid as Environment Pressure at ambient. Apply a Volume Source of heat generation to each cell body, using total heat per cell. Make the enclosure outer walls either adiabatic or with a modest external heat transfer coefficient, and record which you chose.
Checkpoint: The boundary condition and heat source tree shows one fan, one outlet and a heat source on every cell.
- 6
Add goals and mesh
Add Volume Goals for maximum temperature of each cell row and the whole cell set, and a Surface Goal for mass flow and pressure at the inlet. Add an Equation Goal for maximum minus minimum cell temperature. Use automatic global mesh level 4 with a Local Mesh around the cell gaps so that at least three cells span each narrow air passage. Check that the minimum gap size setting matches the smallest cell gap.
Checkpoint: Goals converge in the Goal Plot, and a mesh cut plot shows several cells across each air gap.
- 7
Check convergence and energy balance
Run the baseline at two mesh levels and confirm maximum cell temperature changes by less than 0.5 °C. Then check the energy balance: total heat generated should equal mass flow times specific heat times the air temperature rise from inlet to outlet, plus any loss through walls. Compute this in Excel from the surface goals. A large mismatch usually means an unconverged run or heat escaping through a wall you meant to insulate.
Checkpoint: Mesh change under 0.5 °C and an energy balance that closes within about 5 per cent.
- 8
Study the baseline results
Make Cut Plots of velocity and temperature through the cell mid-height, Surface Plots of temperature on the cells, and Flow Trajectories from the inlet coloured by temperature. Identify rows starved of air and recirculation zones behind cells. Use an XY Plot along the flow direction to show how air temperature rises and how cell temperature follows it. Save these plot settings so every variant is shown the same way.
Checkpoint: Plots that identify the hottest cell and explain why it is hot, consistent with the volume goals.
- 9
Compare layouts and spacing
Run every configuration, Z-type, U-type and tapered plenum, at two cell pitches, on the same mesh rules and fan. Tabulate maximum cell temperature, temperature spread, fan operating flow and pressure drop. Wider spacing usually lowers pressure drop but increases module size, so report volumetric energy density too. Choose the best compromise against your targets and explain the trade-off, including the larger enclosure a wider pitch needs.
Checkpoint: A comparison table where at least one design meets both temperature targets, or a clear statement of how far each misses.
- 10
Report with limits
Write up the model, assumptions on cell properties and heat generation, boundary conditions, mesh and energy-balance checks, and the comparison. State limitations: steady-state constant current, simplified cells, no busbar heating, and fan data at nominal voltage. Recommend a test with thermocouples on a few cells at the predicted hottest and coolest locations to validate the model before relying on it.
Checkpoint: A report with a recommended layout, the test plan to validate it, and every assumption traceable to a source.
Testing & validation
- Mesh-convergence check: maximum cell temperature changes under 0.5 °C between meshes.
- Energy balance between heat generated and heat carried away by air closes within about 5 per cent.
- Fan operating point lies on the datasheet fan curve.
- Hand estimate of air temperature rise from Q = m·cp·ΔT matches the CFD result.
Expected results
A parametric air-cooled module model, converged CFD results for six layout and spacing variants, and a recommended design with its predicted maximum cell temperature, temperature spread and pressure drop.
Take it further
- Run a transient study over a drive-cycle current profile rather than constant current.
- Add busbar Joule heating and tab conduction to the model.
- Compare with a liquid cold-plate design at the same heat load.
- Build a small instrumented test module to validate the hottest-cell prediction.
Viva and review questions
Why is temperature spread as important as maximum temperature?
Cells that run hotter age faster, so a large spread makes cells drift apart in capacity and resistance. The weakest cell then limits the whole pack.
How did you model the cells?
As solid cylinders with effective thermal properties and a uniform volume heat source equal to I²R per cell. It is a module-level simplification that ignores internal layers and non-uniform heat generation.
Why use a fan curve instead of a fixed flow rate?
The flow a fan delivers depends on the pack's pressure drop. A fixed flow would hide the fact that a more restrictive layout gets less air.
How do you know the result is converged?
Goals flattened in the Goal Plot, maximum temperature changed by under 0.5 °C on a finer mesh, and the energy balance closed within about 5 per cent.
Learn it at DIYguru
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