The problem
Immersion cooling promises uniform cell temperatures and better fast-charge and hot-climate performance, which matters for Indian two-wheelers, but it adds fluid mass, sealing, cost and material compatibility questions. There is little open, design-level comparison for small packs. A careful research study can quantify where immersion is worth it and where it is not.
Objectives
- Define an example two-wheeler pack and duty cycles including fast charging in hot ambient.
- Design air-cooled, passive immersion and pumped immersion variants with the same cells.
- Model dielectric fluid properties, natural convection and forced flow with conjugate heat transfer, steady and transient.
- Compare thermal performance, mass, volume and pump power, and set out safety and compatibility questions.
- Propose a validation experiment and the conditions under which immersion is justified.
What you need
Tools and toolboxes
- SOLIDWORKS Part and Assembly — Cell array, holders, busbars, sealed tank, lid, gasket, pump and heat exchanger envelopes, held in configurations for each cooling concept.
- SOLIDWORKS Flow Simulation — Internal analysis with gravity, Heat Conduction in Solids, a user-defined dielectric liquid, transient heat sources and parametric studies.
- SOLIDWORKS Simulation (Static and Frequency) — Tank wall stress under fluid head and pressure, and natural frequencies of the filled pack under vibration loading with added fluid mass as an approximation.
- Mass Properties — Pack mass and centre of gravity including fluid volume.
- SOLIDWORKS Visualize — Section renders explaining the concept.
- Excel or MATLAB — Duty cycle heat profiles, dimensionless analysis (Rayleigh number) and comparison tables.
Software
- SOLIDWORKS 2023 or later
- SOLIDWORKS Flow Simulation
- Excel or MATLAB
Extra parts and data
- Dielectric fluid datasheet — Density, viscosity, conductivity, specific heat, flash point and dielectric strength define the fluid model and safety discussion.
- Cell datasheet and, ideally, measured heat generation — Heat generation during fast charge is the main driver; measured data is much better than an I²R estimate.
Key concepts
- Immersion cooling
- Cells sit in direct contact with a non-conductive liquid that removes heat from their whole surface, by natural convection or forced flow. Single-phase immersion keeps the fluid liquid throughout.
- Rayleigh number
- A dimensionless group that indicates the strength of natural convection. It helps judge whether passive immersion circulates enough fluid on its own.
- Material compatibility
- The fluid must not attack seals, cell wraps, adhesives or plastics over years. This needs testing and cannot be solved in CAD.
- AIS-156
- The Indian standard for traction battery safety of L-category vehicles covers tests a pack must pass. Any new cooling concept must be checked against the current revision.
Step-by-step guide
- 1
Define pack, duty and research questions
Use an example 3 kWh, 60 V two-wheeler pack of 21700 cells. Define duty cases: a hot-ambient city cycle at 40 °C, a sustained high-speed run and a fast charge at an example rate. Generate heat profiles from cell resistance or, better, from measured data, and label their source. Write the research questions: how much does immersion lower peak temperature and spread compared with air, what is the mass and volume penalty, and does passive circulation suffice?
Checkpoint: A study plan with duty heat profiles, three concepts and stated research questions and metrics.
- 2
Model the cell array and air baseline
Build the cell array with holders and busbars as in a conventional pack, then an air-cooled enclosure with fan and plenum as the baseline. Parameterise cell pitch with a global variable. Use the same cell model, heat sources and holders for every concept so differences come only from cooling. Record baseline mass, volume and centre of gravity with Mass Properties.
Checkpoint: A baseline assembly with mass, volume and CG recorded, using the shared cell sub-assembly.
- 3
Design immersion concepts
Create a sealed tank around the same array with a lid, gasket groove and an expansion space or bellows for thermal expansion of the fluid. In the passive concept, add vertical gaps that allow a natural convection loop and fins on the tank walls to reject heat. In the pumped concept, add inlet and outlet ports, a distribution manifold and envelopes for a small pump and liquid-to-air heat exchanger. Calculate fluid volume and mass.
Checkpoint: Two immersion configurations that build cleanly, with fluid volume and total mass tabulated.
- 4
Define the dielectric fluid
Create a user-defined liquid in the Engineering Database from the fluid datasheet, with temperature-dependent viscosity if available, since viscosity drives natural convection. Assign materials to holders and tank. Compute the Rayleigh number for the passive concept's main gap and the expected convection regime. Note that some dielectric fluids have low flash points and that fire behaviour must be part of the safety assessment.
Checkpoint: A fluid definition with sources, and a Rayleigh number estimate that predicts the convection regime.
- 5
Set up passive immersion CFD
Run an Internal analysis with gravity, Heat Conduction in Solids and the dielectric fluid filling the tank. Apply Volume Sources to cells and a heat transfer coefficient to ambient on the outer tank and fins. Begin with steady state at the sustained-run heat level. Refine the mesh in gaps between cells. Check whether the solver converges to a steady pattern or oscillates, which may mean the flow is inherently unsteady and needs a transient run.
Checkpoint: A converged run, or a documented decision to move to transient, with plume patterns visible in velocity cut plots.
- 6
Set up pumped immersion CFD
For the pumped concept, apply an inlet volume flow and environment pressure outlet, and model the heat exchanger as a boundary that returns fluid at a set temperature, with that simplification stated. Sweep pump flow in a Parametric Study. Record maximum cell temperature, spread, pressure drop and pump hydraulic power. Check flow distribution between cell rows with surface goals and tune the manifold if needed.
Checkpoint: Flow-sweep results with pump power, and row flows within an example ±15 per cent of the mean.
- 7
Run transient duty cycles
Run each concept in transient mode with the time-dependent heat profiles: hot city cycle and fast charge. Include the fluid's thermal mass, which slows temperature rise. Use a time step small enough that halving it changes peak temperature by under 0.5 °C. Plot maximum and minimum cell temperatures over time for air, passive and pumped concepts on one chart, and note the time each concept takes to reach its limit.
Checkpoint: Transient plots for all three concepts with a time-step sensitivity check.
- 8
Verify and bound the results
Close energy balances at steady state for each concept. Compare passive heat transfer with a natural convection correlation for a vertical enclosure at the computed Rayleigh number. Run a sensitivity study on uncertain inputs, such as fluid viscosity, cell conductivity and heat generation, by ±20 per cent, and report the range of conclusions. If a conclusion flips within this range, say so.
Checkpoint: Energy balance within 5 per cent and a sensitivity table showing how robust each conclusion is.
- 9
Structural, mass and safety assessment
Run a static study on the tank under fluid head plus an internal pressure from expansion, and a frequency study with fluid mass approximated as distributed mass on the tank, noting that this ignores fluid-structure interaction. Compare mass, volume and energy density across concepts. List safety and compatibility questions for AIS-156 testing, seal and plastic compatibility, leak detection and service. These need testing, not simulation.
Checkpoint: Tank safety factor above the target, a mass and volume comparison, and a safety question list.
- 10
Plan validation
Design a small validation experiment: a module of a few cells or cell-shaped cartridge heaters in a transparent tank with thermocouples at cell surfaces and in the fluid, run at controlled power. Define what agreement would confirm the model, for example peak temperature within 2 °C. Prepare a fluid compatibility soak test plan for seal and wrap materials at elevated temperature.
Checkpoint: A test rig drawing, instrumentation list and acceptance criteria for model validation.
- 11
Write the research report
Present the duty cases, three concepts, CFD set-up, verification, sensitivity, transient comparison and mass, volume and safety assessment. State the conditions under which immersion is justified for a two-wheeler pack, such as high fast-charge rates in hot climates, and where air cooling remains adequate. Separate what the models show from what still needs physical testing, and list the inputs whose uncertainty most affects the conclusions.
Checkpoint: A report or paper draft with clear conclusions, uncertainty and a validation plan.
Testing & validation
- Energy balance within 5 per cent for every steady case.
- Mesh and time-step sensitivity on peak cell temperature within 0.5 °C.
- Natural convection result consistent with a correlation at the computed Rayleigh number.
- Conclusions checked for robustness against ±20 per cent changes in uncertain inputs.
- A physical validation rig defined with acceptance criteria.
Expected results
Three pack concepts with shared cells, steady and transient thermal results across hot-climate and fast-charge duty, mass, volume and pump power penalties, a sensitivity-bounded comparison and a defined validation experiment.
Research angle for your PhD
The contribution is a fair, design-level comparison of air, passive immersion and pumped immersion for small packs under Indian hot-climate and fast-charge duty, with uncertainty bounds. Novelty can come from identifying the fast-charge rate and ambient at which immersion becomes worthwhile, from validated passive-convection results, and from a transparent account of mass, volume and safety penalties. Experimental validation of at least one concept is needed for a strong publication.
Take it further
- Build and test the validation module and calibrate the CFD.
- Study thermal runaway propagation behaviour in fluid with a dedicated test, not CAD alone.
- Compare fluids with different viscosity and flash point.
- Add a cost model with SOLIDWORKS Costing for tank and seals.
Viva and review questions
Why might passive immersion fail to cool enough?
Natural convection depends on fluid viscosity and gap size. A viscous fluid in narrow gaps may barely circulate, so heat leaves mainly by conduction through the fluid, which is slow.
What did you do about uncertain fluid and cell properties?
I ran a ±20 per cent sensitivity study on the key inputs and reported which conclusions stayed the same across that range.
What can CAD and CFD not tell you about immersion?
Long-term material compatibility, leak behaviour, fire performance and thermal runaway propagation. These need physical testing and must be checked against current standards such as AIS-156.
Why include mass and volume in a thermal study?
For a two-wheeler, extra fluid and tank mass reduces range and handling, so a cooling gain is only useful if it outweighs that penalty.
Learn it at DIYguru
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