Periodic Reporting for period 3 - I-BAT (Immersed-cooling Concepts for Electric Vehicle Battery Packs using Viscoelastic Heat Transfer Liquids (I-BAT))
Berichtszeitraum: 2023-04-01 bis 2025-03-31
In the IBAT research project, we explored an innovative approach to battery cell cooling by introducing viscoelastic properties into thermal fluids. This aimed to create a more uniform temperature distribution along the length of a battery pack. The project encompassed the entire development chain—from fluid formulation to the construction of a small-scale battery pack equipped with a thermal management system.
We essentially started from scratch. Drawing from knowledge in other fields, such as transformer oils, we identified dielectric mineral oils as a promising base. Additives that impart viscoelastic properties to fluids were already known, but their application as thermal fluids in battery systems is novel. A key challenge was linking molecular structure to the desired thermal behavior. We addressed fundamental questions such as: Can heat transfer be influenced through elasto-inertial turbulence? And which molecular structures are responsible for these fluid properties?
Extensive testing was conducted to support future commercialization, including material compatibility assessments and initial mechanical stress tests. These efforts led to the elimination of several fluid and additive candidates.
An unexpected discovery was the significant impact of flow geometry on performance. It became clear that there is unlikely to be a universally optimal viscoelastic fluid; instead, optimal performance will depend on the specific pairing of fluid and geometry.
This research project aimed to develop a novel methodology for the efficient cooling of electric batteries in the automotive field. The research activities are structured on three technical objectives and one management objective.
Technical objectives:
• To synthesize novel heat transfer fluids at the molecular and nanoscale level having optimal rheological and thermal properties. The goal is to maximize their thermal and flow performance.
• Fabrication and assembly of linear vortex generators to decisively improve the cooling efficiency by generating specific flow patterns in the cooling system.
• Apply the flow motion concepts in a novel BTMS and develop test fluids in terms of cooling efficiency.
Further objective:
• Management of the project and dissemination of the research findings to academia and industry.
Synthetization of heat transfer (HR) fluids relies on specific estimations provided by various numerical models, which in turn account for the contribution of viscoelastic stresses (i.e. by Phan-Thien-Tanner models) and shear-thinning effects (i.e. through Carreau-Yasuda Modelling approaches). These numerical models should be validated by an experimental dataset related to the rheological and thermal characteristics of the fluids.
We essentially started from scratch. Drawing from knowledge in other fields, such as transformer oils, we identified dielectric mineral oils as a promising base. Additives that impart viscoelastic properties to fluids were already known, but their application as thermal fluids in battery systems is novel. A key challenge was linking molecular structure to the desired thermal behavior. We addressed fundamental questions such as: Can heat transfer be influenced through elasto-inertial turbulence? And which molecular structures are responsible for these fluid properties?
Extensive testing was conducted to support future commercialization, including material compatibility assessments and initial mechanical stress tests. These efforts led to the elimination of several fluid and additive candidates.
An unexpected discovery was the significant impact of flow geometry on performance. It became clear that there is unlikely to be a universally optimal viscoelastic fluid; instead, optimal performance will depend on the specific pairing of fluid and geometry.
This research project aimed to develop a novel methodology for the efficient cooling of electric batteries in the automotive field. The research activities are structured on three technical objectives and one management objective.
Technical objectives:
• To synthesize novel heat transfer fluids at the molecular and nanoscale level having optimal rheological and thermal properties. The goal is to maximize their thermal and flow performance.
• Fabrication and assembly of linear vortex generators to decisively improve the cooling efficiency by generating specific flow patterns in the cooling system.
• Apply the flow motion concepts in a novel BTMS and develop test fluids in terms of cooling efficiency.
Further objective:
• Management of the project and dissemination of the research findings to academia and industry.
Synthetization of heat transfer (HR) fluids relies on specific estimations provided by various numerical models, which in turn account for the contribution of viscoelastic stresses (i.e. by Phan-Thien-Tanner models) and shear-thinning effects (i.e. through Carreau-Yasuda Modelling approaches). These numerical models should be validated by an experimental dataset related to the rheological and thermal characteristics of the fluids.
To deepen our understanding of the heat transfer mechanisms, we developed an in-house CFD code capable of modeling viscoelastic behavior. Additionally, molecular dynamics simulations using LAMMPS allowed us to bridge atomic-scale interactions with mesoscale fluid properties, capturing thermodynamic, transport, and rheological phenomena. This bottom-up approach is currently being integrated with our CFD simulations. Benchmark experiments were used to validate both our in-house CFD model and a newly released OpenFOAM-based open-source code.
A core part of the experimental work involved designing benchmark geometries that induce visco-inertial turbulence. Flow behavior in these geometries was analyzed using Particle Image Velocimetry (PIV) and correlated with heat flux measurements. These measurements were enabled by the novel integration of atom-layer thermopile sensors directly into the test geometries. In these setups, we observed a 2% improvement in heat transfer at Reynolds numbers above 1200.
Building on these insights and validated models, we extended our investigations to real battery cell packs. CFD simulations clearly illustrated the effects of viscoelastic properties: initially, elastic instabilities generate large-scale vortical structures that disrupt boundary layer development near solid surfaces, enhancing heat transfer. In larger battery packs, viscoelasticity helps suppress transient, inertia-driven flow features, reducing drag and promoting a more stable and controllable flow field.
A demonstrator was built to validate the CFD results. However, further validation using actual battery cells was halted due to technical issues. X-ray imaging revealed poor thermal contact between cells and housing, and detached welded tabs indicated subpar manufacturing quality. High tolerances in cell housing fabrication led to misalignment, compromising flow and heat flux measurements. As a result, we designed dummy cells with integrated electrical heaters, though these are not yet complete. Validation experiments will therefore continue beyond the official end of the project.
A core part of the experimental work involved designing benchmark geometries that induce visco-inertial turbulence. Flow behavior in these geometries was analyzed using Particle Image Velocimetry (PIV) and correlated with heat flux measurements. These measurements were enabled by the novel integration of atom-layer thermopile sensors directly into the test geometries. In these setups, we observed a 2% improvement in heat transfer at Reynolds numbers above 1200.
Building on these insights and validated models, we extended our investigations to real battery cell packs. CFD simulations clearly illustrated the effects of viscoelastic properties: initially, elastic instabilities generate large-scale vortical structures that disrupt boundary layer development near solid surfaces, enhancing heat transfer. In larger battery packs, viscoelasticity helps suppress transient, inertia-driven flow features, reducing drag and promoting a more stable and controllable flow field.
A demonstrator was built to validate the CFD results. However, further validation using actual battery cells was halted due to technical issues. X-ray imaging revealed poor thermal contact between cells and housing, and detached welded tabs indicated subpar manufacturing quality. High tolerances in cell housing fabrication led to misalignment, compromising flow and heat flux measurements. As a result, we designed dummy cells with integrated electrical heaters, though these are not yet complete. Validation experiments will therefore continue beyond the official end of the project.
Key outcomes of the project include:
• Development of VE thermal fluids: A unique class of fluids was formulated, offering more stable flow characteristics and improved thermal homogeneity across battery cells.
• Fundamental insights: The project established a clear link between molecular structure and elasto-inertial turbulence, and between fluid properties and flow geometry, enabling targeted fluid-geometry co-optimization.
• Technology Readiness Level (TRL) 7: The concept was demonstrated in a relevant environment, including the construction of a working demonstrator system.
In the area of simulation and modeling:
• A multiscale simulation framework was developed, integrating molecular dynamics (MD) simulations with mesoscale modeling to derive thermodynamic, transport, and rheological properties from atomistic interactions.
• Fluid optimization was guided by this framework, enabling the design of tailored VE fluids for specific thermal management needs.
• Validated CFD models: Both an in-house CFD solver and a recently released open-source code (OpenFOAM) were validated against experimental benchmarks, confirming their accuracy in predicting VE fluid behavior at engineering scales.
In the area of experimental validation:
• Benchmark Geometries: Designed flow channels that link standard flow conditions to heat transfer performance, enabling the derivation of Nusselt number correlations.
• Integral Heat Flux Measurement: Employed Atom-Layer Thermopile (ALTP) sensors for high-resolution, integral heat flux measurements within test geometries.
• Battery Module Flow Bench: Constructed and tested a dedicated flow bench for battery module-level validation, confirming the practical relevance of VE fluids under realistic conditions.
Experimental validation included the design of benchmark geometries that induce visco-inertial turbulence, with flow fields analyzed via PIV and heat transfer measured using integrated atom-layer thermopile sensors. These tests demonstrated up to a 2% improvement in heat transfer at Reynolds numbers above 1200.
Although full validation with real battery cells was limited by manufacturing issues, the project confirmed that immersion cooling with VE fluids significantly outperforms passive air cooling. Continued testing with dummy cells is planned beyond the project’s conclusion.
• Development of VE thermal fluids: A unique class of fluids was formulated, offering more stable flow characteristics and improved thermal homogeneity across battery cells.
• Fundamental insights: The project established a clear link between molecular structure and elasto-inertial turbulence, and between fluid properties and flow geometry, enabling targeted fluid-geometry co-optimization.
• Technology Readiness Level (TRL) 7: The concept was demonstrated in a relevant environment, including the construction of a working demonstrator system.
In the area of simulation and modeling:
• A multiscale simulation framework was developed, integrating molecular dynamics (MD) simulations with mesoscale modeling to derive thermodynamic, transport, and rheological properties from atomistic interactions.
• Fluid optimization was guided by this framework, enabling the design of tailored VE fluids for specific thermal management needs.
• Validated CFD models: Both an in-house CFD solver and a recently released open-source code (OpenFOAM) were validated against experimental benchmarks, confirming their accuracy in predicting VE fluid behavior at engineering scales.
In the area of experimental validation:
• Benchmark Geometries: Designed flow channels that link standard flow conditions to heat transfer performance, enabling the derivation of Nusselt number correlations.
• Integral Heat Flux Measurement: Employed Atom-Layer Thermopile (ALTP) sensors for high-resolution, integral heat flux measurements within test geometries.
• Battery Module Flow Bench: Constructed and tested a dedicated flow bench for battery module-level validation, confirming the practical relevance of VE fluids under realistic conditions.
Experimental validation included the design of benchmark geometries that induce visco-inertial turbulence, with flow fields analyzed via PIV and heat transfer measured using integrated atom-layer thermopile sensors. These tests demonstrated up to a 2% improvement in heat transfer at Reynolds numbers above 1200.
Although full validation with real battery cells was limited by manufacturing issues, the project confirmed that immersion cooling with VE fluids significantly outperforms passive air cooling. Continued testing with dummy cells is planned beyond the project’s conclusion.