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Cross-Plane PIV For The Characterization Of Low Viscosity Viscoelastic Fluids

Lukas Weiss, B Rüppel, Tatiana Weiss, Michael Wensing

Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany


Thermal management of electric high-power devices is more important than ever. The electrification of the transport sector and increasing High-Performance-Computing (HPC) are the main driving forces for the increasing demands. Therefore, immersion cooling concepts arise, where the cooling liquid is in direct contact with the heat source, e.g., the CPU of a computer or the cell of a battery module. The main advantage is to eliminate the heat conduction through a cooling plate or heat pipes in the heat transfer path, which increases cooling efficiency. The downside is, that the cooling system could get very bulky, which is not very critical in huge HPC centers but is a showstopper for mobile applications. The project IBAT investigates radically new dielectric cooling fluids, funded by the EU program horizon 2020. These oil-based synthetic liquids are of very low viscosity. This allows very small gaps between battery cells resulting in a compact battery module design. Naturally appearing vortices at the cell edges quickly dissolve due to the low viscosity liquids. A stable boundary layer forms on the cell surface diminishing the heat transfer. To counteract this phenomenon linear vortex generators (LVGs) could be applied to the cell surface. The downside of this solution is increased flow resistance and thus higher pumping losses in the cooling circuit, partly canceling out the advantages of the low viscosity liquids. The more sophisticated solution to the breakup of the thermal boundary layer is to stabilize the natural vortices. This is achieved by adding long polymer chains to the fluids, which give them viscoelastic properties. The initial results are very promising but show that the fluid properties must be tailored to the specific geometry. The catch is that the viscoelastic fluid properties of such low-viscosity fluids cannot be measured with conventional rheometers. A workaround is provided by optical flow analytics in benchmark geometries. One method is the observation of cross-plane vortices in a 180° bend with circular cross-section. The authors describe and discuss the challenges and uncertainties of cross-plane PIV for such experiments.

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