Experimental analysis of heat transfer to shear-thinning viscoelastic coolants for optimizing surface topographies in immersed battery cooling systems

In many novel applications, batteries undergo high charging and discharging rates, which, among other effects, leads to high thermal wear. Specifically the lifetime of Li-ion batteries reduces significantly under operation outside of a certain range of operating temperatures, which severely impairs the sustainability of the related applications. As air-cooling and pipe-based cooling systems often do not provide enough cooling power or temperature homogeneity, there currently exits strong interest in immersed battery cooling systems. To reach optimum heat transfer at minimum pumping power, the topography of the surface exposed to the coolant, as well as the coolant properties have to be optimized. Especially shear-thinning viscoelastic liquids seem to be promising candidates for coolants, as secondary flow patterns can arise in their flow at low Reynolds numbers. We here present an experimental setup, which allows evaluating different combinations of surface topography and coolant in a flow cavity with a geometry, which is directly relatable to flow between neighboring battery cells. We measure the maximum temperature, as well as the temperature gradient in flow direction of a heated structured sample plate exposed to flow of either pure water or water with 0.1% (w/w) xanthan added. We find that surface topography, which leads to a better cooling performance in combination with the former (Newtonian) coolant, loses this advantage when used with the latter (shear-thinning viscoelastic) coolant. We also find indications for secondary flow arising in the non-Newtonian liquid in combination with another surface geometry. This will help guiding the design of surface topographies for shear-thinning viscoelastic coolants.