Enhancing Heat Transfer in Immersion Cooling of Battery Packs - Using ALTP Heat Flux Sensors

The effectiveness of immersion cooling for the thermal management of Electric-Vehicle (EV) batteries is crucially influenced by the thermophysical and rheological properties of the heat-transfer liquid. This study emphasizes upon the design requirements for such a fluid in terms of bulk properties, i.e., high electrical resistivity and thermal conductivity, low viscosity, but also relevant to the rheological properties maximizing the heat transfer rate. Key concepts of the implemented research constitute: (i) the promotion of vortical motion in the laminar flow regime, which, in turn, enhances heat transfer by disrupting boundary layers; (ii) vortex stabilization through the addition of viscoelasticity-inducing agents in the base heat-transfer liquid. To improve cooling efficiency, the primary objective is to maximize the achievable heat transfer rate for minimal pumping losses. Hence, a multi-objective optimization process must be set in place where the optimal coolant rheology is dependent on the geometrical features of the battery module. The overall framework of interdependent research activities comprises: (i) the characterization of viscoelastic flow with the use of Particle Image Velocimetry (PIV) in a flow loop with benchmark geometries; (ii) heat-transfer measurements employing a novel Atom Layer Thermopile (ALTP) sensor and (iii) dedicated computational fluid dynamics (CFD) modelling using the Phan-Thien-Tanner constitutive equation for elastic stresses. While there are tailored designs for efficient heat transfer in immersion-cooling paradigms needed, in this paper we concentrate on heat flux measurements when cooling a bluff body. In this work results on heat transfer in the wake behind a square rod were analyzed and discussed. High-viscosity liquids have higher heat transfer at equivalent Re-number. An improvement of heat-transfer due to viscoelastic flow behavior is indicated for high-viscosity liquids, but the trend must be proven with additional experiments. PIV based flow analysis shows a mismatch between the flow pattern and the heat transfer surface.