arXiv Physics @arxiv_physics@qoto.org

Hydrogel evaporative cooling has recently emerged as an appealing strategy for personal cooling. However, with the increasing prevalence of extreme heat events featuring high temperatures (above 40$^{\circ}$C) and relative humidity (RH$> 30\%$), hydrogel alone may not achieve thermal comfort under most conditions, as it must be maintained at a sufficiently high temperature-often exceeding the skin comfort temperature ($\sim$35.8$^{\circ}$C)-to achieve effective evaporation in hot, humid environments. This study integrates thermoelectric devices (TEDs) with hydrogels to create a personal cooling solution suited to extreme heat. TEDs pump heat away from the skin, maintaining it at a comfortable temperature, while simultaneously raising the temperature of a hydrogel layer positioned on top of the TEDs to enhance its evaporation rate. The TED-hydrogel tandem device outperforms TEDs or hydrogel alone in extreme conditions (up to 55$^{\circ}$C and RH$> 30\%$). Furthermore, the active temperature control enabled by the TEDs allows the system to adapt to varying thermal loads and environmental conditions. With a manageable hydrogel and battery weight, this cooling system can operate for over six hours. These results demonstrate the potential of hybrid evaporative and thermoelectric cooling as an efficient, adaptable, and sustainable personal cooling solution to combat extreme heat.

This paper presents a novel discovery of a symmetry-breaking effect in porous media with porosity between 0.8-0.9, which we are referring to as the intermediate porosity flow regime. Using large eddy simulation, we studied how heat transfer and turbulent convection occurs within these materials at a microscopic level. We observed symmetry-breaking in porous structures made of regularly spaced circular cylinders, a common design in heat exchangers, immediately following the laminar to turbulent flow transition between Reynolds numbers of 37 and 100. Asymmetric patterns persisted up to Reynolds numbers of 1,000. The initial breakdown of symmetry occurs through a Hopf bifurcation, creating an oscillating flow pattern as shear layers interact around the solid obstacles. When the flow becomes turbulent, random variations in the timing of vortex oscillations (caused by the secondary instability) create asymmetric distributions of fluid velocity and temperature throughout the porous space. This leads to the formation of alternating channels with high and low velocity fluid flow. At the macroscale level, this loss of symmetry creates residual transverse drag force components and asymmetric heat flux distribution on the solid obstacle surfaces. Interestingly, the oscillating flow pattern promotes attached flow on the circular cylinder surfaces, which enhances heat transfer from the cylinders to the fluid. We observe that this secondary flow instability is the primary mechanism of enhanced turbulent heat flux from porous media with circular cylinders compared to those with square cylinders.