Active Heat Transfer Fluids: Enhancement of Convective Heat Transfer by Self-Propelled Particles

Liquid coolants are critical components of many modern technologies, including hybrid electric vehicle batteries, solar receivers, and cooling systems for electronics. Sustaining the growth of these technologies while minimizing their carbon footprint requires coolants that transfer heat efficiently. Since the 1990s, there has been significant research into the use of suspended nanoparticles to improve the heat transfer performance of liquids. The resulting suspensions, known as nanofluids, generally have higher thermal conductivities than the liquids alone, but the improvements provided by nanoparticles are limited. In this work, we investigate the use of self-propelled particles (SPPs), which move autonomously in liquids, and especially the "micro-stirring" they cause in the surrounding fluid, to enhance the transport of heat through liquids. We hypothesize that the enhancement depends primarily on the SPPs' size, speed, material, and volume fraction. We conducted simulations to demonstrate the efficacy of this approach. We also demonstrate this phenomenon experimentally by placing an SPP suspension in contact with a heated surface; for a constant heat flux, a more effective coolant will result in a lower surface temperature. Thus, the heater surface temperature is a figure of merit for cooling performance. In this study, we investigate the heat transfer enhancement performance of several different SPP designs in various geometries. The results will inform the prospective use of coolants containing SPPs, which we term "active heat transfer fluids" (AHTF), in energy and environmental applications.