Effect of Self-Propelled Particles on Convective Heat Transfer: A Simulation Study*

Liquid coolants are critical components of many modern technologies, e.g., electronics cooling, solid-state lighting, particle-based solar receivers, etc. 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 using energy they harvest from their surrounding environment, and especially the "micro-stirring" they cause in the surrounding fluid, to enhance the transport of heat in liquids. We hypothesize that the enhancement depends primarily on the SPPs' size, speed, and volume fraction. To test this hypothesis, we conduct simulations using the discrete element method and finite element method to solve for the particle trajectories and the fluid velocity and temperature fields, respectively. Using the simulations, we investigate the effects of changing SPP size, speed, and volume fraction on the convective heat transfer coefficient as a metric of cooling performance. The results in this work will inform the prospective use of SPP-based liquid coolants, which we term "active heat transfer fluids," in medical, energy, and environmental applications.