Transient Thermophysical Response of Al₂O₃ Nanoparticle-Loaded Fluids in Thick-Walled Microtubes: Conjugate Heat Transfer and System-Level Thermal Filtering

Al₂O₃ nanoparticles are widely dispersed in heat-transfer fluids to form nanofluids with a tailored thermophysical response, because their addition modifies thermal conductivity, viscosity, heat capacity, and consequently the transient thermal behaviour of the fluid–solid system. In this study, the coupled effects of Al₂O₃ nanoparticle loading and solid-wall thickness on the transient response of a thick-walled microtube heated over a finite length at low flow rate, as in heated microfluidic devices, are investigated by numerical simulation. A finite-volume conjugate model with Navier slip is solved directly in the time and frequency domains, with quantified grid and time-step uncertainty. For a wall with ten times the volumetric heat capacity of water, raising the wall-to-radius ratio from to lengthens the response time -fold, lowers the cut-off frequency -fold and conducts of the heat upstream of the heater; for glass and PDMS walls the response time grows only –-fold. This attenuation is capacitive and system-level; up to the trans-wall transfer function stays above . Loading up to vol% shortens the response time by – at fixed Peclet number; at constant flow rate it raises the heat transfer coefficient by but, under the Maiga correlations, nearly triples the pumping power.