Abstract
Al₂O₃ nanoparticles are widely employed to tailor the thermophysical response of heat-transfer fluids 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 thermophysical response of a thick-walled microtube are investigated numerically. A fully implicit finite-volume conjugate heat-transfer model incorporating viscous dissipation and wall slip is employed, with iterative, temporal, and spatial numerical errors quantified separately. The nanoparticles act through the thermophysical properties of the mixture (Maiga correlations), which over the physically valid range ϕ≤0.10 raise the conductivity by 32% and nearly triple the viscosity. Loading raises the dimensional heat transfer coefficient 1.35-fold, almost entirely through conductivity, since the fully developed Nusselt number (≈4.4) changes by less than 3%. It shortens the 99% response time τ99* by only 5%, with no optimum concentration; the optimum of dilute-limit property models is an extrapolation artefact. At constant velocity the property shifts lower the active Peclet number by 26% and triple the active Brinkman number, a genuine conductivity–viscosity trade-off. Wall thickness, not loading, controls the transient: raising d* from 0.2 to 1.0 lengthens τ*99 11.5-fold, as (d*)1.51, and at d*=1.0 about 27% of the interface heat enters upstream of the heated section through axial wall conduction. Under sinusoidal outer-wall heating, solved exactly in the frequency domain, the assembly is a capacitive low-pass filter with ωc≈0.0025 (d*)-1.61 and ωcτ*99≈5.0–5.8.