Abstract

The practical deployment of nanofluids within industrial thermal engineering is frequently constrained by nanoparticle agglomeration and subsequent sedimentation. This experimental study investigates the efficacy of active low-frequency acoustic excitation in enhancing convective heat transfer performance and temporarily mitigating the macroscopic settling of Al₂O₃-water nanofluids within a counter-flow double-pipe heat exchanger. Surfactant-free colloidal suspensions of γ-Al₂O₃ nanoparticles were synthesized at volume fractions ranging from 0.2% to 0.8%, and subjected to continuous acoustic frequencies of 100 Hz, 500 Hz, and 1000 Hz. Statistically validated three-dimensional surface response modeling (p<0.001) reveals that the maximum convective heat transfer coefficient, reaching approximately 620 W/m². K, is attained at the maximum tested volume fraction of 0.8% coupled with a 1000 Hz acoustic frequency. It is hypothesized that the low-frequency acoustic waves generate sustained macroscopic acoustic streaming, which perturb the viscous sublayer and induce forced mixing. Consequently, qualitative visual inspections suggest that this acoustic agitation temporarily delays macroscopic particle sedimentation over the observed short-term operating window without requiring chemical surfactants. While this work demonstrates the fundamental thermal potential of active non-chemical interventions, long-term colloidal stability, net energy benefits, and in-situ hydraulic penalties remain unverified, highlighting critical requirements for future industrial validation.