Thermodynamic and magneto-convective performance of Fe3O4- Al2O3–MWCNT ternary nanofluids in transitional flow regimes

Abstract

The design of advanced heat transfer fluids plays a vital role in improving the thermal efficiency of next-generation cooling systems. This study presents an experimental investigation into the magneto-thermal behavior of a ternary hybrid nanofluid (THNF) formulated from ${\mathrm{Fe}}_3{O}_4$, ${\mathrm{Al}}_2{O}_3$, and multi-walled carbon nanotubes (MWCNTs) dispersed in deionized water (DIW). The study spans transitional and turbulent flow regimes (Re 2300–7000) with nanoparticle volume fractions ranging from 0.025 % to 0.4 %. In the transitional regime, the ternary nanofluid achieved a peak Nusselt number enhancement of 29.49 % at 0.05 vol%, with optimal trade-offs in heat transfer and pressure drop observed near 0.3 vol%. Turbulent regime enhancements ranged from 2.89 % to 14.63 %, with diminishing returns at higher concentrations. To further augment convective performance, externally applied magnetic fields with sine, square, and triangular waveforms were introduced. Among them, square wave excitation yielded the highest thermal gain (up to 39.21 %), followed by triangular (38.13 %) and sine waves (35.5 %) in transitional flows. Magnetic modulation consistently improved heat transfer in both regimes, albeit with increased pressure loss. A Thermal Efficiency Index (TEI) analysis revealed values above unity across all test cases, indicating a net thermohydraulic benefit. Furthermore, an entropy generation assessment showed that waveform-induced mixing mitigated thermal and viscous irreversibilities, thereby enhancing overall thermodynamic efficiency. These findings highlight the potential of waveform-controlled magnetic fields to optimize nanofluid performance in regime-sensitive thermal applications