Boiling heat transfer enhancement by combining novel copper metal foam configurations as rings on a heating horizontal tube with a hybrid nanofluid: An experimental study

This experimental study investigates the enhancement of pool-boiling heat transfer using a novel interrupted copper foam ring configuration on a horizontal tube in an Al2O3–SiO2 hybrid nanofluid. The impact of nanofluid concentrations (0.001%–0.1% Vol.), foam thicknesses (5 and 10 mm), porosities (85% and 90%), and pore densities (30 and 60 PPI) was systematically evaluated. Results demonstrate that optimal performance was achieved with a 5-mm-thick ring (90% porosity, 60 PPI) at 0.05% Vol. hybrid concentration. Under these conditions, a peak heat transfer coefficient (HTC) of 16.74 kW/m2K was recorded at a heat flux of 163.26 kW/m2. A maximum enhancement ratio of 2.01 (101% enhancement over smooth tube) was observed at a moderate heat flux (96.55 kW/m2), attributed to the synergistic effect of improved surface wettability and the strategic ring-gap arrangement. This configuration facilitates spatial decoupling of vapor/liquid paths, providing low-resistance axial and radial channels for rapid vapor escape, thereby eliminating vapor retention. Conversely, at 0.1% Vol., performance gain diminished due to nanoparticle retention and increased hydraulic resistance within the porous matrix. This ring-shaped design offers a superior alternative to foam coverage by balancing high nucleation site density with enhanced permeability, providing a highly efficient solution for advanced thermal management systems.