The urgent and indispensable need to develop new methods of enhancing heat transfer efficiency to improve energy devices’ performance cannot be overlooked in this era of green energy and sustainable technologies. Nanofluids research has proliferated in the past decade, and reports indicate that nanofluids can be used for heat transfer applications in engineering and in general and/or commercial industries. Nanofluid is the dispersion of nanoparticles with high thermal conductivity in common working fluids. Nevertheless, a growing area of research in recent years has involved using two or more nanoparticles in a base fluid, known as hybrid nanofluids. Studies show that hybrid nanofluids exhibited better thermal and rheological characteristics than mono nanofluids. In addition, many researchers have reported on the thermal-fluid behaviors of nanofluids in comparison with hybrid nanofluids on natural convection in cavity flows. This review discusses hybrid nanofluids preparation, stability analysis and characterization, thermal properties and heat transfer characteristics in cavities. Furthermore, hybrid nanofluids demonstrated better heat transfer characteristics than mono nanofluids or conventional fluids even if more research is needed in terms of hybrid nanofluids preparation, stability, characterization and applications.

The use of nanofluids for convectional heat transfer has become a spry area of research in recent years with the aim of improving heat transfer efficiency. Hybrid nanofluids have attracted significant attention and are advancing research and industrial applications since they involve employing more than one type of nanoparticle(s) in a base fluid. They enhance heat transfer by combining the chemical and physical properties of several nanoparticles concurrently and providing the properties in a homogeneous state. However, few experimental studies have focused on natural convective heat transfer using hybrid nanofluids. In this study, the natural convection of alumina – multiwalled carbon nanotube/water hybrid nanofluids formulated using a two-step method at a percentage weight ratio of 10:90 Al₂O₃: MWCNT at various nanoparticles volume concentrations of 0.00, 0.05, 0.10, 0.15, and 0.20 vol% was studied inside a square cavity (AR = 1) with two vertical walls which are isothermal, aimed at the Rayleigh number (Ra) range of 2.81 × 10⁸ to 8.58 × 10⁸. The average Nusselt number (Nu_av), heat transfer coefficient (h_av), heat transfer (Q_av), and Rayleigh number (Ra) were considered at varying temperature gradients of 20°C – 50°C. Al₂O₃-MWCNT/water hybrid nanofluid with 0.10 vol% volume concentration was discovered to have the maximum value for h_av,Q_av, and Nu_av. However, it was also observed that a further increase in the hybrid nanoparticles’ volume concentration led to their deterioration at various temperature gradients. The maximum enhancements of 44%, 49%, and 42% were noted for h_av,Q_av, and Nu_av, respectively, at ∆T = 50 °C, in comparison with the base fluid. Al₂O₃-MWCNT/water hybrid nanofluids application in a square cavity demonstrated enhanced natural convection. This present study concluded that hybrid nanofluids as heat transfer fluid significantly improved heat transfer performance compared to the base fluid.

The need for affordable, clean, efficient, and sustainable solar cells informed this study. Metal oxide TiO₂/NiO heterojunction solar cells were fabricated using the spray pyrolysis technique. The optoelectronic properties of the heterojunction were determined. The fabricated solar cells exhibit a short-circuit current of 16.8 mA, open-circuit voltage of 350 mV, fill factor of 0.39, and conversion efficiency of 2.30% under 100 mW/cm² illumination. This study will help advance the course for the development of low-cost, environmentally friendly, and sustainable solar cell materials from metal oxides.