Mixed Convection Heat Transfer Characteristics of Al2O3 – MWCNT Hybrid Nanofluid under Thermally Developing Flow; Effects of Particles Percentage Weight

In this research, an experimental investigation of hybrid nanofluid mixed convection heat transfer characteristics is conducted. The research specifically investigates the effects of percentage weight composition (PWC) of nanoparticles in the hybrid nanofluids on mixed convection heat transfer characteristics along the lamina, transition and turbulent regions. Transition boundaries, thermal entrance effects, and the influence of tube axial position were also critically investigated and analysed experimentally. Three hybrid nanofluids of Al2O3 – MWCNT (i.e., Al2O3 (60%) – MWCNT (40%), Al2O3 (50%) – MWCNT (50%) and Al2O3 (40%) – MWCNT (60%)) were prepared using two-step method and then subjected to a constant heat flux through a horizontal circular copper tube with an internal diameter of 8mm. Results show a significant change in heat transfer characteristics with different PWCs. Al2O3 (60%) – MWCNT (40%) have shown a better heat transfer enhancement among the three fluids investigated. Its Nusselt number has an enhancement of more than 5 % better than the other two fluids. Along the transition regime, critical Reynold numbers (Recr) of the three nanofluids were found to have differed slightly, with Recr = 2020, 2000 and 2100 for Al2O3 (60%) – MWCNT (40%), Al2O3 (50%) – MWCNT (50%) and Al2O3 (40%) – MWCNT (60%) respectively. Mixed convection effects were found to be more significant with Al2O3 (60%) – MWCNT (40%) than with the other two fluids. At the axial position of 63.75, its mixed convection strength (Ὠ) was about 41%, which is the highest among the three fluids. Its strength of mixed convection was also found to deteriorate to about 14.75% with an increase in axial distance from the tube inlet. A similar observation was also noticed with other fluids. Thermal entrance effects were only found to be significant at x/d = 15 and 31.25, as their influence diminishes with an increased x/d distance from the tube inlet. It was concluded that both mixed convection and thermal entrance effects resulted in heat transfer enhancement, especially in the lamina region. Their influences decrease with an increase in axial distance from the tube inlet. Mixed convection influences were only present in the lamina and transition region, and their strength was reduced with an increase in Reynold number and axial position.