Entropy generation and thermohydraulics of mixed convection of hybrid-nanofluid in a vertical tube fitted with elliptical-cut twisted tape inserts-a computational study

A numerical investigation is carried out to study the mixed convection of Al2O3-Cu/water hybrid-nanofluid in a vertical tube fitted with elliptical-cut twisted tape inserts (TECT). Thermodynamic irreversibilities are evaluated by calculating the Bejan number, as well as the system's local and total entropy generation. Heat transfer, friction factor, thermal performance factor, and entropy generation analyses are conducted for a range of volumetric concentrations of nanoparticles and flow Reynolds numbers between 7000 and 15,000. The realizable k- e model is employed to simulate the turbulent and heat transferring flow computationally. The results clearly demonstrate the influence of mixed convection on heat transfer and entropy generation. In particular, mixed convection simulations predict greater Nusselt number, friction factor, and thermal performance factor than the corresponding forced convection simulations. Further, it is shown that the Nusselt number and the thermal performance factor for mixed convection are 4.6% and 5.5% higher than those for forced convection, respectively. The results also reveal that at Reynolds numbers of 7000, 9000, and 11,000, the thermal entropy production dominates the total irreversibility of the system. Likewise, frictional entropy production is the dominant mode of total irreversibility in the system at high Reynolds numbers of 13,000 and 15,000.