Abstract
Two-dimensional numerical simulations are conducted to study forced convection flow of different water-based nanofluids (ZnO, Al2O3, and SiO2) with volume fractions ( ϕ ) = 0–5% and fixed nanoparticle size (dp) = 20 nm for Reynolds numbers (Re) = 50–225 over a double backward-facing step with an expansion ratio (ER) = 2 under constant heat flux (q″ = 3000 W/m2) condition using the finite volume method. Results indicate that the local Nusselt number increases with volume fraction and Reynolds number for all working fluids. In comparison to water, the maximum heat transfer augmentation of about 21.22% was achieved by using water-SiO2 nanofluid at Re = 225 with ϕ = 5% and dp = 20 nm. Under similar conditions, the Al2O3 and ZnO nanofluids demonstrated 14.23% and 11.86% augmentation in heat transfer in comparison to water. The skin friction coefficient decreases with the increase in Re for all working fluids. No significant differences are observed in the values of skin friction coefficient among all working fluids at a particular Re. These results indicate that the heat transfer enhancement has been achieved with no increased energy requirements. In addition, the velocity increases with the rise in Re, with SiO2 nanofluid exhibiting the highest velocity as compared to other working fluids.
Highlights
Heat transfer enhancement has been the topic of great interest for researchers for the past few decades
Our main concern in this paper is the study of flow and heat transfer using nanofluids over a backward-facing step (BFS) channel, which is one of the fundamental problems in fluid mechanics. e BFS geometry is simple but the associated flow dynamics are complexas evidenced by flow separation caused by sudden expansion, forming a recirculation zone consisting of vortices
The effects of types of nanofluids, volume concentration, and Reynolds number on fluid flow and heat transfer have been investigated in terms of Nusselt number, skin friction coefficient, and velocity distribution
Summary
Heat transfer enhancement has been the topic of great interest for researchers for the past few decades. Referring to the work done by Ekiciler [10], the thermal-hydraulic characteristics were investigated using Al2O3/water nanofluid with volume fractions (φ) = 1–5% at Re = 100–500 in a single backward-facing step with an expansion ratio of 2 He observed that the local Nusselt number rapidly rises to the maximum at the separation point and gradually decreases along the heated wall and an almost fixed shape is attained. A peculiar observation was made regarding the behavior of the local Nusselt number along the heated wall in the study done by Abuldrazzaq et al [11] In their work, they analyzed the thermal performance of different conventional fluids (water, ethylene glycol, and ammonia liquid) in a double backward-facing step channel at Re = 98.5–512 under constant heat flux (q’′′ = 2000 W/m2). The results are presented in terms of Nusselt number, skin friction coefficient, and velocity distribution for all working fluids
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