Double-diffusive natural convection of non-Newtonian nanofluid considering thermal dispersion of nanoparticles in a vertical wavy enclosure

Preetom Nag,Md Mamun Molla

doi:10.1063/5.0058405

Abstract

The flow field, thermal field, and solutal field exposed to thermal and solutal buoyancy forces have been investigated in detail within a wavy enclosure filled with copper(Cu)–water nanofluid incorporating the non-Newtonian characteristics predicted by the power-law viscosity model. During the convection process, the random motion of ultrafine Cu-nanoparticles causing an enhanced energy exchange rate is determined using the thermal dispersion model. The governing equations in a dimensionless form are numerically solved utilizing the finite volume method incorporated with the semi-implicit method for pressure linked equations-revised algorithm. The simulations are carried out with different pertinent parameters, such as the Rayleigh number, Lewis number, power-law index, volume fraction, and buoyancy ratio. The effect of the above parameters on the local Nusselt number (Nu) and the local Sherwood number (Sh) is analyzed to understand the heat and mass transfer properties from the heated wavy surface. Results show that the heat transfer rate from the wavy surface declines, but the mass transfer rate gets stronger with growing Lewis number. Both the heat and mass transfer rates become optimum when the nanofluid behaves as a shear thinning fluid. The distribution of Nu and Sh is found to be periodically attenuated from the lower end to the upper end along the hot wavy surface. The distribution of Nu and Sh is observed to be locally maximum at the crest point of the wavy surface. New correlations to predict the average heat and mass transfer rate concerning the studied parameters are proposed with remarkable accuracy.

Highlights

Double-diffusive convection, recognized nearly six decades ago, has received significant attention in experimental and theoretical studies because of its appearance in various disciplines, such as oceanography, geology, biology, chemical engineering, astrophysics, and material science.1–4 The named convection arises when fluid flow is subjected to the density variation caused by the coexistence of both temperature and solutal gradients
For Rayleigh number (Ra) = 105, ∣ψ∣max rises by 16.16% for shear thinning fluids but decreases by 2.34% for shear thickening fluids when both thermal and solutal diffusion have an equal effect on the flow, i.e., Lewis number (Le) = 1.0. ● The local Nusselt number and the local Sherwood number are extensively high at the lower end of the wavy wall than at the upper end since the buoyancy-driven flow is developed counterclockwise
The distribution of Nu and Sh periodically attenuates from the lower end to the upper end, along with the active wavy surface. ● The convective heat and mass transfer rates from the hot wavy wall decline with increasing the power-law index

Summary

Introduction

Double-diffusive convection, recognized nearly six decades ago, has received significant attention in experimental and theoretical studies because of its appearance in various disciplines, such as oceanography, geology, biology, chemical engineering, astrophysics, and material science. The named convection arises when fluid flow is subjected to the density variation caused by the coexistence of both temperature and solutal gradients. Different convection modes have been numerically investigated concerning the temperature and solutal gradient direction relative to gravity. Several experimental studies regarding thermosolutal convection are performed in a rectangular enclosure considering horizontal temperature and concentration gradient to observe unicellular or multicellular flow structure in a fluid motion for large Lewis numbers and a broad spectrum of buoyancy ratios, the ratio of solutal to thermal buoyancy force. Considering the horizontal temperature and concentration gradient, Gobin and Bennacer studied assisting thermosolutal convection. They reported the attenuation of energy transfer by increasing the ratio of two buoyancy forces, solutal and thermal, at a high Lewis number (Le > 100)

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