Entropy production associated with magnetohydrodynamics (MHD) thermo-solutal natural convection of non-Newtonian MWCNT-SiO2-EG hybrid nano-coolant

Tawsif Mahmud; Taha Chowdhury; Preetom Nag; Md Mamun Molla

doi:10.1016/j.heliyon.2024.e35523

Entropy production associated with magnetohydrodynamics (MHD) thermo-solutal natural convection of non-Newtonian MWCNT-SiO₂-EG hybrid nano-coolant

Heliyon. 2024 Aug 3;10(15):e35523. doi: 10.1016/j.heliyon.2024.e35523. eCollection 2024 Aug 15.

Authors

Tawsif Mahmud^{1

2}, Taha Chowdhury^{1

2}, Preetom Nag^{1

2}, Md Mamun Molla^{1

2}

Affiliations

¹ Department of Mathematics & Physics, North South University (NSU), Dhaka, Bangladesh.
² Center of Applied & Computational Sciences (CACS), NSU, Dhaka, Bangladesh.

Abstract

This article investigates the convective thermal and solutal exchange from the active walls of a trapezium chamber which is filled with multi-walled carbon nanotubes (MWCNT)-silicon dioxide (SiO₂)-ethylene glycol-water hybrid nano-coolant. The hybrid nano-coolant exhibits non-Newtonian shear-thinning rheology and is modeled by the power-law viscosity as per an exploratory report. The convection is generated by both the thermal and solutal buoyancy forces in the presence of a magnetic field. Thermophysical properties of the particular nano-coolants are estimated from the temperature-dependent empirical correlation. An in-house FORTRAN code based on the finite volume method (FVM) has been utilized to simulate the non-dimensional controlling equations representing the physical model. By systematically varying the strength of the magnetic field ( $H a = 0 - 50$ ) and its direction ( $δ_{1} = 0 - 90$ ), the volume fraction of nano-coolant ( $ϕ = 0 - 0.04$ ) and solutal-to-thermal buoyancy ratio ( $N = - 2 to 2$ ), we thoroughly analyzed their impact on the flow field, thermal, and solutal transmission behavior. Significant impacts arising from the shear-thinning nature ( $n < 1$ ) of the nano-coolant were observed, influencing both thermal and solutal transfer rates as reflected in the Nusselt number (Nu) and Sherwood number (Sh). The impact of Ha on Nu and Sh is more substantial for $n < 1$ than the case $n = 1$ . The investigation exposed that when $H a = 30, δ_{1} = 30^{\circ}$ the average Nu ( $\overline{N u}$ ) for nano-ccolants ( $ϕ = 2 %$ ) is maximally enhanced by 80% for $n = 0.6$ compared to the case $n = 1.0$ . For the same nano-coolant with $H a = 30$ and $δ_{1} = 90^{\circ}$ , there is a 128% elevation in the average Sherwood number ( $\overline{S h}$ ) when $n = 0.6$ compared to $n = 1.0$ . The entropy generation ( $\overline{S}$ ) inherent to the heat and mass transfer process and hence a criterion ( $ξ = \overline{S} / \overline{N u}$ ) is computed to address the system thermal performance from the thermodynamics second law. $\overline{S}$ increased as n was reduced, indicating that the shear-thinning effect contributed strongly to the entropy generation.

Keywords: Entropy generation (EGEN); Finite volume method (FVM); Hybrid nanofluids (HNF); Magnetohydrodynamics (MHD); Non-Newtonian rheology; Thermo-solutal convection.