Abstract
A numerical study on heat transfer and entropy generation in natural convection of non-Newtonian nanofluid flow has been explored within a differentially heated two-dimensional wavy porous cavity. In the present study, copper (Cu)–water nanofluid is considered for the investigation where the specific behavior of Cu nanoparticles in water is considered to behave as non-Newtonian based on previously established experimental results. The power-law model and the Brinkman-extended Darcy model has been used to characterize the non-Newtonian porous medium. The governing equations of the flow are solved using the finite volume method with the collocated grid arrangement. Numerical results are presented through streamlines, isotherms, local Nusselt number and entropy generation rate to study the effects of a range of Darcy number (Da), volume fractions (ϕ) of nanofluids, Rayleigh numbers (Ra), and the power-law index (n). Results show that the rate of heat transfer from the wavy wall to the medium becomes enhanced by decreasing the power-law index but increasing the volume fraction of nanoparticles. Increase of porosity level and buoyancy forces of the medium augments flow strength and results in a thinner boundary layer within the cavity. At negligible porosity level of the enclosure, effect of volume fraction of nanoparticles over thermal conductivity of the nanofluids is imperceptible. Interestingly, when the Darcy–Rayleigh number Ra^*gg 10, the power-law effect becomes more significant than the volume fraction effect in the augmentation of the convective heat transfer process. The local entropy generation is highly dominated by heat transfer irreversibility within the porous enclosure for all conditions of the flow medium. The particular wavy shape of the cavity strongly influences the heat transfer flow pattern and local entropy generation. Interestingly, contour graphs of local entropy generation and local Bejan number show a rotationally symmetric pattern of order two about the center of the wavy cavity.
Highlights
Over the last several decades, heat transfer in the natural convection process has received significant impact on various engineering applications, for instance, geothermal systems, heat exchangers, cooling systems for electronic devices, solar energy collector, non-Newtonian chemical processes, chemical reactors and to name a few [1, 2]
Choi and Eastman [5] proposed the concept of nanofluid, mentioning that the nanofluids maintain high thermal conductivity compared to the base fluid used in the study
Hojjat et al [16] did a study on forced convection of nonNewtonian nanofluids inside a uniformly heated circular cylinder; the findings present significant local and average heat transfer coefficients of nanofluid compared to that of the base fluid
Summary
Over the last several decades, heat transfer in the natural convection process has received significant impact on various engineering applications, for instance, geothermal systems, heat exchangers, cooling systems for electronic devices, solar energy collector, non-Newtonian chemical processes, chemical reactors and to name a few [1, 2]. A significant amount of research has been carried out to enhance the heat transfer mechanism in a close enclosure. In this regard, an innovative technique of using nanofluids becomes useful. An innovative technique of using nanofluids becomes useful It has higher thermal conductivity than the conventional fluids. Nanofluids are a homogenous mixture of nanoparticles and base fluid (e.g., water, oil, ethylene glycol). It is made of dispersed nanometer-sized particles where the diameter varies between 1 to 100 nm. In order to acquire knowledge about the enhanced heat transfer mechanism, numerous experiments and simulations have been conducted in the past two decades. Nnanna [6] conducted an experimental study of the heat transfer characteristics of Al2O3-water nanofluid in a differentially heated rectangular cavity and observed that the heat transfer rate is increased even at the small volume fraction (0.2–2%)
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