Natural convection of nanofluid inside a wavy cavity with a non-uniform heating: Entropy generation analysis
21 Mar 2017-International Journal of Numerical Methods for Heat & Fluid Flow (Emerald Publishing Limited)-Vol. 27, Iss: 4, pp 958-980
TL;DR: In this article, the entropy generation in natural convection of nanofluid in a wavy cavity using a single-phase model was analyzed using the finite difference method of the second-order accuracy.
Abstract: Purpose
The main purpose of this numerical study is to study on entropy generation in natural convection of nanofluid in a wavy cavity using a single-phase nanofluid model.
Design/methodology/approach
The cavity is heated non-uniformly from the wavy wall and cooled from the right side while it is insulated from the horizontal walls. The physical domain of the problem is transformed into a rectangular geometry in the computational domain using an algebraic coordinate transformation by introducing new independent variables ξ and η. The governing dimensionless partial differential equations with corresponding initially and boundary conditions were numerically solved by the finite difference method of the second-order accuracy. The governing parameters are Rayleigh number (Ra = 1000-100000), Prandtl number (Pr = 6.82), solid volume fraction parameter of nanoparticles (φ = 0.0-0.05), aspect ratio parameter (A = 1), undulation number (κ = 1-3), wavy contraction ratio (b = 0.1-0.3) and dimensionless time (τ = 0-0.27).
Findings
It is found that the average Bejan number is an increasing function of nanoparticle volume fraction and a decreasing function of the Rayleigh number, undulation number and wavy contraction ratio. Also, an insertion of nanoparticles leads to an attenuation of convective flow and enhancement of heat transfer.
Originality
The originality of this work is to analyze the entropy generation in natural convection within a wavy nanofluid cavity using single-phase nanofluid model. The results would benefit scientists and engineers to become familiar with the flow behaviour of such nanofluids, and will be a way to predict the properties of this flow for the possibility of using nanofluids in advanced nuclear systems, in industrial sectors including transportation, power generation, chemical sectors, ventilation, air-conditioning, etc.
Citations
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TL;DR: In this article, the entropy generation number and the average heat transportation rate of a magnetized Al2O3-H2O nanomaterial natural convection based on entropy generation and L-shaped cavity were calculated through control volume-based finite element method.
Abstract: Natural convected heat transportation attributes can be elaborated better using entropy generation analysis. In current framework, we scrutinized magnetized Al2O3-H2O nanomaterial natural convection based on entropy generation and L-shaped cavity. Non-dimensional forms of governing expressions are computed through Control Volume-based Finite Element Method (CVFEM). Entropy generation number is calculated. Features of active parameters e.g. Rayleigh number, nanoparticles volume-fraction, nanoparticle shape, Hartmann number, magnetic field angle and aspect ratio versus average heat transportation rate (Nusselt number) and the entropy generation number are investigated. For the first time, an economic analysis is introduced for evaluating the performance of the enclosure with consideration cost of nanofluid. Also, in order to assess the performance of the enclosure, six criteria are introduced which two of them are based on the cost of nanofluids. The results were compared with references and a good compromise was seen. According to the results, both the entropy generation number and average heat transportation rate rise when Rayleigh number upsurges. The average heat transportation rate rises with ascending the nanoparticle volume-fraction whereas the entropy generation number declines when nanoparticles concentration ascends. The entropy generation number decreases 15.14% and 8.15% for H a = 25 and H a = 75 , respectively, when ϕ increases from 0 to 0.1.
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TL;DR: In this paper, the transient entropy generation and mixed convection due to a rotating hot inner cylinder within a square cavity having a flexible side wall by using the finite element method and arbitrary Lagrangian-Eulerian formulation was investigated.
Abstract: The current work concentrates on the transient entropy generation and mixed convection due to a rotating hot inner cylinder within a square cavity having a flexible side wall by using the finite element method and arbitrary Lagrangian-Eulerian formulation. Effects of various relevant parameters like Rayleigh number ( 10 4 ⩽ Ra ⩽ 10 7 ), angular rotational velocity ( - 1 ⩽ Ω ⩽ 1 ), dimensionless elasticity modulus ( 10 12 ⩽ E ⩽ 10 15 ) on the convective heat transfer characteristics and entropy generation rates are analyzed for dimensionless time 10 - 8 ⩽ τ ⩽ 3.5 . It is observed that various complex shaped wall deformations are established depending on the non-dimensional elastic modulus of the flexible right wall and cylinder rotation direction. The local and average Nusselt numbers rise with Ra and secondary peaks in the local Nusselt number are established for lower values of Ra. The local heat transfer along the hot cylinder does not change for the case of clockwise rotation of the heated cylinder even if there is a wall deformation in the positive x-direction. The highest average heat transfer and global entropy generation rates are achieved for the case of counter-clockwise rotation of the circular cylinder and for lower values of the flexible wall deformation.
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66 citations
References
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TL;DR: In this article, the authors considered seven slip mechanisms that can produce a relative velocity between the nanoparticles and the base fluid and concluded that only Brownian diffusion and thermophoresis are important slip mechanisms in nanofluids.
Abstract: Nanofluids are engineered colloids made of a base fluid and nanoparticles (1-100 nm) Nanofluids have higher thermal conductivity' and single-phase heat transfer coefficients than their base fluids In particular the heat transfer coefficient increases appear to go beyond the mere thermal-conductivity effect, and cannot be predicted by traditional pure-fluid correlations such as Dittus-Boelter's In the nanofluid literature this behavior is generally attributed to thermal dispersion and intensified turbulence, brought about by nanoparticle motion To test the validity of this assumption, we have considered seven slip mechanisms that can produce a relative velocity between the nanoparticles and the base fluid These are inertia, Brownian diffusion, thermophoresis, diffusioplwresis, Magnus effect, fluid drainage, and gravity We concluded that, of these seven, only Brownian diffusion and thermophoresis are important slip mechanisms in nanofluids Based on this finding, we developed a two-component four-equation nonhomogeneous equilibrium model for mass, momentum, and heat transport in nanofluids A nondimensional analysis of the equations suggests that energy transfer by nanoparticle dispersion is negligible, and thus cannot explain the abnormal heat transfer coefficient increases Furthermore, a comparison of the nanoparticle and turbulent eddy time and length scales clearly indicates that the nanoparticles move homogeneously with the fluid in the presence of turbulent eddies so an effect on turbulence intensity is also doubtful Thus, we propose an alternative explanation for the abnormal heat transfer coefficient increases: the nanofluid properties may vary significantly within the boundary layer because of the effect of the temperature gradient and thermophoresis For a heated fluid, these effects can result in a significant decrease of viscosity within the boundary layer, thus leading to heat transfer enhancement A correlation structure that captures these effects is proposed
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Abstract: Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids that are required in many industrial applications. In this paper we propose that an innovative new class of heat transfer fluids can be engineered by suspending metallic nanoparticles in conventional heat transfer fluids. The resulting {open_quotes}nanofluids{close_quotes} are expected to exhibit high thermal conductivities compared to those of currently used heat transfer fluids, and they represent the best hope for enhancement of heat transfer. The results of a theoretical study of the thermal conductivity of nanofluids with copper nanophase materials are presented, the potential benefits of the fluids are estimated, and it is shown that one of the benefits of nanofluids will be dramatic reductions in heat exchanger pumping power.
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TL;DR: In this paper, an expression for the viscosity of solutions and suspensions of finite concentration is derived by considering the effect of the addition of one solute-molecule to an existing solution, which is considered as a continuous medium.
Abstract: An expression for the viscosity of solutions and suspensions of finite concentration is derived by considering the effect of the addition of one solute‐molecule to an existing solution, which is considered as a continuous medium.
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TL;DR: In this article, a model is developed to analyze heat transfer performance of nanofluids inside an enclosure taking into account the solid particle dispersion, where the transport equations are solved numerically using the finite-volume approach along with the alternating direct implicit procedure.
Abstract: Heat transfer enhancement in a two-dimensional enclosure utilizing nanofluids is investigated for various pertinent parameters. A model is developed to analyze heat transfer performance of nanofluids inside an enclosure taking into account the solid particle dispersion. The transport equations are solved numerically using the finite-volume approach along with the alternating direct implicit procedure. Comparisons with previously published work on the basis of special cases are performed and found to be in excellent agreement. The effect of suspended ultrafine metallic nanoparticles on the fluid flow and heat transfer processes within the enclosure is analyzed and effective thermal conductivity enhancement maps are developed for various controlling parameters. In addition, an analysis of variants based on the thermophysical properties of nanofluid is developed and presented. It is shown that the variances within different models have substantial effects on the results. Finally, a heat transfer correlation of the average Nusselt number for various Grashof numbers and volume fractions is presented.
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