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Numerical Simulation of Natural Convection of a Nanofluid in an Inclined Heated Enclosure Using Two-Phase Lattice Boltzmann Method: Accurate Effects of Thermophoresis and Brownian Forces.

Ahmed M, Eslamian M - Nanoscale Res Lett (2015)

Bottom Line: The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and the base fluid are included in the simulation.The effect of thermophoresis is considered using an accurate and quantitative formula proposed by the authors.Some of the existing results on natural convection are erroneous due to using wrong thermophoresis models or simply ignoring the effect.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical Engineering, Assiut University, Assiut, 71516, Egypt.

ABSTRACT
Laminar natural convection in differentially heated (β = 0°, where β is the inclination angle), inclined (β = 30° and 60°), and bottom-heated (β = 90°) square enclosures filled with a nanofluid is investigated, using a two-phase lattice Boltzmann simulation approach. The effects of the inclination angle on Nu number and convection heat transfer coefficient are studied. The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and the base fluid are included in the simulation. The effect of thermophoresis is considered using an accurate and quantitative formula proposed by the authors. Some of the existing results on natural convection are erroneous due to using wrong thermophoresis models or simply ignoring the effect. Here we show that thermophoresis has a considerable effect on heat transfer augmentation in laminar natural convection. Our non-homogenous modeling approach shows that heat transfer in nanofluids is a function of the inclination angle and Ra number. It also reveals some details of flow behavior which cannot be captured by single-phase models. The minimum heat transfer rate is associated with β = 90° (bottom-heated) and the maximum heat transfer rate occurs in an inclination angle which varies with the Ra number.

No MeSH data available.


Related in: MedlinePlus

Flow streamlines at Ra = 104 and Ra = 106 at various inclination angles. Pr = 7.02 and ϕ = 0.05
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Fig4: Flow streamlines at Ra = 104 and Ra = 106 at various inclination angles. Pr = 7.02 and ϕ = 0.05

Mentions: Flow streamlines for various inclination angles are shown in Fig. 4 for a nanofluid with 5 % particle volume fraction. At Ra = 104, where the induced flow is weak, the effect of the inclination angle is very weak, and in all cases, symmetric concentric single-cell circulations form, consistent with other pure and nanofluid works at low Ra numbers. It is noted that, in these cases, single-fluid models that assume a homogenous flow, also successfully predict the streamlines [9, 10, 12]. At Ra = 106, however, at some inclination angles, particularly at 90° (bottom-heated), predictions of our two-phase flow model are different from refs. [9, 10, 12]. Depending on the magnitude of the Ra number and the extent of the flow perturbation, various solutions may be obtained for a bottom-heated enclosure (β = 90°) filled with a pure fluid; these solutions include a single-cell circulation, double-cell horizontal or vertical circulations, clockwise or counterclockwise [6, 35]. The presence of nanoparticles in our study may be the reason for obtaining a solution other than those for the standard pure fluid single-phase solutions, outlined in ref. [35]. When a single-phase model is used to find the streamlines in a bottom-heated cell at high Ra numbers (106), the presence of nanoparticles makes the fluid more viscous and stable and a single-cell solution is obtained, similar to that reported in ref. [10]. Therefore, one may conclude that, only in some cases, a single-phase model may provide a qualitative solution for the flow patterns in a nanofluid.Fig. 4


Numerical Simulation of Natural Convection of a Nanofluid in an Inclined Heated Enclosure Using Two-Phase Lattice Boltzmann Method: Accurate Effects of Thermophoresis and Brownian Forces.

Ahmed M, Eslamian M - Nanoscale Res Lett (2015)

Flow streamlines at Ra = 104 and Ra = 106 at various inclination angles. Pr = 7.02 and ϕ = 0.05
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4508308&req=5

Fig4: Flow streamlines at Ra = 104 and Ra = 106 at various inclination angles. Pr = 7.02 and ϕ = 0.05
Mentions: Flow streamlines for various inclination angles are shown in Fig. 4 for a nanofluid with 5 % particle volume fraction. At Ra = 104, where the induced flow is weak, the effect of the inclination angle is very weak, and in all cases, symmetric concentric single-cell circulations form, consistent with other pure and nanofluid works at low Ra numbers. It is noted that, in these cases, single-fluid models that assume a homogenous flow, also successfully predict the streamlines [9, 10, 12]. At Ra = 106, however, at some inclination angles, particularly at 90° (bottom-heated), predictions of our two-phase flow model are different from refs. [9, 10, 12]. Depending on the magnitude of the Ra number and the extent of the flow perturbation, various solutions may be obtained for a bottom-heated enclosure (β = 90°) filled with a pure fluid; these solutions include a single-cell circulation, double-cell horizontal or vertical circulations, clockwise or counterclockwise [6, 35]. The presence of nanoparticles in our study may be the reason for obtaining a solution other than those for the standard pure fluid single-phase solutions, outlined in ref. [35]. When a single-phase model is used to find the streamlines in a bottom-heated cell at high Ra numbers (106), the presence of nanoparticles makes the fluid more viscous and stable and a single-cell solution is obtained, similar to that reported in ref. [10]. Therefore, one may conclude that, only in some cases, a single-phase model may provide a qualitative solution for the flow patterns in a nanofluid.Fig. 4

Bottom Line: The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and the base fluid are included in the simulation.The effect of thermophoresis is considered using an accurate and quantitative formula proposed by the authors.Some of the existing results on natural convection are erroneous due to using wrong thermophoresis models or simply ignoring the effect.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical Engineering, Assiut University, Assiut, 71516, Egypt.

ABSTRACT
Laminar natural convection in differentially heated (β = 0°, where β is the inclination angle), inclined (β = 30° and 60°), and bottom-heated (β = 90°) square enclosures filled with a nanofluid is investigated, using a two-phase lattice Boltzmann simulation approach. The effects of the inclination angle on Nu number and convection heat transfer coefficient are studied. The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and the base fluid are included in the simulation. The effect of thermophoresis is considered using an accurate and quantitative formula proposed by the authors. Some of the existing results on natural convection are erroneous due to using wrong thermophoresis models or simply ignoring the effect. Here we show that thermophoresis has a considerable effect on heat transfer augmentation in laminar natural convection. Our non-homogenous modeling approach shows that heat transfer in nanofluids is a function of the inclination angle and Ra number. It also reveals some details of flow behavior which cannot be captured by single-phase models. The minimum heat transfer rate is associated with β = 90° (bottom-heated) and the maximum heat transfer rate occurs in an inclination angle which varies with the Ra number.

No MeSH data available.


Related in: MedlinePlus