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Heterogeneous nanofluids: natural convection heat transfer enhancement.

Oueslati FS, Bennacer R - Nanoscale Res Lett (2011)

Bottom Line: Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection.The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach.The resulting convective flows are found to be dependent on the local particle concentration φ and the corresponding solutal to thermal buoyancy ratio N.

View Article: PubMed Central - HTML - PubMed

Affiliation: ENS-Cachan Dpt GC/LMT, 61, Av du Président Wilson 94235 Cachan Cedex, France. rachid.bennacer@dgc.ens-cachan.fr.

ABSTRACT
Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration φ and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case.

No MeSH data available.


Related in: MedlinePlus

Vertical velocity on the horizontal mid-plan (RT = 104, φ = 2%, Le = 3 and Sr = 2%).
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Figure 12: Vertical velocity on the horizontal mid-plan (RT = 104, φ = 2%, Le = 3 and Sr = 2%).

Mentions: The vertical velocity along the middle plane of the square enclosure using different nanofluids (for RT = 104, Pr = 6.2, Le = 3 and Sr = 2%) is shown on Figure 12. Due to the floating flow inside the enclosure, the velocity shows a parabolic variation near the isothermal walls. The vertical velocity is susceptible to the nature of nanoparticles where two types of nanoparticles (Al2O3 and TiO2) show similar vertical velocity but the third (Cu) is so different. This is explained in Equation 16 where the Brinkman formula shows that the viscosity of the nanofluid is only sensitive to the volume fraction of particles and not influenced by the type of nanoparticles and the expression of the buoyancy ration which is a function of the mass expansion coefficient that depends on the density of the nature of the particle. Indeed, the mass buoyancy force, in addition to the thermal buoyancy force, intensified the flow. Even then, the vertical velocity of nanofluid is higher than that of pure fluid. It means that particle suspension affects the flow field. The flow velocity is almost zero around the centre of the cavity. The profile also gives idea on flow rotation direction.


Heterogeneous nanofluids: natural convection heat transfer enhancement.

Oueslati FS, Bennacer R - Nanoscale Res Lett (2011)

Vertical velocity on the horizontal mid-plan (RT = 104, φ = 2%, Le = 3 and Sr = 2%).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211280&req=5

Figure 12: Vertical velocity on the horizontal mid-plan (RT = 104, φ = 2%, Le = 3 and Sr = 2%).
Mentions: The vertical velocity along the middle plane of the square enclosure using different nanofluids (for RT = 104, Pr = 6.2, Le = 3 and Sr = 2%) is shown on Figure 12. Due to the floating flow inside the enclosure, the velocity shows a parabolic variation near the isothermal walls. The vertical velocity is susceptible to the nature of nanoparticles where two types of nanoparticles (Al2O3 and TiO2) show similar vertical velocity but the third (Cu) is so different. This is explained in Equation 16 where the Brinkman formula shows that the viscosity of the nanofluid is only sensitive to the volume fraction of particles and not influenced by the type of nanoparticles and the expression of the buoyancy ration which is a function of the mass expansion coefficient that depends on the density of the nature of the particle. Indeed, the mass buoyancy force, in addition to the thermal buoyancy force, intensified the flow. Even then, the vertical velocity of nanofluid is higher than that of pure fluid. It means that particle suspension affects the flow field. The flow velocity is almost zero around the centre of the cavity. The profile also gives idea on flow rotation direction.

Bottom Line: Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection.The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach.The resulting convective flows are found to be dependent on the local particle concentration φ and the corresponding solutal to thermal buoyancy ratio N.

View Article: PubMed Central - HTML - PubMed

Affiliation: ENS-Cachan Dpt GC/LMT, 61, Av du Président Wilson 94235 Cachan Cedex, France. rachid.bennacer@dgc.ens-cachan.fr.

ABSTRACT
Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration φ and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case.

No MeSH data available.


Related in: MedlinePlus