Limits...
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

Dynamic, thermal and species fields for different nature of nanoparticle (RT = 104, φ = 2%, Pr = 6.2, Sr = 2% and Le = 3).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3211280&req=5

Figure 8: Dynamic, thermal and species fields for different nature of nanoparticle (RT = 104, φ = 2%, Pr = 6.2, Sr = 2% and Le = 3).

Mentions: Figure 8 presents the comparison of streamlines and isotherms using different nanofluids: TiO2-water, Al2O3-water and Cu-water for RT = 104. However, we varied the Rayleigh number for different types of nanoparticles, from diffusive state to convection state. For all nanofluids, a single cell movement was observed in a clockwise direction. The values of the maximum stream function show that the intensity of flow is higher for Cu-water than that of TiO2-water and Al2O3-water. Hence, in the case of nanofluid heterogeneous solutal forces are in addition to heat one. The importance of solutal gradients, which differs from one type of nanofluid to another, directly affects the dynamic state and heat transfer (illustrated by figure 9 and 10). Indeed, Figure 11 presents the temperatures (a) and concentrations (b) in the middle horizontal plane of the square enclosure, for different nanofluids (RT = 104, Pr = 6.2, Le = 3 and Sr = 2%), illustrates the distinction of each type. From superposed streamlines and isotherms of both TiO2-water and Al2O3-water nanofluids, we find that the dynamic and thermal fields are similar. This reproaches qualitative aspects explained by the fact that the values of thermophysical properties of TiO2-water and Al2O3-water are comparable. In opposition, this is not the case for the other two nanofluids Cu-water and Al2O3-water, which the isotherms and the streamlines show that the distributions are very distinct.


Heterogeneous nanofluids: natural convection heat transfer enhancement.

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

Dynamic, thermal and species fields for different nature of nanoparticle (RT = 104, φ = 2%, Pr = 6.2, Sr = 2% and Le = 3).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 8: Dynamic, thermal and species fields for different nature of nanoparticle (RT = 104, φ = 2%, Pr = 6.2, Sr = 2% and Le = 3).
Mentions: Figure 8 presents the comparison of streamlines and isotherms using different nanofluids: TiO2-water, Al2O3-water and Cu-water for RT = 104. However, we varied the Rayleigh number for different types of nanoparticles, from diffusive state to convection state. For all nanofluids, a single cell movement was observed in a clockwise direction. The values of the maximum stream function show that the intensity of flow is higher for Cu-water than that of TiO2-water and Al2O3-water. Hence, in the case of nanofluid heterogeneous solutal forces are in addition to heat one. The importance of solutal gradients, which differs from one type of nanofluid to another, directly affects the dynamic state and heat transfer (illustrated by figure 9 and 10). Indeed, Figure 11 presents the temperatures (a) and concentrations (b) in the middle horizontal plane of the square enclosure, for different nanofluids (RT = 104, Pr = 6.2, Le = 3 and Sr = 2%), illustrates the distinction of each type. From superposed streamlines and isotherms of both TiO2-water and Al2O3-water nanofluids, we find that the dynamic and thermal fields are similar. This reproaches qualitative aspects explained by the fact that the values of thermophysical properties of TiO2-water and Al2O3-water are comparable. In opposition, this is not the case for the other two nanofluids Cu-water and Al2O3-water, which the isotherms and the streamlines show that the distributions are very distinct.

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