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Enhancing surface heat transfer by carbon nanofins: towards an alternative to nanofluids?

Chiavazzo E, Asinari P - Nanoscale Res Lett (2011)

Bottom Line: Nanofluids are suspensions of nanoparticles and fibers which have recently attracted much attention because of their superior thermal properties.As a result, particles are only needed in a small region of the fluid, while dispersion can be controlled in advance through design and manufacturing processes.Numerical evidences suggest a pretty favorable thermal boundary conductance (order of 107 W·m-2·K-1) which makes carbon nanotubes potential candidates for constructing nanofinned surfaces.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Energetics, Politecnico di Torino, Corso Duca degli Abruzzi, 10129 Torino, Italy. pietro.asinari@polito.it.

ABSTRACT

Background: Nanofluids are suspensions of nanoparticles and fibers which have recently attracted much attention because of their superior thermal properties. Nevertheless, it was proven that, due to modest dispersion of nanoparticles, such high expectations often remain unmet. In this article, by introducing the notion of nanofin, a possible solution is envisioned, where nanostructures with high aspect-ratio are sparsely attached to a solid surface (to avoid a significant disturbance on the fluid dynamic structures), and act as efficient thermal bridges within the boundary layer. As a result, particles are only needed in a small region of the fluid, while dispersion can be controlled in advance through design and manufacturing processes.

Results: Toward the end of implementing the above idea, we focus on single carbon nanotubes to enhance heat transfer between a surface and a fluid in contact with it. First, we investigate the thermal conductivity of the latter nanostructures by means of classical non-equilibrium molecular dynamics simulations. Next, thermal conductance at the interface between a single wall carbon nanotube (nanofin) and water molecules is assessed by means of both steady-state and transient numerical experiments.

Conclusions: Numerical evidences suggest a pretty favorable thermal boundary conductance (order of 107 W·m-2·K-1) which makes carbon nanotubes potential candidates for constructing nanofinned surfaces.

No MeSH data available.


Related in: MedlinePlus

Color online. Eolic parks represent a macroscopic analogy of the proposed nanofin concept: wind towers are slim enough to avoid disturbing the planetary boundary layer, but high enough to reach the region where wind is stronger. Similarly nanofins do not interfere with the thermal boundary layer, but they allow direct energy transfer between the wall and the bulk fluid, thus acting as thermal bridges. The picture of the wind farm is provided as courtesy of the European Commission, October 2010: EU Guidance on wind energy development in accordance with the EU nature legislation.
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Figure 1: Color online. Eolic parks represent a macroscopic analogy of the proposed nanofin concept: wind towers are slim enough to avoid disturbing the planetary boundary layer, but high enough to reach the region where wind is stronger. Similarly nanofins do not interfere with the thermal boundary layer, but they allow direct energy transfer between the wall and the bulk fluid, thus acting as thermal bridges. The picture of the wind farm is provided as courtesy of the European Commission, October 2010: EU Guidance on wind energy development in accordance with the EU nature legislation.

Mentions: Clearly, there are difficulties in the experimental measurements [11], but the previous results also reveal some underlaying technological problems. First of all, the CNTs show some bundling or the formation of aggregates originating from the fabrication step. Moreover, it seems reasonable that CNTs encounter poor dispersibility and suspension durability because of the aggregation and surface hydrophobicity of CNTs as a nanofluid filler. Therefore, the surface modification of CNTs or additional chemicals (surfactants) have been required for stable suspensions of CNTs, because of the polar characteristics of base fluid. In the case of surface modification of CNTs, water-dispersible CNTs have been extensively investigated for potential applications, such as biological uses, nanodevices, novel precursors for chemical reagents, and nanofluids [2]. From the above brief review, it is possible to conclude that, despite the great interest and intense research in this field, the results achieved so far cannot be considered really encouraging. Hence, toward the end of overcoming these problems, we introduce the notion of thermal nanofins, with an entirely different meaning with respect to standard terminology. By nanofins, we mean slender nano-structures, sparse enough not to interfere with the thermal boundary layer, but sufficiently rigid and conductive to allow for direct energy transfer between the wall and the bulk fluid, thus acting as thermal bridges. A macroscopic analogy is given by an eolic park, where wind towers are slim enough to avoid disturbing the planetary boundary layer, but high enough to reach the region where the wind is stronger (see Figure 1). In this way, nanoparticles are used only where they are needed, namely, in the thermal boundary layer (or in the thermal laminar sub-layer, in case of turbulent flows, not discussed here), and this might finally unlock the enormous potential of the basic idea behind nanofluids.


Enhancing surface heat transfer by carbon nanofins: towards an alternative to nanofluids?

Chiavazzo E, Asinari P - Nanoscale Res Lett (2011)

Color online. Eolic parks represent a macroscopic analogy of the proposed nanofin concept: wind towers are slim enough to avoid disturbing the planetary boundary layer, but high enough to reach the region where wind is stronger. Similarly nanofins do not interfere with the thermal boundary layer, but they allow direct energy transfer between the wall and the bulk fluid, thus acting as thermal bridges. The picture of the wind farm is provided as courtesy of the European Commission, October 2010: EU Guidance on wind energy development in accordance with the EU nature legislation.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211310&req=5

Figure 1: Color online. Eolic parks represent a macroscopic analogy of the proposed nanofin concept: wind towers are slim enough to avoid disturbing the planetary boundary layer, but high enough to reach the region where wind is stronger. Similarly nanofins do not interfere with the thermal boundary layer, but they allow direct energy transfer between the wall and the bulk fluid, thus acting as thermal bridges. The picture of the wind farm is provided as courtesy of the European Commission, October 2010: EU Guidance on wind energy development in accordance with the EU nature legislation.
Mentions: Clearly, there are difficulties in the experimental measurements [11], but the previous results also reveal some underlaying technological problems. First of all, the CNTs show some bundling or the formation of aggregates originating from the fabrication step. Moreover, it seems reasonable that CNTs encounter poor dispersibility and suspension durability because of the aggregation and surface hydrophobicity of CNTs as a nanofluid filler. Therefore, the surface modification of CNTs or additional chemicals (surfactants) have been required for stable suspensions of CNTs, because of the polar characteristics of base fluid. In the case of surface modification of CNTs, water-dispersible CNTs have been extensively investigated for potential applications, such as biological uses, nanodevices, novel precursors for chemical reagents, and nanofluids [2]. From the above brief review, it is possible to conclude that, despite the great interest and intense research in this field, the results achieved so far cannot be considered really encouraging. Hence, toward the end of overcoming these problems, we introduce the notion of thermal nanofins, with an entirely different meaning with respect to standard terminology. By nanofins, we mean slender nano-structures, sparse enough not to interfere with the thermal boundary layer, but sufficiently rigid and conductive to allow for direct energy transfer between the wall and the bulk fluid, thus acting as thermal bridges. A macroscopic analogy is given by an eolic park, where wind towers are slim enough to avoid disturbing the planetary boundary layer, but high enough to reach the region where the wind is stronger (see Figure 1). In this way, nanoparticles are used only where they are needed, namely, in the thermal boundary layer (or in the thermal laminar sub-layer, in case of turbulent flows, not discussed here), and this might finally unlock the enormous potential of the basic idea behind nanofluids.

Bottom Line: Nanofluids are suspensions of nanoparticles and fibers which have recently attracted much attention because of their superior thermal properties.As a result, particles are only needed in a small region of the fluid, while dispersion can be controlled in advance through design and manufacturing processes.Numerical evidences suggest a pretty favorable thermal boundary conductance (order of 107 W·m-2·K-1) which makes carbon nanotubes potential candidates for constructing nanofinned surfaces.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Energetics, Politecnico di Torino, Corso Duca degli Abruzzi, 10129 Torino, Italy. pietro.asinari@polito.it.

ABSTRACT

Background: Nanofluids are suspensions of nanoparticles and fibers which have recently attracted much attention because of their superior thermal properties. Nevertheless, it was proven that, due to modest dispersion of nanoparticles, such high expectations often remain unmet. In this article, by introducing the notion of nanofin, a possible solution is envisioned, where nanostructures with high aspect-ratio are sparsely attached to a solid surface (to avoid a significant disturbance on the fluid dynamic structures), and act as efficient thermal bridges within the boundary layer. As a result, particles are only needed in a small region of the fluid, while dispersion can be controlled in advance through design and manufacturing processes.

Results: Toward the end of implementing the above idea, we focus on single carbon nanotubes to enhance heat transfer between a surface and a fluid in contact with it. First, we investigate the thermal conductivity of the latter nanostructures by means of classical non-equilibrium molecular dynamics simulations. Next, thermal conductance at the interface between a single wall carbon nanotube (nanofin) and water molecules is assessed by means of both steady-state and transient numerical experiments.

Conclusions: Numerical evidences suggest a pretty favorable thermal boundary conductance (order of 107 W·m-2·K-1) which makes carbon nanotubes potential candidates for constructing nanofinned surfaces.

No MeSH data available.


Related in: MedlinePlus