27-06-2014, 11:02 AM
Advanced Flow and Heat Transfer Fluids
[attachment=65397]
Nanofluid Structure
Although liquid molecules close to a solid surface are known to form layered structures, little is known about the interactions between this nanolayers and thermo-physical properties of these solid/liquid nano-suspensions.
ANL team (Choi et.al.) proposed that the nanolayer acts as a thermal bridge between a solid nanoparticle and a bulk liquid and so is key to enhancing thermal conductivity.
From this thermally bridging nanolayer idea, a structural model of nanofluids that consists of solid nanoparticles, a bulk liquid, and solid-like nanolayers is hypothesized
Wet-Nanotechnology: nanofluids’ applications
Advanced, hybrid nanofluids:
Heat-transfer nanofluids
Tribological nanofluids
Surfactant and Coating nanofluids
Chemical nanofluids
Process/Extraction nanofluids
Environmental (pollution cleaning) nanofluids
Bio- and Pharmaceutical-nanofluids
Medical nanofluids (drug delivery and functional tissue-cell interaction)
Background
Need for Advanced Flow and Heat-Transfer Fluids and Other Critical Applications
Concept of Nanofluids
Materials for Nanoparticles and Base Fluids
Methods for Producing Nanoparticles/Nanofluids
Characterization of Nanoparticles and Nanofluids
Thermo-Physical Properties
Flow and Heat-Transfer Characterization
Advanced Flow and Heat-Transfer Challenges[/b]
The heat rejection requirements are continually increasing due to trends toward faster speeds (in the multi-GHz range) and smaller features (to <100 nm) for microelectronic devices, more power output for engines, and brighter beams for optical devices.
Cooling becomes one of the top technical challenges facing high-tech industries such as microelectronics, transportation, manufacturing, and metrology.
Conventional method to increase heat flux rates:
extended surfaces such as fins and micro-channels
increasing flow rates increases pumping power.
However, current design solutions already push available technology to its limits.
NEW Technologies and new, advanced fluids with potential to improve flow & thermal characteristics are of critical importance.
Nanofluids are promising to meet and enhance the challenges
Concept of Nanofluids
Conventional heat transfer fluids have inherently poor thermal conductivity compared to solids.
Conventional fluids that contain mm- or m-sized particles do not work with the emerging “miniaturized” technologies because they can clog the tiny channels of these devices.
Modern nanotechnology provides opportunities to produce nanoparticles.
Argonne National Lab (Dr. Choi’s team) developed the novel concept of nanofluids.
Nanofluids are a new class of advanced heat-transfer fluids engineered by dispersing nanoparticles smaller than 100 nm (nanometer) in diameter in conventional heat transfer fluids
Why Use Nanoparticles?
The basic concept of dispersing solid particles in fluids to enhance thermal conductivity can be traced back to Maxwell in the 19th Century.
Studies of thermal conductivity of suspensions have been confined to mm- or mm-sized particles.
The major challenge is the rapid settling of these particles in fluids.
Nanoparticles stay suspended much longer than micro-particles and, if below a threshold level and/or enhanced with surfactants/stabilizers, remain in suspension almost indefinitely.
Furthermore, the surface area per unit volume of nanoparticles is much larger (million times) than that of microparticles (the number of surface atoms per unit of interior atoms of nanoparticles, is very large).
These properties can be utilized to develop stable suspensions with enhanced flow, heat-transfer, and other characteristics
Production of Copper Nanofluids
Nanofluids with copper nanoparticles have been produced by a one-step method.
Copper is evaporated and condensed into nanoparticles by direct contact with a flowing and cooled (low-vapor-pressure) fluid.
ANL produced for the first time stable suspensions of copper nanoparticles in fluids w/o dispersants.
For some nanofluids, a small amount of thioglycolic acid (<1 vol.%) was added to stabilize nanoparticle suspension and further improve the dispersion, flow and HT characteristics
Enhanced Nanofluid Thermal Conductivity
Nanofluids containing <10 nm diameter copper (Cu) nanoparticles show much higher TC enhancements than nanofluids containing metal-oxide nanoparticles of average diameter 35 nm.
Volume fraction is reduced by one order of magnitude for Cu nanoparticles as compared with oxide nanoparticles for similar TC enhancement.
The largest increase in conductivity (up to 40% at 0.3 vol.% Cu nanoparticles) was seen for a nanofluid that contained Cu nanoparticles coated with thioglycolic acid.
A German research group has also used metal nanoparticles (NPs) in fluids, but these NPs settled. The ANL innovation was depositing small and stable metal nanoparticles into base fluids by the one-step direct-evaporation method.
Nonlinear Increase in Conductivity with Nanotube Loadings
Nanotubes yield by far the highest thermal conductivity enhancement ever achieved in a liquid: a 150% increase in conductivity of oil at ~1 vol.%.
Thermal conductivity of nanotube suspensions (solid circles) is much greater than predicted by existing models (dotted lines).
The measured thermal conductivity is nonlinear with nanotube volume fraction, while all theoretical predictions clearly show a linear relationship (inset).
Temperature-Dependent Conductivity
Das et al. (*) explored the temperature dependence of the thermal conductivity of nanofluids containing Al2O3 or CuO nanoparticles.
Their data show a two- to four-fold increase in thermal conductivity enhancement over a small temperature range, 20°C to 50°C.
The strong temperature dependence of thermal conductivity may be due to the motion of nanoparticles
Limitations and Need for TC modeling
The discoveries of very-high thermal conductivity and critical heat flux clearly show the fundamental limits of conventional models for solid/liquid suspensions.
The necessity of developing new physics/models has been recognized by ANL team and others.
Several mechanisms that could be responsible for thermal transport in nanofluids have been proposed by ANL team and others.
CONCLUSION
Development of methods to manufacture diverse, hybrid nanofluids with polymer additives with exceptionally high thermal conductivity while at the same time having low viscous friction.
High thermal conductivity and low friction are critical design parameters in almost every technology requiring heat-transfer fluids (cooling or heating). Another goal will be to develop hybrid nanofluids with enhanced lubrication properties.
Applications range from cooling densely packed integrated circuits at the small scale to heat transfer in nuclear reactors at the large scale
[attachment=65397]
Nanofluid Structure
Although liquid molecules close to a solid surface are known to form layered structures, little is known about the interactions between this nanolayers and thermo-physical properties of these solid/liquid nano-suspensions.
ANL team (Choi et.al.) proposed that the nanolayer acts as a thermal bridge between a solid nanoparticle and a bulk liquid and so is key to enhancing thermal conductivity.
From this thermally bridging nanolayer idea, a structural model of nanofluids that consists of solid nanoparticles, a bulk liquid, and solid-like nanolayers is hypothesized
Wet-Nanotechnology: nanofluids’ applications
Advanced, hybrid nanofluids:
Heat-transfer nanofluids
Tribological nanofluids
Surfactant and Coating nanofluids
Chemical nanofluids
Process/Extraction nanofluids
Environmental (pollution cleaning) nanofluids
Bio- and Pharmaceutical-nanofluids
Medical nanofluids (drug delivery and functional tissue-cell interaction)
Background
Need for Advanced Flow and Heat-Transfer Fluids and Other Critical Applications
Concept of Nanofluids
Materials for Nanoparticles and Base Fluids
Methods for Producing Nanoparticles/Nanofluids
Characterization of Nanoparticles and Nanofluids
Thermo-Physical Properties
Flow and Heat-Transfer Characterization
Advanced Flow and Heat-Transfer Challenges[/b]
The heat rejection requirements are continually increasing due to trends toward faster speeds (in the multi-GHz range) and smaller features (to <100 nm) for microelectronic devices, more power output for engines, and brighter beams for optical devices.
Cooling becomes one of the top technical challenges facing high-tech industries such as microelectronics, transportation, manufacturing, and metrology.
Conventional method to increase heat flux rates:
extended surfaces such as fins and micro-channels
increasing flow rates increases pumping power.
However, current design solutions already push available technology to its limits.
NEW Technologies and new, advanced fluids with potential to improve flow & thermal characteristics are of critical importance.
Nanofluids are promising to meet and enhance the challenges
Concept of Nanofluids
Conventional heat transfer fluids have inherently poor thermal conductivity compared to solids.
Conventional fluids that contain mm- or m-sized particles do not work with the emerging “miniaturized” technologies because they can clog the tiny channels of these devices.
Modern nanotechnology provides opportunities to produce nanoparticles.
Argonne National Lab (Dr. Choi’s team) developed the novel concept of nanofluids.
Nanofluids are a new class of advanced heat-transfer fluids engineered by dispersing nanoparticles smaller than 100 nm (nanometer) in diameter in conventional heat transfer fluids
Why Use Nanoparticles?
The basic concept of dispersing solid particles in fluids to enhance thermal conductivity can be traced back to Maxwell in the 19th Century.
Studies of thermal conductivity of suspensions have been confined to mm- or mm-sized particles.
The major challenge is the rapid settling of these particles in fluids.
Nanoparticles stay suspended much longer than micro-particles and, if below a threshold level and/or enhanced with surfactants/stabilizers, remain in suspension almost indefinitely.
Furthermore, the surface area per unit volume of nanoparticles is much larger (million times) than that of microparticles (the number of surface atoms per unit of interior atoms of nanoparticles, is very large).
These properties can be utilized to develop stable suspensions with enhanced flow, heat-transfer, and other characteristics
Production of Copper Nanofluids
Nanofluids with copper nanoparticles have been produced by a one-step method.
Copper is evaporated and condensed into nanoparticles by direct contact with a flowing and cooled (low-vapor-pressure) fluid.
ANL produced for the first time stable suspensions of copper nanoparticles in fluids w/o dispersants.
For some nanofluids, a small amount of thioglycolic acid (<1 vol.%) was added to stabilize nanoparticle suspension and further improve the dispersion, flow and HT characteristics
Enhanced Nanofluid Thermal Conductivity
Nanofluids containing <10 nm diameter copper (Cu) nanoparticles show much higher TC enhancements than nanofluids containing metal-oxide nanoparticles of average diameter 35 nm.
Volume fraction is reduced by one order of magnitude for Cu nanoparticles as compared with oxide nanoparticles for similar TC enhancement.
The largest increase in conductivity (up to 40% at 0.3 vol.% Cu nanoparticles) was seen for a nanofluid that contained Cu nanoparticles coated with thioglycolic acid.
A German research group has also used metal nanoparticles (NPs) in fluids, but these NPs settled. The ANL innovation was depositing small and stable metal nanoparticles into base fluids by the one-step direct-evaporation method.
Nonlinear Increase in Conductivity with Nanotube Loadings
Nanotubes yield by far the highest thermal conductivity enhancement ever achieved in a liquid: a 150% increase in conductivity of oil at ~1 vol.%.
Thermal conductivity of nanotube suspensions (solid circles) is much greater than predicted by existing models (dotted lines).
The measured thermal conductivity is nonlinear with nanotube volume fraction, while all theoretical predictions clearly show a linear relationship (inset).
Temperature-Dependent Conductivity
Das et al. (*) explored the temperature dependence of the thermal conductivity of nanofluids containing Al2O3 or CuO nanoparticles.
Their data show a two- to four-fold increase in thermal conductivity enhancement over a small temperature range, 20°C to 50°C.
The strong temperature dependence of thermal conductivity may be due to the motion of nanoparticles
Limitations and Need for TC modeling
The discoveries of very-high thermal conductivity and critical heat flux clearly show the fundamental limits of conventional models for solid/liquid suspensions.
The necessity of developing new physics/models has been recognized by ANL team and others.
Several mechanisms that could be responsible for thermal transport in nanofluids have been proposed by ANL team and others.
CONCLUSION
Development of methods to manufacture diverse, hybrid nanofluids with polymer additives with exceptionally high thermal conductivity while at the same time having low viscous friction.
High thermal conductivity and low friction are critical design parameters in almost every technology requiring heat-transfer fluids (cooling or heating). Another goal will be to develop hybrid nanofluids with enhanced lubrication properties.
Applications range from cooling densely packed integrated circuits at the small scale to heat transfer in nuclear reactors at the large scale