25-03-2011, 02:40 PM
NANOFLUIDS.pptx (Size: 1.36 MB / Downloads: 451)
NANOFLUIDS
INTRODUCTION
• Need for efficient working
• Miniaturization
• Proper working
• Low conductivity of conventional fluids
[water, ethylene glycol, mineral oil]
• Limitation of solid-liquid suspensions
• Suspension of nanometer (10-9) sized particles
• Nanofluid technology
(i) Nanoscience
(ii) Nanotechnology
(iii) Thermal engineering
• Coined by Choi
• Less than 100nm
• Low volume fraction
CONVENTIONAL METHODS OF HEAT TRANSFER
Disperse micrometer or millimeter sized particles in heat transfer fluids.
Major problem
• Settling down
• Cause wearing
• Large mass
Increasing surface area
/flow velocity
Advantages
• High surface to volume ratio
• Thermal effectiveness
Cannot be applied to miniaturized products,already been maximized
NANOPARTICLES AND BASE FLUIDS
Nanoparticles
• Aluminum oxide (Al2O3)
• Titanium dioxide (TiO2)
• Copper oxide (CuO)
Base fluids
• Water
• Oil
• Ethylene glycol
PREPARATION OF NANOFLUIDS
Inert gas condensation (2-step).
Schematic of IGC .
1-capacitance manometer
2-LN2 filling tube
3- LN2 exhaust
4-glass vacuum chamber
5-cold plate
6-evaporation boat
7-moisture trap
8-argon gas cylinder
9-disc shutter,
10-high vacuum valve, 11-vacuum pumps, 12-power supply
Advantages of IGC
• Wide variety of nanopowders
• Commercialized
Disadvantages
• Agglomeration
• Poor dispersion
• Direct evaporation
• Chemical vapor deposition
• Chemical precipitation
THERMAL CONDUCTIVITY MODELS
1) Maxwell’s model(Classical model)
Applicable to
• Homogenous
• Isotropic composite material
with randomly dispersed non interacting
spherical particles having
• uniform size
• dilute solutions
Appropriate for predicting properties such as electrical conductivity, dielectric constant and magnetic permeability
The expressions for the ratio of effective conductivity to fluid conductivity
• (ke / kf)=1+([3ϕ (α-1)]/[(α+2)-ϕ(α-1)])
• ϕ -volume fraction or concentration of the dispersed particles
• α -ratio of thermal conductivity of the particle to that of the fluid and
2) Hamilton and crosser model (H&C model)
• Applicable to non-spherical
The expressions for the ratio of effective
conductivity to fluid conductivity is
• (ke / kf)=[α+(n-1)(1+ ϕ(α-1))]/ [α+(n-1)- ϕ(α-1)]
• n -shape factor to account for differences in the shape of the particles