1. PPT by Heliokinesis Research Group
Nanofluids in thermosyphons and heat pipes: Overview of recent experiments and modelling approaches
By Matthias H. Buschmann
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2. Organization of Paper
Focus: aplication of nanofluids in closed two-phase & open thermosyphons, heat pipes, & oscillating heat pipes
Main focus: when nanoparticle are added to working fluids of these gadgets.
4 main parts of the survey discuss:
effects which are related to operatings parameters of gadgets
effects which are related to nanofluid characteristics
the general thermal performance of the gadgets
modelling approaches
Tabel 1: compiles all experimental studies discussed.
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4. Schematics
(a) Heat pipe, (b) and oscillating heat pipe, (c) adapted from Refs

5. Some Statistics Among the experiments:
11 turn their attention to closed two-phase thermosyphons
1 turn their attention to open thermosyphons
18 turn their attention to wicked or grooved heat pipes
8 turn their attention to oscillating heat pipes
51 nanofluids have been tested all together with these experiments (A majority, 41, are water-based. Other basefluids are the refrigan R11, EG, acetone)
Nanoparticles employed are metals namely silver (Ag), gold (Au) and copper (Cu), oxides (Al2O3, TiO2, SiO2, CuO, ZnO, Fe2O3) or variations of carbon (diamond, carbon nanotubes)
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6. Some Statistics Among the experiments:
13 with silver
11 with alumina
5 with CuO
4 with pure TiO2
2 with silica and iron
1 with ZnO
The size of the nanoparticles: 2 nm nm, a maximum 20 nm - 40 nm
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7. Some Statistics
Most authors provide only short comments with respect to the preparation of their nanofluids
91% two-step methods (employ ultra-sonication), only 3 experiments were carried out one-step methods
The sonication time:1 h - 20 h (max 4h-6h)
1/3 of the experimental groups state that they have not employed any stabilization or surfactant
Nanoparticle concentrations: vol. %, wt. %, and ppm
16 experiments are conducted only in vertically orientated gadgets, 9 studies investigate only horizontally positioned apparatus
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8. Some Statistics In seven experiments the inclination angle is varied
Only one publication reports inclination angles between -90O (evaporator on top) and 90O (evaporator at bottom) for an oscillating heat pipe.
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9. Effects with respect to gadget parameters
Filling ratio
It is defined as the ratio of working fluid volume to internal evaporator volume
for oscillating heat pipes the filling ratio is specified as ratio of working fluid volume divided to total internal volume
depending on the design of the gadget.
Inclination angle
Operation temperature
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10. Effects with respect to nanofluid parameters
Basefluid
Particle size, shape, material, and suspension stabilization
Concentration of nanoparticles
Latent heat and surface tension of nanofluids
Nanofluids with magnetic nanoparticles
Two publications present results for magnetic nanofluids or magnetically enhanced heat pipes
Nanofluids with carbon nanotubes
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11. Effects with respect to thermal performance of gadgets
Evaporator
one of the major physical phenomena responsible for the decrease of the thermal resistance is a porous layer built by nanoparticles on the evaporator surface
nearly 90% of all experimental results indicate either a significant decrease of the evaporator temperature or an increase of the evaporator heat transfer coefficient
Condenser
the majority of experiments which indicated that the condenser was more or less not affected by nanofluids, except a group from Shanghai Jiaotong University (China) showed the opposite
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12. Effects with respect to thermal performance of gadgets
Thermal resistance of whole gadget
Thermal resistance is one of the main characteristics of thermosyphons, heat pipes, and oscillating heat pipes.
the majority of the studies indicate a decrease of thermal resistance of the whole gadget
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Experimentally determined reduction of thermal resistance.

13. Effects with respect to thermal performance of gadgets
Specifics of oscillating heat pipes
In all three orientations (vertical, horizontal, and top down positions) the evaporation temperature of the nanofluid filled OHP was lower than the reference case with DI-water
Additional the authors found that the temperature fluctuations at evaporator and condenser were much more pronounced when employing the nanofluid.
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14. Effects with respect to thermal performance of gadgets
Long term tests
the time dependent character of the porous layer has to be considered in more detail.
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15. Modelling and prediction
Semi-empirical models
Models based on conservation equations
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16. Possible mechanisms
nanofluids improve thermal performance due to enhanced thermal conductivity
bubble departure frequency at evaporator is increased due to bubble bombardment by nanoparticles
nanoparticles coat the wick, which enlarges heat transfer area or capillary forces or both of them
the often found concentration optimum follows from counter acting forces
a porous nanoparticle layer forms on the evaporator, which changes wettability
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17. Reference
Matthias H. Buschmann (2013) Nanofluids in thermosyphons and heat pipes: Overview of recent experiments and modelling approaches, International Journal of Thermal Sciences, 72 : 1
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