By Matthias H. Buschmann

1. Nanofluids in thermosyphons and heat pipes: Overview of recentexperiments and modelling approaches By Matthias H. Buschmann PPT by Heliokinesis Research Group

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. PPT by Heliokinesis Research Group

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 togetherwith 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) PPT by Heliokinesis Research Group

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 - 100 nm, a maximum 20 nm - 40 nm PPT by Heliokinesis Research Group

7. Some Statistics • Most authors provide only short comments with respect to thepreparation 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 theyhave not employed any stabilization or surfactant • Nanoparticle concentrations: 0.003 - 5.3 vol. %, 0.1 - 0.5 wt. %, and 1 - 104ppm • 16 experiments are conducted only in vertically orientated gadgets, 9 studies investigate only horizontally positionedapparatus PPT by Heliokinesis Research Group

8. Some Statistics • In seven experiments the inclination angle is varied • Only one publication reports inclinationangles between -90O (evaporator on top) and 90O (evaporator atbottom) for an oscillating heat pipe. PPT by Heliokinesis Research Group

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 isspecified as ratio of working fluid volume divided to total internalvolume • depending on the design of the gadget. • Inclination angle • Operation temperature PPT by Heliokinesis Research Group

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 formagnetic nanofluids or magnetically enhanced heat pipes • Nanofluids with carbon nanotubes PPT by Heliokinesis Research Group

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 PPT by Heliokinesis Research Group

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 PPT by Heliokinesis Research Group 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. PPT by Heliokinesis Research Group

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. PPT by Heliokinesis Research Group

15. Modelling and prediction • Semi-empirical models • Models based on conservation equations PPT by Heliokinesis Research Group

16. Possible mechanisms • nanofluids improve thermal performance due to enhanced thermal conductivity • 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 PPT by Heliokinesis Research Group

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 PPT by Heliokinesis Research Group