1. SURFACE TENSION AND SELF-REWETTING FLUIDS
SPIN-OFF FROM SPACE MICROGRAVITY RESEARCH TO
ELECTRONICS COOLING AND RENEWABLE ENERGIES

2. Outline of the presentation
Origin of research studies on surface tension-driven flows in space microgravity environments in Naples.
Marangoni convection
Systems with non-linear dependence of surface tension
Self-rewetting fluids and applications
Heat pipes
Spin-off from fundamental research
Patents and commercial heat pipe performances improvements
New perspectives: advanced space radiator panels, remote heat exchangers for electronics cooling, thermal management systems for server and mainframes, solar collectors

3. Marangoni Convection in Space Microgravity Environments
First Marangoni convection experiment during Spacelab SL-1 Mission
( ESA Erasmus Experiment Archive)
Science 13 July 1984: Vol no. 4658, pp DOI: /science Prev | Table of Contents | Next Articles
Materials
Marangoni Convection in Space Microgravity Environments
L. G. NAPOLITANO. Institute Umberto Nobile, University of Naples, Naples, Italy
Thermally induced surface-driven convection (thermal Marangoni convection) was investigated in the Spacelab 1 microgravity environment. The configuration studied is related to the floating-zone technique used for crystal growth. The first objective of the experiment was to verify theoretical calculations in a stable floating zone of 10 cm height.
Submitted on March 27, 1984 Accepted on May 23, 1984
Astronaut Ulf Merbold during FPM operations
aboard Spacelab-1 (1983)
                                

4. Marangoni Effect

5. Multicomponent systems
The soap boat. A floating body (length 2.5cm) contains a small volume of soap, which
serves as its fuel in propelling it across the free surface.
Tears of wine. Fluid is drawn from the bulk up the thin film adjoining the walls of
the glass by Marangoni stresses induced by evaporation of alcohol from the free surface.

6. Marangoni Convection around a surface tension minimum under microgravity conditions
(Adv. in Space Research, Vol. 4, Issue 5 (1984), pages 37-41)
J. C. Legros, G. Pétré and M. C. Limbourg-Fontaine
University of Brussels, Chemical Physics Department E. P. (CP.165), 50 Ave. F.D. Roosevelt, B-1050-, Brussels, Belgium
Available online 22 October 2002.
Abstract
At equilibrium, aqueous fatty alcohol solutions presents a surface tension minimum versus temperature. The influence of such an extremum on the Marangoni convection is studied. Two experiments have been performed under microgravity conditions (Texus 8 (1983) and Texus 9 (1984) flights). The velocity fields are determined by following the paths of tracer particles and furthermore, in the Texus 9 experiment, differential interferograms have been recorded

7. Fluids classification according to surface tension behavior
Single component Fluids
<0 (-) -
Negative Binary Mixtures
<0 (-)
Positive Binary Mixtures
>0 (+)
Self-Rewetting Fluids
>0 (+)
* c denotes the concentration of less volatile component 7

8. Ordinary and self-rewetting fluids behaviour
Usual Marangoni effect: Ethanol
Cold side
Hot side
Inverse Marangoni effect:
Water-Heptanol solution
Hot side
Cold side 8

9. Applications to multi-phase heat transfer
For ordinary liquids, the surface tension is a decreasing function of the temperature. For dilute aqueous solutions of high number carbon alcohols the surface tension goes through a minimum and there is a range of temperature in which the surface tension increases. Since these solutions are in non-azeotropic compositions, alcohol preferentially evaporates in the course of liquid/vapour phase change.
Cold
Hot
Heating surface
Surface Tension
Temperature
Ordinary liquids
Surface tension
Self-rewetting fluids
Flow opposite to ordinary liquid Marangoni convection
The surface tension gradient along the liquid-vapour interface, caused by both temperature and concentration gradients, is therefore expected to spontaneously transport liquid towards hot regions on heater surfaces. 9

10. Heat Pipes
Heat pipes are unique devices invented in the early satellites space exploration.
Using latent heat of vaporization of the fluid to transfer heat efficiently at a nearly constant temperature, these systems can be successfully used to control the temperature of spacecrafts components, satellites, electronic devices, propulsion, energy recovery systems and other instruments.
Heat pipes in spacecraft and satellites thermal control
Heat pipes in micro-electronic systems cooling
Heat pipes for air conditioners
Heat pipes in Mobil PC’s system
Heat pipes in gas turbine engine applications
Heat pipes

11. Heat Pipe principle
A heat pipe is a hermetically sealed evacuated tube normally containing a mesh or sintered powder wick and a working fluid in both the liquid and vapor phase.
  When one end of the tube is heated the liquid turns to vapor absorbing the latent heat of vaporization. The hot vapor flows to the colder end of the tube where it condenses and gives out the latent heat. The recondensed liquid then flows back through the wick to the hot end.       
Since the latent heat of evaporation is usually very large, one can transfer several hundred times the amount of heat energy compared to solid copper conductive device for a given temperature difference (e.g W/m/K).
Liquid return by wick

12. Role of gravity and surface tension
In gravity-assisted devices (so called thermosyphons) heat added to the bottom region vaporizes the working fluid. During this phase change process, the fluid picks up the heat associated with its latent heat of vaporization.
Because the vapor in the evaporator region is at a higher temperature and hence at a higher pressure than the vapor in the condenser, the vapor rises and flows to the cooler condenser where it releases the latent heat of vaporization (buoyancy forces assist this process). Gravitational forces then cause the condensate film to flow back down along the heat pipe wall where it can again be vaporized.
Although the inner surface of a thermosyphon may occasionally be lined with grooves or a porous structure to promote return of the condensate to the evaporator or increase the heat transfer coefficient, thermosyphons principally rely upon the local gravitational acceleration for the return of the liquid from the evaporator to the condenser.
top
bottom
A. Heat is absorbed in the evaporating section.
B. Fluid boils to vapor phase.
C. Heat is released from the upper part of cylinder
D. Liquid returns by gravity to the lower part of cylinder.

14. New idea. Heat pipes with self-rewetting fluids.
R. Savino, R. Monti, Space Technology, Vol. 25, n.1, pages (2005)
To improve the heat transfer, one would like to identify additional mechanisms able to pump the liquid phase from the cool to the hot end of the pipe. The basic idea is to help the liquid film to flow away from the condenser by forces other than capillary and gravitational.
]In general, for all the pure working fluids used in conventional heat pipes, the surface tension is a decreasing function of the temperature. Therefore, surface motions due to a surface temperature gradient are directed toward the cold regions of the surface, which may be unfavourable for the return of the liquid to the evaporator.
For binary mixtures the surface tension is a function of the temperature and of the concentration. In this case, two additional forces can be identified in grooved pipes at the film-vapour interface: thermal Marangoni; concentration Marangoni. Both these effects can oppose or favour the film motion towards the hot end. q
wick structure
Hot
Vacuum +
Liquid
Evaporator
Condenser
Adiabatic wall
Evaporation
Condensation
Cold
MARANGONI EFFECT

15. Identification of self-rewetting fluids for heat transfer applications
Selection of the working fluid for heat transfer applications depends on a number of considerations regarding: the operating temperature range, the compatibility with the materials and the fluid thermophysical properties. Different liquids has been considered, including:
Water/alcohols solutions;
Self-rewetting brines with relatively low freezing point (-40°C);
Nanofluids.
In order to compare the different fluids the most relevant thermo-physical properties has been evaluated trought extensive laboratory research activity. Research regarding the following properties:
Surface tension;
Contact angle;
Thermal conductivity. 15

16. Fundamental studies HEATER Video camera CCD THERMO CAMERA CAPILLARY
LIQUID POOL

17. Self-rewetting nanofluids
Suspensions of Ag nanoparticles at very dilute concentrations in water butanol solutions (i.e. self-rewetting nanofluids) have been developed in collaboration with AIST of Japan.
Results on surface tension measurements show the same behavior of self-rewetting fluids. In addition, an improvement in the self-rewetting effect has been found.
SEM images of Ag nanofluids 17

18. Heat pipe performances characterization set-up
Macintosh cpu cooling system
Dummy CPU
Heat pipes
Laptop CPU cooling system

19. Performances of commercial heat pipes
A number of heat transfer performaces tests have been carried out to optimise the composition of the working fluid using a set of commercial heat pipes, filled with different aqueous solutions.
The experiments have been carried out with axially grooved copper heat pipes, with 25 cm length and diameters of 4 mm. The heat pipes were filled with 0.41 cc (or 1.9 cc) of pure water and with the same amount of water solutions of long chain alcohols.
The evaporator is a copper block of 30 mm. Cartridge heaters are introduced inside holes and are heated by a power supply.
WATER
WATER
BINARY MIXTURE
BINARY MIXTURE

20. DIAS laboratory experimental set up
Thermocamera
Data logger
Thermographic images
Temperature profiles
Power supply DC
Experiment
WATER
WATER
BINARY MIXTURE
WATER
WATER
BINARY MIXTURE
Cooling by radiation
and natural convection
BINARY MIXTURE

22. Thermal perfomances of heat pipes
Heat pipes of 250 mm in length with composite wick structure (groove + wire) were manufactured using different working fluids. Experiments were conducted in the horizontal configuration in a vacuum chamber.
4mm diameter
8mm diameter
The dry-out limits of heat pipes with self-rewetting fluids are, in general, more than 50% higher than those of water heat pipes. 22

23. Heat Pipes in remote heat exchangers for Laptop PC and cellular phones

24. Thermal Management systems for server racks and mainframes cooling (Green project sponsored by NEDO, New Energy and Industrial Technology Development Agency, Japan)

25. Thermal Ground Plane (Project sponsored by DARPA and General Electric, USA)

26. Solar Collectors