Analytical and Experimental Investigation of Evaporation from Porous Capillary Structures

2. Analytical And Experimental Investigation of Evaporation from Porous Capillary Structures Presented to ONR Materials Research Review Meeting May 28-30, 2003 Woods Hole, MA G.P. “Bud” Peterson, C. Li and G. Benitiz Department of Mechanical, Aerospace & Nuclear Engineering, Rensselaer Polytechnic Institute Troy, NY 12180

3. What is a Heat Pipe?

4. How does it work?

5. What is it good for?

6. OUTLINE • Analytical Modeling • Formation of the thin liquid film • Evaporation limit • Experimental Investigation • Results and Discussion • Applications and Significance • Acknowledgement

7. BACKGROUND Objective: • To investigate the formation of thin films on capillary surfaces; • To determine the evaporation limit on capillary surface; • To enhance the evaporation limit through optimization of the pore structure, physical properties such as thermal conductivity and wettability; • To maximize the capillary pumping capability through the optimization of the evaporation heat transfer limit.

8. Results and Discussion (7) Comparison of the heat fluxes through thin capillary wick, submerged wick surface and pool boiling.

9. Interfacial Region

10. Heat Flux Distribution Heat Flux & Film Thickness

11. Capillary Structures of Interest • Surfaces investigated include: • Sintered powders • Metal foams • Screen meshes • Micro channel polymers

12. Mathematical Model (1) Physical Model: Evaporation process on a heated surface coated with a single layer of porous material, here metal screen mesh, with liquid supplied by capillary action, producing a wetted surface with saturated liquid in the cells. Cross-section of the screen mesh Screen mesh cell

13. Mathematical Model

14. Mathematical Model The Formation of Bubbles in capillary structures is dominated by the porous structure and superheat between the heated wall and the bulk liquid-phase. Critical bubble radius where For ideal gas

15. Mathematical Model Formation of the bubble in the sharp corner area: a). Superheat b). The geometric shape and size of the cell c). Capillary pressure

16. Mathematical Model Assumptions: 1. Evaporation take places only on the liquid surface 2. Heat transfer through the liquid layer is dominated by conduction Critical boiling heat flux Boundary conditions Conduction through the layer

17. Liquid Distribution Meniscus region Thin film region

18. Results and Discussion (1) Temperature distribution in the thin liquid film formed between the wires at high heat fluxes.

19. Experimental Test Facility (Saturated Structures) • 1. Test article-porous layer 2. Vacuum chamber • 3. Steam Condenser 4. Vacuum pump • 5. Liquid tube. 6. Power supplier • 7. Thermal bath 8. Data acquisition system

20. Test Facility – Test Articles • Advantages: • Changeable porous surfaces • Changeable surface size • Adjustable surface level • Easy to measure the surface temperatures • Using camera to monitor the thin film profile on the porous surface. • Can measure pool boiling on thin porous surface.

21. V Triangular Grooved Polymer Film

22. Experimental Test Facility (Wicking Height Tests)

23. Experimental Investigation - Test Articles Wetting point

24. Results and Discussion Effect of operating temperature (vapor-phase pressure) on the boiling limit of copper screen mesh layer.

25. Results and Discussion Effect of the capillary pressure on the boiling limit of the thin liquid film.

26. Results and Discussion Effect of thermal conductivity of wick layer on the critical boiling heat flux on copper screen mesh.

27. Conclusions • Thin film evaporation has a dramatically higher heat transfer coefficient than pool boiling or submerged surfaces covered with a thin porous layer. • Thin film evaporation can be modeled using a single cell approach; • The formation and profile of the thin film is affected by the wettability and surface tension of the working fluid as well as heat flux; • The majority of the heat transfer occurs in the thin film region of the liquid meniscus resulting in a very high heat flux in this area; • The evaporation heat transfer is significantly affected by the capillary pressure, and increases in the capillary pressure results in a reduction of the evaporation heat transport limit; • Higher thermal conductivity wicking structures have a higher evaporation heat transfer coefficient;

28. Applications and Significance • Electronics applications • Miniature Heat Pipes for Electronic Applications • Wore Bonded Heat Pipes • Spacecraft Thermal Control • Onboard electronics • Deployable radiators • Treatment of Neocortical Epilepsy • Implantable thermal devices

29. PC Wireboard Cooling

30. Miniature Heat Pipes for Electronics Applications

31. Wire Bonded Heat Pipes

32. Transhab Spacecraft (Stowed)

33. Transhab Spacecraft (Deployed)

34. 0.2 mV 20 sec 60C 0.2 mV 5 sec 60C Effect of local cooling on neocortical seizures. A. Control seizure recorded ipsilateral to focal injection of the convulsant 4-aminopyridine lasts over 100 seconds. B. Activation of Peltier directly contacting cortex rapidly terminated a seizure. Treatment of Neocortical Epilepsy Seizure Detection

35. Figure 1. Left – Individual Peltier device used in the experiments described in this application. Right – Two chips glued to the end of copper rod that served as holder and heat sink. Temperature at the brain-Peltier interface was monitored by thermocouple on surface of one of the chips (arrow) Treatment of Neocortical Epilepsy Individual Peltier Device TC

36. Rectangular Grooved Polymer Film

37. Acknowledgement The authors would like to acknowledge the support of the Office of Naval Research.