Investigation of Thin Film Evaporation Limit in Single Screen Mesh Layers

1. Investigation of Thin Film Evaporation Limit in Single Screen Mesh Layers Presented to IMECE 2002 Nov. 19, 2002, New Orleans, LA Yaxiong Wang & G.P. “Bud” Peterson Department of Mechanical, Aerospace & Nuclear Engineering, Rensselaer Polytechnic Institute Troy, NY12180

3. OUTLINE • Background • Modeling Investigation • Formation of the thin liquid film • Evaporation limit • Results and Discussion • Conclusions • Acknowledgement

4. BACKGROUND Objective: • To study the formation of the thin film on a capillary surface; • To evaluate the evaporation limit on a capillary surface; • To investigate the effects of pore structure, physical properties such as thermal conductivity and wettability on the thin film evaporation process; • To determine the effect of capillary pressure on the evaporation heat transfer limit.

5. Mathematical Model (1) Physical Model: Evaporation process on a heated surface coated with a single layer of metal screen mesh. Assuming liquid is supplied sufficiently by capillary action and the liquid is saturated in the cells. Cross-section of the screen mesh Screen mesh cell

6. Mathematical Model (2)

7. Mathematical Model (3) Formation of Bubbles in a wick layer is dominated by the porous structure and superheat between the heated wall and the bulk liquid-phase Critical bubble radius where For idea gas

8. Mathematical Model (4) Formation of the bubble in the sharp corner area: a). Superheat b). The geometric shape and size of the cell c). Capillary pressure

9. Mathematical Model (5) Assumption: 1. Evaporation only take places on the liquid surface 2. Heat transfer through the liquid layer by conduction Critical boiling heat flux Boundary conditions Conduction through the layer

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

11. Results and Discussion (2) Evaporation heat flux through the liquid meniscus region formed between mesh wires

12. Results and Discussion (3) Temperature distribution in a flat liquid film formed between mesh wires.

13. Results and Discussion (4) Evaporation heat flux through the flat liquid film in the cell.

14. Results and Discussion (5) Effect of operating temperature (vapor-phase pressure) on the boiling limit of copper screen mesh layer.

15. Results and Discussion (6) Effect of the capillary pressure on the boiling limit of the thin liquid film.

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

17. Results and Discussion (8) Effect of the contact condition on the critical boiling limit of the thin liquid film.

18. Results and Discussion (9) Effect of thermal conductivity of wick layer on the critical boiling heat flux on copper screen mesh.

19. Conclusions • Thin film evaporation is easily formed in the cells of a single mesh; • 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 goes through 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 meshes have a higher evaporation heat transfer coefficient; • Thin film evaporation has a higher heat transfer coefficient than pool boiling or submerged surfaces covered with a thin porous layer.

20. Pproblems unresolved ? • Experimental verification • Liquid profile in the cells • Boiling limit in the single screen mesh • Stability of the thin film evaporation process

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