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

2. Two-Phase Heat Transfer Lab November 19, 2002

3. Two-Phase Heat Transfer Lab November 19, 2002 l Background l Modeling Investigation Formation of the thin liquid filmFormation of the thin liquid film Evaporation limitEvaporation limit l Results and Discussion l Conclusions l Acknowledgement OUTLINE OUTLINE

4. Two-Phase Heat Transfer Lab November 19, 2002 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. BACKGROUND BACKGROUND

5. Two-Phase Heat Transfer Lab November 19, 2002 Mathematical Model (1) Screen mesh cell Cross-section of the screen mesh 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.

6. Two-Phase Heat Transfer Lab November 19, 2002 Mathematical Model (2)

7. Two-Phase Heat Transfer Lab November 19, 2002 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 where For idea gas Critical bubble radius

8. Two-Phase Heat Transfer Lab November 19, 2002 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. Two-Phase Heat Transfer Lab November 19, 2002 Mathematical Model (5) Critical boiling heat flux Conduction through the layer Boundary conditions Assumption: 1. Evaporation only take places on the liquid surface 2. Heat transfer through the liquid layer by conduction

10. Two-Phase Heat Transfer Lab November 19, 2002 Results and Discussion (1) Results and Discussion (1) Temperature distribution in the thin liquid film formed between the wires at high heat fluxes.

11. Two-Phase Heat Transfer Lab November 19, 2002 Results and Discussion (2) Results and Discussion (2) Evaporation heat flux through the liquid meniscus region formed between mesh wires

12. Two-Phase Heat Transfer Lab November 19, 2002 Results and Discussion (3) Results and Discussion (3) Temperature distribution in a flat liquid film formed between mesh wires.

13. Two-Phase Heat Transfer Lab November 19, 2002 Results and Discussion (4) Results and Discussion (4) Evaporation heat flux through the flat liquid film in the cell.

14. Two-Phase Heat Transfer Lab November 19, 2002 Results and Discussion (5) Results and Discussion (5) Effect of operating temperature (vapor-phase pressure) on the boiling limit of copper screen mesh layer.

15. Two-Phase Heat Transfer Lab November 19, 2002 Results and Discussion (6) Results and Discussion (6) Effect of the capillary pressure on the boiling limit of the thin liquid film.

16. Two-Phase Heat Transfer Lab November 19, 2002 Results and Discussion (7) Results and Discussion (7) Comparison of the heat fluxes through thin capillary wick, submerged wick surface and pool boiling.

17. Two-Phase Heat Transfer Lab November 19, 2002 Results and Discussion (8) Results and Discussion (8) Effect of the contact condition on the critical boiling limit of the thin liquid film.

18. Two-Phase Heat Transfer Lab November 19, 2002 Results and Discussion (9) Results and Discussion (9) Effect of thermal conductivity of wick layer on the critical boiling heat flux on copper screen mesh.

19. Two-Phase Heat Transfer Lab November 19, 2002 l Thin film evaporation is easily formed in the cells of a single mesh; l 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; l 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; l 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; l Higher thermal conductivity meshes have a higher evaporation heat transfer coefficient; l Thin film evaporation has a higher heat transfer coefficient than pool boiling or submerged surfaces covered with a thin porous layer. Conclusions Conclusions

20. Two-Phase Heat Transfer Lab November 19, 2002 l Experimental verification –Liquid profile in the cells –Boiling limit in the single screen mesh l Stability of the thin film evaporation process Pproblems unresolved ? Pproblems unresolved ?

21. Two-Phase Heat Transfer Lab November 19, 2002 The authors would like to acknowledge the support of the Office of Naval Research. Acknowledgement Acknowledgement