Two-Phase Heat Transfer Lab May 28-30, 2003

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

Two-Phase Heat Transfer Lab May 28-30, 2003 What is a Heat Pipe? What is a Heat Pipe?

Two-Phase Heat Transfer Lab May 28-30, 2003 How does it work? How does it work?

Two-Phase Heat Transfer Lab May 28-30, 2003 What is it good for? What is it good for?

Two-Phase Heat Transfer Lab May 28-30, 2003 l Analytical Modeling Formation of the thin liquid filmFormation of the thin liquid film Evaporation limitEvaporation limit l Experimental Investigation l Results and Discussion l Applications and Significance l Acknowledgement OUTLINE OUTLINE

Two-Phase Heat Transfer Lab May 28-30, 2003 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. BACKGROUND BACKGROUND

Two-Phase Heat Transfer Lab May 28-30, 2003 Results and Discussion (7) Results and Discussion (7) Comparison of the heat fluxes through thin capillary wick, submerged wick surface and pool boiling.

Two-Phase Heat Transfer Lab May 28-30, 2003 Interfacial Region

Two-Phase Heat Transfer Lab May 28-30, 2003 Heat Flux Distribution Heat Flux & Film Thickness

Two-Phase Heat Transfer Lab May 28-30, 2003 Capillary Structures of Interest l l Surfaces investigated include: – – Sintered powders – – Metal foams – – Screen meshes – – Micro channel polymers

Two-Phase Heat Transfer Lab May 28-30, 2003 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 porous material, here metal screen mesh, with liquid supplied by capillary action, producing a wetted surface with saturated liquid in the cells.

Two-Phase Heat Transfer Lab May 28-30, 2003 Mathematical Model

Two-Phase Heat Transfer Lab May 28-30, 2003 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. where For ideal gas Critical bubble radius

Two-Phase Heat Transfer Lab May 28-30, 2003 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

Two-Phase Heat Transfer Lab May 28-30, 2003 Mathematical Model Critical boiling heat flux Conduction through the layer Boundary conditions Assumptions: 1. Evaporation take places only on the liquid surface 2. Heat transfer through the liquid layer is dominated by conduction

Two-Phase Heat Transfer Lab May 28-30, 2003 Liquid Distribution Thin film region Meniscus region

Two-Phase Heat Transfer Lab May 28-30, 2003 Results and Discussion (1) Results and Discussion (1) Temperature distribution in the thin liquid film formed between the wires at high heat fluxes.

Two-Phase Heat Transfer Lab May 28-30, 2003 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

Two-Phase Heat Transfer Lab May 28-30, 2003 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.

Two-Phase Heat Transfer Lab May 28-30, 2003 Triangular Grooved Polymer Film V

Two-Phase Heat Transfer Lab May 28-30, 2003 Experimental Test Facility (Wicking Height Tests) Experimental Test Facility (Wicking Height Tests)

Two-Phase Heat Transfer Lab May 28-30, 2003 Wetting point Experimental Investigation - Test Articles Experimental Investigation - Test Articles

Two-Phase Heat Transfer Lab May 28-30, 2003 Results and Discussion Results and Discussion Effect of operating temperature (vapor-phase pressure) on the boiling limit of copper screen mesh layer.

Two-Phase Heat Transfer Lab May 28-30, 2003 Results and Discussion Results and Discussion Effect of the capillary pressure on the boiling limit of the thin liquid film.

Two-Phase Heat Transfer Lab May 28-30, 2003 Results and Discussion Results and Discussion Effect of thermal conductivity of wick layer on the critical boiling heat flux on copper screen mesh.

Two-Phase Heat Transfer Lab May 28-30, 2003 l Thin film evaporation has a dramatically higher heat transfer coefficient than pool boiling or submerged surfaces covered with a thin porous layer. l Thin film evaporation can be modeled using a single cell approach; 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 transfer occurs in 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 wicking structures have a higher evaporation heat transfer coefficient; Conclusions Conclusions

Two-Phase Heat Transfer Lab May 28-30, 2003 l Electronics applications –Miniature Heat Pipes for Electronic Applications –Wore Bonded Heat Pipes l Spacecraft Thermal Control –Onboard electronics –Deployable radiators l Treatment of Neocortical Epilepsy –Implantable thermal devices Applications and Significance Applications and Significance

Two-Phase Heat Transfer Lab May 28-30, 2003 PC Wireboard Cooling PC Wireboard Cooling

Two-Phase Heat Transfer Lab May 28-30, 2003 Miniature Heat Pipes for Electronics Applications

Two-Phase Heat Transfer Lab May 28-30, 2003 Wire Bonded Heat Pipes

Two-Phase Heat Transfer Lab May 28-30, 2003 Transhab Spacecraft (Stowed)

Two-Phase Heat Transfer Lab May 28-30, 2003 Transhab Spacecraft (Deployed)

Two-Phase Heat Transfer Lab May 28-30, 2003 Treatment of Neocortical Epilepsy 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. 0.2 mV 20 sec 0.2 mV 5 sec 60C60C 60C60C Seizure Detection

Two-Phase Heat Transfer Lab May 28-30, 2003 Treatment of Neocortical Epilepsy Individual Peltier Device 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) TC

Two-Phase Heat Transfer Lab May 28-30, 2003 Rectangular Grooved Polymer Film

Two-Phase Heat Transfer Lab May 28-30, 2003 The authors would like to acknowledge the support of the Office of Naval Research. Acknowledgement Acknowledgement