Experimental Investigation of Micro Heat Pipe Radiators in Radiation Environment

1
(No Transcript)
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
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
Treatment of Neocortical Epilepsy
Seizure Detection
35
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.