1. Effectiveness of Linear Spray Cooling in Microgravity
Presented by
Ben Conrad, John Springmann, Lisa McGill
Undergraduates, Engineering Mechanics & Astronautics

2. Heat dissipation requirements
Remove heat fluxes of W/cm2
Applicable to laser diodes, computer processors, etc.
Laser Diode Array
(Silk et al, 2008)

3. Heat dissipation requirements
Current Solutions
Flow boiling
Microchannel boiling
Jet impingement
Spray cooling
Spray cooling is the most promising because it achieves high heat transfer coefficients at low flow rates.
Spray cooling: promising because high heat transfer coefficients for low flow rates. Problem: few examples of practical applications.

4. Limited previous microgravity research
14% variation in the critical heat flux from 0 to 1.8 Gs
Sone et al. (1996): single spray perpendicular to heated surface (100 mm away)
Yoshida, et al. (2001): single spray perpendicular to heated surface (100 mm away)
Microgravity significantly effects critical heat flux
Golliher, et al. (2005): single spray angled 55⁰ in 2.2 sec. drop tower
Significant pooling on the heated surface due largely to surface tension
***NEED TO GET DETAILS ON BAYSINGER/YERKES
The applicability of single spray cooling systems are limited by the difficulty in covering a large area (>1 cm2) and ensuring even spray density over the entire area (Shedd 2007)
Noted a decrease in Nusselt number with acceleration
Yerkes et al. (2004): single spray in micro- and enhanced-gravity.

5. Spray cooling – linear array
Single-spray systems do not cover a large area (> 1 cm2)
Regner and Shedd investigated a linear array of sprays directed 45o onto a heated surface
Directs fluid flow towards a defined exit to avoid fluid management issues
(Shedd, 2007)

6. Experiment basis & hypothesis
Linear spray research showed performance independent of orientation
Performance converged for heat fluxes greater than 20 W/cm^2
BUT still +/- 1G pull at all times
(Regner, B. M., and Shedd, T. A., 2007)

7. Experiment basis & hypothesis
Predict that with similar spray array, spray cooling will function independent of gravity
Performance converged for heat fluxes greater than 20 W/cm^2
Emphasize entire system is in microgravity (pump, etc)

8. Experiment design
Goal: determine variation of heat transfer coefficient h with gravity
q’’: heat flux measured from heater power
Ts: Temperature of heated surface
Tin: Temperature of spray

9. Differential Pressure Sensor
Closed-loop system
Pump
Flow Meter
Pressure Sensor
Bladder
Filter
3 Axis Accelerometer
Spray Box
Heat Exchanger
Differential Pressure Sensor
Therm.
Liquid coolant:
FC-72
Add FC-72 to slide. Highlight the accelerometer.

10. Heater design Ohmite TGHG 1 Ω precision current sense resistor
Four T-type thermocouples embedded in heater
20.6 mm
25.4 mm
8.0 mm
4.3 mm
The material is thin copper, thermocouples embedded near the surface. We assumed thermocouples measure surface temperature.

11. Spray array design Made from microbore tubing: 3.2 cm Shedd, 2007
Picture of actual array, normal to array surface.
Shedd, 2007

12. Spray array & spray box G Fluid inlet & outlet Top half: spray array
Bottom half: heater
Picture of actual array, normal to array surface.
Z-direction

13. Microgravity environment
30 microgravity (nominally 0 g) parabolas lasting 20-25s each
1.8 g is experienced between microgravity

14. Microgravity environment
Add accelerometer data in all directions

15. Procedure: Flow rate Q & heat flux q”
Q (L/min):
0.67
2.67
3.81
q” (W/cm2):
24.9
25.8
26.6
Very conservative heat fluxes used due to experimental nature

16. Epoxy seal failure
Epoxy cracked due to fluid pressure in pre-flight testing
Spray Array
Drain
Epoxy Failure
3.2 cm

17. Epoxy seal failure

18. Visualization shows fluid behavior
Heater
Drain
********Grab two other pictures
Camera

19. Complex fluid behavior
760,758,, ,2216.1,2961.8,3067.8

20. Flight data: flow rate dominates performance
Flowrate determines the heat transfer coefficient
low varies by 2.61%
med by 1.42%
hi by 1.35%
Data fall around a constant heat transfer coefficient
Strongly suggests independence
System experienced acceleration

21. Δh is consistent with Δg for each flow rate
h increases with microgravity
Decreases with enhanced gravity
Plotting the heat transfer coefficient in blue and the system acceleration in green versus flight time for each flow rate shows some interesting relationships:
trends upward in microgravity
downward during enhanced gravity

22. Possible Relationships
h vs. jerk
Increasing variability with flow rate:
Flow rate: L/min L/min L/min
change in acceleration.
increase in variation
Greater turbulence in the spray box?

23. Shedd model for +/- 1 g Shedd (2007) found a correlation of the form:
where the heat transfer coefficient, h, is a function of
the average spray droplet flux, Q”, and constants:
the fluid’s density, ρ,
specific heat, cp,
Prandtl number, Pr,
an arbitrary constant, C in [m.5s-.5], for a linear spray array,
and a constant power, a.
Regner and Shedd earth-based orientation tests
Shedd noted a relationship between:
Constant C accounts for variations between spray arrays
Plotting Regner and Shedd and applying the model,

24. Microgravity results fit trend
Q” is believed to be 10-20% high due to the broken epoxy on the spray array
follow the same relationship that Shedd described
measured flow rates are 10-20% high
improve our agreement with Shedd’s 1g model.
This simple relationship may allow the performance of a microgravity system to be predicted by 1g earth-based orientation tests.

25. Future steps Effect of spray characteristics
Fluid Inlet
Nozzle diameter
Nozzle length
Nozzle edge type
Effect of spray characteristics
Spray hole diameter and length
Hole entrance and exit design
Enhanced surfaces with linear spray cooling?
Spray nozzle design
The extent to which this causes fluid cavitation.
Enhanced surfaces
More orderly draining
(Kim, J. 2007)

26. Conclusion Flow rate Q largely determines h
2.61 % standard deviation of h
Support for a simple relation between h and Q
Ability to predict microgravity performance with a 1g test
Unforeseen correspondence with jerk and Q
Further microgravity studies are needed
Flow rate is the primary driver of the heat transfer coefficient, which had a maximum standard deviation of 2.6%.
Heat transfer coefficient and jerk
Increase in variability with flow rate
Flow loop components
Static, microgravity environment
Near CHF
Linear spray cooling remains a promising method to cool high heat flux and large surface area devices in microgravity.

27. Thanks The authors are thankful to:
the University of Wisconsin ZeroG Team
the Multiphase Flow Visualization
and Analysis Laboratory
the UW Space, Science,
and Engineering Center
the UW Department of Engineering Physics
the Wisconsin Space Grant Consortium
NASA Reduced Gravity Student
Flight Opportunities Program
We would like to thank the University of Wisconsin
Zero Gravity team
Multiphase Flow Visualization and Analysis Laboratory,
UW Space, Science, and Engineering Center,
UW Department of Engineering Physics,
Wisconsin Space Grant Consortium,
and NASA’s Reduced Gravity Student Flight Opportunities Program.

28. Questions

30. FEA confirms broken array & uneven cooling
FEA confirms the rupture caused uneven temperatures:
Top down:
Side with rupture, Side with spray cooling
less cooling
Despite broken spray array, delta_T is small along the spray direction--evidence of uniformity
Cross-section: