Two-phase microgap cooling is proposed as a method to overcome thermal limitations for next generation electronics. A test loop was constructed to analyze the cooling performance of boiling two-phase flow in a 100 micrometer gap channel. Results showed that as heat flux increased, differential pressure rose to over 50 kPa and heat transfer coefficients reached 14,000 W/m2-K. However, at the highest heat fluxes, partial dryout occurred at the rear of the chip, causing a temperature rise. Future work includes testing multiple chip configurations and validating cooling in microgravity conditions. This technique could enable more compact and powerful electronics systems.

1. 0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
-0.10 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30
LocalHeatTransferCoefficient(W/m2-K)
Outlet Quality (-)
Axial Flow Boiling Performance of HFE-7100 in 100μm Gap
Diode 5
Diode 6
Diode 7
Mass flux = 1000 kg/m2 - s
Pumping power ≈ 1.5 W
My Contributions
• Wrote LabView program that runs experiment, records data, and
protects loop hardware
• Ran two-phase tests and analyzed data
• Wired sensors from experiment to the DAQ system
• Built tubes for experiment overhaul
• Gained knowledge of heat transfer, fluid dynamics, microfluidics, signal
processing, thermal vacuum systems, and software implementation
• Learned about the overall running of a lab
Two-Phase Microgap Cooling for Next Generation Electronics
Keith Coulson
Missouri University of
Science and Technology
Frank Robinson
NASA - Thermal Engineering
Goddard Space Flight Center
Abstract
Electronics are getting both smaller and more capable. However, in order
to continue making improvements in small-scale electronics, current
thermal management limitations must be overcome. Remote cooling has
long been the preferred method of cooling computer chips, but it cannot
maintain its effectiveness at higher heat fluxes and heat densities. To
conquer the thermal limitations, embedded cooling is required. Forced
convection using fluids like water provide enough heat transfer, however
two-phase flow, specifically annular flow as opposed to other regimes, is
preferable. The microgap causes the fluid to transition to annular flow (Fig
1) earlier than it would in a larger channel, and it reduces the flow's
sensitivity to gravity. For these reasons, it is preferable to cool the chips
using two-phase microgap cooling. By testing the cooling capabilities of
two-phase flow in a high aspect ratio microgap with various orientations,
gravity independence and cooling capabilities can be verified.
Results
• Channel dimensions (WxHxL): 12.84mm x 100μm x 12.70mm
• Mass flux: 1000 kg/(m2 – s)
• After boiling began, the temperature of the diodes furthest from the
inlet flow saw a temperature drop with large differential pressure
• As the heat flux increased, the two-phase region was pushed further
towards the front of the chip, resulting in temperature and pressure drop
• Differential pressure reached 50+ kPa; HTCs 14,000+ W/(m2 – K)
• At the highest heat fluxes, the back edge of the chip would experience
partial dryout, leading to dramatic increase in temperatures
Approach
In order to properly test this method of cooling, a loop was constructed.
• Reservoir – provide pressure to the loop
• Filter – 7μm filter protects the pump from any possible particles
• Pump – provides specified flow rate to the loop
• Flow meter – feedback to adjust pump to keep constant flow rate
• Preheater – section that heats the liquid so that it enters test section at a
specified temperature
• Test section – thermal test chip with internal heaters and diodes, inside a
housing with a polycarbonate window on top that creates the gap for the
two-phase flow
• Sensors – numerous TC’s, RTD’s, and pressure transducers
Figure 1 – Annular flow visualized1
1http://stsl.gatech.edu/research-bubble.html
Applications and Future Work
• Experiments with multiple test chips
• Microgravity validation
• 3D Integrated Circuits – autonomous robots, heterogeneous integration
• Even smaller and more powerful electronics
Background
• Embedded cooling is the next step in overcoming thermal limitations
• More effective and more compact than remote cooling
• Allows utilization of flow boiling as heat transfer method
• Best regime is annular, which is the most prevalent in microgaps.
• At such small gaps, annular flow is also insensitive to gravity.