1. Thermal management of high power density chips
using a two-phase loop cooling system
December 16, 2011
Jeehoon Choi

2. Background – Trends in Computing H/W Technologies
High performance /Specialization
Education
Personal Gaming
Design
Computing Home Optimization
Notebook, Netbook, Entertainment
High performance, high heat flux
Low-cost/lightweight market change PMP market
Special market
Improved integration of technology
Increase of processing speed
Aggressive miniaturization
Move to the MID
(Mobile Internet Device) market
High
performance
High
efficiency
Power
consumption
Client / Server
Computing Personalization Intelligence Reality

3. Background – H/W cooling in a data center
Cost Reduction – (cost per kWh / DC : 0.0964 USD) Current Efficiency Level Efficiency Goal
Energy Consumed Per Hour Current Efficiency Level Efficiency Goal Electricity used per Year 876,000kW 512,220kW
Enter Total IT Load 50kW 50kW Annual Power Cost 84,446 USD 50,246 USD
Total Facility Load 100kW 60kW Annual Cabon Footprint 528 Tons 314 Tons
Annual Data Center Efficiency Savings
Reduction In Kilowatt Hours Electricity Costs CO2 Emissions Equivalent To
1 Year 354,780 kW 34,201 USD 214 Tons 40 Fewer Cars
5 Years 1,773,900 kW 171,004 USD 1,070 Tons 202 Fewer Cars
10 Years 3,547,800 kW 342,008 USD 2,139 Tons 404 Fewer Cars

4. Background – How can we cool efficiently down servers and data rooms?
Sever industries and markets have
called for the development of enhanced
cooling techniques that are able to meet
these challenging needs under limitations
imposed by small overall volume and
weight, necessary in sever rack-mount or
workstation systems. Besides the cooling
technique is required to attend a large
amount of heat transfer rates over
considerable distances with minimal
temperature drop.
Out-of-control airflow in the data rooms

5. Background – History on thermal management of H/W
CPUs’ Heat Loads
Trends/ Heat Loads
PC Technologies
GPUs’ Heat Loads CPU Over-clocking :
200~250W
The others (RAM & etc.)
Xeon Series Increase in the heat generation
Heat Load Trends
130W 150W operating problems at high
temperatures
100W Requires high- performance
50W thermal systems
Pentium RAM, M/B & etc.
(15W) Increase heat flux
PC
popularization Next- generation
cooing technology
1980 1990 2000 2005 2010
Passive & Active cooling
Personal Water cooling
computing
cooling
Heat pipe application cooling
Research
/Products
Server Active cooling
computing
cooling
Water cooling

6. Background – Taking note of two-phase loop cooling systems
The LHP is a two-phase heat transfer device
with capillary pumping of working fluid. They are
capable of transferring a large amount of heat Q 
mc p T Q 
mh fg
for distances up to several meters.
Sensible heat Latent heat
Reservoir
Loop Heat Pipe
Vapor Line The advantages of LHP
Evaporator Pump
• High heat flux capability
Condenser
• Capability to transfer energy over long distances
without restriction due to routing of liquid & vapor line
Liquid Line
Wick
• Ability to operate over a gravitational field gradient
Conventional Heat pipe
Liquid
• No wick within the transport lines
• Vapor and liquid flows separated;
therefore, no entrainment
Evaporator Condenser
Vapor
This system comprises an evaporator and a condenser, as in conventional heat pipes, but differs in
having separate vapor and liquid line, rather like the layout of the single phase heat exchanger system.

7. Motivation and Objectives
• One sever rack has the same heating power as a heating of a one-family house (~25kW).
• Cooling, power supply and data center infrastructure need twice as much electricity than the computers itself.
• Current techniques reached its physical limit.
High Performance
•Chip temperature - below 70 ℃
from a large amount of heat flux
Space constraints • Stability/Reliability
• Long distance transport lines
• Smaller/ thinner evaporator
• Light-weight system Design For Cost (DFC)
•Design For Manufacturing (DFM) Hot w at er Cold w at er
•Design For Assembly (DFA)
The two-phase loop cooling
system must be developed within
Evaporator Sever
certain constrains, associated, in Condenser
Rack- m ount 4
large part, with its application
high density technologies. Sever
Rack- m ount 3
CPU
Sever
RAM Rack- m ount 2
Power
Supply HDD
Sever
Rack- m ount 1

8. Theory – Principle of operation
When a heat load is applied to the evaporator fluid evaporates from the
surface of the wick. The capillary pumping forces in the wick prevent the
flow of the vapor from evaporator to LCC. As the pressure difference
between evaporator and LCC increases, the liquid is displaced from the
vapor transport line and the condenser and returned to the LCC.
Liquid
4: vapor-liquid interface
Heat out
3: condenser inlet Liquid flow through wick
Vapor
Evaporation Saturation line
Pressure
Liquid P1 1
transport 5 : condenser outlet 2
Condenser
line Vapor P3 3
Liquid compensation transport
Liquid 4
Chamber (LCC) line
P5 5 Pc ,max
6: Liquid 7: wick-liquid
8: wick-vapor inlet interface Vapor removal channel P6 7
interface 6
Pw
Vapor
8
P8
1: vapor 2 : vapor line inlet
T6 T7 T4 T1
Wick
Evaporator Temperature
Evaporating meniscus
The wick structure provides capillary pressure force that transports the
condensate liquid back to the evaporator and ensures working fluid is evenly
distributed over evaporator surface.

9. Modeling – Temperature and Thermal resistance circuit
The maximum heat transfer capacity of electronics cooling systems is
determined based on the maximum permissible temperature at the
Allowable junction temperature semiconductor chips which is normally less than 70°C. With a given constant
value of ambient temperature, the operating temperature is iteratively solved
for the given input value of applied heat load.
Thermal
grease Liquid transport line
Heat source
Tc Ths
Q Te Condenser Q
Tj Evaporator Ta
Temperature profile
Tvi
Chip junction temp.
Vapor transport line
Evaporator temp.
Vapor line inlet temp.
Heat sink temp.
Ambient temp.
Location
Total thermal resistance

10. Modeling – Heat sink design and simulation
Max. heat dissipation capacity of condenser Cu
Al
Overall surface efficiency
of condenser
Long Mean Temperature Difference (LMTD)
Force convection coefficient
Fin thickness and fin height
at given heat transfer coefficients

11. Modeling – Pressure balance
Capillary pressure limits – pressure balance analysis
Ploss Pc ,max Ploss Pw Pv Pc Pl Pg
Maximum capillary pressure Pc ,max
Pv
Pressure profile
Wick Wick
Pc
Pw Vapor removal Pl
channel Pw
Tc Vapor
transport
Vapor transport line Condenser Liquid transport line line
Location
Pw Wick pressure drop, due to liquid flow through the wick thickness
Pressure profile
Vapor pressure drop, necessary to cause the vapor to flow
Pv from the evaporator to the condenser.
Pc Condenser pressure drop
The hydrostatic head due to the unfavorable slopes of the system in
the gravitational field, ΔPg ,which may be zero, positive or negative,
depending on relative positions of the condenser and the evaporator.
Pl Liquid pressure drop, required to return the liquid
from the condenser to the evaporator. Pw Pv Pc Pl

12. Modeling – Driving force of system
Wick structure for capillary pressure
2
Pc ,max
Reff 2
P P2 P 1 1
1
R P P2 P
Qlatent 
mh fg
1
R1 R2
Pressure difference across Geometry of meniscus
a curved liquid surface at liquid-vapor interface
Vapor Tv
removal CNTs
space CNTs
Revp Growth
Twe Cu Wick
Provided
Primary wick Rw Cu Substrate
Alumina layer
Twi Anodization
Rb process
Cu Wick
The copper plate Te Provided
Cu Substrate
Q0

13. Modeling – Total pressure loss calculation (Component design)
The pressure drop due to friction losses in L u2
Pressure loss liquid and vapor flow through the loop (for P f
laminar or turbulent flows, circular or non- D 2
circular pipes, smooth or rough surfaces) is
given by Darcy-Weisbach equation. Q
u Qlatent 
mh fg
Ah fg
Wick pressure drop Vapor line pressure drop Liquid line pressure drop

tw l m tw l Q 32 Lvi vuv2
32 Lvi vQ 32 Lli l ul2 32 Lli l Q
Pw Pv P
Aw l K Aw l h fg K Revl Dvl 4
Dvl v h fg l
Rell Dll 4
Dll l h fg
Hydrostatic head of liquid Condenser pressure drop
Pg l gl sin 2 x2 (1 x) 2 x2 (1 x)
Pcon G G
v (1 ) l z z2 v (1 ) l z z1
K Permeability
2G 2 z z2 z z2
x Mass quality fv x2 2
v dz g sin [ v (1 )]x 2 dz
v Dcon
z z1 z z1
(vapor mass flux/total mass flux)
G Mass flux (G 
m / A) Void fraction
0.36 0.07 1
Correlations from the friction 0.64
1 x v l
pressure drop in each phase 1 0.28
(Ref. the Lockhart-Martinelli method) x l v

14. Outcome – Simulation program on the basis of system pressure balance
The results of the predicted design parameters
help not only to understand the system
performance of at various conditions, but also to
provide details of how the system operates. We
have developed the simulation program for the
system design optimization with the effect of
design parameters using the objective function,
design variables and design boundaries.
Design variable and boundaries
Visualization program for
optimization design and simulation
Parametric studies of the design
variables and boundaries

15. Future work – Improvement factors
Max. Heat transfer capacity System operating temperature
The capillary pumping performance of -Vapor temperature
wick structure has important effect on system. ;less than 40°C level at given heat loads
It is necessary to develop the high performance
evaporator mounted with superior wick structures. n
dP
Pi T
i 1 dT
Straight pores
Liquid flow through wick
Evaporation Saturation line
Pressure
P1 1
2
P3 3
Liquid 4
P5 5 Pc ,max
P6 7
6
Vapor
8
P8
T6 T7 T4 T1
Temperature

16. Future work – Material properties 1
Working fluid and Material selection
As part of a design procedure for a targeted Operating temperature range
application, the selection of the working fluid is the of various working fluids
first step. The working fluid determines the range of
the operating temperature and must satisfy a
number of criteria. In the second step, it is
necessary to decide the compatibility between the
working fluid and the material to avoid chemical
reactions. The chemical reactions would cause the
degradation of the mechanical strength or the
tightness of the LHP, or the existence of non-
condensable gases which could alter the LHP
operation.
Compatibility metals with working fluids
(C: compatible, NC: incompatible)

17. Future work – Material properties 2
Merit number h fg The effect of working fluid on heat transfer
The ranking of working fluid c pl capability and/or system operating temperature
l h fg l h fg l l The friction factor associated
M c
l pl Maximum capillary pressure
c pl with pressure loss of system l
l l l

18. Future work – Material properties 3
Constant
Temperature Tcritical pressure curves
Sub-cooled
liquid region B
A C
D
Superheated
Liquid vapor mix/ vapor region
Saturated
two-phase region Saturated
liquid line
vapor line
Entropy
Vapor temperature vs. Vapor pressure
Change hfg by controlling the evaporator pressure

19. Summary – Research roadmap
Design objective Design requirements
Materials
Sever rack-mount cooling Heat load, Chip temperature, (with working fluid)
Ambient temperature, Weight,
Space constraints, Noise level,
Transport line length, Cost, etc.
Component Models
Experiment System Model
Investigation Condenser, Transport line,
Evaporator, Airflow, etc.
System Simulation
Wick fabrication
Design parameters
(key correlations) Prototype
Theoretical Design and Modeling
Pressure loss, Void fraction
Models Heat transfer coefficient, etc.
Prototype test

20. Summary – Works
Project schedule and plan Detailed design prototypes
Detail design and concept generation Heat source Total heat dissipation condition on one
or multi chips (Specially, CPUs)
-A prototype model with compact condenser with a fan can ; more than 300W
be mounted inside a sever rack-mount. (One CPU is more than 14 W/cm2)
Allowable Below 60 ℃
-Analyzing parameters, operating conditions, constraints,
junction * Steady state condition
etc. to build the systems.
temp. of chips
-Thermodynamic cycle and operating limits to understand Evaporator 40mm (width) x 40mm(length)
the physical concepts and operating principle of the loop dimension x less than 5mm (height)
scheme.
Transport line Between 0.5 m and 1.0 m
Length (Taking note of flexible tubes)
-Selection of working fluid/material/wick structure.
Evaporator sample fabrication and investigation
Evaporator
Condenser
-Theoretical and experimental investigation
(Obtaining wick characters – Nano/Micro application)
Updated system algorism and design refinement CPU
based on previous findings RAM
-To apply new parameters to the optimization program Power
Supply HDD
- Comparison of experimental reviews with simulation results

21. Thank you very much !!!