Overview of high efficiency thermal management technologies being developed at the Centre for Energy Research & Technology at the National University of Singapore.

1. High Efficiency Thermal Management
Technologies
A/P PS Lee
Deputy Director
Centre for Energy Research & Technology
National University of Singapore

2. ∗ Principle Investigator: A/Prof. PS Lee
∗ Research Fellows (6x)
∗ Research Engineers (5x)
∗ PhD Students (7x)
2
Micro Thermal Systems (MTS) Group
– 19 Pax

3. Research Setups
 Liquid Flow Loops (4x)
 For liquid and two-phase
cooling studies
 Compact and Desktop
Wind Tunnels
 For air cooling study
 µ-PIV & µ-LIF System
 Flow and temperature
fields measurements in
micro thermal devices
3

4. Research Resources
∗ Hardware
∗ High speed camera
∗ High speed data acquisition
systems
∗ 3-axis measurement
microscope
∗ Research grade IR Camera
∗ Software
∗ NUS SVU Clusters
∗ In-house Workstation
∗ CFD Packages (Fluent, CFX)
4

5. Awards Winning Technology
Seed Funded S$ 0.5M by SPRING TECS POV grant
2011 IES Prestigious Engineering
Achievement Award

6. Introduction to Oblique Fin Technology
Conventionally, liquid is driven between long straight fins, which are responsible for
the large surface area of contact with the liquid for maximum heat transfer.
However, a slow moving boundary layer forms liquid-solid interface, and thickens
along the flow path
Heat transfer rate from fins to fluid is reduced

7. Introduction to Oblique Fin Technology
The oblique fin design disrupts the formation of this layer, thereby increasing the
effective heat transfer rate
Oblique fin technology utilizes secondary oblique flow to regenerate thermal
boundary layers as well as promote better fluid mixing

8. 8
Conventional Oblique Fins
Liquidflowdirection
Oblique Fin Liquid Cooling
Conventional Oblique Fins
Liquidflowdirection
Temperature contoursVelocity contours

9. 9
0
2
4
6
8
10
12
14
16
0 0.005 0.01 0.015 0.02 0.025
streamwise position, X (m)
Heattransfercoefficient,h(x1000W/m
2
K)
conventional microchannel enhaned microchannel
30
35
40
45
50
55
60
65
70
75
80
0 0.005 0.01 0.015 0.02 0.025
streamwise position, X (m)
Heatertemperature,T(°C)
conventional microchannel enhanced microchannel
Oblique Fin Liquid Cooling

10. 10
0
50
100
150
200
250
300
350
400
450
0 0.005 0.01 0.015 0.02 0.025
streamwise position, X (m)
Staticpressure,P(Pa)
conventional microchannel enhanced microchannel
160
170
180
190
200
210
220
230
240
250
260
0 0.2 0.4 0.6 0.8 1
Dimensionless distance, x 1 '
Staticpressure,P(Pa)
Conventional microchannel Enhanced microchannel
Local coordinate system
Flow direction
x1’=0
x1’=0.22
x1’=0.5
x1’=0.72
x1’=1
Oblique Fin Liquid Cooling

11. 11
Oblique Fin Liquid Cooling – Experimental
Validation
Silicon test pieces Copper test pieces
Aluminum test pieces
Conventional Conventional
Oblique fins Oblique fins
Oblique fins
100µm 1mm0 0

12. 12
Oblique Fin Liquid Cooling – Heat
Transfer Performance
0
5
10
15
20
25
300 400 500 600 700 800
Reynolds number, Re
AverageNusseltnumber,Nuave
conventional microchannel - experiment
enhanced microchannel -experiment
conventional microchannel -simulation
enhanced microchannel - simulation
5
6
7
8
9
10
11
12
13
14
0 0.2 0.4 0.6 0.8 1
Dimensionless channel length, X'
LocalNusseltnumber,Nux
conventional microchannel enhanced microchannel
Flow direction
Global heat transfer performance Local heat transfer performance

13. 13
Oblique Fin Liquid Cooling – Pressure
Drop
• At Re < 400, pressure drop penalty is small and negligible.
• At Re > 400, slightly higher pressure drop is incurred but should be manageable for
the same micropump.
0
500
1000
1500
2000
2500
300 400 500 600 700 800
Reynolds number, Re
Pressuredrop,dP(Pa)
conventional microchannel - experimetal
enhanced microchannel - experimental
conventional microchannel - simulation
enhanced microchannel - simulation
0
0.5
1
1.5
2
2.5
300 400 500 600 700 800
Reynolds number, Re
ENu,Ef
heat transfer enhancement factor
pressure drop penaltyENu = NuEM/NuCM
Ef = fEM/fCM

14. 14
Oblique Fin Liquid Cooling – Hotspot
Mitigation
Finer oblique fin
cluster for hot
spot region
Hotspot (finer fin
pitch)
Finer oblique
fin cluster for
hot spot region
Hotspot (finer fin
pitch)
Background heat
flux (sparser fin
pitch)

15. 15
Temperature Contour: Single Hotspot
Inlet Outlet
0 12.7
12.7
Inlet Outlet
0 12.7
12.7
Conventional microchannel
Uniform pitch
Enhanced microchannel with
variable fin pitch

16. 16
Temperature Contour: Multiple Hotspots
Inlet Outlet
0 12.7
12.7
Inlet Outlet
0 12.7
12.7
Conventional microchannel
Uniform pitch
Enhanced microchannel with
variable fin pitch

17. Value Proposition
High Performance: 50-80% improvement
Scalable: 10mm2 (LED/CPU), 100cm2 (IGBT)
1000cm2 (Battery/Battery Storage)
Different Form Factors: Cylindrical and Flat heat
sources
Hotspot mitigation: Targeted cooling
∗ Biggest advantage – heat transfer improvement at minimal
energy cost

18. • Battery cooling
solution for full EV
bus (~200x300mm)
• Uniform temperature
Inter and intra-cell ΔT
of 1-2°C
Battery TMS
•IGBT solution for one
of the largest Europe
wind turbine maker
Power
electronics
•Successfully cooled
350-400W/cm2 high
heat flux (Chip-size)
High heat flux
• Implementing our
technology on heat
exchangers
Heat exchanger
Experience

19. Oblique Fin Technology
∗ Advantages
∗ Easy to maintain single-phase cooling
∗ Low profile
∗ 50-80% heat transfer enhancement at no pressure
drop penalty compared to conventional liquid
cooling solutions
∗ Hotspot mitigation solution
∗ Cost competitive manufacturing techniques e.g.
MIM and liquid forging

20. Liquid Inlet
Vapor Confined
Vapor has room to expand
span wise (Stable operation)
Flow reversal
(Unstable
operation)
Straight FinStepped Fin
Stabilized Microchannel Two-Phase
Cooling
20

21.  Expanding and stepped channels – enlarged space along
downstream
 Significantly reduced temperature and pressure fluctuations:
stabilized boiling
 Significantly reduced pressure drop – lower pumping power required,
smaller pump can be used, more compact system21
Stabilized Microchannel Two-Phase
Cooling

22. Flow Boiling with Oblique Fins – Critical Heat
Flux
 Oblique fins is able to dissipate higher heat fluxes at the same wall
temperature compared to the straight fins – better heat transfer.
 Delay in the CHF for the oblique fins compared to the straight fins.Page | 22

23.  Significant augmentation in heat transfer for the oblique-finned
microchannels.
 Enhancements in the heat transfer coefficients of between 1.2 to 6.2 times.Page | 23
Flow Boiling with Oblique Fins – Heat Transfer Coefficient

24. Page | 24
 The continuous
interruption of the
boundary layer is
beneficial for single-
phase heat transfer.
 As a result, the
disruption of the thin
liquid-film by the
oblique cuts is also
beneficial for
convective boiling heat
transfer.
Flow Boiling with Oblique Fins

25. Page | 25
 Pressure fluctuations and premature CHF conditions are related with each
other, which in turn, causes boiling instabilities.
 Delay in the incipience of CHF is primarily due to the improved stability offered
by the oblique fins.
2.5×
2.8×
Flow Boiling with Oblique Fins – Critical Heat
Flux

26.  Pressure fluctuations in the oblique fins are smaller than the straight fins, with
reductions of up to 3.6 times are observed.
Page | 26
 Branched secondary channels act as bridges for neighbouring channels to
“communicate”, thus eliminating sudden pressure spikes.
Flow Boiling with Oblique Fins – Pressure
Fluctuations

27. Enhanced Air Cooling – Cross-Connected
Alternating Converging-Diverging Channel
Array
∗ The converging-diverging channel
sections induce secondary flows
through the cross connections,
disturbing the thermal and hydraulic
boundary layers repeatedly.
∗ Reduced boundary layer thicknesses
reduce the overall convection resistance
over the fin walls.
∗ Flow mixing enables a uniform increase
in the air temperature in the streamwise
direction. Therefore, the cooling
potential of the air flow can be utilized
more effectively.
∗ Separation of the secondary flows
generate some flow recirculation
regions within the flow domain. These
vortices reduces the convection heat
transfer efficiency and they increase the
fan power requirement.

28. ∗ Due to the flow separation, two different vortices are generated and
these vortices circulate hot air stream, increasing the overall
convection resistance over the respective fin walls.
∗ A better fin structure is needed to eliminate the vortices that do not
contribute to the overall performance.
Flow directionConverging channel
Diverging channel
Vortex #2
Vortex #1
Boundary layer disruption
Separation
point #2
Separation
point #1
source of the streamlines
Interaction region
Enhanced Air Cooling – Cross-Connected
Alternating Converging-Diverging Channel
Array

29. Improvement in heat sink junction temperature
∗ For a given fan power,
the improvement in the
junction temperature
compared to the
conventional straight
channel heat sink is up to
5°C for 40W of heating
power.
20
25
30
35
40
45
50
55
60
65
0.001 0.01 0.1 1 10
Tjunctionavg[°C]
Fan power [W]
SC HS Exp Q=40W
CCACDC HS Exp Q=40W
SC HS Exp Q=20W
CCACDC HS Exp Q=20W
Enhanced Air Cooling – Cross-Connected
Alternating Converging-Diverging Channel
Array

30. ∗ Oblique fin design is basically a straight channel heat sink with collinear
oblique cuts in the streamwise direction. Compared to the previous
design, manufacturing is relatively simpler.
∗ Single and two phase liquid cooling studies showed that oblique fins can
improve the heat transfer performance more than the pressure drop
penalty compared to the conventional straight channel heat sink.
woblique cut
Main flow direction
Secondary flow directionwfin
Periodic wall
Periodic wall
ϴ
Main flow direction
Fluid mixing
Fluid mixing
wchannel/2
wchannel/2
pf: Fin pitch
Enhanced Air Cooling – Oblique Fin Heat
Sinks

31. ∗ Oblique fin dimensions were adapted to the needs of air cooling, i.e.
thinner fins, lower channel aspect ratio (width/height) etc.
∗ Numerical simulations showed that a significant improvement in
Nusselt number and heat sink junction temperature can be obtained.
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
0 200 400 600 800 1000
ENu,Ef
Reynolds number
30 deg, E_Nu
30 deg, E_f
45 deg, E_Nu
45 deg, E_f
• It was observed that there
exists a critical Reynolds
number. Once Recritical is
exceeded, the heat transfer
enhancement ENu exceeds the
pressure drop penalty Ef.
Enhanced Air Cooling – Oblique Fin Heat Sinks

32. Thanks to a favorable flow field
∗ When the Recritical is exceeded, a
vortex is generated within the
secondary channel, which
increases the advection heat
transfer.
∗ The repetitive disturbance of
the boundary layers over the
other surfaces of the fin walls
maintain a high heat transfer
performance.
𝚹=45°, Re=200 𝚹=45°, Re=400
TopviewSideview
Streamline starting surface
Enhanced Air Cooling – Oblique Fin Heat Sinks

33. Heat sink junction temperature comparison
(Experimental data)
∗ 30° and 45° oblique angle
performance were
compared.
∗ The 30° oblique-finned air
cooled heat sink provided
up to 15°C improvement in
the heat sink junction
temperature at a given fan
power. (Qheater=60W)
30
40
50
60
70
80
90
0.005 0.05 0.5
Tjunction,avg[°C]
Fan power [Pa]
SC HS Q=60W Exp
OF HS 45D Q=60W Exp
OF HS 30D Q=60W Exp
Enhanced Air Cooling – Oblique Fin Heat Sinks

34. Over-PIM Prototypes – Copper
∗ Advantages:
• Can be used to mass produce cheaply
• Tailored material properties
• Near net shape of complex geometry without high tooling
cost design and additional joining process
• Good internal surface finishing
• Enables two-side cooling
∗ Disadvantages:
• Requires much larger orders (50,000- 200, 000)
• Bulk material conductivity lower (poorer performance)

35. Over-PIM
Process
Flow

36. 36
Over-PIM
Prototypes –
Copper

37. Liquid Forged Prototypes –
Aluminum

38. Liquid Forged Prototypes –
Aluminum
∗ Burst Pressure: 2300psi (~158.5 bar)
∗ No deformation witnessed
∗ Visually, a good consistent braze joint

39. Liquid Forged Prototypes –
Aluminum

40. Liquid Forged Prototypes –
Aluminum

41. Liquid Forged Prototypes –
Aluminum

42. Liquid Forged Prototypes –
Aluminum
∗ Brazed setup

43. Liquid Forged Prototypes –
Aluminum
∗ Pressure tested to 100psi (~6.9bar)

44. Completed Projects
IOT Fast Charging
e-Bike Battery
Pack
EV Battery
Cold Plate
Ongoing projects with Bus companies, Defense
contractors, Aviation companies and IGBT manufacturers.
Liquid Cooled
Electric Bus
Road Test

45. 45
Microprocessors
Integrated circuit (IC)
3D ICs
Electric vehicle
Hybrid electric vehicle
High power battery pack
Windmill gear box
Wind turbine waste heat
recovery
Heat exchangers
Radiators
Defense Applications
Avionics
Building heating and cooling
Concentrate photovoltaic
Solar energy collector
Applications

46. Modular Polymer-based Heat Exchanger
for Waste Heat Recovery
46
Cold in
Cold out
Hot out
Hot in

47. Development of a Novel Oblique Fin
Air-Conditioning (OFAC) System
Higher heat transfer &
Lower air pressure drop
ΔP = -14%, Heat Trans. = +6%
Low heat transfer behind the
tube
AirFlow
Advantages
 Enhanced Heat Transfer with Lower Pressure
Drop
 Lower Condensation Temperature
 Lower Compressor Consumption & Higher COP
OFAC Designs Part 1: Oblique-tube condenser coil

48. 48
Increased cooling
capacity up to 25%
Reported COP
improvement by 27%
35°C30°C
40°C
Spray
section
OFAC Technologies COP
Oblique tube coil (Numerical study) +6.5%
Pre-cooled inlet air (Estimated) +27%
Overall Energy Efficiency ∼+30%
OFAC Designs Part 2: Pre-cooled Inlet Air Technology
System Performance
Development of a Novel Oblique Fin
Air-Conditioning (OFAC) System

49. Interested to know more? Please contact PS Lee
at pohseng@nus.edu.sg
49