Recent efforts in cooling of electronic and high heat flux devices
Amit AgrawalAssociate Professor
Department of Mechanical Engineering Indian Institute of Technology Bombay
Motivation: Electronics cooling
• Heat Flux > 100 W/cm2*: IC density, operating frequency, multi-level interconnects
• Impact on Performance, Reliability, Lifetime
• Failure mechanisms: hotspots (static/dynamic), thermal stress
• Very High Heat Flux (VHHF) Transients
*2002 AMD analyst conference report @ AMD.com
Ch. length 0.13 μm 0.09 μm 0.065 μmDie Size 0.80 cm2 0.64 cm2 0.40 cm2
W/cm2 63 78 125
Laptop Thermal Challenges*
CPUTj
GraphicsTj
ComponentTcase
ExhaustTemperature
(thermo limit)
Chassis Temperature
AcousticsAcousticsFan noiseFan noise
*Slide taken from Dr. A. Bhattacharya, Intel Corp
Typical CPU Thermal Solution*
Heat exchanger~ 0.90 oC/W
Package~ 0.17 oC/W TIM
~ 0.20 oC/W
Heat pipe evaporator~ 0.20 oC/W
Heat pipe adiabatic~ 0.03 oC/W
θ TIM θ HP-HX
Tj
θ HX-aθ evapθ j-a ≈ + +θ package + +
THP THX Ta
Θj-a ~ 1.5 oC/W
~0.17 oC/W ~0.20 oC/W ~0.20 oC/W ~0.03 oC/WDepends on
fanand volume
~0.90 oC/W
θ package
θ TIM
θ evap
θ HP-HX
θ HX-a
*Slide taken from Dr. A. Bhattacharya, Intel Corp
Outline
• Flow in microchannel • Synthetic jet for electronic cooling
Setup for boiling flow in microchannel
Microchannel on one side and micro-heater on the
other side of wafer
Flow Visualization (Movies)
Issues to be Addressed
• How to reduce pressure drop?• How to reduce instability in pressure?• How to mitigate hot-spots?• Identify the flow regimes. Quantify their
occurrence.• Demonstrate cooling under steady-state
and transient conditions.
0.25 0.50 0.75 1.00 1.25 1.5025
50
75
100
125
150
Pres
sure
dro
p (m
bar)
Mass flow rate ml/min
Two phase regionSingle phase region
1234b
c
d
e
Onset of boiling
a
Pressure drop versus mass flux
Operating point can be chosen such that pressure drop is same but heat transfer coefficient is maximum
Pressure drop versus aspect ratio
1.0 1.5 2.0 2.5 3.0 3.5 4.00
20
40
60
80
100
120 0.15 ml/min 0.0 W 0.15 ml/min 3.5 W 0.20 ml/min 0.0 W 0.20 ml/min 3.5 W
Pres
sure
Dro
p (m
bar)
Aspect ratio (W/H)
Aspect ratio (AR) = width / height of microchannel
1-phase pressure drop independent of AR
2-phase pressure drop shows a minima
Pressure drop can be minimized by choosing aspect ratio judiciously
Singh, S.G., Kulkarni, A., Duttagupta, S.P., Puranik, B.P., and Agrawal, A., "Impact of aspect ratio on flow boiling of water in rectangular microchannels," Experimental Thermal and Fluid Science, 33, pp. 153-160, 2008.
Channels with different AR but same hydraulic diameter
Instability of TemperatureEffect of surface roughness
Inlet temperature versus time (65 μm microchannel; ε/dh = 0.0018)
Inlet temperature versus time 70 μm microchannel; ε/dh = 0.0171
Instability of TemperatureEffect of channel size
Inlet temperature versus time (65 μm microchannel; ε/dh = 0.0018)
Inlet temperature versus time (45 μm microchannel)
Flow Map Generations
• Total of 413 points are recorded• Regimes identified: annular,
elongated slug/annular, slug,bubbly and single phase
• Maximum quality covered in the experiments 0.3
• Substantial evidence of subcooledboiling
• Low qualities heat flux ↑all the flow regimes exist
• Quality 0.1 and above, elongated slug to annular flow (mostly) with increase in heat flux
• No annular flow at low heat fluxes
Singh, S.G., Jain, A., Sridharan, A., Duttagupta, S.P., and Agrawal, A., "Flow map and measurement of void fraction and heat transfer coefficient using image analysis technique for flow boiling of water in silicon microchannel," Journal of Micromechanics and Microengineering, Vol. 19 (075004), pp. 1-9, 2009
Transient heat flux impact and mitigation test
0 50 100 150 200 250 300
20
40
60
80
100
120
140
Tem
pera
ture
(oC
)
Time (s)
0.0 ml/min0.1 ml/min 0.2 ml/min 0.5 ml/min 1.0 ml/min
Heat flux: 250 W/cm2
for t=10 s
Singh, S.G., Duttagupta, S.P., and Agrawal, A., In-situ impact analysis of very high heat flux transients on non-linear p-n diode characteristics and mitigation using on-chip single-phase and two-phase microfluidics, Journal of MEMS, to appear, 2009.
Synthetic Jet
Synthetic Jet
CavityOrificeVibrating element
Synthetic jet assembly
Flow Visualization
Entrainment of fluid (f = 30 Hz)
Turbulent jet (f = 100 Hz)
Engineering ObjectivesExplore the use of synthetic jet for electronic chip cooling:
1.
Demonstrate their capability for high heat removal with direct impingement.
2.
Study their performance in presence/absence of heat sink, enclosed in a duct, with and without cross-flow.
3.
Compare performance of synthetic jet to continuous jet?
Heat Transfer ExperimentsSchematic of experimental set-up
Nu = f (Re, Pr, z/d, L/d, R/d)
Re: 1500-4200 z/d: 1-30
L/d: 8-22 R/d: 1-7
z (mm)
h avg
(W/m
2 K)
0 50 100 150 200 250 3000
20
40
60
80
100
120
140
160150 Hz250 Hz350 Hz
Heat Transfer Measurements
Max. h is 143 W/m2K
Max. Nusselt number (Nu = hd/k) is 43
Variation of heat transfer coefficient with axial distance
Synthetic versus Continuous jetNuavg vs z/d
Comparable with continuous jet performance at Re = 4000
M. Chaudhari, B. Puranik, A. Agrawal, Heat transfer characteristics of synthetic jet impingementcooling, International Journal of Heat and Mass Transfer, to appear.
Synthetic Jet in Duct with Crossflow
AcknowledgementsStudents: Dr. Shiv Govind Singh, Rohit Bhide, Anshul Jain,
Mangesh Chaudhari, Manu Jain
Colleagues: Dr. B. Puranik, Dr. S. V. Prabhu, Dr. A. Sridharan, Dr. S. Duttagupta
Funding: Department of Information Technology
MicroEE, IIT Bombay: For fabrication of microchannels