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