TFAWS Active Thermal Paper Session T F AWS · PDF file» Cartridge heater and thermocouples » Exchangeable heater surfaces Plain Plain, instrumented with surface-soldered thermocouples

TFAWSMSFC ∙ 2017

Presented By

Marc K. Smith

Acoustic Actuation of Vapor-Liquid

Interfaces in Boiling and

Condensation Processes

Thomas R. Boziuk, Marc K. Smith, and Ari Glezer

School of Mechanical Engineering

Georgia Institute of Technology

Atlanta, GA

Thermal & Fluids Analysis Workshop

TFAWS 2017

August 21-25, 2017

NASA Marshall Space Flight Center

Huntsville, AL

TFAWS Active Thermal Paper Session

Fluid Mechanics Research Laboratory 2

Two-Phase Power Dissipation Applications

Server Farm

Insulated-Gate Bipolar Transistor

Radar

Electric Vehicle Drivetrains

Chemical / Process Engineering


Control of Phase Change Heat Transfer

Boiling heat transfer for high-power, dense

electronic systems

Heat transfer is limited by two primary

processes» Vapor formation and removal rates (critical heat flux)

» Condensation rate

Boiling and condensation present different

design challenges» Boiling: increase CHF, decrease surface superheat

» Condensation: enhance in bulk fluid for efficient

thermal packaging

Acoustic control of 2-phase boiling processes» At heater surface control of vapor growth, spreading,

and advection Surface force engendered by high-frequency

ultrasound

Used in conjunction with complex boiling geometries

» In bulk fluid control of condensation Acoustic actuation couples to surface Faraday waves

or via radiation pressure force and droplet ejection

Pool boiling and nozzle condensation geometries

Boiling

Condensation

1 mm


Acoustic Actuation of Liquid/Gas Interface

Interfacial coupling varies substantially with actuation wavelength

Ultrasonic [O(1 MHz)] liquid/gas interfacial actuation» Short actuation wavelength [O(1 mm)]

Exploits acoustic surface force to effect interfacial deformations and injection of a liquid jet and droplets

» lacoust = 0.9 mm; Dres = 2 mm; lcapillary = O(mm)

» Impedance mismatch Zvapor/Zwater=1.8x10-4

» High acoustic absorption coefficient aH2Ovapor 1,000 aH2Oliquid

» Amplitude = 6.82∙103 kPa peak-to-peak

» Forcing affects vapor bubbles larger than Dres

O(1 kHz) liquid/gas interfacial actuation» Long actuation wavelength [O(1 m)]

Much larger than the characteristic length scale of the vapor bubbles [O(5-10 mm)]

Forces capillary surface waves to enhance mixing of the interfacial thermal boundary layer

» lacoust = 1.5 m; Dres = 5.5 mm; lcapillary = O(mm) Significant disturbances

» Amplitude = 5 kPa peak-to-peak» Bjerknes body forces affect bubble’s path

Video Presented Here



15

2 m

m

Controlled bulk

temperature

Articulated acoustic

transducer

Variation of Critical Heat Flux with Bulk Temperature

Acoustically Controlled Boiling:

Experimental Setup

Heated surface design» Cartridge heater and thermocouples

» Exchangeable heater surfaces Plain

Plain, instrumented with surface-soldered

thermocouples

Microchannel grid

8 160 4 1260

100

140

180

CH

F(W

/cm

2)

Tsubcool(oC)

Distilled water

1 atm

93oC bulk temperature


0

100

200

10 20 30

q’’ (W/cm2)

Ts - Tsat (oC)

0 10050

8

4

q’’ (W/cm2)

DTs (oC)

Ultrasonic Control of Vapor at Surface

High-frequency acoustic actuation» Increases surface temperature (7 oC)

Detaches small scale vapor bubbles

Suppresses vaporization process at most nucleation sites

» Increases CHF by 65% Agreement with wire experiments of Isakoff (1956)

Surface Temperature

110

183

Vapor removal at surface

50 W/cm2



No ActuationAcoustic Actuation

(1.7 MHz)

1 mmq’’ = 100

W/cm2

0

100

10 20 30

q’’ (W/cm2)

Ts - Tsat (oC)

200

110

183190

Ultrasonic Control of the Boiling Curve

Video Presented Here Video Presented Here


Effect of Actuator Incidence Angle

DT (oC)

5

10

f

q

30 60 900

5

10

7

9

8

6

DT

f

DT as function of

Actuator Position

q’’base = 100 W/cm2, Tbase = 110oC, Tbulk = 93oC


microChannel Design: More than Surface Area

Dimpled

200400 1000

Smooth microChannels

0

200

400

10 20 30

q’’ (W/cm2)

Ts - Tsat (oC)10 20 30

Normalized by projected area Normalized by wetted area

CHF (W/cm2)

w/D0 0.1 0.3

100

200

300

(plain)

(400 mm)

(200 mm)

(1000 mm)

0.2

approx. same wetted area


0

250

500

10 20 30

Surface mChannels with Ultrasonic Actuation

mChannel Actuated (1.7 MHz)

100

200

300

110

350

183

1 mm

460

q’’

(W/c

m2)

Ts - Tsat (oC)

Smooth

Small-scale acoustic actuation within mChannels

» Decreases surface temperature (~ 7 oC).

» Increased power dissipation DP 200 W/cm2 at Ts - Tsat =17 oC

» Increases CHF by 31%

» Decreases surface temperature fluctuations.

» Increase CHF by 318% relative to smooth, unactuated case

200 W/cm2200 W/cm2300 W/cm2 300 W/cm2100 W/cm2 100 W/cm2

DT

s(o

C)

0

-5

5 Smooth

mChannel

100 200 300

q’’ (W/cm2)

400 mm

800 mm

10

00

mm

mChannel: actuated

mChannel:

unactuated

Smooth: unactuated Smooth: actuated


O(1 kHz) Acoustic Enhancement of Boiling

Marginal increase in CHF (16%)

Decrease in surface superheat of ~1 oC

Appearance of vapor is markedly different due to surface capillary waves

» Increased condensation has minimal effect on boiling process

a

b

c

d

e

f

g

h

i

j

20

40

60

80

100

q’’ (W/cm2)


Acoustic Control of Vapor Condensation

Pool boiling and condensers both

require enhanced condensation» Pool boiling used in heat sink applications

Vapor boils and condenses in close proximity

» Condensers used in power cycles Vapor is injected; boiling occurs in separate

boiler component

Nozzle geometry interacts with vapor

formation and acoustic enhancement

Condensation is limited by interface

area» Thermal boundary layer surrounds vapor

Boiling

Condensation

1 mm

Condensation

Nozzle Ejection

1 mm


Acoustically Controlled Condensation

Experimental Setup

Vacuum pump sets the

ambient pressure in test cell

Middle plate separates

boiling from condensation» Nozzle geometry can be

varied

» Bulk temperature of upper

tank controlled with coil heat

exchanger (not shown)

» Immersion heater creates

vapor in lower tank

Acoustic actuators:» 1 kHz, placed to sides of

nozzle

» 1.7 MHz, oriented either

above or to side of nozzle

Tank

steam reservoir

d

vacuum pump

nozzle

steam reservoir

nozzle

steam reservoir

nozzle

steam reservoir

10.8 cm

Distilled water

0.15 – 1 atm


nozzle

steam reservoir

Acoustically Enhanced Bubble Condensation

Low Frequency (1 kHz)

Actuated

25 oC subcool 225 W

Continuous Actuation (Atmospheric Pressure)

To: acoustic actuation period 1 msec

0 2 4 6 8 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Volume (25 degrees subcool, kHz continuous)

V/V

o

t/To0

0

1

5 10

Increased thermal interfacial mixing leads to rapid collapse.

A∗ =Vapor Area

Average Baseline Vapor Area

0 200t/T0

1000

0.19

1Base Flow



Boundary Layer Growth - kHz Condensation

Image processing of Schlieren images yields quantitative information on boundary layer growth

Thermal boundary layer in baseline flow does not undergo appreciable growth

» Heat transfer occurs primarily through lower (and subsequently, inner) interface

Acoustic actuation leads to nearly linear growth of boundary layer thickness

» No significant temporal dependence on acoustic actuation

Thermal boundary layer in presence of acoustic actuation is on average 6.7 times thicker

» Up to 17 times thicker

0 5 10

t/To (ms)

dt (mm)

0

1

2 All Instantaneous ThicknessAverage Instantaneous Thickness

dtdt

01-1

x (mm)0 1-1

x (mm)

0 10-10

x (mm)

20 0 10-10

x (mm)

20

{[I’(x

, y

i, t i)

]2}

{[I’(x

, y

i, t i)

]2}

I(x, y

i, t i)

2

4

0

2

4

0

80

160

dtdt


Natural Deformation-Induced Vapor Collapse

nozzle

steam reservoir

Surface tension pinch-off

drives a liquid “spear”

through the center of the

vapor bubble to form a vapor

torus that leads to rapid

condensation.

Schlieren imaging shows

insignificant thermal

gradients in fluid surrounding

bubble.» Inner “spear” enhances heat

transfer

This natural mechanism

indicates that inducing such

a liquid “spear” early in the

bubble formation process

can lead to accelerated

condensation.



Ultrasonic Liquid-Gas Interfacial Actuation

f = 1.7 MHz lacoust = 0.9 mm; Dres = 2 mm

lcapillary = O(mm)

» “Mist” droplets ejected, visible in video

Cavitation and subsequent collapse generates additional droplets

» Larger-scale; not uniformly sized

Acoustic impedance mismatch» Zvapor/Zwater=1.8x10-4

» Surface deforms from acoustic pressure

Deformed surface self-focuses acoustic intensity

1 cm



nozzle

steam reservoir

Condensation Enhancement: Pulsed Ultrasound

6 oC subcool

20 W

20 ms pulse

White LightSchlieren Imaging

Photo

transis

tor

Laser Diodeh

Pulsed actuation.» Saves power

» Minimizes interference with vapor ejection

» Vapor ejection pressure remains unchanged

Pulse actuation is synchronized to “natural” bubble

formation.» Bubble phase reference is obtained using a trigger laser

beam at given height above nozzle

» Actuation wavefronts are monitored using Schlieren imaging

trigger

Significant savings in actuation power



A*

t/Tb

Triggering laser diode

5 mm above nozzle

Q = 225 W

Tb = 50 ms

Subcooling = 25 oC

Axially-Aligned Pulsed Ultrasound Actuation

1

0.27

0 2 4

Base Vapor

Ultrasonic Vapor

nozzle

steam reservoir

High-speed video image processing yields

an estimate of total vapor domain as

function of time.» Subcooling and heater dissipation are

invariant, leading to the relation between

vapor domain and heat transfer coefficient.

𝑞𝑏𝑎𝑠𝑒 = 𝑈𝑏𝑎𝑠𝑒,𝑒𝑓𝑓𝐴𝑏𝑎𝑠𝑒,𝑒𝑓𝑓 𝑇𝑠𝑎𝑡 − 𝑇𝑠 =

𝑈𝐴𝑎𝑐𝑡,𝑒𝑓𝑓 𝑇𝑠𝑎𝑡 − 𝑇𝑠 = 𝑞𝑎𝑐𝑡

𝑦𝑖𝑒𝑙𝑑𝑠 𝑈𝑎𝑐𝑡,𝑒𝑓𝑓

𝑈𝑏𝑎𝑠𝑒,𝑒𝑓𝑓=

𝐴𝑏𝑎𝑠𝑒,𝑒𝑓𝑓

𝐴𝑎𝑐𝑡,𝑒𝑓𝑓

Vapor domain reduced by up to 73% using 20 ms actuation pulses.» HTC increased by 270%

Actuation “regularizes” time-periodic bubble formation.» The base flow a new bubble is ejected while the earlier bubble collapses

» In the presence of actuation, bubble collapse is completed prior to ejection of the

subsequent bubble


0 1t/Tb

0.2 0.4 0.6 0.8

1

0

0.2

0.4

0.6

0.8

1.4

1.2

0 1t/Tb

0.2 0.4 0.6 0.8

1

0

0.2

0.4

0.6

0.8

1.4

1.2

Bubble Volume Tracking

8 oC subcool

20 W

20 ms pulse

Tb = 80 ms

Y = 0.822 cc

Fluid Mechanics Research Laboratory 210 0.2 0.4 0.6 0.8 1

0

40

80

0

2∙104

4∙104

U (W/m2K)

ሶq (W)

t/Tb

Temporal Variation:

Heat Transfer Coefficient and Heat Rate

Peak heat transfer

coefficient occurs

during toroidal breakup

in the absence and

presence of actuation. » Acoustic actuation leads

to near-immediate

doubling of HTC

Peak heat rate occurs

during pinch-off, torus

formation, and toroidal

breakup

Non-spherical effects» Lower peaks and higher

troughs

120

6∙104

Tactuation


High-resolution Schlieren imaging reveals formation of ultrasonically induced

droplet ejection from spear.

Required mass for complete phase change:

mdroplets = E/(cp∙DT) = [(Vo/vg)∙hfg]/(cp∙DT)

mdroplets per pulse: 0.0207 gram/pulse.» Can contribute up to 60% at low subcooling (8 oC), small bubbles or 45% at high

subcooling (25 oC), large bubbles

nozzle

steam reservoir

h

trigger

Interfacial Disturbances by

Secondary Droplet Ejection

Pulsed Ultrasound ActuationBase Flow



Cylindrical Lens

Spherical Lens

Laser

Test SectionVorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

Bubble extent inferred from

masked vectors along

centerline

x

y elevation for interpreting

wake after bubble collapse

y

Particle Image Velocimetry

Fluorescent Particles and optical filter to reduce laser reflections

Algorithmic masking to remove interface/ non-particle-laden flow

Post processing: 10,000 fps flow fields temporally averaged over 0.5 ms (5 frames)» Rightward of bubble is masked to remove interior and bubble shadow

8 oC subcool, 20 W, 20 ms pulse

Top View Side View

Optical Filter,

Camera

Cylindrical Lens

Spherical Lens

Laser

Test Section

Low-Power

Laser

trigger

Low-Power

Laser


Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

0-2 2 0-2 2 0-2 2x/do

0

4

8 Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

0-2 2 0-2 2 0-2 2x/do

0

4

8 Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

0

4

8

0-2 2 0-2 2 0-2 2x/do

y/do

Naturally Condensing Vapor Bubbles

Vorticity Tb/s

1 Vt

Plotting

Threshold:

0.55 Vt

0Tb 0.25Tb 0.30Tb0.25Tb 0.56Tb 0.70Tb0.80Tb 0.85Tb 0.90Tb

Vt = 0.55, Tb = 80 msec, do = 2.3 mm

Vorticity Tb/s

1 Vt


y/do

Acoustically Actuated Vapor Bubbles

Vorticity Tb/s

1 VtVorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

0-2 2 0-2 2 0-2 2x/do

0

4

8 Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

0-2 2 0-2 2 0-2 2x/do

0

4

8 Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

Vorticity 1/s

50

40

30

20

10

0

-10

-20

-30

-40

-50

0

4

8

0-2 2 0-2 2 0-2 2x/do

Plotting

Threshold:

0.2 Vt

0Tb 0.05Tb 0.15Tb0.25Tb 0.30Tb 0.40Tb0.45Tb 0.50Tb 0.60Tb

Vt = 0.55, Tb = 80 ms, do = 2.3 mm

Vorticity Tb/s

1 Vt


Centerline Velocity

v/Vt

0 0.40.2 0.6 0.8 1

-2

2

t/Tb

-1

1

0

y/do

2

4

6

8

10

0

t/Tb

0 0.40.2 0.6 0.8 1

Tb = 80 ms, Vt = 0.55 m/s, do = 2.3 mm

Base Flow Actuated Flow


High-Subcooling, High-Mass Flow Rate Vapor

v/Vt

-2

2

1

-1

0

y/do

0

2

4

t/Tb

0 0.80.4 1.2 1.6t/Tb

0 0.80.4 1.2 1.6

Tb = 50 ms, Vt = 0.70 m/s, do = 5.3 mm

Base Flow Actuated Flow


Acoustic actuation is an effective method for

controlling two-phase flows with heat transfer.

» Interfacial coupling varies with actuation wavelength

Low frequency, O(1 kHz); long wavelength = 1 m

High frequency, O(1 MHz); short wavelength = O(1 mm)

» The acoustic coupling forces liquid-vapor interfacial motion

that affects vapor formation, advection, and condensation.

» Strongly enhances pool boiling heat transfer.

» Accelerates direct-contact vapor condensation.

General Conclusions


Conclusions: Boiling

Ultrasonic actuation (short wavelength).

» Couples to vapor bubbles by a surface force.

» Vapor bubble nucleation, growth, and detachment are

modified.

» CHF increases by 65%; surface temp. increases by 7 °C.

» Condensation increases above the boiling surface.

» Actuation may be turned on and off as needed without a

drop in performance.

Textured surfaces with ultrasonic actuation.

» Microchannels alone increase CHF to 350 W/cm2.

Ultrasound increases CHF to 460 W/cm2.

» Ultrasound reduces surface superheat by 7 °C (in contrast to

smooth heater).


Conclusions: Condensation

O(1 kHz) acoustic actuation (long wavelength).» Interfacial Faraday waves increase condensation in the bulk liquid.

» Interface motion induces a temporally-growing thermal boundary

layer.

» Condensation rate is increased both during growth and after

advection, with increases of up to 425% in the time-averaged

overall heat transfer coefficient.

» Importance of motion in inducing mixing implies effectiveness

scales with surface displacement. Lower frequencies (1 kHz or below).

Prior work used either extremely low (50 Hz) or high (20 kHz)

frequencies, with smaller improvements in HTC.

Ultrasonic acoustic actuation (short wavelength).» Subcooled liquid jet protrudes into vapor bubble, significantly

increasing vapor surface area and heat transfer coefficient.

» Formation of toroidal volume leads to rapid bubble collapse.

» Pulsed actuation causes up to a 73% reduction in vapor extent.

Documents

TFAWS Active Thermal Paper Session T F AWS · PDF file» Cartridge heater and thermocouples » Exchangeable heater surfaces Plain Plain, instrumented with surface-soldered thermocouples