TFAWS August 21-25, 2017 NASA Marshall Space Flight Center MSFC - - PowerPoint PPT Presentation
TFAWS Active Thermal Paper Session 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,
MSFC ∙ 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
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Server Farm Insulated-Gate Bipolar Transistor Radar Electric Vehicle Drivetrains Chemical / Process Engineering
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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
Pool boiling and nozzle condensation geometries
Boiling Condensation 1 mm
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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 Video Presented Here
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152 mm
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 16 4 12 60 100 140 180 CHF (W/cm2) Tsubcool(oC) Distilled water 1 atm 93oC bulk temperature
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6 100 200 10 20 30 q’’ (W/cm2) Ts - Tsat (oC) 100 50 8 4 q’’ (W/cm2) DTs (oC)
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
Video Presented Here
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7 No Actuation Acoustic Actuation (1.7 MHz)
1 mm q’’ = 100 W/cm2
100 10 20 30 q’’ (W/cm2) Ts - Tsat (oC) 200
110 183 190
Video Presented Here Video Presented Here
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DT (oC)
5 10
f
q
30 60 90 5 10 7 9 8 6
DT f
DT as function of Actuator Position
q’’base = 100 W/cm2, Tbase = 110oC, Tbulk = 93oC
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Dimpled 200 400 1000 Smooth microChannels
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/D 0.1 0.3 100 200 300
(plain) (400 mm) (200 mm) (1000 mm)
0.2
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10 250 500 10 20 30
mChannel Actuated (1.7 MHz)
100 200 300
110
350
183
1 mm
460
q’’ (W/cm2)
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/cm2 200 W/cm2 300 W/cm2 300 W/cm2 100 W/cm2 100 W/cm2
DTs (oC)
5
Smooth mChannel
100 200 300 q’’ (W/cm2)
400 mm
800 mm 1000 mm mChannel: actuated mChannel: unactuated Smooth: unactuated Smooth: actuated
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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
a b c d e f g h i j
20 40 60 80 100 q’’ (W/cm2)
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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
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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
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nozzle steam reservoir
Actuated
25 oC subcool 225 W Continuous Actuation (Atmospheric Pressure) To: acoustic actuation period 1 msec
2 4 6 8 10 1 2 3 4 5 6 7 8 9 1
Volume (25 degrees subcool, kHz continuous)
V/Vo t/T
5 10
Increased thermal interfacial mixing leads to rapid collapse.
A∗ = Vapor Area Average Baseline Vapor Area
200 t/T0 100 0.19 1 Base Flow Video Presented Here Video Presented Here
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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
5 10 t/To (ms) dt (mm) 1 2
All Instantaneous Thickness Average Instantaneous Thickness
dt dt
1
x (mm) 1
x (mm) 10
x (mm) 20 10
x (mm) 20 {[I’(x, yi, ti)]2} {[I’(x, yi, ti)]2 } I(x, yi, ti) 2 4 2 4 80 160 dt dt
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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.
Video Presented Here
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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
Video Presented Here
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nozzle steam reservoir
6 oC subcool 20 W 20 ms pulse White Light Schlieren Imaging
Phototransistor Laser Diode
h
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
Video Presented Here
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19 A* t/Tb
Triggering laser diode 5 mm above nozzle Q = 225 W Tb = 50 ms Subcooling = 25 oC
1 0.27 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.
𝑟𝑐𝑏𝑡𝑓 = 𝑉𝑐𝑏𝑡𝑓,𝑓𝑔𝑔𝐵𝑐𝑏𝑡𝑓,𝑓𝑔𝑔 𝑈
𝑡𝑏𝑢 − 𝑈 𝑡 =
𝑉𝐵𝑏𝑑𝑢,𝑓𝑔𝑔 𝑈𝑡𝑏𝑢 − 𝑈
𝑡 = 𝑟𝑏𝑑𝑢 𝑧𝑗𝑓𝑚𝑒𝑡 𝑉𝑏𝑑𝑢,𝑓𝑔𝑔
𝑉𝑐𝑏𝑡𝑓,𝑓𝑔𝑔 = 𝐵𝑐𝑏𝑡𝑓,𝑓𝑔𝑔 𝐵𝑏𝑑𝑢,𝑓𝑔𝑔
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
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1 t/Tb 0.2 0.4 0.6 0.8 1 0.2 0.4 0.6 0.8 1.4 1.2 1 t/Tb 0.2 0.4 0.6 0.8 1 0.2 0.4 0.6 0.8 1.4 1.2
8 oC subcool 20 W 20 ms pulse Tb = 80 ms Y = 0.822 cc
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0.2 0.4 0.6 0.8 1 40 80 2∙104 4∙104 U (W/m2K) ሶ q (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
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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 Actuation Base Flow
Video Presented Here Video Presented Here
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Cylindrical Lens Spherical Lens Laser Test Section
Bubble extent inferred from masked vectors along centerline
x
y elevation for interpreting wake after bubble collapse
y
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
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2
2
2
x/do
4 8
2
2
2
x/do
4 8 4 8
2
2
2
x/do y/do
Vorticity Tb/s
1 Vt
Plotting Threshold: 0.55 Vt
0Tb 0.25Tb 0.30Tb 0.25Tb 0.56Tb 0.70Tb 0.80Tb 0.85Tb 0.90Tb
Vt = 0.55, Tb = 80 msec, do = 2.3 mm
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y/do
Vorticity Tb/s
1 Vt
2
2
2
x/do
4 8
2
2
2
x/do
4 8 4 8
2
2
2
x/do
Plotting Threshold: 0.2 Vt
0Tb 0.05Tb 0.15Tb 0.25Tb 0.30Tb 0.40Tb 0.45Tb 0.50Tb 0.60Tb
Vt = 0.55, Tb = 80 ms, do = 2.3 mm
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v/Vt 0.4 0.2 0.6 0.8 1
2 t/Tb
1 y/do 2 4 6 8 10 t/Tb 0.4 0.2 0.6 0.8 1
Tb = 80 ms, Vt = 0.55 m/s, do = 2.3 mm
Base Flow Actuated Flow
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v/Vt
2 1
y/do 2 4 t/Tb 0.8 0.4 1.2 1.6 t/Tb 0.8 0.4 1.2 1.6 Tb = 50 ms, Vt = 0.70 m/s, do = 5.3 mm
Base Flow Actuated Flow
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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.
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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).
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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
» 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.