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, GA Presented By Marc K. Smith Thermal & Fluids Analysis Workshop TFAWS 2017 TFAWS August 21-25, 2017 NASA Marshall Space Flight Center MSFC ∙ 2017 Huntsville, AL

Two-Phase Power Dissipation Applications Server Farm Insulated-Gate Bipolar Transistor Radar Chemical / Process Engineering Electric Vehicle Drivetrains 2 Fluid Mechanics Research Laboratory

Control of Phase Change Heat Transfer  Boiling heat transfer for high-power, dense electronic systems Condensation  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 Boiling Acoustic control of 2-phase boiling processes  1 mm » 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 3 Fluid Mechanics Research Laboratory

Acoustic Actuation of Liquid/Gas Interface Interfacial coupling varies substantially with  actuation wavelength  Ultrasonic [O(1 MHz)] liquid/gas interfacial actuation Video Presented Here » Short actuation wavelength [O(1 mm)]  Exploits acoustic surface force to effect interfacial deformations and injection of a liquid jet and droplets l acoust = 0.9 mm; D res = 2 m m; l capillary = O( m m) » » Impedance mismatch  Z vapor /Z water =1.8x10 -4 » High acoustic absorption coefficient  a H2Ovap or  1,000 a H2Oliquid Amplitude = 6.82∙10 3 kPa peak-to-peak » » Forcing affects vapor bubbles larger than D res  O(1 kHz) liquid/gas interfacial actuation » Long actuation wavelength [O(1 m)]  Much larger than the characteristic length scale of the Video Presented Here vapor bubbles [O(5-10 mm)]  Forces capillary surface waves to enhance mixing of the interfacial thermal boundary layer l acoust = 1.5 m; D res = 5.5 mm; l capillary = O(mm) »  Significant disturbances » Amplitude = 5 kPa peak-to-peak Bjerknes body forces affect bubble’s path » 4 Fluid Mechanics Research Laboratory

Acoustically Controlled Boiling: Experimental Setup Variation of Critical Heat Flux with Bulk Temperature Articulated acoustic 152 mm 180 transducer Controlled bulk temperature CHF (W/cm 2 ) 140 100 60 12 16 0 4 8 T subcool ( o C)  Heated surface design » Cartridge heater and thermocouples Distilled water » Exchangeable heater surfaces 1 atm  Plain 93 o C bulk temperature  Plain, instrumented with surface-soldered thermocouples  Microchannel grid 5 Fluid Mechanics Research Laboratory

Ultrasonic Control of Vapor at Surface q’’ (W/cm 2 ) 200 183 Video Presented Here 110 100 T s - T sat ( o C) 0 Vapor removal at surface 10 20 30 50 W/cm 2 D T s ( o C) Surface Temperature 8 High-frequency acoustic actuation » Increases surface temperature (7 o C)  Detaches small scale vapor bubbles 4  Suppresses vaporization process at most nucleation sites » Increases CHF by 65%  Agreement with wire experiments of Isakoff (1956) 0 50 100 q’’ (W/cm 2 ) 6 Fluid Mechanics Research Laboratory

Ultrasonic Control of the Boiling Curve Acoustic Actuation q’’ (W/cm 2 ) No Actuation (1.7 MHz) 200 190 183 Video Presented Here Video Presented Here 110 100 0 10 20 30 q’’ = 100 1 mm T s - T sat ( o C) W/cm 2 7 Fluid Mechanics Research Laboratory

Effect of Actuator Incidence Angle D T D T as function of D T ( o C) Actuator Position 10 q 10 9 8 7 f 6 5 0 30 60 90 f 5 q’’ base = 100 W/cm 2 , T base = 110 o C, T bulk = 93 o C 8 Fluid Mechanics Research Laboratory

microChannel Design: More than Surface Area CHF (W/cm 2 ) (400 m m) 300 (1000 m m) Smooth microChannels Dimpled approx. same wetted area 200 (200 m m) (plain) 100 w/D 1000 400 200 q’’ (W/cm 2 ) 0 0.1 0.2 0.3 Normalized by projected area Normalized by wetted area 400 200 0 10 20 30 10 20 30 T s - T sat ( o C) 9 Fluid Mechanics Research Laboratory

800 m m 1000 m m Surface m Channels with Ultrasonic Actuation 400 m m 500 m Channel Actuated (1.7 MHz) 460 m Channel: actuated q’’ (W/cm 2 ) m Channel: 350 unactuated 300 250 200 183 110 100 Smooth 100 W/cm 2 300 W/cm 2 200 W/cm 2 200 W/cm 2 100 W/cm 2 300 W/cm 2 0 1 mm 10 20 30 T s - T sat ( o C) Smooth: unactuated Smooth: actuated Small-scale acoustic actuation within m Channels » Decreases surface temperature (~ 7 o C). Smooth 5 D T s ( o C) Increased power dissipation D P  200 » W/cm 2 at T s - T sat =17 o C 0 » Increases CHF by 31% m Channel » Decreases surface temperature -5 fluctuations. » Increase CHF by 318% relative to smooth, unactuated case 100 200 300 q’’ (W/cm 2 ) 10 Fluid Mechanics Research Laboratory

O(1 kHz) Acoustic Enhancement of Boiling q’’ (W/cm 2 ) a f 20 b g 40 c h 60 d i  Marginal increase in CHF (16%) 80 Decrease in surface superheat of ~1 o C  Appearance of vapor is markedly  different due to surface capillary waves e j » Increased condensation has minimal effect 100 on boiling process 11 Fluid Mechanics Research Laboratory

Acoustic Control of Vapor Condensation 1 mm  Pool boiling and condensers both require enhanced condensation » Pool boiling used in heat sink applications Condensation  Vapor boils and condenses in close proximity » Condensers used in power cycles  Vapor is injected; boiling occurs in separate boiler component Boiling  Nozzle geometry interacts with vapor formation and acoustic enhancement  Condensation is limited by interface area » Thermal boundary layer surrounds vapor Condensation Nozzle Ejection 1 mm 12 Fluid Mechanics Research Laboratory

Acoustically Controlled Condensation Experimental Setup vacuum pump  Vacuum pump sets the ambient pressure in test cell Tank  Middle plate separates 10.8 cm boiling from condensation Distilled water d 0.15 – 1 atm » Nozzle geometry can be varied » Bulk temperature of upper tank controlled with coil heat steam reservoir 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 nozzle nozzle nozzle steam reservoir steam reservoir steam reservoir 13 Fluid Mechanics Research Laboratory

Acoustically Enhanced Bubble Condensation Low Frequency (1 kHz) Increased thermal interfacial mixing leads to rapid collapse. nozzle steam reservoir Base Flow Actuated Volume (25 degrees subcool, kHz continuous) 1 1 1 9 8 Video Presented Here Video Presented Here 7 6 V/V o 5 4 3 2 0.19 1 0 0 0 0 2 4 6 8 10 0 5 10 0 100 200 t/T t/T 0 o Vapor Area A ∗ = Average Baseline Vapor Area T o : acoustic actuation period  1 msec 25 o C subcool 225 W 14 Fluid Mechanics Research Laboratory Continuous Actuation (Atmospheric Pressure)

Boundary Layer Growth - kHz Condensation d t (mm) 160 Image processing of Schlieren  images yields quantitative Average Instantaneous Thickness All Instantaneous Thickness 2 information on boundary layer I ( x , y i , t i ) growth 80  Thermal boundary layer in baseline flow does not undergo appreciable growth x (mm) x (mm) 0 » Heat transfer occurs primarily -10 0 10 20 -10 0 10 20 through lower (and 1 {[ I’ ( x , y i , t i )] 2 } subsequently, inner) interface 4  Acoustic actuation leads to 2 nearly linear growth of boundary layer thickness 0 » No significant temporal {[ I’ ( x , y i , t i )] 2 } dependence on acoustic 4 actuation 0  Thermal boundary layer in 0 d t 5 d t 10 2 presence of acoustic actuation t/T o (ms) is on average 6.7 times thicker 0 -1 1 -1 0 1 x (mm) x (mm) » Up to 17 times thicker d t d t 15 Fluid Mechanics Research Laboratory

Natural Deformation-Induced Vapor Collapse 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 Video Presented Here gradients in fluid surrounding bubble. Inner “spear” enhances heat » nozzle transfer  This natural mechanism steam reservoir indicates that inducing such a liquid “spear” early in the bubble formation process can lead to accelerated condensation. 16 Fluid Mechanics Research Laboratory

Ultrasonic Liquid-Gas Interfacial Actuation  f = 1.7 MHz  l acoust = 0.9 mm; D res = 2 m m  l capillary = O( m m) Video Presented Here “Mist” droplets ejected, visible in » video  Cavitation and subsequent collapse generates additional droplets » Larger-scale; not uniformly sized  Acoustic impedance mismatch » Z vapor /Z water =1.8x10 -4 » Surface deforms from acoustic pressure  Deformed surface self-focuses acoustic intensity 1 cm 17 Fluid Mechanics Research Laboratory