Dr. Dr. HuiHui HUHUAdvanced Flow Diagnostics and Experimental Aerodynamics LaboratoAdvanced Flow Diagnostics and Experimental Aerodynamics Laboratoryry

Department of Aerospace Engineering, Iowa State UniversityDepartment of Aerospace Engineering, Iowa State University2251 Howe Hall, Ames, IA 500112251 Howe Hall, Ames, IA 50011--22712271

Email: Email: huhui@iastate.eduhuhui@iastate.edu

An Experimental Investigation on Surface Water Transport and IceAn Experimental Investigation on Surface Water Transport and IceAccreting Process Pertinent to Wind Turbine Icing Phenomena Accreting Process Pertinent to Wind Turbine Icing Phenomena

Wind Turbine Icing and AntiWind Turbine Icing and Anti--/De/De-- IcingIcing

•• Wind turbine icing Wind turbine icing represents the most significant threat to the integrity of represents the most significant threat to the integrity of wind turbines in cold weather. wind turbines in cold weather. –– Change airfoil shapes of turbine blades.Change airfoil shapes of turbine blades.–– Cause imbalance to the rotating system.Cause imbalance to the rotating system.–– Shedding of large chuck of ice can be dangerous to public safetyShedding of large chuck of ice can be dangerous to public safety..–– Cause errors to anemometers to estimate wind resource.Cause errors to anemometers to estimate wind resource.

•• Some Some thermal dethermal de‐‐icing systems icing systems could consume up to could consume up to 70%70% of the total power of the total power generated by the wind turbine on cold days.generated by the wind turbine on cold days.

• Glaze ice is the most dangerous type of ice.

• Glaze ice form much more complicated shapes and are difficult to accurately predict.

• Glaze ice is much more difficult to remove once built up on aircraft wings or wind turbine blades.

Rime Ice and Glaze IceRime Ice and Glaze Ice

a)a)

b)b)

Oncoming air Oncoming air flow with flow with supercooledsupercooledwater dropletswater droplets

Glaze ice formation

Rime ice formation

Surface Tension Surface Tension IInduced Flow nduced Flow -- MaragoniMaragoni Flow Flow inside Water Dropletsinside Water Droplets

Temperature of Plate, Temperature of Plate, TTWallWall=21.9 =21.9 OOC C Video was taken at f=0.5hz; ReVideo was taken at f=0.5hz; Re--play speed is f=5hzplay speed is f=5hz

((HuHu and Jin, and Jin, Int. J. of Multiphase Flow, Vol. Int. J. of Multiphase Flow, Vol. 36, No.8, pp67236, No.8, pp672––681681, 2010, 2010))

a)a)

b)b)Incoming flow Incoming flow with superwith super--cooled water cooled water dropletsdroplets

Incoming flow Incoming flow with superwith super--cooled water cooled water dropletsdroplets

Molecular Tagging Thermometry Technique (MTT) to Quantify the TiMolecular Tagging Thermometry Technique (MTT) to Quantify the Time me Evolution of the Droplet Cooling and Evaporation ProcessEvolution of the Droplet Cooling and Evaporation Process

0

0.1

0.2

0.3

0.4

0.5

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0.7

0.8

0.9

1.0

1.1

1.2

60 70 80 90 100 110 120 130 140 150 160 17040

45

50

55

60

65

70

Droplet volume, VDroplet heigth, hContact angle

Time (second)

V/V

o , h/

h o

Con

tact

ang

le (d

egre

es)

((HuHu and Huang, AIAA Journal, Vol. 47, No.4, and Huang, AIAA Journal, Vol. 47, No.4, pp813pp813--820, 2009820, 2009 ))

H

R

VWater droplet

Solid surface

)(tan2 1

RH

3

23

sin3)cos2()cos1(

RV

m

m

-400 -200 0 200 400

0

200

400

5.0 7.0 9.0 11.0 13.0 15.0 17.0 19.0 21.0

Temperature(OC)

Test Plate (T=5.0OC)

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2

4

6

8

10

12

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0 5 10 15 20 25 30 35 40

Curve fitExperimental data

Time (s)

Tem

pera

ture

(OC

)

Test plate, T=5.0 Test plate, T=5.0 OOCC

a).a). The first phosphorescence image acquired The first phosphorescence image acquired at 0.5ms after excitation laser pulseat 0.5ms after excitation laser pulse

b).b). The second phosphorescence image acquired The second phosphorescence image acquired at 3.5ms after the same excitation laser pulseat 3.5ms after the same excitation laser pulse

Test plate, Test plate, T=5.0 T=5.0 OOCC

~280~280mm

Phase Change Process within An Icing Water DropletPhase Change Process within An Icing Water Droplet

Test Plate, Tw = -2.0OC Test Plate, Tw = -2.0OC Test Plate, Tw = -2.0OC Test Plate, Tw = -2.0OC

~250mt = 0.5 s

Solid ice

Liquid water

t = 5.0 s t = 20.0 s t = 35.0 s

X (m)

Y(

m)

-400 -200 0 200 400

0

200

4000.0 3.0 6.0 9.0 12.0 15.0 18.0 21.0 24.0

Test Plate (TW=-2.0OC)

t = 0.5 s

Temperature(OC)

X (m)

Y(

m)

-400 -200 0 200 400

0

200

4000.0 3.0 6.0 9.0 12.0 15.0 18.0 21.0 24.0

Test Plate (TW=-2.0OC)

t = 5.0 s

Temperature(OC)

X (m)

Y(

m)

-400 -200 0 200 400

0

200

4000.0 3.0 6.0 9.0 12.0 15.0 18.0 21.0 24.0

Test Plate (TW=-2.0OC)

t = 20.0 s

Temperature(OC)

X (m)

Y(

m)

-400 -200 0 200 400

0

200

4000.0 3.0 6.0 9.0 12.0 15.0 18.0 21.0 24.0

Test Plate (TW=-2.0OC)

t = 35.0 s

Temperature(OC)

Unsteady heat transfer and phase changing process inside small iUnsteady heat transfer and phase changing process inside small icing water dropletscing water droplets

Instantaneous phosphorescence imagesInstantaneous phosphorescence images

((HuHu and Jin, and Jin, International Journal of Multiphase Flow, 36(8):672–681, 2010))

Surface Water Transport in Glaze Ice Accretion ProcessSurface Water Transport in Glaze Ice Accretion Process

•• Collaborating with Dr. Collaborating with Dr. AlricAlric RothmayerRothmayer @ Iowa State University@ Iowa State University

Numerical simulation results of surface water transport Numerical simulation results of surface water transport (Wang & (Wang & RothmayerRothmayer, Computers and Fluids 2009). , Computers and Fluids 2009).

Glaze ice accreting process over a NACA0012 airfoil (Waldman anGlaze ice accreting process over a NACA0012 airfoil (Waldman and d HuHu, 2014), 2014)

Flow Flow directiondirection

VV

Videos of ice accretion

α ≈ 5.0 deg.

ISU Icing Research TunnelISU Icing Research Tunnel

ISUISU--Icing Research Tunnel (ISUIcing Research Tunnel (ISU--IRT)IRT)

•• The ISU Icing Research Tunnel (ISUThe ISU Icing Research Tunnel (ISU--IRT), IRT), originally donated by originally donated by UTC Aerospace System ( formerly Goodrich Corp.), is a researchUTC Aerospace System ( formerly Goodrich Corp.), is a research--grade multigrade multi--functional icing wind tunnel. functional icing wind tunnel.

•• The working parameters of the ISUThe working parameters of the ISU--IRT include: IRT include: •• Test section:Test section: 16 inches by 16 inches by 6 ft16 inches by 16 inches by 6 ft•• Velocity: Velocity: up to 60m/s;up to 60m/s;•• Temperature: Temperature: down to down to --30 30 OOC;C;•• Droplet Size: Droplet Size: 10 to 100 micrometers;10 to 100 micrometers;•• Liquid Water Content (LWC): Liquid Water Content (LWC): 0.05 ~ 20 grams/cubic meter. 0.05 ~ 20 grams/cubic meter.

•• The large LWC range allows ISUThe large LWC range allows ISU--IRT tunnel to be run over a IRT tunnel to be run over a range of conditions from rime ice to extremely wet glaze ice.range of conditions from rime ice to extremely wet glaze ice.

0

2

4

6

0 2 4 6

Linear curve fittingmeasurement data

Phase difference, (radians)

Hei

ght (

mm

)

K=1.0266 mm/pixel

Displacement (pixel)

height (mm

)

Digital Image Projection (DIP) technique Digital Image Projection (DIP) technique (USA Patent Pending)(USA Patent Pending)

Reference imageReference image Deformed image due to the existence of a Deformed image due to the existence of a semisemi--sphere on the reference planesphere on the reference plane

CAKCAdsBDyxZ ),(

Height distributionHeight distribution

CameraProjector

Vertical Translation Stage

Target plate

Host computer

Z

Y

X

Transient Behavior of Wind Transient Behavior of Wind ––Driven Water Film/Rivulet Flows for Icing Physics Study Driven Water Film/Rivulet Flows for Icing Physics Study (Funded by NASA and NSF)(Funded by NASA and NSF)

((HuHu et al. 2011, Experiments in Fluids)et al. 2011, Experiments in Fluids)

-5

0

5

10

15

20

0 1 2 3 4 5 6 7 8 9

rear end of the droplet/rivuletfront end of the droplet/rivulet

Time (s)

Mov

ing

spee

d of

the

cont

act l

ine

(mm

/s)

0

0.5

1.0

1.5

2.0

2.5

0 1 2 3 4 5 6 7 8 9

V/VoS/So

Time (s)

Wet

Sur

face

Are

a (S

/So),

Dro

plet

/Riv

ulet

Vol

ume

(V/V

o)

-2

0

2

4

6

-8 -4 0 4 8 12 16 20 24 28 32 36

Measured Droplet/Rivulet Thickness

Downstream Distance (mm)

Dro

plet

/Riv

ulet

Thi

ckne

ss (m

m)

t = t0+5.0s

-2

0

2

4

6

-8 -4 0 4 8 12 16 20 24 28 32 36

Measured Droplet/Rivulet Thicknessy=0

Downstream Distance (mm)

Dro

plet

/Riv

ulet

Thi

ckne

ss (m

m)

t = t0

-2

0

2

4

6

-8 -4 0 4 8 12 16 20 24 28 32 36

Measured Droplet/Rivulet Thicknessy=0

Downstream Distance (mm)

Dro

plet

/Riv

ulet

Thi

ckne

ss (m

m)

t = t0+1.0s

Contact line moving Contact line moving speed vs. timespeed vs. time

Wet area on the test plate & droplet Wet area on the test plate & droplet /rivulet volume (mass) vs. time/rivulet volume (mass) vs. time

Time Evolution of a WindTime Evolution of a Wind--Driven Droplet/Rivulet Flow over a Flat Surface Driven Droplet/Rivulet Flow over a Flat Surface

•• HuHu et al., AIAAet al., AIAA--20122012--0261,0261, 20122012

Effects of various important parameters:Effects of various important parameters:•• Temperature of the surfaceTemperature of the surface•• Thermal conductivity of the subtractsThermal conductivity of the subtracts•• Roughness of the test surfacesRoughness of the test surfaces•• Surface Surface hydrophobicityhydrophobicity•• Coatings or Coatings or nanonano--structures on the test surfacesstructures on the test surfaces•• ……. .

Boundary layer airflow Boundary layer airflow Boundary layer airflow

Transient Behavior of Wind Transient Behavior of Wind ––Driven Water Film/Rivulet FlowsDriven Water Film/Rivulet Flows(Dry Surface Condition)(Dry Surface Condition)

•• Water flow rate: Water flow rate: Q= 100 ml/minQ= 100 ml/min•• Free stream airflow: Free stream airflow: VV∞∞=20m/s =20m/s

•• Water flow rate: Water flow rate: Q= 100 ml/minQ= 100 ml/min•• Free stream airflow: Free stream airflow: VV∞∞=15m/s =15m/s

•• Water flow rate: Water flow rate: Q= 100 ml/minQ= 100 ml/min•• Free stream airflow: Free stream airflow: VV∞∞=10m/s =10m/s

Experimental conditions:Experimental conditions:smV /20~10

min/0.1 mlQ Incoming flow velocity: Incoming flow velocity: Water flow rate:Water flow rate:Spray droplet size:Spray droplet size: 50um~10D

Incoming Incoming flowflow

Digital image projector

Digital camera

Spray nozzles

Water droplets

NACA0012 airfoil

incoming airflow

Picture of the test section in the icing wind tunnelPicture of the test section in the icing wind tunnel

WindWind--driven Water Runback Flow over a NACA 0012 Airfoildriven Water Runback Flow over a NACA 0012 Airfoil

(Zhang K. and Hu H., AIAA(Zhang K. and Hu H., AIAA--20142014--0741,0741, 2014)2014)

MicroMicro--sized Water Droplets Impinging onto a NACAsized Water Droplets Impinging onto a NACA--0012 Airfoil0012 Airfoil

•• Test ConditionsTest Conditions•• Angle of attack of the airfoil:Angle of attack of the airfoil: αα ≈≈ 0.0 deg. 0.0 deg. •• Temperature of the wind tunnel :Temperature of the wind tunnel : T T ≈≈ 20 20 °°C.C.•• The liquid water content (LWC) : The liquid water content (LWC) : LWC =5.0 g/mLWC =5.0 g/m33

•• Frame rate for Image acquisition: Frame rate for Image acquisition: f = 30 Hzf = 30 Hz

Airflow velocityAirflow velocity

V=20m/sV=20m/sAirflow velocityAirflow velocity

VV =15m/s =15m/s Airflow velocityAirflow velocity

VV =20m/s =20m/s 

Airflow velocityAirflow velocity

VV =25m/s =25m/s 

VV =15m/s=15m/s VV =20m/s=20m/s VV =25m/s=25m/s

Water Runback Flow over a NACA 0012 AirfoilWater Runback Flow over a NACA 0012 Airfoil

• V = 10m/s • V = 15m/s • V = 20m/s • V = 25m/s

Velocity of the oncoming airflow(m/s)

Transition location from film flows to rivulet flows 

(% airfoil chord length)

Averaged width of the rivulet flows(% airfoil chord 

length)

Averaged gap between the rivulet flows (% airfoil 

chord length)

10.0 51.5 13.64 >35

15.0 44.2 9.51 17.41

20.0 35.3 4.92 10.23

25.0 27.1 3.03 6.34

30.0 25.2 1.97 5.19

• V = 10m/s

• V = 20m/s

Water Runback Flow over a NACA 0012 AirfoilWater Runback Flow over a NACA 0012 Airfoil

S x=0.13c

• Feo (2001) and Rothmayer (2003) all suggested that wind- driven water film thickness would follow a x1/4law :

Water Runback Scaling Laws:Water Runback Scaling Laws:

Hu et al., AIAA Journal (2015), submittedHu et al., AIAA Journal (2015), submitted

Glaze Ice Accreting Process over a NACA0012 AirfoilGlaze Ice Accreting Process over a NACA0012 Airfoil

• Test Conditions• Oncoming airflow velocity : V ≈ 20 m/s• Angle of attack of the airfoil: α ≈ 5 deg. • Temperature of  the wind tunnel : T ≈ ‐8 °C.• The liquid water content (LWC) :  LWC =3.0 g/m3

• Total recording time : t = 110 seconds

Frame rate  for Image acquisition,  f = 150Hz, 10X replay

Upper surface

Lower surface

VV

Videos of ice accretion

α ≈ 5.0 deg.

(Waldman R. and Hu H.,(Waldman R. and Hu H., 2015, Journal of Aerocraft, submitted)2015, Journal of Aerocraft, submitted)

Glaze Ice Accreting Process over a NACA0012 AirfoilGlaze Ice Accreting Process over a NACA0012 Airfoil

VV∞∞= 40 m/s= 40 m/s

VV∞∞= 60 m/s= 60 m/s

VV∞∞= 20 m/s= 20 m/s

•• TT ∞∞ = = -- 8.0 8.0 °°C; C; •• = 5 = 5 °°; ; •• LWC = 1.1 g/mLWC = 1.1 g/m33

(Waldman R. and Hu H.,(Waldman R. and Hu H., 2015, Journal of Aircraft , submitted)2015, Journal of Aircraft , submitted)

IR Thermometry to Quantify the Unsteady Heat Transfer ProcessIR Thermometry to Quantify the Unsteady Heat Transfer Process

Incoming Incoming airflowairflow

•• Experimental Conditions: Experimental Conditions: VV∞∞= 35 m/s; = 35 m/s; TT ∞∞ = = -- 8.0 8.0 °°C; C; AoA= 5 AoA= 5 °°; ; LWC = 3.0 g/mLWC = 3.0 g/m33

B AD CE

B AD CE

•• Experimental Conditions: Experimental Conditions: VV∞∞= 35 m/s; = 35 m/s; TT ∞∞ = = -- 8.0 8.0 °°C; C; AOA = 5 AOA = 5 °°; ; LWC = 0.30 g/mLWC = 0.30 g/m33

(Liu Y. and Hu H.,(Liu Y. and Hu H., 2015, AIAA Journal, submitted)2015, AIAA Journal, submitted)

Rime ice accretion

Glaze ice accretion

Hydrophilic, Hydrophobic and Hydrophilic, Hydrophobic and SuperhydrophobicSuperhydrophobic

a). drop on a smooth surface;     b). Wenzel state;        c). a). drop on a smooth surface;     b). Wenzel state;        c). CassieCassie––Baxter state;      d). combined state.Baxter state;      d). combined state.

Measured Measured θθ = 67 [deg]= 67 [deg]

Measured Measured θθ = 170 [deg]= 170 [deg]Measured Measured θθ = 104 [deg]= 104 [deg]

•• Hydrophilic;Hydrophilic; < 90 < 90 oo •• Hydrophobic;Hydrophobic; 90 90 oo << < 150 < 150 oo •• SuperhydrophobicSuperhydrophobic; ; > 150 > 150 oo

Lotus leaves

Effects of Surface Effects of Surface HydrophobicityHydrophobicity on Surface Water Transport and Ice on Surface Water Transport and Ice Accretion ProcessAccretion Process

Anti-icing of superhydrophobicsurface in “freezing rain”

reported by Cao et al. (2009)

•• Surface engineering to change the surface Surface engineering to change the surface hydrophobicityhydrophobicity of the of the airfoils/wings for wind turbine antiairfoils/wings for wind turbine anti--/de/de--icing applicationsicing applications

Anti- icing coating test on wind turbines

Surface Chemistry: Effects of Surface Chemistry: Effects of HydrophobicityHydrophobicity of the Airfoil Surface of the Airfoil Surface on the on the Impingement of Water Droplets ( Weber number ~ 800 )Impingement of Water Droplets ( Weber number ~ 800 )

45 degree slope

With Super-hydrophobic Surface coating

Without Super-hydrophobic Surface coating Without Super-hydrophobic

Surface coating

With Super-hydrophobic Surface coating

Acquired at 12K FPS, replay at 400X slower

Normal impact

Without Super-hydrophobic Surface coating

With Super-hydrophobic Surface coating

Without Super-hydrophobic Surface coating

With Super-hydrophobic Surface coating

Surface Chemistry: Effects of Surface Chemistry: Effects of HydrophobicityHydrophobicity of the Airfoil Surface of the Airfoil Surface on the on the Impingement of Water DropletsImpingement of Water Droplets

Thank you Very Much for Your Time!Thank you Very Much for Your Time!Questions?Questions?

Upper surface

Lower surface

WindWind--driven water film flow over a NACA0012 airfoil pertinent to aircdriven water film flow over a NACA0012 airfoil pertinent to aircraft icing (Zhang and raft icing (Zhang and HuHu, 2014), 2014)