Aircraft Icing“High Speed Flow Effects”

Prof. Dr. Serkan ÖZGENDept. Aerospace Engineering, METU

Fall 2015

• Introduction• Problem formulation and solution method

– Flowfield solution,

– Droplet trajectories and collection efficiencies,

– Convective heat transfer coefficients,

– Extended Messinger Method.

• Results and discussion• Conclusions

Outline

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• In-flight icing on airframe components is one of the most important problems of civil aviation,

• Current procedures (App. C) in FAR 25 and CS 25 havebeen upgraded to account for SLD and mixed ice effects (App. D, O and P),

• Ice formation on the wings, tail surfaces, fan blades or sensors like pitot tubes seriously threaten flight safety as the aerodynamic performance and control characteristics become seriously and often unpredictably degraded.

Introduction-1

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• In order to demonstrate that an airplane can fly safely in icing conditions, certification authorities like FAA and EASA have defined meteorological conditions that the airplane must be tested against in flight tests, laboratory tests and/or computer simulations.

• Compressibility has been shown to be a prominent effect and influences both the accumulated ice mass

and the extent of the iced region.

Introduction-2

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Introduction-3

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Parameters involved in icing

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Parameter Short-hand Symbol

Ambient Temperature Ta (oC or oF)

Freestream Velocity orMach number

V (m/s or knots)

Altitude h (ft)

Liquid water content LWC ρa (g/m3)

Droplet size (median volume diamter)

MVD dp (μm)

Exposure time texp (s)

Horizontal extent HE HE (nm)

Angle of attack α (o)

Shape and size (geometry)

c (m)

• In order to determine the flow velocities required for droplet trajectory calculations, 2-D Hess-Smith panel method is used.

• The solution also provides the external velocity distribution around the wing section required for boundary layer calculations yielding the convective heat transfer coefficients.

Problem formulation - flowfield solution

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• Incompressible Hess –Smith panel solution yields(anywhere in the flow):

• Perturbation velocity components after applyingPrandtl-Glauert compressibility correction:

• Therefore:

Problem formulation - flowfield solution

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.1/ˆ ,1/ˆ 22 MvvMuu

flow. ibleincompressin componentsy on velocitperturbati : and vu

. , vVuVV yx

.ˆ ,ˆ vVuVV yx

Following assumptions are employed for droplets:

– Droplets are assumed to be spherical,

– The droplets do not affect the flow field,

– Gravity and aerodynamic drag are the only forces acting on

the droplets.

Problem formulation - droplet trajectories

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Equations of motion for the droplets:

Droplet drag coefficients are calculated using the drag law given by Gent et al.

Problem formulation - droplet trajectories

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,cosDxm p

,sin mgDym p

,tan 1

xp

yp

Vx

Vy

,2

1 2

pDa ACVD

velocity.relativedroplet :22

ypxp VyVxV

.components velocity airflow local : , yx VV

Droplet trajectories

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s

y

ds

dy oo

Droplet collection efficiency

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• The current study employs a 2-D Integral Boundary Layer Method for the calculation of the convective heat transfer coefficients.

• Previous and present results show that the accuracy achieved with this approach is adequate for the purposes of this study.

Problem formulation -convective heat transfer coefficients

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Problem formulation -Extended Messinger Model

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layer ice in theequation energy ; 2

2

y

T

C

k

t

T

pii

i

layer water in theequation energy ; 2

2

yC

k

t pww

w

balance mass ;

V

t

h

t

Bawi

condition (Stefan) change phase ; y

ky

Tk

t

BL wiFi

• Initial and boundary conditions:– Ice is in perfect contact with the airfoil surface:

– The temperature is continuous at the ice/water boundary and is equal to the freezing temperature, Tf:

– Airfoil surface is initially clean:

Problem formulation -Extended Messinger Model

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.2.01

2.01

2

,,0

2

222

M

rM

C

UVTT

TtT

p

eas

s

.,, fTtBtBT

.0,0 thB

• Initial and boundary conditions (cont’d):

- At the air/water or air/ice interface, heat flux is determined by convection Qc, heat from incoming droplets Qd, evaporation Qe (or sublimation Qs), aerodynamic heating Qa, kinetic energy of incoming droplets Qk, latent heatrelease during solidification Ql and radiation Qr.

Problem formulation -Extended Messinger Model

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• Airfoil: SA 13112,

• Chord: 0.6 m,

• Liquid water content: 0.5 g/m3,

• Median volume diameter: 20 microns

Results and discussion – test cases studied

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Parameter Case 40 Case 41 Case 42

α (o) 10 0 0

V (m/s) 81.3 162.5 249.9

Mach # 0.25 0.50 0.80

Ta (oC) -10 -10 -30

texp (s) 900 450 180

Results and discussion (Case 40, M=0.25)

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Results and discussion (Case 41, M=0.50)

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Results and discussion (Case 42, M=0.80)

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• Compressibility has a significant effect on the mass, shape and extent of accumulated ice.

• Neglect of compressibility effects results in smoother shapes with larger mass.

• Due to aerodynamic heating, stagnation regions have warmer surface temperatures than the freezing temperature preventing ice formation there.

• Inclusion of compressibility effects result in more realistic and conservative ice shapes for aerodynamic

performance prediction.

Conclusions

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