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American Institute of Aeronautics and Astronautics
1
An Experimental Study on Icing Physics for Wind Turbine
Icing Mitigation
Linyue Gao1, Yang Liu
2, Hui Hu
3()
Department of Aerospace Engineering, Iowa State University, Ames, Iowa, 50011
An experimental study was conducted to investigate the dynamic ice accretion process over
the surface of a wind turbine blade with DU96-W-180 airfoil profile in order to elucidate the
underlying physics for wind turbine icing mitigation. The experimental study was conducted
in the Icing Research Tunnel of Iowa State University (ISU-IRT). Six test cases with three
typical liquid water content (LWC) levels (i.e., LWC =0.3 g/m3, 1.1 g/m
3, and 3.0 g/m
3) and
two typical oncoming airflow temperatures (i.e., T∞ = -5 oC, and -10
oC) were selected to
represent rime icing, mixed icing and glaze icing conditions that wind turbines may
experience in winter. In addition to using a high-speed imaging system to quantify the
transient behavior of surface water runback and dynamic ice accretion processes over the
airfoil surface, an infrared (IR) thermal imaging system was also utilized to simultaneously
measure the airfoil surface temperature distributions in the course of the ice accreting
process. The measurement results reveal clearly that, both the length of the surface water/ice
film formed near the airfoil leading edge and the subsequent water/ice rivulet length would
increase as the liquid water content level and the temperature of the oncoming airflow
increase. The thickness of the ice accreted at the airfoil leading edge was found to grow
rapidly with the increasing liquid water content level and decreasing oncoming airflow
temperature for both glaze and mixed icing test cases. The temperature distribution over the
airfoil surface was also found to vary significantly, depending on the type of ice accreted
over the airfoil surface. A stream-wise ‘plateau’ region was observed in the measured
surface temperature distributions for the glaze and mixed icing cases, due to the formation
of surface water runback over the ice accreting airfoil surface.
Nomenclature AoA = angle of attack
C = chord length
IEA = International Energy Agency
IR = infrared
ISU-IRT = Icing Research Tunnel at Iowa State University
LE = leading edge
L, Lf, Lr, LLE = length, stream-wise lengths of ice film, ice rivulet, and ice accretion on LE
LWC = liquid water content
MVD = median volumetric diameter
Q = heat flux
RTV = relative temperature variation
T, T∞ = temperature, ambient temperature
t = time
TE = trailing edge
V∞ = oncoming flow velocity
X,Y = span-wise and stream-wise coordinates
2D, 3D = two-dimensional, three-dimensional
1 Graduate Student, Department of Aerospace Engineering.
2 Postdoctoral Associate, Department of Aerospace Engineering.
3 Martin C. Jischke Professor, Dept. of Aerospace Engineering, AIAA Associate Fellow, Email: [email protected].
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35th Wind Energy Symposium
9 - 13 January 2017, Grapevine, Texas
AIAA 2017-0918
Copyright © 2017 by Linyue Gao, Yang Liu, Hui Hu. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
AIAA SciTech Forum
American Institute of Aeronautics and Astronautics
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I. Introduction
Wind turbines in cold climate are becoming one of the most popular trends for wind energy developments. Cold
climate areas retain wind resources superiority ascribed to the increased air density and wind speeds, providing a
vast market potential, particularly in offshore and hilly projects. But, meanwhile, cold climate exerts profound
negative effects on the aerodynamic performance and lifespan of wind turbines due to the occurrence of icing
events. According to IEA Icing Climate site classification1, five classes are recommended on behalf of the severity
of the icing events whose annual production losses are ranging from less than 0.5% to over 20%. One reason for the
loss of energy production would be the ice accretion on wind turbine blades. The ice accumulation enlarges the
blades’ surface roughness and, more seriously, changes the geometric shapes, which perturbs the aerodynamic
performance of wind turbines2,3
. The downtimes would be the second reason4. Wind turbines without the icing
mitigation equipment have to shut down during the icing events to prevent mechanical damages. The downtimes
usually last days, even weeks, associate with favorably high wind speeds, resulting in the most acute energy loss. In
addition to the power loss, the lifespan of wind turbines subjected to the icing events would be shortened to different
degrees due to the additional loads, unbalance, and edgewise vibration5 or resonance
6. Furthermore, the secondary
induced effects of wind turbine icing, including increased noise and projected ice chunks7, would impair human’s
health and cause damages to their personal and property security.
In order to reduce these dampening effects caused by icing events, multifarious methods for wind turbine icing
mitigation have been proposed and some of them have been commercially utilized. These de-/anti-icing protection
techniques comprise passive and active modes. Hydrophobic/ice phobic coatings8–13
, black paint6 and operational
stops14,15
are most commonly-used passive ice protections, while active ice protections can be subdivided into
chemical, mechanical, thermal, and active pitch control methods. In IEA Ice Class 1 and 2, black paint and active
pitch control are comparably effective and efficient due to their low costs and energy consumption6,16
. For more
intensive icing events, such as those in IEA Ice Class 3, 4, and 5, inside/outside resistive heaters with/without
hydrophobic coatings are most promising in both new and existing wind farms16
. The resistive heaters have both
anti-/de-icing capabilities, very effective but expensive. They are ordinarily installed covering 1/3 leading edge
(LE), 2/3 LE or all blades16
. Recommendations of their mounting positions are imperatively needed, which is greatly
based on comprehensive perceptions with respect to icing physics.
Recent years, researchers pay more attentions to wind turbine icing. Yirtici17
estimated energy production losses
of wind turbines by using Blade Element Momentum theory with a 2D-based ice accretion prediction tool. Hudecz18
numerically analyzed the 2D iced profiles gained from wind tunnel tests by using ANSYS Fluent and then their iced
profiles were further compared with those generated in TURBICE, a numerical ice accretion software from VTT.
Li19
investigated icing area and icing rate considering different angles of attack (AoA) and wind speeds through a
wind tunnel experimental study, and they found that the maximum icing area ratio of icing area against airfoil area is
21.8% at AoA = -80 °. Kraj8 experimentally simulate four typical phases of icing on wind turbine blades, including
the pre-icing, operational-icing, stopped-icing and post-icing phases and further compared the effects of coatings,
heat treatments and some combination thereof. These studies provide variously practicable contributions to the wind
turbine icing issue. However, literatures of more fundamental icing physics are mostly related to aircraft icing20–22
.
Few of them are in the wind turbine field. The significant differences between wind turbines and aircrafts cannot be
neglected. Wind turbines usually operate beneath the Ekman layer, where climate, underlying surface conditions,
and the rainfall intensity, etc. are distinct from those aircrafts may experience. Besides, wind turbine dedicated
airfoils, different from aircraft airfoils, e.g. NACA0012, are thicker and asymmetric, possessing particular
aerodynamic, structural and thermal properties, which would affect icing formation and accretion process. So that an
investigation in terms of the underlying physics during the process of supercooled water droplets impingement,
water runback and ice accumulation, as well as the related thermal phenomena, over a wind turbine blade is valuable
and highly desired for enhancing the fundamental comprehension and providing references on the icing mitigation.
The widely-used icing models, i.e. LEWISE and TURBICE, are proved successful to generate 2D ice shapes
under certain icing conditions, followed with CFD analysis for aerodynamic performance23
. Based upon the given
initial and boundary information, they are efficient to some extent to yield acceptable predictions to meet the
industry requirements, especially in rime ice conditions. But, for more complicated and commonplace icing
conditions, such as glaze mixed icing, experimental techniques have undoubted advantages in providing more
accurate and extensive data to dig into the meticulous physical phenomena.
Therefore, we have conducted a series of experimental tests in the Icing Research Tunnel at Iowa State
University (ISU-IRT). In this paper, we concretely focus on the full life-cycle icing process over a DU96-W-180
airfoil model, as well as the thermal characteristics, to give a better understanding of the underlying physics for wind
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turbine icing. The high speeding imaging technique was used to investigate the transient water runback and dynamic
ice accretion process. Simultaneously, an infrared (IR) imaging thermometry technique was developed and utilized
to quantitatively measure the variations of temperature distributions on the pressure-side surface of the airfoil model.
A thorough experimental setup and case design are illustrated in the Section II. Six cases with two typical ambient
temperatures and three LWC levels were conducted, compared and analyzed in terms of the dynamic behaviors of
windward ice accumulation on the LE, water/ice film and rivulet, and surface temperature variations in Section III.
Section IV concludes the main findings.
II. Experimental Setup and Case Study A. Experimental Setup
Figure 1 shows the configuration of experimental setup for measuring the dynamic ice accreting process and
surface water runback over the DU96-W-180 model in ISU-IRT. The ISU-IRT, a research-grade multifunctional
icing research tunnel, has a capability of generating a maximum wind speed of 60 m/s and a minimum air
temperature of -25 °C. The test section is of 400 mm × 400 mm × 2,000 mm in height, width and length,
respectively, with an approximately 3.0% turbulence level of oncoming flow at its entrance. All the walls of the test
section are optically transparent and interchangeable for various test requirements. The spray system, installed at the
entrance of the contraction section, consists arrays of pneumatic atomizing spray nozzles (Spraying Systems Co.,
1/8NPT-SU11). This system can inject micro-sized water droplets (10 ~ 100 µm in size) into the test section. The
desired LWC levels in the wind tunnel is achieved by regulating the water flow rate while the needed medium
volumetric diameter (MVD) of the oncoming droplets is provided by adjusting the air/water pressure thorough the
spray nozzles. Various icing conditions have been simulated/duplicated in ISU-IRT and a series of
experiments21,22,24
have been successfully completed, which provides an excellent icing environment for wind
turbine icing physics studies.
Figure 1. Schematic diagram of experimental setup.
The high speed camera (PCO Tech, Dimax), aiming at capturing the whole icing dynamic process over the
model, was mounted right above the model with a 24 mm lens (Nikon, 24 mm Nikkor 2.8D). It provides a top view
of 528 × 888 pixels with a resolution of 5.08 pixels/mm. The frame rate of high speed camera is set as 30 frames per
second, the same with the IR camera, triggered by a digital delay/pulse generator (BNC, Model 577) to
simultaneously record the icing process. IR camera (FLIR, A615) and IR inspection window (FLIR, IR Window-
IRW-4C) constitute the IR system for temperature measurements. The IR camera can detect waves within a range of
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7.5-14 µm. It was fixed above the model, providing a field of 640 × 480 pixels with a resolution of 4.77 pixels/mm.
The IR window was embedded in the top panel of the test section. Three materials relevant to the tests are White
Protective Enamel covering the airfoil model surface, water and ice. Their emissivity coefficients are 0.945, 0.980
and 0.960, respectively, which are considered in the temperature calibration of the IR system. A surface
thermocouple (Omega), taped onto the model surface and connected to a J-type thermocouple (Omega), was also
used to record the one-point temperature on the model surface during the test to guarantee that the surface
temperature equals to that of the ambient airflow before each test. The results of this thermocouple were validated
by comparing with the data gathered by IR camera. They are in good agreement and linearly fitted with an
uncertainty less than ± 0.1 °C.
The test model was designed based on a wind turbine dedicated airfoil, DU96-W-180, with dimensions of 152.4
mm and 400 mm in stream-wise and span-wise, respectively, as shown in Fig. 2. DU96-W-180 is an asymmetric,
cambered, low-speed airfoil profile with a thickness-to-length ratio of 0.18, which is widely applied to make the
wind turbine blades as outboard airfoil due to its high lift-to-drag ratio, insensitivity to roughness and low noise25,26
.
It is representative and thus was selected as the objective in this study. The test model, supported by three stainless
steel rods, was mounted at its quarter-chord across the middle of the test section with a desired AoA by using a
digital inclinometer. The model was produced by using the 3D printing technique with a hard plastic material, Vero
White. Its surfaces were coated with Primer and White Protective Enamel, and then polished with up to 2,000 grit
sandpapers to assure the sufficient smoothness.
Figure 2. DU96-W-180 airfoil model. (a) Geometry of DU96-W-180 Profile; (b) Lateral view; (c) Top view.
B. Case Study
Table 1 shows the case design for icing physics study based on DU96-W-300 model. As an outboard airfoil,
DU96-W-180 possesses a relatively high lift-to-drag ratio when its lift-to-AoA curve situates at the linear growth
period where AoA is from 0 ° to 10 °, and one of them AoA = 5 ° is selected for the tests. For most multi-megawatt
wind turbines, their cut-in, rated and cut-out wind speeds, as well as rotor rated rotational speed, are around 3 m/s,
10 ~ 12 m/s, 22 ~ 25 m/s, and 12 ~ 16 rpm, respectively. The relative velocity of a blade element is the resultant
velocity of absolutely oncoming flow velocity and blade rotational linear velocity. The rotational linear velocity is
the product of rotor rotational speed and the local radius. GW87/1500 wind turbine, for example, has a tip speed
ranging from 40 m/s to 80 m/s. DU96-W-180 is designed to be mounted at 60% ~ 99% span-wise position, where
the relative velocity ranges approximately from 24 m/s to 80 m/s, so that the oncoming flow velocity in this study is
set as V∞ = 40 m/s covering the majority situation. MVD is controlled around 10 ~ 100 µm for six cases, which
satisfies the size of impinging water droplets, ranging from 10 µm to 150 µm, onto the wind turbine blades in the
real field environment17,27
. The ice accretion process over the pressure-side surface of the airfoil is highlighted
because the impinging super-cooled water droplets exert more significant influence on it in comparison with the
suction-side surface.
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Besides the parameters mentioned above, ambient temperature and LWC level are the most critical and decisive
factors for the icing issue. Three typical LWC levels, 0.3 g/m3, 1.1 g/m
3 and 3.0 g/m
3, are selected to show the low,
medium and high LWC situations. Two temperatures, -5 °C and -10 °C, are chosen. There are mainly two types of
ice, rime ice and glaze ice. Rime ice is formed when supercooled water droplets hit a surface and freeze
immediately, while glaze ice is constructed when the water droplets that hit the surface do not freeze completely and
the non-frozen water flow over the object1, leading rivulet formation. Compared to glaze icing condition, rime icing
usually occurs at a relative lower temperature with a smaller LWC level. There is also a transition ice type between
rime and glaze ice, named mixed ice. Given the oncoming flow velocity, ambient temperature and LWC level, as
well as the MVD, three typical types of icing conditions, rime, mixed and glaze icing, are expected to be observed
among the following six cases, which will be further validated with the obtained data.
Table 1. Case design for icing physics study on DU96-W-180 airfoil.
Case No. AoA [°] V∞ [m/s] T∞ [°C] (± 1.5 °C) LWC [g/m3] Expected Ice type
1 5 40 -5 0.3 Glaze
2 5 40 -10 0.3 Rime
3 5 40 -5 1.1 Glaze
4 5 40 -10 1.1 Mixed
5 5 40 -5 3.0 Glaze
6 5 40 -10 3.0 Glaze
III. Results and Discussions
To give a concrete to integral concept of the icing process, the dynamic icing process under various ice types is
elaborately illustrated, followed by the time evolutions of surface temperature distributions, dynamic ice accretion
upon the LE, and quantification of the icing impacted regions. And then icing process mechanism is summarized
and discussed.
A. Dynamic Icing Process
Figure 3 shows time evolutions of the dynamic ice accretion process among the three cases with an ambient
temperature of -5 °C. From Fig. 3 (a) to Fig. 3 (c), LWC level increases, and seven typical moments are selected.
Both LE and trailing edge (TE) are portrayed in the figure to help identify the special scale. When t = 0 s, the water
droplets are located upstream near the airfoil model without impinging onto it. The snapshot previous to the first
with water droplets impingement is identified as the beginning when t = 0 s.
It can be seen clearly that, supercooled water droplets impinge onto the airfoil surface and form a thin water film
that runs back over the airfoil surface. The film expands downstream and develops into isolated water rivulets
further downstream, indicating all of these three cases are in glaze icing conditions. Meanwhile, the water film and
rivulets freeze into ice film and ice rivulets. The building up ice icicles block the subsequent water runback and lead
to ice accretion concentrated near the LE. Beyond the basic process, more intuitive ice accreting process are shown.
Two features should be highlighted. The first one is the distinction of water and ice film and rivulets. Water film and
rivulets can be driven by the boundary layer flow due to their liquidity. For three cases, water film splits into rivulets
before 15 s and then continues expanding over the airfoil surface. Approximately at t = 80 s, water film and rivulets
almost completely convert into ice film and rivulets, and ice starts to primarily accumulate upon the region near LE.
Water film/rivulets are adaptable to investigate the transient behavior while ice film/rivulets are more suitable for
identifying of the icing impacted area. Another one is the ice configuration. As LWC level increases, an increasing
number of supercooled water droplets are impinging onto the airfoil, which leads to an enhancement in liquidity and
further results in more complex and coarser surface topography due to the increased uncertainty of ice accumulation.
Meanwhile, more crystal-clear ice deposits with tiny gaps in cases with relatively larger LWC levels are observed. In
Case No.5 where LWC = 3.0 g/m3, icicles can be seen standing closely side by side towards the upstream direction.
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Compared to other cases, it displays several complete water channels from LE to TE. These channels are much
thicker than the rivulets with dead ends shown in Case No.1 and Case No.3 due to the increased LWC level.
Figure 3. Snapshots of dynamic ice accretion process on DU96-W-180 airfoil where AoA = 5°, V∞ = 40 m/s.
Compared to the cases shown in Fig.3, the cases in Fig. 4 have a lower ambient temperature. Due to temperature
change, the appearance of ice accretion on the airfoil surface changes to a great extent. Only in Case No.6, rivulets
are formed and developed, but they are much thinner than those in Case No.5. Lower temperature would lead to a
fast freezing and thus block water runback, as well as the rivulet formation and extension. In Case No.2 and Case
No.4, there is no obvious rivulets. Case No.2 has a uniform ice layer with tiny structures, prone to be rime ice, the
same as expected. Compared to it, Case No.4 has a severe ice accretion, and the configuration of ice aggregated on
airfoil surface is quite different from that in Case No.2. Due to the unique characteristics, Case No.4 pertains to
mixed icing condition where the icicles stand on the surface towards the flow direction with a compact structure.
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Therefore, due to the combined action of ambient temperature and LWC level, the type of icing changes.
Additionally, ice accretion on LE can be observed clearly when t >80 s. But within the first 15 s, the snapshots have
a limitation to show more information due to the tiny changes, especially in Case No.2 and Case No.4.
Figure 4. Snapshots of dynamic ice accretion process on DU96-W-180 airfoil where AoA = 5°, V∞ = 40 m/s.
B. Temperature Variation Analysis
The icing process is along with complicated heat transfers. When the supercooled water droplets impinge onto
the airfoil, they entirely or partially solidify into ice, releasing the latent heat. The heat is immediately absorbed by
the upper unfrozen water layer, leading to a temperature raise in the water/air or ice/air interfaces, and then transfers
to the ambient flows mainly by convection. Due to the surface temperature alteration during the icing process, the
temperature contours over the surface are applied to provide a thorough view of the icing development.
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Figure 5 compares three typical moments of six cases. The increments of LWC level accompanies an
enlargement of icing impacted areas. At t = 8 s, this phenomenon can be observed more clearly. The rivulets that
start to form, stretch and advance downstream beyond 0.5C are shown in Fig. 5 (a), (c) and (e). A higher
temperature occurs near the LE due to a locally larger water collection and freezing rate. The impact of ambient
temperature on the surface temperature distributions can be seen directly, such as the absence of rivulet formation,
but due to the different temperature scales, they cannot be quantitatively compared, which would be further
discussed in the following section. Two interesting phenomena can be observed. Firstly, IR images show a nonlinear
temperature gradient along the chord, particularly in the cases with rivulet formation, which will be further
explained in the next section. Secondly, in the cases without rivulet formation, Case No.2 and Case No.4, their time
evolutions of surface temperature along the chord are also different. A clear expansion of surface temperature
variation in stream-wise is shown in Case No.4 due to the water runback above the ice layer, which help validate
that Case No.4 is in the mixed icing condition while Case No.2 is in the rime icing condition.
Figure 5. Time evolutions of surface temperature distribution on DU96-W-180 airfoil model where AoA = 5°
and V∞ = 40 m/s.
To further analyze the dynamic process of surface temperature variation, time series of temperature distributions
are extracted and shown in Fig. 6. A ‘peak’ can be seen in each curve after a precipitous rise, representing the
stream-wise largest temperature. At downstream locations of ‘peak’, surface temperature drops gradually. The
‘peak’ occurs near the LE and slightly moves downstream as time goes on, indicating the location of largest water
collection (stagnation points) in case without water runback24
. As LWC level increases from 0.3 g/m3 to 3.0 g/m
3,
maximum ‘peak’ value of temperature increases correspondingly due to incremental amount of latent heat released
during the water solidification. The time used for reaching the maximum ‘peak’ temperature also reduces, and all of
the cases can reach stable ‘peak’ values within 10 s.
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Figure 6. Span-wise normalized surface temperature variations along the chord in six cases.
At the early time, the temperature decreases linearly and then a ‘plateau’ region appears in Case No.1, Case
No.3, Case No.4, Case No.5 and Case No.6. This phenomenon is greatly related to the rivulet formation and water
runback. When coming into the second stage, more oncoming water droplets supplement the water layer and the
water film expanding downwards breaks into several isolated rivulets by overwhelming the gravity and water
surface tension. To obtain sufficient surface stress driven by the boundary layer flow, large amount of water
accumulates at the certain position resulting in a thicker water layer and more latent heat release, which in turn
causes a rise of local temperature. From the eight moments selected in the Fig. 6 (a), (c) and (e) with the same
temperature, we can roughly pick up the time of occurrence of the ‘plateau’, t = 8 s, t = 3 s, and t = 2 s, respectively.
This time has a similar trend with the time arriving at the ‘peak’ value, depending on the LWC. In Fig. 6 (d), there is
no obvious bulge on the chord-wise temperature distribution, and it owns the largest ‘peak’ value among the cases
under the same temperature, even larger than the case with a higher LWC ascribed to the intensive ice building
around the LE. Similar findings18,20,28,29
about the more severe effects of mixed ice than both glaze and rime ice have
been presented. In Fig. 6 (b), generally, the curves at different moments are roughly parallel with each other due to
the almost evenly ice building up along the pressure-side airfoil surface, forming a smooth and streamline-shaped
ice layer and indicating a rime icing process. The slight difference between the chord-wise temperatures of two
moments decreases with the increased value of Y/C, opposite with other cases. In Case No.2, the water droplets
impinging onto the airfoil surface freeze immediately. The LE area has higher temperature due to larger water
droplets collection and as distance away from the LE increases, the capability of capturing water droplets smoothly
decreases with respect to the geometry and posture of airfoil model. For other cases, the enlargement of the
difference is caused by water runback. Therefore, according to time series of chord-wise surface temperature
variation, types of icing can be identified more directly than only using the configuration characteristics of them.
In order to achieve a clear comparison of temperature alterations among six cases, a parameter, Relative
temperature variation (RTV), is defined in Eq. (1). It cannot provide the actual value, but it is superior to be applied
to illustrate the dynamic icing process. For the icing conditions without rivulet runback phenomenon, the ice mass
change linearly corresponds to the temperature variation24
, and thus Eq. (1) can be seen as the water collection
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efficiency ratio. But, for the icing conditions with rivulet formation, there is no linear relationship between the
variations of ice mass and temperature so that RTV is not equal to the water collection efficiency ratio. Due to the
icing conditions set up in this study, RTV is appropriate for the further comparison between various types of icing
conditions.
( )
( )
( )
max
t i
jt i
j t i
TRTV
T
(1)
where ( )
j
t i
T
is the difference between span-wise-averaged temperatures ( )
j
t i
T
and ( 0)
j
t
T
at chord-wise location
Y/C = j; ( )
max
t i
T
is the maximum value of ( )
j
t i
T
.
( ) ( ) ( 0)t i t i t
j j jT T T
(2)
( ) ( ) ( 0)
max
t i t i t
j peak j peakT T T
(3)
Figure 7. Comparison of time series of RTV among six cases at different stream-wise locations away from LE.
(a) Y/C = 0, LE; (b) Y/C = 0.1C; (c) Y/C = 0.2C; (d) Y/C = 0.3C; (e) Y/C = 0.4; (f) Y/C = 0.5.
Figure 7 shows the RTV variations at six typical locations on the airfoil model. According to the Eq. (1), (2) and
(3), ( 1) ( 1) ( 1) ( 1) ( 1) ( 1), and constantt t t t t t
j a j b j a j b j a j bRTV RTV RTV RTV RTV RTV
when a <b correspond to the growing,
decreasing and stable trends of curves in Fig. (7). Since the maxT occurs near the LE, RTVs are almost equal to
‘1.0’ at position Y/C = 0 in six cases. As the distance apart from LE increases, RTV values decrease and approach
‘0.0’ at Y/C = 0.5 where the water/ice have not completely developed. Within the time duration of 14 s, cases under
lower ambient temperature generally have larger RTV values than those cases with higher ambient temperature
because the low temperature aids to accelerate the latent heat release and convection between the ice/air or water/air
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interfaces. The RTV values in most cases are fixed after t = 3 s at Y/C = 0.1, as shown in Fig. 7 (b), except for Case
No.1 whose RTV becomes stable when t >12 s. That time equals to the time consumed for reaching the maximum
surface temperature at a certain location. As Y/C increases, duration time for reaching the stable state increases
ascribed to the evolutions of water/ice film and rivulets.
C. Quantification of the Icing Impacted Region
Based on the underlying physics of dynamic icing process and surface temperature distributions, there are
mainly three icing impacted regions, ice accretion upwards the LE, ice film and runback rivulets. Quantification of
them can provide a practical reference for wind turbine icing mitigation, especially the optimization of resistant
heaters.
Figure 8 illustrates the icing impacted region and the parameters used to quantify them. LLE is the length of ice
accretion in the windward direction ahead the LE. Lf and Lr are the lengths of ice film and ice rivulet, respectively.
Notably, the Lr is determined by Eq. (4). As mentioned in Section III A, water rivulets run back over the airfoil
surface, which means the lengths of water rivulet would increase due to the wind-driven boundary layer flow. But
for the ice rivulets, their lengths are constant. Hence, the dimensions of ice film/rivulet, instead of those of water
film/rivulet, are used for the further comparison. Three special shapes of ice rivulets and their length measurement
methods are portrayed in Fig. 8 (b). If there is a large gap within one rivulet and the downstream part is small, its
length is determined as that of upstream part. If the rivulet develops and split into more than one branches, the
lengths of all branches are counted and measured from the dividing line of film/rivulet. If only part of the rivulet
appears in the image, it is not included in the calculation.
( )
1
ni
r
i
r
L
Ln
(4)
where (Lr)i is the length of ith
rivulet and n represents the number of rivulets shown in the image.
Figure 8. Dimensional parameters of icing impacted regions. (a) Main parameters; (b) Three special types of
ice rivulets.
Figure 9 shows ice accreting thickness variation on airfoil LE within 250 s in six cases. The boundary line of ice
is detected by using the method proposed in Ref. [21]. The data are averaged in span-wise and both the mean value
and standard deviation are presented. Generally, the time series of LLE/C are linearly increasing with time, especially
when t is larger than about 80 s. After that time, ice accretion concentrates upon the LE. The nonlinear section is
caused by the water runback over airfoil surface. The final LLE/C when t = 250 s are 2.67%, 1.43%, 3.91%, 5.43%,
4.81%, 7.17% for six cases, respectively. As LWC level increases, the ice thicknesses upon LE correspondingly
increase for both cases in T = −5 °C and T = −10 °C. Since the process of ice accumulation is approximatively
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linear, the linear fitted curves are plotted. The normalized slope by using the slope in Case No.1 are 1.00, 0.54, 1.47,
1.89, 2.40, 3.01 for cases from Case No.1 to Case No.6, respectively. The slopes of the linear curves represent the
growth rate of the ice accretion on LE and inherit the similar trends with values of LLE/C, indicating that LWC aids
both increment of ice thickness and its growth rate. Besides LWC level, ambient temperature is another significant
factor. As shown in Fig. 9 (b), (c) and (d), the cases with lower ambient temperature are compared with the cases
with higher ones. Both Case No.5 and Case No.6 are in typical glaze icing conditions. The LLE/C is larger when
ambient temperature is lower mainly due to two reasons. Firstly, lower temperature leads to an inhibiting effect on
both water rivulet formation and evolution, resulting in the water/ice massing near LE. Secondly, lower temperature
accelerates heat transfers between the water layer and ambient air, which shortens the duration of water turning to
ice, so that water droplets are prone to building up at LE area. Similar phenomenon is observed in Case No.3 and
Case No.4, but differences between of these two cases are smaller. An opposite circumstance occurs in Fig. 9 (b)
where case with lower temperature owns a smaller ice increments on LE. In these two cases, Case No.1 is under
glaze icing condition while Case No.2 in rime icing condition. Due to the different types of icing, ice in Case No.2
with less bubble cells is denser than that in Case No.1 and it almost doesn’t have obvious gaps between the icicles
observed in glaze icing cases, so that the ice has smaller volume and thickness upon LE.
Figure 9. Comparison of ice accretion process on the LE among six cases. (a) Time series of span-wise-
averaged LLE/C; Span-wise-averaged LLE/C and linear fitted curves for (b) Case No.1 and Case No.2; (c) Case
No.3 and Case No.4; (d) Case No.5 and Case No.6. The error bands in light colors are ±1 standard deviation
around the mean.
Figure 10 shows the normalized lengths of ice films and ice rivulets in six cases when t = 300 s. For glaze icing
cases under the same temperature (Case No.1, Case No.3 and Case No.5), ice film length increases correspondingly
with the increment of LWC level. But no obvious pattern occurs in terms of ice film length in Case No.2, Case No.4
and Case No.6 and they are almost the same around 41% due to ice type change. Beyond the environmental factors,
the position of dividing line of ice film/rivulet might be influenced greatly by geometric shape of airfoil due to this
phenomenon.
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Four cases are compared in terms of ice rivulet dimension because Lr is zero in both Case No. 2 and Case No.4.
The number of rivulets are 10, 12, 9 and 15 for Case No.1, Case No.3, Case.5 and Case.6, respectively. When T∞ =
−5 °C, compared with Case No. 1, Lr increases 13.52% and 23.07% in Case No.3 and Case No. 5, respectively,
indicating that Lr increases as LWC level increases. The cumulative percentage of normalized length of ice film and
ice rivulet in Case No.5 is almost reaching 100% due to several completed channels from LE to TE observed in
Fig.3. When LWC levels are the same, such as in Case No.5 and Case No.6, the rivulet length is larger when
temperature increases ascribed to the longer duration of freezing. The lengths of ice films and ice rivulets are
relatively stable when the icing process completely moves into the third stage, and their lengths could be used as the
reference of heater design for wind turbine ice mitigation.
Figure 10. Comparison of both ice film length and ice rivulet length among six cases at t = 300 s.
D. Icing Process Mechanism
To have a more integral concept of the icing process, a full life-cycle icing process from both high-speed
imaging and IR thermal imaging results is extracted and portrayed in Fig. 11. The six cases can be grouped into two
types by their distinct features, i.e. with and without rivulets formation. As shown in the flow diagram, three stages
generally constitute the full life cycle of the glaze icing process. These three stages are not separated with clear
timelines, and they always overlap with their adjacent one or two stages. In the first stage, supercooled water
droplets carried by the oncoming airflow impinge onto the airfoil surface, particularly in the region near the airfoil
LE. The water droplets soon form a thin film expanding downstream along the surface. Notably, the water runback
happens in the upper/water layer of the film while the ice layer is located underneath it. As the film develops, it will
split into several isolated water rivulets stretching further downstream, which is the second stage. The lengths of the
rivulets greatly depend on the temperature, LWC level, and oncoming flow velocity, which is quantitatively
discussed in Section III C. If the lengths of the rivulets are large enough to reach the airfoil TE, water channels will
form and transport the water mass from the LE to the TE. Otherwise, they will freeze before the water mass can
shedding from the TE. After the rivulets formation, a retreating phenomenon is observed because the accumulated
small icicles on the surface impede the subsequent water droplets. The ice is building up around the LE in the
windward direction. For the rime and mixed icing process, there are only two stages without the second stage,
rivulet formation. To further distinguish the rime ice and mixed ice, we should focus on the water runback of the
water layer above the ice layer. For rime ice, there is no water runback while mixed ice has it. One method is to
observe the ice configuration, as mentioned in Section III A. Another method is based on the surface temperature
distribution, particularly the ‘plateau’ region, which is discussed in Section III B.
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Figure 11. Flow diagram for the full life cycle of the icing process over DU96-W-180 airfoil model with and
without rivulets formation.
IV. Conclusions
Icing process on a wind turbine dedicated airfoil model, DU96-W-180, have been experimentally investigated by
simultaneously using high speed imaging and IR imaging thermometry techniques. The full life cycle of icing
process can be roughly divided into three stages and they have overlaps across the timeline. The first stage is
oncoming water droplets impinging onto the airfoil surface followed by water film expansion. The second stage is
the rivulet formation, stretching and freezing. The third one highlights the ice accretion upon LE. Six cases, with
two ambient temperatures (-10 °C, -5 °C) and three LWC levels (0.3 g/m3, 1.1 g/m
3, 3.0 g/m
3), have been conducted
and compared to present the impact of ambient temperature, LWC level and icing conditions on the icing
performance on DU96-W-180 airfoil model. Among the six cases, three typical types of icing conditions are
included, one case for rime icing, one for mixed icing and the rest for glaze icing. For rime icing (Case No.2), there
are no rivulet formation and the ice layer poses a streamline configuration. For mixed icing (Case No.4), no rivulet
is observed under the conditions offered in this study, while icicles building up on the airfoil surface are transparent.
For glaze ice (Case No.1, Case No.3, Case No.5 and Case No.6), four cases with rivulet formation are analyzed and
compared.
The snapshots for dynamic icing accreting process are presented, followed by the quantitative analysis on the
impacted regions. In order to give a reference for wind turbine icing mitigation, the regions are further subdivided
into three parts with respect to the time evolution of dynamic icing process. They are ice film, ice rivulet and ice
accretion upon the LE in the windward direction. Their dimensions are calculated and provided. The 5-min ice film
length achieves about 41% for most cases, and larger LWC level and ambient temperature are inclined to enhance
the ice film length. The length of span-wise-averaged ice rivulets in glaze icing cases increases as LWC increases
and decreases as temperature decreases. Normalized ice accretion thickness on LE towards flow direction within
250 s increases linearly, especially after the first 80 s. As LWC level increases, the ice accretion thickness on LE
increases correspondingly regardless of the ice types. As ambient temperature decreases, the ice accretion thickness
upon LE increases due to the rapid solidification from water to ice for both glaze and mixed icing cases. Rime icing
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case possesses the smallest value of ice accretion thickness upon LE due to its denser structure. The growth rate of
ice accretion thickness has the same trends. Besides, surface temperature distributions are also provided, as well as
the time series of temperature variations. The time series of temperature variations in stream-wise own different
features due to ice types, which can be used for ice type identification. The ‘plateau’ region observed only in glaze
and mixed icing cases is mainly caused by the water runback phenomenon.
In the future study, the thickness of water/ice film and rivulets will be detected to provide a thorough picture of
the icing effects on wind turbine blades.
Acknowledgments
The authors want to thank Dr. Rye Waldman and Mr. Linkai Li of Iowa State University for their help in
conducting for the present study. The support of National Science Foundation (NSF) under award numbers of
CBET- 1064196 and CBET-1435590 is gratefully acknowledged.
References 1 Lehtomäki, V., “Wind Energy in Cold Climates Available Technologies - report,” 2016. 2 Lamraoui, F., Fortin, G., Perron, J., and Benoit, R., “Canadian icing envelopes near the surface and its impact
on wind energy assessment,” Cold Regions Science and Technology, vol. 120, 2015, pp. 76–88. 3 Tammelin, B., Cavaliere, M., Holttinen, H., Morgan, C., Seifert, H., and Säntti, K., “Wind energy production in
cold climate (WECO),” ETSU contractor’s report W/11/00452/REP, UK DTI, 1999, pp. 1–38. 4 Lamraoui, F., Fortin, G., Benoit, R., Perron, J., and Masson, C., “Atmospheric icing impact on wind turbine
production,” Cold Regions Science and Technology, vol. 100, 2014, pp. 36–49. 5 Yin, C., Zhang, Z., Wang, Z., and Guo, H., “Numerical simulation and experimental validation of ultrasonic de-
icing system for wind turbine blade,” Applied Acoustics, vol. 114, 2016, pp. 19–26. 6 Seifert, H., “Technical Requirements for Rotor Blades Operating in Cold Climate,” Proceedings of Boreas VI,
2003, pp. 50–55. 7 Seifert, H., Westerhellweg, A., and Kröning, J., “Risk analysis of ice throw from wind turbines,” Paper
Presented at Boreas VI, 2003, pp. 1–9. 8 Kraj, A. G., and Bibeau, E. L., “Phases of icing on wind turbine blades characterized by ice accumulation,”
Renewable Energy, vol. 35, 2010, pp. 966–972. 9 Wang, Z. J., Kwon, D. J., Lawrence DeVries, K., and Park, J. M., “Frost formation and anti-icing performance
of a hydrophobic coating on aluminum,” Experimental Thermal and Fluid Science, vol. 60, 2015, pp. 132–137. 10
Peng, C., Xing, S., Yuan, Z., Xiao, J., Wang, C., and Zeng, J., “Preparation and anti-icing of superhydrophobic
PVDF coating on a wind turbine blade,” Applied Surface Science, vol. 259, 2012, pp. 764–768. 11
Liu, Q., Yang, Y., Huang, M., Zhou, Y., Liu, Y., and Liang, X., “Durability of a lubricant-infused Electrospray
Silicon Rubber surface as an anti-icing coating,” Applied Surface Science, vol. 346, 2015, pp. 68–76. 12
Lopera-Valle, A., and McDonald, A., “Flame-sprayed coatings as de-icing elements for fiber-reinforced
polymer composite structures: Modeling and experimentation,” International Journal of Heat and Mass
Transfer, vol. 97, 2016, pp. 56–65. 13
Arianpour, F., Farzaneh, M., and Kulinich, S. A., “Hydrophobic and ice-retarding properties of doped silicone
rubber coatings,” Applied Surface Science, vol. 265, 2013, pp. 546–552. 14
Pinar Pérez, J. M., García Márquez, F. P., and Ruiz Hernández, D., “Economic viability analysis for icing
blades detection in wind turbines,” Journal of Cleaner Production, vol. 135, 2016, pp. 1150–1160. 15
Baring-Gould, I., Tallhaug, L., Ronsten, G., Horbaty, R., Cattin, R., Laakso, T., Durstewitz, M., Lacroix, A.,
Peltola, E., and Wallenius, T., Expert Group Study on Recommendations for wind energy projects in cold
climates, 2009. 16
Fakorede, O., Feger, Z., Ibrahim, H., Ilinca, A., Perron, J., and Masson, C., “Ice protection systems for wind
turbines in cold climate: characteristics, comparisons and analysis,” Renewable and Sustainable Energy
Reviews, vol. 65, 2016, pp. 662–675.
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17 Yirtici, O., Tuncer, I. H., and Ozgen, S., “Ice Accretion Prediction on Wind Turbines and Consequent Power
Losses,” Journal of Physics: Conference Series, vol. 753, 2016, p. 22022. 18
Hudecz, A., Koss, H., Hansen, M. O. L., “Ice Accretion on Wind Turbine Blades,” 2014, p. 115. 19
Li, Y., Tagawa, K., Feng, F., Li, Q., and He, Q., “A wind tunnel experimental study of icing on wind turbine
blade airfoil,” Energy Conversion and Management, vol. 85, 2014, pp. 591–595. 20
Gregorio, F., Ragni, A., Airoldi, M., and Romano, G., “PIV Investigation on Airfoil with Ice Accretions and
Resulting Performance Degradation,” Ieee, 2001, pp. 94–105. 21
Waldman, R., and Hu, H., “High-Speed Imaging to Quantify the Transient Ice Accretion Process on a NACA
0012 Airfoil,” 53rd AIAA Aerospace Sciences Meeting, 2015, p. AIAA 2015-0033. 22
Zhang, K., Wei, T., and Hu, H., “An experimental investigation on the surface water transport process over an
airfoil by using a digital image projection technique,” Experiments in Fluids, vol. 56, Aug. 2015, p. 173. 23
Pedersen, M. C., and Sørensen, H., “Towards a CFD Model for Prediction of Wind Turbine Power Losses due
to Icing in Cold Climate,” 2016, pp. 1–6. 24
Liu, Y., and Hu, H., “An Experimental Investigation on the Convective Heat Transfer Process over an Ice
Roughened Airfoil,” 54th AIAA Aerospace Sciences Meeting, vol. 2, 2016, pp. 1–31. 25
Mandell, J. F., Samborsky, D. D., and Miller, D. ., “Effects of resin and reinforcement variations on fatigue
resistance of wind turbine blades,” Advances in wind turbine blade design and materials, 2013, pp. 210–250. 26
Timmer, W. A., and van Rooij, R. P. J. O. M., “Summary of the Delft University Wind Turbine Dedicated
Airfoils,” Journal of Solar Energy Engineering, vol. 125, 2003, p. 488. 27
Agent, O., “Task 19 Work plan 2016-2018,” 2016. 28
Özgen, S., and Canıbek, M., “Ice accretion simulation on multi-element airfoils using extended Messinger
model,” Heat and Mass Transfer, vol. 45, 2009, pp. 305–322. 29
Han, Y., Palacios, J., and Schmitz, S., “Scaled ice accretion experiments on a rotating wind turbine blade,”
Journal of Wind Engineering and Industrial Aerodynamics, vol. 109, 2012, pp. 55–67.
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