Soutenance mémoire
Atmospheric icing
Atmospheric icing is a complex phenomenon that results either from precipitation icing such as freezing rain and wet snow accretion or from in-cloud icing . The occurrence, severity, and type of atmospheric icing depend largely on temperature, wind speed, LWC, and water droplet size. It may take place at ambient air temperatures between -10°C and 0°C, or sometimes, at lower temperatures under particular conditions. In-cloud icing occurs when suspended, supercooled droplets freeze immediately upon impact on an object exposed to the airflow. The occurrence and severity of this type of atmospheric icing strongly depend on the location of the exposed object and on the topography of the surroundings.
Precipitation icing can take the form of freezing precipitation and frozen precipitation. Freezing precipitation occurs when any form of precipitation (freezing rain, freezing drizzle, or freezing fog) freezes upon impact on or contact with an exposed object. Frozen precipitation is any form of precipitation that reaches the ground in frozen form such as snow, snow pellets, snow grains, ice crystals, ice pellets, and hail.
Ice shedding
Ice shedding is the physical phenomenon which is classified as a type of ice mass reduction. The ice mass reduction can be caused by three physical mechanisms: ice melting, ice sublimation and mechanical ice breaking. Ice shedding, or the sudden dropping off of atmospheric ice, is created by mechanical breaking or its combination with the two other mechanisms. In other words, it is responsible for large imbalance forces and moments on supports, impact forces, overloads, vibrations, and in the case of electrical conductors, short-circuits causing flashover when electrical clearances become insufficient. In extreme situations, the increased cable tensions may result in broken insulator assemblies, or short circuits may cause cable breakages that may in turn lead to longitudinal cascading failure of the line .
Damage can occur to structural elements (antennas, etc.) when ice from higher parts fall and hit lower elements in the structure. The height of falling ice is an important factor when evaluating risks of damage, because a greater height means greater dynamic forces from the ice. A method of avoiding or reducing damage from falling ice is the use of shielding structures .
Interaction between ice effects and wind load
The bodies are classified as aerodynamic and bluff bodies with respect to the features of the flow field that they produce when they are immersed in a cross-stream or through fluid . Aerodynamic bodies are characterized by boundary layers completely attached over their whole surface, which leave behind them thin and generally steady wakes containing vortices . The aerodynamic forces acting on these bodies may be evaluated through the simplified potential flow – boundary layer procedure .
Conversely, bluff bodies are characterized by a separation of boundary layer from its surface, and wakes having significant lateral dimensions and normally unsteady velocity fields . For these bodies no simplified mathematical treatment is usually possible, and the forces acting on them may be evaluated either from the solution of the complete Navier-Stokes equations or from the results of experiments . It is well known that structures such as electrical towers are bluff bodies.
The effect of DSD and LWC on aerodynamic coefficients
Meteorological parameters such as LWC and DSD are decisive factors in determining the various types of atmospheric ice accretion and consequently their aerodynamic coefficients. One of the important differences between natural aerosol clouds and its experimental models is their dimensions in relation to a size of the icing structures that are exposed to those clouds. The dimensions of the supercooled aerosol clouds are greater in all directions under natural icing conditions whereas the same order of magnitude is observed for the same factors when simulating ice accretion in wind tunnel.
Therefore, knowing the zone of the uniformity of the artificial aerosol clouds is essential in order to predict the local LWC and DSD. Although the dimensions of zone of uniformity of natural aerosol clouds are not achievable in experimental modeling, it is possible to obtain LWC and DSD having approximately the same level as what is found in natural aerosol clouds. Different combinations of these characteristics produce different types of natural icing phenomena. The LWC is in the range of a few tenth of g/m3 under in-cloud icing conditions; however, values between 1 and 10 g/m3 were also observed under other ambient conditions such as freezing drizzle or freezing rain . The DSD falls in the range of a few μm to about 50 μm under in-cloud icing conditions, whereas it takes significantly greater values under freezing drizzle (in the range of 100 μm) and freezing rain (from the range of 100 μm to several mm) conditions .
CIGELE Atmospheric Icing Research Wind Tunnel (CAIRWT)
The complexities inherent to the study of atmospheric icing phenomena occurring in the atmospheric boundary layer and involving ice accretion on structures make it a difficult task to solve. The absence of a final unified theory for understanding the mechanisms, which trigger and halt the various types of ice accretion, as well as the impossibility of following its complete development in nature make laboratory investigations of this phenomenon more productive than field observations. Such experimental modeling of atmospheric icing in a wind tunnel or in a climatic room is an integral part of examining the phenomenon that includes field measurements, theoretical and experimental modeling.
Experimental modeling of these hazardous weather conditions involves maintaining the aggregate of meteorological parameters forming these conditions within as wide a range as possible in order to be as representative of all the types of atmospheric icing possible. These parameters under consideration comprise air speed, air temperature, air humidity, air pressure, LWC and DSD of the aerosol cloud.
Streamwise and Vertical Variations of Median Volume Diameter (MVD) and LWC of the droplets
The effects of air velocity and nozzle pressures on DSD in the produced spray and on the LWC in the cloud in the middle of the test section of a low-speed horizontal wind tunnel were studied by Kollar and Farzaneh and by Kollar et al. , respectively. The streamwise and vertical variations of these characteristics in the test section were discussed in Kollar and Farzaneh for two specific icing conditions. The following discussion focuses on the streamwise and vertical variations of MVD and LWC for different air velocities and nozzle pressures.
The experiments were carried out in 4 streamwise and 3 vertical positions, and for air velocities. Furthermore, 6 different nozzle air pressures were applied between 180 and 620kPa, when the nozzle water pressure was kept constant. In order to reduce the number of figures, variations along the streamwise directions are presented in one vertical position only, and variations along the vertical direction are shown in one streamwise position only.
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Table des matières
CHAPTER 1 :INTRODUCTION
1-1-Problem definition
1-2-Research Objectives
1-3-Originality and contributions to knowledge
1-4-Methodology
1-4-1-Wind tunnel calibration
1-4-2-Natural Icing Event Measurements
1-4-3- Natural Icing Simulation
1-4-4-Aerodynamic measurements
1-4-5-An introduction to the use of the Particle Image Velocimetry (PIV) method
1-5-Thesis organization
CHAPTER 2 :LITERATURE REVIEW
Introduction
2-1- Atmospheric icing
2-1-1- Types of ice accretion
2-1-2- Accretions on transmission line towers
2-1-3- Ice morphology
2-2- Ice-related loads on transmission line towers
2-2-1- Ice shedding
2-2-2- Galloping
2-2-3- Aeolian vibration
2-2-4- Aerodynamic forces
2-3- Interaction between ice effects and wind load
2-3-1- Aerodynamic forces
2-3-2- Flow field measurements
2-3-3- Blockage effects
2-4- The effect of DSD and LWC on aerodynamic coefficients
Conclusion
CHAPTER 3 :EXPERIMENTAL FACILITIES, TEST MODELS AND TEST PROCEDURES
Introduction
3-1- Experimental Facilities
3-1-1- CIGELE Atmospheric Icing Research Wind Tunnel (CAIRWT)
3-1-1-1- Construction of CAIRWT
3-1-1-2-General Layout of CAIRWT
3-1-1-3-CAIRWT’s Main Systems
3-1-1-3-1-Fan system
3-1-1-3-2-Refrigeration system
3-1-1-3-3-Nozzle spray-bar system
3-1-1-4-Control Panel
3-1-1-4-1-Water Transfer System
3-1-1-4-2-Air Transfer System
3-1-1-4-3-Nozzle Heating System
3-1-1-4-4-Air Heating System
3-1-1-5-Physical parameters of the aerosol cloud produced in the wind tunnel
3-1-1-5-1-Temperature
3-1-1-5-2-Velocity
3-1-1-5-3-Relative and Absolute Humidity
3-1-1-5-4-Liquid Water Content (LWC)
3-1-1-5-5-Droplet Size Distribution (DSD)
3-1-1-6-CAIRWT related equipment
3-1-1-6-1-Integrated System for Icing Studies by Droplets Measurement Technologies
3-1-1-6-2-Particle Image Velocimetry (PIV)
3-1-2- Low speed aerodynamic tunnel
3-1-2-1- General layout
3-1-2-2- Tunnel parameters
3-1-2-3- Tunnel related equipment (External balance)
3-2- Test models
3-2-1-Wind effects on ice accretion
3-2-2- DSD and LWC effects on ice accretion and drag coefficient
3-2-3- Aerodynamic models
3-3- Test procedures and selecting experimental conditions
3-3-1- Wind effects on ice accretions
3-3-2- DSD and LWC effects on ice accretion and drag coefficient
3-3-3- Aerodynamic measurements
Conclusion
CHAPTER 4 :ICE SIMULATION AND WIND EFFECTS ON AN ANGLE MEMBER ICING
Introduction
4-1-Ice simulation
4-1-1-Glaze ice
4-1-2- Rime ice
4-2-Wind effects on horizontal angle member icing in the test section
4-2-1-Sign convention and definition of the variables
4-2-2-Angle of attack
4-2-3-Side slip angle (Yaw angle)
4-2-4-Rolling angle
4-2-5-Estimation of Drag Coefficients of Ice-Covered Angle Bars for different angles of attack
4-3-Wind effects on vertical angle member icing in the test section
4-3-1- Sign convention and definition of the variables
4-3-2- Angle of attack
4-3-3-Sideslip angle (Yaw angle)
4-3-4-Rolling angle
Conclusion
CHAPTER 5 :THE EFFECT OF DSD AND LWC ON ICE ACCRETION AND DRAG COEFFICIENTS
Introduction
5-1- Sign convention and definition of the variables
5-2-DSD and LWC measurements
5-2-1-Streamwise and Vertical Variations of Median Volume Diameter (MVD) and LWC of the droplets
5-3-Ice Accretion Measurements on Tower-Leg Model
5-3-1-Streamwise and Vertical Variations of Ice Accretion on Angle Bar
5-3-2-Calculation of Drag Coefficients of Ice-Covered Angle Bars
Conclusion
CHAPTER 6 :INTERACTION OF WIND WITH A CEMENT ICE PROFILE ON AN ANGLE MEMBER
Introduction
6-1-Sign convention and definition of the variables
6-2-Reynolds number effects
6-3-Ice thickness effects
6-4-Considering different ice profiles
6-5-Different angle member profiles
6-6-Droplet effects
6-7-Effects of projected area
6-8-Drag from standard
6-9-Aerodynamic Forces
6-9-1- Drag Force per unit length
6-9-2- Lift Force per unit length
Conclusion
CHAPTER 7 :AN INTRODUCTION OF USING PIV TECHNIQUE TO STUDY THE EFFECTS OF
ICE SHAPE ON THE VORTEX SHEDDING OF A CYLINDER
Introduction
7-1-Test models
7-2-Test procedure
7-3-Experimental Results
Conclusion
CHAPTER 8:CONCLUSIONS AND RECOMMENDATIONS
8-1-Conclusions
8-2-Recommendations for future study
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