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Modelling of Atmospheric Icing

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Modelling of Atmospheric Icing Modelling of Atmospheric Icing An Introduction essay Petra Thorsson Contents Contents .............................................................................................................................. 2 1 Introduction ............................................................................................................................. 4 2 Problems caused by Icing ........................................................................................................ 5 2.1 Loss of aerodynamic balance and loss of Production ...................................................... 5 2.2 Risks to human life and activities .................................................................................... 5 3 Types of Icing .......................................................................................................................... 7 3.1 General Reasons for Icing ................................................................................................ 7 3.1.1 In-cloud icing ............................................................................................................ 7 3.1.2 Precipitation icing ..................................................................................................... 7 3.2 Types of ice ...................................................................................................................... 7 3.2.1 Glaze .......................................................................................................................... 7 3.2.2 Wet Snow .................................................................................................................. 8 3.2.3 Rime .......................................................................................................................... 8 3.2.4 Other types of ice ...................................................................................................... 8 3.3 Ice Classes ........................................................................................................................ 9 3.4 Height and seasonal variance in icing ............................................................................ 10 3.4.1 Clouds involved in icing ......................................................................................... 13 4 To Measure Ice ...................................................................................................................... 14 4.1 Detection of Ice and Icing .............................................................................................. 14 4.2 ISO 12494 specification (Atmospheric icing of structures) ........................................... 14 4.2.1 Description of standard instrument and output ...................................................... 14 4.3 Ice measuring instruments .............................................................................................. 15 4.3.1 An assessment of different icing instruments ......................................................... 15 4.3.1.1 Methods found unsuitable ................................................................................ 15 4.3.1.2 Methods found suitable .................................................................................... 15 4.3.2 IceMonitor ............................................................................................................... 16 4.3.3 HoloOptics .............................................................................................................. 16 5 Icing in detail ......................................................................................................................... 18 5.1 The Physics Behind Icing ............................................................................................... 18 5.1.1 Influential studies in the 1940' ................................................................................ 18 5.1.2 Droplet size distribution .......................................................................................... 19 5.1.2.1 Median volume diameter .................................................................................. 19 5.1.2.2 Other droplet size distributions ........................................................................ 20 5.1.2.3 Comparison between different droplet size distributions ................................. 20 5.1.3 The liquid water content .......................................................................................... 21 5.1.4 The Icing Growth Rate ............................................................................................ 21 5.1.4.1 The collision efficiency, α1 .............................................................................. 22 5.1.4.2 The sticking efficiency, α2 ................................................................................ 24 5.1.5 Growth regimes ....................................................................................................... 24 5.1.5.1 Dry Growth Regime ......................................................................................... 24 5.1.5.2 Wet Growth Regime ......................................................................................... 25 5.1.6 The heat balance of the icing process ...................................................................... 25 5.1.7 Density and structure of ice deposits ....................................................................... 26 6 Modelling Icing ..................................................................................................................... 28 6.1 Simplifications made to icing models ............................................................................ 28 6.2 Modelling of rime icing in Bulgaria ............................................................................... 28 6.3 Makkonen's in-cloud icing model .................................................................................. 30 6.4 Modelling icing with WRF ............................................................................................ 35 6.5 The axial-growth model by Lozowski et al. (1983a) ..................................................... 39 6.6 The TURBICE model ..................................................................................................... 45 7 Discussion about icing .......................................................................................................... 51 7.1 Methods used in modelling icing ................................................................................... 51 7.2 Problems concerning modelling of icing ....................................................................... 51 8 Planed future work ................................................................................................................ 53 9 References ............................................................................................................................. 54 10 List of pictures ..................................................................................................................... 57 1 Introduction Atmospheric icing is a phenomenon that have an impact on many aspects of today's world. Early influential work on icing was conducted in the 1940' by Langmuir and Blodgett (1946) who studied the trajectories of small water droplets. The work was aimed at, among other things, understanding icing on airplanes. Since then the need for research on icing have grown. Understanding the icing of structures is also an essential part of icing, as icing on structures can lead to difficulties in operating them and can be a threat to human life (Makkonen, 1981). The main part of icing research was aimed at aircraft icing, structural icing and icing on power grids until the early 1990'. Since then icing on wind turbines have been a vital part of icing research (Makkonen et al., 2001). There are many incentives to study icing on wind turbines, one reason for this is that many sites with good wind turbine potential have a high risk of icing. An example of this is the current Swedish government have a goal of 2000 new wind turbines till 2020 (Olofsson, 2010), some of these new turbines will be placed in locations where the icing risk is vast. 2 Problems caused by Icing There are many problems associated with icing. In the case of icing on wind turbines there are three main problems; the risk of harming people nearby when ice falls from the turbines, the loss of production and the reduced lifetime of the components (Ronsten, 2008). 2.1 Loss of aerodynamic balance and loss of Production When icing occurs on a wind turbine the aerodynamics of the blades are changed in such a degree that there is a loss of production. In a study by Antikainen and Peuranen (cited in Dalili et al., 2009) it was shown that imbalance is caused early when the icing is starting. A study done by Jasinski et al. 1997 showed that a small increase in surface roughness can cause an increase in drag coefficient and can thus reduce the power produced by the wind turbine, a small ice accretion at wind speeds close to peak power can lead to a severe loss in production. Even light icing events can cause the surface roughness of the blades to increase in such a degree that there is a loss of aero dynamical efficiency. When the icing is more severe it can cause the torque to become zero and consequently stop the wind turbine all together. When the ice cover is uneven it can cause vibrations, which can lead to a collapse of the turbine. These
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