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  • 1.
    Rydblom, Staffan
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Measuring Water Droplets to Detect Atmospheric Icing2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis describes the exploration of a method to measurethe droplet size and the concentration of atmospheric liquid water. The purpose is to find a cost effective technique to detect the conditions for icing on structures.

    Icing caused by freezing atmospheric water can be a signifi- cant problem for infrastructure such as power lines, roads and air traffic. About one third of the global installed wind power capacity is located in cold climates, where icing of rotor blades is one of the major challenges.

    The icing process is complex and the result depends on a combination of the aerodynamic shape of the structure or airfoil, the velocity of the air and its contained water, the temperature, the mixing of snow and water, the concentration of liquid water and the Droplet Size Distribution (DSD).

    The measurement method is based on a shadowgraph imag- ing system using light emitting diode (LED) light as background illumination and digital image processing. A prototype instru- ment has been constructed. The components were selected keeping the possibility of low-cost volume production in mind. The applications of a commercial instrument based on this tech- nique are e.g. real-time in-situ icing condition measurements and assimilation and verification of data in numerical weather models.

    The work presented shows that measurements of the size and concentration of water droplets using shadowgraph images can be used for the comparison and validation of Numerical Weather Prediction (NWP) models and other instruments. The accuracy of the particle size measurement is high. The accuracy of the concentration measurement has the potential to become high due to the single-particle measurement range calibraiton. The precision of the instrument depends mainly on the number of images that is used to find each measurement value. The real-time performance of the instrument is limited by the image retrieval and processing speed and depends on the  precisionrequired.

  • 2.
    Rydblom, Staffan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thörnberg, Benny
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Droplet Imaging Instrument Metrology Instrument for Icing Condition Detection2016In: 2016 IEEE INTERNATIONAL CONFERENCE ON IMAGING SYSTEMS AND TECHNIQUES (IST), IEEE, 2016, p. 66-71, article id 7738200Conference paper (Refereed)
    Abstract [en]

    An instrument for measuring water droplets is described and constructed. It is designed to measure the volume concentration and the size distribution of droplets in order to detect icing conditions in a natural fog. The instrument works by shadowgraph imaging, with a collimated blue LED as background illumination. We show how to use a reference object to obtain a calibration of the droplet size and the measurement volume. These properties are derived from a measurement of the object's shadow intensity and its edge second derivative. From the size of every measured droplet and its expected detection volume, a measure of the liquid water content (LWC) and the median volume diameter (MVD) can be estimated. The instrument can be used for continuous measurement in a remote weather-exposed location and is tested in a small environment chamber. We also describe this chamber and how we can change the LWC using an ultrasonic fog generator and a fan.

  • 3.
    Rydblom, Staffan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thörnberg, Benny
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Liquid Water Content and Droplet Sizing Shadowgraph Measuring System for Wind Turbine Icing Detection2016In: IEEE Sensors Journal, ISSN 1530-437X, E-ISSN 1558-1748, Vol. 16, no 8, p. 2714-2725, article id 7384444Article in journal (Refereed)
    Abstract [en]

    This study shows that the liquid water content (LWC) and the median volume diameter (MVD) can be derived from images of water droplets using a shadowgraph imaging system with incoherent LED illumination.

    Icing on structures such as a wind turbine is the result of a combination of LWC and MVD and other parameters like temperature, humidity and wind speed. Today, LWC and MVD are not commonly measured for wind turbines. Systems for measuring these properties are often expensive or impractical in terms of location or remote reading. The aim of this study is to gain knowledge about how to design a single instrument based on imaging that has the ability to measure these properties with enough precision and accuracy to detect icing conditions for wind turbines.

    A method to calculate both the LWC and the MVD from the same images is described in this paper. The size of one droplet is determined by measuring the shadow created by the droplet in background illumination. The concentration is calculated by counting the measured droplets and estimating the volumes in which these droplets can be observed.

    In the described study, the observation volume is shown to be dependent on the particle size and the signal to noise ratio (SNR) for each measured particle. An expected coefficient of variation of the LWC depending on the droplet size is shown to be 2.4 percent for droplets 10 µm in diameter and 1.6 percent for 25 µm droplets. This is based on an error estimation of the laboratory measurements calibrated using a micrometer dot scale.

  • 4.
    Rydblom, Staffan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thörnberg, Benny
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Particle Measurement Volume and Light Intensity in a Shadowgraph Imaging System2018In: IST 2018 - IEEE International Conference on Imaging Systems and Techniques, Proceedings, IEEE, 2018, article id 8577170Conference paper (Refereed)
    Abstract [en]

    A method is sought to find the measurement volume of an optical instrument for particle measurement is sought. The study shows that the measurement volume depends on a combination of the optical homogeneity of the illumination light and the camera system. The result from a mix of illumination cases and positions shows that, if the true size and the background brightness are known, the measurement volume can be determined with an average precision of four percent using adual term exponential fit. Using a lens with lower magnification increases the measurement volume since both the field of viewand the depth of field increase. However, a larger field of view increases the required amount of light as well as the sensitivity to other optical errors.

  • 5.
    Rydblom, Staffan
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thörnberg, Benny
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Olsson, Esbjörn
    Swedish Meteorol & Hydrol Inst (SMHI), Sundsvall.
    Field Study of LWC and MVD Using the Droplet Imaging Instrument2019In: IEEE Transactions on Instrumentation and Measurement, ISSN 0018-9456, E-ISSN 1557-9662, Vol. 68, no 2, p. 614-622Article in journal (Refereed)
    Abstract [en]

    The droplet imaging instrument (DII) is a new instrument for cost-effective in situ measurements of the size and concentration of water droplets. The droplet size distribution and the concentration of atmospheric liquid water are important for the prediction of icing on structures, such as wind turbines. To improve the predictions of icing, there is a need to explore cost-effective working solutions. Through imaging, a wide range of droplet sizes can be measured. This paper describes a study of the atmospheric liquid water content and the median volume diameter using the DII and a commercial reference instrument--the cloud droplet probe 2 from Droplet Measurement Technologies Inc. The measurement is done at a weather measurement station in mid-Sweden. For a second validation, the result is compared with predictions using a numerical weather prediction model. The size measurement of the DII is verified using polymer microspheres of four known size distributions. The study shows that the DII measurement is precise, but there is a systematic difference between the two compared instruments. It also shows that droplets larger than 50 μm in diameter are occasionally measured, which we believe is important for the prediction of icing.

  • 6.
    Rydblom, Stefani Alita Leona
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Development and Test of an Imaging Instrument for Measurement of Water Droplets in Icing Conditions2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Structural icing is a persistent challenge for the production of renewable energy from wind. It is mainly caused by supercooled atmospheric droplets of water, which are very common in cold climates. In the most exposed wind parks in Sweden, more than 10 per cent of annual energy production can be lost. Some properties of liquid water are included in current \gls{nwp} models and are used as input parameters for the estimation of icing, but they are rarely measured in-situ for verification or validation.

    To address this problem, a new instrument was developed. This compilation thesis is a collection of five articles describing the development, testing and verification of this instrument. Finally, icing and ice loads are measured and compared with a standard model and a model using \gls{ai} and empirical data.

    The new instrument, called \gls{dii}, is based on shadowgraph imaging using \gls{led} light as background illumination and digital image processing. The components were selected with the possibility of low-cost volume production in mind. The applications of a commercial instrument based on this technique include, for example, real-time in-situ icing condition measurements and assimilation and verification of data in \gls{nwp} models. The instrument, alongside a reference instrument, was tested in two locations with different icing conditions. Shadowgraph imaging and its limitations as a measurement method for droplet size and concentration were investigated.

    The work presented shows that measurements of the size and concentration of water droplets using shadowgraph images can be used for the comparison and validation of \gls{nwp} models and other instruments. The \gls{cv} for a given value of the concentration is lower than \SI{1.6}{\percent} for droplets \SI{25}{\micro\meter} in diameter, based on uncertainty in the size measurement only. The accuracy of the sampling volume can be improved by measuring the background light intensity in the position of the measured droplet.

    A fog chamber was used for initial tests. However, to evaluate models of ice accumulation, in-situ measurements are necessary. These measurements should use a temporal resolution of at least one sample per minute, preferably higher. With a limited amount of data, multivariate data analysis can be used to estimate the level of ice accretion. Together with a heuristic model of erosion/ablation, the resulting figures can be used to simulate the ice load.

    All of the instruments, as well as many other components used during the described field measurements, did at some point break due to the difficult weather conditions. An instrument for measurement of icing conditions needs to be designed with high environmental protection and endurance. The results in the attached papers may help and motivate further technical development of instruments that can measure atmospheric liquid water in icing conditions.

    The full text will be freely available from 2020-07-28 12:05
  • 7.
    Rydblom, Stefani Alita Leona
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thörnberg, Benny
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Measurement of Atmospheric Icing and DropletsManuscript (preprint) (Other academic)
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