Below are some impressions of icing experiments from different research projects with supercooled droplets or ice crystals.
Glaciated and slushy ice layers generated at the icing wind tunnel of TU Braunschweig are characterized as part of this experimental campaign in joint collaboration between TU Braunschweig and TU Darmstadt. The goal is to utilize three different measurement techniques namely Calorimetry, Stereoscopy and Capacitive Sensor. Volumteric liquid water fraction in the ice layers (averaged) is quantified by means of Calorimetry combined with volume of the ice layer obtained through Stereoscopy. The results are compared with measurements obtained from the capacitive sensor technique and analyzed afterwards. Clear trends are obtained for different influential parameters that play a critical role in driving accretion as well as shedding. The large database obtained will be used for more accurate modeling of sticking efficiency as well as modeling of shedding phenomenon.
To characterize the onset of shedding phenomenon, dedicated experiments were performed in the icing wind tunnel of TU Braunschweig. Key influential parameters were varied to study and model the trends of shedding frequency, the size of shed blocks and the initiation time of shedding cycles. Shedding as a result of slushy consistence of ice layer under aerodynamic loads and natural melting of ice layer was investigated in detail. Furthermore, shedding as a result of melting from the interface i.e. between the heated substrate and ice layer was comprehensively investigated. Both wet bulb temperature as well as the supplied heat flux were found to be the main driving parameters for onset of shedding phenomenon. Some impressions from the measurement campaign are shared below.
The determination of the liquid water content of the drop cloud with a rotating cylinder is based on the measurement of the ice accretion under known boundary conditions, such as the icing duration, the temperature and the droplet size.
The measurement technology was developed and applied together with a student for the icing wind tunnel of Braunschweig within the scope of the calibration of the spray system.
For more information on the measurement principle of the rotating cylinder, see J. R. Stallabrass, "An Appraisal of the Single Rotating Cylinder Method of Liquid Water Content Measurement", National Research Council of Canada Low Temperature Laboratory, Report LTR-LT-92, November, 1978. Results of the measurement campaign in the icing wind tunnel of Braunschweig can be found here:https://doi.org/10.5194/amt-14-1761-2021
The most intuitive way of measuring the size of the droplet cloud is measuring the size of the shadow they cast when illuminated by a collimated light source. The setup essentially consists of a high intensity collimated light source (usually a laser, with a fluorescent diffuser) to illuminate the droplets. The droplets act as opaque objects casting a shadow on the opposite side of the illuminating source, which appear as dark spots in the high intensity uniform background. A high resolution camera with a combination of long focal length objectives coupled with telephoto lenses is placed on the opposite side of the light source to capture the afore mentioned shadows produced by the droplets. The droplet velocity can be obtained by tracking the translation of particles between two frames captured with a short delay between the frames, to precisely control the timing of the cameras and light sources a programmable timing unit is used. The size and the effective depth of the shadow is proportional to the droplet size, therefore a precise calibration is needed for shadow graphic droplet sizing. After a through a calibration of size and depth of field, cloud characteristics like size distribution, MVD and droplet flux can be obtained from processing the shadowgrahic images. The ISM is equipped with a powerful processing tool Davis to calibrate, acquire and process shadowgraphs, further details can be found in Knop et al . (2021). Shadowgraphy has been used to calibrate the two modes of the spray nozzles in the BIWT, a parametric model for the size distribution of the newly updated large droplet is obtained, with corrections a good agreement is acheived with widely accepted sizing method PDI.
Particle Image Velocimetry is one of the most robust flow visualization techniques used to retrieve instantaneous velocity field of the flow. The velocity field (in 2D or 3D, stereoscopic) can be obtained by tracking the secondary particles (the seeding particles) embedded in the primary flow. The seeding particles are sufficiently small to follow stokes law at every point in the flow, therefore, the instantaneous velocity of the particle is same as the flow. The high intensity light produced by a laser is converted to a thin sheet of light with a cylindrical lens. The thin sheet illuminates the seeding particles in the interrogation region, the particles scatter the light directly proportional to square their of the diameter. The scattered light is then recorded on camera (usually double frame, with a few micro seconds delay between the frame). The image on each of the frames is partitioned in to multiple interrogation windows and a correlation (cross correlation to estimate similarity in particles) is used to find the translation (displacements) of the particles between the frames. With the displacement vector computed and with the known time lag between the frames the velocity can be computed. Unlike in other applications, no additional seeding particles are needed in an IWT as the water droplets in the IWT cloud serve the purpose of the seeding particles. PIV measurement acquisition and processing at ISM are facilitated through Davis software.
Phase Doppler Interferometry (PDI) from Artium Technologies, Inc. is a droplet measurement technique based on the detection of the characteristic scattered light signal from a spherical particle as it passes through an equidistant interference fringe pattern generated by two intersecting coherent laser beams.
This measurement technique was used as part of the calibration of the spray system in a joint measurement campaign with LaVision GmbH, Göttingen.
For more information on the PDI, please visit the manufacturer Artium Technologies Inc.
Here are the results of the experiments conducted at the Braunschweig Icing Wind Tunnel: https://doi.org/10.5194/amt-14-1761-2021
The Fast Cloud Droplet Probe (FCDP) from SPEC Inc. is a droplet measurement technique based on measuring the intensity of light, scattered by tiny droplets.
The probe was used as part of the calibration of the spray system in a joint measurement with the Institute of Atmospheric Physics of the German Aerospace Center.
For more information on the FCDP, please visit the manufacturer SPEC Inc.
Here are the results of the experiments conducted at the Braunschweig Icing Wind Tunnel: https://doi.org/10.5194/amt-14-1761-2021
The Forward Scattering Spectrometer Probe (FSSP) from SPEC Inc. is a drobplet measurement technique based on measuring the intensity of scattered light by tiny droplets.
The probe was used as part of the calibration of the spray system in a joint measurement campaign with the Institute of Atmospheric Physics of the German Aerospace Center.
For more detailed information about FSSP, please visit the manufacturer's website.
Die Cloud Combination Probe (CCP) ist eine Kombination aus einer Cloud Droplet Probe (CDP) und einer Cloud Imaging Probe (CIP). Die PSD-Bereiche der Sonden betragen 2-50 μm (30 Bins) und 12,5 bis 1550 μm (62 Bins). Damit ergibt sich ein Messbereich von 2-1550 μm, der den CCP ideal für SLD-Messungen macht. Das Funktionsprinzip des CIP basiert auf der Erkennung des Schattens, den ein Tropfen beim Durchgang eines Laserstrahl erzeugt. Das CDP misst die PSD auf der Grundlage der Vorwärtsstreuung, die durch den Tropfen beim Durchgang des Laserstrahl erzeugt wird.
Droplet temperature measurement: Every time light interacts with droplets, it undergoes both reflection and refractions. The light will also undergo internal reflections inside the droplet. For internal reflection, a minimum of deviation exists, which concentrates the light at particular scattering angle called the rainbow angle which is dependent on the size and the refractive index of the droplet then is correlated to droplet temperature. Standard Rainbow Refractometry (SRR) and Global Rainbow Technique (GRT) exploit this principle and simultaneously measure the droplet diameter and the refractive index thus the droplet temperature.