High Speed Visualization of Droplets Impacting with a Dry Surface at High Weber Numbers

The focus of this article is to describe the evolution of the spreading diameter and secondary droplets generated by splashing. High-speed visualization was used to study the time evolution of water droplets impacts with dry surfaces at Weber numbers between 3,500 and 10,000. Different prediction models of the maximal spreading diameter have been compared with each other and with the experimental data. A similarity between the spreading rates was observed in the last stage of the impact at highWeber numbers. The time evolution of the secondary droplets and the formation of the crown was observed and analyzed at the different Weber numbers

Comparison of different droplet measurement techniques in the Braunschweig Icing Wind Tunnel

The generation, transport and characterization of supercooled droplets in multiphase wind tunnel test facilities is of great importance for conducting icing experiments and to better understand cloud microphysical processes such as coalescence, ice nucleation, accretion and riming. To this end, a spray system has been developed, tested and calibrated in the Braunschweig Icing Wind Tunnel. Liquid droplets in the size range of 1 to 150 µm produced by pneumatic atomizers were accelerated to velocities between 10 and 40 m s−1 and supercooled to temperatures between 0 and −20 ∘C. Thereby, liquid water contents between 0.07 and 2.5 g m−3 were obtained in the test section. The wind tunnel conditions were stable and reproducible within 3 % standard variation for median volumetric diameter (MVD) and 7 % standard deviation for liquid water content (LWC). Different instruments were integrated in the icing wind tunnel measuring the particle size distribution (PSD), MVD and LWC. Phase Doppler interferometry (PDI), laser spectroscopy with a fast cloud droplet probe (FCDP) and shadowgraphy were systematically compared for present wind tunnel conditions. MVDs measured with the three instruments agreed within 15 % in the range between 8 and 35 µm and showed high coefficients of determination (R2) of 0.985 for FCDP and 0.799 for shadowgraphy with respect to PDI data. Between 35 and 56 µm MVD, the shadowgraphy data exhibit a low bias with respect to PDI. The instruments' trends and biases for selected droplet conditions are discussed. LWCs determined from mass flow calculations in the range of 0.07–1.5 g m−3 are compared to measurements of the bulk phase rotating cylinder technique (RCT) and the above-mentioned single-particle instruments. For RCT, agreement with the mass flow calculations of approximately 20 % in LWC was achieved. For PDI 84 % of measurement points with LWC<0.5 g m−3 agree with mass flow calculations within a range of ±0.1 g m−3. Using the different techniques, a comprehensive wind tunnel calibration for supercooled droplets was achieved, which is a prerequisite for providing well-characterized liquid cloud conditions for icing tests for aerospace, wind turbines and power networks

The Effect of Impact Angle on the Secondary Droplets at High Impact Velocity

This study focuses on the secondary droplets ejected during splashing at different impact angles. We consider the theory of Riboux & Gordillo [1], which attributes the generation of secondary droplets to a lift force that acts on the spreading lamella, and propose a new approach to handle the oblique impact. This approach is based on previous studies on the lamella formed by impinging jets, where the impacting flow is distributed in the azimuthal direction. To validate the proposed method, we used a flywheel experiment and captured the secondary droplets that are ejected at Weber number larger than 4500 at three different impact angles. In our experimental setup, the droplets were formed by a droplet generator and then let to fall freely due to gravity until impacting the substrate, which was mounted on a flywheel. The small and fast secondary droplets were captured using a shadowgraph technique together with a high-resolution camera and Nd:YAG laser with diffuser optics. The experimental results showed an acceptable agreement with the prediction made by our method in all studied cases. We demonstrate that the shape and droplet size distribution are affected by the impact angle, while the velocity of the ejected droplets remains constant in the azimuthal direction

Role of surrounding gas in the outcome of droplet splashing

This study investigates the influence of the surrounding gas on a droplet impacting a smooth dry glass surface at high Weber and Reynolds numbers. It was performed using a flywheel experiment and different gases at ambient pressure. We analyzed the splashing outcome by measuring the size, velocity, and angle of the secondary droplets and by calculating the total volume ejected. We show that gas entrapment is not the mechanism responsible for splashing at high Weber and Reynolds numbers. We demonstrate that splashing is influenced by the density, followed by the viscosity, and last by the mean free path of the surrounding gas. Furthermore, the surrounding gas primarily affects the number of secondary droplets ejected and their ejection angle, whereas the droplet size and horizontal velocity are independent of the surrounding gas properties. We provide the first theoretical expression for the total volume ejected using the theory of Riboux and Gordillo [Phys. Rev. Lett. 113, 024507 (2014)], which attributes the secondary droplet generation to a lift force experienced by spreading lamella. The relationship between the ejected volume and the splashing parameter is described by a power function

Droplet splashing on thin moving films at high Weber numbers

The influence of a thin moving film on the splashing of droplets was investigated experimentally at high Weber numbers. This study was conducted using a flywheel experiment fitted with a new film gener- ation system, which allows for the production of thin films with variable mean velocity for different liquids. The thickness was measured using a miniature confocal-chromatic sensor during the rotation of the flywheel. Using shadowgraph techniques, the splashing process was analyzed and the evolution of the crown height and diameter were described. It was also demonstrated that the film velocity and thickness influence the development of the crown geometry. The combination of a high-speed and a high-resolution camera allowed us to observe two different instabilities that accelerate the breakup pro- cess, leading to a complete atomization of the crown into secondary droplets. The instabilities observed were: spreading holes and a separation from the crown base. Using the formed holes, we calculated the lamella thickness using two different methods, yielding a constant value of 31 ±3 μm for all the exper- iments. We estimated both the time at which the hole instabilities appeared and the time at which the breakup process began. Moreover, it was demonstrated that small bubbles in the lamella are responsible for the hole formation. We also showed that the entire breakup process is delayed by increasing the film flow velocity, regardless of the Weber number

Detailed atmospheric ice accretion surface measurement using micro-computed tomography

Surfaces exposed to atmospheric cold temperature and humid environments are prone to ice accretion. Airplanes, electrical power transmission cables, and wind turbines are typical examples for which icing has to be considered. The measurement of the resulting ice shapes is a challenging process. While macroscopic characteristics of the ice geometry can be observed using photography and optical scanning techniques, microscopic measurements are difficult to conduct because grooved surface partially occludes the geometry of chasms. To overcome this optical inaccessibility, we propose a method to carry out detailed high-resolution measurements of the accretion surface with micro-computed tomography. This approach provides a unique visualization of the empty spaces in the feather region. The information obtained by this technique can improve the understanding of ice accretion physics and its computational modeling

Porosity formation during atmospheric ice accretion: measurements using micro-computed tomography

Atmospheric ice accretion results from the exposure of technical equipment or facilities to cold and humid environments. Supercooled droplets in a cloud can impact an airplane&#39;s surface and quickly form an ice layer. The presence of air pockets in such a layer is well known and explains the white appearance of some of the accretions. However, estimation of its porosity values and studies on the pore formation mechanics remain limited. In this study, we performed tests in an icing wind tunnel and scans with micro-computed tomography to address these issues. Here, we show that the accretion has closed porosity below 1%, which is mostly produced by the interaction between a spray-like impact on the water surface. The insights we provide here are important to improve ice accretion modelling techniques and establish a different approach to address the interaction between the cloud and the surfaces exposed to atmospheric icing

Comparison of different droplet measurement techniques in the Braunschweig Icing Wind Tunnel

The generation, transport and characterisation of supercooled droplets in multiphase wind tunnel-test facilities is of great importance for conducting icing experiments and to better understand cloud microphysical processes such as coalescence, ice nucleation, accretion and riming. To this end, a spray system has been developed, tested and calibrated in the Braunschweig Icing Wind Tunnel. Liquid droplets in the size range of 1 to 150 µm produced by pneumatic atomizers were accelerated to velocities between 10 and 40 m s-1 and supercooled to temperatures between 0 and -20 °C. Thereby, liquid water contents between 0.07 and 2.5 g m-³ were obtained in the test section. Using the different techniques, a comprehensive wind tunnel calibration for supercooled droplets was achieved, which is a prerequisite to provide well characterized liquid cloud conditions for icing tests for aerospace, wind turbines and power networks

High Velocity Impingement of Single Droplets on a Dry Smooth Surface

The vertical impact of single, mono disperse water droplets on a dry smooth surface was studied experimentally by means of shadowgraphy. A glass substrate was mounted on a rotating wheel to obtain high impact velocities. The droplets were generated on demand. While the Ohnesorge number was kept constant, Weber number and Reynolds number were varied by adjusting the impact velocity. In all performed experiments, splashing was observed. The distinction of the different measurement series was done by the use of the Weber number. The different Weber numbers were, 3,500, 5,000 and 10,000. Phase-locked images were taken and the temporal evolution of the impact was reconstructed by means of the nondimensional impingement time. The outcome of the measurement was analysed by digital image processing to quantify the distribution of the diameter of the resulting secondary droplets in size and time as well as their velocity, and the total deposited mass fraction remaining on the surface after the impingement. In all cases, the greater part of the impinging primary droplet remained on the substrate