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Sprays from droplets impacting a mesh

Published online by Cambridge University Press:  22 May 2019

S. A. Kooij Open the ORCID record for S. A. Kooij [Opens in a new window] ,
A. M. Moqaddam ,
T. C. de Goede ,
D. Derome ,
J. Carmeliet ,
N. Shahidzadeh  and
D. Bonn
S. A. Kooij*
Affiliation:
Van der Waals-Zeeman Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
A. M. Moqaddam
Affiliation:
Chair of Building Physics, Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland Laboratory for Multiscale Studies in Building Physics, Empa, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland
T. C. de Goede
Affiliation:
Van der Waals-Zeeman Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
D. Derome
Affiliation:
Laboratory for Multiscale Studies in Building Physics, Empa, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland
J. Carmeliet
Affiliation:
Chair of Building Physics, Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland
N. Shahidzadeh
Affiliation:
Van der Waals-Zeeman Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
D. Bonn
Affiliation:
Van der Waals-Zeeman Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
*
†Email address for correspondence: [email protected]
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Abstract

In liquid spray applications, the sprays are often created by the formation and destabilization of a liquid sheet or jet. The disadvantage of such atomization processes is that the breakup is often highly irregular, causing a broad distribution of droplet sizes. As these sizes are controlled by the ligament corrugation and size, a monodisperse spray should consist of ligaments that are both smooth and of equal size. A straightforward way of creating smooth and equally sized ligaments is by droplet impact on a mesh. In this work we show that this approach does however not produce monodisperse droplets, but instead the droplet size distribution is very broad, with a large number of small satellite drops. We demonstrate that the fragmentation is controlled by a jet instability, where initial perturbations caused by the injection process result in long-wavelength disturbances that determine the final ligament breakup. During destabilization the crests of these disturbances are connected by thin ligaments which are the leading cause of the large number of small droplets. A secondary coalescence process, due to small relative velocities between droplets, partly masks this effect by reducing the amount of small droplets. Of the many parameters in this system, we describe the effect of varying the mesh size, mesh rigidity, impact velocity and wetting properties, keeping the liquid properties the same by focusing on water droplets only. We further perform lattice Boltzmann modelling of the impact process that reproduces key features seen in the experimental data.

JFM classification

Drops and Bubbles: Aerosols/atomization Drops and Bubbles: Breakup/coalescence Interfacial Flows (free surface): Capillary flows
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 871 , 25 July 2019 , pp. 489 - 509
Copyright
© 2019 Cambridge University Press 

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Kooij et al. supplementary movie 1

Normal mesh of polyester fabric (150 µm) at an impact velocity of 2.7 m/s.

Download Kooij et al. supplementary movie 1(Video)
Video 2.3 MB

Kooij et al. supplementary movie 2

Impact of a droplet on a single row metallic mesh (300 µm) at an impact velocity of 2.7 m/s. The created ligaments can clearly be seen and are very smooth up to the moment of detachment. After detachment the ligaments destabilize showing a clear regular wave disturbance, leading to the formation of satellite drops.

Download Kooij et al. supplementary movie 2(Video)
Video 2 MB

Kooij et al. supplementary movie 3

Impact of a droplet on a single row polyester mesh (150 µm). As in Movie 2 there appears a clear wave disturbance during the destabilization of the (before) smooth detached ligaments. These disturbances determine the breakup of the ligaments and lead to the formation of an abundance of small (satellite) drops.

Download Kooij et al. supplementary movie 3(Video)
Video 1.3 MB

Kooij et al. supplementary movie 4

Simulation of the injection of water through a 300 µm pore with an exponentially slowing injection speed to compare with experiments where such a deceleration is present (see also Fig. 12). The dynamics is very similar to experiments, except that no wave disturbances are observed, which is to be expected since no vibrations are present in the simulations.

Download Kooij et al. supplementary movie 4(Video)
Video 1.3 MB

Kooij et al. supplementary movie 5

Velocity vectors of the middle plain of the computational domain of the simulation of the injection of water through a 300 µm pore (Fig. 12). One can see there is a layer of vapor moving along with the falling ligament.

Download Kooij et al. supplementary movie 5(Video)
Video 2.7 MB

Kooij et al. supplementary movie 6

Simulation of the injection of water through a 300 µm pore (Fig. 12). The color indicates the relative velocity in the z-direction (u_z) to the initial velocity U_0. This shows there is not a lot of stretching after detachment, similar as seen in experiments.

Download Kooij et al. supplementary movie 6(Video)
Video 2.5 MB