Measuring droplet size distributions from overlapping interferometric particle images

Evans, Humberto Bocanegra; Dam, Nico; Voort, Dennis van der; Bertens, Guus; Water, Willem van de

doi:10.1063/1.4909537

Cited by 25 publications

(6 citation statements)

References 15 publications

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“…We now have a closer look at the inside of the spray, using for this the interferometric particle imaging (IPI) technique (see the reviews in Refs. [30,31]). The droplets are indeed too small to be imaged directly but, precisely because they are small, the IPI technique uses their light refraction ability to measure their size.…”

Section: Inadequacy Of the D 2 Lawmentioning

confidence: 99%

Dense spray evaporation as a mixing process

Rivas

Villermaux

2016

Phys. Rev. Fluids

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We explore the processes by which a dense set of small liquid droplets (a spray) evaporates in a dry, stirred gas phase. A dense spray of micron-sized liquid (water or ethanol) droplets is formed in air by a pneumatic atomizer in a closed chamber. The spray is conveyed in ambient air as a plume whose extension depends on the relative humidity of the diluting medium. Standard shear instabilities develop at the plume edge, forming the stretched lamellar structures familiar with passive scalars. Unlike passive scalars however, these lamellae vanish in a finite time, because individual droplets evaporate at their border in contact with the dry environment. Experiments demonstrate that the lifetime of an individual droplet embedded in a lamellae is much larger than expected from the usual d 2 law describing the fate of a single drop evaporating in a quiescent environment. By analogy with the way mixing times are understood from the convection-diffusion equation for passive scalars, we show that the lifetime of a spray lamellae stretched at a constant rate γ is t v = 1 γ ln(1+φ φ), where φ is a parameter that incorporates the thermodynamic and diffusional properties of the vapor in the diluting phase. The case of time-dependent stretching rates is examined too. A dense spray behaves almost as a (nonconserved) passive scalar.

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Section: Inadequacy Of the D 2 Lawmentioning

confidence: 99%

Dense spray evaporation as a mixing process

Rivas

Villermaux

2016

Phys. Rev. Fluids

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“…The diameter of small droplets (d p 5 μm) cannot be measured because these droplets produce less than one interference fringe in our setup. A robust algorithm to measure the droplet size probability density function in case of dense droplet collections is explained elsewhere [35]. A typical probability density function P d p of measured particle diameters is shown in Fig.…”

Section: Dropletsmentioning

confidence: 99%

Making droplets glow in turbulence

et al. 2020

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We present a new technique to study preferential concentration of droplets in a turbulent air flow. Preferential concentration is the tendency of droplets to cluster in regions of strain, while avoiding regions of rotation. We study the properties of the droplet concentration field in zero mean flow turbulence that was created using an array of synthetic jets. The droplets are made of a phosphorescent solution of Europium chelate. They are excited by a laser sheet from a pulsed UV laser, after which the glowing droplets are followed using a high-speed intensified camera. We quantify preferential concentration through measurement of moments of the coarse-grained local droplet density. At the Stokes numbers studied (St ≈ 2) the fractal dimension, a scaling property of this coarse-grained density field, points to clustering. Clustering is a consequence of the compressibility of the droplet velocity field. We also quantify the dynamical behavior of clustering by moving with this velocity field. We find a preference for clustering in the Lagrangian frame during the time interval set by the decay of the phosphorescence.

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“…Furthermore, if large droplets created through primary breakup are evaporating, they will also shift the distribution towards smaller droplet sizes. To investigate the droplet size distributions, a droplet sizing technique called interferometric particle imaging [18][19][20] (IPI) is used. This technique uses the interference between refractions and reflections of coherent light on the droplet to create a fringe pattern on defocused droplet images (see Figure 13).…”

Section: Phosphorescent Spraymentioning

confidence: 99%

“…with λ the wavelength of the light, d p the diameter of the droplet, d s the size of the defocused droplet image, d a the aperture size of the camera lens, v the object distance, and A(θ) a geometric factor which depends on the angle θ between the light sheet and the lens (see Glover et al 18 or Bocanegra Evans et al 20 for more details). Using a frequency-doubled Nd:YAG laser, with a 10-Hz repetition rate and a pulse energy of 15 mJ, a sheet of 532-nm (green) light, is used to illuminate a plane in the area immediately below the nozzle exit.…”

Section: Phosphorescent Spraymentioning

confidence: 99%

Lanthanide-based laser-induced phosphorescence for spray diagnostics

Voort

Maes

Lamberts

et al. 2016

Review of Scientific Instruments

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Laser-induced phosphorescence (LIP) is a relatively recent and versatile development for studying flow dynamics. This work investigates certain lanthanide-based molecular complexes for their use in LIP for high-speed sprays. Lanthanide complexes in solutions have been shown to possess long phosphorescence lifetimes (∼1-2 ms) and to emit light in the visible wavelength range. In particular, europium and terbium complexes are investigated using fluorescence/phosphorescence spectrometry, showing that europium-thenoyltrifluoracetone-trioctylphosphineoxide (Eu-TTA-TOPO) can be easily and efficiently excited using a standard frequency-tripled Nd:YAG laser. The emitted spectrum, with maximum intensity at a wavelength of 614 nm, is shown not to vary strongly with temperature (293-383 K). The decay constant of the phosphorescence, while independent of ambient pressure, decreases by approximately 12 μs/K between 323 and 373 K, with the base level of the decay constant dependent on the used solvent. The complex does not luminesce in the gas or solid state, meaning only the liquid phase is visualized, even in an evaporating spray. By using an internally excited spray containing the phosphorescent complex, the effect of vaporization is shown through the decrease in measured intensity over the length of the spray, together with droplet size measurements using interferometric particle imaging. This study shows that LIP, using the Eu-TTA-TOPO complex, can be used with different solvents, including diesel surrogates. Furthermore, it can be easily handled and used in sprays to investigate spray breakup and evaporation.

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Measuring droplet size distributions from overlapping interferometric particle images

Cited by 25 publications

References 15 publications

Dense spray evaporation as a mixing process

Dense spray evaporation as a mixing process

Making droplets glow in turbulence

Lanthanide-based laser-induced phosphorescence for spray diagnostics

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