Digital holographic measurement of the Lagrangian evaporation rate of droplets dispersing in a homogeneous isotropic turbulence

Abstract

The evaporation rate of diethyl ether droplets dispersing in a homogeneous, nearly isotropic turbulence is measured by following droplets along their trajectory. Measurements are performed at ambient temperature and pressure by using in-line digital holography. The holograms of droplets are recorded with a single high-speed camera (3kHz) and droplets trajectories are reconstructed with an "Inverse problem approach" algorithm (IPA) previously used in Chareyron et al (2012); Marié et al (2014). The thermal-vapor concentration wakes developing around the droplets are visible behind each hologram. A standard reconstruction process is applied, showing that these wakes are aligned with the relative Lagrangian velocity seen by droplets at each instant. This relative velocity is that obtained from the dynamic equation of droplets motion and the positions and diameter of the droplets measured by holography and the IPA reconstruction. Sequences of time evolution of droplets 3D positions, diameter, and 3D relative velocity are presented. In a number of cases, the evaporation rate of droplets changes along the trajectory and deviates from the value estimated with a standard film model of evaporation. This shows that turbulence may significantly influence the phase change process.

Reconstruction of in-line holograms: combining model-based and regularized inversion

“…We illustrate the reconstruction method with an application in fluid mechanics, on holograms of evaporating droplets surrounded by a vapor plume. It was recently proposed on similar types of datasets to combine a parametric estimation of the spherical objects with a simple backpropagation of the residues [9]. This method succeeds in reconstructing a rough orientation and shape of the vapor plume but fails to provide any information about the object phase.…”

“…The parametric part of the reconstruction fits the high contrast fringes thereby enabling the non-parametric part to retrieve the faintest parts of the object. These reconstructions are performed jointly, meaning that their interferences are taken into account, in contrast to the previous method that works on subtracted intensities [9].…”

“…The proposed method is applied to the experimental holograms of diethyl ether droplets evaporating in a turbulent flow. As further described in [9,19,24], a piezoelectric jetting device injects droplets of ∼ 100 µm in a turbulent flow generated and controlled by three pairs of woofers. The in-line holographic setup is composed of a monochromatic beam of wavelength λ = 532 nm and a Phantom V611 high speed camera with a framerate of 3 kHz.…”

“…These interferences are sensitive to the amplitude changes and phase shifts induced by the objects, making this technique particularly suitable for image and study absorbing and/or phase samples. Today, this high sensitivity is used in numerous fields including biology [2][3][4][5][6][7], fluid mechanics [8][9][10][11], and particle characterization [12][13][14].…”

Robust object characterization from lensless microscopy videos

“…Other evaporating particles passing close to the particle of interest also act as outliers. The evaporation rate of these particles was recently studied using the least squares approach (2) [16]. Figure 6(b) shows two frames from the video and the 2D trajectory of the particle over time (superimposed color line).…”