Lagrangian measurements of the fast evaporation of falling diethyl ether droplets using in-line digital holography and a high-speed camera

Abstract

measurements of the fast evaporation of falling diethyl ether droplets using in-line digital holography and a high-speed camera. Experiments in Fluids, Springer Verlag (Germany), 2014, 55, 1708 (13 p.). 10.1007/s00348-014-1708-6. ujm-01020697 Experiments in Fluids manuscript No. (will be inserted by the editor) Lagrangian measurements of the fast evaporation of falling Diethyl Ether droplets using in-line digital holography and a high-speed camera Abstract The evaporation of falling Diethyl Ether droplets is measured by following droplets along their trajectories. 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 and reconstructed with an "inverse problems" approach algorithm previously tested (Chareyron et al 2012). Once evaporation starts, the interfaces of the droplets are surrounded by air/vapor mixtures with refractive index gradients that modify the holograms. The central part of the droplets holograms is unusually bright compared to what is expected and observed for non-evaporating droplets. The reconstruction process is accordingly adapted to measure the droplets diameter along their trajectory. The Diethyl Ether being volatile, the droplets are found to evaporate in a very short time: of the order of 70 ms for a 50 − 60 µm diameter at an ambient temperature of 25• C. After this time, the Diethyl Ether has fully evaporated and droplets diameter reaches a plateau. The remaining droplets are then only composed of water, originating from the cooling and condensation of the humid air at the droplet surface. This assertion is supported by two pieces of evidence: (i) by estimating the evolution of droplets refractive index from light scattering measurements at rainbow angle, and (ii) by

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

“…Typical evaporation rate measured along trajectories are presented. While without turbulence this evaporation rate is close to that predicted by the standard "d 2 " law (Marié et al, 2014), it may significantly fluctuate and deviate from this law in presence of turbulence. The thermal-vapor concentration boundary layers and wakes visible around each hologram of droplets have also been reconstructed with a standard back propagation method, providing a convincing picture of the vapor film around the droplet.…”

“…It is caused by the deflection of the light by the refractive index gradient in the thermal -vapor concentration boundary layers and wakes developing around the droplet as discussed in Seifi et al (2013); Méès et al (2013). It was shown that the wake image visualizes well the Lagrangian relative motion of the air around the droplet (Chareyron et al, 2012;Marié et al, 2014). The effect of the vapor is more pronounced in the center part of the hologram and is not taken into account in the hologram Fraunhofer formation model.…”

“…Unlike in Chareyron et al (2012); Marié et al (2014) where the "drop on demand mode" was used, it is operated in continuous droplet jet-mode. The ether jet created at the capillary exit by a syringe pump is disrupted, which delivers droplets of approximately 1.8 times the orifice diameter (Lee, 2003), that is about 110 µm, with a velocity of the order of 2 m.s −1 .…”

“…The reason is that it provides the high accuracy on droplet position and size that is required to track them and to measure their evaporation rate at large distance. The IPA method is described in Soulez et al (2007a,b) and has been successfully applied in Chareyron et al (2012); Seifi et al (2013); Marié et al (2014). It consists in minimizing the difference between each recorded hologram and a model hologram.…”

“…In the present configuration, the magnification ratio is about 1.544 for z = 0 and it varies between 1.51 and 1.58 in the measurement volume. The magnification factor has been deduced from calibration by using a linear scale glass reticle (Edmunds Optics, 62 − 252), following the same procedure as in Marié et al (2014). Practically, a spherical droplet with coordinates x, y, z and diameter d is therefore reconstructed as an object located at x e = m(z)x, y e = m(z)y, z e = m(z)z with a diameter d e = m(z)d.…”