Statistical Lagrangian evaporation rate of droplets released in a homogeneous quasi-isotropic turbulence
Résumé
Turbulence has long been suspected to increase the evaporation rate of droplets via the convective effects it generates. The experimental data reported in this paper provide evidence of this increase and statistically quantify these effects. Measurements have been performed following the same Lagrangian approach as in Chareyron et al. [1], Marié et al. [2, 3]. Ether droplets have been released in a quasi isotropic homogeneous turbulence generated by synthetic jets and tracked using in-line digital holography. Their Schmidt number is typically of the order of 2 and their Reynolds number is moderate (≤ 3). Their instantaneous positions and diameters have been measured by processing the holograms with an inverse problem approach (IPA) that has been implemented in Python language for a high performance computer. This has allowed to drastically reduce the processing time and to reconstruct a high number of trajectories for various turbulence conditions. The Lagrangian statistics computed from these trajectories, totaling 1.3 million samples, show that the relative mean motion and turbulence seen by the droplets on average increases their evaporation rate. Within the parameter range investigated, we find that this increase is not well predicted when estimating the convective effect in the Sherwood number with the norm of the instantaneous relative velocity seen by the droplets. In contrast, this increase is very well predicted when the Sherwood number is calculated using a Reynolds number based on the norm of the mean relative velocity plus its RMS fluctuation.
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