Evaporation of a freely floating droplet in an airstream: effects of temperature, humidity, and shape oscillations

TL;DR

This study combines experiments and modeling to analyze how temperature, humidity, and shape oscillations influence the evaporation of freely levitated water droplets in airflow, revealing complex dynamics and developing an extended evaporation model.

Contribution

It introduces a modified evaporation model incorporating shape oscillations and convective effects, validated against experimental data for various environmental conditions.

Findings

Larger droplets exhibit persistent shape oscillations affecting evaporation rates.

The extended model accurately predicts droplet size evolution and evaporation time.

A regime map illustrates droplet lifetime variations across temperature and humidity.

Abstract

We present a comprehensive experimental and theoretical investigation of the evaporation dynamics of freely levitated water droplets in an upward airstream under varying temperature and relative humidity conditions, using a custom-designed wind tunnel that replicates natural rainfall scenarios. A high-speed imaging system captures the temporal evolution of morphology, shape oscillations, and size reduction of the droplet undergoing evaporation. Our observations reveal that larger droplets exhibit persistent shape oscillations due to the interplay between inertia and surface tension in the presence of convective airflow, which significantly alters the evaporation rate compared to that of a stationary spherical droplet in quiescent air. To quantify the effects of air convection, complex morphology, and shape oscillations of the levitated droplet at different temperatures and humidity, we develop a modified evaporation model that extends the classical $d^2$-law. This model incorporates (i) a generalized Sherwood number that accounts for the variation in Reynolds number, Schmidt number, temperature, and relative humidity and (ii) a shape factor that captures the time-averaged surface area of oscillating droplets. The model is validated against experimental findings across a wide range of droplet sizes and environmental conditions, showing excellent agreement in predicting the temporal evolution of droplet diameter and total evaporation time. Furthermore, we construct a regime map showing the variation in the lifetime of the droplet in the temperature-humidity space. The present study establishes a framework that integrates convective transport and morphological deformation, offering new insights into the microphysics of raindrop evaporation.