Detailed numerical analysis of evaporation of a micrometer water droplet suspended on a glass filament

- Prediction of local evaporation rate across the droplet surface.
- Investigation of shape evolution of an evaporating droplet.
- Investigation of the effect of glass knob size on droplet drying dynamics.
- Internal recirculation induced by shear force and surface tension gradient.

For a long time, the single droplet drying (SDD) equipment has been utilized to investigate droplet drying behaviour relevant to spray drying. Typical of the device is a global measurement of droplet's drying kinetics such as evaporation rate, temperature history, diameter changes, etc., and has been extensively used for such purposes. Utilization of computational fluid dynamics (CFD) enables SDD experiment to be interpreted and examined closely as well as for better exploration of the device with views of improving it in future. Here, for the first time, we have provided detailed numerical simulation of the laboratory condition of SDD using a pure water droplet suspended on a glass filament tip. With CFD, it is therefore possible to determine the local flux of vapour or global evaporation rate across the droplet-air interface using the equations of transport. The Arbitrary Lagrangian-Eulerian (ALE) procedure embedded inside Navier-Stokes ensures interface tracking as well as visualization of the droplet shape evolution. As a concrete demonstration of the SDD situations, effects of different glass knob sizes for different sizes of suspended pure water droplet (0.98-1.56 mm initial diameter), upward inflow of humid air and temperature (0.01% and 358.15 K), and air velocity 1.11 m s−1 were examined on evaporation rate; shape formation; flow velocity and temperature distribution within and around the droplet. Our model is based on the first principles, without adjustable parameters, the predictions made are in good agreement with the available experimental SDD data.

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