The Evaporation of Water Droplets. A Single Droplet Drying Experiment

Abstract

The heat transfer coefficient for single suspended water droplets has been measured over an air temperature range of 23–200°C. Experiments were performed under both natural and forced convection. Air velocities ranged from 0.25 to 1.00 m/s producing Re _D numbers from ˜30 to 100. Results were correlated by the Ranz and Marshall equation obtaining for all data: Nu = 0.8 Pr^0.33 + 6.4. Droplet nonsphericity and droplet oscillation were observed in many of the experiments. However, it was not possible to directly ascertain, particularly in the case of droplet oscillation, whether droplet heat and mass transfer had increased as a result. Droplet viscosity, surface tension and droplet mass were identified as important factors in determining the frequency of droplet oscillation and the degree of droplet nonsphericity. These parameters were correlated by a dimensionless group. The methodology and limitations of the single droplet drying technique have been discussed.

Keywords:

• Heat transfer coefficient
• Natural and forced convection
• Ranz and Marshall equation
• Droplet nonsphericity
• Droplet oscillation
• Filament suspension

Acknowledgments

The author would like to thank Dr. E. L. Smith, Reader in Chemical Engineering at the Department of Chemical Engineering and Applied Chemistry, Aston University, for his contribution to this article.

Notes

^aNot calculated here.

^bDue to the fragile nature of the filament, a new filament was used for each new drying temperature.