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Evaporation from freely falling droplets

Published online by Cambridge University Press:  04 July 2016

J. J. Spillman
J. J. Spillman*
Affiliation:
College of Aerodynamics, Cranfield Institute of Technology
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Summary

An analysis of results from reports on droplet evaporation indicated a marked difference in behaviour depending upon the suspension technique used. Few results were obtained with freely falling droplets, the condition which is of interest in aerial spraying operations. Test results obtained at Cranfield by dropping droplets down a climatic tower showed two unexpected results.

The diameter of water droplets reduced linearly with time until they reached a size of about 150 micrometers and then the rate of reduction in diameter increased by about 27%. It is shown that this change occurs when the fall Reynolds number of the droplet becomes less than about 4.

Above this Reynolds number the airflow is separated from the base of the droplet and the trapped air in the toroidal vortex becomes fully saturated and no evaporation occurs over the base region. Below a Reynolds number of 4 the flow is attached everywhere and evaporation occurs from the whole surface area.

When small amounts of molasses are added to a spray formulation the droplets evaporated initially in a way similar to water droplets. As a result of falling, a toroidal vortex motion is induced within the droplet. This causes the same small fraction of the volume of the droplet to cycle close to the surface.

Hence, this fraction of the volume looses all its water, becomes far more viscous as a result and the consequence is the toroidal motion almost stops. In effect the molasses forms a skin over the droplet surface inhibiting further evaporation of the remaining volume of water.

Type
Research Article
Information
The Aeronautical Journal , Volume 88 , Issue 875 , May 1984 , pp. 181 - 185
Copyright
Copyright © Royal Aeronautical Society 1984 

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References

1. Frossling, N. On the evaporation of falling drops. Gerlands Beitrage zur Geophysite, 52. 170, 1938.Google Scholar
2. Ranz, W. E. and Marshall, W. R. Evaporation from drops. Chemical Engineering Progress 48, 3, 1952 ,141-146 and 48(4), 173–180.Google Scholar
3. Roth, L. O. and Porterfield, J. G. A photographic spray sampling apparatus and technique. Trans. ASAE 8, 4, 1965, 493–496.Google Scholar
4. Goering, C. E., Bode, L. E. and Gebhart, M. R. Mathematical modelling of spray droplet deceleration and evaporation. Trans ASAE 15, 1972. 220–225.Google Scholar
5. Hosseinpour, M. Behaviour of single drops and sprays in differing climatic conditions in a climate tower. British Crop Protection Council Symposium on Controlled Droplet Application. 1978, 83–89.Google Scholar
6. Wanner, R. Evaporation of water based pesticides formulations in aerial applications. Proceedings of VIth IAAC Congress, Turin, 1980, 252–271.Google Scholar
7. Picot, J. J. C., Chitrangad, R. and Henderson, G. Evaporation rate correlation for atomised droplets. Trans ASAE 24, 1981. 552–554.Google Scholar
8. Hamey, P. Y. The Evaporation of airborne droplets. MSc Thesis. Bio-Aeronautics, Cranfield Institute of Technology, 1982.Google Scholar
9. Whistlecroft, S. Y. The use of a climate tower for studies of evaporation rates of water droplets. MSc Thesis, Bio-Aeronautics, Cranfield Institute of Technology, 1983.Google Scholar
10. Davies, C. M. Aerosol Science. Academic Press, London, 1966.Google Scholar
11. Bachelor, G K. An Introduction to Fluid Dynamics. Cambridge University Press, 1967 Google Scholar
12. Prandtl, L. Applied Hydro-and Aeromechanics. McGraw- Hill, 1934.Google Scholar
13. Taneda, S. J. Phys. Soc Japan, 1956. 11, 302.Google Scholar
14. Taneda, S. Rep Res Inst Appl Mech, Kyushu Univ. 1956. 4, 99.Google Scholar