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National Advisory Committee for Aeronautics, Technical Notes - Vaproization Rates and Heat Transfer Coefficients for Pure Liquid Drops

The design of high~output combustors for Jet—propelled aircraft
and equipment such as spray dryers and cooling towers requires an
accurate knowledge of liquid vaporization rates. The liquid may
evaporate from spheres, cylinders, or flat surfaces depending on the
type of equipment. In Jet engines, the fuel is frequently injected
as liquid droplets at a point upstream of the combustion zone, and
the concentration of vaporized fuel in the fuel-air stream entering
this zone is determined by the rate of vaporization of the droplets.
In order to determine this rate, a study was made at NASA Lewis
laboratory of droplets vaporizing under conditions similar to those
encountered in aircraft combustion systems.

Two general methods may be used to calculate the vaporization
rate of a pure liquid drop. mass—transfer equations employ the dif-
ference between the partial pressure of the vapor at the drop surface
and the partial pressure of the vapor in the streaming air as the
driving potential. The other method is based on a heat-transfer equa-
tion and the driving potential is given.as the difference between the
air temperature and the surface temperature of the drop. An equa-
tion based on heat transfer as the controlling process and showing
the effect of mass transfer on the heat~transfer coefficient was
chosen for investigation.

In order to obtain a better understanding of the effect on
vaporization of air temperature, a wetted cork sphere simulating a
drop of liquid was placed in an air stream of constant mass-flow
rate and varying temperature. Liquid was injected into the sphere
at a rate equal to the vaporization rate, and the loss of liquid by
vaporization from the sphere's surface was determined. The mass-
flow rate of one liquid was varied to give values of Reynolds numbers
from 1600 to 5700. Air temperatures were varied from 30° to 500° C
in the tests of nine pure liquids having latent heats of vaporization
ranging from 50 to 500 gram-calories per gram. The resulting data
on vaporization rates were found to correlate with a semiempirical
theory of heat and mass transfer. Surface-temperature data were
obtained for each liquid and used in the corresponding heat-transfer
calculations.