53    ENTRAINMENT BY LIGAMENT-CONTROLLED
EFFERVESCENT ATOMIZER-PRODUCED SPRAYS
J.J. Sutherland, P.E. Sojka, and M.W. Plesniak
Thermal Sciences and Propulsion Center
School of Mechanical Engineering
Purdue University
West Lafayette, Indiana 47907-1003
ABSTRACT
Entrainment of ambient air into sprays produced by a new type of effervescent atomizer is reported. Entrainment data were obtained using a device similar to that described by Ricou &
Spalding.1 Entrainment data were analyzed using the model of Bush & Sojka,2 in concert with
measured momentum rate data that were also acquired as part of this study. The analysis shows
that entrainment by sprays produced using this type of atomizer is predicted to within about 35%
by the expression
m.
(1)
^PeMo
where me is the entrained gas mass flow rate, x is the distance along the spray axis measured
from the dispenser exit orifice, pe is the density of the entrained air, M() is the spray momentum
rate at the exit orifice, and E is the experimentally determined entrainment number whose value
is 0.15 ±0.056(2o).
INTRODUCTION
Effervescent atomization is characterized by actively introducing gas bubbles into a liquid
stream immediately upstream of an exit orifice, thereby forming a two-phase flow. This allows
efficient transfer of energy between the atomizing gas and the liquid. The result is a high quality
spray (i.e., small mean drop size) produced at air-to-liquid mass flow rate ratios substantially
lower than most atomizers.
A number of investigators have studied sprays produced by effervescent atomizers. They have
been concerned almost exclusively with atomizer performance; i.e., mean drop size, and have reported their results in terms of the spray Sauter mean diameter (SMD or, equivalently, D32).
One topic which has not been studied in detail is entrainment. Entrainment, for the purposes of
this discussion, is defined as the quantity of ambient gas which is drawn in through the interface
of a spray as it expands downstream of the nozzle. It is the result of momentum transfer, from
both liquid drops and any atomizing gas used, to the ambient air surrounding the spray.
Entrainment has important implications in many engineering applications. In consumer product sprays, the entrained mass flow rate has a cause and effect relationship on carrier liquid evaporation. In combustion applications such as gas turbines and diesel engines, entrainment has a
significant effect on the local equivalence ratio and, therefore, a direct impact on NOx formation.
In furnaces, entrainment has a large impact on droplet residence time, since higher entrainment
rates lead to greater rates of spray deceleration and, therefore, slower moving drops. Entrainment
is also important in spray drying, where the goal is to remove as much liquid as possible from a
droplet during the drying process. Finally, entrainment can influence the finish quality of painted
or coated surfaces through both transfer efficiency and solvent evaporation.
52nd Purdue Industrial Waste Conference Proceedings. 1997. Ann Arbor Press. Chelsea, Michigan 48118. Printed in
U.S.A.
525