Estimation of the diameter–charge distribution in polydisperse electrically charged sprays of electrically insulating liquids

Shrimpton, John S. and Rigit, A.R.H. (2009) Estimation of the diameter–charge distribution in polydisperse electrically charged sprays of electrically insulating liquids. Experiments in Fluids, 46 (6), 1151-1171. (doi:10.1007/s00348-009-0626-5).

Abstract

The majority of scientific and industrial electrical
spray applications make use of sprays that contain a
range of drop diameters. Indirect evidence suggests the
mean drop diameter and the mean drop charge level are
usually correlated. In addition, within each drop diameter
class there is every reason to suspect a distribution of
charge levels exist for a particular drop diameter class. This paper presents an experimental method that uses the joint
PDF of drop velocity and diameter, obtained from phase
Doppler anemometry measurements, and directly obtained
spatially resolved distributions of the mass and charge flux
to obtain a drop diameter and charge frequency distribution.
The method is demonstrated using several data-sets
obtained from experimental measurements of steady polydisperse
sprays of an electrically insulating liquid produced
with the charge injection technique. The space charge
repulsion in the spray plume produces a hollow cone spray
structure. In addition an approximate self-similarity is
observed, with the maximum radial mass and charge flow
occurring at r/d * 200. The charge flux profile is slightly
offset from the mass flux profile, and this gives direct
evidence that the spray specific charge increases from
approximately 20% of the bulk mean spray specific charge
on the spray axis to approximately 200% of the bulk mean
specific charge in the periphery of the spray. The results
from the drop charge estimation model suggest a complex
picture of the correlation between drop charge and drop
diameter, with spray specific charge, injection velocity and
orifice diameter all contributing to the shape of the drop
diameter–charge distribution. Mean drop charge as a
function of the Rayleigh limit is approximately 0.2, and is
invariant with drop diameter and also across the spray
cases tested.

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