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National Advisory Committee for Aeronautics, Technical Notes - Fluctuations in a Spray Formed by Two Impinging Jets

The spray resulting from the impingement of two jets of water was
investigated to determine the characteristics of the instability asso-
ciated with this method of spray formation. -It was found that upon
impingement of_two Jets a ruffled sheet of liquid was formed perpen-
dicular to the plane of.the two jets. The liquid sheet disintegrated
intermittentlg'forming groups of drops, which appeared as waves-
propagatinngrom the point of impingement. The intermittent disinte—
gratiOn of the liquid sheet occurred with irregular spacing between
waves and with variable wave intensity; however, the frequency of
wave formation was a constant over a finite time interval under con-
stant operating conditions. There was_an abundance of small waves,
with the number of waves of a given intensity decreasing as the
wave intensity increased. From the photographic and frequency_data
obtained, the ruffling of the_liquid sheet appears to be-caused by both
irregularities in the jets and interaction with the air. The ruffling
of the liquid Sheet persists to the point of disintegration of the sheet
and establishes the frequency of the wave formation.

Frequency measurements obtained by a photoelectric technique
showed that wave frequency varied between l000 and 4000 cycles per
second for test conditions of jet velocities from 20 to 80 feet per
second, impingement angles up to 100°, and jet diameters of 0.025,
0.040, and 0.057 inch. Frequency was found to increase with jet
velocity with approximate direct proportionality. An increase in
impingement angle resulted inoa decrease in wave frequency for
impingement angles of from so to 100° with the decrease being
approximated by the decrease in the cosine of one-half the impingement
angle. Both the diameter of the jets and the length of the jets
before impingement had negligible effects on wave frequency compared
to the effect of jet velocity and impingement angle.

The combustion process in rocket engines has, under certain
operating conditions, exhibited conditions of instability. This
instability is characterized by sustained oscillations in combustion-
chamber pressure at frequencies that vary from approximately 20 up to
several thousand cycles per second. An explanation of such oscil-
lation at low frequencies may be resonance in a circuit where the
propellant feed system and the rocket chamber are dynamically coupled.
The instability at—higher frequencies, however, is not readily explained
on the same basis. Combustion instability of this rapidly reoccurring
type appears to originate in the injection and mixing process. In view
of this relation of the injection process to instability, the flow
characteristics and the patterns of sprays formed by the impingement of
two liquid jets, a typical method of injection, have been investigated
at the NASA Lewis laboratory with the intent that such an evaluation
may be useful in the study of combustion instability.