This paper examines the effect of small downward inclinations on the format
ion of slugs. Experiments were conducted with air and water at atmospheric
pressure, in a pipe with a diameter of 0.0763 m, a length of 23 m and incli
nations of -0.2, -0.5 and -0.8 degrees. Measurements of the variation of th
e interfacial displacement were made simultaneously at a number of location
s. For low gas velocities in a horizontal configuration waves with lengths
of 16-20 cm, grow until they reach of the top of the pipe. These waves evol
ve from smaller wavelength waves (8-10 cm) through a non-linear growth mech
anism. At high gas velocities, the liquid height is not large enough for th
is mechanism to be operable. In these cases slugs evolve from the coalescen
ce of roll waves. Surprisingly, the large amplitude small wavelength waves
observed in horizontal flows, at the transition to slug flow, are damped in
pipelines that are inclined slightly downward. The transition is associate
d with the initiation of long wavelength, small amplitude waves, whose appe
arance is predicted by a viscous long wavelength linear stability analysis.
A local Kelvin-Helmholtz instability at the crest of a growing long wavele
ngth wave is observed when a slug forms. The frequency of slugging is equal
to frequency of these long wavelength waves. (C) 2000 Elsevier Science Ltd
. All rights reserved.