This article describes a study of a two-dimensional two-inlet side-dum
p combustor fed with a mixture of air and propane. The present results
concern symmetric operating conditions with respect to the two inlets
. Stable and unstable regimes which depend on the inlet velocity and t
he equivalence ratio have been identified. Schlieren visualization, ra
dical imaging with an intensified CCD camera, and simultaneous pressur
e, inlet velocity and C2 emission light measurements, have been used t
o characterize the combustor behavior. Imaging of the flowfield has pr
ovided an insight on the flame structure and its interaction with the
entering jets. The geometry of the flowfield inside the combustion cha
mber with or without instability was symmetric with respect to the com
bustor centerline. For stable combustion, the flowfield was characteri
zed by the presence of two zones of intense heat release located on bo
th sides of the jet impingement region and were distributed along the
combustor centerline. Two low-frequency unstable modes (a fuel-rich re
gime and a fuel-lean regime with an instability frequency around 500 H
z) were studied using a conditional imaging technique. These instabili
ties were characterized by the excitation of the quarter-wave mode of
the combustor and were associated with a complex evolution of the jets
and the flame. Jet oscillations were due to the kinematic superpositi
on of the lateral entering jets and longitudinal velocity fluctuations
generated by heat release oscillations in the dome region. It was fou
nd that unsteady heat release occurs in two different ways: pulsating
combustion in the dome region and convection of reaction zones downstr
eam of the jet-impingement region. Flame oscillations were induced by
a periodic impingement of the jets on the centerplane of the chamber.
Pressure fluctuations in the test section were roughly in phase with t
he global C2 emission, indicating that the in stabilities were sustain
ed by energy addition to the acoustic field. A two-dimensional distrib
ution of the Rayleigh index computed for each unstable mode indicated
that the fuel-lean mode was driven by the unsteady heat release in the
dome region whereas the fuel-rich mode was driven by the flame oscill
ations downstream of the jet-impingement region. The transition from t
he fuel-lean to the fuel-rich instability featured a shift of driving
mechanism. This study shows that even in our idealized geometry the co
upling mechanisms leading to low-frequency combustion instabilities ar
e not unique and illustrates the difficulty of devising predictive mod
els.