  We have studied experimentally particle suspension when injecting a gas at
the bottom of an immersed granular layer confined in a Hele-Shaw cell. This
work focuses on the dynamics of particles slightly denser than the surrounding
fluid. The gas, injected at constant flow-rate, rises through the granular bed
then forms bubbles which entrain particles in the above liquid layer. The
particles settle down on the edges of the cell, avalanche on the crater formed
at the granular bed free surface, and are further entrained by the continuous
bubbling at the center. We report the existence of a stationary state,
resulting from the competition between particle entrainment and sedimentation.
The average solid fraction in the suspension is derived from a simple
measurement of the granular bed apparent area. A phenomenological model based
on the balance between particle lift by bubbles at the center of the cell and
their settling on its sides demonstrates that most of the particles entrained
by bubbles come from a global recirculation of the suspension.
