  A new two-phase model for concentrated suspensions is derived that
incorporates a constitutive law combining the rheology for non-Brownian
suspension and granular flow. The resulting model exhibits a yield-stress
behavior for the solid phase depending on the collision pressure. This property
is investigated for the simple geometry of plane Poiseuille flow, where an
unyielded or jammed zone of finite width arises in the center of the channel.
For the steady states of this problem, the governing equations are reduced to a
boundary value problem for a system of ordinary differential equations and the
conditions for existence of solutions with jammed regions are investigated
using phase-space methods. For the general time-dependent case a new drift-flux
model is derived using matched asymptotic expansions that takes into account
the boundary layers at the walls and the interface between the yielded and
unyielded region. The drift-flux model is used to numerically study the dynamic
behavior of the suspension flow including the appearance and evolution of an
unyielded or jammed region.
