  The stability of buoyant flows occurring in the mixed convection regime for a
viscous fluid in a horizontal plane-parallel channel with adiabatic walls is
investigated. The basic flow features a parallel velocity field under
stationary state conditions. There exists a duality of flows, for every
prescribed value of the mass flow rate across the channel cross-section, caused
by the combined actions of viscous dissipation and of the buoyancy force. As
pointed out in a previous study, only the primary branch of the dual solutions
is compatible with the Oberbeck-Boussinesq approximation. Thus, the stability
analysis will be focussed on the stability of such flows. The onset of the
thermal instability with small-amplitude perturbations of the basic flow is
investigated by assuming a very large Prandtl number, which is equivalent to a
creeping flow regime. The neutral stability curves and the critical parametric
conditions for the onset of instability are determined numerically.
