  We investigate the creation of dark matter particles as a result of the decay
of the scalar field in the framework of warm inflationary models, by using the
irreversible thermodynamics of open systems with matter creation/annihilation.
We consider the scalar fields, radiation and dark matter as an interacting
three component cosmological fluid in a homogeneous and isotropic
Friedmann-Lemaitre-Robertson-Walker (FLRW) Universe, in the presence of the
curvature terms. The thermodynamics of open systems as applied together with
the gravitational field equations to the three component cosmological fluid
leads to a generalization of the elementary scalar field-radiation interaction
model, which is the theoretical basis of warm inflationary models. Moreover,
the decay (creation) pressures describing matter production are explicitly
considered as parts of the cosmological fluid energy-momentum tensor. A
specific theoretical model, describing coherently oscillating scalar waves, is
considered. In particular, we investigate the role of the curvature terms in
the dynamical evolution of the early Universe, by considering numerical
solutions of the gravitational field equations. Our results indicate that
despite the fact that the Universe becomes flat at the end of the inflationary
era, the curvature terms, if present, may still play an important role in the
very first stages of the evolution of the Universe.
