  The present work deals with irreversible Universal thermodynamics. The
homogenous and isotropic flat model of the universe is chosen as open
thermodynamical system and non-equilibrium thermodynamics comes into picture
due to the mechanism of particle creation. For simplicity, entropy flow is
considered only due to heat conduction. Further, due to Maxwell-Cattaneo
modified Fourier law for non-equilibrium phenomenon, the temperature satisfies
damped wave equation instead of heat conduction equation. Validity of
generalized second law of thermodynamics (GSLT) has been investigated for
Universe bounded by apparent or event horizon with cosmic substrutum as perfect
fluid with constant or variable equation of state or interacting dark species.
Finally, we have used three Planck data sets to constrain the thermal
conductivity \lambda and the coupling parameter b^2. These constraints must be
satisfied in order for GSLT to hold for Universe bounded by apparent or event
horizons.
