In this article, a composite parallel-plate channel whose central portion is occupied by a clear fluid and whose peripheral portion is occupied by a fluid saturated porous medium, is considered. The flow in the porous region of the channel is assumed to be laminar, governed by the Brinkman-Forchheimer-extended Darcy equation, while the flow in the clear fluid region of the channel is assumed to be turbulent. The validity of this laminar/turbulent assumption is validated by estimating Reynolds numbers in the clear fluid and porous regions of the channel. Although the flow in the porous region remains laminar, it is still fast enough for the quadratic drag (Forchheimer) effects to be important. In this situation, hydrodynamic mixing of the interstitial fluid at the pore scale becomes important and may cause significant thermal dispersion. It is shown that thermal dispersion may result in some counterintuitive effects, such as the increase of the Nusselt number when the width of the clear fluid region in the center of the channel is decreased.