The mixed convection flow onset conditions of viscous fluid for internal symmetric sink flow between two nearly parallel disks stationary similar to the fluid motion in the collector of a Solar Updraft Power Plant is discussed. For that, the system of equations regarding the numerical steady state turbulence model for buoyant incompressible fluids exploiting the variation of the duct cross-section (roof shape coefficient) and the corresponding Richardson number (or mixed convection parameter) is embraced here. Mixed convection flow onset conditions are demonstrated graphically through the theoretical calculation of the Richardson number and the numerical derivation of the Nusselt number. A Richarson number limit (Ri > 10) is suggested for mixed convection onsets, where thermo-hydrodynamic aspects revealed a flow surrounded by complex phenomena. Effects of an important geometrical parameter, namely, roof shape coefficient, could be also detected. (C) 2016 Elsevier Ltd. All rights reserved.