The friction factor at laminar flow in vertical parallel plate ducts with uniform heat flux has been determined as function of Reynolds number and Rayleigh number using a numerical model. The velocity field and the wall shear stress are modified as compared to forced convection due to buoyancy effects when the Rayleigh number is increased, and for sufficiently large Rayleigh numbers the friction factor becomes increased. The duct height to width ratio has been 10, 20, and 50, and combinations of Reynolds number and Rayleigh number have been chosen in order to study mixed convection situations when the Reynolds number is larger as well as smaller as compared to free convection. The Prandtl number has been set to 0.72 corresponding to e.g. air and to 70 which is typical for e.g. some transformer oils, which are fluids that are frequently applied in buoyancy driven cooling applications.

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