Natural convection flow in vertical flat ducts with heated face is used to intensify the transfer in technical systems, such as ventilated gaps of facade designs. Understanding of physical processes that accompany the air flow in vertical flat parallel-plate ducts gives ameliorating the structures designing process and increasing its operating characteristics.

The aim of this work is evaluation the average speed of natural convection air flow in vertical parallel-plate duct with different temperature of walls.

It is enough for barotropic natural convection flow in the vertical parallel-plate ducts that the polytropic index in the barotropic state do not exceed the polytropic index in the equilibrium state. Polytropic index in the uniform and barotropic natural convection flow is almost proportional to the length of the channel. It is established that the shorter the channel, the greater must be the heat flux that creates vertical traction, and vice versa.