Combined convection vertical duct

Convection is the heat transfer in the fluid from or to a surface (Fig. 11.28) or within the fluid itself. Convective heat transport from a solid is combined with a conductive heat transfer in the solid itself. We distinguish between free and forced convection. If the fluid flow is generated internally by density differences (buoyancy forces), the heat transfer is termed free convection. Typical examples are the cold down-draft along a cold wall or the thermal plume upward along a warm vertical surface. Forced convection takes place when fluid movement is produced by applied pressure differences due to external means such as a pump. A typical example is the flow in a duct or a pipe. 

M. Iqbal, A. K. Khatry, and B. D. Aggarwala, On the Second Fundamental Problem of Combined Free and Forced Convection through Vertical Non-Circular Ducts, Appl. Sci. Res., (26) 183-208, 1972. 

Z. F. Dong, and M. A. Ebadian, Analysis of Combined Natural and Forced Convection in Vertical Semicircular Ducts with Radial Internal Fins, Numer. Heat Transfer, part A, (27) 359-372,1995. 

This is a case of free convection. The combined heat transfer will involve two vertical plates (the two sides of the duct) and a colder horizontal plate (upward) with warmer air (top of duct) as well as a cooler horizontal plate (downward) with warmer air. 

---