الفهرس 
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Abstract
Heat transfer is one of the particular interests to engineers, who attempt to understand and control the flow of heat through the use of thermal insulation, heat exchangers, and other devices.
The present thesis aims to study the effect of mixed free and forced convections to the velocity profiles changes of the fluid flow in the vertical tubes.
A mathematical model is presented to simulate the concerned case. The input parameters to the mathematical model are the Reynolds, Grashof and Prandtl numbers as well as tube dimensions, while the results are the velocity profiles at different levels of the tube.
The results show that the velocity profiles have been changed from the fully developed profile to the saddle shaped profile due to the mixed convection. For high values of Grashof number and low values of Reynolds and Prandtl numbers, a reversal zone has been generated to form a recirculation cell.
The formation of the recirculation cell was studied thoroughly. A mathematical relation was deduced to describe the formation of this cell.