This work with numerical and experimental studies of specific issues of natural convection flow around a horizontal cylinder in a space limited by rectangular cavity. Physical models of laminar unsteady flows have been used for numerical calculations of heat transfer coefficients of four different diameters of horizontal cylinders and for several temperature gradients. Procedures and results of the experimental studies are contained in the next section. All results were compared with different authors and different approaches, However the Nusselt numbers can be estimated due to the temperature interval, the results indicate need for further investigation. and Obsahuje seznam literatury