Free convection phenomenon from a heated circular cylinder is commonly seen in the industry. For this case it can be noted to heat transfer from power transmission lines, heat exchangers and heat transfer from pipes. In this study, free convective flow and heat transfer from a heated circular cylinder inside a channel using PIV technique was investigated. In fact, by taking two vertically plates around the cylinder in the water, the free convective between the two plates is flowed. For different channel’s geometry the flow field patterns and surface temperature of the cylinder’s surface distribution are obtained and the aim is finding appropriate channel’s geometry to increase heat transfer from the cylinder. Geometric parameters of the study, including water level, channel width, length and angle of channel plates for convergent channel, divergent channel and convergent-divergent channel. Flow field pattern is obtained by PIV technique and the cylinder surface temperature is measured by thermocouples. In each case, by changing the mentioned parameters, flow rate at the top of the cylinder, surface temperature of the cylinder ,total volume of fluid passing through the channel, the position of the vortex and uniform flow pattern within the channel, is discussed. A vortex formed alongside of the plume and rise to the free surface, where it detached from the plume structure and moved along the free surface. Vortex dynamics and the physical characteristics of the swaying and meandering motions appeared to be interdependent. The relationship between the flow pattern and the heat transfer characteristics at the cylinder surface is studied and the origin of the oscillation discussed. Diagram of flow velocity from cylinder and the cylinder surface temperature versus time, is plotted .By choosing appropriate geometry for the channel, flow around the cylinder and through the channel is well-formed and by increasing the flow velocity over the cylinder, the surface temperature of the cylinder decreased. At last, the heat transfer coefficient and the Nusselt number for cylinder surface in channels with suitable geometry are achieved and compared with the free convective flow from cylinder without channel. The results indicate increase in heat transfer coefficient for the cylinder surrounded with channel. According to the results, the appropriate geometry to increase the heat transfer, is convergent-divergent channel. In this case, comparing by the channel with vertical plates the Nusselt number and free convection heat transfer coefficient of the cylinder surface have increased 1.4%, and in comparison with the cylinder without channel they have increased 6.6%. The results are compared with valid experimental results and good agreement was observed. Keywords: Free convective flow, Heated circular cylinder, Nusselt number, PIV technique
