Natural convection around horizontal tubes with smooth, rough, and machined surfaces

This study concerns natural convection around horizontal tubes with smooth, rough, and machined surfaces. The study is motivated by the need for understanding the machining effect or the use of a rough surface layer on the natural convection process. An experimental system is constructed that includes a thick wall metal tube equipped with thermocouples for measuring the surface temperature. A heating element is inserted inside the tube and is coupled with a power supply that can be adjusted to achieve surface temperatures of 60 - 160 degrees C. The tube surface is machined at various depths of 1 - 3 mm. Also, four grades of sandpaper are used to cover the tube surface. An analysis of measured data is based on variations in the Nusselt number as a function of the Rayleigh number and surface condition. Results show that the measured data for the smooth tube are consistent with literature results. Although surface machining increases the heat transfer area, it lowers the heat transfer rate because of the low thermal conductivity of air, which replaces the removed metal in the machined grooves. Similarly, covering the tube surface with sandpaper reduces the rate of heat transfer from the tube surface because of contact resistance and the thermal resistance of the sandpaper. Data analysis that takes into consideration the above resistances, where the contact and sandpaper thermal resistances are eliminated, show enhancement of up to 30%. This implies that the direct roughening of a metal surface would enhance the heat transfer rate by 30%.