Comparison of Heat Transfer and Friction in Pipes with Various Internal Roughness

The heat transfer and friction factor of turbulent pipe flows with different internal roughness are experimentally investigated. Three types of roughness in forms of a mesh, hemispherical elements, and a coil are added to the interior of pipes with a nominal diameter of two inches. The working fluid is air, and the Reynolds numbers varies from 20,000 to 90,000 in increments of 10,000. For investigating the heat-transfer properties the pipe wall is heated to 375°C while the inlet air remains at the room temperature. The measurements show that the mesh-type roughness results in a maximum Nusselt number, Nu, increase of approximately 6%, the pipes with hemispherical roughness increased the Nu by a maximum amount of 30%, and the coil increased Nu by up to 60% compared with the smooth pipe. The maximum increase of friction factor is 40% for the pipes with mesh-type roughness, 30% for pipes with hemispherical roughness, and 67% for pipes with coil roughness. The experimental results indicate that adding hemispherical and coil roughness to the internal surface of the pipe can lead to a significant improvement in the rate of heat-transfer while adding a mesh-type roughness can have marginal improvements and comes with a large frictional loss penalty. The analysis shows that the highest thermohydraulic performance is achieved using the hemispherical roughness elements.