Mixed Convection Heat Transfer Experiments in Smooth and Rough Vertical Tubes

摘要

The mixed convection regime is a transitional heattransfer regime between forced convection and naturalconvection, where both the forced component of flow,and the buoyancy induced component are important.Aiding flow is when buoyancy forces act in the samedirection as the forced flow (heated upflow or cooleddownflow), while opposing flow is when the buoyancyforce is in the opposite direction of the forced flow(cooled upflow or heated downflow). For opposing flowthe buoyancy always increases the rate of heat transferover the forced convection value. For aiding flow, as theheat flux increased, a reduction in heat transfer isencountered until a condition known as laminarizationoccurs, where the heat transfer is at a minimum value.Further increases in the wall heat flux causes re-transitionto turbulence, and increased heat transfer. For the firsttime experiments were performed to characterize theeffect of surface roughness on heat transfer in mixedconvection, for the case of aiding flow. A correlation wasdeveloped to allow calculation of mixed convection heattransfer coefficients for rough or smooth tubes.