In this experimental investigation the influence of buoyancy driven secondary flow on the effective length-wise heat transfer coefficient in a circular horizontal tube is considered for liquid water at different applied external circumferential heat flux distributions. The test section consisted of a 2 m long stainless steel tube with an inner diameter of 27.8 mm and a wall thickness of 2.77 mm. Hydrodynamic fully developed inlet flow at a uniform temperature of approximately 20°C was considered for a Reynolds number range from 650 to 2600. Three externally imposed heat flux distributions were examined: a fully uniform heat flux condition along the length and the circumference of the tube and another two, where only the lower or upper halves of the tube experienced uniform heat flux, while the non-heated halves were externally adiabatic. Results showed that the heat flux distribution had a significant influence on the effective Nusselt number. The fully uniform heat flux distribution cases produced the highest Nusselt numbers. The cases where only the lower half of the tube was heated had higher Nusselt numbers than the cases where only the upper half was heated.
Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016.