Local and Average Heat Transfer Characteristics for Turbulent Airflow in an Asymmetrically Heated Tube

Turbulent airflow experiments were performed with a specially fabricated test section tube which facilitated nonuniform heat transfer around the tube circumference. Heating was accomplished by passing electric current axially through half the tube wall (subtending a 180 deg arc), while the other half of the wall was not directly heated. Measurements were made both in the thermal entrance region and the fully developed region, and the Reynolds number was varied from about 4400 to 64,000. The results of the experiments underscored the strong interaction between circumferential tube-wall conduction and fluid convection when a gas flowing in a tube is heated nonuniformly around its circumference. The effects of the wall conduction were shown to be significant at low Reynolds numbers but diminished as the Reynolds number increased. Owing to the circumferential nonuniformities, the thermal development was much slower than that for a uniformly heated pipe flow. By use of a suitably defined circumferential average heat transfer coefficient, the present fully developed results agreed well with a literature correlation for uniformly heated flows. At any cross section, the local coefficients varied around the tube circumference, with the smallest value at the mid-point of the heated arc. Buoyancy effects at low Reynolds numbers were investigated and found to be undetectably small.