Experimental investigation of circumferentially non-uniform heat flux on the heat transfer coefficient in a smooth horizontal tube with buoyancy driven secondary flow

In this experimental investigation the influence of non-uniform heat flux distributions on the internal heat transfer coefficient in a horizontal circular tube was studied for liquid water. The tube had an inner diameter of 27.8 mm and a length to diameter ratio of 72. Different outer wall heat flux conditions were studied for Reynolds numbers ranging from 650 to 2600 at a Prandtl number of approximately 6.5. Heat flux distributions included fully uniform heating (which had a circumferential angle span of 360°) and different partial uniform heat flux distributions with angle spans of 180° or 90° at different circumferential positions. Depending on the angle span, local heat flux intensities ranging from 1658 W/m2 to 6631 W/m2 were tested. Results indicate that the average steady state Nusselt number is greatly influenced by the applied heat flux position and intensity. Highest average heat transfer coefficients were achieved for cases where the applied heat flux was positioned on the lower half (in terms of gravity) of the tube circumference, while the lowest heat transfer coefficients were achieved when the heating was applied to the upper half of the tube. Smaller angle spans produced lower heat transfer coefficients. The relative thermal performance of the different heating scenarios where characterised and described by means of newly developed heat transfer coefficient correlations for angle spans of 180° and 90° which correlated 92% and 96% of the data respectively within 3% of the measured Nusselt number.