Turbulent heat transfer augmentation and friction in a circular pipe flows with ribs detached from the wall

الملخص EN

The turbulent heat transfer and the friction in a circular pipe with ring ribs detached from the pipe wall have been studied experimentally.

The effects of detached distance-to-rib height ratio, rib thickness-to-rib height ratio, and flow Reynolds number are examined for fixed rib height-to-pipe diameter ratio and rib pitch-to-rib height ratio.

The clearance ratio (C/h) is varied from 0.222 up to unity and the Reynolds number ranged from 2.0 * 104 to 8.85* 104 at clearance ratio of 0.378, the rib thickness-to-rib height ratio is varied from 0.25 up to unity.

The rib height-to-pipe diameter and the pitch-to-rib height ratio are fixed at h/D = 0.1 and Pi/h = 10, respectively.

The air fluid flow is heated, under uniform heat flux condition, by means of an electric heater wrapped around the wall of the testing pipe.

It is found that the detached ribbed pipe has the advantage of eliminating the hot spots that occur behind the attached ribs.

The average heat transfer coefficient was found to be three to five and half times that of smooth ducts, whereas friction factor measurements showed an increase approximately 14 to 19 times over those of the smooth pipe for the present tested conditions.

The results show that at lower Reynolds number the detached ribbed pipe give a higher thermal performance than those at higher Reynolds number.

The detached ribs with C/h = 0.378 and t/h=0.25 give the best thermal performance among the clearance and rib thickness-to-rib height ratios investigated.

New correlations are obtained for the influence of detached distance-to-rib height ratio, rib thickness-to-rib height ratio, and flow Reynolds number on both Nusselt number enhancement and flow friction factors ratios.