Local heat transfer coefficients in an annular passage with flow turbulation

Abstract

In this experimental and numerical investigation, the use of flow turbulation was considered as a method to increase local heat transfer coefficients in annular heat transfer passages. Experimental data was obtained for cases with and without inserted ring turbulators within a horizontal annular test section using water for average Reynolds numbers ranging from 2000 to 7500 and average Prandtl numbers ranging from 6.73 to 6.79. The test section was heated uniformly on the inner annular wall and had a hydraulic diameter of 14.8mm, a diameter ratio (inner wall diameter to outer wall diameter) of 0.648, and a length to hydraulic diameter ratio of approximately 74. A set of circular cross sectioned ring-type turbulators were used which had a thickness of 1mm, a ring diameter of 15.1mm and a pitch of 50mm. It was found that the presence of the flow turbulators increased the average Nusselt number by between 33.9% and 45.8%. The experimental tests were followed by numerical simulations to identify the response in the heat transfer coefficient by changing the geometry of the turbulators. For this, the turbulator diameters were ranged from 0.5 mm to 2 mm, and the gap size (between the inner wall and a turbulator ring) ranged from 0.125 mm to 4 mm at a pitch of 50 mm. The results showed that the use of turbulators increased the Nusselt numbers by a maximum of 34.8% and that the maximum can be achieved for a turbulator diameter of 2 mm and a gap size of 0.25 mm, for all the Reynolds numbers tested. From the numeric determined pressure drop values it was found that the smaller gap size had the lowest pressure drop and the smallest turbulators also produced the lowest pressure drop.