Influence of ring type flow turbulators on the local heat transfer coefficients in an annular passage – An experimental and numerical investigation

Abstract

In this experimental and numerical investigation, the use of circular cross sectioned ring-type flow turbulators 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 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 14.8 mm, a diameter ratio (inner wall diameter to outer wall diameter) of 0.648, and a length to hydraulic diameter ratio of approximately 71. A set of turbulators were inserted which had a thickness of 1 mm, a ring diameter of 15.1 mm and a pitch of 50 mm. Local heat transfer coefficients were obtained using liquid crystal thermography to map the effects of adding the set of ring turbulators. This method provided results which could be used to compare the turbulator effects between turbulators. It was found that the presence of the flow turbulators increased the average Nusselt number by between 34% and 54%. 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 can significantly increase the Nusselt number. Heat transfer was optimised in terms of the turbulator diameter and the wall gap size. 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.