Augmentation of heat transfer rates in turbulent annular tube flow using spiral spring inserts

Abstract

Nomenclature The present research \vork studies the effect of using spiral spring inserts to augment heat transfer rates in turbulent annular tube flow. In the present work, the test section outer tube is a brass tube of 35 nun ID and 3500 mm length. Three different inner tubes were used, each of 3700 mm length and 10 mm. 14 mm and 19 mm diameter, respectively. Nine spring inserts designs were tested; with wire diameter of 0.6 mm . 0.95 mm and 1.5 mm and spiral pitch of 2()nmi. 40 nun and 60 mm. The flowing air is heated by an electrical heater wound uniformly around the test section tube to give uniform wall heat flux conditions. The outer tube inner surface and the flowing air axial temperatures distributions are measured. The plain annular tube Nusselt number Niip.0 and the axial static pressure drop App.0 are related to the Reynolds number, based on the hydraulic diameter. The heat transfer enhancement for the different cases are presented in the form of the Nusselt number ratio (Nu/Nup.0) versus Reynolds number. A general relation was derived, which represents the heat transfer enhancement due to die various parameters. The frictional loss ratio (A p / A pp 0) is related to both the inserted wire diameter and the spiral pitch. It is concluded that: increasing both the tube ratio and/or wire diameter augments both the heat rate and the frictional losses. On the other hand, increasing the wire spiral pitch decreases the heat transfer rates. The maximum achieved heat transfer enhancement was 3.58 times the basic plain annular tube value. On the other hand, the corresponding pressure drop is about 9.6 the plain annular tube value.