Dimples in turbulent pipe flows: experimental aero-thermal investigation

Abstract

An experimental investigation is performed to study the effects of turbulent flow structures on the wall heat transfer, specifically inside the spherical cavities of partially dimpled tubes. The geometry under investigation consists of an array of spherical cavities distributed over the inner surface of half a tube. Each row has five indentations separated by an angular offset of 36° over the half circumference. The dimples are characterized by a depth-to-print diameter (d/Dp) of 0.18, a print diameter-to-pipe diameter (Dp/D) of 0.25, and a streamwise distance (xd/D) between consecutive cavities of 0.56. At Re=20000, flow separation, reattachment, and secondary flows are captured using both planar Particle Image Velocimetry (PIV) and stereo-PIV. The time-averaged flow field, the vorticity levels, and Turbulent Kinetic Energy (TKE) fields inside the dimples can be used for numerical validation. Moreover, the local heat transfer field inside the cavity is measured for Re=20000−80000, by using Liquid Crystals Thermography (LCTG). At the same Re and Pr, an overall heat transfer enhancement of 1.8 (1.8 times the Nusselt number value (Nu) in bare pipes) has been measured. The use of dimples in heat exchangers thus results in a rise in thermal efficiency at the cost of an increased pressure drop. Comparing the skin friction coefficient (Cf) of the dimpled pipe with the one measured in ribbed tubes for Re=20000−80000, it is shown that higher levels of the aerothermal performance (high heat transfer and low-pressure losses) are achieved by employing the dimple technique.