Experimental studies on hydrodynamic resistance and flow pattern of a narrow flow channel with dimples on the wall

Abstract

A systematic flow visualization study on flow structures inside dimples with different relative depths (h/d) and with round or sharp edges is carried out in the first part of this investigation; three flat test plates are made and each one has two dimples with the same print diameter (d) at 40 mm and the same h/d, which is set at 4%, 20% and 50% respectively. Then friction factor is measured on a test channel with two types of test plates. One has dimples array with h/d at 4% and round edge and the second is a flat plate used as reference; surface roughness can be changed for both test plates by painting and covering with water-proof paper. Flow visualization is also done on a dimple located at the center of the plate in the channel. Results show that shallow dimple (h/d=4%) produces non-separated flows at Reynolds number Re δ1500. Dimple with h/d at 20% produces two symmetric vortices at Reδ1700. Round edged dimple changes its fl pattern at different Reynolds number comparing to its sharp edged counterpart. For the dimple with h/d at 50%, at Reδ 1800, it is unstable with its rotating direction changing frequently. In this part of preliminary work, dimpled plate with roughness reduces the friction factor by at least 2% in the flow channel comparing to the reference plate at Reynolds number Re Dh≈8,500-24,000. The friction factor curve of the channel with roughened dimpled plate approaches a hydraulically smooth channel with the increase of Reynolds number. No increase in friction factor is observed on the channel with dimpled plate having smooth surface in the comparison to the reference channel without dimples. Copyright © 2004 by ASME.