Heat Transfer in a Channel with Dimples and Protrusions on Opposite Walls

Abstract

Local and spatially averaged Nusselt-number data are presented forthe dimpled surface of a channel, both with and without protrusions (with the same shapes as the dimples ) on the opposite wall. Channel aspect ratio is 16, ratio of channel height to dimple print diameter is 0.5, Reynolds numbers based on channel height ranges from 5 ££ 10 3 to 3:5 ££ 10 4 , and ratio of channel inlet stagnation temperature to wall temperature ranges from 0.73 to 0.94. Because of the added vortical, secondary e ow structures and e ow unsteadiness induced by the protrusions, local and spatially averaged Nusselt numbers are augmented considerably and have greater Reynolds-number dependence, compared to a channel with a smooth top wall and dimples on one opposite wall. Nusselt-number variations arealso observed asthelocation ofthearray of protrusionsis changed with respect to thedimples. Form drag and channel friction factors are augmented, and thermal performance factors are then generally lower when protrusions are employed, compared to a channel with a smooth top surface and a dimpled bottom surface. Nomenclature D = dimple print diameter f = channel friction factor f0 = baseline friction factor in channel measured with smooth top and bottom channel surfaces H = channel height k = thermal conductivity Nu = Nusselt number based on channel hydraulic diameter Nu0 = baseline Nusselt number based on channel hydraulic diameter and measured with smooth top and bottom channel surfaces ReDh = Reynolds number based on bulk mean channel velocity and channel hydraulic diameter ReH = Reynolds number based on bulk mean channel velocity and channel height Toi = spatially averaged stagnation temperature at test section inlet Tw = surface temperature X = streamwise coordinate measured from upstream edge of test surface Z = spanwise coordinate measured from spanwise centerline of test surface