ABSTRACT
Heat transfer and flow friction measurements were carried out for a fully developed
turbulent flow in a rectangular duct with a dimpled bottom wall. The test duct was
designed with a cross-section of (40 mm height x160 mm width) and its bottom wall
was made of a removable aluminum plate. This plate was dimpled in a uniform
stagger arrangement with three different pitches. Identical dimples with 25 mm
diameter, 17 mm print diameter and 3 mm depth were investigated within a range of
Reynolds number (based on the duct hydraulic diameter) from 22,000 up to 90,000
for three different pitch to dimple print diameter ratios of 1.7, 2.0, and 2.35.
It was found that the presence of dimples on flat surfaces enhances heat transfer in
general due to shear layer reattachment, vortex pairs and the vortical fluid generation
and periodic unsteadiness. A maximum value for Nu/Nuo of about 3.8 was
approached corresponding to a value of F/Fo of about 2.38 within the investigated
ranges Re and P/Dp ratio. This demonstrates that an enhancement can be achieved
by using dimpled surfaces superior to that when using continuous ribbed surfaces.
New correlations for both (Nu/Nuo)/(F/Fo) and (Nu/Nuo)/(F/Fo)1/3 were obtained as
functions of Re and P/Dp ratio. |
