Investigation of flow characteristics and heat transfer augmentation in square ducts with turbulence promoters

Abstract

An experimental investigation has been carried out to observe flow characteristics and heat transfer augmentation in the developing region of a 50mm square duct having one heated rough wall. Room air is entered into the duct through a bell mouth contraction. The overall length of the duct is 5.75m which includes the test section of iength 3.0m. The heated wall is made of both plane as well as rough surfaces. The roughness is made by making 1mm x 1mm parallel ribs of different pitch to height ratio (pie) across the section. Heat is supplied to the heated wall by an electric heater designed for the purpose. The experiments have been carried out both for non-ribbed and different ribbed walls at different Reynolds numbers. Velocity and temperature of air are recorded at various sections along the length of the test duct. The bottom surface temperature of the wall is also measured and from these the wall surface temperatures is calculated. The mean velocity and temperature of air along the duct bisectors are recorded at different sections of the test duct. Pressure drops along the iength of the test duct are also recorded. Heat transfer and friction characteristics like heat transfer coefficient, Nusselt number, Stanton number, local and mean friction factors, universal velocity and temperature distributions etc. are evaluated for different wall roughness in both the developing and the developed region. The result shows that for constant pie, Nusselt number increases with Reynolds number but consequently for constant Reynolds number Nusselt number decreases as pie increases. Again for constant pie, Stanton number decreases. It is observed that the secondary flow starts generating from the entry of the duct and its influence on the axial mean velocity profile increases as the flow field develops. At X1Dh=57 the velocity field seems to attain nearly fully developed condition. Due to the influence of secondary velocity the profiles takes the saddle shaped form at the centre.