Enhancement of forced convection heat transfer in ribbed roughened square ducts

Abstract

The temperature, heat transfer and fluid flow characteristics in a 50 rom x 50 mm square duct with one replaceable heated wall have been experimentally investigated. In the experiment thirteen different heated walls are used of which one is non-ribbed and twelve are ribbed with four types of rib geometries viz triangular, trapezoidal, saw tooth forward and saw tooth backward and each with pie ratios of 6, 8 and 10. Both the base and the height of all the rib geometries are I mm. The wall is made ribbed by machining out the above geometries from a solid aluminum plate parallel to each other at different pie ratios across the duct section. The room air is used as the working fluid which enters into the duct through a bellmouth contraction. The overall length of the duct is 5.755m, which includes the test section of I .83m. Heat is supplied to the bottom wall of the test section by an electric heater designed for the purpose. Measurements of velocity and temperature in the duct cross sections at XlDh = 3 and 33 down stream from the inlet of the heated test section are taken in the region of -I < (ZIB) < I and -I < (YIB) :s; I at seven different Reynolds numbers varying from 5.0 x 104 to 5.6 X 104 The experiments have been carried out both for one non-ribbed and twelve ribbed walls at different Reynolds numbers. Velocity and temperature of air are recorded at two sections one at XlDh = 3 and other at XlDh = 33 i.e. near the inlet and the outlet of the test duct. The bottom surface temperature of the wall is also measured and from these the wall surface temperatures are calculated. Pressure drops along the length 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 the developed regIOn. The result shows that for constant pie, Nusselt number increases with the increase of Reynolds number and for constant Reynolds number Nusselt number decreases as pie increases. The effect of rib shape is not very significant both on friction factor and heat transfer characteristics. But nevertheless the Saw tooth forward rib geometry produces highest values while the trapezoidal geometry produces the lowest values.To examine the similarities between the velocity and temperature fields their normalized values of (Twi - T) / (Twi - T",) and UfU", have been plotted. The shape of the profiles show clearly the effect of both heating and turbulence promoters. The temperature profiles show higher val ues near the comers than those near the centre indicating higher heat transfer rate there where the inlluence of secondary Ilow is prominent.