Local heat/mass transfer measurements in a rectangular duct with discrete ribs

The influence of arrangement and length of discrete ribs on heat/mass transfer and friction loss is investigated. Mass transfer experiments are conducted to obtain the detailed local heat/mass transfer information on the ribbed wall. The aspect ratio (width/height) of the duct is 2.04 and the rib height is one tenth of the duct height, such that the ratio of the rib height to hydraulic diameter is 0.0743. The ratio of rib-to-rib distance to rib height is 10. The discrete ribs were made by dividing each continuous rib into two, three, or five pieces, which were attached periodically to the top and bottom walls of the duct with a parallel orientation. The combined effects of rib angle and length of the discrete ribs on heat/mass transfer are considered for the rib angles (α) of 90 and 45 deg. As the number of the discrete ribs increases, the uniformity of the heat/mass transfer distributions increases. For α=90 deg, the heat/mass transfer enhancement with the discrete ribs is remarkable, while the heat/mass transfer performances are slightly higher than that of the transverse continuous ribs due to the accompanied high friction loss penalty. For α=45 deg, the average heat/mass transfer coefficients and the heat/mass transfer performances decrease slightly with the discrete ribs compared to the case of the angled continuous ribs.