ENHANCED HEAT TRANSFER AND FLOW FEATURES IN A DUCT MOUNTED WITH VARIOUS RIBS

The present experimental work is a comprehensive study of heat transfer and flow field characteristics inside a rectangular duct integrated with ribs of variable cross sections on the bottom wall. Four kinds of rib configurations, i.e., square (Case 1), pentagonal (Case 2), trapezium (Case 3), and truncated prismatic (Case 4), were deployed in the cooling passage to improve the thermal performance. The optical techniques, i.e., particle image velocimetry for flow field measurements and liquid crystal thermography for surface temperature measurements, were used to obtain reliable and accurate information about the flow and heat transfer features without disturbing the actual flow conditions. Transient experiments were conducted at a typical Reynolds number value, i.e., 42,500. The influence of the rib configurations and their relative spacing (P/e = 8, 10, and 12) were studied through the mapping of local- and spanwise-averaged enhanced heat transfer. Furthermore, the thermal performance parameters (overall Nusselt number augmentation, friction factor ratio, and thermohydraulic performance) are discussed in detail to select the best rib design. The truncated prismatic rib provided the best values of the thermal (27.91%), friction (32.75%), and thermohydraulic performance (45.16%) at P/e = 8, P/e = 12, and P/e = 8, respectively, when compared with square ribs. Moreover, the reasons for the variations in the heat transfer distribution inside the ribbed passages are elucidated through the flow structure information (mean velocities, mean streamlines, fluctuation statistics, mean vorticity, and turbulent intensity) at a relative spacing of 12. The heat transfer results are in harmony with the flow features, which helps in understanding the fundamental heat transfer mechanism.