Detailed measurements of heat transfer coefficients in a 180-deg rectangular turned channel with the perforation divider

This work measured the heat transfer coefficients of air flow through the 180-deg rectangular turned channel by using the transient liquid crystal method. The cross section of the channel was square. The divider of the 180-deg rectangular turned channel had several perforations with equal interval, then the fluid in the first duct can early flow into the second duct through the perforations to adjust the local heat transfer. Variable parameters were the Reynolds number (Re = 2468–12,340), the ratio of the perforation’s diameter to the channel hydraulic diameter (d/Dh = 1/4 and 2/4), the angle of perforations (θ = −45°, 0° and +45°), and the number of perforations (n = 1, 2 and 3). The results indicated that, at the turn and the neighborhood, the effects of the flow inertia and impingement induced the peak values of heat transfer on the bottom and outer side surfaces. Excluding the heat transfer amount at the inner side surface and the perforations of the divider, the non-perforation configuration had the best total heat-transfer performance image among all the test configurations. The image declined when the number or size of perforations increased; while the angle of perforations was insensitive to image. Besides, the distribution of the Nusselt number (Nu) was strongly influenced by the angle of perforations (θ) as d/Dh = 2/4. Different from others configurations, the one with (d/Dh = 2/4 and θ = +45°) grew the Nu along the main flow direction in the second duct due to the bigger amount of bypass fluid from perforations, greatly improving the heat transfer at the downstream of the second duct. It demonstrated that the perforations of the divider did adjust the spatial heat transfer of the 180-deg rectangular turned channel.