Experimental and numerical analysis of heat transfer and friction factors in ribbed rectangular convergent and divergent channels

This study experimentally and numerically investigates the heat-transfer and flow-resistance characteristics of ribbed rectangular tapered channels for turbine-blade internal cooling. Infrared thermography was employed to obtain full-surface temperature and Nusselt-number distributions on the one-side ribbed wall of convergent and divergent channels with 60° parallel ribs. Experiments were conducted at Reynolds numbers ranging from 13,845 to 41,535, and the measured data were used to validate steady Reynolds-averaged numerical simulations. Among the tested turbulence models, the V2F model showed the closest agreement with the measured local Nusselt-number distributions and friction factors. Based on the validated numerical method, the local heat-transfer distribution, friction factor, secondary-flow intensity, and overall thermal performance were compared among the convergent, divergent, and straight ribbed channels. The convergent channel produced a pronounced increase in local heat transfer from the middle region to the outlet because of flow acceleration and enhanced near-wall mixing, but it also caused the largest friction loss. Its friction factor was approximately 1.5 times that of the straight channel and about 4.2 times that of the divergent channel. In contrast, the divergent channel showed a more uniform heat-transfer distribution and a much lower friction factor owing to flow deceleration. As the Reynolds number increased, the influence of channel tapering on heat-transfer redistribution became weaker. Based on the adopted thermal-performance factor, the divergent channel achieved the best overall thermal-hydraulic performance among the three configurations, with η approximately 1.4-1.5. These findings provide useful guidance for the thermal-hydraulic design of ribbed tapered cooling passages.