Flowfield measurements in a ribbed channel relevant to internal turbine blade cooling

The internal cooling of turbine blades is an important part of overall blade cooling that is necessary to survive current turbine inlet temperatures. This internal cooling is often performed by means of rib-turbulated passages designed to provide maximum heat transfer coefficients. One determining factor in the cooling effectiveness is the flow pattern created by the ribbed channel. This paper presents a comparison of the effect of two different channel blockage ratios on the fully developed flow pattern in a stationary ribbed channel. For the experimental investigation, a scaled-up ribbed passage was tested using a closed-loop channel facility. The facility included a passage consisting of a developing region, a fully developed region, a 180° bend, and a second developing region following the bend. Friction factors were determined through the use of pressure taps throughout the channel, and flow fields were measured using a two-component laser Doppler velocimeter. A majority of the testing performed was done under conditions of in-line (i.e. non-staggered) ribs with ratios of rib height to channel hydraulic diameter of 0.10 and 0.17. All LDV measurements were performed at a Reynolds number of 20,000. Results indicate that both blockage ratios create significant recirculation regions immediately before and immediately following the rib. While the smaller rib results indicate a reattachment downstream of the rib, the larger rib results indicate that the flow is separated along the entire streamwise distance between the ribs. The smaller blockage ratio also indicated a small recirculation region directly on top of the rib, whereas the larger blockage ratio showed no such region. Additionally, the increased friction factor created by the larger ribs can be attributed to the larger recirculation regions between ribs.