Flow structure and heat transfer in a square passage with offset mid-truncated ribs

The enhancement of heat transfer attributed from rib turbulators relative to the increased pressure drop in the channel is a crucial design parameter. Thus, the design of the truncated ribs (whose length is less than the passage width) provides options to address such cooling requirements when the pressure loss is a critical factor. Considering different types of truncated ribs, mid-truncated ribs (which are truncated in the central part of the rib) have been proved to show better thermal performance than other types of truncated ribs. A numerical study of mid-truncated ribs with different offset placements or angles on turbulent heat transfer inside a non-rotating cooling passage of a gas turbine blade is performed for inlet Reynolds number ranging from 10,000 to 50,000. The overall performance characteristics of six different types of mid-truncated ribbed arrangements are also compared: Cases A1, A2, A3 utilize mid-truncated spanwise ribs with different relative rib stagger positions at different spanwise locations; Cases B, C, D utilize mid-truncated angled ribs with the ribs oriented at ±45° (Case B), +45°/+45° (Case C), and ±135° (Case D). In all cases, a gap is present between the ribs placed on opposite spanwise parts of the channel, to provide the mid-truncation. It is found that the 135° mid-truncated rib (Case D) has the highest heat transfer coefficient, while the 90° mid-truncated ribs with no offset placement (Case A1) behave best in reducing pressure loss penalty. Although Case A shows larger heat transfer augmentation, Case D is advantageous for augmenting side-wall heat transfer when the pressure loss is considered and the Reynolds number is comparatively large. Case C exhibits the best overall thermal performance over the largest range of experimental conditions when the pressure drop is also considered. Staggered arrangement for 90° mid-truncated ribs can enhance heat transfer efficiently and makes a good overall performance at low Reynolds numbers. Case A1 can be used in practical operation because of reduced weight and good thermal performance at high Reynolds numbers. This is the first study on various offset mid-truncated ribs aiming to improve the heat transfer of turbine blade internal cooling passages with reduced pressure loss penalty.