Experimental investigation of internal cooling passages of gas turbine blade with pin-fins and rib-turbulators

Heat transfer and pressure characteristics in a rectangular channel are experimentally explored in detailed. The study consisted of 3 parts: 1) effects of detached pin space, 2) combined effects of detached pin space and ribs, and 3) effects of pin-fin geometry on heat transfer. The overall channel geometry (W=76.2 mm, E=25.4 mm) simulates an internal cooling passage of wide aspect ratio (3:1) in a gas turbine airfoil. With a given pin diameter, D=6.35 mm= ¼E, three different pin-fin height-to-diameter ratios, H/D = 4, 3, and 2, were examined. Each of these three cases corresponds to a specific pin array geometry of detachment spacing (C) between the pin-tip and one of the endwalls, i.e. C/D = 0, 1, 2, respectively. The Reynolds number, based on the hydraulic diameter of the un-obstructed cross-section and the mean bulk velocity, ranges from 10,000 to 25,000. The experiment employs a hybrid technique based on transient liquid crystal imaging to obtain distributions of the local heat transfer coefficient over all of the participating surfaces, including the endwalls and all the pin elements. Pressure drop of each test case is also measured in order to evaluate the performance of each case based on a non-dimensional parameter, performance index, PI. Experimental results reveal that the presence of a detached space between the pin-tip and the endwall have a significant effect on the convective heat transfer and pressure loss in the channel. The presence of pin-to-endwall spacing promotes wall-flow interaction, generates additional separated shear layers, and augments turbulent transport. In general, an increase in detached spacing, or C/D leads to lower heat transfer enhancement and pressure drop. Addition of broken ribs and full ribs has significant impact on heat transfer enhancement at the endwall only. Due to the geometry of the ribs, that is relatively low as compared to the overall height of the channel, the pressure loss seems to be insensitive to the presence of the ribs. Results showed that ribs underperform as compared to the cases without ribs. Triangular pin-fins with sharp edges have the advantages of generating additional wakes and vortices compared to circular and semi-circular pin-fins which contribute to higher heat transfer at the downstream region. However, heat transfer at the leading region of the triangular pin-fins are lower due to a more streamlined geometry at the leading region and without the presence of horseshoe vortices, that is one of the major contributing factors of heat transfer enhancement for circular and semi-circular pin-fins. Having the largest number of pin-fins and arranged in a dense configuration, the TRI3 case has the highest overall heat transfer enhancement ranging between 3.5-3.8, that is approximately 5%-20% higher than that of the circular pin-fin array. As the TRI1 and TRI2 cases show comparable heat transfer enhancement, this suggests that the heat transfer performance of the triangular pin-fin arrays is insensitive to the transverse spacing. In addition, more uniform heat transfer is also observed on the endwall and neighboring pin-fins in all triangular shaped pin-fin arrays. The semi-circular pin-fin array has the lowest heat transfer performance ranging from 2.7-3.4. However, triangular pin-fin arrays give the highest pressure loss due to the largest induced form drag among all cases, while circular pin-fin array exhibits the lowest pressure loss.