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4.3 Turbine Blade Aerodynamics the endwall where the quantities are affected the most. Location P1 is located along a pitchwise plane just downstream of the first row of holes and location P2 is located along a pitch plane just downstream of the 3rd and last row of holes. The coolant concentration is defined as the ratio of coolant density to the free-stream density67. The coolant concentration is the highest at the core of jet. Also, the coolant jets have forced vortex motions at the core68. Thus, the locations of the coolant jets in figure 36 can be readily identified at the locations of high turbulence intensity and coolant concentration. At location P1, ten distinct jets with high magnitudes from the ten holes upstream are clearly identifiable. The four jets near the suction side are attached to the endwall while the jets nearest the pressure side appear to be slightly lifted up from the endwall. Local blowing ratio is high near the suction side because the wall static pressure is low there and local blowing ratio is low near the pressure side because the wall static pressure is high there. Thus, the jets have higher momentum near the suction side than the jets near the pressure side. High momentum reduces mixing of the jets with the surrounding flow. Also, the suction side main flow has higher kinetic energy than the pressure side main flow. The high kinetic energy bends the suction side jets toward the endwall and high momentum aids the process. On the other hand, the low momentum jets near the pressure side easily penetrate the low energy main flow and lifts up from the endwall. At location P2 of figure 36, turbulence intensity Tu>11% near the endwall across the pitch is caused by the combined effects of all coolant jets as Tu is maximum 11% at this location without any coolant injection. Besides the signatures of three jets (Tu≥19%) from the last row of holes near the pressure side, no other jets are distinct at this location. The higher coolant concentration near the pressure side is the result of large number of jets near the pressure side compared to the number of jets near the suction side. In addition, some jets in the first and second rows are directly lifted away from the endwall by the up-wash flows of the pressure side leg vortex and suction side leg vortex. This action mixes the jets easily with the main stream and the coolant concentration from these jets reduces significantly. Some jets on the pressure side may also have been swept toward the middle of the passage by the cross flow and pressure side leg vortex. Thus, the coolant concentration is the highest near y/P=- 0.45 at location P2. Similar results about the locations of the coolant jets are reported in a linear vane passage69. The effectiveness of individual coolant jet is largely dependent upon its ability to stick persistently to the endwall to provide the maximum coverage. The location chosen for a coolant hole is therefore very important in this respect. Figure 37 provides evidence by how strongly the secondary flows deflect and block some coolant jets simply because of their location70. The coolant holes shown in figure 37 are arranged in four pitchwise rows at upstream of leading edge, 30% axial chord, 60% axial chord, and 90% axial chord. Four individual holes are also located at the pressure side of the blade passage. All the holes have same shape and geometry. The dark traces on the endwall are produced by the ejected coolant jets as they travel along the endwall. The length, level of darkness, and lateral spreading of the traces indicate the distance traveled by the jets, level of consistency of coolant, and lateral coverage by the jets, respectively, before they are mixed with the main fluid. Ammonia gas mixed with the coolant air stream reacts with the Diazo coating on the endwall and produces such traces71. Surface flow visualization as the coolant jets ejecting indicates the separation or lift-off lines of the pressure side leg/passage vortex and suction side leg vortex in figure 37. The five holes from the pressure side at 30% axial chord and the holes at the last two rows are located downstream of the lift-off line for the pressure side leg vortex. Friedrichs et al. (1996) shows that this line has moved downstream compared to that without coolant injection. The 4th and 5th Fig. 37. Visualization of surface flow and coolant jet trajectories along endwall in a linear blade passage at Minlet=1.0. Source: See Note 70. Fig. 38. Passage vortex and total pressure losses at exit flow with (Minlet=2.0) and without coolant injection in a linear blade passage. Source: See Note 65. 384PDF Image | Turbine Blade Aerodynamics
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