| Summary: | 博士 === 國立成功大學 === 航空太空工程學系碩博士班 === 96 === Raising the turbine entry temperature is a common method to increase the performance and power output of a gas turbine engine. However, the level of turbine entry temperature is much higher than the blade material can endure. Thus, it is necessary to guard the blade against the entering hot gas by guiding cooling air from the compressor into the turbine. The purpose of this research is to numerically simulate the flowfield and temperature distribution of gas turbine blades with film cooling, and then to investigate the influence of coolant quantity on the blade heat transfer. The finite element based finite volume method CFD software, CFX-TASCflow, was adopted as the numerical tool. To validate the numerical tool, thermal simulations in a 2D channel with symmetric ribs on the walls, a rotating square channel, and a turbine nozzle without film cooling were conducted, and the numerical results were in good agreement with the experimental measurements. In the thermal simulations of a rotating square channel, the models with round corners were also tested to reduce the pressure loss resulting from the separations at the sharp corners in the original model. Though it was successful in decreasing the pressure loss by replacing the sharp corners with the round corners, the wall heat transfer was also reduced at the turn and in the second pass with the modifications.
In order to improve the numerical prediction of heat transfer on a turbine blade with film cooling, the SST turbulence model was adopted to simulate the turbulent flow. The computational domain included the blade passage, the coolant holes, and the plenum. The test model was a turbine nozzle blade and two staggered rows of conical holes drilled on the suction side as the cooling holes. The results indicated that the averaged heat transfer coefficient distribution agreed with the experimental measurements satisfactorily at a blowing ratio of m=0.45. Noticeable discrepancies on are discovered near the rear hole at m=0.6 and m=1.0, even though the results agree with the measured data downstream away from the holes. The average heat transfer coefficient in the region near the holes is even overpredicted at m=1.0. The discrepancies resulted from the lift-off phenomenon which occurred in the heat transfer prediction at high blowing ratios, but not in the heat transfer measurements. It would be discovered at a higher blowing ratio than 1.0, but occurs too early at this relatively low blowing ratio in this computation.
In order to investigate the influence of the blowing ratio on the turbine blade heat transfer, the numerical predictions were performed in the range of a blowing ratio m=0.15-1.0. It was found that with increasing the blowing ratio, the average heat transfer coefficient decreased on the wall downstream away from the holes because the cooling air injected for wall protection from the ambient hot gas was increased. But the average heat transfer coefficient decreased first and then increased in the vicinity of the holes. In addition, a peak and a valley value also appeared in the average heat transfer coefficient distribution as m>0.45. After analyzing the thermal field in the vicinity of the holes, it was discovered that a pair of counter-rotating vortices formed while the cooing air was ejected out of the holes. The scales of the induced vortices became greater as the blowing ratio was increased. At m=1.0, the cooling air with high momentum lifts off the surface, and the ambient hot gas completely flows below the cooling air. This implies that an excess blowing ratio causes wall heat transfer to increase while poor cooling effectiveness near the film cooling holes is due to the lift-off phenomenon. This also means that increasing the blowing ratio is not always a good way to improve cooling at any given position on the suction side. Choosing an adequate blowing ratio can make the cooling air cool the turbine blades more effectively. In addition, the entrainment by the induced counter-rotating vortices also degrades the cooling effectiveness near the film cooling holes. Cooling air lift-off and hot gas entrainment by induced vortices result in high wall heat transfer and poor cooling effectiveness at high blowing ratios. Finding the cooling configuration to prevent cooling air lift-off and hot gas entrainment will be the focus for the future research.
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