The augmentation of film cooling effectiveness of gas turbine (GT) blades by incorporating novel techniques

Abstract

Modern gas turbine engines operate at significantly high temperatures to improve thermal efficiency and power output to a greater extent. The enhancement in rotor inlet temperature (RIT) increases the heat transfer rate to the turbine blades which requires sophisticated cooling schemes to maintain the blade temperature in acceptable levels. Therefore, this research refers to the numerical investigation of film cooling technique applied in gas turbines. The first part of this research deals with the implementation of symmetric airfoil deposition in 2D-flat plate computational domain to acknowledge its influence on the cooling performance of conventional air-film cooling techniques. Furthermore, water droplets were injected (mist injection) in cooling jet to concede the augmentation of local and average centerline film cooling effectiveness in downstream regions. This prediction of two-phase flow (continuous and discrete) was investigated by utilizing discrete phase model (DPM). The comprehensive investigation on variation of various ratios of density, mass flux, momentum flux and velocity and their influences on cooling effectiveness was also performed. Results demonstrated the significant enhancement of low temperature regions in downstream due to the inclusion of airfoil deposition and hence higher cooling effectiveness was achieved. Moreover, substantial increment in cooling effectiveness was achieved with a small amount of mist injection (2% mist) into the coolant jet. The evaporation of mist in downstream regions increased lower temperature regions and enhanced the cooling performance. Lastly, it was concluded that higher density ratio (DR=2.74) and moderate blowing ratio (BR=3.01) with the insertion of airfoil deposition and mist injection yield 13.6% higher average centerline film cooling effectiveness (𝜼̅) than conventional film cooling technique without the presence of mist injection and airfoil deposition. While the second part of this research aims at the investigation of cooling performance of three different shaped holes namely (Dome Forward (DF), Ginkgo iii | P a g e Forward (GF), and Ginkgo Reverse (GR)) in terms of centerline and local lateral effectiveness and establishment of comprehensive comparison of these novel shaped holes with the cooling performance of cylindrical (CY) shaped hole. In order to avail the complete possible view of the specific effects of varying operating conditions the investigations were performed at different density ratios (DR=1.2, 1.6 and 2.0), and blowing ratios (BR=0.78, 1.0, 1.5 and 2.0). At all the operating conditions, the results demonstrated significant augmentation in centerline and lateral effectiveness when GR shaped hole was employed followed by the GF, DF, and CY cooling holes. The increment of 92.6% in centerline effectiveness was found at x/D=20, DR=2.0 and BR=1.0, and 110.49% at x/D=30 when GR shaped hole was employed. For shaped cooling holes, the low velocity gradient through the film alleviated the jet lift off and turbulence intensity resulting in a decreased entrainment of hot gas to bottom surface. To conclude, the prominent lateral coverage due to the shaped cooling holes significantly enhanced the thermal protection and overall cooling performance.