Industrial aeroderivative gas generators used to drive pipeline compressors had experienced significant scrap rates of high pressure turbine (HPT) blades due to oxidation on the aftmost row of film cooling holes at mid-airfoil on the pressure side. The oxidation attack resulted in a significant loss of base material and effective thickness, and eventually became the life limiting factor and a major decision point in determining the engine overhaul period. A quick comparison of the aeroderivative HPT blade with its aero-version design showed that the cooling air entering each aeroderivative HPT blade is restricted by a metering plate at the blade root. Using the aero-engine design as a baseline, a compressible flow network analysis (CFNA) was conducted to identify the root cause of the oxidation and develop design upgrades to extend the aeroderivative HPT blade life.
In noncontact annular labyrinth seals used in turbomachinery, fluid prerotation in the direction of shaft rotation effectively increases fluid velocity in the circumferential direction and generates fluid forces with potential destabilizing effects to be exerted on the rotor. Swirl brakes are typically employed to reduce the fluid prerotation at the inlet of the seal. The inlet flow separates as it follows the swirl brakes, and the ratio between tangential component of the velocity at the seal, and the velocity of the rotor surface varies consequently. Effective swirl brakes can significantly suppress the destabilizing fluid forces as it is effectively reducing the tangential velocity. The literature shows that leakage rate can also be reduced by using swirl brakes with “negative-swirl.” In this study, a labyrinth seal with inlet swirl brakes is selected from the literature and considered the baseline design. The seal performance is evaluated using ANSYS-cfx. The design of experiments (DOEs) approach is used to investigate the effects of various design variables on the seal performance. The design space consists of the swirl brake's length, width, curvature at the ends, the tilt angle, as well as the number of swirl brakes in the circumferential direction. Simple random sampling method with Euclidean distances for the design matrix is used to generate the design points. Steady-state computational fluid dynamics simulations are then performed for each design point to analyze the performance of the swirl brakes. Quadratic polynomial fitting is used to evaluate the sensitivity of the average circumferential velocity with respect to the design variables, which gives a qualitative estimation for the performance of the swirl brakes. These results assist in creating a better understanding of which design variables are critical and more effective in reduction of the destabilizing forces acting on the rotor, and thus will support the swirl brake design for annular pressure seals.
Hole-pattern annular gas seals have been proven to be very effective in reducing leakage flow between high and low pressure sections in turbomachinery. This type of seal has two distinct flow regions: an annular jet-flow region between the rotor and stator, and cylindrical indentions in the stator that serve as cavities where flow recirculation occurs. As the working fluid enters the cavities and recirculates, its kinetic energy is reduced, resulting in a reduction of leakage flow rate through the seal. The geometry of the cylindrical cavities has a significant effect on the overall performance of a hole-pattern annular gas seal. Previous studies have been primarily focused on cylindrical cavities that are perpendicular to the axis of the seal and have indicated that the performance may be improved by varying the depths, spacing, and diameters of the cavities. However, to date the effects of elliptical cylinder cavities has yet to be investigated. In this study, the effects of elliptical shape hole pattern geometry on the leakage and dynamic response performance of an industry-relevant hole-pattern seal design are investigated using a combination of computational fluid dynamics (CFD), hybrid bulk flow/CFD analysis, and design of experiments techniques. A CFD model of the baseline hole-pattern seal was first developed and validated against experimental data. A design of experiments (DOE) study was then performed to investigate the effect that various elliptical shape cavities had on the leakage rate through the seal. CFD simulations were run for multiple geometry configurations of the cylindrical cavities to evaluate the seal performance at each of the design points. The design space was defined by varying the values of five geometrical characteristics: the major and minor radius of hole, the angle between the major axis and the axis of the seal, the spacing between holes along the seal axis, and hole spacing in the circumferential direction. Quadratic polynomial fitting was then used to analyze the sensitivity of different design variables with respect to the different outputs. This detailed analysis allowed for a greater understanding of the interaction effects from varying all of these design parameters together as opposed to studying them one variable at a time. Response maps generated from the calculated results demonstrate the effects of each design parameter on seal leakage as well as the relationships between the design parameters. The data from this analysis was also used to generate linear regression models that demonstrate how these parameters affect the leakage of the seal. The results of this study could aid in improving future designs of hole-pattern annular gas seals.
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