Preliminary Design Investigations for the Selection of Optimum Reaction Degree for 1st Stage of a High Pressure Gas Turbine
Abstract: One-dimensional (1D) turbine design calculation phase requires a handful of input data and choice of design parameters to provide the blade flow path geometry along with the flow kinematics and thermodynamics properties at the blade mid-span. The choice of important aerodynamics design parameters namely reaction degree, nozzle guide vane NGV exit flow angle or flow coefficient and stage loading defines the mid-span flow velocity triangles. Despite being a very initial turbine design phase the implication of 1D calculation are such that the design choices made based on the design parameters at this design phase cannot be altered as the design proceeds towards detailed three-dimensional (3D) flow field analysis. Thus an optimum choice of the design parameters is essential for maximum turbine performance. There exist certain design recommendations for the selection of reaction degree, stage loading and flow coefficient for uncooled turbines. The rationale and underlying flow physics is straight forward for an uncooled case but a highly cooled case can benefit from a lower relative flow velocity. The aerodynamic design parameters have their own implications on the design of a cooled turbine, where the choice of reaction degree and flow coefficient has a strong impact on the stage design for a given stage loading. For a design of a cooled turbine, selection of a lower flow coefficient and lower reaction degree seems opportune from the heat transfer and the performance point of view. The flow coefficient has traditionally, in some cases, been set to a higher value on basis of the Smith charts which were originally devised for uncooled turbines. The reaction degree sets the relative rotor inlet temperature (hence cooling requirements) and should be carefully chosen for a high performance. However, presently there do not exist recommendations for the selection of optimum reaction degree for cooled turbine for given stage loading and NGV exit flow angle.This thesis work aims to contribute in developing the recommendations for the choice of optimum reaction degree for a cooled turbine. The goal is to determine the range of optimum values for reaction degree for given stage loading and NGV exit flow angles. A parametric study has been formulated to perform this goal. 1D meanline design tool (LUAXT) is used to implement different loss models. These models are validated using experimental results. The validation showed that Craig & Cox is the most accurate when tested against the test data obtained from two different stage geometries. A discussion on flow physics as represented in different loss models is presented to develop further understanding of loss physics. Craig & Cox loss model is further considered for the parametric design investigations using LUAXT 1D design tool to develop design recommendations for optimum reaction degree values.The performed design investigations indicated that a choice of low reaction value along with a low stage loading and a low flow coefficient reduces the overall stage coolant consumption and results in overall increased stage performance. Since for a HPT 1st stage, the interest lies in a high stage loading, a range of reaction degree has been recommended to be between 0.20 to 0.37 to provide the optimum stage design when chosen for stage loading in between 1.40 to 1.80 and the stator exit flow angle in range of 74o to 78o. A two-dimensional (2D) blade profiling and blade to blade flow field analysis is carried out for one of the recommended cases to verify the velocity triangles as obtained from meanline design. Small differences in the flow velocities were found mainly due to the difference in fluid properties and differences in throat calculations which can be resolved with 1D-2D design iterations. The profiling and the blade to blade flow field analysis for one of the recommended design justified it to have a reasonable cascade. The recommendations on optimum reaction degree for cooled turbine as obtained from the performed calculations can be used for future 1D design investigations of a high pressure cooled turbines.Keywords: 1D design, aerodynamic design parameters, flow kinematic, thermodynamics, rotor inlet temperature, cooled turbine, reaction degree, flow coefficient, stage loading
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