Abstract

In designing steam turbine airfoil, a continuous slope (second derivative) is desirable to prevent local flow circulation and a minimum curvature is necessary to decrease turbulence. To meet this requirement, the design program uses fifth and sixth order. Berstein polynomials to define for both the pressure-side and the suction-side profile curves. Two sets of data are required to the design program. The first set of data is relevant to the geometric parameters that include radius of nose and tail, inlet and exit metal angles, length and orientation of chord, throat and pitch, bend angle, and wedge angle. The second set of data consists of five control variables to determine the suction-side profile and three control variables to determine the pressure-side profile. The program was successfully applied to modify an existing airfoil. Geometric parameters were obtained by direct measurement. Trial and error procedures were employed to determine the value of those eight control variables so that the airfoil can be curve-fitted by running the design program. The airfoil reproduced by the design program were continuous smooth, which were utilized in a subsequent computational fluid dynamics performance analysis. A close examination of reproduced airfoil reveals that the curve-fitting result is satisfactory as it eliminates certain discontinuities in the measured data. A more efficient airfoil could then be made by adjusting geometrical parameters and control variables. Results obtained by using this program indicate that it can not only effectively optimize airfoil shape to achieve better aerodynamic performance but also preserve original design intents. Through-flow calculations were also made to determine an optimum flow path of the entire turbine. A viscous flow program was used to aerodynamically analyze the original and the new designs.

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