Flow directing element for the turbine of a rotary machine and method of operation therefor

A flow directing element for the turbine of a rotary machine having an airfoil which includes leading edge and a trailing edge which extend spanwisely and a convex side surface and a concave side surface which extend chordwisely from the leading edge to the trailing edge, the improvement which comprises:

• a convex side surface having a critical roughness characteristic, and a roughened surface having a surface roughness that is greater than 60 AA microinches, that is within twenty percent of the critical roughness for the airfoil and that is spaced chordwisely from the leading edge and the trailing edge;
• wherein the roughened convex side surface decreases aerodynamic flow losses and the spacing of the convex side surface with respect to the leading edge and trailing edge avoids the decrease in fatigue life of the element which is associated with like flow directing elements having increased roughness in the suction side surface and increased roughness in the leading edge and trailing edge region;
• wherein the flow directions element has a leading edge region formed by an elliptical surface and a trailing edge region which is formed by a circular surface and wherein the suction surface has a surface tangent to the leading edge region on the suction side and tangent to the trailing edge region on the suction side and wherein the roughened surface of the suction side surface lies between the leading edge region and the trailing edge region and does not extend into these regions of the airfoils;
• wherein the flow directing element has an end having an end wall which adapts the elements to bound a flowpath for working medium gases, and wherein the roughened surface is spaced spanwisely from the end wall.

A method of operating a gas turbine engine installed in an aircraft, the aircraft having a sea level take off operative condition and a cruise operative condition, the gas turbine engine having a turbine and an array of airfoils disposed therein, each airfoil having a suction side surface and a pressure side surface, comprising:

• flowing working medium gases through the array of airfoils at a first Reynolds number at a sea level take off operative condition and flowing working medium gases at a second Reynolds number which is less than the first Reynolds number at the cruise operative condition of the aircraft;
• flowing the working medium gases over the suction side surface and pressure side surface of the airfoil, the suction side surface of the airfoil being roughened such that the surface roughness is within twenty percent of the critical roughness characteristic of the airfoil for the cruise operative condition;
• wherein the roughened suction side surface increases aerodynamic flow losses at the cruise operative condition and wherein the spacing of the roughened suction side surface and roughness of the suction side surface with respect to the leading edge and the trailing edge avoids the decrease in fatigue life of the flow directing element which is associated with like flow directing elements having increased roughness over the suction side surface at the leading edge and trailing edge of the airfoil.