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Networking and blade mechanics.

When 100 experts have a chance to exchange ideas on a highly focused topic, the result is a very successful seminar. The 2011 program of the 16th annual Blade Mechanics Seminar [1] gathered over 100 engineers and scientists to discuss various HCF (High Cycle Fatigue) issues of aero and industrial turbine and compressor airfoils. A keynote speech on the "Impact of Rotordynamics on Blade Vibrations" opened the seminar and explained the state of the art of rotor-blade interactions. This lecture also pointed out future challenges for rotor train and blade dynamics due to anticipated instabilities in an electrical network powered by a combination of steam and gas turbines as well as solar- and wind-farms. Afterwards the 11 presenters demonstrated different HCF problems and their mitigations based on either the traditional redesign approaches or advanced solutions, such as frictional damping technology or intentional blade mistuning.

The HCF problems of turbine blades are most likely as old as the first design of Parson's steam turbine in 1884. Based on a five-year investigation of 227 GE steam turbines with ratings above 5 MW, Wilfred Campbell (1924) created the first resonance criteria for bladed discs. At that time, disc assemblies were analyzed with Rayleigh's or Dunkerley-Southwell's approach, simple analytical methods that either overestimated or underestimated the real blade frequency, respectively. Therefore, the resonance frequencies of the analytically designed freestanding blades were often measured in the spin pit. To avoid those frequencies, the redesign strategies of a M-mass reduction or a K-stiffness increase of the vibrating blade (Fig. 1) were generally applied.

The blade mass was usually decreased by slightly cutting the airfoil tip. However, this approach generated bigger radial clearances between the blade tip and casing, which downgraded the stage performance. In the 70s and 80s, ID perturbation methods were developed; they removed thin layers of material from the suction and pressure sides of the airfoil.

A commonly used method for increasing stiffness was a lacing wire, which was circumferentially threaded through a hole of every airfoil. Sometimes as many as three lacing wires could be found in old steam turbine stages. Another method, applied to avoid large diameter wires, was staggered zigzag pins. However, each blade required two holes, which reduced the airfoil section and could generate significant notch stresses. Therefore, winglets were later employed on the longer blades, or cover bands at the airfoil tip. These bands were attached to the blade tip by riveting. To avoid problems of rivet creep or accelerated corrosion at the interface, blades with integrally machined shrouds are employed today. Shrouded blades enclose the flow and improve the aerodynamic efficiency of the stage better than a lacing wire. But even today, lacing wires and damping pins are applied to engines operating with variable rotational speed, such as turbochargers or industrial steam turbines.

Nowadays HCF blade analysis is still based on the Campbell diagram (Fig. 1). Taking advantage of the continual increase in computational capabilities, analysis has advanced from the Transfer Matrix Approach used in the 60's and 70's through the 3D Finite Element Method, which appeared during the 80's and on to the latest methods, such as the Modal Synthesis approach, which includes frictional sliding on the blade interfaces. For that reason, the new strategies of D-damping leverage or/and Ddetuning (Fig. 2) are of importance in the modern dynamic analysis of turbine and compressor blades.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Friction damping technology has already gathered 30 years of design experience (Szwedowicz, 2010) and can effectively reduce the resonance amplitude of interest by using under-platform dampers or by optimizing the shroud coupling. The most recent detuning technology for the disc assembly is based on intentional mistuning of every blade with respect to the critical engine order (see EO in Fig. 1). This design strategy breaks up the excitability criterion of the disc assembly expressed with EO = n N [+ or -] j, where j = {0, 1, 2 ...., J*} with j* = N/2 or J*=(N-1)/2 for an odd or even number N of blades in the stage, respectively. In other words, the intentional blade mistuning disorders the n-th nodal diameter mode shape of the vibrating disc assembly. Then, instead of the major resonance response of the tuned disc assembly (Fig. 1), individual resonances of the mistuned blades are excited; these superimposed responses result in smaller resonance amplitudes, as illustrated in Figure 2. This technique is currently used for blisks whose vibrations depend mainly on the airfoil geometry and are not influenced by the manufacturing tolerances of the blade assembly. The successful application of this method requires sophisticated simulation methods to confirm that other resonances of the mistuned disc assembly are not amplifted, as they would be in cases of uncontrolled mistuning (Whitehead, 1988).

Besides discussion on these topics, the participants also visited an exhibition organized by 8 seminar sponsors. Everybody was able to examine real turbine vanes representing different phases of the reconditioning process used for hot gas turbine components.

References

Campbel L., W., 1924, "The Protection of Steam Turbine Disk Wheels From Axial Vibrations", General Electric Review, Part I, pp. 352 - 360, Vol. XXVII, No 6, June; Part II, pp. 459 - 484, Vol. XXVII, No 7, July; Part III, pp. 511 - 504, Vol. XXVII, No 8, Aug.

Szwedowicz, J., 2010, "30-Year Anniversary of Friction Damper Technology in Turbine Blades", Global Gas Turbine News [A Supplement to Mechanical Engineering Magazine], Vol. 50, No. 1, ASME International. Gas Turbine Institute, Atlanta, USA, pp. 6-7, Feb.

Whitehead, D. S., 1988, "The Maximum Factor by Which Forced Vibration of Blades Can Increase Due to Mistuning", ASME Journal of Engineering For Gas Turbines and Power, Vol.. 120, pp. 115-119.

Bunker, S. R., 2011, "View From The Chair," Global Gas Turbine News (A Supplement to Mechanical Engineering Magazine), Vol.. 51, No. 2, ASME International Gas Turbine Institute, Atlanta, USA, pp. 2, April

By Dr. Jaroslaw Szwedowicz, Program Manager Technology and Methods, Alstom Power, Switzertand I Associate Editor, ASME Journal of Engineering for Gas Turbines and Power | Chair of IGTI "Structures & Dynamics" Committee, ASME Swiss Section
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Author:Szwedowicz, Jaroslaw
Publication:Mechanical Engineering-CIME
Geographic Code:1USA
Date:Dec 1, 2011
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