PSI - Issue 17

Shahnawaz Ahmad et al. / Procedia Structural Integrity 17 (2019) 758–765 Shahnawaz Ahmad/ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

The gas turbine is a complex unit of the power generation machinery of an aero engine. The turbine module of a gas turbine, experiences very high temperature-pressure conditions, resulting in high stresses throughout the run of the engine which in turn damages the components and thus limits the life of the engine. The gas turbine components are subjected to some of the harshest conditions inside the engine. The turbine blades are prone to damage and crack formation as they experience extreme pressures and temperatures resulting in high stress. Various aspects of the problem of blade vibrations have been subjects of extensive research, and the overall review of the problem has been done by Rao, Srinivasan and Rieger[1]. The transient vibration analysis using strain life approach following the methodology mentioned below (Figure 1) has been undertaken by Irretier and Vyas [2]. The turbine blades are prone to high cycle fatigue failures, and these failures are primarily due to resonance that occurs at the blade critical speeds during startup and shutdown conditions. As the blade passes through critical speeds, even if it does not fail catastrophically, it accumulates fatigue. There are several theories that estimate the damage in the structure as it passes through resonance and these theories prove to be significant in studying transient loading on the structures. Rao, Pathak and Chawla [3], compared various cumulative damage theories of predicting blade life. Since a crack is a discontinuity, its presence alters the stiffness and consequently the natural frequency of the component. W. M. Ostachowicz and M. Krawczuk[4] studied the effect of cracks on natural frequencies of the cantilever beam. Ming-Chaun Wu et. al.[5]examined the effect of crack on rotating cantilever beam. We adopted the FEA approach to study the effect of crack on the natural frequencies of the blade and performed sweep tests on an LDV setup to validate the analytical results. Presence of discontinuity may also lead to change in the damping parameters. An algorithm is also developed with the help of FEA software and Lazan ’ s law to compute the modal damping and explore if cracks influence modal damping in any significant manner.

2. Life Estimation

The methodology adopted in this study for life prediction is shown in the flowchart (Figure 1)

Figure 1 Methodology for estimating life of a turbine blade

2.1. Aerodynamic Load Assessment on the Turbine Blade Turbine blades are exposed to relatively high levels of aerodynamic and centrifugal forces. The steady and periodic aerodynamic loads due to steady and unsteady components of gas, coupled with steady centrifugal forces due to rotation create distributed loads along the blades. These distributed loads have steady and periodic components. The gas dynamic forces on the blades are periodic with the instantaneous nozzle passing frequency, s v n  = where, s n is the number of nozzles, and  is the rotational frequency of the bladed disc. For determining this periodically varying force for the bladed disc, a coupled analysis was carried out on the ANSYS 14.5 workbench. The flow through two nozzles was modelled in ANSYS fluent under appropriate boundary conditions. The flow model involves the blade moving through a control volume between two nozzle inlets (Figure 2) and the pressure variation on the blade surface is approximated by a FLUENT analysis in ANSYS 14.5 environment. The flow was taken as glazing to the leading edge, Figure 3. The pressure variation is periodic as the blade passes through all the nozzles, one by one, experiencing the same pressure variation. The pressure distribution in the control volume around the blade profile is shown in Figure 3. The variation of maximum pressure on blade surface is shown