Unsteady Aerodynamics of Low-Pressure Steam Turbines Operating Under Low Volume Flow

Abstract

Nonsynchronous excitation under low volume operation is a major risk to the mechanical integrity of last stage moving blades (LSMBs) in low-pressure (LP) steam turbines. These vibrations are often induced by a rotating aerodynamic instability similar to rotating stall in compressors. Currently extensive validation of new blade designs is required to clarify whether they are subjected to the risk of not admissible blade vibration. Such tests are usually performed at the end of a blade development project. If resonance occurs a costly redesign is required, which may also lead to a reduction of performance. It is therefore of great interest to be able to predict correctly the unsteady flow phenomena and their effects. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. 3D computational fluid dynamics (CFD) has been applied to simulate the unsteady flow in the air model turbine. It has been shown that the simulation reproduces well the characteristics of the phenomena observed in the tests. This methodology has been transferred to more realistic steam turbine multistage environment. The numerical results have been validated with measurement data from a multistage model LP steam turbine operated with steam. Measurement and numerical simulation show agreement with respect to the global flow field, the number of stall cells and the intensity of the rotating excitation mechanism. Furthermore, the air model turbine and model steam turbine numerical and measurement results are compared. It is demonstrated that the air model turbine is a suitable vehicle to investigate the unsteady effects found in a steam turbine.

On Turbulence and its Effects on Aerodynamics of Flow through Turbine Stages

“…Various aerodynamic effects, such as wake shedding at the trailing edge, secondary flows in radial direction, blade vibrations, flow leakage in axial gaps, shock waves and effects at trailing edge in transonic stages, angles of attack at the leading edges, etc., can seriously affect the rotor blades and their efficiency performance. Blades loading and forces, acting on blades, are significantly lower under aforementioned conditions, leading to less efficiency [7].…”

Investigation of Rotating Instability in the Last Stage of Low Pressure Turbine during Low volume flow Operation

“…However, a number of LSMB failures occurred in order to maximize the heat extraction, the mass flow through the LP turbine section is practically cut off, leading to an unacceptable heating-up of the turbine blades and also combined with high dynamic stresses in the turbine blades. Due to the highly complex nature of the flow field under ventilation most of the research in the past has been performed using either model turbines or measurement in power plants [1]. .…”

“…This region is sometimes called ventilation region because the rotor blade is swirling in the globally slow moving fluid with small pressure differences. Here the term Low Volume Flow (LVF) operation is used [1]. Such off design conditions may cause aero elastic problems mainly to the LSMBs and also putting them at a risk of damage due to windage .i.e.…”

Effects of a Single Blade Incidence Angle Offset on Adjacent Blades in a Linear Cascade

“…Blade flutter problems started to appear with increasing frequency in high-efficiency power-generating turbines with slender and lighter blades, which are more prone to suffering from flutter [2]. In addition, the increased requirements for the flexibile operation of power plants employing steam turbines lead to off-design operation regimes, which often result in an onset of blade flutter [3]. It was reported that although 90% of the potential flutter and high-cycle fatigue problems are covered during development testing, the remaining few problems account for nearly 30% of the total development cost and are responsible for over 25% of all engine distress events [4].…”