Numerical and Experimental Investigation of the Aerodynamic Excitation of a Model Low-Pressure Steam Turbine Stage Operating Under Low Volume Flow

Abstract

The diversification of power generation methods within existing power networks has increased the requirement for operational flexibility of plants employing steam turbines. This has led to the situation where steam turbines may operate at very low volume flow conditions for extended periods of time. Under operating conditions where the volume flow through the last stage moving blades (LSMBs) of a low-pressure (LP) steam turbine falls below a certain limit, energy is returned to the working fluid rather than being extracted. This so-called “ventilation” phenomenon produces nonsynchronous aerodynamic excitation, which has the potential to lead to high dynamic blade loading. The aerodynamic excitation is often the result of a rotating phenomenon, with similarities to a rotating stall, which is well known in compressors. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. The analysis revealed that the rotating excitation mechanism observed in operating steam turbines is reproduced in the model turbine. A 3D computational fluid dynamics (CFD) method has been applied to simulate the unsteady flow in the air model turbine. The numerical model consists of the single stage modeled as a full annulus, along with the axial-radial diffuser. An unsteady CFD analysis has been performed with sufficient rotor revolutions to obtain globally periodic flow. The simulation reproduces the main characteristics of the phenomenon observed in the tests. The detailed insight into the dynamic flow field reveals information on the nature of the excitation mechanism. The calculations further indicate that the LSMB tip clearance flow has little or no effect on the characteristics of the mechanism for the case studied.

On the Prediction and Theory of the Temperature Increase of Low Pressure Last Stage Moving Blades During Low Volume Flow Conditions, and Limiting it Through Steam Extraction Methods

“…Due to the increased incidence on the LSB tip region, stall cells can occur in the tip region of the LSB, normally at the operating point at which the maximum pressure rise across the tip occurs. As described by Megerle et al [7], if both the number and the frequency of the rotating stall cells coincides with one of the blade mode shape's nodal diameters and its associated natural frequency, then large amplitude vibration occurs. Nonsynchronous excitation due to dynamic stall has also been reported in Refs.…”

“…If a direct study on the stall cells is required, then a full annulus transient calculation, using the SAS turbulence model should be used as described in Ref. [7].…”

A time and ensemble equivalent linearization method for nonlinear systems under combined harmonic and random excitation

“…Many engineering vibration problems are approximated as systems that are either harmonically or randomly excited. However, in reality many dynamic systems are subjected to a combination of broadband noise and harmonic excitation, for example floating crane systems, 1 turbine blades under turbulent flows, 2 or energy harvesting devices. 3 A variety of methods exist to predict vibration of structures under purely harmonic or random excitation.…”

Numerical analysis of a nonlinear double disc rotor-seal system

“…The effect of the fluid exciting force in a multi-stage centrifugal pump, shrouded turbine, steam turbine, and other rotating machinery that includes a multi-level sealing structure can be very significant (Yuan et al, 2007;Megerle et al, 2013). Research on the mechanisms of fluid-solid interaction and the influence of nonlinear seal force on rotor-seal systems has become a priority for the safe operation of rotating machinery.…”