Experimental and Numerical Study of Flow and Dynamics on LSB at 34 MW Steam Turbine

Abstract

The paper deals with experimental research of the flow and dynamics of the blades in the last stage of a steam turbine with nominal output of 34 MW and a connected axial exhaust hood. The experiments were carried out on a turbine with relatively low inlet steam parameters “- 64 bars and 445 °C. It was possible to change the operating modes of the turbine during the course of measurement so that significant ventilation would be achieved in the last stage up to the point when aerodynamic throttling occurred in the last stage. In other words the turbine output varied from about 2 to 35 MW. The output of 2 MW was for the case of the island mode turbine operation. The experiments were carried out using static pressure taps and measurements of temperatures at the root and tip limiting wall. In addition to static pressure taps and temperature measurement, it was also possible to carry out probing by pneumatic probe with a diameter of 30 mm. Blade vibration monitoring sensors, so called last stage blade tip-timing, were also installed. The blade tip-timing acquisition hardware was used to monitor rotor blades tip amplitude. Due to the obtained experimental data, it was possible to verify the behaviour of the last stage and the connected exhaust hood for four measured variants. The courses of pressures and steam angles along the length of the LSB were determined. Furthermore, basic parameters of the last stage were determined, i.e. reactions of the stage, Mach and Reynolds numbers and values of pressure recovery coefficients. Based on experimental data the boundary conditions for CFD calculations were determined. Comparison of CFD calculations done for ventilation modes and for a nominal mode was also included. Another phenomenon which occurred during the probing of the flow parameters, particularly in ventilation modes, was the inability to determine parameters of steam due to low values of measured dynamic pressure in the vortex area at the root of the blade. The probe was able to detect dynamic pressure at the level of 50 Pa and more. In other words the transition point between backward and forward flows was identified. This limit point was used for further analysis of ventilation character of the steam flow depending on the ventilation coefficient c2x/u. where c2x is the average axial velocity at the LSB outlet, calculated from volumetric mass flow and u is LSB circumferential velocity calculated at LSB middle diameter. Due to the fact that it was also possible to measure vibration amplitudes of blades using the tip-timing method for a variety of modes, the relationships between pressure ratio over the tip and root of the last moving blade and vibration amplitude were also determined. This verified that the highest amplitude of blade tips occurred just when the compression of the medium on the blade tip was maximum, i.e. c2x/u = 0.05.

Numerical Investigation of Potential Flow Induced Vibrations of Steam Turbine Last Stage Rotor at Low Load Operation - Part 2: Rotating Instabilities Detection

“…The present work enabled the identification of a mechanism of instabilities linked to the hub separation vortex, alternative to the tip RI widely observed in literature. The findings of this study are also in line with numerical and experimental evidence proposed in the literature [7,9,24], which reported an increase of blade vibration related to the onset of the hub separation vortex in the diffuser.…”

“…The OPs have been selected with two different strategies: TEST A starts from the off-design point and decreases the condenser pressure up to reaching a safe condition within the stability range, while TEST B starts from the design condition (OP0) and gradually increases the condenser pressure up to unsafe operations from the stability range. It is worth highlighting how OP1 and OP4, and OP5 presents flow coefficients within the range of instability identified by both Megerle et al [18] and Hoznedl et al [24].…”

“…A bounded high-resolution scheme has been employed to calculate the advection fluxes of continuity, momentum and total energy equations, resulting in second-order accuracy. In order to reduce the computational costs, steam is considered as an ideal gas in light of findings of previous investigations, which demonstrated that during LVF conditions, the steam is superheated due to the ventilation effect [18,24].…”

“…To the author's knowledge, few previous studies are available in the literature concerning axial exhaust systems during LVF conditions, and this is the first time that they are being investigated with unsteady simulations. Recently Hoznedl et al [24] presented experimental and steady state numerical results of flow in the LSB of a steam turbine with an axial diffuser. Temperature and pressure measurements highlight how the steam at L1 inlet is superheated during LVF conditions.…”

Experimental steam turbine T10MW cold end cooling by water spraying

“…Due to the fact that ventilation regimes influence also the dynamic behaviour mainly of the last rotor blades, it is necessary to monitor the interaction of flow and dynamic load [5]. Similar results are rare, when experiments were performed on work (in the described case on 34 MW turbine in waste incinerator plant) [6]. Even in this case the flow parameters around the last stage are related to the amplitude of blade vibration defined by the BTT system (blade -tip -timing).…”