Steady and Unsteady Flow Measurements Under Low Load Conditions in a Low Pressure Model Steam Turbine

2012 ◽  
Author(s):  
Kiyoshi Segawa ◽  
Shigeki Senoo ◽  
Hisashi Hamatake ◽  
Takeshi Kudo ◽  
Tateki Nakamura ◽  
...  
Keyword(s):  
Steam Turbine ◽  
Reverse Flow ◽  
Pressure Fluctuations ◽  
Flow Region ◽  
Outer Side ◽  
Flow Measurements ◽  
Unsteady Pressure ◽  
Large Pressure ◽  
Low Load ◽  
Load Conditions

Four-stage low pressure model steam turbine tests are carried out under the low load conditions of 0% to 20% load. In such low load conditions, the reverse flow is generated from turbine exit. Steady pressure measurements using multi-hole pneumatic probes are made to specify the outer boundary of the reverse flow region. The reverse flow regions are determined from the flow angles measured by the multi-hole pneumatic probes, traversing in the radial direction which rotates 360 deg around the longitudinal axis. The outer boundary of the reverse flow regions varies depending on turbine loads and has good agreement with the results of the numerical analyses. The pressure fluctuations are measured using unsteady pressure transducers installed on both the inner and outer side walls of the outlet stage and on the next-stage stationary blade surfaces to investigate the relation between pressure fluctuation and volumetric flow. It is found that the pressure fluctuations, which are defined by the standard deviation of unsteady pressure, become larger with decreased volumetric flow at the outer side as well as the inner side which is the same as the tendency seen for blade dynamic stress characteristics. The authors have previously reported good agreement between the experimental and numerical results. The unsteady pressure probe as another measurement technique is employed to investigate the spanwise pressure fluctuations at the outlet of the moving blade. The results show that as the load decreases, large pressure fluctuations are observed in the vicinity of the outer side after the stages where the reverse flow is observed. This is the same tendency as the results of wall pressure measurements. The generation of large pressure fluctuations, detected by the two different measurement techniques, might have a relationship with the effects of not only the vortex motion in the reverse flow region but also the overall flow field (including main forward flow) oscillated by the multiple vortex motions in the reverse flow region as seen in both experiments and computations. The large pressure fluctuations in the vicinity of the outer side after the blade lead to the increase of exciting force and vibration stress on moving blades. Detailed aerodynamic investigations of these part-load conditions are needed to analyze a blade excitation for further improvement of reliability and availability of steam turbines. The complicated flow structures at low load conditions in a steam turbine can be understood with the aid of both the steady and unsteady flow measurements and calculations.

Computations for Unsteady Compressible Flows in a Multistage Steam Turbine With Steam Properties at Low Load Operations

2011 ◽  
Vol 133 (10) ◽  
Author(s):  
Shigeki Senoo ◽  
Kiyoshi Segawa ◽  
Hisashi Hamatake ◽  
Takeshi Kudo ◽  
Tateki Nakamura ◽  
...  
Keyword(s):  
Thermodynamic Properties ◽  
Steam Turbine ◽  
Internal Energy ◽  
Compressible Flows ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Unsteady Pressure ◽  
Low Load ◽  
Pressure Steam ◽  
Load Conditions

A computational technique for multistage steam turbines, which can allow for thermodynamic properties of steam, is presented. Conventional three-dimensional multistage calculations for unsteady flows have two main problems. One is the long computation time and the other is how to include the thermodynamic properties of steam. Ideal gas is assumed in most computational techniques for compressible flows. To shorten the computational time, a quasi-three-dimensional flow calculation technique is developed. In the analysis, conservation laws for compressible fluid in axisymmetric cylindrical coordinates are solved using a finite volume method based on an approximate Riemann solver. Blade forces are calculated from the camber and lean angles of blades with momentum equations. The axisymmetric assumption and the blade force model enable the effective calculation for multistage flows, even when the flow is strongly unsteady under off-design conditions. To take into account steam properties including effects of the gas-liquid phase change and two-phase flow, a flux-splitting procedure of compressible flow is generalized for real fluid. Density and internal energy per unit volume are selected as independent thermodynamic variables. Pressure and temperature in a superheated region or wetness mass fraction in a wet region are calculated by using a steam table. To improve computational efficiency, a discretized steam table matrix is made in which the density and specific internal energy are independent variables. For accuracy and continuity of steam properties, the second order Taylor expansion and linear interpolation are introduced. The computed results of the last four-stage low-pressure steam turbine at low load conditions show that there is a reverse flow near the hub region of the last stage bucket and the flow concentrates in the tip region due to the centrifugal force. At a very low load condition, the reverse flow region extends to the former stages and the unsteadiness of flow gets larger due to many vortices. Four-stage low-pressure steam turbine tests are also carried out at low load. The radial distributions of flow direction downstream from each stage are measured by traversing pneumatic probes. Additionally, pressure transducers are installed in the side wall to measure unsteady pressure. The regions of reverse flow are compared between computations and experiments at different load conditions, and their agreement is good. Further, the computation can follow the trends of standard deviation of unsteady pressure on the wall to volumetric flow rate of experiments.

Aerodynamic Numerical Investigation of Low-Pressure Steam Turbine Last Stage Under Low Load Conditions With Mode Decomposition Methods

2019 ◽  
Author(s):  
Ping Hu ◽  
Tong Lin ◽  
Rui Yang ◽  
Xiaocheng Zhu ◽  
Zhaohui Du
Keyword(s):  
Steam Turbine ◽  
Frequency Spectrum ◽  
Low Frequency ◽  
Low Pressure ◽  
Unsteady Pressure ◽  
Mode Decomposition ◽  
Low Load ◽  
Last Stage ◽  
Operating Points ◽  
Load Conditions

Abstract It is common that steam turbine works at different operating points, especially under low load conditions, to cater to complex and varied demands for power generation recently. Considering the long and thin shape of last stage moving blades (LSMBs) in a low-pressure (LP) steam turbine, there are many challenges to design a suitable case which balances global efficiency against sufficient structure strength when suffering excitations at low load operating points. In present work, the aim is to extract specific aerodynamic excitations and recognize their distribution and propagation features. Firstly, steady 3D computational fluid dynamics (CFD) calculations are simulated at 25GV and 17GV (25% and 17% of design mass flow conditions) and corresponding unsteady calculations are performed with enough rotor revolutions to obtain integrated flow periodicities. Unsteady pressure signals near tip region of LSMBs are monitored circumferentially in both static and rotating coordinates. The fast Fourier transformation (FFT) results of unsteady pressure signals show that there are broadband humps with small disturbance amplitudes in low frequency spectrum at 25GV, however, a sharp spike is shown in low frequency spectrum at 17GV. Further, circumferential mode decomposition (CMD) method has been applied to distinguish different fluctuations in frequency and the mode numbers and circumferential propagating pace of which have been obtained. Finally, dynamic mode decomposition (DMD) method has been performed to describe detailed mode shapes of featured flow perturbances both in static and rotating coordinate system. These analyses indicate that at 25GV, a band of unsteady responses with very low amplitude was noted which has some features similar to rotating instability (RI). However, distribution and propagation features of flow unsteadiness at 17GV are in good agreement with rotating stall (RS) in compressor.

Unsteady Flow Features and Vibration Stresses of an Actual Size Steam Turbine Last Stage in Various Low Load Conditions

2013 ◽  
Author(s):  
Naoki Shibukawa ◽  
Yoshifumi Iwasaki ◽  
Mitsunori Watanabe
Keyword(s):  
Steam Turbine ◽  
Pressure Fluctuation ◽  
Load Condition ◽  
Test Facility ◽  
Stress Increase ◽  
Unsteady Pressure ◽  
Low Load ◽  
Vibration Stress ◽  
Last Stage ◽  
Load Conditions

Experimental investigations with a six stage real scale low pressure steam turbine operated at a very low load conditions are presented in this paper. Although the tested 35 inch last stage blades are circumferentially coupled at both tip and mid span with an intention to reduce the vibration stress, still its increase was observed at extremely low load condition. The pressure fluctuations were measured by several silicon diaphragm sensors which were mounted on both inner and outer casings of the stator inlet, exit and blade exit position. The measurement of the vibration stress was performed by strain gauges on several blades. The power spectra of unsteady pressures were precisely investigated considering both their location and steam flow condition. And the results implied that huge reverse flow and re-circulation started in the same location as a blade-to-blade CFD predicted. In terms of the correlation between vibration stress and the flow feature, the pressure fluctuation around the blade tip produces dominant effects on the vibration stress. The unsteady pressure frequency were also investigated and compared with those of the blade resonance and rotational speed. Basic trends observed in the results are similar to what other researchers reported, and on top of that, the continuous trends of pressure fluctuation and blade vibration stress were systematically investigated. Even the wall pressure, not the pressure on blade surface, showed the effective fluctuations which excited the several nodes of natural frequencies of the last stage blade. A series of FFT of fluid force by a full annulus quasi-steady CFD simulation seems to predict dominant mode of the excitation which account for the behavior of vibration stresses. The mechanism of the rapid stress increase was examined by considering CFD results and measured unsteady pressure data together. As the test facility takes a responsibility as an independent power producer, the tests were conducted in real plant operations which include multi stage effects, inlet distortions, Reynolds Number effect and so on. The obtained data and the particular indicator of vibration stress increase can be used as a part of design tool validation with neither aerodynamic nor mechanical corrections.

Computations for Unsteady Compressible Flows in a Multi-Stage Steam Turbine With Steam Properties at Low Load Operations

2010 ◽  
Author(s):  
Shigeki Senoo ◽  
Kiyoshi Segawa ◽  
Hisashi Hamatake ◽  
Takeshi Kudo ◽  
Tateki Nakamura ◽  
...  
Keyword(s):  
Steam Turbine ◽  
Compressible Flows ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Computation Time ◽  
Steam Turbines ◽  
Multi Stage ◽  
Low Load ◽  
Pressure Steam ◽  
Load Conditions

A computational technique for compressive fluid in multistage steam turbines which can allow for thermodynamic properties of steam is presented. The understanding and prediction of flow field not only at design conditions but also at off-design conditions are important for realizing high-performance and high-reliability steam turbines. Computational fluid dynamics is useful for estimations of flows. However, current three-dimensional multi-stage calculations for unsteady flows have two main problems. One is the long computation time and the other is how to include the thermodynamic properties of steam. Properties of the ideal gas, such as equations of state and enthalpy formula, are assumed in most computational techniques for compressible flows. In order to shorten the computation time, a quasi-three-dimensional flow calculation technique is developed. In the analysis, system equations of conservation laws for compressible fluid in axisymmetric cylindrical coordinates are solved by using a finite volume method based on an approximate Riemann solver. Blade forces are calculated from the camber and lean angles of blades using momentum equations. The axisymmetric assumption and the blade force model enable the effective calculation for multi-stage flows, even when the flow is strongly unsteady under off-design conditions. In order to take into account steam properties including effects of the gas-liquid phase change and two-phase flow, a flux-splitting procedure of compressible flow is generalized for real fluid. Density and internal energy per unit volume are selected as independent thermodynamic variables. Pressure and temperature in a superheated region or wetness mass fraction in a wet region are calculated by using a steam table. To improve computational efficiency, a discretized steam table matrix is made in which the density and specific internal energy are independent variables. For accuracy and continuity of steam properties, the second order Taylor expansion and linear interpolation are introduced. The computed results of last four-stage low-pressure steam turbine at low load conditions show that there is a reverse flow near the hub region of the last (fourth stage bucket and the flow concentrates in the tip region due to the centrifugal force. At a very low load condition, the reverse flow region extends to the former (i.e. the first to third) stages and the unsteadiness of flow gets larger due to many vortices. Four-stage low pressure steam turbine tests are also carried out at low load or even zero load. The radial distributions of flow direction downstream from each stage are measured by traversing pneumatic probes. Additionally pressure transducers are installed in the side wall to measure the unsteady pressure. The regions of reverse flow are compared between computations and experiments at different load conditions, and their agreement is good. Further, the computation can follow the trends of standard deviation of unsteady pressure on the wall to volumetric flow rate of experiments. The validity of the analysis method is verified.

Wavenumber-Frequency Spectrum Analysis of Pressure Fields Around an Automobile

2019 ◽  
Author(s):  
Xifeng Wang ◽  
Kenta Mizushiri ◽  
Hiroshi Yokoyama ◽  
Akiyoshi Iida
Keyword(s):  
Boundary Layer ◽  
Flow Field ◽  
Frequency Spectrum ◽  
Spectrum Analysis ◽  
Reverse Flow ◽  
Pressure Fluctuations ◽  
Flow Region ◽  
Vehicle Body ◽  
Frequency Spectrum Analysis ◽  
Pressure Fields

Abstract In order to evaluate the interior noise caused by the flow around automobiles, it is necessary to clarify the nature of the pressure fluctuations on the surface of vehicle body. The pressure fluctuations around the vehicle which are caused by the fluid motion can be solved by unsteady-compressible Navier-Stokes equation. However, the differences between the scales and intensity of the pressure fluctuations related to the hydrodynamic pressure fluctuation (HPF) of the flow field and the aerodynamic sound (acoustic pressure fluctuation APF) are quite large, these phenomena can be considered separately as two different phenomena. This assumption can help us to understand the contributions of these two components of pressure fluctuations to the structural vibration and interior sound of automobiles. Since both the HPF and the APF are pressure fluctuations, they cannot be separated only by measuring with a single pressure sensor. In this study, we divided these pressure fluctuations by using wavenumber-frequency spectrum analysis. Wind tunnel experiment showed that the HPF and the APF have different wavenumber fields in the wake of a rear-view mirror, and the intensity and wavenumber of the HPF are larger than that of the APF. Flow field was also investigated by using the incompressible flow simulation. As a result of wavenumber-frequency spectrum analysis based on the pressure fields around the vehicle body, the HPF and the APF have different wavenumbers in the case of a boundary layer flow field with no separation such as boundary layer on the vehicle roof. On the other hand, very small wavenumber components of the HPF were observed in the recirculation flow around the rear-view mirror downstream, despite incompressible simulation was done. This is probably due to the flow fields excite the vehicle body in the direction close to the vertical with respect to the vehicle body surface (side shield) in the separated flow region, and the wavenumber vector project on the shield surface apparently become smaller. The wavenumber vector becomes short but the frequency is constant, which leads the speed of pressure propagation apparently increases. In the reverse flow region, even if the uniform flow velocity is smaller than the speed of sound, the HPF may still contribute to vibration and sound generation. At the same time, since the flow velocity is actually slowed in the reverse flow region, large wavenumber components were also observed. Therefore, the wavenumber spectrum was observed in a wide range of the wavelength region. In conclusion, by investigating the wavenumber frequency spectrum, it is possible to estimate the flow field contributing to the interior noise of automobiles.

Blade Excitation by Broad-Band Pressure Fluctuations in a Centrifugal Compressor

1988 ◽  
Vol 110 (1) ◽  
pp. 129-137 ◽  
Author(s):  
U. Haupt ◽  
M. Rautenberg ◽  
A. N. Abdel-Hamid
Keyword(s):  
Centrifugal Compressor ◽  
Reverse Flow ◽  
Pressure Ratio ◽  
Pressure Fluctuations ◽  
Broad Band ◽  
Rotating Stall ◽  
High Mass ◽  
Blade Vibration ◽  
Flow Angle ◽  
Unsteady Pressure

The mechanism of blade excitation during the operation of a high-mass-flow, high-pressure-ratio centrifugal compressor has been investigated. This was carried out in the compressor operating range below 60 percent of design speed and in the zone of unsteady flow occurrence, where considerable blade vibration has been measured but no periodic unsteady pressure pattern such as rotating stall could be identified. Experiments conducted to study the mechanism of interactions between flow and blades were accomplished using several measuring methods simultaneously, such as measurements of blade vibration, flow angle at impeller inlet, unsteady pressure at different meridional and peripheral locations, as well as flow visualization by means of oil pattern. Analysis of the measurements showed typical broad-band characteristics of the unsteady pressure field and also for the blade vibration behavior. Results of flow angle investigations at the impeller inlet together with the analysis of oil pattern show that the broad-band pressure fluctuations and blade excitation can be attributed to a strong reverse flow near the suction side of the radial blade in the shroud zone. This reverse flow has its source downstream of the impeller and is extending back up to a location ahead of the impeller inlet. Similar results were obtained when the compressor was operated with vaneless and vaned diffuser configurations.

scholarly journals Research on Water Droplet Movement Characteristics in the Last Two Stages of Low-Pressure Cylinder of Steam Turbine Under Low Load Conditions

2021 ◽  
Vol 9 ◽  
Author(s):  
Shuangshuang Fan ◽  
Ying Wang ◽  
Kun Yao ◽  
Yi Fan ◽  
Jie Wan ◽  
...  
Keyword(s):  
Steam Turbine ◽  
Turbine Blades ◽  
Low Pressure ◽  
Water Droplets ◽  
Movement Characteristics ◽  
Low Load ◽  
Pressure Cylinder ◽  
Last Stage ◽  
Two Stages ◽  
Load Conditions

In the operating process of the coal-fired generation during flexible peaking regulation, the primary and secondary water droplets in the steam flowing through the last two stages of the low-pressure cylinder could influence the efficiency and safety of the steam turbine definitely. However, systematic analysis of the movement characteristics of water droplets under low-load conditions is scarcely in the existing research, especially the ultra-low load conditions below 30%. Toward this end, the more novel algebraic slip model and particle transport model mentioned in this paper are used to simulate the primary and secondary water droplets. Taking a 600 MW unit as a research object, the droplets motion characteristics of the last two stages were simulated within four load conditions, including 100, 50, 40, and 30% THA. The results show that the diameter of the primary water droplets is smaller, ranging from 0 to 1 µm, during the flexible peak regulation process of the steam turbine. The deposition is mainly located at the entire moving blades and the trailing edge of the last two stator blades. With the load decreasing, the deposition effect decreases sustainably. And the larger diameters of secondary water droplets range from 10 to 300 µm. The erosion of secondary water droplets in the last stage is more serious than that of the second last stage for different load conditions, and the erosion of the second last stage could be negligible. The pressure face and suction face at 30% blade height of the last stage blade have been eroded most seriously. The lower the load, the worse erosion from the secondary water droplets, which poses a potential threat to the fracture of the last stage blades of the steam turbine. This study provides a certain reference value for the optimal design of steam turbine blades under flexible peak regulation.

Investigation of Unsteady Pressure Fluctuations in a Simplified Steam Turbine Control Valve

2020 ◽  
Author(s):  
Christian Windemuth ◽  
Martin Lange ◽  
Ronald Mailach
Keyword(s):  
Steam Turbine ◽  
Power Plants ◽  
Pressure Fluctuations ◽  
Control Valve ◽  
Steam Turbines ◽  
Unsteady Forces ◽  
Part Load ◽  
Large Pressure ◽  
Head Surface ◽  
Flow Through

Abstract Steam turbines are among the most important systems in commercial and industrial power conversion. As the amount of renewable energies increases, power plants formerly operated at steady state base load are now experiencing increased times at part load conditions. Besides other methods, the use of control valves is a widely spread method for controlling the power output of a steam turbine. In difference to other throttling approaches, the control valve enables fast load gradients as the boiler can be operated at constant conditions and allows a quicker response on variable power requirements. At part load, a significant amount of energy is dissipated across the valve, as the total inlet pressure of the turbine is decreased across the valve. At these conditions, the flow through the valve becomes trans- and supersonic and large pressure fluctuations appear within the downstream part of the valve. As a result, unsteady forces are acting on the valve structure and vibrations can be triggered, leading to mechanical stresses and possible failures of the valve. Besides more complex valve geometries, a spherical valve shape is still often used in smaller and industrial steam turbines. Because of the smooth head contour, the flow is prone to remain attached to the head surface and interact with the flow coming from the opposite side. This behaviour is accompanied by flow instabilities and large pressure fluctuations, leading to unsteady forces and possible couplings with mechanical frequencies. The spherical valve shape was therefore chosen as the experimental test geometry for the investigation of the unsteady flow field and fluid-structure-interactions within a scaled steam turbine control valve. Using numerical methods, the test valve is investigated and the time dependent pressure distribution in the downstream diffuser is evaluated. The evolution of the flow stability will be discussed for different pressure ratios. Pressure signals retrieved from the control valve test rig will be used to compare the numerical results to experimental data.

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