Numerical and Experimental Comparison of Forced Response of Free-Standing and Single-Connected Last Stage Blades

2017 ◽  
Author(s):  
Francesco Piraccini ◽  
Tim Rice ◽  
Jury Auciello ◽  
Alexey Evtushenko ◽  
Michael Mossom
Keyword(s):  
Steam Turbine ◽  
Initial Phase ◽  
Forced Response ◽  
Volume Flow ◽  
Experimental Comparison ◽  
Experimental Measurements ◽  
Aerodynamic Design ◽  
Free Standing ◽  
New Development ◽  
Last Stage

In this paper a comparison is made between numerical forced response simulations and experimental measurements for two popular Steam Turbine Last Stage Blade (LSB) architectures: a free standing LSB and a single-connected LSB with a mid-span wing (also known as a ‘snubber’). These variants share the same aerodynamic design in operation, i.e. they have the same ‘operating’ geometry. The focus of this study is the level of vibration response induced on the blades by resonance with a non-synchronous excitation. This can reduce the maximum condenser pressure safely attainable by a Steam Turbine during low volume flow (LVF) operation. The study develops a common method to effectively represent the LVF excitation in FEM harmonic analysis for both free standing and single-connected LSB’s. This is valuable to LSB designers during the initial phase of a new development (when an assessment of the LVF capability is required before experimental measurements have been taken). In addition, the method can be used to evaluate the suitability of an existing LSB design to a revised environment, as it is often the case in retrofit applications.

Forced Response Prediction for Steam Turbine Last Stage Blade Subject to Low Engine Order Excitation

2011 ◽  
Author(s):  
Bin Zhou ◽  
Amir Mujezinovic ◽  
Andrew Coleman ◽  
Wei Ning ◽  
Asif Ansari
Keyword(s):  
Steam Turbine ◽  
Low Pressure ◽  
Forced Response ◽  
Operating Conditions ◽  
Low Flow ◽  
Analytical Prediction ◽  
Unsteady Pressure ◽  
Engine Order ◽  
Last Stage ◽  
Cfd Analyses

Low Engine Order (LEO) excitations on a steam turbine Last Stage low-pressure (LP) Bucket (or Blade) (LSB) are largely the result of flow unsteadiness (e.g. flow circulation and reversal) due to low steam exit velocity (Vax) off the LSB at the off-design conditions. These excitations at low frequencies impose major constraints on LP bucket aeromechanical design. In this study, bucket forced response under typical LEO excitation was analytically predicted and correlated to experimental measurements. First, transient CFD analyses were performed at typical low flow, low Vax operating conditions that had been previously tested in a subscale low pressure turbine test rig. The unsteady pressure distribution on the bucket was derived from the transient CFD analyses at frequencies corresponding to the bucket’s modes of vibration. Subsequently, these computed unsteady pressure were mapped onto a LSB finite element model, and forced response analyses were performed to estimate the bucket dynamic response, i.e. the alternating stresses and strains. The analytically predicted bucket response was compared against measured data from airfoil mounted strain gages and good correlation was found between the analytical prediction and the test data. Despite uncertainty associated with various parameters such as damping and unsteady steam forcing etc., the developed methodology provides a viable approach for predicting bucket forced response and in turn High Cycle Fatigue (HCF) capability during early phases of steam turbine LSB design.

CFD Modelling of Steam Turbine Last Stage Blades at Low Load Using Multiple Mixing Plane Approach

2020 ◽  
Author(s):  
Antonio Mambro ◽  
Francesco Congiu ◽  
Enzo Galloni
Keyword(s):  
Steam Turbine ◽  
Flow Behavior ◽  
Volume Flow ◽  
Operating Conditions ◽  
Continuous Increase ◽  
Multiple Mixing ◽  
Wide Range ◽  
Low Load ◽  
Last Stage ◽  
Low Volume

Abstract The continuous increase of variable renewable energy and fuel cost requires steam turbine power plants to operate with high flexibility. This situation leads to steam turbines running at very low volume flow (LVF) for an extended time. Ventilation power and temperature predictions have a significant impact on the thermo-economic optimization of the power plant and lifetime assessment of the ventilating stages. In the last decade with increasing capabilities of CFD and computational resources, significant steps have been made in assessing complex flow behavior. Full size or scaled experimental testing of different last stage blades for a wide range of low load operating conditions is expensive, therefore CFD provides new opportunities in low load assessment. However, prediction of the flow structure of the ventilating stages still represents a challenge for the current CFD tools in terms of calculation time and reliability of the results. There are many different approaches in assessing this phenomenon, which require different computer resources and may not be necessary for most industrial applications. This paper presents the validation of the multiple mixing plane approach (MMP) presented by [9] for low-pressure steam turbine running at low load. Through a comparison with measurements results and more sophisticated methods, it is shown that this approach is able to sufficiently accurately predict the flow field and hence the ventilation power and temperature at low volume flow.

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

2014 ◽  
Author(s):  
Adam Beevers ◽  
Said Havakechian ◽  
Benjamin Megerle
Keyword(s):  
Steam Turbine ◽  
Temperature Increase ◽  
Low Pressure ◽  
Volume Flow ◽  
Flow Conditions ◽  
Steam Extraction ◽  
Last Stage ◽  
Low Volume ◽  
Critical Regions ◽  
Ventilation Conditions

During extreme low volume flow conditions, the last stages of a low pressure steam turbine operate in ventilation conditions that can cause a significant temperature increase of critical regions of the last stage moving blade. Under some conditions, the blade temperature may rise above a safe operating temperature, requiring the machine to be shut down. Limiting the heating effect on the last stage moving blade increases the allowable operating range of the low pressure turbine. One common method is to spray water droplets into the low pressure exhaust. As the length of last stage moving blades continues to increase, this method reaches its limit of practical operating effectiveness due to the amount of water required and its impact on the erosion of the LSB. An investigation into complimentary solutions to limit the temperature increase was conducted using CFD. An appropriate CFD setup was chosen from a sensitivity study on the effect of geometry, mesh density, turbulence model and time dependency. The CFD results were verified against steam turbine data from a test facility. The proposed complimentary solutions to limit the temperature increase include low temperature steam extraction, targeted for critical regions of the moving blade. From the test turbine and CFD results, the drivers of the temperature increase during ventilation conditions are identified and described.

Effect of Steam Injection on Last-Stage Blade Temperature of Low Pressure Steam Turbine at Low Volume Flow Condition

2017 ◽  
Author(s):  
Can Ma ◽  
Jun Wu ◽  
Yuansheng Lin
Keyword(s):  
Steam Turbine ◽  
Nuclear Power ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Steam Injection ◽  
Low Pressure ◽  
Volume Flow ◽  
Pressure Steam ◽  
Last Stage ◽  
Low Volume

In the nuclear power plant, the last stage of the low pressure steam turbine is characterized by long blades. These long blades operate under severe working conditions with wet steam flow and strong mechanical stress. At the start up and shut down operating condition where the volume flow is extremely low, the last stage blades operate in ventilation conditions where there is significant reverse flow in the exhaust and the last stage. In such a condition, the reverse flow would cause significant increase in the blade temperature. In addition, the rotating reverse flow would increase the vibration of the rotor blade. Such temperature increase and enhanced vibration can cause blade damage and force the machine to be shut down. In previous work, the steam injection in the last stage has been proposed as a promising method to decrease the reverse flow at low volume flow conditions, which reduces the stall cell size in the last stage blade. This work investigates the effect of the steam injection process on the blade temperature distribution by conducting three-dimensional flow simulations. Various steam injection configurations are compared in this work and the major consideration to be noted in the design process is discussed.

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

2015 ◽  
Vol 137 (10) ◽  
Author(s):  
Adam Beevers ◽  
Said Havakechian ◽  
Benjamin Megerle
Keyword(s):  
Steam Turbine ◽  
Temperature Increase ◽  
Low Pressure ◽  
Volume Flow ◽  
Flow Conditions ◽  
Steam Extraction ◽  
Last Stage ◽  
Low Volume ◽  
Critical Regions ◽  
Ventilation Conditions

During extreme low volume flow conditions, the last stages of a low pressure steam turbine operate in ventilation conditions that can cause a significant temperature increase of critical regions of the last stage moving blade (LSB). Under some conditions, the blade temperature may rise above a safe operating temperature, requiring the machine to be shut down. Limiting the heating effect on the LSB increases the allowable operating range of the low pressure turbine. One common method is to spray water droplets into the low pressure exhaust. As the length of LSBs continues to increase, this method reaches its limit of practical operating effectiveness due to the amount of water required and its impact on the erosion of the LSB. An investigation into complimentary solutions to limit the temperature increase was conducted using CFD. An appropriate CFD setup was chosen from a sensitivity study on the effects from geometry, mesh density, turbulence model, and time dependency. The CFD results were verified against steam turbine data from a scaled test facility. The proposed solutions include low temperature steam extraction, targeted for critical regions of the moving blade. From the test turbine and CFD results, the drivers of the temperature increase during ventilation conditions are identified and described.

scholarly journals Analysis on flow separation characteristics of last stage blade in steam turbine under small volume flow condition

2019 ◽  
Vol 23 (5 Part B) ◽  
pp. 3239-3250
Author(s):  
Lihua Cao ◽  
Wenlong Wang ◽  
Huanhuan Luo ◽  
Pengfei Hu ◽  
Zhengwen Li ◽  
...  
Keyword(s):  
Steam Turbine ◽  
Flow Separation ◽  
Flow Condition ◽  
Small Volume ◽  
Thermal Power ◽  
Peak Load ◽  
Volume Flow ◽  
Blade Height ◽  
Last Stage ◽  
Variation Rule

The inlet steam flow of steam turbine is obviously reduced and even in the state of small volume flow when the thermal power unit is involved in peak load operation, which greatly influences the safety of steam turbine. The last stage flow field of steam turbine under small volume flow condition is calculated by the CFX to study the formation, development, influencing range, and the variation rule of the flow separation vortex cores. The results show that the flow separation vortices appear near last stage blade at 30% of the rated volume flow. With the decrease of volume flow, the flow separation vortices gradually spread to the root of blade. When the volume flow decreases to 8% of the rated volume flow, the flow separation vortices almost occupy the whole blade. The position of the flow separation vortex cores shifts from 72.7% of the relative blade height to 49.1%. The results of this paper lay a foundation for the safety analysis of the last stage blade in the steam turbine under the small volume flow condition.

Practical Optimization of Mistuned Bladed Disk of Steam Turbine With Free-Standing Blade Structure for Forced and Self-Excited Vibration

2018 ◽  
Author(s):  
Yasutomo Kaneko ◽  
Kazushi Mori ◽  
Hiroharu Ooyama
Keyword(s):  
Steam Turbine ◽  
Forced Vibration ◽  
Optimization Method ◽  
Forced Response ◽  
Free Standing ◽  
Bladed Disks ◽  
Bladed Disk ◽  
Excited Vibration ◽  
Random Mistuning ◽  
Blade Flutter

Although bladed disks are nominally designed to be cyclically symmetric (tuned system), the vibration characteristics of all the blades on a disk are slightly different due to the manufacturing tolerance, deviations in the material properties, and wear during operation. These small variations break the cyclic symmetry and split the eigenvalue pairs. Bladed disks with small variations are referred to as a mistuned system. Many researchers suggest that while mistuning has an undesirable effect on the forced response, it has a beneficial (stabilizing) effect on blade flutter (the self-excited vibration). Therefore, it is necessary to optimize a bladed disk for forced vibration and blade flutter. In this study, firstly, the stability analysis of a mistuned bladed disk of a steam turbine that experienced the blade flutter in the field is carried out by use of the reduced order model, the Fundamental Mistuning Model. It is reported that the bladed disk analyzed failed due to unstalled flutter of the 1st mode, and the problem was solved by alternating mistuning. By comparing the analysis results with these field experiences, the analysis method is validated. Secondly, a parametric study on the mistuning effect is carried out for typical mistuning patterns, such as periodic and random mistuning, for both forced and self-excited vibrations. Finally, based on the above-mentioned results, a practical optimization method considering both forced vibration and self-excited vibration with respect to the bladed disk of a steam turbine with a free-standing blade structure is proposed.

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