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Steam turbines belong to the most important systems in power conversion. In order to control their power output at steady state base load or part load conditions, the use of control valves is a widely used method. The flow in a pair of high-pressure steam turbine control valves which are situated in one valve chamber is analyzed in this paper from the pressure losses point of view. The experiments were carried out on a valve model installed in a wind tunnel for high speeds in the Aerodynamic laboratory of the Institute of Thermomechanics of the Czech Academy of Sciences. The flow at different valve cone lifts in both control valves was investigated and results were generalized in the form of so-called valve characteristics. Investigations were carried out on the model with control valve strainers and without them. In order to further understand the reasons of detected pressure loss differences, the numerical simulations were carried out in the Doosan Skoda Power Company using ANSYS software tools. The comparison of results shows which cases of the valve setup are preferable with regard to pressure losses and thus efficiency. Furthermore, an existing pressure loss prediction model for valve pressure loss estimation for each investigated case was updated. This model helps to better design the valve geometry in order to increase overall turbine efficiency.

This paper describes the effect of the incidence angle of the bucket on the performance of the axial turbine stage. The off-design operating conditions of the turbine stage result in a change in the velocity diagrams. This results in a positive or negative incidence. One of the basic objectives in turbine stage design is to estimate the flow behaviour of the turbine stage during off-design operation as accurately as possible and to quantify the corresponding decrease in efficiency. The available loss prediction models allow the estimation of the loss distribution when the incidence angle changes in the preliminary design phase. However, the incidence angle is actually affected by the velocity of circulation, which changes the input velocity vector (induced incidence). Thus, the blade row angle of attack is different from the angle based on the velocity diagram calculations. The result of our paper is a relation for estimating the angular difference of the input flow angle to the bucket defined as a function of the turbine operation mode.

Stall flutter of long blades influences the operation safety of the large steam turbines in off-design conditions. As angles of attack are typically high, a partial or complete separation of the flow from the blade surface occurs. The prediction of stall flutter of turbine blades is a crucial task in the design and development of modern turbomachinery units and reliable design tools are necessary. In this work, aerodynamic stability of a linear turbine blade cascade is tested experimentally at high angle of attack +15°, Ma = 0.2 and the reduced frequency of 0.38. Controlled flutter testing has been performed in a travelling wave mode approach for the torsion with the motion amplitude of 0.5°. In addition, ANSYS CFX with SST k-ω turbulent model is used for URANS simulations of a full-scale computational domain. A separation bubble formed on suction surface near the leading edge has been found in CFD results for each blade. Excellent agreement between the experimental and numerical results in stability maps has been achieved for this case under investigation. This is encouraging and both experimental and numerical techniques will be tested further.

A new design of an intercept valve assembly of the intermediate-pressure turbine part of greater power output is investigated in terms of pressure losses and flow fluctuations by using measurement on an experimental valve model. In addition, numerical simulations are used to further clarify measured phenomena. For such valve assemblies, it is important to exactly predict pressure losses and avoid danger of vibrations, which are caused by undesirable flow fluctuations, in order to guarantee valve’s efficiency and operational reliability. For this type of valve, it is especially important for turbine operations in partial loads (off-design conditions). Measurements were carried out in the Aerodynamic laboratory of the Institute of Thermomechanics of the Czech Academy of Sciences (IT) in a modular aerodynamic tunnel. Numerical simulations were carried out in the Doosan Skoda Power Company (DSP) by using a package of ANSYS software tools. The experimental valve model is a scaled model of a real valve assembly. It consists of an inlet pipeline, a stop valve and a control valve including its diffuser and outlet pipeline. Measured regimes were defined by a mass flow rate and a control valve cone lift which can be precisely changed. In order to investigate pressure loses, total and static pressures at valve characteristic locations were measured by using Prandtl probes and wall static pressure taps. In order to measure pressure fluctuations, Kulite fast response pressure transducers were used. They were situated near the valve throat where the flow fluctuations, which are strongly related to a flow separation, are the most visible and influential. Measurement results are compared with numerical results and locations with a flow separation were found. In order to reduce this phenomenon, different valve seat angles were also tested. As a result, a valve design could be optimized and, for a pressure loss prediction, a pressure loss model for this new intercept valve assembly could be created. Therefore, pressure losses in similar valve assemblies can be predicted with required accuracy for each new turbine where modern intercept valves are used. This helps to increase steam turbine efficiency and reduce fuel consumption. Based on pressure fluctuations results, operating conditions at which dangerous flow instabilities occur were identified. It was concluded that there is an operating condition border where the flow field starts to be unstable. As a result, the areas of safe and dangerous operating conditions can be predicted so that the operational reliability of the valve can be guaranteed.

Aerodynamic measurements and numerical simulations carried out on a model of a high-pressure valve assembly used for nozzle governing of a turbine with 135MW output are described in this paper. Aim of the study is to investigate effects of control valve’s strainers on pressure losses and unsteadiness in the flow field. It is an important task since undesirable flow fluctuations can lead to operational reliability issues. Measurements were carried out in the Aerodynamic laboratory of the Institute of Thermomechanics of the Czech Academy of Sciences (IT) where an aerodynamic tunnel is installed. Numerical simulations were carried out in the Doosan Skoda Power (DSP) Company using ANSYS software tools. The experimental model consists of one of two identical parts of the real valve assembly. It means it consists of an inlet pipeline, a stop valve, a valve chamber with two independent control valves, its diffusers and outlet pipelines. The numerical model consists of both assembly parts and includes also an A-wheel control stage in order to simulate the real turbine operating points. The different lifts of the main cone in each control valve for its useful combinations were investigated. Results were evaluated on the model with control valve’s strainers, which were historically used in order to stabilize the flow, and without them. The results of the experimental measurement were compared with the numerical results in the form of pressure losses prediction. From measured pressure fluctuations, it was found out where and for which conditions a danger of flow instabilities occurs. It can be concluded that there is a border, in terms of operating conditions, where the flow field starts to be unstable and this border is different dependent of the fact whether the control valve’s strainers are used or not. Therefore, the areas of safe and danger operational reliability can be predicted. The influence of the control valve’s strainers on the maximal amplitude of periodic fluctuations appears only for the cases when valves are highly overloaded. For normal operating conditions, there is no difference. As a result, the control valve’s strainers do not have to be used in standard applications of valve assemblies. Furthermore, a loss model for valve pressure loss estimation could be updated. Therefore, a pressure loss should be predicted with a sufficient accuracy for each new turbine bid with similar valve assemblies.

Valves are integral parts of modern steam turbines. They provide flexible operations and fast load changes which have become more often due to the increasing share of renewable energy. Along with this, the low pressure loss is required for guaranteed operation conditions. This paper is concerned with investigation of pressure losses in the compact valve, which is used as a valve for the intermediate-pressure turbine part, and its geometry modification to lower overall pressure losses. The investigation is based on results of numerical simulations which were carried out in the Doosan Skoda Power Company using a package of ANSYS software tools. Source of increased pressure losses in the compact valve is identified and a way of its reduction is proposed based on the comparison of the current results with data from a typical control valve model. Data analysis is provided in the paper.

For modern steam turbines with large operating range and enhanced efficiency an ultra-long last stage rotor blade in a low pressure turbine part for high backpressure and 50 Hz fixed speed operation have been developed. An advanced design approach was used to create the blade shape with a high thermodynamic efficiency, high natural frequencies and a very high safety factor of average radial static stress in the blade span. Furthermore, the tip section of the bucket was improved to decrease a static tension and the hub section was refined to simplify the assembly. Tip and hub airfoils were experimentally validated in a supersonic wind tunnel. An advanced in-house procedure using numerical analysis to predict a potential danger of unstalled flutter was carried out for wide range of operating conditions.

In order to eliminate occurrences of flutter of low pressure turbine blades it is necessary to understand the associated unsteady aerodynamics. For this reason, an experimental and numerical study of controlled flutter (travelling wave mode) in a linear turbine blade cascade oscillating in a torsional as well as translation motion is conducted. Unsteady aerodynamic forces and moments were measured on a subsonic eight-blade turbine cascade rig where central four blades are flexibly mounted each with two degrees of freedom. Thin blades in the cascade represent the tip section of the last stage rotor blades, which defines the turbine overall performance. A commercially available 3D CFD software ANSYS CFX is used to simulate the unsteady aerodynamic loading in the blade cascade. Experimental data and simulations are compared and influence of aerodynamic forces and moments on flutter is analysed.

Design of a new generation of the large power steam turbines leads to very long last rotor blades with tip sections operating often in supersonic region. The flow field in such tip sections is not fully explored and it is believed to be fairly complex and very sensitive even on minute changes of flow parameters. This stimulated the study of the performance of profiles suitable for rotor tip sections. Numerical simulations have been carried out using TU Prague’s code based on the solution of the RANS equations and SST turbulence model by implicit finite volume method with AUSMPW+ scheme in high resolution formulation. Experimental data have been gathered in the intermittent high-speed wind tunnel of IT CAS CR for 2D cascade measurements equipped by an adjustable supersonic inlet nozzle, perforated inserts at side walls and adjustable perforated tailboard.

Recent trends in the electric energy market such as biomass, waste incineration or combined cycle power plants require innovative solutions in steam turbine design. Variable operating conditions cause significant changes in flow field surrounding the steam turbine last stage blades. Therefore, the enlargement of operating range for last stage blades presents new challenges in design of turbine cascades. Several turbine cascades were designed and analyzed by commercial and in-house software of CTU Prague. Selected profiles were experimentally validated in the high-speed wind tunnel for 2D cascade measurements of the Institute of Thermomechanics of the Czech Academy of Sciences which is equipped by an adjustable supersonic inlet nozzle, perforated inserts at side walls and adjustable perforated tailboard. Comparisons are presented of numerical results with optical and pneumatic measurements for a wide range of inlet and outlet Mach numbers for optimized hub and tip profile cascades.