Giorgio Zanazzi

Validation of Conjugate Heat Transfer Predictions on Labyrinth Seals and Novel Designs for Improved Component Lifetime

Cyclic lifetime assessment of steam turbine components has become increasingly important for several reasons. In the last years and decades the nominal steam temperatures and pressures were further increased to improve cycle efficiency. In addition, the market constantly demands increased flexibility and reliability for given lifetime exploiting the limits of the existing materials. A number of components in a steam turbine are critical in the focus of lifetime predictions such as the rotor and front stage blades, the inner casing and the area of labyrinth seals connected to the life steam. For this reason, it becomes extremely important to rely on accurate predictions of local temperatures and heat-transfer-coefficients of components in the steam path. The content of this paper aims on the validation of the numerical tools based on CHT (conjugate heat transfer) approach against experimental data of a labyrinth seal regarding discharge coefficients and measured heat transfer coefficients. Furthermore, a real steam turbine application has been optimized in design and operation to improve lifetime. The improved prediction of temperature and heat transfer allowed novel designs of labyrinth seals of a single flow high-pressure turbine and a combined intermediate and low-pressure turbine, which helped to strongly increase the component lifetime of a steam turbine rotor by more than 100%.Copyright

Experimental and Numerical Investigation Into the Aerodynamics of a Novel Steam Turbine Valve and its Field Application

Control valves are one of the key steam turbine components that guarantee operational safety in a power plant.There are two aerodynamic aspects, which are the current focus for the development of Alstom’s valves. One is the reduction of the aerodynamic loss to increase the efficiency of the power plant. The other is operational flexibility, which is increasingly demanded to react faster to load requirements from the electric grid. This is becoming more important as power generation becomes increasingly decentralized, with a growing contribution from renewable energy sources. For this reason, a fast control loop is required for valve operation, which depends on an accurate linearization of the valve characteristic.In this paper the flow fields in an existing steam control valve have been analysed and subsequently optimized using CFD techniques. The approach specifically designed for drilled strainers is further illustrated. Following the validation of the baseline design with model testing, an improved diffuser has been designed using CFD analysis and the resulting performance benefit has been confirmed with further testing.The required grid frequency support requires control valve throttling. For this reason, an accurate prediction of the linearization table is extremely important to support the required response time limits. Further numerical work has been carried out with various opening positions of the valve, leading to an improved valve linearization characteristic. It is demonstrated that the numerical prediction of the linearization curve agrees very well with data obtained from operating power plants.Copyright

Unsteady CFD Simulation of Control Valve in Throttling Conditions and Comparison With Experiments

The operational flexibility of steam power plant is becoming more important as power generation becomes increasingly decentralized, with a growing contribution from renewable energy sources. In a power plant the control valve is a key component to guarantee the control of the plant of which is increasingly demanded to extend the operational capability.At specific operating conditions, the control valve could experience vibrations. In this paper, the physical phenomena of the unsteady aerodynamic excitation force have been investigated by means of CFD techniques.An in-house code was used to simulate the flow-induced vibration. Unsteady transonic 3D simulation generally requires huge computational effort. A novel unsteady quasi-3D approach has been developed and applied as pre-design tool to establish the qualitatively operational map of the valve and to detect the critical operational range, to reduce the number of detailed 3D simulations.The numerical results are compared with experimental test undertaken in the Central Research Institute of Electric Power Industry [4] and full 3D simulation performed with the commercial tool CFX, using the Scale-Adaptive Simulation (SaS) turbulence model. Different pressure drops at certain lift have been selected from the operational map and reproduced numerically. Different modes have been identified, from stochastic behavior with wide width of frequency to periodic flow with one dominant frequency.Results indicate good agreement between the predicted frequency and amplitude and benchmark experiments. The quasi-3D simulation is able to reproduce the principle behavior of the flow field for different drop of pressure and capture the different operational mode. Similar behaviour has been detected also for the selected operating condition in the full 3D analysis.In addition, flutter calculation of the downstream pipe is carried out. It has demonstrated that the implementation of oscillating discharge piping influences the amplitudes and frequency of the upstream flow region.Copyright