J. C. García

Numerical Analysis of the Blade Forces Caused by Wake/Blade Interaction in the Last Stage of a Steam Turbine

In a steam turbine stage there is an interaction between blades and the flow field. The blades are subjected to the forces caused by the flow field, but also the flow field is affected by the blades and its movement. The nozzle wakes cause uneven pressure field downstream and produce alternating forces on blades which lead to blade vibrations. Some of the vibrations originated in this way may damage the blades and affect the turbine performance. The results of numerical computations about the forces acting on the blades as a result of the variations in the flow field in the axial clearance rotor-stator in the last stage of a 110 MW steam turbine are presented. The analysis is focused on discussing the pressure field because it is necessary for further computation of the useful life time. The flow field was resolved using computational fluids dynamics and the computed pressure field was integrated around the blades to get the forces acting on blades. These computed dynamical forces will be used in the blade useful life estimation and in the investigation to the failure causes of these blades. The Navier-Stokes equations are resolved in two and three dimensions using a commercial program based on finite-volume method. 2-D and 3-D geometry models were built to represent the dimensional aspects of the last stage of the turbine. Periodic boundary conditions were applied to both sides of a periodic segment of the 2-D and 3-D models with the purpose of reducing computational efforts. The computations were conducted in steady state and transient conditions. The results show that the force magnitude acting on blades has an harmonic pattern. Finally a Fourier analysis was used to determine the coefficients and frequency of a Fourier equation which can be used to calculate the alternating stresses on the blade in order to predict the useful life of the blades. Also, the pressure and velocity fields are shown between the diaphragm and rotor blades along the axial clearance.Copyright

Failure of last-stage turbine blades in a geothermal power plant

This paper presents an investigation into causes of failure of geothermal steam turbine blades. Several L-0 blades of geothermal steam turbines of 110 MW capacity suffered failures, causing forced outages of the turbines. To assess the causes of failure, the natural frequencies of the blades installed on the rotor were measured in the laboratory. The measured frequencies were compared with the natural frequencies calculated through a finite-element analysis (FEA) of the blade. The FEA was also used to calculate the vibratory stresses on the blade numerically. Also, the investigation analyzed the operational data and the history of the blade failures on several rotors of different units from the same system. The results of previous repairs were reviewed, and metallurgical investigations were conducted to identify the mechanical and metallurgical modes of failure. The results of the investigation showed that the fracture of two blades was attributed to installation and manufacturing errors and aggravated by general deterioration of the blades. The deterioration was caused by the erosion and corrosion process that resulted from moisture condensation in the steam.

Prediction of Film Cooling Effectiveness on a Gas Turbine Blade Leading Edge Using ANN and CFD

Abstract In this work, the area-averaged film cooling effectiveness (AAFCE) on a gas turbine blade leading edge was predicted by employing an artificial neural network (ANN) using as input variables: hole diameter, injection angle, blowing ratio, hole and columns pitch. The database used to train the network was built using computational fluid dynamics (CFD) based on a two level full factorial design of experiments. The CFD numerical model was validated with an experimental rig, where a first stage blade of a gas turbine was represented by a cylindrical specimen. The ANN architecture was composed of three layers with four neurons in hidden layer and Levenberg-Marquardt was selected as ANN optimization algorithm. The AAFCE was successfully predicted by the ANN with a regression coefficient R2<0.99 and a root mean square error RMSE=0.0038. The ANN weight coefficients were used to estimate the relative importance of the input parameters. Blowing ratio was the most influential parameter with relative importance of 40.36 % followed by hole diameter. Additionally, by using the ANN model, the relationship between input parameters was analyzed.

A Computational Analysis of Multiphase Flow Cyclonic Separator for Clean Combustion in Power Plants

In this paper a numerical computation of the flow dynamics in a compact cyclonic separator (CCS) for multiphase mixtures is presented. The study is directed to power plants consumption requirements where fuel gas must be free of solid particulate. A finite volume approach has been employed with body-fitted coordinates in a 3-D solution of the CCS dynamics. The cylindrical geometry under study includes aspect ratios in the range 2.5<R<3.8 (where R = height/diameter). The CCS has three exits as follows: one on the top for gas; one on the bottom for low particle concentration liquid; and the last one tangentially located on the lower part of the CCS for high particle concentration liquid. The turbulence was resolved using a RNG model, while the interactions between each component of the flow were addressed using a mixture slip model. The three-phase liquid-gas-solid mixture considered was gasoil-propane-mineral coal particles with the composition in volume fraction of liquid to gas of 0.9 to 0.1 in addition to 109 kg/m3 of 40 microns coal particles as the disperse phase. The results indicate that reversible flow of liquid through the upper gas-outlet may be a function of the outlet pressure conditions. Also, velocity conditions of the income mixture flow at the inlet defined the residence time of the flow during the operation of the CCS, which affects the separation too. In this work density profiles are shown to indicate the regions of up flow for gas and liquid drag. The presence of a third phase in the form of solid particles affects the flow patterns in a CCS.Copyright

Film cooling optimization on leading edge gas turbine blade using differential evolution

This article reports the optimization of film cooling on a leading edge of a gas turbine blade model, with showerhead configuration, it is based on five input parameters, which are hole diameter, hole pitch, column holes pitch, injection angle, and velocity at plenum inlet. This optimization increased the Area-Averaged Film Cooling Effectiveness ( η Aav ) and reduced the consumption of coolant flow. Differential Evolution assisted by artificial neural networks was used as optimization algorithm. Reynolds Averaged Navier–Stokes computations were carried out to getting the net database and to evaluate the optimized models predicted by artificial neural network. The results show an effective increment of η Aav by 36% and a mass flow reduction by 66%. These results were reached by means of a better distribution of cooling flow at blade surface as function of the input parameters. To assure the reliability of the numerical model, particle image velocimetry technique was used for its validation.

Effect of Rotor Diameter on the Thermal Stresses of a Turbine Rotor Model

Abstract Thermal stresses in a simplified steam turbine rotor model during a cold startup are analyzed using finite element analysis (FEA). In order to validate the numerical model, an experimental array is developed in which a hollow cylinder is heated with hot air in the external surface. At the thick wall of the cylinder, temperature distribution is measured in real time, while at the same time an algorithm computes thermal stresses. Additional computational fluid dynamics (CFD) calculations are made to obtain magnitudes of velocity and pressure in order to compute convective heat transfer coefficient. The experimental results show good agreement with the FEA computations. To evaluate the effect of rotor diameter size, FEA computations with variation in external and internal diameters are performed. Results show that thermal stresses are proportional to rotor diameter size. Also, zones of higher stress concentration are found in the external and internal surfaces of the rotor.

Effect of the Modification of the Start-Up Sequence on the Thermal Stresses for a Microgas Turbine

Microgas turbines (MGT) are an alternative for small-scale energy production; however, their small size becomes a drawback since it enhances the heat transfer among their components. Moreover, heat transfer drives to temperature gradients which become higher during transient cycles like start-up. The influence of different start-up curves on temperature and thermal stresses of a microgas turbine was investigated. Stationary and rotational blades of the turbine were numerically simulated using CFD and FEM commercial codes. Conjugated heat transfer cases were solved for obtaining heat transfer from fluid toward the blades. Changes of temperature gradients within the blades during the start-ups were calculated under transient state with boundary conditions according to each curve to assess accurate thermal stresses calculations. Results showed that the modification of the start-up curves had an impact on the thermal stresses levels and on the time when highest stresses appeared on each component. Furthermore, zones highly stressed were located near the constraints of blades where thermal strains are restricted. It was also found that the curve that had a warming period at the beginning of the start-up allowed reducing the peaks of stresses making it more feasible and safer for the turbine start-up operation.

Computational fluid dynamics simulation and geometric design of hydraulic turbine draft tube

Any hydraulic reaction turbine is installed with a draft tube that impacts widely the entire turbine performance, on which its functions are as follows: drive the flux in appropriate manner after it releases its energy to the runner; recover the suction head by a suction effect; and improve the dynamic energy in the runner outlet. All these functions are strongly linked to the geometric definition of the draft tube. This article proposes a geometric parametrization and analysis of a Francis turbine draft tube. Based on the parametric definition, geometric changes in the draft tube are proposed and the turbine performance is modeled by computational fluid dynamics; the boundary conditions are set by measurements performed in a hydroelectric power plant. This modeling allows us to see the influence of the draft tube shape on the entire turbine performance. The numerical analysis is based on the steady-state solution of the turbine component flows for different guide vanes opening and multiple modified draft tubes. The computational fluid dynamics predictions are validated using hydroelectric plant measurements. The prediction of the turbine performance is successful and it is linked to the draft tube geometric features; therefore, it is possible to obtain a draft tube parameter value that results in a desired turbine performance.

Failure Analysis to Blades Of Steam Turbines At Normal Conditions Of Operations And Resonance

The need to provide quality services, accurate diagnosis, timely rehabilitation and improvement in materials and operation of the turbines, have given way to scientific research crack initiation and crack propagation, the impending fracture and estimating life of the turbine components at normal conditions and resonance. The L-1 stage blades suffer high alternating stresses, for its size, exposure to high temperatures, and mechanical loads under repetitive strain of cyclic load. The lasts stages of blades are also subjected to severe centrifugal loads stress that, when combined with the alternating stress, is responsible for fatigue failures. In this work, the last stage L-1 blades of a steam turbine under cyclic load or fatigue were analyzed, the first instance to observe and measure the initiation crack and propagation crack under normal conditions operation and conditions on resonance; making the comparison to estimate the life time of the blade at both conditions.

Numerical Simulation of Mixed Convection in a Rotating Cylindrical Cavity: Influence of Prandtl Number

A numerical study of the flow and heat transfer on a rotating cylindrical cavity solving the mass, momentum, and energy equations is presented in this work. The study describes the influence of the Prandtl number on flow in critical state on a cavity which contains a cooling fluid. Problem studied includes Prandtl numbers 0.001 < Pr > 15, aspect ratios 0.25 < A > 1, and Reynolds numbers 0.001 < Re > 600. Differential equations have been discretised using the finite differences method. The results show a tendency followed by heat transfer as the Reynolds number increases from 300 to 600; in addition, emphasis on the critical values of the Rayleigh number for small Prandtl numbers shows that thermal instability in mixed convection depends on the Prandtl number.