G. Urquiza

Inverse Heat Transfer Using Levenberg-Marquardt and Particle Swarm Optimization Methods for Heat Source Estimation

The present work uses the Levenberg-Marquardt Method (LMM) and a Particle Swarm Optimization (PSO) for estimating the heat generation function for a Guarded Hot-Plate Apparatus (GHPA). This device is used for thermal conductivity determination of insulating materials. The problem is one-dimensional in cylindrical coordinates. Geometries are a disc (Hot-Plate) and an annulus (Guard). A heat generation function is estimated considering one to five parameters. Capability of each method for recovering the analytical function is tested. Results are satisfactory for this kind of problem.

Numerical analysis of erosion of the rotor labyrinth seal in a geothermal turbine

Excessive erosion of the labyrinth seal of a 110 MW geothermal turbine has been investigated. This study used computational fluid dynamics (CFD) and aims to identify one cause of erosion and a possible solution for substantially reducing it. The predictions were based upon a numerical calculation using a CFD model of the labyrinth seal with a water/steam flow containing hard solid particles and solved with a commercial CFD code: Fluent V5.0. The results confirmed the existence of flow conditions that play a major role in the rotor labyrinth seal erosion. Afterwards, the flow path was simulated with changes of rotor labyrinth seal geometry, which are indeed feasible of being implemented. The results confirmed that it is possible to reduce the erosion process by approximately 80% by incorporating a steam flow deflector in the fourth stage diaphragm, which changes the steam flow direction in the inlet zone to the rotor labyrinth seal channel, resulting in a reduction in steam volumetric mass flow and hard particle velocity by about 44%.

Artificial Neural Networks for Inverse Heat Transfer Problems

We present a solution for an inverse heat transfer problem involving internal heat generation using artificial neural networks. The problem involves a heat conduction problem with internal heat source in cylindrical coordinates. The network is a feedforward with backpropagation algorithm. We compare the results with the Levenberg-Marquardt method and discuss advantages and disadvantages. The two methods recover very well the optimum parameters.

Special Instrumentation and Hydraulic Turbine Flow Measurements Using a Pressure-Time Method

The objective of the work was to evaluate the efficiency of a hydraulic turbine by means of the flow measurement, for a given water head. The hydraulic turbine of 180 MW output has been in service for 20 years. The real value of efficiency was needed in order to proceed with minor/mayor modifications to improve it. In a case of a runner deterioration the pressure-time (the Gibson) method was chosen to proceed with a test for flow determination. However, to measure the pressure in the penstock no access from the external space of the penstock was found, so the special instrumentation had to be developed, which could be installed inside different sections of the penstock for determination of the pressure as required by the Gibson method. After the successful installation of the pressure transducers and a special hermetic capsule, from which a cable was laid through the manhole to the control room, the test was carried out at different loads applying the Gibson method. Simultaneously, the instrumentation for the Winter-Kennedy method was installed and calibrated during the test. In the paper all the turbine measured characteristics are given and discussed. It was concluded that the efficiency of the hydraulic turbine was still high and no modifications were necessary. Having instruments calibrated for the Winter-Kennedy method other curves can be obtained at different heads.Copyright

Modelling of the flow at the last stage blade tenon in a geothermal turbine using renormalization group theory turbulence model

A bi-dimensional modelling investigation of the flow in the last stage of a 110 MW geothermal turbine has been conducted. The study was based upon a Renormalization Group Theory turbulence model. The results confirmed the existence of flow conditions which may play a main role in the erosion of the L-0 stage blade tenon, which had been detected in periodic overhauls. According to predicted results the relationship between erosion and flow patterns might exist due to: (1) a vapour jet hitting directly on tenon surface at velocities around 65 m/s; (2) a low-pressure region identified with recirculating flow, which may be causing cavitation on the damaged surface. Afterwards, the flow was simulated with changes on the geometry and grid. These changes are, indeed, practically feasible of being implemented. The simulations showed that it is possible to reduce the erosion process by enlarging a flat region close to the L-0 rotor stage. Namely, this change of geometry produces a flow pattern that diminishes the strength of recirculation flow making it possible to reduce both the flow rate through tenon region and its velocity on tenon surface. The pressure drop diminishes as well, clearly reducing a risk of cavitation.

Experimental and numerical simulations predictions comparison of power and efficiency in hydraulic turbine

On-site power and mass flow rate measurements were conducted in a hydroelectric power plant (Mexico). Mass flow rate was obtained using Gibsons water hammer-based method. A numerical counterpart was carried out by using the commercial CFD software, and flow simulations were performed to principal components of a hydraulic turbine: runner and draft tube. Inlet boundary conditions for the runner were obtained from a previous simulation conducted in the spiral case. The computed results at the runners outlet were used to conduct the subsequent draft tube simulation. The numerical results from the runners flow simulation provided data to compute the torque and the turbines power. Power-versus-efficiency curves were built, and very good agreement was found between experimental and numerical data.

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

Combating of the Solid Particle Erosion in a Steam Turbines

The causes and possible solutions of solid particles erosion of the steam path components to reducing it substantially are presented. Applying numerical CFD simulations excessive solid particle erosion damage of the blades tenons, wheel filets, rotor labyrinth seals and main stop valves were studied and steam/solid particles flow conditions which play a main role in the erosion process determined. Afterwards, the flow was simulated introducing specific changes to the geometry of components in order to modify the flow pattern favourably. In each case analyzed, the results confirmed that is possible to reduce the erosion process significantly due to reduction of the volumetric flow rate and velocity of vapour (case of blade tenons), velocity reduction and to modify the angle of particle flow incidence on the surface by increasing the distance from the labyrinth seal to the rotor wheel (case of wheel filets) and flow velocity reduction due to incorporating a steam flow deflector to the diaphragm, which changing flow direction in the inlet zone to the rotor labyrinth seal channel (case of rotor labyrinth seal). In the case of the main steam valve, introducing profile off-set on the valve spherical surface, the steam/particle flow was separated from the valve critical zone, changing the particles trajectories and angle of impact on the surface, resulting in erosion rate reduction. Also, a passive methods of combating solid particle erosion of tenons in the form of weld deposits around them as a protective barrier and wheel filets, rotor labyrinth seals and blades and shrouds coating are presented. All analyzed design modifications and other improvements were implemented and its performance monitored.© 2002 ASME

Measurement of the Flow in a 170 MW Hydraulic Turbine Recording the Pressure-Time Rise in One Section of the Penstock

The objective of the present work is to evaluate the performance of a hydraulic turbine by means of the measurement of flow using the Gibson method based on recording pressure–time rise in one section of the penstock and relate it to the pressure in the upper reservoir to which the penstock is connected. Volumetric flow is determined by integration of the time function of a differential pressure (between the section and the inlet to the penstock). Flow measurement was possible this way because the influence of penstock inlet was negligible as far as an error of the measurement is concerned. The paper presents the results obtained with this method for the case of a 170 MW hydraulic turbine. The length of the penstock was 300 m. Previous experience and a standard IEC-41-1991 were the criteria adopted and applied. An efficient and fast acquisition system including a 16 bit card was used. The flow rate was calculated using a computer program developed and tested on several cases. The results obtained with the Gibson method were used for calibration of the on-line flow measuring system based on the Winter-Kennedy principles. This last method is used for continuous monitoring of the turbine flow rate. Having calculated the flow rate and output power the efficiency is calculated for any operating conditions. A curve showing the best operating conditions based on the highest efficiency is presented and discussed. Flow simulation allowed having an estimation of a flow recirculation region size.Copyright

Numerical prediction of the microstructure of weld heat affected zone (HAZ) in SMAW weld deposits on Cr‐Mo‐V steel

The refining of a metallurgical structure is generally seen as an advantage for material that is of interest and it is often a chosen solution as a means of obtaining the required mechanical properties. In the heat affected zone (HAZ) of the weld, achieving the refinement of grains also offers greater resistance to cracking during the production process (welding process and heat treatment after welding) and it can improve the effectiveness of the weld during operations. In Cr-Mo-V steels applied for the principal components of steam and gas turbines (rotors, casing, valves and piping), the microstructure of the HAZ typically comprises a thick-grained strip of bainite immediately adjacent to the fusion line of the base metal, a strip of fine grains and an intercritical strip (decorated bainite), which contains a mixture of ferrite and bainite. Due to the high operating temperature of the aforementioned components, they show signs of temper embrittlement after a prolonged operating period. Material afflicted with temper embrittlement can be very difficult to weld, as it can show a high sensitivity to heat cracking and to cracking during the elimination of stress after welding.