Using Design to Teach Thermodynamic Cycles: Designing a 250 MW Steam Power Plant Using the Rankine Cycle

Power generation is increasingly important in the turbine industry. Students need exposure to the complexities of such systems as found in this design project. This project is part of the second of a two-course thermodynamic sequence designed to provide a foundation in thermodynamics and expose the students to various power generation cycles. One way to teach students the Rankine Cycle is to involve them in the various aspects of the cycle through a design project. Students, in teams of four or five, are given the task of designing a 250 MW steam power plant based on the Rankine Cycle. Calculations are made using the software of choice, usually Engineering Equation Solver (EES). Students are required to make an oral and written presentation. In addition to the presentation of calculations and graphs, an emphasis is placed on describing the general considerations of the design problem and the presentation of the unique advantages of the design. Students gain valuable experience in system optimization and better learn to justify their design decisions. Based on student evaluations the project was well received and increased student interest in the field of power generation. However, there is a need to include an economic component to the problem, and more time must be spent in class discussing typical component operating parameters.

High-Temperature Fatigue Properties of Single Crystal Superalloys in Air and Hydrogen

Hot section components in high performance aircraft and rocket engines are increasingly being made of single crystal nickel superalloys such as PWA1480, PWA1484, CMSX-4 and Rene N-4 as these materials provide superior creep, stress rupture, melt resistance and thermomechanical fatigue capabilities over their polycrystalline counterparts. Fatigue failures in PWA1480 single crystal nickel-base superalloy turbine blades used in the Space Shuttle Main Engine (SSME) fuel turbopump are discussed. During testing many turbine blades experienced Stage II non-crystallographic fatigue cracks with multiple origins at the core leading edge radius and extending down the airfoil span along the core surface. The longer cracks transitioned from stage II fatigue to crystallographic stage I fatigue propagation, on octahedral planes. An investigation of crack depths on the population of blades as a function of secondary crystallographic orientation (β) revealed that for β = 45+/- 15 degrees tip cracks arrested after some growth or did not initiate at all. Finite element analysis of stress response at the blade tip, as a function of primary and secondary crystal orientation, revealed that there are preferential β orientations for which crack growth is minimized at the blade tip. To assess blade fatigue life and durability extensive testing of uniaxial single crystal specimens with different orientations has been tested over a wide temperature range in air and hydrogen. A detailed analysis of the experimentally determined Low Cycle Fatigue (LCF) properties for PWA1480 and SC 7-14-6 single crystal materials as a function of specimen crystallographic orientation is presented at high temperature (75 F – 1800 F) in high-pressure hydrogen and air. Fatigue failure parameters are investigated for LCF data of single crystal material based on the shear stress amplitudes on the 24 octahedral and 6 cube slip systems for FCC single crystals. The max shear stress amplitude [Δτmax] on the slip planes reduces the scatter in the LCF data and is found to be a good fatigue damage parameter, especially at elevated temperatures. The parameter Δτmax did not characterize the room temperature LCF data in high-pressure hydrogen well because of the noncrystallographic eutectic failure mechanism activated by hydrogen at room temperature. Fatigue life equations are developed for various temperature ranges and environmental conditions based on power-law curve fits of the failure parameter with LCF test data. These curve fits can be used for assessing blade fatigue life.

The Use of Electrostatic Charge Measurements as an Early Warning of Distress in Heavy-Duty Gas Turbines

The idea of measuring the electrostatic charge associated with the debris contained in the exhaust gases of a gas turbine (sometimes named EDMS, Engine Debris Monitoring System, or EEMS, Electrostatic Engine Monitoring System) has been demonstrated by several authors as an interesting diagnostic tool for the early warning of possible internal distresses (rubs, coating wear, hot spots in combustors, improper combustion, etc.) especially for jet engines or aeroderivative gas turbines. While potentially applicable to machines of larger size, the possibility of transferring this monitoring technology to heavy-duty gas turbines, which have exhaust ducts much bigger in size and different operating conditions, should be demonstrated. The authors present a synthesis of their experience and of the most significant data collected during a demonstration program performed on behalf of ENEL, the main Italian electric utility. The purpose of this program was to test this concept in real operating conditions on large turbines, and hence to evaluate the influence of the operating conditions on the system response and to assess its sensitivity to possible distresses. A good amount of testing has been performed, during this program, both on a full scale combustion rig, and on two machines rated at about 120 MW, during their normal and purposely perturbed operating conditions in a power plant. The authors, on the basis of the encouraging results obtained to date, comment on the work still required to bring this technology to full maturity.

Estimation of Distributed Unbalance of Rotors

Mass unbalance commonly causes vibration of rotor-bearing systems. Lumped mass modeling of unbalance was adapted in most previous research. The lumped unbalance assumption is adequate for thin disks or impellers, but not for thick disks or shafts. Lee et al. (1993) proposed that the unbalance of shafts should be continuously distributed. Balancing methods based on discrete unbalance models may not be very appropriate for rotors with distributed unbalance. A better alternative is to identify the distributed unbalance of shafts before balancing. In this study, the eccentricity distribution of the shaft is assumed in piecewise polynomials. A finite element model for the distributed unbalance is provided. Singular value decomposition is used to identify the eccentricity curves of the rotor. Numerical validation of this method is presented and examples are given to show the effectiveness of the identification method.

Numerical Investigation of Blade Flutter at or Near Stall in Axial Flow Turbomachines

During the design of the compressor and turbine stages of today’s aeroengines, aerodynamically induced vibrations become increasingly important since higher blade load and better efficiency are desired. In this paper the development of a method based on the unsteady, compressible Navier-Stokes equations in two dimensions is described in order to study the physics of flutter for unsteady viscous flow around cascaded vibrating blades at stall. The governing equations are solved by a finite difference technique in boundary fitted coordinates. The numerical scheme uses the Advection Upstream Splitting Method to discretize the convective terms and central differences discretizing the viscous terms of the fully non-linear Navier-Stokes equations on a moving H-type mesh. The unsteady governing equations are explicitly and implicitly marched in time in a time-accurate way using a four stage Runge-Kutta scheme on a parallel computer or an implicit scheme of the Beam-Warming type on a single processor. Turbulence is modelled using the Baldwin-Lomax turbulence model. The blade flutter phenomenon is simulated by imposing a harmonic motion on the blade, which consists of harmonic body translation in two directions and a rotation, allowing an interblade phase angle between neighboring blades. Non-reflecting boundary conditions are used for the unsteady analysis at inlet and outlet of the computational domain. The computations are performed on multiple blade passages in order to account for nonlinear effects. A subsonic massively stalled unsteady flow case in a compressor cascade is studied. The results, compared with experiments and the predictions of other researchers, show reasonable agreement for inviscid and viscous flow cases for the investigated flow situations with respect to the Steady and unsteady pressure distribution on the blade in separated flow areas as well as the aeroelastic damping. The results show the applicability of the scheme for stalled flow around cascaded blades. As expected the viscous and inviscid computations show different results in regions where viscous effects are important, i.e. in separated flow areas. In particular, different predictions for inviscid and viscous flow for the aerodynamic damping for the investigated flow cases are found.

Hybrid Magnetic/Foil Bearing System for an Oil-Free Thrust Bearing Test Rig

A novel, high-speed, high temperature, oil-free, foil thrust bearing test rig has been developed with a critical element being a double-acting, active magnetic thrust bearing. The magnetic thrust bearing is used to react against loads applied to the foil thrust bearing under test. The magnetic bearing has the capability of reacting against thrust loads of up to 2224 N (500 pounds) at speeds to 80,000 rpm, while the rotor is supported by foil journal bearings. Two issues that are especially challenging for this test rig are magnetic material selection and the electronic control system. The magnetic material selection is critical due to the high centrifugal stresses that occur at 80,000 rpm. The electronic control system must handle the non-linear variation in stiffness and damping that is seen by the magnetic thrust bearing as the foil thrust bearing is loaded, as well as maintain rotor system stability as the foil bearing is purposefully overloaded to the point of failure to discover maximum load and performance capabilities. This paper describes the design of the active magnetic thrust bearing, the material selection process, and the development of a digital signal processor based control system. Typical experimental data obtained during operation of the test rig will also be presented.

Global Nonlinear Modelling of Gas Turbine Dynamics Using NARMAX Structures

This paper examines the estimation of a global nonlinear gas turbine model using NARMAX techniques. Linear models estimated on small-signal data are first examined and the need for a global nonlinear model is established. A nonparametric analysis of the engine nonlinearity is then performed in the time and frequency domains. The information obtained from the linear modelling and nonlinear analysis is used to restrict the search space for nonlinear modelling. The nonlinear model is then validated using large-signal data and its superior performance illustrated by comparison with a linear model. This paper illustrates how periodic test signals, frequency domain analysis and identification techniques, and time-domain NARMAX modelling can be effectively combined to enhance the modelling of an aircraft gas turbine.

Dynamic Simulation of Full Start-Up Procedure of Heavy Duty Gas Turbines

A simulation program for transient analysis of the start-up procedure of heavy duty gas turbines for power generation has been constructed. Unsteady one-dimensional conservation equations are used and equation sets are solved numerically using a fully implicit method. A modified stage-stacking method has been adopted to estimate the operation of the compressor. Compressor stages are grouped into three categories (front, middle, rear), to which three different stage characteristic curves are applied in order to consider the different low-speed operating characteristics. Representative start-up sequences were adopted. The dynamic behavior of a representative heavy duty gas turbine was simulated for a full start-up procedure from zero to full speed. Simulated results matched the field data and confirmed unique characteristics such as the self-sustaining and the possibility of rear-stage choking at low speeds. Effects of the estimated schedules on the start-up characteristics were also investigated. Special attention was paid to the effects of modulating the variable inlet guide vane on start-up characteristics, which play a key role in the stable operation of gas turbines.

Experimental Evaluation of a Metal Mesh Bearing Damper in Parallel With a Structural Support

In a previous ASME paper experiments were reported on metal mesh bearing dampers (MMD) that were tested in a power turbine rotor at speeds up to 12,000 rpm. They were made of 0.229 mm stainless steel 304 wire mesh, compressed to 57% density, which is close to the maximum density that was economically available. After balancing, a level of vibration was achieved similar to that previously observed with squeeze film dampers. These experiments showed that the MMD could suppress vibration amplitudes of the 22.7 kg rotor at critical speeds of 4,000 rpm and 9,300 rpm. Much of the testing showed the rotor having little or no response to unbalance on coastdown through the critical speeds. The donut-shaped MMD in those tests were the only bearing supports; no squirrel cages were used. A question was raised about the feasibility of using MMD in parallel with a squirrel cage bearing support so that the stiffness can be controlled independently of the damping. This paper presents experimental results for metal mesh dampers with a squirrel cage as a parallel bearing support. Experiments with copper mesh as seal elements (on another project) had indicated that copper mesh has higher damping than stainless steel, so copper was chosen for these experiments. Both a linear viscous damping model and a hysteretic damping model were investigated. Some hysteretic damping models predict that damping depends on stiffness. A different hysteretic model turned out to be useful and promising as a prediction model for two reasons: a) it fits the measured data, and b) it predicts that the damping is not lost if the MMD is put in parallel with a steel structure such as a squirrel cage bearing support. The measurements reported here support the validity of that prediction.

Experimental Investigation of Misaligned Rotors

This paper is concerned with an experimental investigation of misaligned rotors. Phenomena observed in the field by different authors are substantiated through these experimental results.