A Parametric Analysis for Optimal Selection of a Gas Turbine Cogeneration System

Cogeneration is a simultaneous production of heat and electricity in a single plant using the same primary energy. Usage of a cogeneration system leads to fuel energy saving as well as air pollution reduction. A gas turbine cogeneration plant (GTCP) has found many applications in industries and institutions. Although fuel cost is usually reduced in a cogeneration system but the selection of a system for a given site optimally involves detailed thermodynamic and economical investigations. In this paper the performance of a GTCP was investigated and an approach was developed to determine the optimum size of the plant to meet the electricity and heat demands of a given site. A computer code, based on this approach, was developed and it can also be used to examine the effect of key parameters like pressure ratio, turbine inlet temperature, utilization period, and fuel cost on the economics of GTCP.

The Coarsening Kinetic of γ′ Particles in Nickel-Based Superalloys During Aging at High Temperatures

Nickel-based superalloys are widely used in applications requiring strength at high temperature, and in particular in manufacturing of several important components of both aeronautics and land based gas turbines. The main property of these materials is due to their particular microstructure consisting of a fcc lattice nickel matrix (γ phase), strengthened by precipitation of a second phase Ni3(Ti,Al) (γ′ phase), having fcc lattice. During aging at high temperatures, γ′ precipitates increase their size, following a kinetic law described by the classical LSW theory. In this work the growth kinetic of γ′ precipitates for the superalloy GTD 111 has been investigated by SEM. Samples of the alloy have been aged in the typical range of service temperatures for times up to 8000 hours. For each sample a large number of images has been acquired and the size and distribution of γ′ particles have been evaluated. Plotting the average size values, corresponding to the different times and temperatures analyzed, it was possible to obtain the growth kinetic of these particles, finding that the above mentioned theory gives a good description of the observed behaviour. The data obtained for GTD 111 have been also compared with other data referring to INCONEL 738, obtained from samples aged in the same conditions and analyzed in the same way. GTD 111, when compared to IN738, resulted to have a much slower growth kinetic, resulting in a much higher creep resistance. From the collected data it was possible to calculate also the activation energy for the diffusion process for both alloys, finding out values in agreement with those obtained by other authors and very close to the activation energies of Ti and Al in Ni matrix.

Strain-Life Assessment of Grainex Mar-M 247 for NASA’s Turbine Seal Test Facility

NASA’s Turbine Seal Test Facility is used to test air-to-air seals for use primarily in advanced jet engine applications. Combinations of high temperature, high speed, and high pressure limit the disk life, due to the concern of crack initiation in the bolt holes of the Grainex Mar-M 247 disk. The primary purpose of this current work is to determine an inspection interval to ensure safe operation. The current work presents high temperature fatigue strain-life data for test specimens cut from an actual Grainex Mar-M 247 disk. Several different strain-life models were compared to the experimental data including the Manson-Hirschberg Method of Universal Slopes, the Halford-Nachtigall Mean Stress Method, and the Modified Morrow Method. The Halford-Nachtigall Method resulted in only an 18% difference between predicted and experimental results. Using the experimental data at a −99.95% prediction level and the presence of 6 bolt holes it was found that the disk should be inspected after 665 cycles based on a total strain of 0.5% at 649°C.

Probabilistic Analysis of a Turbofan Secondary Flow System

This paper documents a probabilistic analysis of the secondary flow system in a modern commercial turbofan engine. The purpose of this analysis is to investigate the variability in the high and low rotor bearing loads and total secondary flow due to the inherent uncertainty in manufacturing processes and engine performance. In addition to quantifying the variability in bearing load and secondary flow, the sensitivity of the parameters to individual input variables is determined. The system was found to behave linearly, resulting in negligible mean shifts due to input variation. The importance of correlation among the performance parameters will be addressed, as well as the effects of different correlations. Methods used to reduce the time required for the analysis will also be discussed. This type of analysis has many applications in cost reduction, engine design, optimization, and root cause analysis that will be covered in this paper.

Thermodynamic Analysis of Intercooled Gas-Steam Combined and Steam Injected Gas Turbine Power Plants

The current emphasis on the development of gas turbine related power plants such as combined and steam injected is on increasing the plant efficiency and specific work while minimizing the cost of power production per kW and emission. The present work deals with the thermodynamic analysis of intercooled (both surface and evaporative) gas/steam combined and steam injected cycle power plants. The intercooling has a beneficial effect on both plant efficiency and specific work if the optimum intercooling pressure is chosen between 3 and 4. The evaporative intercooler is superior to surface type with reference to plant efficiency and specific work.

Oil Pumping in High Speed and High Loaded Ball Bearings

Trends in aircraft engine design cause increased mechanical stress requirements for rolling bearings. Consequently high amounts of heat are rejected which results in high oil scavenge temperatures. The direction of oil flow in the bearing can considerably affect the heat transported by the oil. An RB199 turbofan bearing and its associated chamber were modified to carry out the survey. The test bearing was a 124mm PCD ball bearing. The bearing has a split inner-ring employing under-race lubrication by two individual jets. The total oil flow could be devided to any ratio through the jets. This had an impact on the oil scavenge temperatures and the scavenge flows on both sides of the bearing. Significant reduction in the ‘heat to oil’ was achieved when oil was fed at certain proportions (ratio). This work is part of the European Research programme Brite Euram ATOS (Advanced Transmission and Oil Systems).

Influence of High Fogging Systems on Gas Turbine Engine Operation and Performance

High fogging is a power augmentation device where water is sprayed upstream of the compressor inlet with higher mass flow than that which would be needed to saturate the intake air. The main focus of this paper is on applications of high fogging on the ALSTOM gas turbine engines of the family GT24/GT26. Engine operation and performance are illustrated based on test results obtained from four different engines that have meanwhile accumulated more than 12’000 operating hours (OH) in commercial operation with ALSTOM’s ALFog® high fogging system. The effect of internal cooling (water evaporation inside the compressor) is investigated considering actual compressor boundaries matched within the complete engine. Changes in the secondary air system (SAS) and corresponding movement of the engine operating line have been taken into account. Power output gain as high as 7.1% was experimentally demonstrated for injected water mass fraction (f = mH2O/mair) equal to 1% and considering internal cooling effects only. Higher figures can be obtained for operation at low ambient relative humidity and partial evaporation upstream of the compressor inlet.

Optimization of the Managing Strategy of a Cogenerative Power Park in a Liberalized Market

The paper addresses the optimization of the managing strategy of a cogenerative power park in a liberalized market characterized by great time variability of the electricity sale price. Besides electric tariffs and marginal costs, a variety of other factors affects the selection of operating mode of the prime movers, such as environmental conditions, O&M costs, range of plant output regulation capability and performance deterioration of the components. The optimum plant operating schedule is found with reference to a tariff scenario where the base-load, mid-term and peak-load electricity costs are set by economical competition among a variety of power stations. The first part of the paper focuses on the case of a single cogenerative combined cycle, generating heat and electricity for an industrial load. The paper then deals with the extension of the optimization strategy to the more complex case of a combination of various prime movers (including combined cycle, combustion turbines and reciprocating gensets), all feeding the same district heating network. The results indicate the utmost importance of a correct management of the power stations for the achievement of best energy ad economic results.

Optimal Operational Planning of Cogeneration Systems With Microturbine and Desiccant Air-Conditioning Units

Economic and energy-saving characteristics of cogeneration systems with microturbine and desiccant air-conditioning units are investigated on system operational planning. An optimization approach is adopted to rationally evaluate these characteristics. In this approach, on/off and rated/part load status of operation of equipment and energy flow rates are determined so as to minimize the hourly energy charge subject to satisfaction of energy demand requirements. In this optimization problem, performance characteristics of the microturbine and desiccant air-conditioning units are modeled in consideration of the influence due to ambient air temperature. Moreover, the influence due to ambient air humidity is also considered in the desiccant air-conditioning unit using the psychrometric diagram. The implementation of the numerical analysis method, discussed in this paper, to two cogeneration systems, clearly shows economic and operational benefits of using desiccant air-conditioning.

Studies on Bypass Transition of a Boundary Layer Subjected to Localized Periodic External Disturbances

This study aims at clarification of wake-induced bypass transition process of a boundary layer on a flat plate with no pressure gradient. Special attention is paid to inception as well as growth of a turbulent spot created by the incoming wake as an external disturbance. To meet this goal a unique wake generator is invented to create an isolated turbulent spot. A multi-probe sensor with seven single-hot-wire probes is used to measure wake-affected boundary layer. The wake generator consists of a disk, pillars and a very thin wire with a small sphere on it. The sphere on the wire generates periodic wakes behind it when it passes across the main flow in front of the test flat plate. These sphere wakes impinge the flat plate in a spatially and timewisely localized manner so that the wakes periodically leave narrow affected zones inside the boundary layer. The observations confirm that an isolated turbulence spot emerges from each of those wake-affected zones. It is also found that the turbulent spot observed in this study bears a close resemblance to the conventional turbulent spot that takes a shape of arrowhead pointing downstream.