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Coal, Biomass and Alternative Fuels

1997;():V002T05A001. doi:10.1115/97-GT-004.
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The moisture level in biomass fuels potentially impacts efficiency in conversion to power. This paper examines the efficiency and net power output of a circulating fluidized bed gasifier-combined cycle with flue gas drying for a range of as-received raw biomass moisture contents and levels of pre-gasification drying. Due to the lack of empirical data available, a modeling approach is used to simulate the effect of varying moisture content in the gasifier feed biomass. Below 30%, the raw-biomass moisture content has a negligible effect on the cycle efficiency and net power output. Higher moisture contents significantly reduce cycle efficiency. For a specified as-received biomass moisture content, drying prior to gasification increases overall efficiency, but the gains in efficiency decrease with increasing levels of drying.

Commentary by Dr. Valentin Fuster
1997;():V002T05A002. doi:10.1115/97-GT-005.
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The Biomass Power Program of the US Department of Energy (DOE) has as a major goal the development of cost-competitive technologies for the production of power from renewable biomass crops. The gasification of biomass provides the potential to meet his goal by efficiently and economically producing a renewable source of a clean gaseous fuel suitable for use in high efficiency gas turbines. This paper discusses the development and first commercial demonstration of the Battelle high-throughput gasification process for power generation systems. Projected process economics are presented along with a description of current experimental operations coupling a gas turbine power generation system to the research scale gasifier and the process scaleup activities in Burlington, Vermont.

Commentary by Dr. Valentin Fuster
1997;():V002T05A003. doi:10.1115/97-GT-014.
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Consideration of a hydrogen based economy is attractive because it allows energy to be transported and stored at high densities and then transformed into useful work in pollution-free turbine or fuel cell conversion systems. Through its New Energy and Industrial Technology Development Organization (NEDO) the Japanese government is sponsoring the World Energy Network (WE-NET) Program. The program is a 28-year global effort to define and implement technologies needed for a hydrogen-based energy system. A critical part of this effort is the development of a hydrogen-fueled combustion turbine system to efficiently convert the chemical energy stored in hydrogen to electricity when the hydrogen is combusted with pure oxygen. The full-scale demonstration will be a greenfield power plant located sea-side. Hydrogen will be delivered to the site as a cryogenic liquid, and its cryogenic energy will be used to power an air liquefaction unit to produce pure oxygen.

To meet the NEDO plant thermal cycle requirement of a minimum of 70.9%, low heating value (LHV), a variety of possible cycle configurations and working fluids have been investigated. This paper reports on the selection of the best cycle (a Rankine cycle), and the two levels of technology needed to support a near-term plant and a long-term plant. The combustion of pure hydrogen with pure hydrogen with pure oxygen results only in steam, thereby allowing for a direct-fired Rankine steam cycle. A near-term plant would require only moderate development to support the design of an advanced high pressure steam turbine and an advanced intermediate pressure steam turbine.

Commentary by Dr. Valentin Fuster
1997;():V002T05A004. doi:10.1115/97-GT-037.
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Naphtha fuel for combustion turbines possesses some unique physical properties that must be considered in the design of the fuel delivery system for trouble free operation. The fuel system must be designed to start the turbine on natural gas; distillate or naphtha, transfer to the secondary fuel and back to the original fuel; over a defined load range. The timing and permissives required for these events to occur smoothly, without tripping the unit, demand full control over the flow, temperature and pressure of all fuels involved. The same delivery system is often used to deliver other fuels that differ in density, volatility, vapor pressure and flow, compounding the design process. This paper examines some of the design attributes employed in Westinghouse combustion turbines that are fueled by naphtha and natural gas. The design considerations and modifications to the conventional fuel delivery system are the subjects of this paper.

Commentary by Dr. Valentin Fuster
1997;():V002T05A005. doi:10.1115/97-GT-038.
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Advanced coal based power generation systems, such as the Air Blown Gasification Cycle (ABGC), offer the potential for high efficiency, electricity generation with minimum environmental impact. An important component of the ABGC development is the gas turbine combustion system. It must bum low calorific value (LCV) coal derived fuel gas, at high turbine inlet temperatures with minimum pollutant emissions. A phased development programme has been completed burning LCV fuel gas (3.6–4.1 MJ/m3) with low emissions, particularly NOx derived from fuel bound nitrogen. Performance tests were carried out on a generic tubo-annular, prototype combustor, at Mach numbers generally lower than those typical to engine applications, with encouraging results. Five design variants, operating at conditions selected to represent a particular medium sized industrial gas turbine each returned an improvement in combustor performance. A further five variants were investigated to establish which design characteristics and operating parameters most affected NOx emissions.

Commentary by Dr. Valentin Fuster
1997;():V002T05A006. doi:10.1115/97-GT-039.
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Due to their high efficiency and flexibility, aeroderivative gas turbines were often considered as a development basis for intercooled engines, thus providing better efficiency and larger power output. Those machines, originally studied for natural gas, are here considered as the power section of gasification plants for coal and heavy fuels. This paper investigates the matching between intercooled gas turbine, in complex cycle configurations including combined and HAT cycles, and coal gasification processes based on entrained-bed gasifiers, with syngas cooling accomplished by steam production or by full water-quench. In this frame, a good level of integration can be found (i.e. re-use of intercooler heat, availability of cool, pressurized air for feeding air separation units, etc.) to enhance overall conversion efficiency and to reduce capital cast. Thermodynamic aspects of the proposed systems are investigated, to provide an efficiency assessment, in comparison with mare conventional IGCC plants based on heavy-duty gas turbines. The results outline that elevated conversion efficiencies can be achieved by moderate-size intercooled gas turbines in combined cycle, while the HAT configuration presents critical development problems. On the basis of a preliminary cost assessment, cost of electricity produced is lower than the one obtained by heavy-duty machines of comparable size.

Commentary by Dr. Valentin Fuster
1997;():V002T05A007. doi:10.1115/97-GT-040.
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Westinghouse began the development of a compact, entrained, slagging gasifier technology utilizing in-situ fuel gas cleaning for combustion turbine power cycles in 1986. The slagging gasifier is air-blown, and produces a hot, low-heating value fuel gas that can be combusted and quenched to combustion turbine inlet temperatures while maintaining low levels of NOx emissions. The U.S. Department of Energy sponsored engineering studies and pilot testing during the period 1986 to 1992. This work has shown that the technology has promise, although performance improvements are required in some key areas. A major challenge has been the development of in-situ removal of sulfur, alkali vapor, and particulate to low enough levels to permit its use in combustion turbine power systems without additional, external gas cleaning. This paper reviews the Westinghouse slagging gasifier, direct coal-fired turbine power generation concept; the pilot test results; and the current development activities that Westinghouse is engaged in.

Commentary by Dr. Valentin Fuster
1997;():V002T05A008. doi:10.1115/97-GT-044.
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The performance and practicality of heavy duty combustion turbine power systems incorporating thermochemical recuperation (TCR) of natural gas has been estimated to assess the potential merits of this technology. Process models of TCR combustion turbine power systems based on the Westinghouse 501F combustion turbine were developed to conduct the performance evaluation. Two TCR schemes were assessed — Steam-TCR and Flue Gas-TCR. Compared to conventional combustion turbine power cycles, the TCR power cycles show the potential for significant plant heat rate improvements, but their practicality is an issue. Significant development remains to verify and commercialize TCR for combustion turbine power systems.

Commentary by Dr. Valentin Fuster
1997;():V002T05A009. doi:10.1115/97-GT-062.
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Gas turbines (GT) have emerged as the most efficient means of transforming heat into mechanical work and with efficient generators are serving as major components of new electricity generation systems. The CCTL research and development efforts are directed towards developing a low cost solid fuel (SF) cogasifier fed by low cost local feedstocks to be coupled with smaller GT systems. The benefits of such systems can be enhanced if valuable by-products are produced or additional community purposes are served. We consider cogasification of biomass with other domestic fuels as a long term strategy for effective utilization of biomass. Our theoretical and experimental work indicate that blending oxygenated fuels such as biomass, MSW, RDF and dried sewage sludge with carbonaceous fuels such as coals, coke and chars in a small cogeneration system will have technological, economic and environmental advantages.

Commentary by Dr. Valentin Fuster
1997;():V002T05A010. doi:10.1115/97-GT-063.
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Advanced integrated gasification combined cycle (IGCC) plants promise to be efficient and environmentally friendly systems to utilise solid fuels for the production of electricity and heat. An IGCC system consists of a gasifier, producing a low calorific value (LCV) fuel gas, and a gas turbine in which the LCV fuel gas is being combusted. At this time some demonstration IGCC plants have been commissioned in the United States and Europe. A sound understanding of the interaction between the gasifier and the gas turbine combustor is critical for successful operation of an IGCC system. Reliable theoretical and experimental information on the characteristics of the gas turbine as a whole and the combustor as such, leading to this information is needed prior to commercialisation of these IGCC systems. The combustion of natural gas in gas turbine combustors has been studied extensively. The combustion of coal-derived LCV fuel gas however has been studied in much less detail.

To obtain more fundamental data on the combustion of LCV fuel gas, a 1.5 MW pressurised fluidised bed gasifier (PFBG) with a separate pressurised topping combustor (PTC) has been designed, built and operated at Delft University of Technology (The Netherlands). The maximum system pressure is 10 bar. Experiments have been performed at 8 bar, using recirculated flue gas, steam and oxygen as gasifying agents. The produced LCV fuel gas is combusted in an oxygen blown PTC. In this way a flue gas with a high carbon dioxide concentration can be obtained from which the carbon dioxide can be removed more easily than from flue gases. A numerical model has been constructed to simulate the combustion of the LCV fuel gas in the PTC.

A detailed description of the test rig will be given. The first experimental results will be described and compared with simulation results obtained with the commercial Computational Fluid Dynamics code Fluent version 4.3. Finally the future work will be described.

Commentary by Dr. Valentin Fuster
1997;():V002T05A011. doi:10.1115/97-GT-217.
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Ceramic candle filtration is one of the few technologies for high temperature particulate removal which can meet both gas turbine manufacturer’s inlet particulate requirements and U.S. Clean Air Act requirements. Similarly, advances made in sorbent technology, especially metal oxide sorbents, have resulted in effective sulfur emissions mitigation. Current use of metal oxide sorbents, however, has focused on, regenerable formulations which have excellent sulfur affinity but high physical attrition, resulting in poor overall economics.

This paper suggests the use of spent metal oxide and calcium based materials for use in sulfur removal as an alternative to regenerable metal oxide sorbents. It becomes even more attractive when used in combination with ceramic candle filtration technology. When such sorbents are classified to a desired particle size and injected into a high temperature coal utilization process, such a “once-through” sorbent can effectively remove sulfur and simultaneously increase the permeability of dust collected at a downstream ceramic filter station, in a highly cost effective manner.

Commentary by Dr. Valentin Fuster
1997;():V002T05A012. doi:10.1115/97-GT-221.
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Recent market place activity over the past years has shown a renewed trend in using steel mill waste gas for combustion in gas turbines. To effectively use steel mill gas, as well as other industrial off gases and syngas from gasification, more than just the combustion process has to be considered. With steel mill gases some of the more critical areas involve gas clean up and compression, as well as integration with the steam system of the host industrial plant.

This paper will summarize some of the special considerations for applying a gas turbine/combined cycle power plant to a steel mill. Due to the similarities of the fuels, this paper will also discuss how this is common to other industrial gases applications. GE has substantial experience in combustion of low calorific fuels as well as industrial process gases and more recently steel mill gases.

Commentary by Dr. Valentin Fuster
1997;():V002T05A013. doi:10.1115/97-GT-273.
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The kraft process dominates pulp and paper production worldwide. Black liquor, a mixture of lignin and inorganic chemicals, is generated in this process as fiber is extracted from wood. At most kraft mills today, black liquor is burned in Tomlinson boilers to produce steam for on-site heat and power and to recover the inorganic chemicals for reuse in the process. Globally, the black liquor generation rate is about 85,000 MWfuel (or 0.5 million tonnes of dry solids per day), with nearly 50% of this in North America. The majority of presently-installed Tomlinson boilers will reach the end of their useful lives during the next 15 to 20 years. As a replacement for Tomlinson-based cogeneration, black liquor-gasifier/gas turbine cogeneration promises higher electrical efficiency, with prospective environmental, safety, and capital cost benefits for kraft mills. Several companies are pursuing commercialization of black liquor gasification for gas turbine applications. This paper presents results of detailed performance modeling of gasifier/gas turbine cogeneration systems using different black liquor gasifiers modeled on proposed commercial designs.

Commentary by Dr. Valentin Fuster
1997;():V002T05A014. doi:10.1115/97-GT-290.
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Considering the expected climate change, biomass is one of the promising energy sources for the future. However, burning and only producing low temperature heat means wasting exergy. To utilise this renewable fuel, highly efficient cogeneration plants are requierd. Existing, small and medium sized power generation systems using gas turbines, have, either complicated flow schemes, or problems with fuel charging of pressurised combustion chambers, which are yet to be solved.

Here is a solution is presented requiring no preparation of the biomass fuel. In several publications (Jericha 1991, Fesharaki 1995, Fesharaki 1996) the inverted gas turbine cycle has been presented. This project deals with the investigation and the development of an atmospheric biomass combustion chamber, combined with an inverted gas turbine cycle. The system consists of a combustor with wood grate firing, working at atmospheric pressure. The exhaust gas from the combustor, with a temperature of 1050 °C, is cooled by water or steam injection, to a temperature of 730 °C. The exhaust gas is purified in a cyclone and expanded in a gas turbine to a pressure of 0.3 to 0.4 bar. The exhaust gas of the turbine can be used for operating a single-pressure steam generator, and for district heating. The exhaust gas is purified again by condensing the moisture in the exhaust gas, which stems from the biomass and the steam, or water injection after the combustion chamber. The purified gas is recompressed to atmospheric pressure, which is used for combustion-air preheating. With such a system a high electrical efficiency can be achieved, dependent upon the turbo machines, and the use of an additional bottoming steam cycle.

Considering the Austrian situation a feasibility study is being carried out. Special attention has been paid to:

Turbine design,

Exhaust gas purification,

and the cycle control. The feasibility study shows, that this concept is practicable, and that several biomass existing heating systems in Austria could be converted for production of electricity.

Topics: Biomass , Gas turbines
Commentary by Dr. Valentin Fuster
1997;():V002T05A015. doi:10.1115/97-GT-291.
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World-wide, biomass is the most used nonfossil fuel and is expanding from its traditional thermal applications to more usage for liquid fuels and electricity. More than 9 gigawatts of biomass electrical generation capacity have been installed in the United States, primarily by forest products industries, since the Public Utilities Regulatory Policy Act (PURPA) was passed. Combined heat and power (CHP) technologies promise to improve power-to-heat efficiencies to strengthen the economic viability of these electrical generating methods. These technologies, which are now being tested and demonstrated, employ industrial and aeroderivative gas turbines; use a variety of feedstocks including agricultural wastes, residues, and dedicated energy crops; and range in size from 8 MW to 75 MW. Specific demonstrations with the U.S. Department of Energy Biomass Power Program and partners in Vermont and Hawaii are discussed.

Commentary by Dr. Valentin Fuster
1997;():V002T05A016. doi:10.1115/97-GT-292.
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The goal of the Advanced Turbine Systems (ATS) program is the design and development of high thermal efficiency gas turbines with pollutant emissions at single digit levels, through the development of advanced recuperated gas turbines. Following successful subscale catalytic reactor testing, a full scale catalytic combustion system was designed to be representative of a single can in a multi-can gas turbine combustor configuration. The full scale catalytic combustion system is modular in design and includes a fuel/air premixer upstream of the catalytic reactor and a post catalyst homogeneous combustion zone downstream of the catalyst bed to complete the homogeneous gas-phase reactions. System start-up is accomplished using a lean-premixed (LP) low emissions fuel injector. The system transitions to catalyst operation using a variable geometry valve that diverts air flow into the catalyst at loads greater than 50% of full load. The variable geometry valve is used to operate the catalyst within the narrow operating window due to limited fuel/air turndown allowed by the catalyst. A catalyst design with preferential catalyst coating on a corrugated metal substrate to limit catalyst substrate temperatures was selected for the system. Mean fuel concentration measurements at the inlet to the catalyst bed using an instrumented catalyst module showed the fuel/air premixing to be within catalyst specifications. Preliminary combustion tests on the system were completed. The catalytic combustion system was tested over the 50-to-100% load range. Using variable geometry control, emissions goals (< 5 ppmv NOx, < 10 ppmv CO and UHC corrected to 15% O2) were achieved for catalyst operation between 50-and-100% load conditions. The system was started and operated under part-load conditions using the LP injector. Efforts are under way to accomplish successful transition from LP mode of operation to catalytic mode of operation using the variable geometry system.

Commentary by Dr. Valentin Fuster
1997;():V002T05A017. doi:10.1115/97-GT-300.
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Indirectly-fired cycles provide one means of using a fuel other than natural gas or distillates of various purities to generate power using a gas turbine. In a closed cycle, the fuel typically is used to heat a clean working fluid which is then expanded through a gas turbine, after which it is cooled and recompressed before being recirculated through the heating circuit. In an open cycle, the heated working fluid (usually air) is exhausted to the atmosphere after expansion in the turbine and passage through heat recovery devices. In both cases, the temperature of the working fluid may be boosted before entry to the turbine by supplementary firing of a premium fuel such as natural gas in a topping combustor. A major advantage of such indirectly-fired cycles is that the concerns arising from the use of a dirty fuel in other advanced cycles are confined to the fireside surfaces of the heat exchange equipment, whereas the gas turbine is exposed to a relatively benign environment. One limitation of such systems is that the emissions problems are the same as for a conventional coal-fired boiler although, on an power output-normalized basis, the emissions from an indirectly-fired cycle may be lower. The requirements of the potential candidate materials for the various components in the circuit are discussed, and the critical issues for each are identified.

Commentary by Dr. Valentin Fuster
1997;():V002T05A018. doi:10.1115/97-GT-377.
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To meet the challenge of deregulation and customer demands for a free competitive market, the electric utility industry in the U.S. (and, for that matter, throughout the world) will experience tremendous changes over the next five years. These changes will be driven by two major forces: the deregulation of the industry and, therefore, no guaranteed return on investment but more importantly, the demands of customers for a free competitive market in the electric utility industry where they can achieve the lowest cost for the commodity. This will force utility companies to position themselves as low-cost producers. Although low cost does not necessarily mean success, it is obvious that cutting and/or reducing capital expenditures will play the most important role.

Unregulated markets encourage product diversity, as firms look for “niche” profit opportunities. A pervasive lesson from other industries that have recently been deregulated clearly shows that unless properly planned, these companies will not only do poorly but may be completely wiped out from the market Generation Planning (base load vs. peak load, long-term vs. short-term) will become more important since two-thirds of the capital investment is tied to generation facilities. While low-cost utilities will have greater flexibility in adapting to competition, they will be far from immune to industry changes.

Under a fully competitive marketplace, all generating plant assets/investments will come out of a rate base. Since all companies will be exposed to competition, high-cost generating assets would no longer be subsidized by ratepayers. This will force the utility companies to invest in low capital cost generation only, at least during the next ten to fifteen years.

This paper will briefly discuss the status of various advanced generation technologies with respect to their costs, applicability and limitations, where these technologies are expected to be cost effective and finally how these technologies compare with the state-of-the-art combined cycle gas-turbine technology. It is predicted that as environmental regulations tighten on pollution, advanced generation technologies may benefit at the expense of current fossil fuel technologies. However, it is not certain whether economic growth in the U.S. can be sustained if new regulations on pollution force to add new plants with advanced generation technologies, compared to continuing with today’s generation mix. It will be examined how, when and where the advanced generation technologies would play an important role in penetrating the market on their own merits.

Commentary by Dr. Valentin Fuster
1997;():V002T05A019. doi:10.1115/97-GT-378.
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A number of options for power generating unit repowering by installing topping gas turbine units, using the novel natural gas “partial oxidation” technology on basis of heavy duty and aeroderivative gas turbines, intended for modernization of existing natural gas fired steam power plants have been examined. A comparative thermodynamic, technical and economic analysis of these repowering options has been made. The additionally generated useful power and the efficiency of production of additional electricity have been used as the most important parameters for comparison with traditional repowering options. Pages 8, Figures 8, Tables 6.

Commentary by Dr. Valentin Fuster
1997;():V002T05A020. doi:10.1115/97-GT-497.
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The goal of the Advanced Turbine Systems (ATS) program is to develop a high thermal efficiency industrial gas turbine with ultra-low emissions (<10 ppmv NOx, CO and UHC @ 15% O2) over the 50 to 100% load range. Catalytic combustion was chosen as an approach likely to meet ATS emissions goals. A subscale catalytic combustor development program was designed to develop a technical knowledge base for catalyst design (catalyst construction, length), performance (ignition, activity and emissions) and operating limitations (fuel-air turndown and sensitivity to combustor operating variables). A novel catalyst design with preferential catalyst coating to limit substrate temperatures was used in the tests. The catalytic combustor consists of a fuel-air premixer, catalytic reactor and a post-catalyst zone for completion of homogeneous gas phase reactions. In situ measurements of mean fuel concentrations at the exit of the premixer were completed to characterize fuel-air premixing levels. Performance of the catalyst was monitored through global emissions measurements at the exit of the post-catalyst combustor under simulated engine conditions, and measurement of catalyst substrate temperatures. Ultra-low emissions were achieved for relatively uniform fuel-air premixing (<10% peak to peak variation in fuel concentration) with higher inhomogeneities (>10% peak to peak variation) leading to either locally high or low substrate temperatures. Regions with low substrate temperatures led to high CO and UHC emissions. Modeling of post-catalyst homogeneous reactions using a standard stationary, one-dimensional, laminar premixed flame formulation showed good agreement with measurements. In short term tests, the catalysts showed the desired chemical activity and ability for multiple light-off. The subscale combustor development work provided the necessary technical information for full scale catalytic combustion system development for the ATS gas turbine.

Commentary by Dr. Valentin Fuster

Combustion and Fuels

1997;():V002T06A001. doi:10.1115/97-GT-056.
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Lean premixed prevaporized combustion (LPP) is a promising approach for the reduction of emissions in gas turbine combustion. Typically, modern gas turbines operate at high pressure and high temperature flow conditions, giving rise to very short self ignition times. Consequently, the residence time of the mixture in the premix duct has to be minimized. Complete fuel vaporization and mixing can only be achieved by an improvement of the atomization process towards a fine droplet size spectrum over the whole range of operating conditions. In this paper an air assisted pressure swirl atomizer is introduced and analyzed. The special design of the nozzle under investigation enhances the interaction between the liquid sheet of the atomizer and the co-flowing air around the nozzle. The visualization of the atomization process gives detailed insight into the fundamental atomization phenomena of the atomizer. In addition, extensive measurements of droplet size distributions describe the dependence of the atomization process on liquid flow rate and air velocity, respectively. All phenomena occurring are explained in detail by means of a theoretical analysis of the flow pattern. With the help of this kinematic model, it can be shown why co-rotating swirls of atomization air and liquid cone lead to better atomization than counter rotating swirls.

Topics: Optimization , Ducts
Commentary by Dr. Valentin Fuster
1997;():V002T06A002. doi:10.1115/97-GT-057.
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Catalytic combustion offers the possibility of attaining the firing temperatures of current and next generation gas turbines [up to ∼1450°C (2640°F)] with nitrogen oxides (NOx) production as low as 1 part per million by volume (ppmv). Such catalytic combustion technology has been under development at Catalytica for several years, and the first full scale test of the technology took place at the General Electric Company under TEPCO sponsorship in 1992. The results of the most recent and most successful full scale test in this program are reported in this paper.

The catalytic combustor system was designed for the GE Model MS9001E gas turbine fired with natural gas fuel. The 508-mm (20-in) diameter catalytic reactor was operated at conditions representative of the startup and load cycle of that machine. It was verified that the observed NOx levels were produced not in the catalyst, but in the diffusinn flame of the preburner used to start the system and maintain the necessary catalyst inlet temperature. Even so, NOx levels below 5 ppmv (at 15% O2) were achieved at the simulated base load operating point. Carbon monoxide (CO) and unburned hydrocarbons (UHC) emissions were likewise below 10 ppmv at that condition. Single digit emissions levels also were recorded at conditions representative of the combustor operating at 78% load, the first such demonstration of the turndown capability of this system. Throughout the test, dynamic pressure measurements showed the catalytic combustor to be quieter than even the diffusion flame combustors currently in commercial service.

Commentary by Dr. Valentin Fuster
1997;():V002T06A003. doi:10.1115/97-GT-059.
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Dry low NOX combustion technology has been successfully applied to the EGT Tornado and Tempest industrial gas turbines. This lean-premix technology has been based on that being employed in the EGT Typhoon gas turbine, as reported by Norster & De Pietro (1996) but with a number of modifications to suit the individual engines.

The Tornado is a 6.1 MWe machine designed in the late 1970’s for power generation and mechanical drive applications. The worldwide emissions legislation of recent years has provided the requirement to reduce NOX emissions in the exhaust, both for new machines and for those already in operation. Hence a system suitable for retrofitting as well as new production was required. The Tornado utilises similar burners to the Typhoon but with different combustion chambers and a different centre casing from the standard Tornado. Due to the differing cycle conditions, a different reaction zone stoichiometry has been used. A short rig test program followed by engine testing have achieved NOX emissions at base load significantly lower than the initial program target of 42 ppmv and led to the program target being revised to 25 ppmv.

The Tempest, launched into the market in 1995 produces 7.49 MWe in single shaft configuration and is aimed at the electrical power generation market. To comply with current emissions legislation, a DLN system has been developed. The Tempest is a 25% scale up of the Typhoon but its mechanical design incorporates a simplified main and pilot burner arrangement and a fully fabricated combustor. At base load, the Tempest operates at a higher turbine entry temperature than the Typhoon but has been designed such that the equivalence ratio in the reaction zone is slightly lower. A comprehensive test programme has demonstrated hardware which significantly improves upon the target emissions limit of 25 ppmv NOX.

Commentary by Dr. Valentin Fuster
1997;():V002T06A004. doi:10.1115/97-GT-060.
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This paper describes the development of an ultra low NOx (i.e. sub 10 ppmvd) combustion system from the successful completion of a high pressure rig programme, to the installation of the system on a gas turbine in commercial operation. It includes a detailed description of early screening tests for DLN concepts and an explanation of how these were developed on both rig and engine test bed.

Specific discussions include the concepts and technology for:

1) NOx reduction

2) Optimisation of premix turndown for carbon monoxide control at part load conditions

3) Control of combustion driven pressure oscillation (combustor dynamics) and the failures resulting from the lack of control.

In addition to the DLN aerothermal development, an outline description of the combustor mechanical design and the control and systems modification from the conventional combustion system is given.

Commentary by Dr. Valentin Fuster
1997;():V002T06A005. doi:10.1115/97-GT-061.
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The second Generation of EGT’s G30 DLE combustion system was introduced after a successful series of high pressure rig and engine tests. This paper covers how operational problems with field commissioning hardware on the lead DLN machine were dealt with, leading to achievement of reliable low NOx hardware. Several changes were applied to the early design which improved the mixing and reduced the effects of high temperature distortion and combustor dynamics. This resulted in increased life of the burner and changed the characteristics of dynamics. It also led to very low emission levels with an outstanding capability for turndown of CO with NOx below 25 ppmvd (15% O2) over the whole load range. Further coverage is given to the effect of field tuning, and of fuel composition on the amplitudes and frequencies of dynamics. The installation has been supported by on-line condition monitoring of engine parameters, emission levels and ambient conditions, which are also discussed. The general overview of site history is followed by a summary of lessons learnt in field comparison to development test bed.

Commentary by Dr. Valentin Fuster
1997;():V002T06A006. doi:10.1115/97-GT-071.
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New tests were established early in the development of JP-8+100 fuel to provide system design information in addition to evaluating the JP-8+100 additives. Preliminary evaluation of the JP-8+100 additives indicated that a thermal stability improvement for JP-8 of 100°F (56°C) was achieved for the wetted wall temperature. Specific tests were conducted to evaluate various, additive concentrations and combinations for meeting the bulk fuel goal. Screening type tests were conducted in the Extended Duration Thermal Stability Test System. The promising additive candidates were then tested in the Advanced Reduced Scale Fuel System Simulator. These test systems were designed to provide information that is directly applicable to aircraft/engine fuel system designs. Based on the tests results of both the Extended Duration Thermal Stability Test and the Advanced Reduced Scale Fuel System Simulator, there were essentially zero deposits experienced at 385°F (196°C) bulk fuel temperatures with recirculation of JP-8+100 fuel. There were slight deposits experienced during tests of this fuel at 400°F (204°C) with recirculation. The description of the test systems and their operating characteristics along with the test results of the additive evaluations will be covered in this paper. A discussion of future fuel system applications and the potential payoff for JP-8+100 fuels will also be included.

Topics: Temperature
Commentary by Dr. Valentin Fuster
1997;():V002T06A007. doi:10.1115/97-GT-072.
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A genetic algorithm, coupled with a versatile preliminary design tool, is employed to demonstrate the concept of an autonomous design procedure for gas turbine combustors with user specified performance criteria.

The chosen preliminary design program utilises a network based approach which provides considerable geometric flexibility allowing for a wide variety of combustor types to be represented. The physical combustor is represented by a number of independent, though interconnected, semi-empirical sub-flows or elements. A full conjugate heat transfer model allows for convection, conduction and radiative heat transfer to be modelled and a constrained equilibrium calculation simulates the combustion process. The genetic algorithm, whose main advantage lies in its robustness, uses the network solver in order to progress towards the optimum design parameters defined by the user. The capabilities of the genetic program are demonstrated for some simple design requirements, for example zone fuel/air ratio, pressure drop and wall temperatures.

Commentary by Dr. Valentin Fuster
1997;():V002T06A008. doi:10.1115/97-GT-073.
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Lean premixed combustor manufacturers require premixer concepts that provide homogeneity (mixedness) of the fuel which burns in the main flame. Ideally premixer evaluation would be conducted under realistic combustor operating conditions. However, current techniques typically are limited to cold—flow, low pressure (<14 atm) conditions or comparison of measured NOx emissions with others obtained in premixed systems. Thus, a simple, consistent method for quantifying unmixedness in lean premixed combustors operating at high pressure, fired operating conditions is proposed here, using the characteristic time model developed in the companion paper.

Commentary by Dr. Valentin Fuster
1997;():V002T06A009. doi:10.1115/97-GT-074.
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The design and development of a pilot assisted dry low-NOx LPP (Lean Premix Prevaporized) combustor for the VT100 turbogenerator is described. The combustor was designed for and tested on ethanol, a renewable biofuel chosen to minimize the contribution to atmospheric CO2.

Engine specifications for pressure drop, operability and emissions have been met. The combustor has successfully demonstrated ultra low NOx and CO emissions below 10 and 20 ppmv respectively at the best engine conditions.

The rig testing was performed at reduced inlet temperatures and the fuel flow was scaled to achieve main combustion zone flame temperatures as in the engine. The results show good agreement for NOx vs. flame temperature between the rig and engine tests.

The dry low-NOx combustor for the VT100 turbogenerator meets the emission specification and gives the hybrid propulsion system the potential of meeting the foreseen (2010) NOx emission targets for urban delivery vehicles of 1 g/kWhe compared to today’s 7 g/kWhe.

Commentary by Dr. Valentin Fuster
1997;():V002T06A010. doi:10.1115/97-GT-076.
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Contamination levels of liquid fuel entering the combustor of a gas turbine must be low to avoid high temperature corrosion and fuel system fouling. Accordingly, each of the major industrial gas turbine manufacturers has strict contamination limits which must be met in order to comply with the warranty of the turbine. The responsibility to assure compliance lies with the owner of the power plant and can only be guaranteed with on-site fuel analysis for contaminants and ash-forming metals.

This paper discusses the various fuel analysis techniques available to the gas turbine user. It will review each technique’s ability to meet the need for fast and precise on-site analytical data. It will be demonstrated that the rotating disc electrode (RDE) atomic emission technique is the preferred analytical method for on-site fuel analysis. Actual field experience will be used to illustrate and discuss compromises that may be necessary to meet the objectives of the gas turbine user, fuel treatment supplier and turbine manufacturer.

Commentary by Dr. Valentin Fuster
1997;():V002T06A011. doi:10.1115/97-GT-077.
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The US Air Force is developing an additive package to improve the thermal stability of JP-8 fuels by 100°F. Consequently, JP-8 fuels containing the developed additive package are referred to as JP-8+100 fuels. Field tests of the JP-8+100 fuels have shown that the additive package greatly reduces maintenance cost and labor in comparison to JP-8 fuels by minimizing fuel system malfunctions caused by fuel deposition, e.g., fuel control changeouts, combustor damage, etc. The developed additive package contains three components: antioxidant, dispersant/detergent, and metal deactivator.

This paper presents simple analytical techniques that can be performed on-site or in the laboratory to determine the dispersant capacity and metal deactivator additive concentrations of JP-8+100 fuels. Since several dispersant/detergent candidates are being evaluated for use in the JP-8+100 additive package, the analytical techniques were developed to measure the dispersant capacity of the additive package instead of the concentration of one particular dispersant/detergent. The dispersant capacity test measures the ability of a fuel sample to suspend a metal oxide powder/water/isopropanol mixture. The dispersant capacity test can be used to identify jet fuels which contain the JP-8+100 additive package and to rate the dispersant capacity of a JP-8+100 fuel.

In contrast to the dispersant capacity test, the metal deactivator additive (MDA) tests were designed to determine the concentration of N,N′-disalicylidene-1,2-propanediamine which is the primary MDA used in jet fuels. The MDA tests use fuel soluble compounds or aqueous extraction to chemically react MDA to form colored species. The color of the MDA compound is measured visually for qualitative determinations or spectrometrically for quantitative determinations. Combination of the different MDA tests allows MDA to be detected down to 0.1 ppm regardless of fuel color, age, or type.

Topics: Metals , Jet fuels
Commentary by Dr. Valentin Fuster
1997;():V002T06A012. doi:10.1115/97-GT-126.
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A gas turbine combustor is modeled using a two-reactor, finite-rate mixing and chemistry gas particle approach. The first reactor, used to simulate combustion in the primary zone, permits independent definition of the rates of macromixing and micromixing within the reactor, and the amount of premixing of fuel and air entering the reactor. Finite-rate macromixing is simulated by consideration of the fluid particle residence time distribution frequency function and the ages of the particles in the reactor. Finite-rate micromixing is simulated using a modified Coalescence-Dispersion (C-D) model. The second reactor model simulates combustion in the dilution zone of the combustor, and is modeled as a plug flow reactor with cross-flowing jets of dilution air and co-flowing streams of cooling film air. The primary zone reactor model predicts physically reasonable trends in mean temperature, and CO and NOx emissions as the macromixing and micromixing parameters are varied with respect to the perfectly-stirred reactor limit. The model also has shown to predict the correct trends in modeling NOx and CO emissions from aircraft engine gas turbine combustors.

Commentary by Dr. Valentin Fuster
1997;():V002T06A013. doi:10.1115/97-GT-127.
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The General Electric Company has developed and successfully tested a full-scale, ‘F’ class (2550°F combustor exit temperature), rich-quench-lean (RQL) gas turbine combustor, designated RQL2, for low heating value (LHV) fuel and integrated gasification combined cycle applications. Although the primary objective of this effort was to develop an RQL combustor with lower conversion of fuel bound nitrogen to NOx than a conventional gas turbine combustor, the RQL2 design can be readily adapted to natural gas and liquid fuel combustion.

RQL2 is the culmination of a 5 year research and development effort that began with natural gas tests of a 2″ diameter perforated plate combustor and included LHV fuel tests of RQL1, a reduced scale (6″ diameter) gas turbine combustor. The RQL2 combustor includes a 14″ diameter converging rich stage liner, an impingement cooled 7″ diameter radially-stratified-quench stage, and a backward facing step at the entrance to a 10″ diameter film cooled lean stage. The rich stage combustor liner has a novel double-walled structure with narrow circumferential cooling channels to maintain metal wall temperatures within design limits. Provisions were made to allow independent control of the air supplied to the rich and quench/lean stages.

RQL2 has been fired for almost 100 hours with LHV fuel supplied by a pilot scale coal gasification and high temperature desulfurization system. At the optimum rich stage equivalence ratio NOx emissions were about 50 ppmv (on a dry, 15% O2 basis), more than a factor of 3 lower than expected from a conventional diffusion flame combustor burning the same fuel. With 4600 ppmv NH3 in the LHV fuel, this corresponds to a conversion of NH3 to NOx of about 5%. As conditions were shifted away from the optimum, RQL2 NOx emissions gradually increased until they were comparable to a standard combustor. A chemical kinetic model of RQL2, constructed from a series of ideal chemical reactors, matched the measured NOx emissions fairly well. The CO emissions were between 5 and 30 ppmv (on a dry, 15% O2 basis) under all conditions.

Commentary by Dr. Valentin Fuster
1997;():V002T06A014. doi:10.1115/97-GT-143.
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This paper describes recent results of AF-sponsored research in the thermal stability of high temperature fuels. At temperatures of 550 °C (1000 °F) and above, both thermal -oxidative and pyrolytic deposition are important. A brief discussion of deposition characteristics and mitigation measures is presented.

Commentary by Dr. Valentin Fuster
1997;():V002T06A015. doi:10.1115/97-GT-144.
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This paper describes major results of combustor envelopment activities conducted under 1 MW industrial engine (Turbo Green 1200) program at Samsung Aerospace. The program was initiated in 1992 partially supported by Korean government. The main application of the industrial engine is a standby duty unit with a future derivative engine considered for a cogeneration unit. During the engine development, various tests of the engine and its components were carried out to confirm the performance target or the engine. This paper concentrates on the component test activities of the combustion section.

Commentary by Dr. Valentin Fuster
1997;():V002T06A016. doi:10.1115/97-GT-145.
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The social responsibility for the environment, in conjunction with the threat of more stringent emissions regulations requirements, initiated a comprehensive NOx-emissions reduction programme in BMW Rolls-Royce. The achievements of the first step for NOx-emissions reduction by optimisation of the single annular combustor stoichiometry and mixing are presented. The combustor development programme is described, and rig and engine test results are compared. The NOx-certification levels achieved with the BR 710 single annular low NOx-combustor are down to 55 % of the actual ICAO limit.

Commentary by Dr. Valentin Fuster
1997;():V002T06A017. doi:10.1115/97-GT-146.
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In this research work the potential of rich quench lean combustion for low emission aeroengines is investigated in a rectangular atmospheric sector, representing a segment of an annular combustor. For a constant design point (cruise) the mixing process and the NOx formation are studied in detail by concentration, temperature and velocity measurements using intrusive and non-intrusive measuring techniques.

Measurements at the exit of the homogeneous primary zone show relatively high levels of non-thermal NO. The NOx formation in the quench zone is very low due to the quick mixing of the secondary air achieved by an adequate penetration of the secondary air jets and a high turbulence level. The NOx and CO emissions at the combustor exit are low and the pattern factor of the temperature distribution is sufficient.

Commentary by Dr. Valentin Fuster
1997;():V002T06A018. doi:10.1115/97-GT-147.
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Current progress in gas turbine performance is achieved mainly by increasing the turbine inlet temperature. At high temperature levels (>2000K), the hot combustion gases can no longer be considered as chemically inert, and it becomes important to account for dissociation and recombination reactions occurring not only in the combustion chamber but also within the expanding gas stream in the turbine. In this paper, the authors present a two-dimensional numerical study of chemically reactive flow of hot combustion gases through the first guide vane of a gas turbine. For this initial study, simplified boundary conditions are assumed: blade cooling air mixing is neglected, the blade wall temperature is assigned a fixed value, and uniform inlet conditions are assumed. This study investigates the effect of turbulence on chemical reaction kinetics and presents pollutant emission levels at the nozzle exit. Particular attention is also focussed on chemical reactivity near the pressure and suction sides of the turbine guide vane blades.

Commentary by Dr. Valentin Fuster
1997;():V002T06A019. doi:10.1115/97-GT-148.
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This paper presents two different soot production models and their predictions in a practical combustion chamber. These predictions are compared with detailed internal measurements of soot concentration by probe sampling. Both models solve two additional transport equations for soot mass concentration and number density, and incorporate representations of source terms for particle nucleation, surface growth, coagulation and oxidation. In one approach these rates are inferred from soot property measurements in a confined turbulent jet flame, and in the other from a flamelet-based approach employing computations of a kerosene laminar counter-flow flame which incorporates detailed reaction kinetics. Preliminary results from both models are encouraging. Although both over predict the peak levels of soot measured in a gas turbine combustor at 7 bar, these computations have neglected the effects of radiation, which will be included in subsequent calculations. The approach based on detailed chemistry captures more accurately the very high rate of oxidation towards the combustor exit which is a pronounced feature of the measurements and is missing in earlier reported studies. This is related to differences in the coagulation model. Calibration of model parameters for the empirical model from simple experiments leads to exaggerated coagulation at the higher combustor temperatures, reduced soot number densities and under-estimates effective surface area for oxidation. The suitability of laminar flamelet counter-flow flames as a route to generating the soot production relationships necessary for combustor predictions are carefully reviewed in the light of strain rate and residence time effects on detailed hydrocarbon pyrolysis.

Commentary by Dr. Valentin Fuster
1997;():V002T06A020. doi:10.1115/97-GT-149.
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A method is presented for predicting soot in gas turbine combustors. A soot formation/oxidation model due to Fairweather et al [1992] has been employed. This model has been implemented in the CONCERT code which is a fully elliptic three-dimensional (3-D) body-fitted computational fluid dynamics (CFD) code based on pressure correction techniques. The combustion model used here is based on an assumed probability density function (PDF) parameterized by the mean and variance of the mixture fraction and a β-PDF shape. In the soot modeling, two additional transport equations corresponding to the soot mass fraction and the soot number density are solved. As an initial validation, calculations were performed in a simple propane jet diffusion flame for which experimental soot concentration measurements along the centerline and along the radius at various axial downstream stations were available from the literature. Soot predictions were compared with measured data which showed reasonable agreement. Next, soot predictions were made in a 3-D model of a CF6-80LEC engine single annular combustor over a range of operating pressures and temperatures. Although the fuel in the combustor is Jet-A, the soot computations assumed propane to be the surrogate fuel. To account for this fuel change, the soot production term was increased by a factor of 10X. In addition, the oxidation term was increased by a factor of 4X to account for uncertainties in the assumed collision frequencies. The soot model was also tested against two other combustors, a CF6-80C and a CFM56-5B. Comparison of the predicted scot concentrations with measured smoke numbers showed fairly good correlation within the range of the soot model parameters studied. More work has to be performed to address several modeling issues including sensitivity to oxidation rate coefficients and scalar diffusion.

Commentary by Dr. Valentin Fuster
1997;():V002T06A021. doi:10.1115/97-GT-150.
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The Air-Blast Simplex (ABS) nozzle may have significant mechanical design advantages when compared to Pure Air-Blast (PAB) designs, and attractive cost benefits. The major barrier to implementing ABS nozzles is spray collapse at high ambient pressures. The present study addresses this issue, and presents the results in a manner that is useful to the gas turbine combustor designer. The results reveal that spray collapse is not significant as long as the fuel-to-air mass ratio is maintained below about 0.3. The results also reveal two distinct curves for air effective area that are attributed to the presence or lack of flow separation in the vane/shroud assembly. In the case of the separated flow, a larger rate of decrease in effective area with increasing fuel air mass or momentum ratio is observed. These results help address ABS spray angle collapse at high pressure, and identify strategies that may adequately mitigate, or even eliminate, spray collapse in a suitably designed combustor.

Topics: Fuels , Nozzles
Commentary by Dr. Valentin Fuster
1997;():V002T06A022. doi:10.1115/97-GT-151.
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Due to the continuous increase of pressure ratios in modern gas turbine engines the understanding of high pressure effects on the droplet evaporation process gained significant importance. The precise prediction of the evaporation time and the movement of the droplets is crucial for optimum design and performance of modern gas turbine combustion chambers. Numerous experimental and numerical investigations have been done already in order to understand the evaporation process of droplets in high pressure environments. But until now, all high pressure experiments were carried out with droplets attached to a thin fiber resulting in the impairment of the droplet evaporation process due to the suspension unit.

In the present study, a new experimental set up is introduced where the evaporation of free falling droplets is investigated. Monodisperse droplets are generated in the upper part of the test rig and fall through the stagnant high pressure gas inside the pressure chamber. Due to the relative velocity between droplet and gas, convective effects have to be considered in this study which are taken into account by experimental correlations. The droplet diameter and the droplet velocity are measured simultaneously by means of video technique and a stroboscope lamp. Detailed measurements with heptane droplets are presented for different pressures (p = 20 bar, 30 bar and 40 bar), gas temperatures (T = 550 K and 650 K) and initial diameters (d0 = 680 μm, 780 μm and 840 μm). The experiments were carried out with single component droplets.

The experimental results are compared with numerical calculations. For this a theoretical model was developed based on the Conduction Limit model and the Uniform Temperature model. Good agreement for all conditions investigated is observed when using the Conduction Limit model. The Uniform Temperature model predicts incorrectly the evaporation process of the droplet.

Commentary by Dr. Valentin Fuster
1997;():V002T06A023. doi:10.1115/97-GT-152.
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An experimental investigation is carried out on modeling of fuel atomization for the purpose of simulating the idle regime of a gas turbine combustor through atmospheric testing. If the simulation is successfully applied, it will significantly reduce the cost of testing. The simulation must sustain nearly the same fuel spray characteristics and the same aerodynamics at the exit of the frontal device. Air assisting through the main stage of a dual orifice fuel nozzle is employed to match the fuel spray characteristics. Optical diagnostic methods including flow visualization and Adaptive Phase/Doppler Velocimetry are used for the investigation of spray characteristics. Once the fuel spray characteristics are matched by air assisting, the combustor characteristics may then be matched by maintaining the loading parameter constant. The possibility of modeling with air assisting is shown and appropriate conditions for air assisting are found.

Commentary by Dr. Valentin Fuster
1997;():V002T06A024. doi:10.1115/97-GT-153.
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A unique idea of premixture jet swirl combustor (PJSC) was proposed for the ultra low NOx combustor of a Mach 3 turbojet. The combustor installed six simple premixing chambers which were arranged at certain angles to the center axis also to the circumference axis on the combustor dome. This arrangement formed large and strong recirculating flows necessary to stabilize flame at lean fuel air ratio conditions. The fuel mixing study revealed that the radial fuel injectors inserted in a premixing chamber exhibited a high degree of uniformity. Single can combustors of PJSC with three types of main fuel injectors were manufactured for the high temperature and high pressure combustion test program. All combustors performed stable combustion for a wide range of FAR and obtained combustion efficiency of 99.9 % at Mach 3 cruise conditions, namely inlet temperature of 1008 K, inlet pressure of 830 kPa and fuel air ratio of 0.0223. HTHPC-01 combustor, which installed the radial fuel injectors and had long mixing length, presented the best NOx emissions and achieved emission index of 2 g/kg fuel at that design condition. PJSC met the emission goal of HYPR project, and concept validation test was completed in success.

Commentary by Dr. Valentin Fuster
1997;():V002T06A025. doi:10.1115/97-GT-180.
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Infrared (IR) spectra of the exhaust emissions from a static gas turbine engine have been studied using Fourier Transform (FT) spectroscopic techniques. Passive detection of the infrared emission from remote (range ∼ 3 m) hot exhaust gases was obtained non-intrusively using a high spectral resolution (0.25 cm−1) FTIR spectrometer. Remote gas temperatures were determined from their emission spectra using the total radiant flux method or by analysis of rotational line structure. The HITRAN database of atmospheric species was used to model the emission from gas mixtures at the relevant temperatures.

The spatial distribution of molecular species across a section transverse to the exhaust plume −10 cm downstream of the jet pipe nozzle was studied using a tomographic reconstruction procedure. Spectra of the infrared emission from the plume were taken along a number of transverse lines of sight from the centreline of the engine outwards. A mathematical matrix inversion technique was applied to reconstruct the molecular concentrations of CO and CO2 in concentric regions about the centreline.

Quantitative measurements of the molecular species concentrations determined non-intrusively were compared with results from conventional extractive sampling techniques.

Commentary by Dr. Valentin Fuster
1997;():V002T06A026. doi:10.1115/97-GT-196.
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Indirect fired multistage combined/cogeneration turbine system, fed with pulverized coal, has been considered. Net electric power of the system amounts to about 46 MW. Such a system has already been considered in former paper of the author, No 95-CTP-60, except that water/steam tube radiation screens were discussed. In this paper, on the other hand, a course of calculation as well as an example of air tube radiation screen is presented. The radiation screen supplys heat to the working air for the high-pressure turbine. The middle- and low-pressure turbines are fed with air heated in two convective hot-heat-exchangers placed just behind the combustion chamber.

Commentary by Dr. Valentin Fuster
1997;():V002T06A027. doi:10.1115/97-GT-206.
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The characteristic time model (CTM) represents the dominant physical subprocesses related to combustor performance in terms of characteristic times. Properly formulated, these characteristic times account for variations in combustor geometry, fuel characteristics, and operating conditions. Here, a CTM for piloted–lean premixed combustor NOx emissions is used to investigate the sensitivity of NO formation in such combustors to fuel/air unmixedness and suggests an experimental method of evaluating premixed performance under fired conditions that is discussed in the companion paper.

Commentary by Dr. Valentin Fuster
1997;():V002T06A028. doi:10.1115/97-GT-218.
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Ten aviation turbine fuels (five Jet-A fuels, three JP-5, one JP-8, and one JPTS) were stressed at 185 and 225°C in a single-pass heat exchanger. On the basis of several criteria applied at 185°C, these fuels cover a broad thermal-stability range from lesser-quality fuels to the most stable JPTS fuel. Three of these fuels contain significant concentrations of dissolved metal (copper, > 30 ppb). The surface and bulk insolubles formed from each fuel have been quantified using surface-carbon burnoff of tubing sections and of in-line filters. The total insolubles measured at 185 and 225°C fall in the range 0.3–7.5 μg/mL and 0.1–2 μg/mL, respectively. In general, the greater the quantity of insolubles formed at 185°C, the greater its reduction at 225°C. Possible explanations for this effect are offered, and implications relative to understanding surface fouling are discussed.

Commentary by Dr. Valentin Fuster
1997;():V002T06A029. doi:10.1115/97-GT-219.
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A quartz crystal microbalance (QCM)/Parr bomb system with a headspace oxygen sensor is used to measure oxidation and deposition during thermal oxidative stressing of jet fuel. The advantages of the oxygen sensor technique in monitoring fuel oxidation is demonstrated. Simultaneous measurement of deposition using the QCM shows a strong correlation between oxidation and deposition in jet fuels. Studies performed over the temperature range 140 to 180°C show that surface deposition peaks at an intermediate temperature, while bulk deposition increases with temperature, in studies of jet fuel antioxidants, we find that rapid increases in oxidation rate occur upon consumption of the antioxidant. The antioxidant appears to be consumed by reaction with alkylperoxy radicals. In studies of metal deactivator (MDA) additives, we find that MDA is consumed during thermal stressing, and this consumption results in large increases in the oxidation rate of metal containing fuels. Mechanisms of MDA consumption are hypothesized.

Topics: Jet fuels , oxidation
Commentary by Dr. Valentin Fuster
1997;():V002T06A030. doi:10.1115/97-GT-224.
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Global reaction mechanisms and rate constants are commonly used in computational fluid dynamics models which incorporate chemical reactions to study aviation fuel thermal and oxidative thermal stability. Often these models are calibrated using one set of conditions, such as flow rate and temperature. New conditions are then calculated by extrapolation using the global expressions. A close inspection of the origin of global oxidation rate constants reveals that in systems that undergo autocatalysis or auto inhibition, a simple over-all global activation energy and reaction order are not good descriptors of the reaction process. Furthermore, pseudo-detailed chemical kinetic modeling of a fuel which experiences autocatalysis shows that the observed reaction order for oxygen consumption varies with initial oxygen concentration, extent of reaction, and temperature. Thus, a simple global rate expression used to describe oxygen consumption in an autoaccelerating system is insufficient to allow extrapolation to different temperature or time regimes.

Commentary by Dr. Valentin Fuster
1997;():V002T06A031. doi:10.1115/97-GT-225.
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Stationary gas turbines for power generation are increasingly being equipped with low emission burners. By applying lean premixed combustion techniques for gaseous fuels both NOx and CO emissions can be reduced to extremely low levels (NOx emissions <25vppm, CO emissions <10vppm). Likewise, if analogous premix techniques can be applied to liquid fuels (diesel oil, Oil No.2, etc.) in gas-fired burners, similar low level emissions when burning oils are possible. For gas turbines which operate with liquid fuel or in dual fuel operation, VPL (Vaporised Premixed Lean)-combustion is essential for obtaining minimal NOx-emissions. An option is to vaporise the liquid fuel in a separate fuel vaporiser and subsequently supply the fuel vapour to the natural gas fuel injection system; this has not been investigated for gas turbine combustion in the past.

This paper presents experimental results of atmospheric and high-pressure combustion tests using research premix burners running on vaporised liquid fuel. The following processes were investigated:

• evaporation and partial decomposition of the liquid fuel (Oil No.2);

• utilisation of low pressure exhaust gases to externally heat the high pressure fuel vaporiser;

• operation of ABB premix-burners (EV burners) with vaporised Oil No.2;

• combustion characteristics at pressures up to 25bar.

Atmospheric VPL-combustion tests using Oil No.2 in ABB EV-burners under simulated gas turbine conditions have successfully produced emissions of NOx below 20vppm and of CO below 10vppm (corrected to 15% O2). 5vppm of these NOx values result from fuel bound nitrogen. Little dependence of these emissions on combustion pressure bas been observed. The techniques employed also ensured combustion with a stable non luminous (blue) flame during transition from gaseous to vaporised fuel. Additionally, no soot accumulation was detectable during combustion.

Commentary by Dr. Valentin Fuster
1997;():V002T06A032. doi:10.1115/97-GT-226.
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For aircraft gas turbines as well as for industrial gas turbines current and future developments aim at the implementation of lean premixed-prevaporized (LPP) combustor techniques. For the development and optimization of these combustors powerful CFD-codes are required. A new code developed at the Institut für Thermische Strömungsmaschinen (ITS), University of Karlsruhe, provides detailed information on the gas flow as well as on the propagation and evaporation characteristics of liquid wall films inside combustors. The flow characteristics of the gas phase are predicted using a Finite-Volume 3D-Navier-Stokes code with k-ε turbulence modeling. To calculate the evaporation characteristics of a propagating wall film, a two-dimensional wall film model based on the boundary layer equations is proposed.

The present paper comprises a comparison between calculations and experiments for the verification of the code and a detailed study on the evaporation characteristics of fuel films. The results obtained allow judgement to be made on the risk of coke formation on the prefilming surface and suggest that in some operating points a LPP combustor can be operated utilizing solely film evaporation. In addition, the computer code developed also accounts for many familiar types of shear driven film flows such as internal prefilming air blast atomizer flows for example.

Commentary by Dr. Valentin Fuster
1997;():V002T06A033. doi:10.1115/97-GT-244.
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The use of premix combustion in stationary gas turbines can produce very low levels of NOx emissions. This benefit is widely recognized, but turbine developers routinely encounter problems with combustion oscillations during the testing of new pre mix combustors. Because of the associated pressure fluctuations, combustion oscillations must be eliminated in a final combustor design. Eliminating these oscillations is often time-consuming and costly because there is no single approach to solve an oscillation problem.

Previous investigations of combustion stability have focused on rocket applications, industrial furnaces, and some aeroengine gas turbines. Comparatively little published data is available for premised combustion at conditions typical of an industrial gas turbine.

In this paper, we report experimental observations of oscillations produced by a fuel nozzle typical of industrial gas turbines. Tests are conducted in a specially designed combustor, capable of providing the acoustic feedback needed to study oscillations. Tests results are presented for pressures up to 10 atmospheres, and with inlet air temperatures to 588 K (600 F) burning natural gas fuel.

Based on theoretical considerations, it is expected that oscillations can be characterized by a nozzle reference velocity, with operating pressure playing a smaller role. This expectation is compared to observed data, showing both the benefits and limitations of characterizing the combustor oscillating behavior in terms of a reference velocity rather than other engine operating parameters. This approach to characterizing oscillations is then used to evaluate how geometric changes to the fuel nozzle will affect the boundary between stable and oscillating combustion.

Commentary by Dr. Valentin Fuster
1997;():V002T06A034. doi:10.1115/97-GT-265.
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Environmental compatibility requires low emission burners for both gas turbine power plants and jet engines. In the past, significant progress has been made in the development of low NOx and CO burners by introducing lean premixed techniques. Unfortunately these burners often have a more pronounced tendency than conventional burner designs to produce combustion driven oscillations. The oscillations may be excited to such an extent that strong pulsation may occur, and this is associated with a risk of engine failure.

In order to describe the acoustical behaviour of the complete burner system, it is crucial to determine the unit function response of the flame itself. Using a new method which was presented in 1996 by Bohn et al. [1] the dynamic flame behaviour can be predicted by means of a full Navier-Stokes-simulation of the complex combustion process for both the steady-state and transient case.

The authors have successfully used this method to obtain the frequency response of turbulent diffusion flames which are mainly controlled by the mixing process. Chemical kinetics become dominant for premixed flames. Therefore, the combustion process of a premixed methane-air mixture is modelled using a systematically reduced 6-step reaction mechanism which takes account of a set of 25 elementary reactions. This reduced mechanism was implemented in the 3D-Navier-Stokes solver in order to perform a combined flow and combustion computation.

The dynamic combustion process of a laminar premixed methane flame in a matrix burner configuration has been investigated. At first, the steady-state combustion process was simulated using the code described above. The results are compared with experimental data. Very good agreement over a wide range of equivalence ratios has been found for quantities such as laminar burning velocity or adiabatic flame temperature. The steady state results are then used as an operating point from which the transient flame behaviour after a sudden jump in the mass flow at the burner inlet has been obtained. Finally, these data lead to the unit function response which can be transferred into frequency space by a Laplace transformation.

The frequency response of the premixed methane flame obtained by a Navier-Stokes simulation has been compared with both experimental as well as analytical solutions. It must be stressed that a pure delay time element which is often used as an analytical formulation is not suitable to describe the dynamic flame behaviour in detail. The frequency response shows the characteristics of a higher order delay time element with several important details.

Parametric studies on the influence of equivalence ratio and the flow pattern of the internal burner fluid flow which are of interest for gas turbine applications, show the importance of the detailed knowledge of the dynamic flame behaviour for the stability analysis of a gas turbine combustor.

Topics: Flames
Commentary by Dr. Valentin Fuster
1997;():V002T06A035. doi:10.1115/97-GT-266.
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Recent regulations on NOx emissions are promoting the use of lean premix (LPM) combustion for industrial gas turbines. LPM combustors avoid locally stoichiometric combustion by premixing fuel and air upstream of the reaction region, thereby eliminating the high temperatures that produce thermal NOx. Unfortunately, this style of combustor is prone to combustion oscillation. Significant pressure fluctuations can occur when variations in heat release periodically couple to acoustic modes in the combustion chamber. These oscillations must be controlled because resulting vibration can shorten the life of engine hardware.

Laboratory and engine field testing have shown that instability regimes can vary with environmental conditions. These observations prompted this study of the effects of ambient conditions and fuel composition on combustion stability. Tests are conducted on a subscale combustor burning natural gas, propane, and some hydrogen/hydrocarbon mixtures. A premix, swirl-stabilized fuel nozzle typical of industrial gas turbines is used. Experimental and numerical results describe how stability regions may shift as inlet air temperature, humidity, and fuel composition are altered. Results appear to indicate that shifting instability regimes are primarily caused by changes in reaction rate.

Topics: Stability , Combustion , Fuels
Commentary by Dr. Valentin Fuster
1997;():V002T06A036. doi:10.1115/97-GT-267.
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The results of the investigation of the flow in a transparent (quartz tube) downscaled (≈1:3) model of a lean-premix type gas turbine combustion chamber are presented and discused. The model was tested at atmospheric pressure in reacting conditions; flow measurements were taken by a two-channel fiber-optic laser doppler velocimeter, using Al2O3 seeding of the air flowrate. The measurements cover a wide flow field inside the combustion chamber, including flame development and recirculating regions. Long-time samples (10–20 s) were used in order to achieve a good accuracy in the measurement of average flow conditions over the whole flow field; this involved a limited capability of representation of high-frequency components of turbulence, which could be locally obtained with optimization of the data rate and seeding conditions. Fast measurements were also locally performed where the seeding conditions were favourable. Integral variables and power spectra for reacting conditions show some distinctive aspects for the turbulence structure of reacting turbulent flows in confined spaces. Further measurements cover the outlet throat section of the premix combustor, demonstrating the persistence of a radial flow component on account of wall curvature effects and a certain degree of asimmetry in the inlet velocity distribution.

Commentary by Dr. Valentin Fuster
1997;():V002T06A037. doi:10.1115/97-GT-276.
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A double concentric premixed swirl burner is used to examine the structure of two different methane-air premixed flames. Direct flame photography together with local temperature data provides an opportunity to investigate the effects of swirl number distribution in each annulus on the global and local flame structure, flame stability and local distribution of thermal signatures. An R-type thermocouple compensated for high-frequency response is used to measure the local distribution of thermal signatures in two different flames, each of which represents a different combination of swirl number in the swirl burner. In order to improve the accuracy of the temperature data at high-frequency conditions, information on the thermocouple time constant are also obtained under prevailing conditions of local temperature and velocity by compensating the heat loss from the thermocouple sensor bead. These results assist in quantifying the degree of thermal nonuniformities in the flame signatures as affected by the distribution of swirl and to develop strategies for achieving uniform distribution of temperatures in flames.

Topics: Combustion , Flames
Commentary by Dr. Valentin Fuster
1997;():V002T06A038. doi:10.1115/97-GT-277.
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Developing integrated coal gasification combined cycle (IGCC) systems ensures cost-effective and environmentally sound options for supplying future power generation needs. In order to enhance thermal efficiency of IGCC and to reduce NOx emission, a 1500 °C -class gas turbine combustor for IGCC was designed, tested and the performance of the combustor was evaluated under pressurized conditions. The designed combustor had three characteristics: 1) In order to assure the stable combustion burning low-Btu gas (LBG), an auxiliary combustion chamber was installed at the entrance of the combustor. 2) To reduce fuel NOx emission that was produced from the ammonia (NH3) in the fuel, the rich-lean combustion method was introduced. 3) To compensate for the declined cooling-air associated with the higher temperature of the gas turbine, the tested combustor was equipped with a dual-structure transition piece so that the cooling air in the transition piece can be recycled to cool down the combustor liner wall. As a result of combustor tests, it is confirmed that CO emission is less than 20ppm, the conversion rate of NH3 which contains about 1000ppm in the coal gasified fuel to NOx shows 40 percent or below, and the liner wall temperature remained below almost 850 °C under high pressure (1.4MPa), rated load condition.

Commentary by Dr. Valentin Fuster
1997;():V002T06A039. doi:10.1115/97-GT-302.
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Current progress in gas turbine performance is achieved mainly by increasing the turbine inlet temperature. State-of-the-art military aircraft gas turbines operate with turbine inlet temperatures exceeding 2000 K, and future development plans call for even higher temperature levels. At such high temperatures, the hot combustion gases can no longer be considered as chemically inert, and it becomes important to account for the chemically reactive nature of the expanding flow. In this paper, the authors present a one-dimensional model of the chemically reactive flow through the first turbine stage of an aircraft turbo-jet engine. The model is used to study the impact of chemical reactivity on pollutant emission characteristics and engine performance (i.e., overall efficiency and specific thrust). Three different flight conditions are considered: sea-level static operation (take-off), subsonic cruising at 10000 meters altitude, and supersonic flight at 20000 meters altitude. The results of this study show that typical flight conditions and operating parameters of turbo-jet engines produce high pollutant emission levels and decrease overall efficiency and specific thrust.

Commentary by Dr. Valentin Fuster
1997;():V002T06A040. doi:10.1115/97-GT-304.
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An experimental investigation has been carried out to assess the aerodynamic effects of locating radial struts within the pre-diffuser of a modern combustor dump diffuser system. Engine representative inlet conditions were generated by a single stage rotor, with the diffuser system incorporating various compressor outlet guide vane (OGV)/pre-diffuser assemblies and an annular flame tube with representative porosity. Stagnation and static pressure measurements were obtained at numerous locations and included assessment of the upstream pressure field, associated with the struts, which impacts on the rotor and OGV aerodynamics. Measurements were also obtained within the feed annuli, surrounding the flame tube, with attempts also being made to assess the stagnation pressure distributions presented to a simulated flame tube burner. Initial tests were performed with an OGV row attached to a conventional 1.45 area ratio pre-diffuser, this providing the datum to which all other systems were assessed. These included systems with thin or thick struts with the strut blockage, at pre-diffuser exit, being 5% and 11% of the gas passage area respectively. For the geometries tested it was shown that the method of adjusting each pre-diffuser passage area, to account for the strut blockage, was successful in providing similar levels of reduced kinetic energy at pre-diffuser exit. Despite this, however, the presence of strut wakes and their effect on the dump cavity flow produced increases in stagnation pressure loss. These loss variations were evaluated for both the feed annuli and burner flows, with the magnitudes depending on whether the struts were aligned or midway between burners. Also assessed was the impact of the increased circumferential flow non-uniformity that was observed for the flow within the inner feed annulus. A beneficial effect produced by the struts was the significant reductions in flow swirl, within the diffuser system, relative to the datum. This improved axial alignment of the flow, provided a more uniform pressure distribution to the burners and a more stable feed to the various flame tube features.

Commentary by Dr. Valentin Fuster
1997;():V002T06A041. doi:10.1115/97-GT-305.
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Time-accurate CFD analysis is used to model combustion instability in a premixed axisymmetric combustor typical of industrial gas turbine engines. The experiment of Richards and Janus (1997) is modeled; the hardware consists of a fuel injector similar to industrial premix fuel nozzles, a water-cooled can combustor, an uncooled refractory plug that reduces flow area, and a long exhaust duct. The CFD calculation domain extends from the air swirler within the fuel nozzle to the exhaust duct exit. Two cases are modeled using 2D time-accurate axisymmetric CFD analysis: a low nozzle air velocity (u = 30 m/s) case that exhibits combustion instability and a high nozzle air velocity (u = 60 m/s) case that does not. The CFD analysis agrees well with the experimental measurements, including peak-to-peak pressure variation and instability frequency for the unstable case. For the unstable case, the airflow through the swirler actually flows upstream part of the time, and hot combustion products are forced into the premix annulus. The potential of using a time-accurate CFD approach for modeling combustion instability in complex 3D combustors is discussed.

Commentary by Dr. Valentin Fuster
1997;():V002T06A042. doi:10.1115/97-GT-306.
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Experimental results on the influence of temporal unmixedness on NOx emissions are presented for both non-catalytic and catalytically stabilized, lean premixed combustion. The test rig used for the experiments consists of a fuel/air mixing section which allows variation of the degree of temporal unmixedness while maintaining a uniform “average over time” concentration profile over the cross section at the inlet to the combustion chamber. The unmixedness is measured as “rms fluctuations in fuel concentration” by an optical probe using laser absorption at 3.39μm over a 9mm gap. “Average over time” measurements are taken with “conventional” suction probe analyzers. The combustion chamber is an insulated, tubular reactor (i.d. 26.4mm). At the inlet to the combustion chamber a honeycomb monolith section is inserted. This monolith is either catalytically active or inactive for catalytically stabilized or non-catalytic combustion respectively. For both modes, the exact same inlet conditions are applied. In catalytically stabilized combustion a fraction of the fuel is consumed within the catalyst and the remaining fuel is burnt in the subsequent homogeneous combustion zone.

It is shown that catalytically stabilized combustion yields lower NOx emissions and, more important, that the effect of temporal fuel/air unmixedness on NOx emissions is much smaller than with non-catalytic combustion under identical inlet conditions. Experimental evidence leads to the conclusion, that the catalyst is capable of reducing temporal fluctuations in fuel concentration and/or temperature in the combustion process, thereby preventing excess NOx formation. As a result, the requirements on mixing quality are less stringent when using catalytically stabilized combustion instead of conventional, non-catalytic combustion.

Commentary by Dr. Valentin Fuster
1997;():V002T06A043. doi:10.1115/97-GT-308.
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Noise generated in gas turbine combustors can exist in several forms — broadband noise, sharp resonant peaks, and regular or intermittent non-linear pulsing. In the present study, dynamic pressure measurements were made in several JP-5-fueled combustor configurations, at various mean pressures and temperatures. The fluctuating pressure was measured at mean pressures from 6 to 14 atm and inlet temperatures from 550 K to 850 K. The goal of the present work was to study the effect of changes in mean flow conditions on combustor noise: both broadband noise and sharp tones were considered. In general, the shape of the broadband noise spectrum was consistent from one configuration to another. The shape of the spectrum was influenced by the acoustic filtering of the combustion zone. This filtering ensured the basic consistency of the spectra. In general, the trends in broadband noise observed at low mean pressures were also seen at high mean pressures; that is, the total sound level decreased with both increasing equivalence ratio and increasing inlet temperature. The combustor configurations without a central pilot experienced higher broadband noise levels and were more susceptible to narrow peak resonances than configurations with a central pilot. The sharp peaks were more sensitive to the mean flow than was the broadband noise, and the effects were not always the same. In some situations, increasing the equivalence ratio made the sharp peaks grow, while at other conditions, increasing the equivalence ratio made the sharp peaks shrink. Thus, it was difficult to predict when resonances would occur, however, they were reproducible. Noise was also observed near lean blow out. As with other types of noise, lean blow out noise was affected by the combustion chamber acoustics, which apparently maintains the fluctuations at a uniform frequency. However, the actual conditions when this type of noise was experienced appeared to simply follow the lean blow out limit, as it varied with mean temperature and pressure.

Commentary by Dr. Valentin Fuster
1997;():V002T06A044. doi:10.1115/97-GT-309.
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A numerical method is presented for predicting steady, three-dimensional, turbulent, liquid spray combusting flows in a gas turbine combustor. The Eulerian conservation equations for gas flow and the Lagrangian conservation equations for discrete fuel liquid droplets were solved. The trajectory computation of the fuel droplets provided the source terms for all the gas-phase equations. A standard k-ε submodel was used for turbulence. The combustion submodel used was a global local equilibrium model, where chemical species (CxHy, O2, CO2, H2O, CO, H2 and N2) approached local thermodynamic equilibrium with a rate determined by a combination of local turbulent mixing and global chemical kinetics times. The numerical methodology for gas-phase calculations involved a staggered finite-volume formulation with a multi-block curvilinear orthogonal coordinate computational grid, and the PISO algorithm. This numerical code was applied to a model gas turbine combustor similar to that of the Allison 570KF currently in use by the Canadian Navy. The combustor was equipped with an advanced airblast fuel nozzle. The calculations included the analysis of the internal passages of the fuel nozzle. Through the numerical study at full-power and low-cruise operating conditions, a better understanding of the physical processes of flow and temperature fields inside the primary zone was obtained. Predicted hot spots corresponded to locations where deterioration of the combustor liner has been observed in practice.

Commentary by Dr. Valentin Fuster
1997;():V002T06A045. doi:10.1115/97-GT-310.
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The provision of an ignition source in the central region of a liquid-fired combustor reduces the requirement for wide spray angles, rich primary zones, and their associated performance drawbacks such as high levels of soot and NOx formation and high liner wall temperatures. Various ignition devices have been considered for providing centrally located ignition sources. The current paper presents a study of one alternative concept — the integral catalytic torch ignitor/injector — as a means for providing both combustor light-off and enhanced flame stability in the combustor primary zone.

An integral catalytic torch in a fuel injector offers the potential to significantly improve ignition arid flame stability and thus the opportunity to operate combustor primary zones at leaner conditions, which may improve emissions, pattern factor, and combustor liner durability. This paper presents computational and experimental results for a conventional liquid-fired combustor with the addition of a catalytic torch (replacing the pilot pressure atomizer) down the centerline of an air-blast fuel injector. The benefits of a lean primary zone with an integral catalytic torch/injector were investigated both computationally and experimentally by comparing combustor performance with standard and successively leaner primary zones. Pattern factor and emissions are compared with different primary zone jet configurations to observe if the central torch can enhance the operability of leaner primary zones in conventional combustor geometries. The experimental and computational results suggest that the integral catalytic torch can provide more than adequate ignition capabilities with improved combustor emissions when it is combined with a relatively lean operating primary zone.

Commentary by Dr. Valentin Fuster
1997;():V002T06A046. doi:10.1115/97-GT-311.
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This paper describes the development of an ultra-low emissions combustion system for Allison’s Advanced Turbine System (ATS) engine, which is being developed in cooperation with the U.S. Department of Energy. The simple cycle engine is designed to have a thermal efficiency that is 15% better than today’s best in class engine, and exhaust emissions of 9 ppm NOx, 20 ppm CO, and 20 ppm UHC. The approach taken to meet the low emissions goals is based on ultra-lean premixed fuel-air combustion supported by a catalyst. The progress toward development and integration of lean premix (LPM), catalytic and post-catalytic stages, and the combustor-to-turbine transition duct into an overall ATS combustion system is presented.

A parametric computational fluid dynamics (CFD) study was conducted on the performance of lean premix modules at ATS conditions. Various lean premix modules were tested extensively under atmospheric conditions to determine airflow capacity, flashback propensity, lean blowout (LBO) fuel-air ratios, and fuel concentration profiles at the module exit. Kinetic modeling using the GRI mechanism has been used to estimate ignition delay times in the post-catalytic zone. Comparison between the modeling results and experimental data at high pressure shows good agreement. A detailed computational analysis was performed to design the combustion-to-turbine transition duct. The results indicate that the scroll duct configuration produces an acceptable mass flow uniformity and swirl angle exiting the duct into the turbine section. High pressure sector rig tests have been performed to evaluate staging interaction issues. The results indicate that the series staged approach can facilitate incorporation of the catalytic combustion system by expanding the operability range. NOx emissions levels of 9 ppm or less can be sustained over a wide range of equivalence ratios.

Commentary by Dr. Valentin Fuster
1997;():V002T06A047. doi:10.1115/97-GT-334.
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The evolution probability density function (PDF) method provides a framework for the simulation of both diffusion and premixed turbulent flames. With this method, the chemical reaction rates are treated without approximation. In contrast, the conventional Reynolds-average methods need to model the mean reaction rates in turbulent flame calculations. In addition, conventional methods require special models for premixed flames that are developed under restrictive assumptions and rely on ad hoc expressions for the rate of reaction progress. The present work demonstrates the capability of the PDF method in realistic combustor design calculations. A lean premixed flame swirl combustor is simulated using the scalar PDF method, and the results are compared with experimental data. It is shown that the PDF method can correctly predict the turbulent flame speed and location of the flame. The ability of the PDF method to handle finite-rate complex chemistry of any number of reaction steps makes it an ideal candidate for emissions predictions in low emission combustor designs.

Commentary by Dr. Valentin Fuster
1997;():V002T06A048. doi:10.1115/97-GT-335.
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A probability density function/chemical reactor model (PDF/CRM) is applied to study how NOx emissions vary with mean combustion temperature, inlet air temperature, and pressure for different degrees of premixing quality under lean-premixed (LP) gas turbine combustor conditions. Inlet air temperatures of 550, 650 and 750 K, and combustor pressures of 10, 14 and 30 atm are examined in different chemical reactor configurations. Primary results from this study are: incomplete premixing can either increase or decrease NOx emissions, depending on the primary zone stoichiometry; an Arrhenius-type plot of NOx emissions may have promise for assessing the premixer quality of lean-premixed combustors; and decreasing premixing quality enhances the influence of inlet air temperature and pressure on NOx emissions.

Commentary by Dr. Valentin Fuster
1997;():V002T06A049. doi:10.1115/97-GT-336.
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The effect of fuel composition on NOx formation in lean premixed prevaporized (LPP) combustion is examined using an atmospheric pressure jet-stirred reactor fitted with a prevaporizing-premixing chamber and liquid fuel atomizing nozzle. Four liquid fuels are studied, including the pure hydrocarbons n-heptane (C7H16) and n-dodecane (C12H26), No. 2 low sulfur diesel fuel oil (LSDFO#2) with 0.0195% sulfur and 0.0124% nitrogen by weight, and n-dodecane doped with n-ethylethylenediamine (C2H5NHCH2CH2NH2 or C4H12N2) to give 0.0096% nitrogen by weight in the doped fuel. For comparison, propane (C3H6) is burned. The combustion temperature range of the experiments is 1625 to 1925K, and the nominal residence time of the reactor is 3.5ms. The first objective of the work is to determine the effect which increasing fuel carbon number has on the NOx yield of high-intensity LPP combustion. For combustion at 1800K, an increase of 15 to 20% is measured in the NOx yield when the fuel is changed from C3H6 to C12H26. Comparison to earlier work on CH4 and C3H6 combustion in the jet-stirred reactor operating at 1800K shows essentially an identical increase in NOx yield between CH4 and C3H6 as between C3H6 and C12H26. The second objective of the work is to determine the conversion of fuel-nitrogen to NOx for the combustion of low nitrogen content fuels in high-intensity LPP combustion. The measurements indicate a fuel-nitrogen to NOx conversion of 70 to 100%. These conversion values should be regarded as preliminary since only two nitrogen-containing fuels have been examined and only one prevaporizer-premixer system has been used.

Commentary by Dr. Valentin Fuster
1997;():V002T06A050. doi:10.1115/97-GT-362.
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The main aim or the present work is to explore computational fluid dynamics and related turbulence and combustion models for application to the design, understanding and development of gas turbine combustor. Validation studies were conducted using the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) scheme to solve the relevant steady, elliptical partial differential equations of the conservation of mass, momentum, energy and chemical species in three-dimensional cylindrical co-ordinate system to simulate the gas turbine combustion chamber configurations. A modified version of k-ε turbulence model was used for characterization of local turbulence in gas turbine combustor. Since, in the present study both diffusion and pre-mixed combustion were considered, in addition to familiar bi-molecular Arhenius relation, influence of turbulence on reaction rates was accounted for based on the eddy break up concept of Spalding and was assumed that the local reaction rate was proportional to the rate of dissipation of turbulent eddies. Firstly, the validity of the present approach with the turbulence and reaction models considered is checked by comparing the computed results with the standard experimental data on recirculation zone, mean axial velocity and temperature profiles, etc. for confined, reacting and non-reacting flows with reasonably well defined boundary conditions. Finally, the results of computation for practical gas turbine combustor using combined diffusion and pre-mixed combustion for different combustion conditions are discussed.

Commentary by Dr. Valentin Fuster
1997;():V002T06A051. doi:10.1115/97-GT-366.
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Some results of the NPP “EST” activities on improvement of the environmental record of the stationary gas turbines, viz.GTG-1500, FRAME 5, GTN-16, which are in use nowadays in Russia, are presented. Toxic emissions reduction is achieved through combustors update. The technique of directed dosed air blow-in into the maximum fire temperature zones allows to reduce the NOx emissions to 70–130 mg/Nm3 (converted to NO2 at 15% O2 concentration) via minimal combustor design modifications. Lower NOx emissions, amounting to 15–25 mg/Nm3, can be produced through combustion of lean premixed air and fuel in the 4-burner low-toxic module. When retrofitting the GTN-16 unit combustor, the standard dome was replaced with 20 such modules.

Commentary by Dr. Valentin Fuster
1997;():V002T06A052. doi:10.1115/97-GT-370.
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The exhaust plumes of modern gas turbine engines are of great concern due to the emission of atmospheric pollutants, such as carbon monoxide, unburnt hydrocarbons and oxides of nitrogen (NOx) and visibility caused by the presence of black carbonaceous smoke and nitrogen dioxide (NO2) giving rise to a new plume visibility phenomena of “yellow smoke”. A detailed hydrocarbon oxidation and NOx scheme was used to simulate chemical reactions occurring through the gas turbine engine and near-field plume. In addition limited experimental measurements have been made directly behind a reheated gas turbine engine to measure gaseous emissions and to quantify the rate of conversion of nitric oxide to nitrogen dioxide. Two experimental methods were employed to measure emissions; the first a conventional probe technique, the second a non-intrusive method. Results show a fair agreement between experimental data and predicted emissions, showing the maximum conversion of NO to NO2 at low reheat fuel flowrates. These detailed results can be used as an input to atmospheric modelling codes.

Commentary by Dr. Valentin Fuster
1997;():V002T06A053. doi:10.1115/97-GT-371.
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This paper describes the methodology and application of Computational Fluid Dynamics (CFD) to Dry Low NOx (DLN) combustion systems throughout the range of small industrial gas turbines produced at European Gas Turbines (EGT) Lincoln UK.

The use of CFD in the development of such systems has been encouraged not only by the availability of a variety of general purpose CFD codes, but also by the inherent difficulties associated with direct measurement in such a harsh environment. Combusting flow analyses provide detailed predictions of local temperature and velocity fields together with exhaust emissions, enabling numerous conceptual studies to be undertaken without the usual associated mechanical difficulties.

In particular, the work EGT has concentrated on concerns the prediction of fuel / air mixing quality upstream of the flame front, in order to assess the effect of fuel injector design variables on NOx production. This methodology has accelerated injector development resulting in less than 10 ppmV NOx combustors.

Validation of the detailed features of the flow field is currently underway, though parametric comparisons have already proved consistently accurate in displaying the trends necessary for the development of an ultra low NOx combustion system. Correlations of rig emissions data with overall predictions have shown to be in good agreement.

Commentary by Dr. Valentin Fuster
1997;():V002T06A054. doi:10.1115/97-GT-395.
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Theoretical background, details of implementation and validation results of a computational model for turbulent premixed gaseous combustion at high turbulent Reynolds numbers are presented. The model describes the combustion process in terms of a single transport equation for a progress variable; closure of the progress variable’s source term is based on a model for the turbulent flame speed. The latter is identified as a parameter of prime significance in premixed turbulent combustion and is determined from theoretical considerations and scaling arguments, taking into account physico-chemical properties of the combustible mixture and local turbulent parameters. Specifically, phenomena like thickening, wrinkling and straining of the flame front by the turbulent velocity field are considered, yielding a closed form expression for the turbulent flame speed that involves, e.g., speed, thickness and critical gradient of a laminar flame, local turbulent length scale and fluctuation intensity. This closure approach is very efficient and elegant, as it requires only one transport equation more than the non-reacting flow case, and there is no need for costly evaluation of chemical source terms or integration over probability density functions.

The model was implemented in a finite-volume based computational fluid dynamics code and validated against detailed experimental data taken from a large scale atmospheric gas turbine burner test stand. The predictions of the model compare well with the available experimental results. It has been observed that the model is significantly more robust and computationally efficient than other combustion models. This attribute makes the model particularly interesting for applications to large 3D problems in complicated geometries.

Commentary by Dr. Valentin Fuster
1997;():V002T06A055. doi:10.1115/97-GT-396.
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Swirling flames are used in many industrial applications like process furnaces, boilers and gas turbines due to their excellent mixing, stability, emission and burnout characteristics. The wide-spread use of swirl burners in the process and energy industries and, in particular, new concepts for the reduction of NOx-emissions raise the need for simple-to-use models for predicting lean stability limits of highly turbulent flames stabilized by internal recirculation.

Based on recently published experimental data of the first author concerning the reaction structures of swirling flames operating near the extinction limit, different methods for predicting lean blow-off limits have been developed and tested. The aim of the investigations was to find stabilization criteria that allow predictions of blow-off limits of highly turbulent recirculating flames without the requirement for measurements in those flames.

Several similarity criteria based on volumetric flow rates, burner size and material parameters of the cold gases, were found to be capable of predicting stability limits of premixed and (in some cases) nonpremixed flames at varying swirl intensities, burner scales and fuel compositions. A previously developed numerical field model, combining a k,ϵ-model with a combined “assumed-shape Joint-PDF”/Eddy-Dissipation reaction model was also tested for its potential for stability prediction.

Commentary by Dr. Valentin Fuster
1997;():V002T06A056. doi:10.1115/97-GT-439.
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This paper summarizes experimental and computational results on the mixing of opposed rows of jets with a confined subsonic crossflow in rectangular ducts. The studies from which these results were excerpted investigated flow and geometric variations typical of the complex 3-D flowfield in the combustion chambers in gas turbine engines.

The principal observation was that the momentum-flux ratio, J, and the orifice spacing, S/H, were the most significant flow and geometric variables. Jet penetration was critical, and penetration decreased as either momentum-flux ratio or orifice spacing decreased. It also appeared that jet penetration remained similar with variations in orifice size, shape, spacing, and momentum-flux ratio when the orifice spacing was inversely proportional to the square-root of the momentum-flux ratio. It was also seen that planar averages must be considered in context with the distributions. Note also that the mass-flow ratios and the orifices investigated were often very large (jet-to-mainstream mass-flow ratio >1 and the ratio of orifices-area-to-mainstream-cross-sectional-area up to 0.5 respectively), and the axial planes of interest were often just downstream of the orifice trailing edge. Three-dimensional flow was a key part of efficient mixing and was observed for all configurations.

Topics: Jets , Ducts , Orifices
Commentary by Dr. Valentin Fuster
1997;():V002T06A057. doi:10.1115/97-GT-440.
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The combustor for 300kW class ceramic gas turbine (CGT301) has been developed to achieve high thermal efficiency of 42%. The combustor inlet air temperature and the turbine inlet temperature(TIT) are higher compared with the conventional same class gas turbine. The “pilot” CGT system, with a recuperator to recover the waste heat in the exhaust gas, can heat combustor inlet air up to 1013K(740°C) at TIT of 1623K(1350°C). Low NOx emission is one of the important problems for the CGT combustor. To solve the problem, the lean premixed combustion method was applied. Stable combustion is another important problem for the lean premixed combustor. To solve the problem, this combustor employed the variable geometory system and the fuel staging system. The former controlled the temperature in the combustion zone. The latter optimized the distribution of air/fuel concentration in the mixing zone. The CGT system has achieved both stable combustion and low NOx emission. NOx emission was reduced to 17.6 ppm against the “primary type” CGT target of 70 ppm under the full load condition at TIT of 1200°C in combustor rig test. The engine tests of “primary type” CGT with a ceramic combustor, which did not employ the variable geometry system and the fuel staging system, were conducted successfully for a total of 32 hours without any damage of the ceramic combustor under the full load condition.

Commentary by Dr. Valentin Fuster
1997;():V002T06A058. doi:10.1115/97-GT-459.
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The geometric and operational features of gas turbine engine combustors are receiving increased scrutiny due to a growing concern regarding environmental impact, performance, durability, and manufacturability. To minimize the risk associated with new projects, optimization of designs which are similar to those in current operation is attractive. To achieve this goal, a methodology that is efficient and can reveal interactions between parameters that affect performance is necessary. An approach which addresses these requirements is statistically designed experiments (e.g., multivariate experiments or “design of experiments”), which offers efficiency as well as the ability to identify interactions between variables. This approach was adopted and demonstrated in the present study utilizing a set of hardware specifically developed to allow multivariate experiments to be conducted. A radial mixer geometry consisting of four parameters (primary and secondary swirl vane angles, the presence of a venturi, and the co-/counter swirl sense) was examined. The design was developed to maintain constant effective area and overall dimensions. The responses selected for study were stability (i.e., reaction lean blow-out) and fuel distribution.

The results reveal that (1) the multivariate approach is effective in the present application, (2) the swirl sense between the primary and secondary swirlers play an influential role in determining the uniformity of the spray, (3) the size of the fuel spray area is affected by the mixer venturi and the swirl sense, (4) the symmetry of the fuel presentation is affected by the interaction of the swirler angles, (5) Lean Blow Out (LBO) is not affected by the parameters selected, and (6) the parameters affecting fuel distribution and combustion stability differ, indicating that the combustion performance is not described entirely by fuel distribution.

Commentary by Dr. Valentin Fuster
1997;():V002T06A059. doi:10.1115/97-GT-460.
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Gas turbine fuel atomization nozzles are subject to internal erosion damage caused by the flow of the liquid fuel and its inevitable contaminants. Nozzles must be sufficiently erosion resistant, or erosion tolerant, to insure acceptable performance throughout their design life. Recently developed macrolamination (ML) technology for construction of pressure-swirl nozzles shows potential performance, cost, and size benefits when compared to conventionally constructed nozzles. The objective of this study is to compare macrolaminated and conventional nozzles under identical accelerated erosive conditions to determine their relative wear characteristics. To date, twelve nozzles have been compared with some being disregarded because of anomalies. It is shown that erosion damage results in increased flow rates. Also, the tendencies shown are for conventional nozzles to erode to narrower spray angles and macrolaminated nozzles to erode to wider spray angles. ML nozzles exhibit some important failure modes that differ from nozzles of conventional design.

Topics: Pressure , Erosion
Commentary by Dr. Valentin Fuster
1997;():V002T06A060. doi:10.1115/97-GT-477.
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A cooling entrance region for a modern annular combustor has been successfully tested. Because of high heat load at the entrance part of the liner, a good design is very important. The design presented here offers ultra-low dilution, robustness and high performance. The cooling system includes convective, film and impingement techniques, which were experimentally investigated in a plastic (perspex) model of a full scale 60° sector of a simplified combustor design. The importance of a high performance entrance is obvious. It generates low pressure drop, better combustion (circumferential even flow for the liner cooling) and it gives lower flame temperature (maximizes the air in the flame). Guidelines for the design of such an entrance are presented in this paper.

Commentary by Dr. Valentin Fuster
1997;():V002T06A061. doi:10.1115/97-GT-478.
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The development of a dry, low-emissions combustor for the AlliedSignal Model ASE120 industrial gas turbine engine is in progress. The combustor is designed to provide 10MW of engine power output and also meet all current exhaust emissions requirements. The combustion system has a single-stage premixer and a novel, yet simple, variable geometry to control the flame temperature over the entire operating range of the ASE120 gas turbine engine. Design concepts of this lean premix-prevaporize combustor operating on air staging technology are presented. Preliminary results from computational fluid dynamics (CFD) analyses of the system are discussed.

Commentary by Dr. Valentin Fuster

Oil and Gas Applications

1997;():V002T07A001. doi:10.1115/97-GT-508.
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Gas Turbine (GT) performance seriously deteriorates at increased ambient temperature. This study analyses the possibility of improving GT power output and efficiency by installing a gas turbine inlet air cooling system.

Different cooling systems were analyzed and preliminary cost evaluations for each system were carried out.

The following three cooling systems were considered in detail:

a) Traditional compression cooling system;

b) Absorption single-acting cooling system using a solution of lithium bromide;

c) Absorption double-acting cooling system using a solution of lithium bromide.

Results clearly indicate that there is a great potential for GT performance enhancement by application of an Inlet Air Cooling (IAC). Technical and economical analyses lead to selection of a particular type of IAC for significant savings in capital outlay, operational and maintenance costs and other additional advantages.

Commentary by Dr. Valentin Fuster
1997;():V002T07A002. doi:10.1115/97-GT-509.
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The Cooper-Bessemer Rotating Products group of Cooper Energy Services has designed an all-new industrial gas turbine / compressor package based upon the Allison Engine Company 501-KC5 gas generator with a two-stage industrial power turbine. The latest project management techniques were employed to reduce design cycle time while optimizing total product quality, manufacturability, and reliability. The resulting gas turbine / compressor package is a low-risk, technologically conservative approach, designed to avoid the problems often associated with new product development.

Commentary by Dr. Valentin Fuster
1997;():V002T07A003. doi:10.1115/97-GT-510.
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Thermodynamic/environmental assessment of a CRGT system was carried out on an RB211 based compressor station. The configured cycle was evaluated using the commercial process simulation software ASPEN PLUS®. The results of the thermodynamic assessment are presented in terms of overall thermal efficiency, specific work, unit duties, conversion rates and other relevant parameters. The above were determined for varying fuel split ratios, steam to carbon ratios in the reformer, and part loads. Comparison between CRGT operation and STIG operation is given. A NOx model was also constructed based on a one dimensional model of the combustor using ASPEN PLUS® which showed substantial reduction of the NOx emissions from the CRGT as compared to the basic cycle. Additionally, the CRGT system is evaluated economically in terms of capital and operating costs of the added equipment to a typical compressor station operation. Preliminary results of these environmental and economic analysis are presented.

Commentary by Dr. Valentin Fuster
1997;():V002T07A004. doi:10.1115/97-GT-511.
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The 6000 shp class Allison 501-KC5S gas turbine will be introduced to the industrial power generation market in 1997 as a low-risk upgrade of the 501-KC5 engine. The aero-derivative and industrial background of the 501-KC5 engine is discussed along with the 501-KC5S product definition and product description. In addition, near-term horsepower increase plans for a 501-KC7 are outlined.

Commentary by Dr. Valentin Fuster
1997;():V002T07A005. doi:10.1115/97-GT-512.
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This paper presents the authors’ perspective regarding the growth of gas turbine technology as applied to the industrial market for the next two decades. Although emphasis is placed on off-shore (platform and floating production) applications, the effects of the advance in technology of gas turbines for land based operations is included. Past trends in the advancement of basic gas turbine technology are utilized as the basis to establish this forecast.

An introduction and a description of the Air Bottoming Cycle, the Intercooled Gas Turbine Cycle, and a hybrid gas turbine combining aeroderivative and heavy frame design are also included.

Commentary by Dr. Valentin Fuster
1997;():V002T07A006. doi:10.1115/97-GT-513.
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This paper presents the Kværner design concept for an LM2500 Gas Turbine Package, with combined engine interfaces for both the LM2500 PE and the LM2500 Plus. The paper also presents the Kværner Modularized Auxiliary System concept, where the lube oil module and the fuel modules are located in separate compartments integrated in the turbine skid, protected from soak-back heat and blade-out conditions.

Commentary by Dr. Valentin Fuster

Cycle Innovations

1997;():V002T08A001. doi:10.1115/97-GT-123.
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High generating efficiency has compelling economic and environmental benefits for electric power plants. There are particular incentives to develop more efficient and cleaner coal-fired power plants, to permit use of the world’s most abundant and secure energy source. This paper presents a newly-conceived power plant design, the Dual Brayton Cycle Gas Turbine PFBC, that yields 45% net generating efficiency and fires on a wide range of fuels with minimum pollution, of which coal is a particularly intriguing target for its first application. The DBC-GT design allows power plants based on the state-of-the-art PFBC technology to achieve substantially higher generating efficiencies while simultaneously providing modern gas turbine and related heat exchanger technologies access to the large coal power generation market.

Commentary by Dr. Valentin Fuster
1997;():V002T08A002. doi:10.1115/97-GT-125.
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The main performance features of a semiclosed cycle gas turbine with carbon dioxide-argon working fluid are described here. This machine is designed to employ coal synthetic gas fuel and to produce no emissions.

The present paper outlines three tasks carried out. Firstly the selection of main engine variables, mainly pressure and temperature ratios. Then a sizing exercise is carried out where many details of its physical appearance are outlined. Finally the off-design performance of the engine is predicted.

This two spool gas turbine is purpose built for the working fluid, so its physical characteristics reflect this requirement. The cycle is designed with a turbine entry temperature of 1650 K and the optimum pressure ratio is found to be around 60. Two major alternatives are examined, the simple and the precooled cycle.

A large amount of nitrogen is produced by the air separation plant associated with this gas turbine and the coal gasifier. An investigation has been made on how to use this nitrogen to improve the performance of the engine by precooling the compressor, cooling the turbine nozzle guide vanes and using it to cool the delivery of the low pressure compressor.

The efficiencies of the whole plant have been computed, taking into account the energy requirements of the gasifier and the need to dispose of the excess carbon dioxide. Hence the overall efficiencies indicated here are of the order of 40 percent. This is a low efficiency by current standards, but the fuel employed is coal and no emissions are produced.

Commentary by Dr. Valentin Fuster
1997;():V002T08A003. doi:10.1115/97-GT-142.
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An approach to improving the thermal cycle efficiency of combustion turbine (CT) based power plants is to develop thermal cycles with interceding, reheat, recuperation and humidification. Until recently, this was viewed by combustion turbine manufacturers as cost prohibitive and involving new operating and maintenance challenges. Also, early attempts by some manufacturers to develop sophisticated thermal cycles resulted in the realization that significant funds, personnel, and time are required. This investment could not be justified, particularly considering the availability of efficient and economical combined cycle (CC) plants.

Therefore, increased efficiency for both simple and CC power plants has been achieved by raising the firing temperature and pressure of the basic Brayton cycle. However, every increase in the CT firing temperature required progressively higher development cost and increased NOx control challenges, which has re-awakened interest in advanced cycles.

The Department of Energy’s Federal Energy Technology Center (FETC), in cooperation with combustion turbine manufacturers, is working on an Advanced Turbine Systems Program. The program goal is to develop technologies to provide a significant increase in natural gas-fired CC power generation plant efficiency with thermal efficiency target values in excess of 60%. Materials published in the program show that participating large CT original equipment manufacturers (OEM), are relying heavily on an increase in the CT’s firing temperature to approximately 2600 F (1700 K), with associated advancement in materials and cooling techniques, to achieve the target efficiency. Also employed are some improvements in component efficiency and various methods of utilization of heat in the bottoming cycle. The development of the necessary sophisticated materials and cooling techniques requires very significant development costs and is based on long duration and expensive experimental investigations and field demonstrations. Increasing the bottoming cycle efficiency primarily depends on the practicality of engineering solutions and capital vs operating cost trade-offs, and not on technology advancements.

The current Cascaded Humidified Advanced Turbine (CHAT) technology, which utilizes existing, commercially proven combustion turbine and industrial hardware integrated in sophisticated thermal cycles, offers an achievable, practical and cost-effective alternative to a current CC plant. Current CHAT plants require relatively minimal engineering developments associated primarily with a) modification of the power shaft CT’s compressor discharge and turbine inlet plenums — for interfacing the HP shaft and other thermal cycle components — a very important engineering task, but not comparable in complexity to the development associated with further increase of the CT inlet temperature; and b) engineering of an HP expander with an inlet temperature of 1600 F (1145 K), essentially integrating steam turbine and industrial expander technologies.

As it was shown collectively in the previously published references, in addition to an efficiency equal to that for CC plants (based on the same CTs), CHAT plants have significantly lower (10–20%) specific capital costs and have important operating advantages (higher than CC efficiency at part-load operation, with excellent load following and dynamic benefits, including rapid start capability). Those features reduce both the CHAT plant cost of electricity and offer a method to improve improve the economics of power generation systems due to the operational flexibility added by a CHAT plant.

One of the most effective ways to increase the CHAT plant efficiency is to increase the HP expander inlet temperature from the current level of 1600 F (1145 K), which represents the level of the combustion turbine technology of the late 1960’s – early 1970’s. EPRI and ESPC have identified that a CHAT plant, based on the current combustion turbine technology (with turbine inlet temperature (TIT) of 2550 F (1670 K)), could achieve the ATS Program target efficiency of 60% with an HP expander inlet temperature of approximately 2000 F (1365 K). HP expanders with this relatively low turbine inlet temperature, as shown later, will require cooling of only the first stage nozzles and stage blades, if the newest single crystal alloys are used. However, the increase of the HP expander inlet temperature will be complicated by the relatively-high inlet pressure. This presents a significant engineering challenge, particularly if one would like to preserve the excellent start-up characteristics and other dynamic benefits of the CHAT plant.

EPRI and ESPC are co-sponsoring the development of a high pressure expander with a target inlet temperature of 2000 F (1365 K). It will be shown that this HP expander, when integrated with a power shaft based on W 501 FA modified for the CHAT application, results in a power plant that will achieve the ATS Program target efficiency of 60%.

This paper presents a) current CHAT plant’s performance and cost characteristics, b) findings of the project for the development of the HP expander with target temperature of 2000 F (1365 K), and c) a comparison of the advanced CHAT concept’s performance and development costs to those of the ATS program. This paper also shows how, in the future, the new ATS technology can be incorporated into even more efficient, cost-effective and reliable CHAT power plants.

Topics: Combustion , Turbines
Commentary by Dr. Valentin Fuster
1997;():V002T08A004. doi:10.1115/97-GT-285.
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The design-point performance characteristics of a wide variety of combined-cogeneration power plants, with different amounts of supplementary firing (or no supplementary firing), different amounts of steam injection (or no steam injection), different amounts of exhaust gas condensation etc, without limiting these parameters to present-day limits are investigated. A representative power plant with appropriate components for these plant enhancements is developed. A computer program is used to evaluate the performance of various power plants using standard inputs for component efficiencies; and the design-point performance of these plants is computed. The results are presented as thermal efficiency, specific power, effectiveness, and specific rate of energy in district heating. The performance of the simple-cycle gas turbine dominates the overall plant performance; the plant efficiency and power are mainly determined by turbine inlet temperature and compressor pressure ratio; increasing amounts of steam injection in the gas turbine increases the efficiency and power; increasing amounts of supplementary firing decreases the efficiency but increases the power; with sufficient amounts of supplementary firing and steam injection the exhaust-gas condensate is sufficient to make up for water lost in steam injection; and the steam-turbine power is a fraction (0.1 to 0.5) of the gas-turbine power output. Regions of “optimum” parameters for the power plant based on design-point power, hot-water demand, and efficiency are shown. A method for fuel-cost allocation between electricity and hot water is recommended.

Commentary by Dr. Valentin Fuster
1997;():V002T08A005. doi:10.1115/97-GT-286.
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Methods to analyze, improve and optimize thermal energy systems have to take into account not only energy (exergy) consumption and economic resources but also pollution and degradation of the environment. The term “environomics” implies a method which takes thermodynamic, economic and environmental aspects systematically into consideration for the analysis and optimization of energy systems. For optimization of energy systems, the environmental aspects are quantified and introduced into the objective function.

In this particular work, the environomic approach is followed for the analysis and optimal design of a combined-cycle plant. In addition to the basic configuration, two alternatives for NOx abatement are studied: Selective Catalytic Reduction (SCR) and steam injection. The optimization problem is solved for each configuration and the results are compared with each other. The effect of the unit pollution penalties and of the limits imposed by regulations is studied. Some general conclusions are drawn.

Commentary by Dr. Valentin Fuster
1997;():V002T08A006. doi:10.1115/97-GT-287.
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Improvements to the steam bottoming cycles hold the promise of raising the combined cycle thermal efficiency to values near and above 60%. Up to now, steam bottoming cycles with three pressure levels of steam evaporation have been realised. A further advantage seems possible by the use of double fluids, such as mixtures of steam and ammonia.

In the cycle proposed here, the authors limit Themselves to the use of steam and water only, in order to avoid all the difficulties, that may arise from such mixtures. The solution given here, relies on multiple evaporation levels, more than three up to five and even more. They should be to be achieved with the help of newly developed steam turbochargers, which allow the unification of the steam flow from three different neighbouring pressure levels, into one steam flow to be transmitted via the live steam line to the main turbine. This large number of evaporation levels, together with the required economisers for feed water heating and the ensuing superheaters arranged in the proper way, gives a steam water heat acceptance curve, which can be closely matched to the exhaust gas cooling line, so that the heat transfer from the gas turbine exhaust to the steam bottoming cycle can be effected with a minimum of temperature differences.

It should be pointed out that the steam pressures are selected in the undercritical region, and that a total combined cycle efficiency very near to 60% can be achieved. Using most modern gas turbine models together with this novel bottoming cycle will even allow to exceed the value of 60%. Examples given have been calculated for standard gas turbine models.

Topics: Evaporation , Cycles , Steam
Commentary by Dr. Valentin Fuster
1997;():V002T08A007. doi:10.1115/97-GT-288.
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Closed loop steam cooling schemes have been proposed by a number of manufacturers for advanced Combined Cycle Gas Turbine (CCGT) power plant (see for example Corman (1996) and Briesch et al. (1994)) asserting that thermal efficiencies in excess of 60% (LHV) are achievable combined with significant improvements of ∼15% in specific power (see Corman (1995)). In understanding the efficiency advantage however, the relative performance of each cooling system (subject to the same practical constraints and technology levels) is a better indicator then the absolute value.

Assessment of the performance of such novel schemes generally involves a detailed numerical analysis of an integrated cycle which may often prevent validation of the results or obscure an understanding of the physical basis for the claimed improvements. Here, to overcome this, a group of simplified expressions are defined for the variation of each cycles efficiency due to cooling which show where the differences come from. These expressions are based simply on a calculation of the marginal increase in heat rejected, to the environment from the cycle, due to an increase in the level of cooling. After these relationships are validated using detailed heat balance calculations they are used to compare the main cooling options, namely open loop air, closed loop air and closed loop steam when subject to the same practical constraints and assumptions. Based on these results it is proposed that the relative advantage of closed loop cooling may not be as significant as previously thought. Furthermore, it is shown that the closed loop cooling efficiency gain is heavily dependent on the performance and reliability of substantial Thermal Barrier Coatings (TBCs).

Finally, although the majority of recent interest in closed loop cooling schemes has focused upon CCGT plant, there are other systems where the benefits of closed loop steam cooling appear to be greater, in particular cycles involving steam injected gas turbines. Such a cycle is analysed here with a number of advanced cooling options.

Topics: Cycles
Commentary by Dr. Valentin Fuster
1997;():V002T08A008. doi:10.1115/97-GT-324.
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Dynamic instabilities become increasingly one of the design criteria for natural-circulation evaporators in Heat Recovery Steam Generators (HRSGs) of combined cycle power plants, especially in systems with a vertical gas duct. To calculate such instabilities, the computer program DEDYN is being developed. The calculation method has been presented at the ASME Cogen-Turbo Power 1995 in Vienna (Payrhuber, 1995).

This publication deals with modeling of a natural-circulation evaporator. The evaporator investigated in this paper consists of 340 parallel finned tubes in 4 layers of 16 meters (total tube length of one tube is 64 meters) and a total heat transfer surface of 16400 square meters. This heat exchanger bundle of a HRSG with a vertical duct is divided in 8 sub-ducts, which permits a calculation with local heating medium (= flue gas) temperatures. The inlet and outlet headers of this bundle are connected to a drum.

Several variations have been investigated like changing the operation pressure, different heights of the drum, subcooling when entering the downcomer, as well as parallel flow or counter flow through the evaporator tube bundle. The paper shows the boundary conditions for preventing or at least damping mass flow oscillations in the evaporator.

Commentary by Dr. Valentin Fuster
1997;():V002T08A009. doi:10.1115/97-GT-340.
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A number of cycles have been proposed in which a solid oxide fuel cell is used as the topping cycle to a gas turbine, including those recently described by Beve et al. (1996). Such proposals frequently focus on the combination of particular gas turbines with particular fuel cells. In this paper, the development of more general models for a number of alternative cycles is described. These models incorporate variations of component performance with key cycle parameters such as gas turbine pressure ratio, fuel cell operating temperature and air flow. Parametric studies are conducted using these models to produce performance maps, giving overall cycle performance in terms of both gas turbine and fuel cell design point operating conditions. The location of potential gas turbine and fuel cell combinations on these maps is then used to identify which of these combinations are most likely to be appropriate for optimum efficiency and power output. It is well known, for example, that the design point of a gas turbine optimised for simple cycle performance is not generally optimal for combined cycle gas turbine performance. The same phenomenon may be observed in combined fuel cell and gas turbine cycles, where both the fuel cell and the gas turbine are likely to differ from those which would be selected for peak simple cycle efficiency. The implications of this for practical fuel cell and gas turbine combined cycles and for development targets for solid oxide fuel cells are discussed. Finally, a brief comparison of the economics of simple cycle fuel cells, simple cycle gas turbines and fuel cell and gas turbine combined cycles is presented, illustrating the benefits which could result.

Commentary by Dr. Valentin Fuster
1997;():V002T08A010. doi:10.1115/97-GT-351.
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The combustion gas turbine, operating in both simple and combined cycle modes, is rapidly becoming the preferred prime-mover for electrical power generation for both new plants, and in the repowering of old power stations. In replacing Rankine cycle plants the combustion gas turbine could become dominant in the power generation field early in the next century. Fired currently with natural gas, and later with gasified coal these gas turbines will operate for many decades with no concern about resource depletion. This paper addresses an extension of high efficiency gas turbine technology but uses a combustion and emission-free heat source, namely a high temperature gas cooled nuclear reactor. The motivation for this evolution is essentially twofold, 1) to introduce an environmentally benign plant that does not emit greenhouse gases, and 2) provide electrical power to nations that have no indigenous natural gas or coal supplies. This paper presents a confidence-building approach that eliminates risk towards the goal of making the nuclear gas turbine a reality in the 21st century.

Commentary by Dr. Valentin Fuster
1997;():V002T08A011. doi:10.1115/97-GT-387.
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Specification of a turbocharger for a given engine involves matching the turbocharger performance characteristics with those of the piston engine. Theoretical considerations of matching turbocharger pressure ratio and mass flow with engine mass flow and power permits designers to approach a series of potential turbochargers suitable for the engine. Ultimately, the final choice among several candidate turbochargers is made by tests. In this paper two types of steady-flow experiments are used to match three different turbochargers to an automotive turbocharged-intercooled gasoline engine. The first set of tests measures the steady-flow performance of the compressors and turbines of the three turbochargers. The second set of tests measures the steady-flow design-point and off-design-point engine performance with each turbocharger. The test results show the design-point and off-design-point performance of the over-all thermodynamic cycle, and this is used to identify which turbocharger is suitable for different types of engine duties.

Commentary by Dr. Valentin Fuster
1997;():V002T08A012. doi:10.1115/97-GT-408.
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Humidification of the flow through a gas turbine has been proposed in a variety of forms. The STIG plant involves the generation of steam by the gas turbine exhaust in a heat recovery steam generator [HRSG], and its injection into or downstream of the combustion chamber. This increases the mass flow through the turbine and the power output from the plant, with a small increase in efficiency. In the evaporative gas turbine [or EGT] cycle, water is injected in the compressor discharge in a regenerative gas turbine cycle [a so-called CBTX plant-compressor [C], burner [B], turbine [T], heat exchanger [X]]; the air is evaporatively cooled before it enters the heat exchanger. While the addition of water increases the turbine mass flow and power output, there is also apparent benefit in reducing the temperature drop in the exhaust stack. In one variation of the basic EGT cycle, water is also added downstream of the evaporative aftercooler, even continuously in the heat exchanger. There are several other variations on the basic cycle [e.g. the cascaded humidified advanced turbine (CHAT)].

The present paper analyses the performance of the EGT cycle. The basic thermodynamics are first discussed, and related to the cycle analysis of a dry regenerative gas turbine plant. Subsequently some detailed calculations of EGT cycles are presented. The main purpose of the work is to seek the optimum pressure ratio in the EGT cycle for given constraints [e.g. fixed maximum to minimum temperature]. It is argued that this optimum has a relatively low value.

Topics: Gas turbines , Cycles
Commentary by Dr. Valentin Fuster
1997;():V002T08A013. doi:10.1115/97-GT-409.
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After industrial gas turbines have been in production for some amount of time, there is often an opportunity to improve or “uprate” the engine’s output power or cycle efficiency or both. In most cases, the manufacturer would like to provide these uprates without compromising the proven reliability and durability of the product. Further, the manufacturer would like the development of this “Uprate” to be low cost, low risk and result in an improvement in “customer value” over that of the original design. This paper describes several options available for enhancing the performance of an existing industrial gas turbine engine and discusses the implications for each option. Advantages and disadvantages of each option are given along with considerations that should be taken into account in selecting one option over another.

Specific options discussed include dimensional scaling, improving component efficiencies, increasing massflow, compressor zero staging, increasing firing temperature (thermal uprate), adding a recuperator, increasing cycle pressure ratio, and converting to a single shaft design. The implications on output power, cycle efficiency, off-design performance engine life or time between overhaul (TBO), engine cost, development time and cost, auxiliary requirements and product support issues are discussed. Several examples are provided where these options have been successfully implemented in industrial gas turbine engines.

Commentary by Dr. Valentin Fuster
1997;():V002T08A014. doi:10.1115/97-GT-441.
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LNG regasification process needs a considerable quantity of thermal energy that is usually obtained by cooling sea water or by burning a fraction of the evaporated natural gas. These systems, though offering low cost and high reliability, are thermodynamically inefficient: they require energy for water pumping or fuel to provide heat and do not exploit the physical exergy related to the initial conditions of LNG to produce mechanical work.

The present paper aims to assess the performances of various gas turbine based cycles which use the LNG regasification process as a low temperature heat sink for power cycles. In particular it will focus on the following configurations:

• Closed loop gas cycles

• Gas-gas combined cycles

• Combined gas-organic Rankine cycles

Two different sendout pressure (70 and 30 bar, corresponding respectively to the supply of long-distance pipelines or power plants based on heavy-duty gas turbines) are considered. Their performances are calculated and proper effectiveness indexes (e.g. thermal and exergetic efficiency) are introduced to carry out a comprehensive comparison among the systems considered. A simple economical analysis completes the discussion.

Commentary by Dr. Valentin Fuster
1997;():V002T08A015. doi:10.1115/97-GT-442.
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Due to techno-economic assets, the demand of combined cycles (CC) is currently growing. Nowadays, in a diversified electricity mix, these plants are often used on a load cycling duty or in the intermediate load range. The ability to start quickly and reliably may be a decisional criterion for the selection of the plant, in addition to the design performance, the cost and the pollutant emissions. Therefore, together with the simulation of CC transients, a proper monitoring system aimed at keeping high plant performance during the transients is required.

With the help of advanced measurement and monitoring devices, artificial intelligence (AI) techniques as expert systems (ES) and neural networks (NN) can fulfill this duty.

The goal of this paper is to show that a NN technique can be used reliably to obtain the response of a complex energetic system, such as CCs, during a slow transient and consequently as part of an on-line monitoring system.

In this work, a CC power plant is simulated by dividing it into three blocks, which are representative of the three main elements of the CC: namely the gas turbine (GT), the heat recovery steam generator (HRSG) and the steam turbine (ST). To each of them a NN is associated. Once the training and testing of the NNs is carried out, the blocks are then arranged in a series cascade, the output of a block being the input of the subsequent one. With this solution, the NN-based system is able to produce the transient response of a CC plant when the input information are the GT inlet parameters.

The transient data, not easy to obtain from measurements on existing plants, are provided by the CCDYN simulator (Dechamps, 1995). The performance obtained by the NN based system are observed to be in good agreement with those given by CCDYN, the latter being validated on the basis of measurements in an existing plant. The NN code, providing the departures of the measured data from the predicted ones, can be considered as a proper system for on-line monitoring and diagnosis.

Commentary by Dr. Valentin Fuster
1997;():V002T08A016. doi:10.1115/97-GT-490.
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Advanced gas turbine designs require revisiting the optimization process to provide maximum competitiveness of new generating installations. This counts specifically for those designs created for combined cycle applications. Gas turbine performance and its associated exhaust temperature has been increasing at a rapid pace over recent years. The conventional method of selecting a GT based upon price and performance, and then designing a complex bottoming cycle does not provide sufficient solutions for power generation in an open access marketplace. The optimal solution takes into account the interrelation between the GT and WS cycle, leading to a more efficient, simplified and flexible power plant. This analysis shows how different levels of GT exhaust energy lead to different optimum cycle solutions. It shows, as postulated above, that considering the WS cycle demands in gas turbine design leads to a simpler cycle with inherent advantages in efficiency, reliability and flexibility.

Commentary by Dr. Valentin Fuster
1997;():V002T08A017. doi:10.1115/97-GT-491.
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The new thermodynamic cycle using hydrogen energy is now under investigation by many engineers. The Japanese World Energy Network research program is also hydrogen technology development program. One of the target of the program is to develop the high thermal efficiency power plant without emissions. The H2-O2 fired gas turbine is the key technology of the project and the new RANKINE cycle is suggested as one of the most effective cycle.

The new RANKINE cycle is based on the direct steam expansion cycle and the performance calculation has been examined to find the optimal operating point. For the cycle development, further investigations of the component development, the operational ability, and the cost competitiveness are important. Among these investigations, this paper reports the operational ability, especially the start up performance.

In this analysis, the algorithm and flow line for start up is developed. And the investigation finds that the new RANKINE cycle has the good possibility for the practical use.

Topics: Gas turbines , Cycles
Commentary by Dr. Valentin Fuster
1997;():V002T08A018. doi:10.1115/97-GT-492.
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A study has been carried out to assess the performance improvement of a combined cycle used for an industrial power plant when ceramic turbine components are employed. This paper presents the details of this study. Performance improvement is obtained as a result of reduced blade cooling air. In this study four different kinds of combined cycles were investigated and these are listed below:

A. Combined cycle with a simple gas turbine.

B. Combined cycle with an inter-cooled gas turbine.

C. Combined cycle with a reheat gas turbine.

D. Combined cycle with an inter-cooled reheat gas turbine.

Results of this study indicate that the combined cycle with a simple gas turbine is the most practical of the four cycles studied with an efficiency of higher than 60%. The combined cycle with reheat gas turbine has the highest efficiency if a higher compressor exit air temperature and a high gas temperature (over 1000°C) to reheat the combustion system are used. A higher pressure ratio is required to optimize the cycle performance of the combined cycle with the ceramic turbine components than that with the metal turbine components because of reduced blade cooling air. To minimize leakage air for these higher pressure ratios, advanced seal technology should be applied to the gas turbines.

Commentary by Dr. Valentin Fuster
1997;():V002T08A019. doi:10.1115/97-GT-493.
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Combined gas turbine-steam turbine cycles have gained widespread acceptance as the most efficient utilization of the gas turbine for power generation, particularly for large power plants. In order to maximize the achievable thermal efficiency, more than one exhaust heat recovery boiler is used. The current trend is to use three boilers at three different operating pressures, which improves thermal efficiency but significantly increases the initial cost of the plant.

There are advantages in replacing an exhaust heat recovery system using multiple boilers by a single heat exchanger in which the water side pressure is above the critical pressure of water; we shall refer to such a heat exchanger as a supercritical heat exchanger. The supercritical steam leaving the heat exchanger is expanded in a two phase turbine and then fed into the engine combustor. A condenser and a water treatment system are used to recover most of the water in the exhaust stream. A turbine system identical to the basic engine turbine system is added in parallel in order to allow for the operation with increased mass flow due to the steam injection. To achieve maximum efficiency such a turbine should be provided with variable area nozzles. With this arrangement, it becomes possible to inject sufficient steam to produce stoichiometric combustion at the desired turbine inlet temperature. We shall refer to this cycle as the Water Injected Stoichiometric Combustion (WISC) gas turbine cycle. The various components described above can be added to any existing gas turbine engine to change it to the WISC configuration.

The WISC engine offers significant economical advantages. The specific power output per pound of air for the WISC engine is more than five times that of the basic engine, the thermal efficiency is 75% higher than that of the basic engine. This produces a significant reduction in the initial investment in the plant as well as its operating expenses.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster

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