Gas Turbine Combustion

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Combustion

The background information is essential to produce the understanding of the gas turbine combustion process. During the combustion process, as the fuel and oxidizer are turned into exhaust products, heat is generated. Interestingly, some source of heat is also necessary to start combustion. Fuel is introduced at the front end of the burner in a highly atomized spray from the fuel nozzles.

Combustion air flows in around the fuel nozzle and mixes with the fuel to form a correct fuel-air mixture. This is called ‘primary air' and represents approximately 25 percent of total air taken into the engine. The fuel-air mixture which is to be burned is a ratio of 15 parts of air to 1 part of fuel by weight. The remaining 75 percent of the air is used to form an air blanket around the burning gases and to lower the temperature. This temperature may reach as high as 3500° F. By using 75 percent of the air for cooling, the temperature operating range can be brought down to about half, so the turbine section will not be destroyed by excessive heat. Mattingly [5] The air used for burning is called primary air- and that for cooling is secondary air. The secondary air is controlled and directed by holes and louvers in the combustion chamber liner.

Historical Overview

Combustion instabilities were identified at the start of the 1950s as an endemic disease and were then the subject of research aimed at understanding their origins, explaining how they developed and, eventually, predicting their levels. The researchers were very quickly convinced of the difficulty of the problem, which is essentially due to two factors: firstly, the difficulty of taking detailed measurements of the internal flow in engines, because of the extremely severe physical conditions inside them, and secondly, the close coupling between numerous unsteady mechanisms related to fluid mechanics, combustion, two-phase flows, etc. Unfortunately, this report doesn't allow accounting for the personal efforts of individuals in the field.

Combustion Instability

The term combustion instability refers to a wide variety of oscillatory phenomena observed in combustion systems. Unstable combustion is not desirable and it can reduce efficiency and increase pollution. Considering that the combustion process leads to a localized heat release with high energy, there is no surprise that instability occurs. Combustion takes place inside a chamber with inlet-outlet conditions, where the acoustic system can also introduce oscillatory effects, like a Helmholtz oscillator. The coupled heat-release process and acoustic dynamics can produce a large amplitude pressure oscillation, which is called a “thermo acoustic instability”. The growth of the pressure oscillation degrades the system performance of a combustion engine. There are a variety of other sources of combustion instabilities including thermodiffusive instabilities, chemical-kinetic instabilities, convective instabilities, and Kelvin-Helmholtz instability. [2][3]

Combustion instabilities have been found in all types of systems. The reason is very simple and fundamental i.e. by design the combustion processes generate high power densities under conditions when the losses of energy are small. [Figure 4], Only weak coupling between fluctuations of the combustion power and the flow of the medium is sufficient to produce undesirable fluctuations of pressure and kinetic energy in the flow. It is therefore possible to construct a general analytical framework sufficiently comprehensive to capture most of the main features of instabilities in any combustion system.

Gas Turbine Combustion Chamber

Today, three basic combustion chambers are in use. They are the annular combustion chamber, the can type, and the combination of the two called the can-annular. The combustion section contains the combustion chambers, igniter plugs, and fuel nozzles or vaporizing tubes. Igniter plugs function only during starting, being cut out of the circuit as soon as combustion is self-supporting. The combustion chamber must be of light construction and is designed to burn fuel completely in a high velocity air stream. Mattingly [5] provides a sound understanding of the subject. The combustion chamber liner is an extremely critical engine part because of the high temperatures of the flame. The liner is usually constructed of welded high-nickel steel. The most severe operating periods in combustion chambers are encountered in the engine idling and maximum rpm ranges. Sustained operation under these conditions must be avoided to prevent combustion chamber liner failure.

Annular combustion chamber shown in figure 1 is used in engines of the axial-centrifugal-flow compressor design. The annular combustion chamber permits building an engine of a small and compact design. Instead of individual combustion chambers, the primary compressed air is introduced into an annular space formed by a chamber liner around the turbine assembly. A space is left between the outer liner wall and the combustion chamber housing to permit the flow of secondary cooling air from the compressor. Primary air is mixed with the fuel for combustion. Secondary (cooling) air reduces the temperature of the hot gases entering the turbine to the proper level by forming a blanket of cool air around these hot gases. The annular combustion chamber offers the advantages of a larger combustion volume per unit of exposed area and material weight, a smaller exposed area resulting in lower pressure losses through the unit, and less weight and complete pressure equalization.

Can-type combustion chamber is made up of individual combustion chambers. This type of combustion chamber is so arranged that air from the compressor enters each individual chamber through the adapter. Each individual chamber is composed of two cylindrical tubes, the combustion chamber liner and the outer combustion chamber, shown in figure 2.

Can-annular combustion chamber: This combustion chamber uses characteristics of both annular and can-type combustion chambers. The can-annular combustion chamber consists of an outer shell, with a number of individual cylindrical liners mounted about the engine axis as shown in figure 3. The combustion chambers are completely surrounded by the airflow that enters the liners through various holes and louvers. The fuel-air mixture is ignited by igniter plugs, and the flame is then carried through the crossover tubes to the remaining liners.

Combustion instability Mechanism

It is well known that NOx emissions from gas turbines can be reduced by premixing the fuel and air and burning at lean equivalence ratios. However, under these conditions, combustors are highly susceptible to undesirable combustion instabilities. Combustion instabilities are caused by a coupling between unsteady heat release and acoustic waves.

Instabilities in gas turbines had received much less attention than in other systems. Relatively little information has been available about such instabilities in practical systems, for proprietary reasons. However it is probably true for several reasons, mainly relatively large acoustic losses in the combustors that until fairly recently serious combustion instabilities had been quite rare in gas turbines.

One of the more promising ultra low NOx gas turbine combustor concepts is the lean, premixed combustor that has demonstrated the potential for significant reductions in NOx emissions over what can be achieved by current designs.[3] details the concept, difficulties and the research ongoing into the field.

Proposed Design of Combustor

It has been understood the importance of developing a combustor model to further analyze the project with more engineering elaboration. It has been proposed to develop a simple combustor model on Solidworks , which require the basic and advanced research in the development of gas turbine combustor. After looking at the possible benefits of annular combustor over the other types it is understood that the annular combustor will be one of the major contender of design. The detail about the construction needs to be further researched and clarified. The different complex parameters which are considered by engineers being researched by the author. It is also been proposed to concentrate the effort in the development of premixed combustor as they are prone to the instability related to thermo acoustic vibrational issues.

Analysis of Project

There is a need of more technical support from EON UK. The author has requested the company to provide him with the condition monitoring and flame instability data. The author has so far been able to get the OEM and Part Breakdown handbook of LM6000 PB and PD engines. The project has achieved the definite direction in there 8 weeks. The logbook is operated on daily weekly basis and the meetings with the supervisor are on the schedule. There is a need of more precise approach towards the project and more scholarly researched material needs to be found.

Project Schedule

The following paragraph discusses the time line of the project, the author has shown his project management skills and has used the Microsoft Project Software to develop and manage the schedule of activities. However, at this stage of the project, it is assumed that he project will carry out at the pace and the project achievements will be monitored using MS Project. The level of detail has been defined on the basis of planned routine and planned outcomes as expected. The progressive elaboration of the individual tasks has not been done at this stage of the project; as it require more explanation and more understanding of the project. This issue has been added into the agenda by the author in the next semester. The project phases are clearly been defined as instructed in the module guide, the schedule has been set on the milestone basis. The importance of following the project has clearly been understood.

The project schedule ‘grant chart' is attached with the report.

Conclusions

This report has focused on combustion dynamics. This topic has been extensively investigated over the past few decades. While the initial effort has been limited by experimental techniques, lack of technical expertise and a limited level of understanding so far. Many aspects of the problem were understood, but the prediction of combustion instability still poses important scientific and technical challenges. The project is going on time and the additional time left will be utilized in further research in the next semester.

References

[1] K. Kailasanath and E. J. Gutmark, Propulsion combustion: fuels to emissions edited by G.D. Roy, pp.129-172, Taylor & Francis (1998).

[2] Semerjian, H. and Vranos, V., Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1976, pp. 169-179.

[3] Tim Lieuwen, Vigor Yang, Combustion Instabilities in Gas Turbine Engines: Operational Experience, Fundamental Mechanisms, and Modeling Vol. 210, Progress in Astronautics and Aeronautics Series, 210 Published by AIAA, 2006, 600 pages, Hardback ISBN-10: 1-56347-669-X, ISBN-13: 978-1-56347-669-3

[4] Philip P. Walsh, Paul Fletcher Gas Turbine Performance. Blackwell Science, pp 191-198

[5] Jack D. Mattingly, Elements of Gas Turbine Propulsion. Mc Graw-Hill ISBN-0-079121-969

[6] Xinming Huang, Development of Reduced-Order Flame Models for Prediction of Combustion Instability Dissertation submitted to the Faculty of the Virginia Polytechnic Institute and State University

[7] Lesson 2 Major Engine Sections: http://www.globalsecurity.org/military/library/policy/army/accp/al0993/le2.htm (visited on 16/11/2007)

[8] Burner Thermodynamics NASA Glynn Research Institute:

http://www.grc.nasa.gov/WWW/K-12/airplane/burnth.html

Visited on 12/11/2007

[9] Turbine Engine Basics: Purdue AAE Propulsion www.cobweb.ecn.purdue.edu/.../basics/burner.html (visited on 12/11/2007)

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