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Applied Thermal Engineering 26 (2006) 766–771

An expert system concept for diagnosis and monitoringof gas turbine combustion chambers

N.H. Afgan a,*, M.G. Carvalho a, P.A. Pilavachi b, A. Tourlidakis b,G.G. Olkhonski c, N. Martins d

a Instituto Superior Tecnico, Mechanical Engineering, Av. Rovisco Pais, 1049 Lisbon, Portugalb Aristotle University of Thessaloniki, 2461 Kozani, Greece

c All Russian Thermal Engineering Institute, 095 Moscow, Russiad University of Aveiro, 234 Aveiro, Portugal

Received 27 August 2004; accepted 24 April 2005Available online 29 November 2005

Abstract

In this paper, the main principles of operation, the conceptual design and the development of an expert system for fault diagnosisand monitoring of gas turbine combustion chambers are presented. The concept of the gas turbine chamber expert system is basedon the monitoring of the spatial and temporal distribution of the heat flux inside the combustion chamber and the simultaneouscomparison of respective readings of diagnostic variables with values obtained through numerical model simulation of different sit-uations which lead to the malfunction of combustion chamber and deterioration of its performance. The demonstration of theexpert system will be performed at the VTI (All Union Energy Institute, Russia) gas turbine combustion chamber test facility.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Expert systems; Gas turbines; Combustion chambers; Diagnostic strategy

1. Introduction

The future energy strategy in Europe will be based onthe increased use of gas for electricity production. Gasturbine electricity generation units consist of the founda-tions of this strategy [1]. The number of gas turbineplants will increase in all power ranges. In particular,small and mini gas turbine plants will become strategi-cally an important part of the energy systems. The effi-ciency and reliability of those units will requiresophisticated surveillance and control. It would be verydifficult to have a large number of different expert sys-tems to support these units. For this reason, the de-velopment of prognostic and diagnostic systems will be

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* Corresponding author. Tel.: +351 21 8418082; fax: +351 218475545.

E-mail address: nafgan@navier.ist.utl.pt (N.H. Afgan).

of great importance. The deficiency in expert manpowerfor gas turbine surveillance may become a limiting fac-tor in the further development of energy systems basedon small and mini gas turbine units. Bearing in mindthat reliability of gas turbine units is very importantfor their introduction into the energy systems, the devel-opment of expert systems similar to the one described inthis paper may actively contribute to the confidence insmall and mini gas turbine introduced in local, regionaland global power generation systems. The Europeanadded value of this concept is realised by the promotionof efficient and reliable small and mini gas turbines withprognostic and diagnostic systems supported by remoteexpert surveillance.

The combustion process in a gas turbine combustionchamber is a very complex process with a number ofphysical and geometrical variables affecting its perfor-mance. The total heat flux on the combustion chamber

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walls is an integral parameter reflecting the changes ofthe combustion process inside the chamber. The varia-tion of the heat flux in space and time is a result ofthe spatial and temporal distribution of a number ofparameters affecting the combustion process. In thisrespect, the aforementioned changes may lead to unde-sired operating conditions and to the malfunction ofthe system. The diagnosis of these situations is of greatimportance to the user of the gas turbine who aims tomaximise its efficiency and reliability. In particular, mal-function of the gas turbine combustion chamber is asso-ciated with instability of the flame and its distortion dueto burner malfunction.

Consequently, the present research work contributestowards the utilisation of monitoring the heat flux asthe main input to the expert system instead of usingalternative variables. The above monitoring process iscarried out with the utilisation of a new heat flux sen-sor. The expert system performs a number of taskssuch as diagnosis of pulsating regimes inside the com-bustion chamber, control of low-emission operation forexit concentration of NOx less than 50 mg/Nm3, mon-itoring of the reliability of the combustion process inseveral regimes of combustion chamber operation,monitoring of the lifetime of the combustion cham-ber, control of the flame flashback into the premixingzones.

2. The concept of the diagnostic and monitoring system

The expert system of the gas turbine combustionchamber diagnostic system is based on a new heat fluxsensor [2,3] and the corresponding evaluation of thereadings of the monitoring system. The new method isbased on the use of a convectively cooled porous sensingelement, as illustrated in Fig. 1.

Fig. 1. Convectively cooled

The high temperature gas flows over the porous ele-ment and promotes heat transfer to the element surfacethrough convection and radiation. The cooling gas,flowing through the porous element, is heated due tothe heat transfer from the hot gas, via the porous matrixso that

qtot ¼ qcon þ qrad ¼ j � cp � DTAt the critical gas flow rate jcrit the hot gas boundary

layer will be blown off so that the heat flux due to con-vection will disappear, qcon = 0, and the total heat fluxwill become equal to

qtot ¼ qrad ¼ jcrit � cp � DTR

Repeating the same measurement procedure at thegas flow rate j < jcrit, the following relation is valid:

qtot ¼ qcon þ qrad ¼ j � cp � DTso that the convection part of the heat flux could bedetermined by

qcon ¼ qtot � qrad

Experimental demonstration of the heat flux sensorhas proven its versatility for various potential applica-tions at the required accuracy.

The combustion process in the gas turbine combus-tion chamber is a very complex process with a numberof physical and geometrical variables influencing its per-formance. The total value of the heat flux on the com-bustion chamber walls is an integral parameterreflecting the changes that may occur in the combustionprocess inside the chamber. The spatial and temporaldistribution of the heat flux is the result of the valuesof the aforementioned parameters that determine thecombustion process. In this respect, certain parametersettings may lead to undesired operating conditions ofthe system. The diagnosis of those situations is ofgreat importance for maintaining high efficiency and

porous element sensor.

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reliability of the gas turbine system. In particular, mal-function of the gas turbine combustion chamber isaffected by instability of the flame and its distortiondue to burner malfunction.

The gas turbine control system includes sequencing,control, protection and operator information, whichprovides for orderly and safe start-up of gas turbine,control of proper loading and an orderly shutdown pro-cedure. In addition, it includes an emergency shutdowncapability, which can operate automatically by suitablefailure detectors or manually [4]. Coordination betweengas turbine control and driven equipment must be pro-vided for start-up, operation and shutdown.

3. The concept of the expert system

Presently available knowledge-based systems used fordiagnostics of gas turbine system performance are basedon the monitoring and processing of a number of con-trol variables [5]. These variables reflect integral pro-cesses in the gas turbine system. Fault diagnosis inthese systems is aimed towards detecting those situa-tions, which are of general nature and reflect events,which have either already taken place in the system orhave been degrading the gas turbine system elements.

The stability of the flame in the gas turbine combus-tion chamber is an immanent problem in the gas turbineoperation. There are two approaches to the flame stabil-ity problem, the first one is based on the organisation ofthe combustion process in the chamber with high gasvelocity and respective flow arrangement inside thechamber and the second approach is based on introduc-tion of a diagnostic system which recognises the changeof characteristic parameters in the combustion chamberresponding to flame instability. The current diagnosticsystem of gas turbine combustion chambers detectsand also predicts flame instability. In addition, diagnos-tic variables are used for the lifetime assessment of com-bustion chambers and of the first blade row of thedownstream turbine unit.

The diagnostic strategy is organised to recognise thefollowing main operational situations:

1. Diagnosis of pulsating regimes inside the combustionchamber (combustion noise).

2. Control and diagnosis of low-emission combustionchamber with outlet NOx concentration less than50 mg/Nm3.

3. Diagnosis of low-NOx combustion chamber opera-tion with temperature level up to 1500–1700 K.

4. Monitoring of reliable combustion at various regimesof combustion chamber operation.

5. Assessment of lifetime of combustion chamber andfirst row of gas turbine blades by monitoring gastemperature.

6. Control and monitoring of the circumferential gastemperature distribution at the turbine outlet.

7. Control of the flame breakthrough to the premixingzones.

4. The expert system for the gas turbine combustion

chamber

The basic concept of the gas turbine chamber expertsystem is based on the monitoring of the space and timedistribution of heat flux and temperature in the combus-tion chamber and the comparing of diagnostic variableswith values obtained through numerical model simula-tion of different situations that may occur and can leadto the malfunction of combustion chamber perfor-mance.

Monitoring systems, knowledge bases, knowledge-based systems and display systems are elements of thediagnostic system. The composition of the diagnosticsystem structure is designed to perform the followingfunctions:

• Continuous monitoring and diagnosis of gas turbinecombustion chamber.

• Comprehensive fault detection.• Multilevel diagnostics to permit time dependent fore-cast of failure.

• Assessment of the effect of the fault on the perfor-mance of the gas turbine system.

• Special feature of the diagnostic system with lifetimeassessment of gas turbine combustion chamber andfirst row of turbine blades.

The expert system is composed of the followingelements.

4.1. Monitoring system

The monitoring system comprises of heat flux sensorsdistributed at specific locations in the combustion cham-ber and an acquisition system with respective softwarefor signal conditioning and verification. It also includesbuffers for data storage.

4.2. Knowledge base

The knowledge base represents a set of data obtainedthrough three-dimensional numerical simulation of thegas turbine chamber and organised in the structure ofelements representing situations to be analysed by thesystem. The 3-D numerical model of a gas turbine com-bustion chamber comprises a numerical solution tool ofa set of discretised equations including mass, momen-tum and energy balances for the control volume ofdiscrete elements of the system. The model represents

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the generic simulation of space distribution of parame-ters in the system. This simulates different malfunctionsituations of the combustion chamber and defines theknowledge-based system. The data obtained representthe possible situations to be retrieved by the diagnosticsystem.

4.3. Knowledge-based system

The knowledge-based system is a data processingsoftware with a logically structured inference enginefor data retrieval. The data obtained at every cycle ofdata acquisition represents the input data for theinference engine to be processed in accordance with aretrieval procedure. The retrieval procedure of theknowledge-based system is a set of rules to recognisethose situations which are the most probable situationscorresponding to the individual reading. Every situationobtained in the retrieval procedure is displayed withits respective probability assessment of diagnosticconfidence.

4.4. Display system

The final stage of the diagnostic procedure is the dis-play of the obtained situations and prognoses for thefurther development of the failure. In this respect, thedisplay elements consist of a subsystem of the diagnosticsystem.

5. Set-up of the expert system

The concept is set-up as follows.

5.1. The basic concept of the diagnostic system based

on the heat flux pattern in the gas turbine combustion

chamber

A new method for heat flux measurement [4] isadapted for practical deployment as a heat flux sensorin the gas turbine combustion chamber. The method isbased on a new sensor, whose reliability in long-termoperation at high temperatures has been already demon-strated. Thanks to its small overall dimensions and veryshort response time, the sensor can be easily adjusted forgas turbine on-line monitoring. The heat flux patternrecognition is represented in object form and used forretrieval of specific conditions in the gas turbine com-bustion chamber.

5.2. The monitoring system

The gas turbine monitoring system is based on theheat flux measurements integrated with the help of a val-idation procedure, specifically designed according to a

diagnostic strategy. A selected number of instrumentsare positioned in accordance with guidelines suggestedby the structure of the specific domain knowledge-basedsystem gained by expert operators [5]. The data valida-tion procedure essentially consists of a statistical analy-sis of the quality of the data.

5.3. The knowledge-based system for the gas turbine

combustion chamber diagnostic system

The design of the knowledge base comprises selectionof the potential situations, which are to be recognised inthe retrieval procedure. The respective number of theheat flux pattern readings describes these situations. Inorder to obtain the heat flux patterns for the specific sit-uations, a 3-D numerical simulation code for gas turbinecombustion chambers is used. In accordance withthe diagnostic strategy of the system, a number of situ-ations with respective heat flux patterns are generatedand presented in the knowledge base [6,7]. The test facil-ity combustion chamber is equipped with a number oftemperature measurement sensors for the control ofthe gas outlet condition.

Among the situations to be investigated are thefollowing: instability of LHV gas combustion; burnerfailure development; lifetime monitoring of combus-tion chamber and gas turbine first row blades; varia-tion in combustion regimes; low NOx combustionoperation; high temperature regimes of combustionchamber.

5.4. The diagnostic procedure

The diagnostic procedure includes the monitoringsystem for reading and conditioning of the diagnosticparameters, the development of an appropriate numeri-cal model for the gas turbine combustion chamber, thedesign of the knowledge base of fault detection, and adecision-making procedure to handle quality assurance,design reliability and to support the persistence of infor-mation [8]. The test facility with the combustion cham-ber is used as the standard pattern for the numericalmodel to be used in gas turbine diagnostic system. Thefuzzification of the diagnostic variables leads to theuse of fuzzy logic for the assessment of respectivesituations.

The diagnostic and monitoring system for the gas tur-bine combustion chamber [9,10] is shown in Fig. 2.

5.5. Implementation and calibration of the prognostic

and diagnostic system

The actual implementation and calibration of theprognostic and diagnostic system for the on-line moni-toring of the combustion chamber of a turbo-gas set willbe performed on an experimental facility. As shown in

Fig. 2. Diagnostic and monitoring system for gas turbine combustion chamber.

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Fig. 2, the VTI test facility will be specially designed forexperimental work including a gas and liquid fuelledcombustion chamber. This is equipped with a controlsystem with corresponding instrumentation for the con-trol of integral parameters of the turbine combustionchamber. It includes verification of the monitoring sys-tem and a corresponding validation procedure. Theamount of data in the knowledge base is kept withinworkable bounds simply by limiting the number ofselected situations, which describe the most probablefaults.

6. Conclusions

The concept of a prognostic and diagnostic expertsystem for the gas turbine combustion chamber is a tool,which supports efficient and reliable gas turbine opera-tion as well as remote surveillance and monitoring ofgas turbine systems. Since high-level expertise is becom-ing scarce in energy systems, the development of theprognostic and diagnostic expert system for gas turbinecombustion chambers will become an ultimate goal ofthe future energy system.

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