Handbook of Combustion, Volume 2 (Combustion Diagnostics and Pollutants)

546 Pages·2010·94.79 MB·English

by  Maximilian Lackner

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An Overview of Combustion Diagnostics
AlfredLeipertz,SebastianPfadler,andRobertSchießl
1.1
Introduction
Duringthepastthirtyyears,tremendousprogresshasbeenmadeinthedevelopment
ofdiagnostictoolsforcombustionresearch.Theresultsobtainedbyapplyingthese
diagnostic techniques has contributed greatly to the current understanding of
combustion, to the improvement of practical combustion devices with a lesser
impactontheenvironment,andtothedevelopmentofnewcombustionprocesses.
Thefieldhasgrownquickly,especiallyduringthepasttwodecades,suchthattoday
combustiondiagnosticsareusedformanifoldpurposesthatincludefundamental
researchinacademia,powerplantcontrol(fordiagnosingpollutantemissionsand
ensuringreliableoperation),orproductiontechnology(tooptimizetheproduction
processesforcommercialgoods).Asitisdifficulttoprovideabriefoverviewthatfully
meetstherequirementsofallthesegroups,thischapterneednotincludeacomplete
anddetailedoverview.Rather,itincludesthedetailsofdedicatedtechniquesandtheir
use in both scientific and technical applications. Consequently, only the essential
propertiesofparticulardiagnostictechniques,togetherwithsomeexamplesoftheir
uses,arediscussedhere.Hence,amoregeneraldiscussionoftheroleofdiagnostics
withinthefieldofcombustionscienceandtechnologyisprovided.
First,thosequantitiesandphenomenathatarerelevanttocombustionprocesses,
and which are therefore targets for diagnostic approaches, are outlined. It is also
shown that the appropriate choice of suitable measurement techniques depends
heavily on the particular properties of the combustion process to be investigated.
Basedonthesegeneralaspects,severaldiagnostictechniquesaredescribedthatare
commonly used to investigate combustion phenomena. These techniques can be
splitintotwoclasses,namelythosebasedonmechanicalprobing,andthosewhich
utilize optical diagnostics. The discussion of each technique includes a short
treatmentoftheunderlyingphysics,anassessmentoftheapplicabilityofthemethod
incombustionprocessesatbothlaboratoryandtechnicalscale,andoftheabilityof
HandbookofCombustionVol.2:CombustionDiagnosticsandPollutants
EditedbyMaximilianLackner,FranzWinter,andAvinashK.Agarwal
Copyright(cid:1)2010WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
ISBN:978-3-527-32449-1
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themethodtofulfilltheneedsforspatialandtemporalresolution,nonintrusiveness,
androbustness.
Afurthersectionisdedicatedtoimagingandmultidimensionaldiagnosticsand,in
describingthesimultaneoususeofthedifferentmeasurementtechniques,thestate
of the art of laser diagnostics in combustion research is highlighted. Finally, the
potentialoflaser-diagnosticstovalidatethoseapproachesusedinnumericalsimula-
tionsisdiscussed.Theoverviewiscompletedbytheprovisionofaccessiblequantities
andrelevantdiagnostictechniques,detailingcomprehensiveinformationrelatingto
theapplicationofdifferentcombustiondiagnostictechniques.
Theadventoflasersmayberegardedasthemainstartingpointforthetremendous
developmentofspectroscopicmethodsforcombustiondiagnostics.Previously,both
emissionandabsorptionspectroscopyhadcontributedtothecombustionsciences
well before the onset of laser technologies. Indeed, the existence in flames of
(cid:1) (cid:1) (cid:1) (cid:1) (cid:1)
important intermediate species such as OH , CH , HCO , NH or C had been
2
provenbyusingspectroscopicmethods.Yet,spectroscopybasedonlasersoffered
novel,vastlyrefinedwaysinwhichthecombustionphenomenacouldbeobserved.By
employing the small spectral bandwidth of laser light, it became possible to
distinguishortoisolatedifferentspectralfeaturesofatomsandmoleculesatmuch
greaterresolutionthanintheabsenceofacoherentlightsource,whiletheabilityto
produceultra-short laserpulsesmadepossibletheresolutionandanalysis ofvery
rapidandtransientprocesses.Notably,laserbeamscanbeformedveryeasilyinto
well-defined,sharpgeometricshapesthataremorepointlike,line-likeandplane-like
thanispossiblewithconventionallightsources,whilstthedirectionofpropagationof
laserlightcanbecontrolledwithgreatprecision.Consequently,itbecamepossibleto
defineirradiated(probed)volumeswithgreataccuracy,andoververylargedistances.
Taken together, this collection of unique properties led to laser diagnostics
becomingthemostwidelyusedprocedures,notonlyinfundamentalandapplied
combustionresearchbutalsofortheinvestigationandcontrolofpracticalcombus-
tionsystems.
1.2
DiagnosticsinCombustion:TasksandRequirements
Asitisdifficulttounderstandthetasksandneedsofcombustiondiagnosticswithout
any prior knowledge of the properties of combustion, a brief description of such
properties shouldhelpto explain how certain requirements are imposed ondiag-
nostictechniques.
The desired temporal and spatial resolution of a measurement depends on the
governingtimeandlengthscalesofthesystemtobecharacterized.Anestimateof
these scales for a small collection of different combustion applications is shown
inTable1.1.
Combustionoftenoccursasaspatiallyextremelyinhomogeneousprocessinthin,
sheet-likeregions(flames).Since,inmostrealisticcombustionsystems,thethick-
ness of these layers is on the order of some ten to hundred micrometers, an
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1.2 DiagnosticsinCombustion:TasksandRequirements 3
Table1.1 Timeandlengthscalesofsomedifferentcombustionsystems[1].
Application Timescale(ms) Lengthscale(m)
Flamemeasurement–laminar 105–106 10(cid:2)4–10(cid:2)2
Flamemeasurement–turbulent 10(cid:2)2–102 10(cid:2)5–10(cid:2)2
Fireresearch 102–103 10(cid:2)5–10(cid:2)2
Jetengine;compressorinlet 103–104 10(cid:2)4–10(cid:2)1
Gasturbineburner 10(cid:2)1–101 10(cid:2)5–10(cid:2)4
Afterburner 10(cid:2)1–100 10(cid:2)5–10(cid:2)4
investigationofthesetinystructureswillrequireahighspatialresolution,aprecise
alignment,andanaccuratedefinitionofthemeasurementvolume.Figure1.1shows,
foranadditionalillustrationofthelengthscales,thecomplexstructureofasteady
laminar flame (taken from a numerical simulation) by species mass fraction and
temperatureprofilesacrosstheflamefront.
Combustion also involves a wide range of time scales; some relevant chemical
reactionsoccurwithinafewnanoseconds,whilemanychemicalspeciesthatplaykey
rolesincombustionchemistryexistforonlyveryshorttimeintervals,intherangeof
hundredsofnanosecondsormicroseconds.
Aninabilitytoresolvetheseshorttimescalesmeansthatmanyoftheessential
aspects of combustion measurements may be missed. However, the use of lasers
allowsthetemporalresolutionorshortmeasurementtimesrequiredtoobservethese
rapidprocessestobeachieved,oneexamplebeingthestructureshiddeninaflame
(Figure1.2).
Otheraspectsofcombustionthatimposehighdemandsondiagnostictechniques
include the complexity and high parametric sensitivity of many combustion pro-
cesses.Duetothestrongnonlinearityoftheequations(mostlythechemicalsource
Figure1.1 (a)Structureofapremixedmethane/airflameat20bar(simulation).Thespatialextent
ofthesceneshownis0.1mm;(b)Sketchofthetemporalandspatialscalesrelevantincombustion.
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Figure1.2 Temperaturestructureofaturbulentflametakenbytwo-dimensionalRayleigh
scattering(falsecolorrepresentation,topright)whichishiddeninflamephotography(bottom
left)[2].
term)thatgovernthedynamicsofacombustionsystem,thosespeciesthatexistin
tinyamounts[downtomassfractionsintheparts-per-million(ppm)scale]mayexert
a greater influence on the system’s behavior than those which are abundantly
present,inthepercentrange.Theabilitytodetectandmeasurethese“key”species
(cid:1) (cid:1)
(e.g.,OH ,CH inFigure1.1),withoutinterferencefromthemuchmoreabundant
“bath”ofhundredsorthousandsofotherspecies,representsamajorrequirementfor
thesuccessfulexperimentalanalysisofcombustion.
Thepotentiallyhighsensitivityofcombustionwithrespecttovariationsinphysical
boundaryconditionscanrenderprohibitivetheuseofmeasurementtechniquesthat
greatly alter the physical conditions during an experiment. Clearly, a diagnostic
technique should not alter the system that it is attempting to measure; indeed, a
majorindicatorforthequalityofadiagnostictechniqueinthisrespectwouldbeits
level of nonintrusiveness. Recognized diagnostic techniques can be quite sharply
separated,basedontheirlevelofintrusion,withmostopticaltechniquesconsidered
nonintrusive,andthoseinvolvingmechanicalprobeshighlyintrusive.
Finally,majorconstraintsexistwhenadiagnostictechniqueistobeusedwithina
technical combustion environment. Unlike laboratory systems, most practical en-
vironmentsarenotdesignedspecificallytoalloworalleviatediagnosticproceduresto
becarriedout.Hence,diagnostictechniquesmustoftenbeappliedunderconditions
thatarenotoptimallysuitedtotheunderlyingmeasuringprinciples.Animportant
propertyofadiagnostictechniqueis,therefore,its“robustness”–thatis,itsabilityto
functionreasonablywellunderconditionsthatmaydeviatestronglyfromthosefor
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which the technique was originally designed. Robustness also implies that the
instruments used for diagnostics can operate under the adverse conditions (e.g.,
hightemperaturesandpressures,presenceofhighlyreactivesubstances)encoun-
tered in combustions. The ideal diagnostic technique must be sufficiently robust
enoughtosurvivehostileenvironments,andtoprovidealargeamountofcombus-
tion-relatedinformation.
1.3
InvasiveTechniques
Traditionalapproachesforcombustiondiagnosticsaretypicallybasedontechniques
whereamechanicalprobeisinserteddirectlyintoaregionofinterest.Thismaybe
either a sensitive part of the measurement system, or a device that samples the
mediumofinterest(eithercontinuouslyorbatchwise)atthemeasurementpointfora
subsequentexsituanalysis.Althoughthefollowingapproachesareoftenusedfor
measurementsunderpracticalconditions,themostundesirablepropertyofinvasive
probes–thatis,alocalinteractionwiththemeasurementsystemtobeanalyzed–
remainsunavoidable,andtheoftennon-negligibleinherentsystematicerrorsmust
be carefullyweighed if themeasured quantity istobe rated.Therelevance ofthe
problemscausedbymechanicalprobesdependsonthemechanicalsizeoftheprobe
relativetothesizeofthecombustionfieldunderinvestigation.
1.3.1
Temperature
Fortechnicalapplications,thetemperatureinsideacombustionchamberisprobably
themostimportantstatevariabletobemeasured.Thesameholdsforfundamental
investigationsindownscaledmodelflames,wheretremendousprogresshasrecently
beenmadewithregardstotheanalysisoftemperaturedistributions.
1.3.1.1 Thermocouples
Byfarthemostwidelyuseddevicesfordeterminingtemperaturesincombustion
systems are thermocouples. Here, the underlying physical principle is the Seebeck
effect,whichdescribestheoccurrenceofamaterial-dependentelectricvoltagewhena
temperaturedifferenceexistsbetweenthejunctionpointsoftwodifferentconductor
materials that form a closed loop. In practice, the two different materials provide
standardized (albeit nonlinear) voltage–temperature characteristics over the appli-
cabletemperature ranges.Thedetails ofsome different materialpairsofthermo-
couplesandtheirapplicationrange,thatcanbeusedincombustionsystemsarelisted
inTable1.2.Dependingonthetemperaturerangeencountered,themeasurement
accuracyis0.5–0.75%oftheindicatedtemperature,althoughatleast(cid:3)2K[3]canbe
improveduponbyusinganappropriatecalibration.
Aswithallinvasivetechniques,themaindrawbackofthermocouplesisthatthe
probe(s) must be placed directly into the measurement volume, where it can
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Table1.2 Pairsofthermocouplematerialsandtheirapplicationranges(typesJ,K,R,BafterRef.[3];
typeDafterRef.[5]).
Internationalcodeletter Material Applicationrange((cid:4)C)
J Fe/Constantan (cid:2)210–1200
K Ni-Cr/Ni-Al (cid:2)270–1372
R Pt-(13%)Rh/Pt 0–50–1768
B Pt-(30%)Rh/Pt-(6%)Rh (cid:2)000–1820
D W-(26%)Re/W-(5%)Re 00-0–2315
influencetheflameviaanumberofmechanisms.However,inviewoftherobust
measurementprinciplethesewell-knowndisadvantagesareoftenacceptedformany
applications.Dependingonthedimensionsoftheprobeandtheflowconditionsof
the measurement object, the local flow field in the probed volume can be greatly
influenced;themixingoffuelandoxidizerortheturbulence–chemistryinteraction
canalsobeaffectedbysuchnonisokineticsampling.Furthermore,theheatbalance
of the flame may be affected by heat conduction and radiation, mainly from the
surrounding burning chamber walls. In addition, certain materials on the sensor
surfaceareknowntoactaschemicalcatalysts,andthismustbedifferentiatedagainst
thereactionofthethermocouplematerial(mostlyplatinum;seeTable1.2)byusing
chemically active species that increase the decomposition of the normally stable
refractoryceramicswhich,inmostcases,serveasaninsulationmaterialintheflames
or at high-temperature atmospheres. Heitor and Moreira [4] have provided an
excellent overview of the drawbacks and successful applications of these sensors
inthefieldofcombustiondiagnostics.
1.3.1.2 ResistanceThermometry
Resistance thermometry represents another invasive technique for temperature
measurements,inwhichtemperature-dependentchangesintheelectricalresistance
of a conductor material are used to determine local temperatures with point-wise
precision. Despite the perfect linearity of resistance-based thermometers and the
availabilityofstandardizedtypes(e.g.,withplatinum;Pt100),themaindrawbackof
resistancethermometry,whencomparedtothermocouples,isitslimitedapplication
rangeofuptoabout1300Kandthelongresponsetimesrequired.Theaccuracyofthe
techniqueexceedsthatofthethermocouple,however[6].
1.3.1.3 ThermochromePaintings
Inrecentyears,thermochromepaintingshavebecomepopularforprovidingamore
crudeestimationof,forexample,thesurfacetemperaturesofcomponents.Thebasic
principlebehindsuchcolorformationistheabilityofcertainmaterialstoreversibly
change their color as a function of the ambient temperature, this being due to
changesinthematerials’crystalloidormolecularstructures.Anadditional,recently
developed method of temperature measurement employs temperature-sensitive
paints(TSPs);thisisbasedonthesensitivityoftheluminescentmoleculestotheir
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thermalenvironmentfollowing(ultraviolet)excitation[7].Ariseintemperatureof
the luminescent molecule raises the probability that the molecule will return to
groundstateviaanonradiationprocess,ratherthanemitaphoton.Thisthermally
inducedphosphorescencequenchingformsthebasisofthetemperaturemeasure-
mentviaaquantitativedetectionoftheemissionattenuation.
1.3.2
FlowVelocity
1.3.2.1 PitotTubes
Traditionally, Pitot tubes have been used to measure the total pressure of moving
fluids;bycombiningtheinformationacquiredwiththatrelatingtothesurrounding
staticpressure,theflowvelocitycanbededucedfromthedynamicpressure(whichis
thedifferencebetweenthetotalandstaticpressures).Prandtltubes(Pitotstatictubes)
areusedtomeasurethetotalanddynamicpressuresimultaneously.Whereas,Pitot
statictubesarestillusedinmodernairplanestomeasurethepressureandpressure
differences simultaneously to determine both altitude and airspeed, the invasive
probingtechniquesused tomeasure flowvelocityonapointwise basishavebeen
largelysupersededbytheadventofnoninvasivelaserdiagnostics.
Dibble et al. [8] have compared the velocities measured by laser Doppler ane-
mometry(LDA;seeSection1.4.6.1)withPitottubemeasurements,inanonreacting
jetflowandnonpremixedjetflames.ThediscrepanciesbetweenthePitotprobeand
LDA measurements were within the range of 5% for the jet flow. However, an
additionalsystematicerrorwasobservedintheturbulentflamewhenmeasuringthe
dynamicpressurethatisattributedtoheattransfertothePitottube.Despitethese
problems,Pitot-typesensorsarestillusedtodayinavarietyofapplications,onesuch
examplebeingtosupportthenumericalsimulationsofdustexplosions[9].Inthis
case, dust- and temperature-resistant Pitot-type anemometers used to measure
turbulenceintensitieswerecombinedwithahigh-speedcameratovisualizeflame
displacementandsimultaneouslyaccesstheratiobetweenturbulentflamespeedand
turbulenceintensityunderdifferentoperatingconditions
1.3.2.2 HotWireAnemometry
Thebasicprincipleofhotwireanemometry(HWA)canbeusedtodetermineflow
velocitiesbymeasuringthecoolingofheatedwiresbyforcedconvectioninaflowthat
isdependentontheflowvelocity(e.g.,Ref.[10]).AlthoughHWAisrecognizedasa
classicdiagnosticmethodforturbulenceresearch,itcanonlybeusedtoinvestigate
combustion systems in the unburned region of the flame to be analyzed. This is
mainlyduetothefactthat,eveniftheanemometermaterialwereabletowithstand
thehightemperaturesintheflames,therequiredtemperaturecorrectionswouldbe
impracticable. Nevertheless, HWA is often used to characterize the turbulence
conditions of combustion systems pointwise in the reactant zone. For example,
HWAhasbeenusedforin-cylindermeasurementsinaninternalcombustionengine
inacar[11],aswellastostudythepropagationofpremixednaturalgasairflamesina
rectangularductwhichwasclosedatoneend[12].
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1.3.2.3 IonizationProbes
Analternativeinvasiveapproachfortheexaminationofflame-relevantvelocitiesis
thatofionization(alsoLangmuir)probes,whereanelectricalfieldisappliedacross
theflame.Thesubsequentlymeasuredcurrentsandpotentialscanthenbeusedto
measureionconcentrations(e.g.,Ref.[13]),flamepositions(e.g.,Ref.[14]),orflame
propagationspeeds(e.g.,Ref.[15]).Alternatively,forenginediagnosticsasparkplug
hasbeenusedastheprobe[16].
1.3.3
SpeciesConcentrations
Measurementoftheconcentrationsofminorityandmajorityspeciespriorto,during,
and/or after the actual combustion process, is essential not only for investigating
fundamental combustion phenomena but also for process control applications.
Differentprobingstrategiesareappliedingeneral,mainlydependingonthemea-
surementprinciple,anddueinpartalsototheregularizedorhistoricallyestablished
methodofsampling.Forexample,constantvolumesampling(CVS)hasbeenusedfor
manydecadestosupportthetestingofvehicleemissions,wherebyaconstanttotal
flowrateofavehicleexhaustplusdilutionairismaintained.Inorderforthistooccur,
astheexhaust flowincreases, the dilutionair must be automaticallydecreased. In
this case, a bag measurement of the emissions represents the key method for
legislativepurposes,asitprovidesasinglevalueforthemajorexhaustspeciesofa
completetestcycle,includingthedifferentphasesofaccelerationandconstantspeed.
Although the sampling may be conducted in a continuous manner by using a
suitableejectororprobingdevice(e.g.,Pitottubeorlow-pressuresampling),followed
byadditionaldilutionstages,thequantitativeanalysisofthespeciesconcentration
canbeperformedbatchwiseinthecaseofsomediagnostictechniques.Anexample
ofthisisthegas-chromatographicmeasurementofspeciesinengineexhaustgases,
althoughthisisalsooftenappliedtoananalysisofbasicpropertiessuchasheating
valueandthecompositionofgaseoushydrocarbonfuelsforcombustioninstationary
gasturbines.
1.3.3.1 FlameIonizationDetectors
Traditionally, flame ionization detectors (FIDs) [17] are used to measure the con-
centrations of hydrocarbons downstream in coiled columns that separate the
inflowingspeciestobeanalyzedonthebasisoftheirdifferentretentiontimeson
the column surface [18]. For exhaust gas analysis, these detectors can be used as
stand-alone systems in order to quantify the amounts of unburned hydrocarbons
(UHCs)asacomposite.ThephysicalprincipleofFIDsdependsonthefactthatapure
hydrogenflameproducesverylittleionizationoftheinvolvedspecies,whereasthe
addition of traces of hydrocarbons causes a drastic increase in the degree of
ionization. Consequently, FIDs are composed of a hydrogen diffusion flame into
whichthegastobeanalyzedisinserted;thisresultsinameasurabledirectcurrent
(DC) signal of an applied electrostatic field in close vicinity to the flame in the
presenceofUHCs.
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1.3 InvasiveTechniques 9
1.3.3.2 ChemiluminescenceDetectors
Theanalysisofoxidesofnitrogen(NO )inexhaustgasesistypicallyconductedusing
x
chemiluminescence detectors, where the primary principle is a photo-physical
(cid:1)
detectionofthesignalemittedaselectronicallyexcitedNO decaystotheground
2
statefollowingthechemicalreactionofNOwithozone.TheNO concentrationsare
2
acquiredindirectlyviaanadditionalmeasurementoftheconcentrationofNOaftera
preceding total conversion of the complete NO -load to NO; this is followed by a
x
simultaneous measurement of the NO concentration, without conversion and
subtractionoftheobtainedconcentrations.
1.3.3.3 NondispersiveInfraredAnalysis
The nondispersive infrared (NDIR) analyzer is a robust device which is used to
measurecarbonmonoxide(CO)concentrationsinexhaustgases,bymakinguse
ofthestrongbroadbandabsorptionnear4.7mm.Thedevicetypicallyconsistsof
a reference and a probe cuvette, both of which are irradiated with IR light. The
useofachopperwheelpermitsanalternatingIRradiationofboththegastobe
analyzed and the reference gas mixture, such that no absorption occurs at the
probed band of the spectrum. Often, an opto-pneumatic detector is applied that
measures the flow between two additional absorption chambers located
beneath the probe and the reference cell. Due to the different absorption rates
in the latter cells, the IR-absorbing gases in the detector chambers feature
different heating rates (and thus volumetric expansions) which are balanced by
the flow through a microchannel. The latter effect is converted to an electric
current that is proportional to the CO concentration in the probe volume. With
this apparatus it is also possible to monitor CO , SO , NH , H O and other
2 2 3 2
exhaust gas components, by using appropriate absorption wavelengths (see
Section 1.4.3).
Concentrationsofoxygencanbemeasuredbymakinguseoftheelectrochemicalor
paramagneticpropertiesofthedimer.Theelectrochemicalapproachisutilizedinthe
exhaust ducts of almost all Otto engines with integrated three-way catalysts, for
measuringandcontrollingthecompositionofthefuel–airmixture.Theprincipleof
thepotentiometriclambdaoxygensensor(seealsoVol.2,Ch.4)isbasedontheoxygen
ion conductivity of a ZrO -membrane with exhaust gas passing by on one side
2
(outer Pt electrode) and ambient air on the other side as a reference (the “inner
Pt-electrode”).
Theparamagneticapproachtomeasuringoxygenconcentrationsisbasedonthe
strong magnetic susceptibility. In this case, a magnetomechanical principle is
implemented that is based on a sensor in which a rotator comprising two nitro-
gen-filledspheres(diamagnetic)isarrangedwithinasymmetricmagneticfield.Ifan
oxygen-containing gas passes through the sensor, the oxygen is drawn into the
magnetic field due to its paramagnetic property, thus strengthening the field. In
contrast, any nitrogen inside the glass spheres will have an opposite magnetic
polarizationandbeforcedoutofthefield,causingarotation.Thedegreeofdeflection
thuscausedisproportionaltotheoxygenconcentration.
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1.3.3.4 GasChromatography/MassSpectrometry
Inadditiontotheabove-mentionedspecies-specificdiagnostictechniquesthatare
used widely in commercial sensor systems to characterize species concentrations
under technically relevant conditions, gas chromatography (GC) and mass spec-
trometry (MS) are each applied to investigate fundamental phenomena (see also
Vol.2,Ch.2).Both,GCandMSsystemsoffertheopportunityofquantitativetrace
analysis for a large number of species, based on a single-sensor system. Axford
etal.[19]haveappliedMStothesamplingofionsfromatmosphericflamestostudy
theeffectsofappliedelectricfields.Thekineticaspects[20]orspecialissuesinthe
analysisofexhaustgascompositionofinternalcombustionengineshaveeachalso
beeninvestigatedusingMSconcentrationmeasurements,anexamplebeingthatof
oilemissions[21].
Flame studies have been performed using GC alone (e.g., Ref. [22]) or in
combinationwithMS[23].
1.3.4
Pressure
Inthecaseoftechnicalapplications,measurementoftheburningchamberpressures
(respectivelythepressureatfiringconditionsasavariableofstate)playsanimportant
role,notonlywithregardstostabilityissuesbutalsotosecurityissues.Moreover,
knowledgeofthelowerandupperprocesspressures,forexampleoftheidealJoule
cycle, would provide direct information on the thermal efficiency. In the case of
automotiveapplicationsinparticular,themeasurementofengineandcompressor
cylinderpressurescanprovideadecisiveindicationofcompleteengineperformance,
sinceitcanberelatedtotheengine’spower,efficiency,throughput,andleakstatus.
Thetime-basedevolutionofcylinderpressureisalsoindicativeofthe“health”ofa
machine,andenablescondition-basedmaintenance.
Depending on the apparent temperature range, working atmosphere and pres-
sure,andalsoonthetemporalresolution,differenttypesofpressuremeasurement
methodsmaybeused.Foralmostallpressurerangesandlowsamplefrequencies,a
simplemanometer-typepressuregaugecanbeusedinconjunctionwithresistive,
capacitive,orinductivetransducers.Agoodexampleoftheapplicationofthissensor
typeisinthemeasurementoffuelpressureinthesupplylinesofadomesticgasoroil
burner.
Piezoelectricorthin-filmstraingauge-typepressuresensorsareoftenusedwhen
high sampling frequencies are necessary. This may apply especially to in-cylinder
measurements,wherethesensorsmaybeintegratedintothesparkorglowplug(e.g.,
Ref. [24]). Another specific application for pressure measurements within harsh
environmentsisintheinvestigationofcombustion-drivenpressureoscillationsin
lean premixed gas turbines, where a feedback between heat release fluctuations
and the burning chamber acoustics can result in severe combustion noise, with
pressure amplitudes of several hundreds of millibars. In this case, piezoelectric
pressuresensorsarebestsuitedfordetectingandmeasuringthedynamicpressure