Characterization of the spectral signature of dual-fuel combustion luminosity: implications for evaluation of natural luminosity imaging

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Chan::1ctcrizution of the Spectral Signature or Dual-fuel Combustion Lurninosity: Implications for t;;v.ihmtion of Natural Luminosity Imaging

Authors:

• Ald Srun, Paul Scherrnr lnstilulc, OVGA l 19A, Cll-5232 Villigcn PSI, Switzerland o Email: ales.srnaûi!psi.ch: Tel.: 1·41 79 310 87 74, Fax: +41 56 310 26 24 • Rolf Rombach, Paul Scherrer lnstitute, OVGA l 07, CH-5232 Villigen PSI. Switzerland

• K:ti Herrmiuu,, lnstitute of Thermal and Fluid Engineering. School of Engineering, University of Applied Sciences North western Swit:,.erland, Klostcrzelgslrnsse 2. Cl 1-5210 Windisch, Switzcrland

• Gilles

B1·uncnux, IFP Energies nouvelles, 1 et 4 avenue de Bois Préau. 92852 l{ueil·Malmaison, France; Institut Carnot If.PEN Transports Energie

Abstract

N-dudecane pilot-ignited lean-premixcd nnlurnl-gas combustion has heen invcstigulecl in a rapid compression-expansion machine. The aim or this study was to characterize the combustion spectral footprint: identify the main sources or na(ural luminosity, characterize the lcmpoml brightness evolution, and providc guidance for the evaluation of nmural luminosity imaging acquisitions. Nutural luminosity spectra in the range of 280 - 610 nm were acquired, 1 D-rcsolvcd along the injector axis, using an imuging spectrograph and intensified high-specd camera. Four significant contributions to lhe noiural luminosity wcre idcntificd: Soot,

OH

* and

CH* chemilumincsçcnce,

as

well as overlapping hrnadbaml chemilumincscence of C02*, CHO* and CliiO• species. The CH* chemiluminescence could be only dctccted nt ignition and during the pilot-fuel combustion period. Similarly, iniliul OH* .and broadhand luminosity wcrc also detected at ignition. 1-lowevcr. this lumino~ity .idditionally incnmscd lale in the cycle, when methane. cnrichcd wilh diluted pilot-fuel, lbrms an extensive burnl zone with close-to-stoichiomctry conditions. For the ignition delay dctcction, imnging of broadband luminosity hus 10 be recomrnended sincc m ignition il shows a highel' 1-Lsc-rate thun the OH* chemiluminescence. 1t was shown llrnt. aller ignition, in dual-fuel combustion. the coupling hetween the nalurnl luminosity and heat release-ratc is too wcak to extract usefül information.

Introduction

The optical investigations of combustion oflcn rcly on the îmaging of flame natural luminosity. Such detection is state of the art und utilizes llnme imaging in various wavclcngth ronges: around 310 nm frir OH* chemiluminescence detection, around 430 nm for the detection of CR*, or bruudband/color imaging for the dcteciion or all luminescent specics in the llumc. Nevertheless, the chemiluminesccncc or nll species in flames strongly dcpcnds on the ternperature, reaction rates, cquivalence ratio, pressure as well as the strnin rnte. Furthermore, such imaging is prone to the interference by soot incandescence. Whcn con1hustion processcs arc investigated optically, a study of the spectral signature of combustion is or high vnlue 10 identil)' possihle interfcrenccs as well as to estahlish a framcwork for the interpretation of resuhs. During the vuliclulion of CFD studies nflcn CH* or OH* chemîcal mechanisms are uscd to prcdicl the chemiluminescencc signal. Fur comparison with expel'iments, it is or parnmount irnportance that the experimenls indeed detect only the target spccies emission.

The natural llame luminosily originales !rom the emission or sevcral spccies emittîng i1, a wide range of wavclcnglhs. In spnrk-Îgnited premixed-comhustion cngines. several studies investigaled the llume and spark·plasma spcclra [ 1]. Within the varîety of combustion concepts fenturing auto-ignition. the spectral footprînl of non·sooting Homogcm:ous-Chargc Compression· Ignition (HCCI) is must thoroughly investigate<l. In the visible range, studies report broadban<l chcmiluminescence between 300 nm - 600 nm 10 be the primat')' cmission source l2-6]. Additionally. the chemilumincsccnce

of

CH• and OH* hecomcs visible at sufficiently high equivalcncc ratio 14,5]. The ratio of CH*/01-1* as wcll us the intcnsity of the hroadband (88)

luminosity. were shown to corrclatc wcll with the local equivalcncc ratio, and hoth, the 011+ and 1313 signais werc found to be proportional to the Hcat Rclense Rate (I IRR) [2]. Wilh incrcasing level of spatial mixture stratilicaliun. the cumbustion regime

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changes to parlially prcmixcd combustion until lhe conventional diesel combustion rcgime is reached. Al diffusion combustion Singe. soot bccomcs the dominant source or luminosity al longer wavelengths [2.7-9). Nevel'theless. al shoJ'tet w,1velengths, a considerable portion of luminosily slill origimncs from OH*. CH* and BB luminosity. During the auloignition and premixecl burning part, BB contributions frorn bolh, CO,* us 1vcll fis CH20'-' and CHO* chcmiluminescence were identified, while at later stages lhc HB luminosit)' is again doinin.ited by C02+ (7,81. Thi;; proporlionulity or HRR and 01-1*/BB intensity does not apply anymorc.

The chernical 1nechanisms describing the formation or the cxcitcd lumincscing specics can aid the understanding of the expcrimcntally observed trends. A shol't revicw of the liternlurc suggcsls the dillcrcnl chcmilumincsccnl spccies to be produced at different reaclion stages during the combustion. Fitsl. exciled CH*. uriginating from the oxidation reactions of

two-cnrbon-ulom specics, is t:xpected to form 110.11 j. Therefore, considerably stronger CH+ chcmilumincsccncc in thunes

or

higher hydrocarbor,s is expecled relative lo mclhanc. Ncxl, CH20 and CHO species will he lhrmed. 1.itcrature invesligating thcse particular specics i.;hemilumincscencc is scarcc. CH20* is suggestcd to be fonned in the recombination rcactions of 1-carbon atom radicals [ 12.131 and CHO* during lhc oxidation rcaclions or CI-JiO with the OH radical [ 13, 12]. At las!, the formation of C02* and OH• species is cxpcc1ed. COl is formecl mostly lhrough the oxidation or CO with O atoms, while the 01-1* originales from the recomhination reactions or J-1 and O aiorns as well as from the oxidation or CH radical with 02 113, 14, 10.151. Contnu)' lo CH*. CHO*, and Cl-120*, which are ahundanl only during the combustion. the CO+O- CO/' and O+H-.OH* rc,1cliuns rcmain active also in the hot burnt zones with close to stoichiometric conditions [13,14]. This leuds to a pe1·sisten1 CO,+ and OH* luminosity also in the burnt regit.ms.

The dual-lue! combustion process comprises of pilot-fuel auw-ignition in methane/air mix1t1rc, rollowcd by turbulent premixed

llome propagation through the renrnining charge. The lùel equivalence ratio. os well .is the ralio of premixed l'ucl and pilot· fücl. is strongly slrmilïcd in the combustion chambcr. Severa! optical studies investigated 1his combuslion using che111ilu111inescence ond schlieren imaging 116·2 II, The commun observation is lhal mcthane strongly defürs lhe ignition deloy, and during the pilot-fuel combustion n higher HRR is obscrvcd duc lo the simullancous combustion of premixed methane-. The rno1îva1ion lor this study was the observation of a decoupled evolution of HRR and 01 I'" brighlness, perccivcd in our previous work [16,17J and olhcr studics 1191, Figure I illustrates the combustion 1-m.R rate compared to the field-of-view integrated 01-1"' chemîluminescencc for a swcep of bnckground mcthanc cquivalcncc ratios in the RCEM (16]. Adding methanc into the charge air leads to an increased peak I IRH. (by a foctor up 10 2) relative lu lhc pilol-fucl-only case. The 01-1* chcmilumincsccncc signal appcars to he decoupled from the HRR. ln dual-fuel cases, lhe highest OH+ chemilumincsccncc signal has becn dctcctcd during the prcmixed !lame propagation stage ( l ·2 ms after ignition) and up 10 10 limes hîghi;:r OH*

signais were observcd for highcr mcthanc background concentrations. This behavior has also heen reported by Schlatter et al. (19] for 11-hcphme pilot Ille!. Up tu date, il was nol invcstigatcd whcther this signal originales from OI I*. 138 chemiluminescence 01· possibly sooi blnck-bocly radialion.

400 350 2 2.5 3 3.5 Time aSOI (rns] (tj c Cl '<n 'Methane: --- i/Jaa0.66 - - -</i=0.48

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=

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FÎf!lll"I" l: Comh11stî1111 IIRH (fl!ll·lillc) uni! fidcl·ul'·,·il'\\' li1lci,:1·u1cil 011'· i11111;,:i11~ ,ii,:rrnl (d:1.~hcd) for .1 rnrh1tio11 ot'pn~mix~1l 111cth;1nc cqui\ :ill!rm.: 1·a1io, ilC<1Hit'Nl usin(.: !Ill' ll'~I ~l'lll(I frnm 11

<il

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(omliti1111~; T~o1 = 81 () h.:, p~rn = 2S hal". injcclion d11rn1ion: li.SN Ill~. i11jc1·1ioii 1H'C·~•111'l'; <,OO lrnr. pilnt-J'ucl; 11-tiotlt•t·anc.

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Figure J indicates a considcrahly slowcr risc rnlc or OH* sign.il ,it ignition in dual-fuel cases. This <lependcnci;: of the intensity

and tempmal evolulion of the 01-I* chcmiluminescence might have a srrong implication on thi;: detection of ignition delay using a thresholding approach. Similnr might also apply for the general natural luminosity im,,ging. The correlation hetween

the local ignition and chcmiluminescence is presented in Figure 2 hy comparing the simultaneously acquii·ed schlieren and

OH* inrngcs from diesel ond dual-füel case with the samc ignition delay. Thi;: op1icol system sensitivity was unchangcd

l

l 7J. At ignition. the first ()11+ signal is expected to he dctcctcd. The temporal cvolution nf the Schlieren sigm1I is more complcx 1221: in the diesel case, flrst schlieren signais with high i.:ontrast are detected in the fucl-vapor zones duc lu the evaporative cooling. The low-temperature combustion gcnerated heat compensates for the cvaporativc cooling und leads to a softening of the schlicren signal, which thcn n:nppcnrs aller ignition.

-10 0 10 Radial die1. JrnmJ

:

::

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6 0.10 ~ 1.33 . .,.20 - -Î 30 t; 5 40 1.35 ] 50 _{....____} -10 0 10 Rlid131 âi~I. J1111nJ 1.3e 1,4Q 1,43 1.4!i _f.

(a) Tso1 • 770 K. <lic1-1~ = 0 (b) Tso1 = 850 K. 4'c1H = 0.66

1.48 1.60 m•

Figure 2: l'imc-sc.-ic, 11f .~chlicrcu 1111<.I 011~ dtl"'miluminr.'Hl'IICI' i111:1g~s fo1· (a) 11il·scl. uml (h) <111111-fuêl rnsl· with app1·o~im:ih!I~.-the samc ig11itinn {lt•la)'·

In the diesel case (Figure 2a), the first appcarnncc or the 01-1"' signal ( l.06 ms) correlates well with the fïrst schlieren signal reappearance upstream of the spray lip. The OH"' intensity quickly rises and cxcccds 30% or lhi;: camera dymnnic-range

already 50 ~1s aficr ignition. ln contras!. the dual-ll1el case shows a very slow risc or 01-1* signal. First photons are visible on the OH* images at 1.02 ms aller the start of injection. lt takes more than 125 ~1s to reach the intensity comparable lo 30% of the camera dynamic-nmgc used in the diesel case. Schlieren imaging is more sensitive to detect ignition in the dual-fuel case -the lïrst schlicrcn signnl reappears already 1-2 image frames prior to the fïrst 01-1+ photons being detected (0.99 ms) This is considerably earlier than the ignition dclay ( 1.06 ms) dctccted by the common thresholding technique used in [16J. Depcndi11g

on the sensitivity of the employed deteelion system, the slow tise rate of the OH* signal can lead to signilicanl overprediction

ofthe ignition delay, as reportcd in [191.

Ohscrving the pcculiarity of ignition detection in dual-fuel cases, tht: objectives of the present investigations wcrc:

( 1) charactcrize the dual-füel combustion natural luminosity relative to the luminosity of conventional diesel combustion, (2) idcntify the species contributions to the natural luminosity, (3) explain the observed temporal evolulion or OH* signal intensity. and (4) prnvide guidelines for the cvalumion or dual-J\lel naturnl luminosity imaging.

This publication is structurcd as follows: First, the t!Xperimental test rig, the I D-spectrosr.;opy setup for the time-resolved

acquisition

o

r

llamc-lurninosity spectra. as well as the proccssing approach. ure pi-esented. ln the results section, the spccics

conlribuling to the nntural luminosity were identified, and the dcpcndence of spectral emission on the charge and injection conditions is presented. The discussion focuses on the implications or the observed luminosity trends for the cvalmHion of nutural luminosity imaging in the dual-fuel comhustion investigations.

2 Expcrimental AjJparatus

2. 1 Rapid Compressio11-Exprmsio11 Machine

(

RCEM)

The Rapid Compression Expansion Machine (RCEM) used in thîs study is a free-lloating piston nrnchine with a bore of

84 mm and stroke of up to 249 mm, Mcthunc-uir mixture is conditioned in the combustion chnmber prior 10 the rapid

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3 threshold level ([lso1). To admit the pilot fuel. a single-hole coaxial diesel injector with a I OO pm conical nozzlc is used,

4 mounted at the cylinder pcriphcry. with the injcctor axis in the radial direction relative Lo lhc RCEM cylindcr axis. Rcadcrs 5 interested in the detailed information on the RCEM combustion chambcr gcomctry arc rclcrred lo 117]. The dctailcd 6 description of the RCEM opcnüion principlc und 5lrulegy ure uvuilnblc in 1231.

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2.2 Experimental condition.~·

ln this study. the RCEM has heen operated at the maximal HDC displacement setting. ( 1.38 dm3 _displacement_,_{249 mm Bl)} C

clearance to the cylinder head) to have a sufficient TDC clearance for minimizing the pilot spray interaction with walls. The operation strategy of the RCEM was sclectcd based on prcvious opcration expericncc to assure high-repeatability of the conditions at Stan Of Injection (SOI): A mixturc of mcthanc und nir with a BDC prcssurc or 1.2 bar ha~ bccn conditioned in the cylinder beforc lhc rnpid compression to assure homogeneous mixture [17). The machine was opcr.1ted at a compression

rutio of about 20, and the pilot fuel in_iected o;:(lrly in the cycle (c.1. J.5 111s before TDC) when the cylinder pressure reached a threshold of 25 bar. The bulk charge temperature at ignition was controlled hy adapting the cylinder head and walls

tempe1·.:1ture .it the BDC. Consequentl)1, the HIJC charge tempcraturn (equal to the wall tcmperature [24]) and the temperaturc at the 80! (Ts01) were changcd.

The rneusun:ment mmrîx co11sis1ed or four main v,1rh1tions: V,1!'ialion of the premixed meth(lne cquiv:;ilence ratio (1t1çH4) in the range betwccn 0-0.66, vorimion of' Tso, (770 K, 810 K, 850 K ), variation ol' pilot injection durarion (0.J8 ms, 0.58 ms, J.21 ms) as well as the charge oxygen content ( 15%,. achieved hy dilutinn ni' air with nitrogen.). The injection pressure was kept constant al 600 bar (steady fuel injection rate: 2.43 mg/ms). Table I summarizcs the expcrimental matrix and conditions ofthis study.

T,tbl(• 1: On•,·vi\!I\ of rlll' v~pcrimcntal m:111·i,. \ aine~ in br;1ccs 11 crç 11111 acq11i1·cll for all (Tos,-, aria rions. l'hc \':tlue, ur lhc stn11cl111·d cu~c 111·,· lll'i11trd holll,

Tsm and lf>c11~ variation Injection duration Charge oxygen variation

vai·ia

tion

Chante BDC pressure 1.20 bnl'

Stroke 236,5 mm :1: 1 mm

Cômorcssion ratio 20

BDC temp_eraturn 343 K 363 K 383 K 363 K

JJressure at SOI 25hm·

Tcmperature al SOI (Tso1) 770 K 810

I<,

850 K 810 K

Charge oxvi!:cn content 21% 15%

Premixed mcthanc 0, 0.33. (0.48), (0.53), 0, 0.33, 0.59 0, 0.33, 0.59 cauivalcncc ratio ( dlr;,u) 0.59. (0.66) (relative to 21 % oxygen)

Pilot injection duration 0.58 ms 0.38 ms, 0.58 ms. 0.58 ms

1.21 ms

Pilot-fuel mass pér injection 1.18 mg 0.64 mg. 1.18 mg, 1.18 mg

2.67 mu

Pilot injcctor cncrgizing Lime 400 µs

300

~15, 400 µs, 700 ~lS 400 µs ŒTl

Pilot inicctiôn oressurc 600bM

2.3 Optical

setup

Two di fl'ercnt sets of optical diagnostics wcrc uscd in this study. The introduction ligures 1\11d Figure 9 prcscnt rcsults ucquired with a simultancous Schlicrcn and 01-1* chcmilumincsccnec imaging selup dcscribed in 117]. Since both techniques represent stnte of the art, 1hey ore no1 l\1rther i11troduced here. Second, the combustion spectral footprin1 w,is characterized with a ID -spectroscopy sctup described below.

Figure 3 prcsents the I D-spcctroscopy sctup cmploycd Lo invcstigate combustion luminosity spccirn. Emission wfls acquired

thruugh the pi5Lon window ovcr two UV uluminum mirrors. A port

or

cmittecl light w.is ,·ellectecl by the dichroic beamsplitter

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towards the spcctrugroph and projected on the spectrugrnph slit using an UV objective (Sodern Ccrco 94 mm IÏ4.1 ). The application or an im.:iging spectrograph (Acton SP-300i. gruting 300 lines pc,· mm, hlazed for 500 nm) cnublcd ID resolution of the spectrllln along the slit. The spectrnlly dispcrsed light was detected with an intcnsifïcd high-speed camer;i system (La Vision 11!::i-lRO coupled with Photron FustCam !::>A 1.1 l operated at a frame rate or 20 kHz (intensifie,· g.ite 45 ~ts). A slit with of J OO µm was used, reduccd to 40 µm for strongly sooting cases. and the intcnsi lier gain was ndapted hetween 7

-20 counts/photoelectrnn dcpcnding on the flome hrîghtness to fully exploit the camcn1 dynomic-r.inge. The wavelength range between 250 - 600 nm was ai.quired. Below 300 nm only wcak radiation was detected. and thereforc, separation of diffèrent grating ordcrs was not necessory. The spectral resolution of about 2 11111 and the resolution along the slit of about I mm were limited by the spcctrogrnph aberr.itions and not by the camcrn rcsolwion. The 11eld of vie,v of the ID spcctruscopy was set to spcctrnlly resolve the tfome emission and soot luminosity in one dimension along the pilot-injcctor axis.

"-UV-enhanced alu. mirror Beamsplltter 50% 300nm-550nm

~

!

. Equipment

-

- --

--- \ : not in use : ~ ____ ,..

...

: Cerco 94 mm

,

₌

.

_{;;;..._}_f/4_.1_UV_Jens Spectr graph IRO

HSC

Pilot inJeclor

Figure J; 01Hlrnl M'IUJl l'or I D·sp~ctrn~cnp)' ac11uisilin11s ut lhc HCE\I. A r<•ngh ~kil!t;h nf the RCEi\J :/,l'om~ti-y 1111!.I optic,11 ;icccs~ i~

shown as wcll.

2.4 Spec:tra proccssiug

Spectral images were processcd to cxtrni.t the spectra and identify the contributing specics. First. the images wcre averagcd over 10 experimental rcpclitions. and across l O pixels along the axial direction (corresponding to 2 mm). The so obtaincd rnw spectra were furthcr proccssc;;d os demonstrated in Figure 4.

The raw spectra were corrected for the system spectn1l nnd spatial dependence of transmission und quantum cfficiency ( more detaîled description in (17]). In the ncxt stage, the presence of soot incandescence has bcen detected (Figure 4 (a) sooting combustion, (h) non-sooting combustion). For this pur1~ose, a soot-cmissivi1y corrected Planck's law curvc is littcd 10 the corrected spectrum in the wavclenglh range of 470 - SR5 nm. The soot cmissivity relation proposed by Musculus

L

25 J has bccn used. ln this spectral range. the contribution of name chcmilumincsccncc to the overall intensity is cxpectcd lobe smullest in the availablc spectral mnge. The result of tilting procedure is a value or the apparent soot temperaturc at the spcctrum spatial location, and the estimated brightness of sool incandescence over the complete wavelength range invçsligated here. The blue dashed curve in rïgure 4a demonstratcs the rcsulting Planck's law curve fit. Two criteria wcrc dcvised to distînguish sooting combustion from non·sooting cases bascd on the Planck's law 11t results. The first critcrion proves the apparent soot temperature for unreasonable values: if the apparent temperaturc lies outsidc the 1500- 2800 K range the emission was assumed not to originale from sool incandescence, therethre, the case was dccmcd non-sooting. The second critcria rcquircd that soot incandescence considcrnbly contributes to the total flamc luminosity al shorter wavelengths (450 nm): The Planck-fittcd soot incandescence brightness at 450 nm (bitte dashcd lin..: Figure 4a) was compared to th..: total cmission nt 450 nm (black line Figure 4a). Cases, where soot incandescence did not exceed an arhitrary thrcshokl or 50% of the total emission, were assumed to he non-sooting. Thcsc critcri11 were deemed to reliably detcrrninc sooling combustion by analysis of spectra with the naked eyc: il was strnightforward lo dislinguîsh sooling combustion spectral fcatures from the non·sooting.

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·ii> · - - -Spline fit

c:;

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Q)

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01...J.,~.;..:;;;~ .:_~ ~ ~ ~ ~ 300 400 500 600 Wavelength [nm] (nl Spectrum ofn milclly sooting case

....-, ::i ~ 0.8 è ·~ 0.6 Il>

ê:

=

_rn0.4

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u,~c

speclrum

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(i.;) SuhlnlC!c.:d BB luminosily

600

Fi:,:u1·c -l: l ll-~11cclrnscop)" Jll"ot·c,sin:r 11ppnrnch: (i!J Th1· nm spcl'trnm (sp.) ancl trnnsmis,ion c111Tl'l'tcd (Trnns. l'OIT.) spct·trum of a

soorin~ 1·.0111h11sti1111 l'H'n(. wilh dcmo11~1ra1ctl l'lanl, antl spli111·-Jh 1:u1·,ç~. (b) lt:1\1 spèt't1·u111 a1ul tr:rn~mb~ion to1·1·c('!~cl ~pcct1·11111

or

a non-~cwting 1·nmhu,ti1111 c,·cnt, wilh a (lcm1.111.~n·:ifç1J .~plinf,lil rrsul(. (~) The ~p~çtrum from (b) 11ilh ~uhtrnct~d h!'lnldhanll J11minMitr.

ln the sooling cases, the liued incundescence luminusity wns subtracted J,·um the spc.:ctra. ln non-suoling c.:asc.:.s, thi.s pruc.:e.ssing

step was skipped. The l'emainîng signal can be attributed to the llame nnturnl Juminosity. Flome Juminosity (Figun: 4o) shows

well distinct peaks at around 310 nm, 430 nm, and 510 nm on a pedestal nf bl'oadband lu111inosity. The separation of the OH*.

Cl-1* and C2

*

chcmiluminescence from the broadband (BB) luminosity follows the appl'oach proposed in l2). A,spline fit with

a long kcrnel (MATLAB func.:lion 'fit' using lhc 'SmoothingSpline' functionality with the smoothing parameter set to 0.00 J) has been used 10 approximate the spectrun, oJ'the background luminosily - whc.:rea.s the.: points lrom the bands 305 - 320 11111, 423

-4J~J nm, and 510 - 520 nm were not given to the fil function. ln this way the bro,ldbnnd luminosity al wavelcnglhs with

ovel'lapping nal'rowband species chemilutninescence was approximated. ln the second step. the splint-approximated broadbnnd luminosity lrns bccn subtracted from the spectrum (Figure 4b). The integrated signal in the spectral band of 325 - 350 11111 has bc.:cn uscd as a mc.:asurc for broadband luminosity intcn.sity. The sclection ofthis spcdra range was motivatcd by minimizing the interference from possible soot lumînosity. The imensily oJ' the remaining pcaks (aller the splinc.:-lît .subtraction) in the.:

range of 305 -320 11111, 423 - 439 nm, and 510 - 520

nm

li.1s been integrnted over the corresponding bonds and nssigned os the intensity of OH*, CH* and Cl* chcmiluminescence, l'espectively (Pigul'e 4b), However. the C/ chcmiluminescence wos too weak to be dctccted rcliably and was not fürthcr proccssed.

ln sooting cnscs, this method cannot distinguish bctwc.:c.:n the.: suot blm;k-body lmninosity and the.: broadban<l luminosity in the.:

upper wavelcngth range (above 450 nm). This results in an underestimation oJ' the broadband luininosily in souting cases since ail light emission in the range or 470- 590 nm is considered to originale from the blackbody luminosity. ln the shorter

wavelength range used for broadband luminosity estimation (325 - 350 nn1) this eJTOI' is suhstantially smaller (estitnates lcss

than 20% in the most sooting considercd c.:ases) and is not cxpectcd to influence the interpretation of the l'esults in this work.

3 Re

s

ults

J. I

Temp

oral evo

lutin11

nftlteflame

e

111i

ssio11 spec

t

ra

The.: temporal cvolution of the spectral cmission or a non-suuting diesel ($c114 = OJ and 11011-sooting dual-fuel ($c1.14 = 0.66)

combustion c.:asc at Ts01=810 K i.s prescntc.:d in Figure 5. ln both cases, al the lime of ignition, luminosity is detected across the completc spectral range acquired herc. Bi;;sidcs thi;; 01-1* chc.:miluminc.si.;c.;ncc c.:mis.sion in the range or 305 - 320 nm also

iwo bands of broadband lum inosity are visible: A band extending ll'om approximutely 300 - 500 nrn. and a band exicnding

from 500 nm towards longel' wavele11gths. ln nccordance with the liierature. the emîssion in th,;: lirsl bond was uttrlbu1c.:d to

C0₂*. CHO*. and Cl-hO* cmissions. The emission of the second hand persists longe1' Jhan the hl'oadband (138) and Oii*

luminosily. ln the lileralurc this cmission is commonly attributcd to the ovcrlapping 1-120 vibration-rotation bands 500

(7)

2 3 4 5 6 7 8 9

10

11

12

13 14 l!:l

16

17

18

1

9

20

2

1

22

23

2

4

2

5

26

27 28

29

30

:n

32 33

34

35 36

3

7

38 39 40

41

'12 43 44

4

5

46

4

7

48

4

9

50 51 t>2

53

54

55

56 5 "/ 58 59 60 61 62 63 64 6!:l

580 11111, ami the N0-0 continuum 400 - 800 11111 [26J. In somc

or

the duul-füel cycles (c.r Figure Sb) al a lalcr slugc, an

addilional cmission at 589 11111 was detected and allributcd tu sodium D-line emission. The sodium is bclicvcd lu originnte from the Iubricatlon agent used in RCEM.

The e1t1ission of CH* (430 nm) is short-Iived and was only detected in the vcry carly phase of combustion -tàintly visible for the diesel case in Figure Si1 inlay. The emission of CH* in the diesel cHSC is also accotnpanied with a bright cmission in the wavelcnglh band from 300 - 450 nm. Contrary lo the light cmitted laie in the cycle, during this transicnl the cmission in blue

wavclengths is brighter than at wavelengths longer than SOU nm. ln accordance to [7.8] authors attributed this to the light emilled from Cl-120* and CHO*, which arc nul expected to be formed aller the pilot li.ici is consumed. Therefore. the emission

Iater in the cycle 111ight be primarily duc to CO~"". However. no spectral features could be dcti:cted 10 confirm this .issu111ption. /\ very large dilkrence in the brightness of the diesel and dual-li.lei case required different color scalc rcprcscntation: diesel

case was multiplied by a factor of 1 O. The brighl transicnt Iuminosity at ignition seen in the diesel case was not visible in the dm1I-ruel

cnse

(Figul'e 5h). /\t late stages

or

combustion no significant difference in the spectral footprint ufthe cases in Figure 5 is npparem. ln the following, the analysis focuses on the dependence of OH*, CH*, and bruadband emission (BR) on the rnethane equivalencc ratio, charge tcmpcrnture. nnd pilot injection duration.

600 ---··- 600 10 Ë 550

·;

~

Ë 550 ₈ .S.500 0.8 .S.500 :ë 0.6 ~ :ë 6 0>450

₄

_~J

_-

g>4

50

:=; c: _~

_.i

Q) Q) 4 "&ï 400 0.4 ai 400 > 42~.8 _ 1__].:~ > ~ 350 ₀_.2 ~ 350 2

-

~ 300 0 300 0 2 3 4 5 6 2 3 4 5 Time aSOI [rns] Tîme aSOI [ms] (a) 4ic1i4.,0 (b) $c114=0.66

Fil,!llr'é 5: Télll)l•lntl t•vnlntinn nf anrni.:ctl (acrn,s th<' illj<'CIOJ' 11\ÎS) thune luminnsity for (a) tlit·scl. a11d (h) d1111l-ft1cl CIISC. Coutlitions: Tsoi = 8JO h .. l::T = -100 µ; •. rhc hrigh111c~s ni' the Jie~l'I t11se i~ mnllipliti1l b~' :1 facitll" ol' i Il lo ntilizl' the l"Olllplctl' l'ill!~ti or rolnr

rcr11"csc11tt1iion. ln lhr dic~cl rasv (al tht• inlay plo1 sium~ 11111::_11ilitd the s1>1:ctr,1l b:111() arnuntl .J.l(J 11111 al a lime around ignition.

The spatially I D-resolyed temporal evolution of the combustion spectral întensity of BB, OH* and CH* chcmilurninescence a long the injecter axis for a variation of methanc cquivalcm:c rmio îs presented in Figure 6. A comparison of the 01-1* and BIJ ehcmiluminescence intensity shows only 111inor diffcrences betwecn the spatial signal distribution ofbolh specîes. Only during the pilot·fuel combustion a diffornncc bctwcen the OI I• and broadband chcmilumincsecncc cvolutîon

wns

observed - the broadband signal pcaks and shows a highcr inlensity than the. OH* signal.

The signal

o

r

CH* shows a considernbly difforent hehavior than the 01-1* and BB. Il is only present early in combustion and not dclcclable durîng the premixed !lame propagation phase. Il shows

a

high rise-rate at ignition and lhus aids the delection of ignition dclny. ln the Jute phase of combustion. CH* inlcnsity is very weak, overlaid with bright 88 Iuminosity signal. This

bchnvior is in agreement wîth the litcraturc suggcsling lhal the Cl .1* is fonned during the oxidmion ol"C2 and C11 I species. ln the combustion of long hydrocarbon pilol·fücl and prernixed methane, considcrably mure C2 is expectcd during the combustion of pilot fuel.

The dil1crcnt rise rotes of species chemilumincscencc at ignition will have a strong inlluence on the mcasurcd ignition delay using

a

thresholding technique (as already obscrvcd in Figure 2). This is demonstrated hy the dashcd vertical lînes in Figure 6.

When detecting the ignition based on an 01-1* imuging thresholding approach, the slow OH signnl rise rate at ignition might lead to an imprecise ignition dclay dctcction, depending on the system sensitivity and the selected thrcshold level. This 111ight be especially critical in dual-fücl cases where .i very slow risc of the 01-1* signal alh:r the ignition has been ohscrved. On the

(8)

1 2

'3 other hand, CH* and BB signais show a faster rise rate al ignition and appear Lo enable a more preeisc ignition delay dctcclion. 1 This will be fürlhcr assesscd in the discussion section.

s

6 7 8 9 l O 11 12 13

14

15 16 l'/

18

19 20 ?. l 22 23

2

4

25 26 ?.7 28 29 30 31 32 33 34 35 36 37 3$ 39 40

41

'12 13 44 4 t, 46 47

48

,19 50 ~l b2 53

54

55 %

S7

58

59 60 61 62 6::l 64 65

Ë

<PCH4"' O.O §.20 . ...:.,30 -~ 40

~

50

~ 60 -~ 20 1~ -0 301'

50

40

1

60

i

20 ~

:

,

30

l

ti.

401~~ ,,._.,

so

1 •

.

_•· _._·_:'.-

_.

r

_...

.

:.

• ..

_-

_,

_·

_~

_·

_.

_~_. 601

l

"

··/LI,-.,· 1' QJCH4 = 0.33

!

'

.

,

!

) (

, .

.

-

·•

.

! :~_..... ! .. -.₁~ .. _,~_,··_{< -}. ,r· _• - 1

"JI"

.

...

··j :

• t,

• '

!

--tt.,l\~.,..it-. 1

1.

5 2 2.5 3

3.5 1.5 2 2.5 3 3.5

Time aSOI [ms] cj,CH4 = 0.66 I 0

J~

.

l~tr.t:~

~

.

:

·~.'r:

:-

~li(,'J

1.

5

2

2.

5 3 3.5

l~igun· (,: T~mpornlly a111I spati;1lly r,·~n!,·l'll inl\'n~ity or 1!1c· IJ1·oudb;t1HL 011". :11111 Cil'' ('hc111i!u111inc~cc1H:~ !'or ,1 vari:111011 or the h:ll'kgi·111rnd mcth:t!!l' t•t1uh·:1!~11cc ratio. Cor1ditio11s: T~rn

=

l!lll l,, l'ilot i11,jccli1111: 1-:T

=

-1011 µ~. l11j1•1·ti1111 from top tn hottom. Figure 7 extends the analysis 0fthe tempor(II species chemiluminescence evolution 10 n vt1rintion of charge .ind pilot-injection

conditions. The evolution of CH*, 011*. and AB-chemiluminescence intensities for an extensive variation of background mc!hane cquivalencc ratio, charge tenipcn1turc, injection duraiion, and simulatcd exhausl gas rccirculation (EGR) is prcscntcd.

The duration of CH"' emissîon moderately im;reast:s with the addition of metlrnne into lht: hackground mixture. This împlies a prolonged duration of the pilot-lùel burning. JJowever, the injection duration does not show a sign1ticant influence on the duration of CH* signal. This also applies to $c1-14 = 0 cases with reduced charge oxygen content (hottom row. Figure 7), while

tu the eonlrary, a combination of rcduced oxygen and methanc addition considerably incrcases the duration or the CH* emissiun, indicaling prolonged pilot-li.tel combustion.

For ail variation of conditions in this study, a consîderable increase of 011• signal with the addition or methune has been observed. f.'ocusing first on the Q>nH variation at Tsot =

8

1

0

K and ET=

400

µs ( upper row). the diesel case shows a fast rise of OH* signal al ignition, l'ollowed by a plateau for I ms and consequent signal decrease. Wilh increasing q>u14• the OH* signal

l'ises later (longer IP) and shows lower rise rates. Nevertheless, following the initial faster rise. the signal keeps slowly rising for 1-3 ms depcnding on the $c114 and drops considerably latcr. As already observed above. a several limes highcr peak

emission of OH* is detected with incre.ising $c114. This influence is especially pronounced under the Tsot = 770 K conditions

and for short injections. For thosc lower T501 cases (Figure 7. 2"d row). wilh $ciM = 0, the maximal cquivalcnce ratio or pilot

f"uel rapidly Jeans-out, lcading Lo a rapid decrease or OH* brightncss. With lht: addition or mclhanc, the mcthanc and pilot-fuel

cumbinaliun J'orm long pcrsisting zones wilh elusc lo stoichiûmclric cundilions. This is aligncd with the longer persisicncc uf' the high 01-1* signol observed in lht: spectral Jootprint.

At latc :auges atlcr ignition, 01-1~ und BB signais show a vcry similar temporal evolution as already obscrvcd in Figure 6. This

applies Lo ail variations in lhis study. Ncvcrlhclcss. signilicant dillcrcnccs between the brightncss ur OH* and BB were dcleelcd during lhc pilut-J"ucl burning period. wilh a slccper risc ur BB cmissiun al ignition. Thcse rcsulls suggest BB

chemîluminescencc to be a better marker of the HRR durîng the pilot-combustion phflsc than lhe Ol l* sîgnnl. The ditlèrcnces

in the rise rate become prnnounced at higher <~nt~ concentrations.

(9)

2 3 4 5 6 7 8 9 10 11

12

13 ] 4 1~ 16

1

7

1

8

19 7.0 2] 22 23

2

4

25 26 27 28 29 30 31 32 33 34 35 36 37

38

39 40

41

42

43 '14 45

4

6

47

48 49 50 51 52 '.l3 54 55 56 57

58

:i9 60 61 62 63 6'1 6~

ln case or n:duccd charge-oxygen-concenlrntion, u suhstantial reduction of ail chemiluminescence intensitics has hccn

ohserved. For l 5'Yo 0~, in comparison to 21 % 02 at T~01 = 770 K (similar ignition delays), .ibnul two times lower CH"'

intensity hns been ohserved. OH* and bnmdhrincl lurninosity m·e even more inllucnced: about a factor 4 lower OH*. ami làclor

or 2 lower BB signais were observcd in the diesel-case. With the addition of methane al 15% 02, the peak 01-1"', nnd BI~

luminosity do nol inerease lik..: for 21 % 02. Theref'orc. al ~c,14 = 0.59, going from 15% 02 to 21 % 02• lh<.: !lame brightness for

both. OH* and HB chemilurninescence increases hy a factor

or

about 30. This w..is attributed lo the dei.:rcused adiab..itic !lame

lemperature al rcduced charge oxygen content.

Olten rc:porled correlations of CH*. or CH* /01-1~ ratio. to the comhustion cquivolence ratio obviously do nul apply in lhese

cases. Morcover. no correlation of the cumulative CH* cmission lo the total pilot-fuel quantity has heen observed. On the nther

hancl, the peak CH* intcnsity seems to increase with rising Tsoi, and slightly decreases with highcr rnethane content: up to 30%

of CH* signal dcercase was observed with a metlmne wncentrntion or $c:;114 = 0.66. This complex dependence of Cl-1*

lurninosity on the charge porameters renders any quanti lieation bnsed on Cl I* impossible.

4 ₂

v-,~

~

T

=

850K

,1

~ ET• 400j,, 3 2 T • 810K

;

~__d

"

"'""

/ / ... ..._ Î "810K

,

:

w:

____{_

-

·

" -~ ET •

ao

o

,

'

l~

1 T

=

810K 0_·2 _E_T_{= 400}_µs 0.1 - :. : (0 (•15% 0 .

=:..:...-

2 0.5 1 1.5 2 2.5 2 3 4 5 1 2 3 d 5

Tirné aSOI [ms) Time aSOI (ms] Tlme aSOI [ms)

Fi(!Lll"C 7: F\'oluiiou 111' Cil~. Oil". ;1nd BB hu11î1111~i1>

l'o,·

"

vu.-iillioli

o

r

~m •• T~rn· în,jcclion 1".T illHI <'h1t!'gc O'<Yf!fll cunrcnt. l .inr

coin,· marks lhc ~n lJ•

4 Discussion

The discussion section focuses primarily on the implicntÎMS

o

f

the observed spectral tentures for the evaluation of dual-fuel

combustion n.itural luminosity imaging. li lrns bi:en ah'eady established in lhc resulls section that the 1m:asun:mcnt of 13B signal is, in general, more rcliable lo detecl ignition than OH*, duc lo the steeper rise rate of BB signal al ignition.

Furthermore, the BB cmission spcctra overl.ip with the cmission bands of

O

J

I+ and CH*. Thcrdore, conventional bandpass

filter imaging of 01-1* and CH* mîght primarily dctcct BB luminosily instead nf the targclcd spccies.

(10)

1 2 3 ~ 5 6 7 8 9 10 11 12 13 1'1

15

16 17 18 19

?

O

21

22 23

24

25 26 27 28 29 30 31 32 33 34 35 36

:n

38 39 10 4 l 42

4J

44 45 46 ,17 48

49

50 51 52 53

5

4

S:i 56 57 58 59 60 61 62 63 64 65

Figure 8 prcscnts the temporal cvolution or the pcrccntage of" 01-1* and CH* contribution lo the ovcrall signal passing a bandpass fillcr lor corrcspomling spccics dct..:clion. The hcal rck:asc ml..: (HRR. bollom nxcs) is plollr.:d for rdcrcncc. This processing lollows the nn.ilysis proposecl hy N,\julilhucli et ni.

1

2

1 :

iclenl OH* uncl CI-lot. bundpnss filters 1rn11smitting in the

wavelength bands of 305 - 320 11111 and 425 - 435 nm were îlssumed lhr 011+ and Cl 1•, respectively. The portion of the luminosity originating from BB chemiluminescence was separmed from the nmTow-band 01-1* and CH* emission hased on spline fïtting (section 2.4).

~12 ~ a, c. 6

•

::r: ₀ u 400 1 - - -- - . . - - . ; ~ 300 =-200 cc

a:

100

:r:

0

~~====l

2 3 4 Time aSOI [ms] 5 ::0 80 ~ Q) 60

g>

40 ê: 20

~

Cl. 0 • :t: 0 Pilot ET: --300,,s --400,,.s

--soo,,s

'1'cH4: 2 3 4 Time aSOI [ms] 80 ~ ~ Q) 60 0 ) <1l 40 'E Q) 20 0 qÎ ()

.

Cl. :t: 0 5

[

1

2

~ Q) Q. 6

:r:

0

u

0 µ...~41-...µ_4-'-,....] ,_____,__._,,_ , 80 ~ Q) 60

_Ë

.--"'---rl40

fü

2 3 4 Time aSOI [ms] 5 u 20 iii Cl. 0 • :t: 0

(a) ~c-1-14 variation, 8T = 400 µ~ (b) ~c-1-14 and ET variation (cl 4>c-114 and EOR variation

Figui\• Il: Tc111purnl cvol11tiun of H1t• ~ont,·i1J11tion of llll· 11111·,·1111·-l.1111ul Cil* (U(l!lCI' 11\t•s), ,11Hl O!F (ntidtllc il~t·~) cilcmil11mi11~,ccnrr

to ilil' on·nill lu11il110,ity l0111iikd i11 tin· spcdrnl h1111d, u~t'd l'or 011'' }llld Cii" clicmilumi11t~r<'rH:c ddC'rtiou. Tht' 1

rn

n

_(hnll<111₁:tn~) i~ plotlC·d l'or n·fcrclicc. \'f1ria1io11s: (il) _{'Îlt 11,}1•111·i,Hion, (11) if>rn, 1111d ET "1,·intio11, 1111(1 (C) ~c,ii 1llid E<iH l'1H'i11tio11.

ln lhr.: liicrnturr.: a strong dr.:pr.:ndcncc of the C'l-1* and 01-1* contribution on lhr.: lcvd or prr.:mixr.:d-ncss in purlially-prr.:mixcd

compression ig11ilio11 combustion was observed [2J: only BB lumînosity (no OH* or CH") was observed for lùlly premixed combustion or heptane (q> up to approximately 0.40). ln 12). for conditions with higher mixture stratification leading to a considerahle po11ion of diffusion combustion. na1Tow·band species luminosity contl'Îbuting up to 50"!., rOI' 011*, and up to 15'% for CH*, in the corresponding wavelength bands. were reported. lndeed. similar maximum values of the narrnw-band species contributions wcre obscrvcd in the prescnt study - peak values in the range of 60-70% OH* contribution and 10-13% Cl-!* r.:ontribulion wcn: obsr.:rvcd for ail invcstigmcd conditions. This is cxpcctr.:d sincc the dual-lüd combustion r.:xhibils a considr.:rnblc lcvd or mixing inhomogr.:ncity and highcr cquivalcm:c ratios than invcstiguted in [2[.

h is interesting to compare the tempmal evolution or the narrowband chemiluminescence contribution to the liltered signal. Analyzing the

$e

114 variation (Figure 8), a good agreement of the CH* contribution with pcak HRR is obscrvcd: the fast rlse of

CH* portion at ignition and disappearance of the signal aftcr the pilot-fuel is consume<.! during the peak 1-IRR. This is in agreement wilh the conclusion thal CI-1* cmission only occurs during ihe pilot-fuel comhusiion. On the contrary, the portion of 01-1* starts to inr.:rcasr.: alkr igniiion and rcachcs a stcftdy lcvcl or abolit 65% only nlicr lhr.: pilot-lücl is consttmcd. This lcvr.:I or ül 1• contribution remnins al a plntcm1 levcl l'or severnl mîllîseconds ofler the peîlk 1-JRR and begîns to decre.ise very late în the cycle when the total flnme hrightness rapidly decreases.

Thc tr.:mporal cvoluiion of specir.:s contribution to luminosity was found moslly inscnsilivc to injr.:clion duration and thr.:reforr.: insr.:nsitivr.: to the pilot-rue( mixture fraction al ignition (Figurr.: 8b). Dr.:spitr.: the four timr.:s largcr l'uel mass in casr.: or the

longest injection. nîll.ximul CH* l:ontribution remains constant nt about 12% of illl luminosîty in the 430 nm band. The snme

was also observecl in the dual-1\tel Cîlse (dilshed lincs, different 11.Ts). Neverthelcss, a slightly lower peak C:l l't contribution of

(11)

7,

J I O'.V., was reachcd. Similarly, also OI I* again rises Lo a plati.:flu vnlue of 65% which is ri.:achcd nlîer the pilot-füel is burnl. The

4 contribution or OH* st,1rts to deviate from this plakm1 only latc in the cycle when the mixture considcrably leans oul.

~ 6 7 f3 9 10 11 12 13 1'1 15 16 l ï 18 19

?O

21 22 23

2

4

25 26 27 28 29 30 31 .3 7.

33

34 35 36 37 38 3q 40 41 42

4

3

44

45 46

4ï

48 49 50 51 ~2 ~3

54

55 56 57 ~8

~9

60 61 62 63 64 6~

A more s11bstantial depcmlcni.:i.:

of

CH"' and 01

r+

contributions on

<l>rn

4 was observed only in thosc cases with reduced charge

oxygen concentration (Figure 8c). Rcduced pe.ik CH* contributions wcre observcd at lower charge 02 (dnshed and dot-dashed

li11es) .is wcll as with im;reosil1g 1J1<.:₁₁₄. This is most probably due lo slower pilot-fuel combustion (implied by longer CH* signal

persistence) lcading to a simultnneous luminosity or alrcady burnt and burning pilot-fuel zoni.:s. ln udclition. the evolution or

Of l~ contribution appi.:ms to be dependent on pilot-füi.:I hurning duration. For ail 02 contents, the snrne plateau lcvcl of OH* contribution is m;hieved - however, considcrably luter for lower charge 02, corrcsponding to the time when CH* contribution

di.:cri.:ases.

üverall, this analysis conlirms the implications for m1lurHl-luminosi1y imaging evaluation statcd nbove. At ignition, the 88

lun,inosity mostly dominmes the ~ignal in OH* spectral rnnge. Therefore. the acquisition of BB luminosity is prcfe1rnd for thi.:

detection of ignition. This intluence has to be ai.:counlecl particularly also for validation CFD simulations employing an OH*

chemilumim:sci.:nce chemical mechanism - during the pilot-fuel buming considi.:rnbli.: dirtèrenccs to the expcrimcntal d!\lfl

might bi.: ohserved since befmc the pilot-fuel is burnt the camera acquires mostly BB luminosity. J;urthcrmori.:, this data shows

that simple CH* chcmilumini.:sci.:nce imaging in the 430 nm band moslly captures other specics Lhan CH* (Figure 8).

Simultoneous imaging with u second camer.i in an adjacent wavelcngth band could be used Lo subtrncl the Bl3 luminosity.

Neve1thelcss, this woulcl rcquirc precise calibration and possibly lead to noisy data.

The ohsi.:rvi.:d platenu value of 01-1* contribution in oil voriations might indicatc thai this spectral emission originati.:s frum the

burnl zones with a similar composition. This would be in agreemml with the OH~ and C02* mcchanisms, which prcdict

persistent chemiluminescenec from the hot burnt zones with the highi.:st intensity at around stoichiomclric condîtions and a rapid drop of intcnsity at fücl richer/leaner conditions. The obscrved temporal evolution of thi.: chi.:miluminesccnce emission is

cxpl.iined by this assumplion as well, funhcr confirmi.:d by plouing the temporal cvolution or the Abel-invcrtcd OH*

chemillllnim;scencc i111iige~ for a variation or

~

c

tt

4 ( Figure 9). Abel invi.:rsion trnnsfonns the line-of-sight ima~i.:s 10 cross

-section imugcs bnsed on the axial symmclry assumption. These images werc obtnined by ensemble-avcraging ovcr ten

cxperiment.il repetitions followi.:d by fi Fourier-transform bascd inverse Abel-transformation mcthod proposi.:d by Pretz.lei· [27].

Such transformation minimizcs lhi.: noise common with the simpli.:r Ahcl·inversion methods.

The diesel casi.: (Fîgure 9, 11ppe1· panels) alter ignition shows .i t)'pical diesel liftcd-tlan,e two-branch 01-1* distribution 1281.

The conditions at the spray axis arc bclicvi.:d tu bi.: too rich tn produce chemilumincscence. Some time after the end or injection (EOI) the two !lame branches mcrgc (inrnge at 0.9 ms) as the EOI i.:ntrninment wave proceeds to rcduci.: the pilot-füel cum;cntration upstream of thi.: spray Lip. At the spray tip, the mixture is still füel-rich, thus resulting in a lower emission from the core of the sprny. /\s the EOI entrainmc11t fürther procccds. the 1\.lel-mixture continuously kans out li.:ading to .i recession

of the

OJ-1

*'

from upstrcam Lo the tip of the spray. This rcsults in a horseshoe shape of the cmission dui.: to the spray-tip vortex.

(12)

2 3 15 0.6 0.9 1.0 1.1 1.3 1.6 2.0 2.4 2.6 ms 4 25 5 35 6 45 7 8 9 10 1)

12 u

14

15

16

1 7 ] 8 19

20

L1 22 ?.3 24 2!:\

26

27 28 29

JO

31.

32

3J 34 3:i 36

37

38 39 4 () 41

4

2

tl3 11 45

4

6

4 "/ 48 tl 9 50 51

52

:i3

5

4

55

56

~>7 :i8 !:>9 60 fil 62 63 64 55 65 .... ~~---...--,---,---_.---t--..;.--;---:=---ii--·.:, 25 35 45 55 6b 'Ï'cH~ = .33

Î

25 ;35 'Q 45 ~ 55 ~ 65 ?o·M : .66 ·10 0 10 Radial disl. [mm] 300

i'

200 .,,; ji' 100 ~ 0

~]

__

Time aSOI (ms]

Fig111·r 'J: 'l'cmporal .:, ol11tir1n of l'hc ,\bcl·itn'tl'WI 011 ·' i1ll11gcs Îf11' ,·:u·i111iun of $rn ,, Conditiu11~; T~rn = 850 K. ET= .JllO 11~-The h(l(l.(1111 plot ~ho"~ lht' t'VOltlli(lll 11f lht' IIR!l (full lim·), und f1cld·of·vit11 inw~nllcll Oii~ inrn)(ill;! ,ig11al (du~lm.ll. Cirrlcs indkale lhc

timc in~t.1111~ uf lht itt1<11,(c p1111d~ alJtJ\ l\,

The dual-füel cuses (Figure 9, miùdlt.: anù bottom rôws) show a diffürcmt cvolution of the OH* emission. Due to the 111'emixed

mcthane. a longer pcrsistence of the 011* emission in the wukc or the spray is observcd. Similarly. it also takes longer for the

spray tip tn lean-out. Overall, this res11lts in ltlÎ'ge spray-plun1e volumes with closc-to stoichio1m:tric conditions, pcrsisting long

aller the cnd-ol~injcction. This also explains the decoupled rlRR rate and 01-1* emissîon (Figure 9. bottom plot). Prcmixcù flnmi.: propugation is barely visible on 01-1* images due to the lean $("114 inwstigated in this study. Nevertheless. the overall hcat· rclcasc incrcascs crmsidcrably duc to the increased pilot-füel q11antity.

The findings imply fi loo low coupling betwci.:n the OH* and 1-IRR frir any prnctical application. ln othcr words, under dual·

füel engines conditions, the natural lumi11osity canuot be tnken as a measure for the local HRR or HRR location as commonly

ùone in HCCI/PCCI combustion f2, 1

Il

-

Fu11hennore, 1he vast range of different signnl i111i:nsi1ic~ originating from diffcrcnt arcas or the combustion chamhcr requires a high camera dynamic range to resolve both, the premi>iecl and pilot-fuel regions. Undcr somc conditions, light rcllcctions frorn luminous prcmixed regions might exhihit higher intensities than the premixed

name propagation brightncss.

5 Conclusions

The spectral rootprint or diesel and dual-l'ucl combustion was invcstigatcd I D-resolvcd along the injector axis using. a spectrogrnph couplcd to an intensilled high-spei:cl camera. ln sumnrnry, the study rcvcalcù lhrcc primary chcmiluminesccnce sources in hoth, diesel and dual-fuel combustîon: OH•. CHt and.broadband (138) chcmilumincsccncc. ln the bandpass arounù 3 l O 11111 used for OH* imaging. about 60'X, of the luminosîty originales from OH•. 138 chcmill1rnincsccni.:c (mtributcd mustly lo Cl-10*. Cl-120*. and C02*) was iùcntilicd as the source of the remaining signal. /\t laie stages or co111hustîon. the 813 lurnino:;ity Jüllow:; similar lcmpornl and spatial cvolution as OH*. Nevcrthelcss, at carly stages during the pilot-fuel burning

(13)

1 2 3 4 5 6 1 8 9 10 11 12 13 14 15 1 6 1 7 ] 8 19 20 21 22 23 ?. 4 25 26 27 28 29 30

31

32 33 34 3:i 36

the tcmpornl evolutions or 01-1* and BB chcmilumincsr.:cnce werc difterent. ln this stage, evcn the cmission ,li 31 O nm is dorninated by the BB luminosity. Both crnissions onset sinrnlt,meously at ignition and thcrcfon.: do not interfère with the ignition delay detections. The cmission orthi;; CH* rndical is

a good

tracer fr>r the combustion

of

the pîlot-ll1el. 1 lowevcr. due to the strong BB luminosily intcrli.:rencc;:, it

is

challcnging to detect CH* using bandpa$S-lillered Îlntlging - an acquisition using a spectrogrnph mighl be rc;:quired.

8asccl on the analysis of experimental data the lollowing i.;onclusions and implications for the cvalualion or mitural-luminosity images can be stated:

• OH*. CH* and HB luminosity show simultaneous onsel at ignition and therefore equally qualify as a lrnccr for the deterniination or the ignition dclay (ID). lntegrntccl over the spectral domain the 88 luminosity is the most intense. rurthermore. nt ignition. th(' 88 luminosily shows the steepcst ri~e and can he thcrefrm: recornmended for the future investigations of ID.

• The risc nitc of luminosity

at

ignition decreases considerably wilh highcr

4icl!4

and with decreasing charge oxygcn content. Large range ofOH*/BIY intensity fr()lll ignition to late combustion stages poses a subst:mtial prnhlem duc lo the limitcd detcctor dynumic range and sensitivity. Il is therefore impcrntivc 10 use the lowest possible thrcshold whcn dctccling ID based on Ol l*/IH3'" images in ordcr 10 avoid ovcr,prediction. If the sensitivity of the acquisition system

is insufficient.

th

e

ignition delay might be considerably over.predicted.

• During the pilot-fuel hurning, the light 1:mission around 310 11111 is dominated by the 88 chemiluminescence. Therefore caution is nccdcd whcn comp,1rîng the 01 l* temporal evolution wilh simulations considering OH*

mechaoism only.

• Commonly uscd CH* detection using 10 11111 FWHM handpass li lier ccntered

m

430 nm foils to distinguish the wcuk CH* emission from the hroadhand luminosity. Both. in the diesel and dual-fuel cases, Jess than 13% of the 111tered light originates from CH*. Spectroscopie und/or two-wavelength detection would be nccdcd to separote the CH*' from the BB luminosity.

• Burned regions wilh dose-to stoichiometric conditions dominatc the llnme emission at late stages of combustion. Especially al lenn <~rn4 considerably more than an ordcr or magnitude lbwer brightness is expcctcd from the premixed !lame rcgions. This explains the ohservcd temporal evolution or OH* being decoupled from the HRR and poses

a

challenge for the detection of prcmixcd burning zones.

37

38

Acknowledgements

3 9

Financinl support from

CCEM

(prnject #803 "SchcDual'') and the Swiss Federal Ofncc of' Encrgy (gnuil Sl/501 12:l-O 1)

is

4 0 grntefol ly admowledged.

41 42 43 44 45 46 47 48

49

50 51 52 53 54 5~ 56 57 58 59

60

61 62 63 64 65

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