RD-R149 360 EXCIPLEX SYSTEMS FOR VAPOR/LIQUID VISUALIZATION OF FUEL 1/1SPRAVS(U) TEXAS UNIV AT DALLAS RICHARDSONL A MELTON ET AL. 1984 UTD-CHEM-8401 AFO5R-TR-94-ii78

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FIE-= GRovp i S.S SOR fuel sprays, fluorescence diagnosticsl, liquid/vapor3

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Under Grant 83-0307 sponsored by the Air Force Office o: Scientific Research',TheUnierstyof Texas at Dallas has conducted research On potential new organic exciplex

systems for use in fluorescent visualization of the vapor and liquid portions of evaporatin:fuel sprays. These exciplex systerns represent a novel and potential!%, powerful diagnostictoo! 'or the analysis of the vaporization processes in gas turbine fuel sprays, and as suchpossess considerable relevance to Air Force propulsion needs. This Final report describesthe five best exciplex systems developed to date, the methods used to screen nearlyv onehundred of potential sys'tems, and the characterization of the naphthalene/tetramethyl-p--e-.e . dla:ne svste-z. -_is last s-,stem has already been _,sed in demonstration exner:-

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Exciplex systems for Vapor/Liquid Visualization pw

TABLE OF CONTENTS

page

m SUMMARY... ....... 2

I NTRODUCT ION ...... 3

EXCIPLEX PHOTOPHYSICS AND SEARCH STRATEGIES ...... 5

EXPERIMENTAL APPARATUS AND PROCEDURES ..... 10

RESULTS AND DISCUSSION ..... 12

AREAS OF FUTURE WORK ....... 19

REFERENCES ............. 20

APPENDIX (SPECTRA) ......... 22

PUBLICATIONS ........ 23

PERSONNEL ASSOCIATED WITH GRANT.... 24

INTERACTIONS (COUPLING) ..... 25

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SUMIMARY

Under grant 83-0307, sponsored by the Air Force Office of ScientificResearch, the University of Texas at Dal lIas has conducted research on

* potential new organic exciplex systems for use in fluorescent visualizationof the vapor and l iquid portions of evaporating fuel sprays. These exci-plex systems represent a novel and potential ly powerful diagnostic tool forthe analysis of the vaporization processes in gas turbine fuel sprays, andas such, possess considerable relevance to Air Force propulsion needs. Thisfinal report describes the five best exciplex systems developed to date,the methods used to screen nearly one hundred of potential systems, and thecharacterization of the naphthalIene/tetramethylI-p-phenylIene diamine system.This last system has already been used in demonstration experiments tophotograph the separate l iquid and vapor patterns in an evaporating fuelspray.

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INTRODUCTION

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Gas turbinr and diesel engine combustions are extremely difficult prob-lems, from both a theoretical and experimental standpoint, because bothinvolve chemical ly reacting, turbulent flows with large mass and tempera-ture gradients. because the fuel is introduced as a spray, initially theflow is heterogeneous, and the problem is, therefore, even more complex.Literal ly hundreds of studies, as detailed in recent reviews(I-6), havebeen made of the behaviour of droplet distributions in these sprays, butthere have been very few studies in which both the liquid and vapor distri-butions have been determined.

The separate measurement of liquid and vapor concentrations by opticaltechniques, which can be virtually non-intrusive, has been difficultbecause, for fuel-I ike molecules, most spectral characteristics are vir-tual ly the same in the liquid and vapor phase. Johnston investigatedspontaneous Raman scattering but concluded that, in the C-H stretchingregion near 3000 cm- 1, there were insufficient differences between thespectra of the vapor and liquid to al low their separate measurement(7,8).Other optical methods have exploited the index of refraction differencesbetween the I iquid droplets and the less dense ambient vapor/air surround-ing the droplets; these studies have used direct photography(9,10), laserlight scattering(11), or two wavelength laser absorption/scattering(12,13).An alternate strategy, used in this work, is to develop fuel additiveswhich interrogate the spray, and reveal its characteristics in theirfluorescence.

. In previous work, it has been shown that certain organic exciplex sys-tems, when added to fuel-like liquids and excited with ultraviolet light,result in a blue-green fluorescence from the liquid phase and a purplefluorescence from the vapor phase(14). Thus, these systems possess the

U unique characteristic that their emissions distinguish the liquid and vaporphases from one another, and hence offer the possibi I ity of seeing thevapor and liquid portions of the evaporating fuel spray separately.

One exciplex system, based on tetramethyl-p-phenylene diamine (M) andnaphthalene (N), has already been used to demonstrate the diagnostic poten-

tial of the exciplex visualization idea. These molecules were added tohexadecane, and the fuel mixture was sprayed through a simplex nozzle intohot nitrogen. The spray was irradiated with a sheet of laser light (266nm), and the resulting fluorescence was photographed through fi Iters.Photographs were obtained which showed the liquid portion of the spraywithout the vapor and vice versa(15,16).

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This initial exciplex system is, however, not optimal. The exciplexfluorescence decreases at high temperatures because the exciplex binding

Kf energy is not sufficiently high, the emission bands of the monomer M and.the exciplex E are partial ly overlapped, the vapor phase emission from Mis subject to quenching by molecular oxygen, and the two exciplex compo-nents have significantly different volatil ities. Under Air Force sponsor-ship, grant 83-0307, the University of Texas at Dal las has examined manypotential organic exciplex systems in a search for better diagnostic addi-tives. Five systems were found to be promising, and their spectra areincluded in this report.

The next section summarizes the relevant exciplex photophysics as wel Ias the basic screening criteria and methods. Subsequent to that, the exper-imental apparatus and techniques are described. The results of tne studiesare then presented and are discussed in light of the criteria for usefuldiagnostic systems. Suggestions for future work are outlined and, final ly,selected spectra obtained during this investigation are attached as anappendix.

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EXCIPLEX PHOTOPHYSICS AND SEARCH STRATEGIES

An exciplex (excited state complex) is formed when an excited state 0molecule reacts with itself or another molecule to form a complex, which is. 0bound only in the excited state. The binding of the exciplex may be quitesignificant, up to 100 kJ/mole, i.e., 30% of a normal chemical single bond.The ground state is, however, repulsive and the two reactant molecules haveno special Dinding once the excited state has decayed. There is a richarea of organic photochemistry associated with exciplexes as reactionintermediates; however, for the purpose of developing fuel spray diagnos-tics, we are concerned only with those exciplexes which emit strongfluorescence(17,18).

In Fig. 1, the relevant photophysics for such an exciplex visual izationsystem is shown. A fluorescent organic molecule, M, can absorb light toform the excited species M*, which may then fluoresce and return to theground sta e or may react with another ground state molecule N to form theexciplex E . The exciplex is formed in a reversible chemical equilibriumwith M and may either dissociate back to M and N, fluoresce, or undergofurtner chemical reaction. Only the first two paths are of interest heresince fast chemical reaction will disqualify an exciplex system as a diag- Snostic for fuel sprays.

Two concepts are important in understanding how such a fluorescentexciplex system can be used to distinguish the vapor an liquid portions ofan evaporating fuel spray. First, since the exciplex E is bound withrespect to separated M and N, and ground state M and N have virtual ly no Sinteraction, the exciplex fluorescence is necessarily at lower energy thanthat of M*; indeed this spectral shift of the fluorescence is approximately 1equal to the binding energy of the exciplex and may be as much as 10000 cm- 1.

Second, the concentration of N may be adjusted so that in the liquidphase the M + N = E* equi I ibrium Iies far toward the right, i.e., theratio of the population of E* to that of M* is roughly 100. In the vapor Sphase, the densities are much less and the sol vent stabilization of therelatively polar exciplex is much less, hence, the equilibrium lies farToward the left, i.e., the ratio of the M population to that of E becomesE*roughly 100(19). In this situation, E emission, at its distinct positionin the spectrum, is only seen if liquid is present; M* emission, at itsseparate spectral position, is only seen if vapor is present. This ideal-ized situation is rarely achieved in full, but nonetheless, a system basedon tetramethyl-p-phenylene diamine (M) and naphthalene (N) has been used tophotograph the liquid and vapor portions of a hol low cone fuelspray(15,16).

The exciplex systems offer several advantages over the conventional Isystems based on the analysis of droplets and subsequent calculation ofvapor concentrations. The vapor is measured directly, and, since fluores-cence is inherently linear in the density of fluorescers, the detectedfluorescent intensity should be directly proportional to the mass concen-Tration of the liquid or vapor portion of the spray, as appropriate. Theexciplex systems are wel I adapted to use in modern laser visualizationtechniques since a slice of the fuel spray may be irradiated with a sheetof laser light, and the resulting fluorescence may be imaged onto an array

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detector. The f uorescent lifetimes of the emitting species are typically -.less than 100 nanoseconds(20), and hence independent measurements on afuel spray can be made at rates greater than one megahertz -- limited notby the photophysics of the detection system but rather by the availablerepetition rates of laser excitation sources and the capability of high 0speed cameras and/or digital imaging systems.

The exciplex systems are, however, not without tneir drawbacks. Tneequilibrium between M and E is temperature dependent, and systems must bedeveloped for which the binding of E relative to M and N is sufficientlystrong that the exciplex emission does not vary strongly with temperatureduring the evaporation of the fuel spray. Excited organic molecules suchas M are very sensitive to quenjhing by molecular oxygen, and currentmeasurements indicate that any M wi I I be quenched by oxygen on every gas -

kinetic col I ision. If this occurs, then the vapor intensity measurementsfor a fuel spray evaporating in air wi I I be confounded with the penetrationof air into the spray, and the results w I Il not be readi ly interpretable asvapor concentrations. Finally, it should be noted that the additives M andN are not the fuel itself, and hence the emissions from M* and E* serve asmarkers for the fuel vapor and liquid fractions, respectively, but are notthe major fuel component themsel ves. It wi I I be necessary to establ ishthat these emissions are indeed good linear markers in order to interpretthe intensity measurements quantitatively.

The initially successful exciplex system was compounded with naphatha-lene (N) and tetramethyl-p-phenylene diamine (M) with the balance beinghexadecane. In light of the promise shown by the demonstration experimentsperformed with this system and in light of the concerns outlined in theprevious paragraph, a search was undertaken to find alternate exciplexsystems which would relieve some of the potential drawbacks. In particularit was desired to create systems which would satisfy the fol lowingcriteria:

i) There snould be a large spectral shift between the M and Eemissions so that the spectral overlap of these features would be mini-mized. It should be remembered that the absorption and emission bands oforganic molecules are inherently broad and may easi ly be 5-Z,000 cm- I wide,so large spectral shifts are required if the emissions of M and E are tobe separated.

ii) The exciplex E* should be strongly bound with respect to theseparated M* and N molecules. If the binding is too weak, then the exci-plex equl I ibrium wi I I shift toward the separated reactants as thetemperature is raised. The result is that the fraction of the emissionresulting from E* drops and that resulting from M* rises. One would then 5infer that less liquid and more vapor was present. Ideal ly, the bindingenergy should be sufficiently high that E remains the dominant emittingspecies from the liquid up to temperatures at which the fuel evaporates,i.e., its boi I ing point at ambient pressure.

iii) The quenching of the vapor phase fluorescence of M* by molecular 0oxygen should be minimized. Al I present experimental measurements of thefluorescence quenching of organic molecules indicate that molecular nitro-yen and al iphatic nydrocarbons are virtual ly inert(21,22) and tnat

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molecular oxygen is indiscriminately efficient(23,24). Thus, rather thansearching for candidates for M* which might have lower probabilities ofquenching in a given bimolecular encounter, we have chosen to examinecandidates which have short radiative lifetimes since this parameter deter-mines the number of col I isions an excited state molecule undergoes at agiven pressure.

iv) In order to make the exciplex system a good marker for the vaporand liquid, one must insure that the ratio of marker to fuel remainsconstant during the evaporation of the spray droplets. If the vapor pres-sures of the several components are equal, then the marker and fuelspecies wi I I rigorously coevaporate. We have chosen to match the boilingpoints of the three species at atmospheric pressure as closely as possiblein order to minimize potential differential evaporation. Recent work byWang et al supports this matching of boiling points as an effective criter-ion for coevaporation(25).

v) The exciplex components should be commercially available if pos-sible. Ideal ly these investigations will result in exciplex systems whichwi I I be used by a wide variety of non-chemists and, were specialty synthe-ses required, the use of the diagnostic systems would be retarded.

Although each proposed exciplex system must be tested individual ly to

determine its suitability, individual systems can be screened for theconstraints i) and i i) fairly easi ly. Rehm and Wel ler have developed aseries of correlations which enable one to calculate the energy at whichthe maximum of the exciplex fluorescence occurs and the binding energy of[ the exciplex, if one knows the singlet excitation energy of M (availablefrom absorption or emission spectra) and the oxidation/reduction potentialsof the donor/acceptor components of the complex as measured in acetonitri le(19). These parameters are available in tabulations, and thus a widevariety of potential exciplex systems can be rapidly screened.

hvcmax [E(D/D + ) - E(A-/A)] -. 15eV

AH = E o - [E(D/D + ) - E(A-/A)] - .13eV

where 'H is tne binding energy of the exciplex, 6E is the singlet excita-

tion energy, and E(A-/A) and E(D/D +) are the oxidation/reduction poten-tials. Tne equi librium constant for exciplex formation is given by

[E*J L?K - = exp [-(AH - TLS)/RT]

[M [NJ

where AS is found to be approximately -85 J/K-mole over a wide range ofexciplexes(20). Consequently, use of the fluorescence spectra compiled byBerlman(26), tables of oxidation and reduction potentials(19,27) and tab esof boiling points(28) al lows one to assess the probable usefulness of awide variety of systems prior to testing.

8

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The relative fluorescence yields of the exciplex and the monomer aregi ven by

e Kte [E*] Kfe==K fN]

m Kfm [M Kfm K[N]

The radiative rate constant Kfe for exciplexes is typical ly in the rangeof 106-107 sec-1 (20) and Kfm can be obtained from Berlman's compilation.

I Using the techniques outlined here we have examined nearly one hundred 6potential exciplex systems, of which 31 yield fluorescence (a propertywhich is not determined by the correlations given above and which must beseparately tested). These systems were then tested against the criteriagiven above and fluorescence spectra as a function of temperature weremeasured. Of the 31 potential systems, five showed strong exciplex fluo-rescence which dimbnished bt less than a factor of two as the temperaturewas raised from 23 C to 250 C. These systems are presented in detail inthe results section.

0

9

EXPERIMENTAL APPARATUS AND PROCEDURES

The main body of experimental work in this project was the measurement

of fluorescence spectra for test mixtures. Tne spectra were run on one ofThree devices: a) a Farrand Mark I spectrofluorimeter, which has a contin-uously tunable excitation source; b) a special ly assembled fluorimeterconsisting of a XeCl excimer laser (308 nm), high temperature fluorescencecel I, monochromator, pnototube, and associated electronics; or c) a spec-ial ly assembled fluorimeter consisting of a Nd:YAG laser set to produce thefourth harmonic at 266 nm, high Temperature-high pressure fluorescencecell, spectrometer, phototube, and boxcar integrator. The Mark 1 fluori-meter was useful for survey scans, particularly at room temperature, butbecause of its relatively cramped sample compartment and relatively lowexcitation intensity, it was rarely used for high temperature measurements.The two special ly assembled fluorimeters operated with conventional rightangle and front face fluorescence geometries and are described briefly in

the fol lowing paragraphs.

The excimer laser excited fluorimeter was used at UT-Dal las. The exci-mer laser source (Lumonics 861S) can produce 80 md/pulse at 10-20 hertz,altnough the typical excitation energies used were only 1-5 mJ/pulse. Thebeam is focussed onto a high temperature fluorescence cel I in such afashion that, when the 10 x 20 mm quartz cuvet is oriented with the 10 mmwide surfaces perpendicular to the laser beam, the beam does not hit the

side walI s of the cuvet. If the beam is al owed to fal I on these surfaces,unacceptable amounts of scattered light result.

Tne 10 x 20 mm cuvet is held in an aluminum support so that the assemblycan be raised to bring the liquid in front of the laser beam or lowered soThat The vapor is in front of the laser beam. The cel I holder assemblywi I I also pivot so that the vapor spectra, in which spurious emission fromliquid on the wal Is is a problem, can be taken in a conventional right

I angle fluorescence geometry and liquid spectra, which must be taken off thefront surface since the solutions are optically thick, can be taken withthe cel I aligned at approximately 45 degrees to the incident laser beam.Tne cuveT holder assembly can also be translated perpendicular to the laserbeam so that the irradiated spot in the front surface experiments wi I I I ieon tne optical axis of the monochromator. The temperature in an experi-ment is monitored with an iron-constantan thermocouple inserted in thealuminum cuveT holder approximately I mm behind the cuvet. The entire cellassemoly is suspended in an insulated box, with quartz entrance and exit

wiqdows, and is heated by air which has passed through an electric oven.In some cases, where reaction with or fluorescence quenching by oxygen isthought to oe a problem, The air flow is replaced by nitrogen.

In a typical preparation, a few mi I Ii liters of solution are mixed in thecel l, with the solvent and sol ids being weighed on an analytical balanceanJ the minor liquid additives being added with a cal ibrated syringe. In

" al I cases, the solution is purged with nitrogen for at least ten minutes toremove dissolved oxygen and is then stoppered with a teflon plug. Thisprocedure is probably adequate during tne .)eatup phase of an experimentsince the increasing vapor pressure inside the cuvet acts to drive air out;

S""however, during tne cooldown phase, the pressure in the cuvet becomesSubatmospheric and air is drawn into the cuvet. The result is that most

10

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test solutions, which are initially colorless, end up with a color anywherefrom amber to deep brown. The thermal oxidation chemistry described hereis a minor interference in taking the spectra of these solutions, but would

S[ have no effect on the use of the solutions in fuel spray diagnostics sinceit is orders of magnitude slower than the 50-100 mi I I isecond lifetimes of

droplets in typical fuel sprays.

The fluorescence from the cel I was focussed through a quartz lens ontothe entrance slit of a quarter-meter monochromator (Jarel I-Ash) equippedwith an RCA 1P28 phototube. The output was taken to an electrometer andthence to a strip chart or X-Y recorder.

In the measurements on the 2.5% naphtnalene/1.0% tetramethyl-p-phenylenediamine exciplex system, (performed at United Technologies Research Centerwith James F. Verdieck), the fourth harmonic of a Nd:YAG laser at 266 nmwas used to excite fluorescence. This fluorescence was then focussedthrough a quartz lens into a Spex 1702 monochromator equipped with an EMI

9829QA phototube, and the signal was taken to a PAR 162/164 boxcar, whichcould be operated in the scanning mode for lifetime measurements or with afixed delay for spectral measurements. Spectra were taken at temperatureswel I above the boiling points of the individual components. The cuvet wasplaced inside a pressure cel I and pressurized to 100 psig with nitrogen.The cell had a 7.5 cm diameter by 20 cm high cylindrical cavity and hadfour quartz windows located at its midsection. The cuvet was placed in anelectrically heated 5 cm diameter brass cylinder, which was insulated fromthe pressure cell with zirconia felt. Since the cuvet was not strongenough to stand a pressure differential of 100 psi, iI could not be sealed;

1. instead, a 50 cm piece of thin wal one-eighth inch diameter stainless steeltubing was wound in a conical spiral and fitted into the teflon stopper inthe top of the cuvet. This tubing al lowed the pressure inside and outside . ,of tne cuvet to equal ize and at the same time served as a reflux condenser .to prevent the solution from getting out of the cuvet and onto the pressurecel I windows.r -

The spectra were digitized and replotted in a standard format by compu-ter. These normalized spectra are collected in the Appendix.

In the exciplex systems described in the later sections, the followingcommercially available chemicals were used (abbreviation is given in paren-tneses): 1) Aldrich; 1-methoxynaphthalene (I-MeON) (98+%), 1-methylnaphtna-lene (1-MeN) (97%), N,N,N',Nf-tetramethyl-p-phenylene diamine (TMPD) (98%,subl imed oefore use), 1-cyanonaphthalene (1-CNN) (tech., sub limed beforeuse), naphthalene (N) (99+%), fumaronitri le (FN) (98+%), trihexyl amine(THA) (98%), 1,2,4-Trimethyl benzene (124TMB) (99+%), hexadecane (C) (99%);2) Pfaltz and Bauer; heptyl butyrate (purity not stated), N,N-dibutylani line (DBA) (purity not stated). Except as noted, the chemicals wereused without further purification.

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RESULTS AND DISCUSSION

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The results resented here fal I into two broad catagories: 1) the searchfor new exciplex systems which would not have the I imitations of the naph-thalene/TMPD system; and 2) the attempts to thoroughly characterize thenaphthalene/TMPD system.

A. New Exciplex Systems

As mentioned previously, through use of the correlations developed byRehm and Wel Ier, and tables of oxidation-reduction potentials, boilingpoints, fluorescence spectra and emission properties, one can choose poten-tial exciplex systems which will have approximately the right spectralseparation of monomer and exciplex emissions and approximately the rightbinding energy. Each system must be individually tested however to deter-mine whether it emits strongly and whether that emission persists at hightemperatures. Nearly one hundred potential exciplex systems were tested

for emission at room temperature; thirty-one systems yielded emitting ,exciplexes, and five of those systems have exciplex emission at 2500C whichis no less than a factor of two weaker than their room temperature emis-sions. These five best systems are summarized in Table I and are discussedindividual lybelow.

1) Most Promising Exciplex Systems -

Table I summarizes the characteristics of the five most promising exci-plex systems found in the search. The spectra for these systems are col-lected in the Appendix, and each system is discussed briefly below. Ineach case, the designations M and N are used to indicate whether the -

molecule is the primary vapor phase emitter and exciplex former (M) or theground state reactant (N).

a)15% Naphtnalene(N)/0.5% Tetramethyl-p-phenylene diamine (M)(Spectra A1-6,19-21)

This system, which has already been used in demonstration experiments, 5forms a strongly emitting exciplex, whose maximum fluorescence fal Is near516 nm at room temperature. The major vapor phase emitter, TMPD, has ashort radiative lifetime (4.3 nsec), and hence is only modestly sensitiveto quenching by molecular oxygen (fluorescence reduced by roughly a factorof six in air at atmospheric pressure). The main drawback of this systemis that naphthalene Doi Is at 2180C and TMPD boils at 260°C, hence, the

components may differential ly distil I out of an evaporating droplet, andthus distort tne measurements. In addition, the small spectral separationand smaI I binding energy means that the system is only useful up to about

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b)10% 1-Methylnaphtnalene(N)/1% TMPD(M) (Spectra A4-9,22-24)

This system, in which I-Methylnaphthalene is used to raise the boilingpoint of the N component (BP=2400 C), produces a slightly weaker exciplex

fluorescence, but otherwise is very similar to the naphthalene/TMPD sytem.

c) 10% I-Methoxynaphthalene(N)/1.5% TMPD(M) (Spectra A4-6,10-12,25-

27)

This system, which is nearly coevaporative, displays strong fluores-

cence and the best thermal stability of any of the exciplex systems -

tested. In al I other respects, it is also very simi lar to the naphtha- 9

Iene/TMPD system.

d) 0.4% 1-Cyanonaphthalene (M,N)/ O% N,N-Dibutylani line (M,N)(Spectra A13-18,28-30)

In this system, both monomers emit at approximately 350 nm, and hencethe vapor phase emission cannot be unambiguously assigned to either spe-

cies. The spectral separation and thermal stability are typical ofthe other systems. A closer match of the boil ing points could easily be

made by substituting appropriate alkyl groups on the ani line.I

e) 10% Trihexylamine(N)/1% 1-Cyanonaphthalene (M) (Spectra A13-15,31-33, no spectra were obtained for THA since it does not absorb at 308 nm)

This system possesses excel lent separation between the monomer andexciplex fluorescence maxima, has medium strength exciplex fluorescence and

[ has typical thermal stability. It is, however, more sensitive to oxygen

quenching than the TMPD systems. As with the previous system, a vaporphase exciplex emission appears and wi I I complicate the use of this system

for diagnostic work. This is the only system for which the calculated

fluorescence maximum differs significantly from the observed maximum; it is

liKely that Rehm and Wet ler's correlations apply only to aromatic systemsand not to the aliphatic amines. p

Optimal concentrations nave not been determ;ned for systems b), c), ande); it is therefore possiole that they can sti I I be improved modestly.

In summary, we now have a set of potential exciplex diagnostic systems,each of which has strong and weak points. It is likely that, rather thanhaving one outstanding and universal system, it wi I I De necessary to matchthe exciplex system characteristics to the demands of a particular problem.

2. Interfering Photochemistry: Examples of Two Promising Systems Which* Failed

a) 1,2,4-TrimethylDenzene (M)/Fumaronitrile(N)

On the basis of calculations based on tabulated parameters and the Rehm

and Wel ler correlations, this system ,ds thought to be an excel lent candi-

, date for a visualization system. It was estimated to have a large spectralseparation, and hence a large binding energy, as wel I as being a relatively

low-boiling (175 C), nearly coevaporative system. The system does show alarge spectral separation: the 124 TMB fluorescence peaks at 290 nm and the

15 p

exciplex emission peaks at 520 nm. The fumaronitrile itself is not fluo-rescent, and hence there is no fluorescence interference from that sourcein the monomer fluorescence region. The fumaronitri le is not very solublein nonpolar solvents such as hexadecane, and hence a mixed solvent such ashexadecane/heptyl butyrate is used to dissolve the fumaronitri le. Thismore polar solvent stabilizes the relatively polar exciplex and results inan additional red-shift of the exciplex fluorescence by 10-20 nm. However,as the temperature is raised, both the monomer and exciplex fluorescence

0decrease rapidly, and above 125 C there is virtual ly no emission fromeither species. The electronical ly-excited alkyl benzenes undergo athermal ly-activated internal rearrangement to form a variety of Dewar

benzene-l ike molecules(30), and hence at higher temperatures the monomerexcited state disappears before it can fluoresce or react to form an exci-plex. The alkyl benzene rearrangement is a unimolecular reaction, andhence, its rate cannot be manipulated by changing the concentration of

species in the solution. The system had to be abandoned as a candidate for

a vapor/I iquid visual ization diagnostic.

b) Cyanonaphtha I ene/TMPD

This potential exciplex system also looks very good on the basis of theRehm and Wel ler correlations. It should show a spectral shift of some 180nm between the maxima of the monomer and therefore, it should be possible p

to develop mixtures for which the M + N =E eqi I ibrium would e dominatedby E* even for temperatures as high as the boi I ing points (265 C) of thisnearly coevaporative system. However, when the components are mixed atroom temperature, tne solution immediately turns yellow: the selection of

a highly effective electron donor and a highly effective electron acceptor,[ chosen in order to develop a system with large binding energy and spectral

separation, resulted in the formation of a ground state charge transfer* complex which has its own absorption (and possibly fluorescence), and hence ..-

this system was not investigated further.

B. Characterization of the Naphthalene/TMPD Exciplex System

This exciplex system was used in the initial demonstration of the exci-plex vapor/I iquid visual ization technique, and with different concentra-tions, in the demonstration that the exciplex techniques can, even at thisstage of development, be used to photograph a two-dimensional slice ofan evaporating fuel spray. In order to develop a standard for the charac-terization of new systems, and in order to understand the underlying photo-physics of these demonstration experiments, a closer study of the naphtha-lene/TMPD system was made.

Using 266 nmoexcitation, vapor spectra (A42-45) were taken at tempera-tures Sp to 225 C and liquid spectra (A34-41) were taken at temperatures upto 350 C. The Key element to be noted in these spectra is that the spec-tral shapes of the vapor and liquid spectra change as a function of tem-perature; as a result, broad band fi Iters should not be used to separatethe vapor and I iquid emissions since the intensity measured wi I I representa confounding of the density of fluorescers with the changing spectral

shape.

16

The vapor spectra change shape as the temperature rises, primarilybecause they are a sum of naphthalene and oTMPD emissions. Naphthalene (BP2180C) is more volati le than TMPD (BP 265 C) and its spectrum dominates thelow temperature spectrum; TMPD is the stronger emitter and as the tempera-ture rises so that more TMPD is evaporated, its emission eventual lydominates the high temperature emission. In addition, however, the firstexcited singlet state of TMPD lies below that of naphthalene and hencethere is efficient energy transfer from excited napththalene to TMPD; as

* the vapor density rises, these energy tranfer processes result in theeventual disappearance of naphthalene vapor fluorescence in favor of TMPDfluorescence.

It should also be noted that, in earlier work, the vapor emission wasshown to have a long tai I in the visible and this was suggested to be vaporphase exciplex emission(14). These spectra do not confirm that observa-tion; it was liKely due to liquid phase emission. This latter result isthe more desirable, for it means that the spectral overlap between thevapor and liquid emission spectra for the naphthalene/TMPD system is signi-ficantly less than it was thought to be.

In the I iquid phase spectrum the changes are more complicated. Thesolutions witn 2.5% naphthalene, 1.0% TMPD (w/w) show low temperaturephenomena not seen in earlier measurements on solutions with 10% and 15%

naphthalene concentrations(14). At low temperatures there is a broad bandpeaking at about 500 nm due to the exciplex, which is strongly favoredthermodynamically at these temperatures. However, there are also signifi-cant minor emissions at 340 nm and 380 nm from excited naphthalene andTMPD; these emissions occur because the solvent, at these low temperatures,

[ Iis too viscous for the excited species to diffuse to a reaction partnerduring its fluorescence lifetime(31). Thus the Kinetics of exciplex forma-.Tion, as wel as the tnermodynamics become important. As the Temperature

* rises, the viscosity of the hexadecane drops and these monomer emissionsbecome less important relative to the exciplex emission. In addition, asthe temperature rises, the exciplex emission itself shifts toward higher

U energies. This effect, which is not ful ly understood, has been observed inother exciplex systems(32). Finally, the M* + N = E* equi librium is tem-perature dependent, and so as the temperature rises, the fraction of theexcited state population in the exciplex decreases and the ratio of exci-plex emission to TMPD emission decreases. It is this shift in the exciplexequilibrium which fundamentally determines the upper temperature limit fora particular exciplex system. For this system, the highest useful tempera-ture is approximately 250 C.

The napnthalene/TMPDDolutions in these experiments were taken to tem-peratures as high as 400 C, and when returned to room temperature showedonly barely detectable discoloration. Hence, the thermal stability of thesolutions, in the absence of molecular oxygen, should not be a problem.

In addition to these spectra, the fluorescence lifetime of the naphtha-lene/TMPD exciplex at room temperature for a 2.5% naphthalene, 1.0% TMPD(w/w) solution in hexadecane was determined to be 51±5 nanoseconds. Thisvalue is in the middle of the range of typical exciplex lifetimes, and isconsistent with the estimates made in earlier papers.

17

.*.~~~ --- ..

These results obtained with the naphtha ene/TMPD system have impli ca-

tions for the evaluation of other exciplex systems. The following conclu-

sions can be drawn:

1) Reliable high temperature spectra can probably be obtained if molecu-

lar oxygen is excluded from the system. Temperature and irradiation with

ultraviolet light by themselves do not cause serious degradation of the

system.

2) Vuantitative work with exciplex systems wi I I probably require that

they be compounded so that only one of the monomers is fluorescent. In

* tnis manner, one can assure That the vapor phase emission is proportional

to the monomer concentration. At present, visualization of the liquid phase

is restricted to temperatures at which the exciplex emission dominates that

of the monomer; extension to higher temperatures wi I I probably require

extensive deconvol ution.

3) It is not necessary to compound the exciplex systems so that the

monomer emission is completely suppressed at lower temperatures. The

kinetic effects shown in tne lower temperature 2.5% naphthalene/1.0% TMPD

spectra did not interfere significantly with the demonstration fuel spray

visual ization pnotographs taken earlier.

I!:

18

.* . ... - . . .

AREAS OF FUTURE WORK

The worK descr i bed in this report was or ig i na I y part of a three yearproposed program for the design and development of fluorescent sensors forfuel sprays. The characterization and testing of new exciplex systems wil Icontinue under a subcontract to the University of Texas at Dallas fromUnited Technologies Research Center( USA prime contract DAAG29-84-C-O010).

' Included also in this subcontract is research on ways to use similar fluo-rescent systems to determine other fuel spray properties such as droplettemperatures, absolute oxygen concentrations, and fuel/oxygen equ iva enceratios.

iii-9

,d._..........................................-. . .

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REFERENCES

1. G. M. Faeth, Prog. Energy Combust. Sci. 9,1 (1983).

I L 2. C. K. Law, Prog. Energy Combust. Sci. 8, 171-201 (1982).

3. N. A. Chigier, Prog. Energy Combust. Sci. 2, 97-114 (1976).

4. W. A. Sirignano, and C. K. Law, Adv. in Chemistry Series 166,Evaporation-Combustion of Fuels, J. T Zung, ed., 1-26 ACS (1978).

5. W. Sirignano, Prog. Energy Combust. Sci., 9 291-322 (1983).

6. Krier, H. and Foo, C. L., Oxidation Combust. Rev. 6, 111 (1973).

7. S. C. Johnston, in Fluid Mechanics of Combustion Systems, T. Morrel I,ed. Proceedings of the ASME Fluids Engineering Conference, Boulder Colo.22-28 June 1981 (ASME, New York, 1981), 107-118.

8. S. C. Johnston, SAE paper 80-0136, SAE Transactions 89 Sec. 1, 786

(1981).

9. J. J. Sangiovanni and M. Labowski, Comb. and Flame 47, 15 (1982).

10. R. C. Peterson and A. C. AlKidas, Comb. and Flame 53, 65 (1983).

11. A. J. Yule, C. Ah Seng, P. G. Felton, A. Ungut, and N. A. Chigier, in* 5 Proceedings, Eighteenth International Symposium on Combustion (The

Combustion Institute, Pittsburgh, 1981), 501.

12. A. R. Chraplyvy, Appl. Opt. 20, 2620 (1981).

13. J. M. Tisnkoff, D. C. Hammond, Jr., and A. R. Chraplyvy, J. FluidsI r Eng. 104, 313-317 (1982).

14. L. A. Melton, App I. Opt 22, 2224 (1983).

15. L.A. Melton and J. F. Verdieck, Combustion Sci. and Tech. (1984), tobe publ ished.

16. L. A. Melton and J. F. Verdieck, in Proceedings, Twentieth

International Symposium on Combustion (The Combustion Institute,Pittsburgh, to be publ ished 1985).

17. J. B. BirKs, "Excimer Fluorescence in Organic Compounds," in Progressin Reaction Kinetics 5, G. Porter, ed. (Pergamon, London, 1969).

. 18. T. Forster, Angew. Chem. Int. Ed. (Eng.) 8, 333 (1969).

19. D. Rehrn and A. Wel er, Z. Phys. Chem. Frankfurt am Main 69, 183(1970).

20. R. A. CaIdwel I and D. Creed, Acc. Chem. Res. 13, 45 (1981).

21. T. J. WnitaKer and B. A. Bushaw, J. Phys. Chem. 85, 2180 (1981).

20

..........................................

22. S. Hirayama and D. Phi Ilips, J. Phys. Chem. 85, 643 (1982).

23. R. A. CovalesKie, D. A. Dolson, and C. S. Parmenter, J. Chem. Phys. 72,5774 (1980).

24. K. Chihara and H. Baba, Chem. Phys. 25, 299 (1977).

25. C. H. Wang, X. Q. Liu, and C. K. Law, Combust. and Flame 56, 175(1984).

26. I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules(Academic Press, New YorK, 1971).

27. Tecnnique of Electroorganic Synthesis, part II, N. L. Weinberg, ed.,vol. 5, in Techniques of Chemistry. A. Weinberge, ed. (Wiley-

InTerscience, New York, 1975) p 667ff.

28. Handbook of Chemistry and Physics, 62nd ed., CRC Press, Boca Raton,(1982).

29. A. Zweig, A. H. Maurer and B. G. Roberts, J. Org. Chem., 32, 1322-9(1967). 9

30. D. Bryce-Smith and A. Gilbert, Tetrahedron 32, 1309-26.

31. T. Forster and H-P. Seidel, Zeit. Phys. Chem. (N.F.) 45, 58-71 (1965).

32. R. A. CaldwelI , private communication.

21

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PUBLICATIONS

I 1. L. A. Melton and J. F. VerdiecK, "Vapor/Liquid Visual ization for Fuel

Sprays", Combustion Sci. and Tech., (1984), to be published.

2. L. A. Melton and J. F. Verdieck, "Vapor/Liquid Visual ization in Fuel

Sprays", in Proceedings, Twentieth International Symposium on

Combustion (The Combustion Institute, Pittsburgh, 1985), to be

published.

3. A. M. Murray and L. A. Melton, "New Exciplex systems for Vapor/Liquid

Visual ization", AppI. Opt., (1985) in preparation.

4. L. A. Melton, J. F. Verdieck, and A. M. Murray, "Characterization of

Napnthalene/TMPD Exciplex SysTems Used in Vapor/Liquid Visual ization",

Appl. Opt., (1985) in preparation.

I23

I

. . . . . . . . . . . . . . . -

PERSONNEL ASSOCIATED WITH GRANT

rk 1. L. A. Melton, principal investigator

2. A. M. Murray, Ph.D, University of Arizona, postdoctoral research

associate

p 24

INTERACTIONS (COUPLING)h •1. L. A. MelTon and J. F. Verdieck, "Vapor/Liquid /visualization for Fuel

Sprays", paper presented at tne Fal I Meeting of the Eastern Section ofthe Combustion Institute, Providence, R.I. , Nov. 1983.

2. L. A. Melton and J. F. Verdieck, "Vapor/Liquid Visual ization in Fuel* Sprays", presentation of paper #2 above at the Twentieth International 0

Symposium on Combustion, Ann Arbor, Michigan, August 1984.

3. L. A. Melton, seminars on vapor/ liquid visualization at the fol lowinginstiTutions:

a) Department of Mecndnical Engineering, Carnegie-MelI on University, 0

Nov. 1983.

b) Combustion Research Faci I ity, Sandia National Laboratory, May 1984.

c) Department of Mechanical Engineering, Stanford University, May 1984.

d) Molecular Physics Group, SRI International, Menlo ParK, CA, May

1984.

e) Department of Mechanical Engineer ng, University of Cal ifornia-

Berkeley, May 1984.

f) Department of Chemical Engineerin , Yale University, July 1984.

4. L. A. Melton spent July 5-August 10, 1984, working with J. F. Verdieckin tne Chemical Physics Group (Alan Eckbretn, manager).

25

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