Carbonaceous aerosols from diesel fuel pyrolysis in shock tube

~ Pergamon

PH: 50021-8502(96)00330-8

J. A"OJbI Sci.• Vol. 27. Suppl. I. pp. SSIS-SSI6. 1996 Copyright \C 1996 EI....ner Science LId Printed in Great Bril. in. All righll reserved

0021·8S02/96 SI5.00 + 0.00

CARBONACEOUS AEROSOLS FROM DIESEL FUEL PYROLYSIS IN SHOCK TUBE J. D'ALESSIO, M. LAZZARO, P. MASSOLI Istituto Motori CNR. Via Marconi 8. 80125 Napoli, Italy

The pyrolysis of a complex liquid fuel, whose characteristics are listed in Table L has been investigated in high temperature and pressure conditions (P=9-12 bar, T=17QO-2500 K) behind reflected shock waves in argon atmosphere using a conventional diaphragm-type shock tube [1]. The measurement section, placed 50mm from the end plate, has been provided with optical accesses (Fig. I). Light extinction measurements were performed in the visible (A,=514.5nm) and in the infrared ().=1300run) using an Ar+ enhanced laser (P=3W) and a diode laser (p=SmW) respectively. Scattering measurements in the visible were also performed. To discriminate the scattered and extincted light against thermal radiation, a high speed turbine chops at about 15kHz the Ar+ laser beam, while the diode laser is internally modulated at 20kHz. The fuel was fed in the shock tube as a finely dispersed aerosol. At this aim, a commercial aerosol system for biomedical applications was used, argon being the carrier gas. Mie theory [2] was applied to determine the droplet size and number concentration of the fuel aerosol before the shock arrival. The scattered light from the particle cloud was simultaneously collected under three fixed angles (9F30°, 60° and 120°) and the size and number concentration of droplets were inferred measuring the scattering cross sections CVV<9 i) and the dissimetry ratios CVV<9i)/CVV<9j)' The droplets number density N p was about 106 em-) and their mean diameter dp was around IJ.Lm. Shocks were started before any appreciable evaporation and/or settling of aerosol droplets were observed. Droplets rapidly evaporate behind the incident shock as revealed by the sudden decrease of the scattering signal and the concurrent increase of both visible and infrared extinction signals (Fig.2). The fuel concentration in the shocked gas was obtained as follows: [fuel]=Np(1td:16)pfuelr (gIan3 ), r being the density ratio across the shock. Carbon atom concentrations of about 10 18 an-) were obtained. Time t=O corresponds to the arrival of the reflected shock in the measuring section. Then. behind the reflected wave, fuel vapours pyrolyze and form soot particles, as revealed by a steep decrease in the infrared extinction signal and the simultaneous increase of thermal emission. Assuming the soot particles behave as Rayleigh scatterers, the soot volume fraction was obtained from the

relationship

r.=.!.ln-.!../6~Im{ m: -I}, where L is the optical path length (tube diameter) and m is L I. A. m +2

the soot refractive index ( the value m=1.9-iO.6 was adopted). Soot concentration profiles were obtained as (Clooc = f v Peoot' where PIlQ is the soot density (the value 1.86 pjan3 was adopted). The results of our invesigation are shown in Figg, 3-4 . In Fig.3 the soot yield at 1.3 milliseconds has been reported as a function of temperature, showing a bell shape as found tipically for pure lighter hydrocarbons [3,4] . A description of kinetics of the overall precess can be obtained by the measurement of the induction time 't of soot formation, defined as the time elapsing since the establishment of experimental conditions to the appearance of first soot particles (fig.2). In Fig.4 the product of the induction time and initial carbon atom concentration Pc (moVan3 ) has been plotted in an Arrhenius type diagram. The process can be described by the expression 'tPc =Aexp(E A /RT) , obtaining an activation energy E A of about 220 kJ/mot. SSIS

SSI6

Abstra cts of the 1996 European Aerosol Conference

Table I : Fuel Characteristics

mp eC)

174.0

FBP eC)

372.0

Paraffins (% wt.)

46.1

Napht. DOD condo (%wt.) Napht. cond, (%wt.)

17.5 11.3

Mono aromatics (% wt.) Di-aromatics (% wt.) Tri-aromatics (%wt.) C (% wt.)

14.7

H (% wt.)

9.1 0.9 84.7 12.6

o (%wt.)

0.9

Fig. 1 1.0 Soal yield 1111 • 1.3 1M

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1i

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C

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c

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1000

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2000

.....;

400

T(K)

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300

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20400

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200

100

o

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25

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IO·rr(K)

Fig.4

[1] Massoli, P.•Bertoli. C.• Lazzaro. M.: IMEcHE C485. London, 1994.pp.187-192. [2] Bohren. C.F., and Huffman. D.R.: Absorption and Scattering ojLight by Small Particles. John Wl1ey & Sons. 1983. [3] Bauerle.St., Karasevich, Y.• Slavov.St., Tanke. D., Tappe. M.• Thienel, Th.• and w~er. H.Og.: Twenty-Fifth Symposium (International) on Combustion, p. 627. The CombustIon Institute. 1994. [4] M.Frenklach, S.Taki, M.B.Durgaprasad,R.MatuIa: Comb. and Flame. 54, 81 (1983)