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Experimental study and modeling of dodecane ignition in a diesel engine
Experimental Study and Modeling of Dodecane Ignition in a Diesel Engine K. SAHETCHIAN,* J. C. CHAMPOUSSIN, M. BRUN, N. LEVY, N. BLIN-SIMIAND, C. ALIGROT, F. JORAND, M. SOCOLIUC, A. HEISS, and N. GUERASSI Laboratoire de M~canique Physique, CNRS URA 879, 2 Place de la Gate de Ceinture, 78210 Saint-Cyr-l'Ecole, France (K.S.; N.B.-S.; F.J.; A.H.) and Laboratoire de Machines Thermiques, Ecole Centrale de Lyon, BP 163, 69131 Ecully Cedex, France (J.C.C.; M.B.; N.L.; CA.; M.S.; N.G.) Two experiments have been performed under conditions as close as possible to those existing in a diesel engine. The first is oxidation of n-dodecane in a motored diesel engine running under conditions close to ignition but avoiding it. The progress of chemical reactions is followed by measurements of the global temperature increase AT of the exhaust gases, and by continuous sampling of the combustion chamber gases, to measure the concentrations of hydroperoxides and molecular hydrogen; about 4.2% of the energy introduced as hydrocarbon is consumed, thus showing significant transformations during the ignition delay of n-dodecane. The location of the maximum concentration of hydroperoxides coincides with the fuel jet's edge. Tarlike compounds are present in the unburnt dodecane at the engine exhaust. The second experiment is the study of ignition delay of an n-dodecane spray in an oxidation chamber filled with air, between 715 and 760 K and 15 and 25 bar. A reduced mechanism of 32 reactions, with three types of branching due to the species (RO2, RO2H) , (HO2, H202), and H, enable one to predict the ignition delay. Computer simulations are made with the KIVA II code. They show good agreement between the experimental and the calculated ignition delays. They also indicate that, during the ignition delay, reactions occur first at the boundary of the fuel spray. A temperature increase of about 100 K takes place at the hottest points, which correspond to concentration maxima of the three branching species. Time-dependent evolutions of average concentrations show that RO2H reaches a maximum first, then H202, and lastly the H atom.
INTRODUCTION Diesel combustion is fraught with chemical kinetic problems. Global mechanisms are often used for modeling oxidation in diesel engines [1, 2] and include only a few reactions, but the various parameters must be calibrated for each fuel and operating condition. The purpose of this paper is to understand better the basic phenomena occurring during the ignition delay and to obtain experimental results to establish and validate a chemical mechanism describing this delay, so that all experimental results may be reproduced without additional modifications of the parameters. Later, a link might be established between these reactions and the formation of particulates or carbonaceous pollutants. In a previous study [3] of engine knock, a chemical kinetic model was set up to reproduce ignition in an experimental, motored, CFR engine, operating with different compres-
* Corresponding author.
sion ratios and homogeneous mixtures of air and hydrocarbon containing more than five carbon atoms: octane, heptane, and pentane. The importance of isomerization reactions of RO 2 radicals and of hydroperoxides has been emphasized in the occurrence of ignition [3, 4]. Similar hydroperoxides have been recently observed in a fired, experimental, diesel engine [5]. This fact suggests the use of a similar mechanism to describe ignition in diesel combustion. To establish a chemical mechanism and to choose values for its input parameters, it was necessary to perform experiments under conditions as close as possible to those existing in a diesel engine. The fuel under test was ndodecane; this hydrocarbon has a long linear molecule, like those of diesel fuels, and in this case isomerization is important [3]. Three types of experiment have been made. To determine chemical parameters in the mechanism it is useful to have a reduced number of experimental variables. This is why, as a first step, a constant volume oxidation chamber, in which temperature and pressure are controlled, was
COMBUSTION AND FLAME 103:207-220 (1995) Copyright © 1995 by The Combustion Institute Published by Elsevier Science Inc.
0010-2180/95/$9.50 SSDI 0010-2180(95)00093-L
208
K. S A H E T C H I A N E T AL.
used. But concentrations of the products present during the ignition delay are difficult to measure under these conditions because of the very short reaction time. In fact, a motored engine enabled mean quantities of peroxides in the cylinder to be measured, but many intermediates were difficult to separate. These products were trapped continuously under conditions avoiding ignition, but very close to it. Finally, a flow apparatus has been used as a reactor in which dodecane is oxidized. This apparatus enabled hydroperoxides to be produced at the highest possible concentrations. In this case, analyses were easier and comparisons were possible between these compounds and those formed in the engine.
measured, respectively, with a thermocouple (type J, 4' = 2 mm) and a water-cooled piezoelectric transducer. Acquisition and processing of these signals were on a PC computer. The test conditions used in the chamber are summarized below: initial temperature: initial pressure: one-hole injector: preset opening pressure: injected mass: injection time:
715 to 760 K, 15 to 25 bar; 4, = 0.25 mm; 225 bar; 19.7 mg; 3.3 ms.
A more detailed description of the experiment and of the procedure has been given [6].
Experimental Diesel Engine EXPERIMENTAL Oxidation Chamber The experimental apparatus (Fig. 1) consists of a cylindrical, steel, combustion chamber ( L = 100 mm, 4' = 40 mm) filled with air from an accumulator and preheated by electrical resistance heaters imbedded in the steel. A singleshot injection pump supplied the fuel through a one-hole nozzle. The injection pressure and the needle lift signal were measured to determine the injection conditions (mainly the velocity at the nozzle output). Mean temperature and instantaneous pressure in the bomb were
I AIR I IN~
The experimental diesel engine used is a motored version derived from an industrial Lombardini, four-stroke, single cylinder, direct injection, diesel engine (100-mm bore, 90-mm stroke, 0.7-L displacement and compression ratio 8). The cylinder head was equipped with large optical accesses. The fuel was supplied by an in-line injection pump and a single hole (0.2 mm diameter) injector nozzle. The spray direction coincided with the horizontal axis of the parallelepiped-shaped combustion chamber. Dodecane was used as a typical fuel. The temperatures of the cooling water and intake air were monitored; it was thus possible to ap-
COOLING SYSTEM ~[ tIEE~LFT ECHLI ~I ~.-~
IS~T ~~A U
EXH COMBUSIO TN CHAMBER
~'N ELECTR ICAL RES ISTANCE HEATERS
P,INJ, PUMP~ . ~
CONTROL UNIT ACQUISITION SYSTEM
Fig. 1. Oxidation chamber.
D O D E C A N E I G N I T I O N IN A D I E S E L E N G I N E proach progressively ignition which was detected by the internal pressure rise and seen through the window of the cylinder head. Reactions inside the cylinder were followed by the temperature and concentrations of the products. Measurements of temperature could not be made inside the cylinder; the only temperature measurement possible was that of the exhaust gases using a thermocouple located in the exhaust pipe as seen in Fig. 2. Due to variations in gas velocity during a cycle, this exhaust gas temperature is very close to the average value of the temperature within the cylinder at the exhaust valve opening. Local concentrations of products were determined by continuous sampling [7] over the whole running cycle of the engine for 2 or 3 min, depending on the position of the probe in relation to the jet. The probe is perpendicular to the liquid fuel jet axis at 13 mm from the injector exit. The end of the probe is located at any point between the jet axis and the wall. It is conical with a 30 ° half-angle and a leak of 0.4-ram diameter. The concentration of hydrogen was measured in gases sampled from the exhaust pipe and kept inside cylindrical rubber balloons of capacity of 1 or 2 litres.
Flow Apparatus Figure 3 shows this apparatus, n-Dodecane was vaporized by making it stream along a rod located at the center of a heated tube through which nitrogen flows. In this way, vapours of n-dodecane were obtained in the temperature range of 468-493 K without any oxidation or
209
Exhaust
Thermocoeupl
Combuisot i h
J ~
"~'~ i Injector ~ P r o b e f Cylinder
Fig. 2. Setup for sampling in the cylinder and temperature measurement in the exhaust pipe of the experimental diesel engine.
decomposition. Oxygen was added close to the vessel inlet. Afterwards, an Archimedes screw brought about mixing and also a core placed at the inlet provided flow homogeneity inside the vessel, which was located in a thermoregulated oven. Products were recovered at the vessel outlet after reaction. The residence times were about 5 s. The products and dodecane were
Dodecane
Fig. 3. Schematic of the flow apparatus: A vaporizer, B variable heating set, C vessel, D thermoregulated oven, E Archimede's screw, F core, G bubbler.
210
K. S A H E T C H I A N E T AL.
collected by bubbling the gases through a solvent (bubbler diameter: 8 ram). Acetonitrile, used for H P L C chromatographic analyses, was the solvent, with the gases maintained at ~ 333 K until contacting the acetonitrile. Its partial vaporisation keeps the solvent's temperature slightly lower than that of the room. The final sample volume can be adjusted by progressive additions of acetonitrile to compensate for evaporation. The concentration obtained this way was too low for analysis by mass spectrometry. Therefore, the acetonitrile from about ten such trappings was gathered and evaporated until sufficiently high H P L C peaks were observed. To have fewer secondary products and a fair quantity of hydroperoxides, the following operating conditions were chosen: * flow of liquid dodecane * flow N 2 * flow 0 2 * oven temperature *vaporizer temperature * trapping time *vessel volume * residence time of reactants * diameter of the bubbler tube
6 ~l/mn ]500 N m l / m n 300 N m l / m n 503 K 483 K about 20 mn 152 ml 4.7 s 10 mm
Purification and Analysis of Hydroperoxides from the Oxidation of n-Dodecane As described in the previous paragraph, a sample was obtained from the flow system consisting of two phases, one mainly of dodecane and the other mainly of acetonitrile. Analyses were made on the acetonitrile phase because some other products are there in a minor concentration. Preparative chromatography of hydroperoxides, previously used in case of heptyl hydroperoxides [8], could not be used, since these heavier molecules were decomposed on silica. Samples were analyzed further by liquid chromatography. When oxidation was not in the flow system, but in the engine, additional problems had to be solved. These were: (a) water present in the
air sucked up by the engine is trapped; this presents an additional difficulty in product identification, (b) the pressure variations inside the cylinder led to significant and sudden irregularities of the outflow; this needed a 36 mm diameter bubbler and a large volume of acetonitrile solvent. The probe and the pipe were at the temperature of the engine block, ~ 333 K, in order to avoid any condensation, which might lead to errors in the concentrations of the hydroperoxides. The sensitivity of the chromatographic analysis led us to concentrate the sample at the same volume by evaporation at room temperature. Analyses are then similar to those described above.
Chromatographic Techniques for Products Analyses Gases collected from the diesel engine are analyzed by gas chromatography. A 5-A molecular-sieve packed column (length 2 m, carrier gas Argon U containing ~ 5 ppm oxygen, temperature 313 K) was used; a zirconia cell constitutes the detector [9]. Chromatographic separation of hydroperoxidic compounds is made by High Pressure Liquid Chromatography HPLC on a Lichrosorb Merck RP18 column, 200 × 4.6 mm, 7 /~m, with an eluent acetonitrile-water 6 5 % - 3 5 % and a flow 0.7 m l / m n , detection is made by UV spectrophotometry at 220 nm.
EXPERIMENTAL RESULTS
Ignition Delay Measurements in the Oxidation Chamber The experimental results concern the evolution of pressure during combustion and ignition delay. The beginning of ignition is experimentally detected by the sudden pressure change. Of the data obtained in the oxidation chamber, only those in the temperature range of 715 to 760 K are gathered in Table 3. For lower temperatures, there is significant impingement at the wail. For higher temperatures, the ignition delay is too short to obtain an accurate value. The experimental delay of each test (No. 1-5) is an average obtained from four experi-
DODECANE IGNITION IN A DIESEL ENGINE ments under the same operating conditions (temperature and pressure).
211
650
Gas Temperature (K)
Estimates of Chemical Changes Before Ignition in the Experimental Engine To estimate the importance of chemical reaction during the ignition delay and also the average temperature in the combustion chamber of the experimental engine,• measurements of the instantaneous pressure in the cylinder were performed. A classical thermodynamical diesel model allowed us to deduce from the pressure loop (see Figure 4) an indicated work of 29 J per cycle (that is 4.2% of the injected energy) for a typical run with an exhaust gas temperature increase of AT = 6.1 K. It also allows us to calculate the average gas temperature inside the cylinder during such a run, as plotted in Fig. 5. Temperature Increase of the Exhaust Gases from the Experimental Engine A temperature increase of the exhaust gases AT occurs when fuel is introduced into the combustion chamber of the running engine. Depending on the temperatures of both the cooling water and air intake, all values of AT lie in the range 0-12 K. Ignition occurs if AT > 12 K. The value of AT is the guiding parameter for our experiments. This temperature increase of the exhaust gases shows that
Engine Pressure
15 •
(bars)
12
TDC
300
Crank Angle
l
i
,
~
,
~
)
A
320
340
360
380
400
420
440
460
400
480
Fig. 5. Average calculated gas temperature inside the cylinder with pressure loop of Fig. 4 temperature (K) versus crank angle.
reactions occur in the compressed mixture, even without ignition. Hydroperoxides Characterization of hydroperoxides in the products was performed with the sample from the flow apparatus. The hydroperoxides from the diesel engine were deduced from a comparison of chromatograms. The products from the flow apparatus have been analyzed by several techniques. These were
(a) Thin layer chromatography (TLC) The presence of hydroperoxides can be deduced from the analogy observed by TLC, between the oxo-heptyl hydroperoxide retention factor and that of products formed in the reactor, the first one and the others giving the same pink coloration with the peroxide reagent N,N dimethyl para-phenylene-diamine [8]. (b) High-Pressure Liquid Chromatography
(HPLC)
9 6 3
Inttantaneous )
0 0.05
45O
0.2
i
i
i
i
035
0.5
0.65
08
Fig. 4. Pressure loop inside the cylinder without ignition. M e a s u r e m e n t s of instantaneous pressure versus volume inside the combustion chamber. Conditions: stoichiometric richness, air intake temperature 298.2 K, water cooling temperature 332.4 K, temperature increase AT = 6.1 K.
Figure 6 shows a typical analysis by HPLC of products obtained and detected at 210 nm. Peaks A, C, and D, have retention times analogous to those of the heptyl hydroperoxide and the oxo-heptyl hydroperoxide [8]. These peaks show a maximum of the UV absorption at 210 rim. (c) LiquM Chromatography~Mass Spectrome-
try (LC/MS) A Nermag Mass Spectrometer associated with liquid chromatography (same column, eluent composition and flow rate as for the HPLC
212
K. S A H E T C H I A N E T AL.
<
eJ
g
m
Fig. 6. HPLC
•
o
eo
~
m
o0
.
chromatogram of hydroperoxides obtained
in the flow system; column Lichrosorb, 7/xm, eluent acetonitrile/water 65/35, flow 0.7 mL/mn, time in mn, UV detection at 220 nm. analyses), with a thermospray interface and ammonium acetate as ionizing salt, was used. The double simultaneous detection at 5.30 min. and 7 rain of peaks at m / z 203 and 220 (Fig. 7) on one hand, and at 6.20 min. and 8.20 min at m / z 217-234 (Fig. 8) on the other hand, was noticed. These peaks are interpreted as M H +, [MNH4] + and M ' H +, [M'NH4] + with M = 202, the molecular weight of dodecyl hydroperoxide and M' = 216, the molecular weight of oxododecyl hydroperoxide. The detection of two pairs of peaks in each case is probably due to isomers. Identity of L C / M S and H P L C peaks cannot be entirely assured due to the different modes of elution and modes of detection; in H P L C detection is made at atmospheric pressure and under low pressure in case of L C / M S .
Abundance
Ion
203.00
(202.70
to
H P L C analyses of samples from the Diesel engine show many peaks, but among them peaks A, C and D in Fig. 9 appear clearly. Another peak B, which, unlike A, C and D, exists only with tests on the engine, appears in the same area (see Fig. 9). The results of the probe samples are illustrated in Figs. 10 and 11. Figure 10 (for a distance from the jet axis of 3.4 mm) illustrates the increase and decrease of peroxides with the exhaust gas temperature increase AT, having a maximum at ~ 10 K. Figure 11 shows that the maximum concentrations (for a given exhaust gases temperature increase AT = 6 K) seem to be located on the jet's edge, where the flame appears; this is in agreement with previous rapid cinematography visualization by Fluzin [10], which showed the first appearance of ignition in an area situated between 3 and 6 mm from the jet's axis, depending on the running conditions. The observed coincidence between the maximum of the hydroperoxides and the localization of the onset of ignition at the edge of the fuel jet, leads us to the conclusion that these hydroperoxides play a significant role in this ignition starting.
Formation of Heavy Products After several hours running the engine without any ignition, the unburned dodecane collected in the exhaust is slightly yellow; also there is a
203.70)
3000
2000
i000
Time-->
2.00
Abundance
Ion
4.00
6.00
220.00
219.70
8.00
to
i0.00
220.70)
3000
2000
i000
' ' ' Time-->
I .... 2.00
I 4.00
6.00
8.00
I0.00
Fig. 7. LC/MS analysis of hydroperoxides recoveredfrom the flowsystem; columnLichrosorb Merck RP18, 200 x 4.6 mm, 7 /zm, eluent acetonitrile/water 65/35, ammonium acetate thermospray; dodecylhydroperoxide.
DODECANE IGNITION IN A DIESEL ENGINE Abundance
6
0
Ion
0
217.00(216.70to 217.70) 0 ~
213 IPeroxidesl Arbitrary unit
4000
2000 +
Time--> Abundance
2.00
4.00
Ion
6.00
_~-~\
\
8.00 i0.00
234.00(233.70 tO 234.70) o
e peak D
24000 00060]~I 00 AT(K )
Fig. 10. Peroxides concentration versus temperature in-
i--i, , 4.00 Time--> ....2.00
crease at the exhaust pipe; the sampling probe is located at
6.00
8.00 I0.00
Fig. 8. L C / M S analysis of hydroperoxides recovered from the flow system; column Lichrosorb, 7 ~ m , eluent acetonitrile/water 6 5 / 3 5 , flow 0.7 m L / m n , time in mn, a m m o nium acetate thermospray; oxo-dodecyl hydroperoxide.
3.4 m m from the fuel jet axis.
brown viscous phase, insoluble in dodecane. In the cylinder itself, small amounts of a tar-like, burnt deposit were observed. It is likely that these solid particles make a major contribution to the laser beam attenuation, measured in this engine during the ignition delay [11].
exhaust, and that amount of H 2 rises exponentially when AT is increasing. This is seen in Fig. 12, where the quantity of H 2 (in arbitrary units) is plotted versus AT. The highest value observed, close to ignition, corresponds to about 10 ppm of H 2. It should be noticed that samples collected from the exhaust pipe show hydrogen concentrations lower than those collected through the probe.
Formation of Molecular Hydrogen
MODELING
Gaseous samples taken at the point where the amounts of hydroperoxide were highest, have clearly shown that hydrogen exists as soon as a temperature increase of 3 K is reached at the
Basic Postulates of the Kinetic Model The regime prior to ignition in a diesel engine is characterized by significant heterogeneities
IPeroxidesl
~A
i
Arbitrary unit
I.L
9.46 10.48
B
11.54
13.90
/
, S .,6
A peakD peakC peak
o
Fig. 9. H P L C c h r o m a t o g r a m of hydroperoxides and products from the diesel engine; column Lichrosorb Merck RP18, 200 × 4.6 m m , 7 /~m, eluent acetonitrile/water 65/35, U V detection at 220 nm.
fuel j e t axis
fuel j e t edge
d(mm)
Fig. 11. Peroxides concentration versus distance of the
sampling probe from the fuel jet axis; the temperature increase at the exhaust pipe is AT = 6 K.
214
K. SAHETCHIAN ET AL, ]H21 / / / / / / / / / /
A r b i t r a r y unit
/autoignition / / /
/* o
I
•
5
io
aT(K )
Fig. 12. Concentration of molecular hydrogen versus the temperature increase AT.
regarding the fuel phase (liquid or gas), the fuel mixture (rich or lean), the temperature and the pressure. In the constant volume combustion chamber, the same phenomena arise. At any time and any location in the chamber, the concentration of reactants and radicals, the temperature and pressure, the generated heat and the mass transfer of the chemical species, all depend both on chemical processes and turbulence. A detailed mechanism, including every possible elementary reaction, would be time consuming in a multidimensional computational code and uncertain because several reactions are questionable. The classical models of diesel ignition are based on very simplified chemistry, including one or several global reactions [1, 2, 4, 12, 13]. The reduced mechanism proposed in this work is based on a chemical kinetic model consisting of elementary reactions, emphasizing those preceding ignition, mainly the isomerization of R O 2 radicals. This model, initially developed to predict ignition data in a CFR engine [3], is modified to describe properly the phenomena occurring in the combustion chamber of a diesel engine or an oxidation chamber. Modifications are necessary because of the structure of the fuel and of the heterogeneous nature of diesel combustion. To predict various phenomena in
the combustion process, such as exothermicity and the formation of solid particles, reactions are added, especially global ones. It should be pointed out that these global reactions are not real ones and are introduced to keep a reduced mechanism, even though fuel consumption, solid particle formation and reaction exothermicity are maintained. Ignition delay includes temperature heterogeneities from 500 up to 2000 K or more; therefore, the simulation of this process needs more than one branching and one termination reaction. This is the reason why the proposed model is based on the assumption of three types of branching reactions, each corresponding to a determined temperature range in which a major role is devoted to different species. Table 1 shows the species involved for the different temperature levels. A similar approach has already been taken by Westbrook et al. [14] and Morley [15]. In the previous study [3], the formation of hydroperoxides by isomerization reactions was shown to be important up to 1000 K. When the temperature increases, the importance of these reactions decreases. This could be explained by the following mechanism (see Table 2), in which the R radical can react in different ways: R + 0 2 --~ R O 2 , R+O 2~P+HO
(2) 2,
(11)
R --* decomposition products.
(26)
Reaction 2 and reaction 11 have a low activation energy and are not sensitive to temperature. The R O 2 radical reacts in: R + 02,
(3)
RO 2 --* ROOH'.
(4)
RO 2 ~
TABLE 1 Species Involved in Branching Reactions for Different Temperature Ranges Involved Species
T (K)
RO2, RO2H HO2, H 2 0 2 H
300 < T < 900 700 < T < 1100 800 < T
DODECANE
IGNITION
IN A DIESEL
ENGINE
215
TABLE 2 Chemical Reactions and Kinetic Constants for Dodecane Oxidation (Units: mol, cm, s, K, caD, where R designates an alkyl radical, P an intermediary compound, and N 2 is considered as an inert species No.
Reaction
EA/R
z
ai
Ref.
R H q-
2 3
0 2 -+ R + HO 2 R -k 0 2 - + R O 2 RO 2 ~ R + 0 2
3.16 × 1013 5.64 X 1012 2.50 × 1013
24000 0 13700
0 0 0
1-1 1-1 1
[3] [3] [3, 18]
4 5
R O 2 --+ ROOH" RO 2 + P ~ RO2H + R
7.90 X 1011 2.00 X 1013
8700 5100
0 0
1 1-1
13] [3]
ROOH" ~ R O 2 ROOH" -~ P + O H ROOH" + O 2 ~ O O R O O H OOROOH ~ RO2H + OH RO2H ~ OH + R R + 0 2 ~ P + HO 2 HO 2 + HO 2 ~ H20 2 + 0 2 HO 2 + P ~ H20 2 + R HO 2 + RH ~ H20 2 + R H O 2 + H O 2 + N 2 --* H 2 0 2 + O 2 + N 2 H 2 0 2 + N 2 --~ 2 O H + N 2 O H 4- H 2 --* H 2 0 4- H OH + RH ~ R + H20 2 O H 4- C ~ C O 2 4- H 2 OH + P ~ R + H20 H + 0 2 -~ O H + O H + 0 2 + N 2 ~ HO 2 + N 2 l H + C ~ C + H 2 + gH 2 H + H + N2 ~ H e + N2 H 4- O H 4- N 2 --+ H 2 0 4- N 2 R --+ 12 C + 12 H 2 4- H R + 18 0 2 --+ 12 C O 2 4- 12 H 2 0 + H O + C ~ CO O + R H --+ O H + R O 4- H 2 --~ O H + H C O + H O 2 ~ CO 2 + O H CO + O H -~ CO 2 + H
7.50 × 109
2.45 1.08 1.99 8.00 1.50 4.20 5.03 9.62 7.68 1.50 1.02 3.75 3.00 1.13 1.99 7.70 5.00 3.00 1.40 1.00 6.00 1.30 6.02 5.10 1.51 1.45
5500 15500 0 8500 21000 3000 603 6160 8000 0 22700 1660 435 0 435 8460 0 0 0 0 14700 6500 0 2270 3160 11900 0
0 0 0 0 0 0 0 0 0 0 0 1.6 2 0 2 0 --0.8 0 0 -2 0 0 0 0 2.67 0 0
1 1 1-1 1 1 1-1 2 1-1 1-1 2-1 1-1 1-1 1-1 1-1 1-I 1-1 1-1-1 1-1 1-1-1 1-1-1 1 1-0 1-1 1-1 1-1 1-1 1-1
1
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
An increase
of temperature
of the importance i.e.,
A
reaction
3.
ROOH"
radicals
ROOH"
~
entails an increase
of the inverse Reactions
of reaction
introduced
N 1014
X × X × N x × x X × N
1012 10 It 1015 1011 1014
1013 1013 1012 1017 108
107 × 1012
× 108 × 1014 × 1017
× × )< ×
101° 1015 1023 1013
× 106
× 1011 x 10 u × 104
× 1014 x 10 u
branching
follows
2, for
are
UO 2 ÷ RH H20
(6)
R O 2,
--* H 2 0
2 + M ~
above
2 + R,
(14)
+ M,
(16)
2OH
700 K, where
For high pressures, ROOH"
---, P + O H ,
ROOH"
+
Reactions ture.
0
2 --+
OOROOH.
in the
engine
the
increase
M is a n i n e r t s p e c i e s . the reaction
H + 0 2 --*
(7)
OH
(8)
8 0 0 K . A t 8 0 0 K a n d a p r e s s u r e o f 15 b a r , t h e r a t e o f t h e b r a n c h i n g r e a c t i o n 21 is a b o u t 1 %
6 and 7 are likelier at high tempera-
However,
[3] [3] [3] [3] [3] [3] [3] [3] [3] [3] [3] [19] this work this work [19] [19] [19] this work [20] [19] this work this work this work [21] [19] [22] [23]
of
÷ O ( 2 1 ) is c o n s i d e r e d
to play a part above
of that of HO 2 formation in reaction 22. During the delay, the importance o f r e a c t i o n 21 increases
with temperature.
Under
these
con-
pressure entails larger O2 concentrations, which partially offset the effect of tempera-
ditions,
ture.
delay, reactions are assumed to be so slow, that t h e p r o c e s s is g o v e r n e d by chemistry, turbu-
ROOH
° is t h u s
t i o n s 11 a n d 2 6 b e c o m e
stabilized. more
When
reac-
important,
chain
tion
we consider
21 is n o t
that
negligible.
the
branching
During
the
reacignition
216 lence being created only by disturbance due to the spray. It has been observed that, in the diesel engine, ignition first occurs at the spray's periphery [16, 17]. After ignition, the difficult problem concerning the simultaneous representation of turbulence and chemistry still remains. Chemical Mechanism
The proposed mechanism reproducing ignition in an oxidation chamber is described with 32 reactions, whose rates are given by the generic Arrhenius equation:
where A is the rate constant's preexponential factor, C i is a concentration, E A the activation energy, R the universal gas constant, T the temperature, cei is a partial order, generally, the stoichiometric coefficient, and z a constant. In Table 2, all the chemical reactions are given with their kinetic parameters. In this mechanism, the first 16 reactions come from the previous mechanism of ignition [3]. These reactions are sufficient to describe oxidation during the ignition delay of an homogeneous mixture. They contain R O 2 and H O 2 radicals, as well as peroxides. Their rate constants are derived considering the length of the linear molecule, especially for reactions 2, 4-11, and 14. For diesel conditions, the mixture is heterogeneous and other reactions (17-32) have been introduced to reproduce the evolution of different variables. The heavy products experimentally observed probably contain atoms of C, H and O. To simplify all solid species containing C, H and O or C and H or only C atoms these are all symbolized by C. Likewise molecular H 2 is supposed to be created at the same time as solid C, whereas during the ignition delay H 2 is probably formed through R radicals reacting to form alcenes. These heavy products have been introduced to reproduce their possible effect on the ignition delay, which can interfere with the effect of the peroxides. This is the reason why kinetic parameters have been estimated for reactions 19, 23, 26, and 27.
K. S A H E T C H I A N E T AL. In the mechanism previously proposed [3], the following transformation of R radicals into smaller non isomerizable alkyl radicals is a termination reaction: R --, R' + P.
(a)
In this work, a higher hydrocarbon fuel is used and reaction a is not considered as a termination step, R' being still isomerizable, because of the number of carbon atoms. Furthermore, the observation of heavy products leads us to introduce a termination reaction between a radical and solid C. Several studies have been made of reactions between soot and H, O H [24, 25, 26] or O [24, 25, 27]. These reactions are considered here to be heterogeneous. The oxidation of C by O atoms has a rate constant estimated from previous work [24, 25, 27] to have a mean reaction probability of 0.23. The oxidation of long hydrocarbon chains is very complex, because of the variety of products, so that the global reactions 19 and 26-28 are introduced to the mechanism. Reaction 19 is also an heterogeneous termination step. Some reactions are not elementary processes. Two global reactions explain hydrocarbon consumption, viz 26 and 27. The global reaction 26 is the degradation without oxidation of the hydrocarbon into species noted as C atoms and molecular hydrogen, whereas in reaction 27, the whole oxidation is introduced, with formation of H 2 0 and CO 2. This global reaction 27 is highly exothermic and does not lead to an increase in the active centers, the H radical taking the place of the R radical. The formation and oxidation of particles in diesel combustion are very complex. To describe them, a simplification is introduced by means of C atoms, the mass of particles being equivalent to the global mass of C atoms. These C atoms appear in two types of reaction: the oxidation reactions 19 and 28 with CO formation and the heterogeneous termination reactions 19, 23, and 28. Concerning hydrogen, reactions 17, 21, 22, 24, 25, and 30 are branching or termination reactions. The oxidation of CO appears in reactions 31 and 32. In order to limit the number of species and reactions, a few simplifications have been introduced. In
D O D E C A N E I G N I T I O N IN A D I E S E L E N G I N E reaction 9, the hydroperoxide R O 2 H appears instead of a ketohydroperoxide O R O 2 H. This assumption was made because the two decomposition rates are very close and radicals appear in these reactions. It is validated by R R K M studies [28]. In reaction 10, R corresponds to the RO radical which is assumed to be decomposed [29]. In reactions 5, 13, and 20, the radical created from the intermediate compound is assumed to be R. The global reaction 27 is first order with respect to the radical R and zeroth order with respect to oxygen. Reaction 19 is first order with respect to O H and C. Computational Code
The numerical three-dimensional, time-dependent, code KIVA-II [30] uses the Arbitrary Lagrangian Eulerian method through a finite volume grid to solve the conservation equations for the fluids. KIVA was first developed to describe combustion in a reciprocating engine, so this code is designed to accept chemical kinetic, multistep, models in a constant volume combustion chamber. The computation is pseudo-3-D. The calculation is not stochastic, but the distribution of the droplets at the output of the injector is. The spray is assumed to be composed of several "particles," each of them being formed by a set of droplets which have the same physical and chemical properties. KIVA lI incorporates break-up, collisioncoalescence and evaporation of the droplets. The turbulent effects due to the fuel spray are taken into account, and the turbulent gas flow is computed with the k-e compressible model. The thermodynamical data of the unknown species, especially all radicals, are added to the inner program. They were calculated by means of the NIST data base [31]. For the axisymmettic geometry of the combustion chamber, the
217
numerical solution of the set of equations is computed only for a fraction of the chamber, defined by an angular sector of 0.5 degree (one cell), its radius and height being equal to those of the combustion chamber (respectively 20 and 100 cells). As pointed out above, during the ignition delay, the chemical time scale is assumed to be small enough for the density fluctuations for the species to be neglected, when modeling the mean reaction rate [32]. However, once ignition itself is achieved, the rates of change of temperature and chemical rates increase strongly. Thus, turbulent combustion takes place; but such a model does not exist in the original KIVA II and must be developed. Thus, even if the chemical time scale is small due to the high pressure, we make the assumption that it remains large compared to the turbulence time scale, i.e., the Damk6hler number is small [32]. It is worth noting that for some years, the flamelet concept [33, 34] seems to be a powerful tool to describe the interaction between chemistry and turbulence. However for diesel engines, few models are available and taking into account more than one Arrhenius equation in the existing modeling [16, 32] does not appear to be easy. C O M P U T E D RESULTS AND DISCUSSION Validation
Five experiments in the oxidation chamber were chosen, as explained above to validate the mechanism (see Table 3), in such a way that the calculation would lead to a better understanding of the combustion process. The experimental delay of each test (No. 1 to 5) is an average value obtained for the same operating conditions (temperature and pressure). The rate constants in the mechanism have been
TABLE 3
Comparison Between the Values of Ignition Delay Measured in the Bomb and Computed by KIVA with the Kinetic Model (Each Experimental Datum Is an Average Value) Experiment Initial pressure (bar) Initial temperature (K) Calculated delay (ms) Mean measured delay (ms)
1 15 719 3.3 3.5
2 15 741 2.3 2.6
3 20 715 3.4 3.2
4 20 740 2.4 2.2
5 25 760 1.7 1.5
218 chosen to simulate ignition with agreement on average for the five experiments. Table 3 presents a typical comparison of the measured and computed delays for different temperatures and pressures. The chemical kinetic model reproduces experimental values with good agreement. Indeed, the average of the five relative errors is smaller than 8% for each experiment, but the relative error is of the same order as the measurement error. Before fuel injection the fluid is at rest and the temperature field is homogeneous. The delays, validated by pressure measurements, give our mechanism complete coherence. However, it should be noted that self-ignition delay results from a physical delay, including mechanical and thermal phenomena (atomization, heating, evaporation... ) described by KIVA II and a chemical delay, described by the kinetics. The spray atomization model was validated using measurements on a PDA (Particle Dynamic Analyser) [35]. Taking into account this result, the good agreement between experimental and calculated total delays shows that the chemical mechanism suitably describes the real process.
K. SAHETCHIAN ET AL. t,0 Normalized mass fractions & temperature
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(b) Fig. 13. Calculated time-resolved evolution of temperature and mass fractions of: (a): ROmH, H202, H; (b): ROm, HOm, OH; Tmax = 930 K. (Experiment No. 5:P0 = 25 bar; TO = 760 K). The normalized temperature is defined by T * = ( T - T o ) / ( T m a x - T o ) , where T O is the initial temperature and T the mean instantaneous thermodynamical temperature in the chamber defined by:
Simulation Results T =
Figure 13 reproduces the time-resolved evolution of the mean temperature and species concentration in the whole bomb volume for experiment No. 5. During the delay, an increase in the concentration of RO 2 and R O z H is observed while the temperature increase is still insignificant; then the hydroperoxide concentration increases rapidly and reaches a maximum at a time of 1.70 ms. Afterwards, the concentrations of RO 2 and R O z H decrease continuously but are still important. At 1.75 ms, maxima in the concentrations of H O 2 and H 2 0 2 appear, followed by a fall within the flame, while the mean temperature and amounts of H and OH both rise. The histories of T and [OH] are identical. The two-dimensional representation of the results in Fig. 14 shows that these radicals and peroxides are concentrated near the spray's periphery. In this region, a particular point exists where heat generation is particularly important. This "hot spot" corresponds to the
Tin i \i:1
ni, ]/i=l
where T/ and n i are, respectively, the temperature and the number of moles in the cell i, and N the total number of cells in the chamber.
highest radical accumulation and to the start of ignition. Figure 15 shows the time-resolved evolution of this "hot spot." Before ignition, a temperature rise AT of about 100-150 degrees can be locally observed. This can be linked with experimental results obtained in the motored diesel engine without autoignition, showing a mean temperature increase of about 12 degrees in the exhaust gases for the highest air intake temperature. This region, changing from one simulation to another, corresponds approximatively to the radial and axial limits of the liquid spray. It shows good agreement with high-speed cinematography of the reactive spray, presently under development. The first pressure increase has been well predicted and
DODECANE
IGNITION IN A DIESEL ENGINE
Fresh Air (T0=760,0K)
AutoignitionPoint(Tmax=900,5K)
,
Nozzle
Two-PhaseMedium(Train=486,7K)
r e d u c e d c h e m i c a l kinetic m e c h a n i s m o f 32 reactions is p r o p o s e d to i n t e r p r e t t h e o b s e r v a tions. T h e c h e m i c a l m e c h a n i s m correctly p r e dicts ignition delay, for d i f f e r e n t t e m p e r a t u r e s a n d pressures. T e m p o r a l e v o l u t i o n s o f the conc e n t r a t i o n s o f d i f f e r e n t species w e r e o b t a i n e d . A v e r a g e t i m e - r e s o l v e d values show m a x i m u m c o n c e n t r a t i o n s for R O 2 H , t h e n for H 2 0 2 a n d the H atom. A rapid increase of OH concent r a t i o n a n d t e m p e r a t u r e c o r r e s p o n d s to the a p p e a r a n c e o f ignition. T h e m o d e l shows t h a t c h e m i c a l r e a c t i o n s o c c u r d u r i n g t h e ignition d e l a y at t h e b o u n d a r y o f the spray. T h e s e t r a n s f o r m a t i o n s could b e t h e s o u r c e o f p o l l u t a n t s likely to play a p a r t in the c o m b u s t i o n process.
[
Fig. 14. Calculated isotherms in the chamber at 1.70 ms. This instant corresponds exactly to the numerical delay. t h e spatial d i s t r i b u t i o n o f the species confirms that the flame, in the first stage, is p r e m i x e d in n a t u r e . It p r o p a g a t e s in a r e g i o n w h e r e mixing o f fuel a n d o x i d i z e r b e c o m e s b e t t e r , i.e., in the o u t l i n e o f t h e g a s e o u s spray, T h e t h r e e - d i m e n sional h i s t o r i e s p r e s e n t several m a x i m a at the s a m e time. CONCLUSION In this work, e x p e r i m e n t s w e r e p e r f o r m e d on the ignition o f a d o d e c a n e s p r a y u n d e r conditions close to t h o s e existing in a diesel engine. Significant c h e m i c a l c h a n g e s have b e e n shown to o c c u r d u r i n g the ignition delay. M e n t i o n was especially m a d e o f t h e significant h e a t release, a t e m p e r a t u r e i n c r e a s e o f the exhaust gases a n d t h e f o r m a t i o n o f h y d r o p e r o x i d e s . A
"2200 Hottesttemperaturein the chamber(K) 1900I600-
219
/
/
Aut°igniti°idela~
1300" 1000" , AT 700 . . . . . . . . . ~. . . . 1,0 1,1 1.2 1,3 1,4 1,5 1,6
REFERENCES 1. Westbrook, C. K., and Dryer, F. L., Prog. Ener. Combust. Sci. 1:1 (1984). 2. Zellat, M., Rolland, Th., and Poplow, F., SAE Paper 900254 (1990). 3. Blin-Simiand, N., Rigny, R., Viossat, V., Circan, S., and Sahetchian, K., Combust. Sci. Technol. 88:329 (1993). 4. Kong, S. C., and Reitz, R. D., ASME 93, ICE 22, 6. 5. Blin-Simiand, N., Rigny-Gautrand, R., Sahetchian, K., and Brun, M., Entropie 17:115 (1993). 6. Aligrot C., Th~se de Doctorat, Ecole Centrale de Lyon (1994). 7. Sahetchian, K., Rigny, R., and Blin, N., Combust. Sci. Technol. 60:117 (1988). 8. Sahetchian, K., Rigny, R., and Circan, S., Combust. Flame 85:511 (1991). 9. Sahetchian, K., Heiss, A., and Rigny, R., Can. J. Chem. 60:2896 (1982). 10. Fluzin, G., Th~se de doctorat, Ecole Centrale de Lyon (1990). 11. Ayachi, N., Laurent, P., and Brun, M., Entropie 16l:45 (1991). 12. Theobald, M. A., Ph.D. thesis, Department of Mechanical Engineering, M.I.T. 1986. 13. Miiller, U. C., Peters, N., and Linan, A., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, p. 777. 14. Westbrook, C. K., and Pitz, W. J., SAE Paper SAlE 872107 (1987). 15. Morley, C., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p. 911. 16. Baritaud, T., Henriot, S., N6raud, E., and Veynante, D., Colloque du Groupement Scientifique Moteur,
1,7
time (ms) 1.8 1.9 2,0
Fig. 15. Time-resolved evolution of the "hot spot" (Experiment No. 5:P0 = 25 bar; To = 760 K).
Technip Eds. 1994, p. 45. 17. Edwards, C. F., Siebers, D. L., and Hoskin, D. H., SAE Paper 920108 (1992). 18. Benson, S. W., The Foundations of Chemical Kinetics, McGraw-Hill, New York, 1960.
220 19. Baulch, D. L., Cobos, C. J., Cox, R. A., Esser, C., Frank, P., Just, Th., Kerr, J. A., Pilling, M. J., Troe, J., Walker, R. W., and Warnatz, J., J. Phys. Chem. Ref. Data 21:3 (1992). 20. Lynch, K. P., Schwab, T. C., and Michael, J. V., Int. J. Chem. Kinet. 8:651 (1976). 21. Herron, J. T., and Huie, P. E., J. Phys. Chem. 2:467 (1973). 22. Tsang, W., and Hampson, R. F., J. Phys. Chem. Ref. Data 15:1087 (1987). 23. De More, W. B., Golden, D. M., Hampson, R. F., Howard, C. I., Kuzylo, M. J., Molina, M. J., Ravishankara, A. R., and Sander, S. P., J.P.L. Publication, 87-41, 1 (1987). 24. Bradley, D., Dixon-Lewis, G., El-Din Habik, S., and Mushi, E. M. J., Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1985, p. 931. 25. Puri, R., Santoro, R. J., and Smyth, K. C., Combust. Flame 97:125 (1994). 26. Shaknazarian, I. K., Sahetchian, K., Philipossian, A. C., and Nalbandyan, A. B., Int. J. Chem. Kinet. VIII:23 (1975). 27. Von Gersum, S., and Roth, P., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, p. 999.
K. S A H E T C H I A N
ET AL.
28. Tardieu de Maleissye, J., Rigny-Gautrand, R., and Sahetchian, K., 13th International Symposium on Gas Kinetics, Dublin, September 11-16 (1994). 29. Heiss, A., Tardieu de Maleissye, J., Viossat, V., Sahetchian, K., and Pitt, I.G., Int. J. Chem. Kinet. 23:607 (1991). 30. Amsden, A. A., O'Rourke, P. J., and Butler, P. D., Los Alamos Laboratory LA-11560-MS (1989). 31. Stein, S. E., Rukkers, J. M., and Brown, R. L., NIST Structures and Properties Database and Estimation Program, National Institute of Standards and Technology, Gaithersburg, MD 20899 (1991). 32. Dillies, B., Marx, K., Dec, J., and Espey, C., S.A.E. Paper 930074 (1993). 33. Marble, F. E., and Broadwell, J. E., Project Squid, Technical Report TRW-9-PU (1977). 34. Peters, N., Twenty-First Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1986, p. 1231. 35. Guerrassi, N., Champoussin, J. C., and Aligrot, C., ll~me Congr~s Fran~ais de M6canique, Lille, France (6-10 Septembre 1993).
Received 20 June 1994; accepted 20 March 1995.
INTRODUCTION Diesel combustion is fraught with chemical kinetic problems. Global mechanisms are often used for modeling oxidation in diesel engines [1, 2] and include only a few reactions, but the various parameters must be calibrated for each fuel and operating condition. The purpose of this paper is to understand better the basic phenomena occurring during the ignition delay and to obtain experimental results to establish and validate a chemical mechanism describing this delay, so that all experimental results may be reproduced without additional modifications of the parameters. Later, a link might be established between these reactions and the formation of particulates or carbonaceous pollutants. In a previous study [3] of engine knock, a chemical kinetic model was set up to reproduce ignition in an experimental, motored, CFR engine, operating with different compres-
* Corresponding author.
sion ratios and homogeneous mixtures of air and hydrocarbon containing more than five carbon atoms: octane, heptane, and pentane. The importance of isomerization reactions of RO 2 radicals and of hydroperoxides has been emphasized in the occurrence of ignition [3, 4]. Similar hydroperoxides have been recently observed in a fired, experimental, diesel engine [5]. This fact suggests the use of a similar mechanism to describe ignition in diesel combustion. To establish a chemical mechanism and to choose values for its input parameters, it was necessary to perform experiments under conditions as close as possible to those existing in a diesel engine. The fuel under test was ndodecane; this hydrocarbon has a long linear molecule, like those of diesel fuels, and in this case isomerization is important [3]. Three types of experiment have been made. To determine chemical parameters in the mechanism it is useful to have a reduced number of experimental variables. This is why, as a first step, a constant volume oxidation chamber, in which temperature and pressure are controlled, was
COMBUSTION AND FLAME 103:207-220 (1995) Copyright © 1995 by The Combustion Institute Published by Elsevier Science Inc.
0010-2180/95/$9.50 SSDI 0010-2180(95)00093-L
208
K. S A H E T C H I A N E T AL.
used. But concentrations of the products present during the ignition delay are difficult to measure under these conditions because of the very short reaction time. In fact, a motored engine enabled mean quantities of peroxides in the cylinder to be measured, but many intermediates were difficult to separate. These products were trapped continuously under conditions avoiding ignition, but very close to it. Finally, a flow apparatus has been used as a reactor in which dodecane is oxidized. This apparatus enabled hydroperoxides to be produced at the highest possible concentrations. In this case, analyses were easier and comparisons were possible between these compounds and those formed in the engine.
measured, respectively, with a thermocouple (type J, 4' = 2 mm) and a water-cooled piezoelectric transducer. Acquisition and processing of these signals were on a PC computer. The test conditions used in the chamber are summarized below: initial temperature: initial pressure: one-hole injector: preset opening pressure: injected mass: injection time:
715 to 760 K, 15 to 25 bar; 4, = 0.25 mm; 225 bar; 19.7 mg; 3.3 ms.
A more detailed description of the experiment and of the procedure has been given [6].
Experimental Diesel Engine EXPERIMENTAL Oxidation Chamber The experimental apparatus (Fig. 1) consists of a cylindrical, steel, combustion chamber ( L = 100 mm, 4' = 40 mm) filled with air from an accumulator and preheated by electrical resistance heaters imbedded in the steel. A singleshot injection pump supplied the fuel through a one-hole nozzle. The injection pressure and the needle lift signal were measured to determine the injection conditions (mainly the velocity at the nozzle output). Mean temperature and instantaneous pressure in the bomb were
I AIR I IN~
The experimental diesel engine used is a motored version derived from an industrial Lombardini, four-stroke, single cylinder, direct injection, diesel engine (100-mm bore, 90-mm stroke, 0.7-L displacement and compression ratio 8). The cylinder head was equipped with large optical accesses. The fuel was supplied by an in-line injection pump and a single hole (0.2 mm diameter) injector nozzle. The spray direction coincided with the horizontal axis of the parallelepiped-shaped combustion chamber. Dodecane was used as a typical fuel. The temperatures of the cooling water and intake air were monitored; it was thus possible to ap-
COOLING SYSTEM ~[ tIEE~LFT ECHLI ~I ~.-~
IS~T ~~A U
EXH COMBUSIO TN CHAMBER
~'N ELECTR ICAL RES ISTANCE HEATERS
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CONTROL UNIT ACQUISITION SYSTEM
Fig. 1. Oxidation chamber.
D O D E C A N E I G N I T I O N IN A D I E S E L E N G I N E proach progressively ignition which was detected by the internal pressure rise and seen through the window of the cylinder head. Reactions inside the cylinder were followed by the temperature and concentrations of the products. Measurements of temperature could not be made inside the cylinder; the only temperature measurement possible was that of the exhaust gases using a thermocouple located in the exhaust pipe as seen in Fig. 2. Due to variations in gas velocity during a cycle, this exhaust gas temperature is very close to the average value of the temperature within the cylinder at the exhaust valve opening. Local concentrations of products were determined by continuous sampling [7] over the whole running cycle of the engine for 2 or 3 min, depending on the position of the probe in relation to the jet. The probe is perpendicular to the liquid fuel jet axis at 13 mm from the injector exit. The end of the probe is located at any point between the jet axis and the wall. It is conical with a 30 ° half-angle and a leak of 0.4-ram diameter. The concentration of hydrogen was measured in gases sampled from the exhaust pipe and kept inside cylindrical rubber balloons of capacity of 1 or 2 litres.
Flow Apparatus Figure 3 shows this apparatus, n-Dodecane was vaporized by making it stream along a rod located at the center of a heated tube through which nitrogen flows. In this way, vapours of n-dodecane were obtained in the temperature range of 468-493 K without any oxidation or
209
Exhaust
Thermocoeupl
Combuisot i h
J ~
"~'~ i Injector ~ P r o b e f Cylinder
Fig. 2. Setup for sampling in the cylinder and temperature measurement in the exhaust pipe of the experimental diesel engine.
decomposition. Oxygen was added close to the vessel inlet. Afterwards, an Archimedes screw brought about mixing and also a core placed at the inlet provided flow homogeneity inside the vessel, which was located in a thermoregulated oven. Products were recovered at the vessel outlet after reaction. The residence times were about 5 s. The products and dodecane were
Dodecane
Fig. 3. Schematic of the flow apparatus: A vaporizer, B variable heating set, C vessel, D thermoregulated oven, E Archimede's screw, F core, G bubbler.
210
K. S A H E T C H I A N E T AL.
collected by bubbling the gases through a solvent (bubbler diameter: 8 ram). Acetonitrile, used for H P L C chromatographic analyses, was the solvent, with the gases maintained at ~ 333 K until contacting the acetonitrile. Its partial vaporisation keeps the solvent's temperature slightly lower than that of the room. The final sample volume can be adjusted by progressive additions of acetonitrile to compensate for evaporation. The concentration obtained this way was too low for analysis by mass spectrometry. Therefore, the acetonitrile from about ten such trappings was gathered and evaporated until sufficiently high H P L C peaks were observed. To have fewer secondary products and a fair quantity of hydroperoxides, the following operating conditions were chosen: * flow of liquid dodecane * flow N 2 * flow 0 2 * oven temperature *vaporizer temperature * trapping time *vessel volume * residence time of reactants * diameter of the bubbler tube
6 ~l/mn ]500 N m l / m n 300 N m l / m n 503 K 483 K about 20 mn 152 ml 4.7 s 10 mm
Purification and Analysis of Hydroperoxides from the Oxidation of n-Dodecane As described in the previous paragraph, a sample was obtained from the flow system consisting of two phases, one mainly of dodecane and the other mainly of acetonitrile. Analyses were made on the acetonitrile phase because some other products are there in a minor concentration. Preparative chromatography of hydroperoxides, previously used in case of heptyl hydroperoxides [8], could not be used, since these heavier molecules were decomposed on silica. Samples were analyzed further by liquid chromatography. When oxidation was not in the flow system, but in the engine, additional problems had to be solved. These were: (a) water present in the
air sucked up by the engine is trapped; this presents an additional difficulty in product identification, (b) the pressure variations inside the cylinder led to significant and sudden irregularities of the outflow; this needed a 36 mm diameter bubbler and a large volume of acetonitrile solvent. The probe and the pipe were at the temperature of the engine block, ~ 333 K, in order to avoid any condensation, which might lead to errors in the concentrations of the hydroperoxides. The sensitivity of the chromatographic analysis led us to concentrate the sample at the same volume by evaporation at room temperature. Analyses are then similar to those described above.
Chromatographic Techniques for Products Analyses Gases collected from the diesel engine are analyzed by gas chromatography. A 5-A molecular-sieve packed column (length 2 m, carrier gas Argon U containing ~ 5 ppm oxygen, temperature 313 K) was used; a zirconia cell constitutes the detector [9]. Chromatographic separation of hydroperoxidic compounds is made by High Pressure Liquid Chromatography HPLC on a Lichrosorb Merck RP18 column, 200 × 4.6 mm, 7 /~m, with an eluent acetonitrile-water 6 5 % - 3 5 % and a flow 0.7 m l / m n , detection is made by UV spectrophotometry at 220 nm.
EXPERIMENTAL RESULTS
Ignition Delay Measurements in the Oxidation Chamber The experimental results concern the evolution of pressure during combustion and ignition delay. The beginning of ignition is experimentally detected by the sudden pressure change. Of the data obtained in the oxidation chamber, only those in the temperature range of 715 to 760 K are gathered in Table 3. For lower temperatures, there is significant impingement at the wail. For higher temperatures, the ignition delay is too short to obtain an accurate value. The experimental delay of each test (No. 1-5) is an average obtained from four experi-
DODECANE IGNITION IN A DIESEL ENGINE ments under the same operating conditions (temperature and pressure).
211
650
Gas Temperature (K)
Estimates of Chemical Changes Before Ignition in the Experimental Engine To estimate the importance of chemical reaction during the ignition delay and also the average temperature in the combustion chamber of the experimental engine,• measurements of the instantaneous pressure in the cylinder were performed. A classical thermodynamical diesel model allowed us to deduce from the pressure loop (see Figure 4) an indicated work of 29 J per cycle (that is 4.2% of the injected energy) for a typical run with an exhaust gas temperature increase of AT = 6.1 K. It also allows us to calculate the average gas temperature inside the cylinder during such a run, as plotted in Fig. 5. Temperature Increase of the Exhaust Gases from the Experimental Engine A temperature increase of the exhaust gases AT occurs when fuel is introduced into the combustion chamber of the running engine. Depending on the temperatures of both the cooling water and air intake, all values of AT lie in the range 0-12 K. Ignition occurs if AT > 12 K. The value of AT is the guiding parameter for our experiments. This temperature increase of the exhaust gases shows that
Engine Pressure
15 •
(bars)
12
TDC
300
Crank Angle
l
i
,
~
,
~
)
A
320
340
360
380
400
420
440
460
400
480
Fig. 5. Average calculated gas temperature inside the cylinder with pressure loop of Fig. 4 temperature (K) versus crank angle.
reactions occur in the compressed mixture, even without ignition. Hydroperoxides Characterization of hydroperoxides in the products was performed with the sample from the flow apparatus. The hydroperoxides from the diesel engine were deduced from a comparison of chromatograms. The products from the flow apparatus have been analyzed by several techniques. These were
(a) Thin layer chromatography (TLC) The presence of hydroperoxides can be deduced from the analogy observed by TLC, between the oxo-heptyl hydroperoxide retention factor and that of products formed in the reactor, the first one and the others giving the same pink coloration with the peroxide reagent N,N dimethyl para-phenylene-diamine [8]. (b) High-Pressure Liquid Chromatography
(HPLC)
9 6 3
Inttantaneous )
0 0.05
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0.2
i
i
i
i
035
0.5
0.65
08
Fig. 4. Pressure loop inside the cylinder without ignition. M e a s u r e m e n t s of instantaneous pressure versus volume inside the combustion chamber. Conditions: stoichiometric richness, air intake temperature 298.2 K, water cooling temperature 332.4 K, temperature increase AT = 6.1 K.
Figure 6 shows a typical analysis by HPLC of products obtained and detected at 210 nm. Peaks A, C, and D, have retention times analogous to those of the heptyl hydroperoxide and the oxo-heptyl hydroperoxide [8]. These peaks show a maximum of the UV absorption at 210 rim. (c) LiquM Chromatography~Mass Spectrome-
try (LC/MS) A Nermag Mass Spectrometer associated with liquid chromatography (same column, eluent composition and flow rate as for the HPLC
212
K. S A H E T C H I A N E T AL.
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.
chromatogram of hydroperoxides obtained
in the flow system; column Lichrosorb, 7/xm, eluent acetonitrile/water 65/35, flow 0.7 mL/mn, time in mn, UV detection at 220 nm. analyses), with a thermospray interface and ammonium acetate as ionizing salt, was used. The double simultaneous detection at 5.30 min. and 7 rain of peaks at m / z 203 and 220 (Fig. 7) on one hand, and at 6.20 min. and 8.20 min at m / z 217-234 (Fig. 8) on the other hand, was noticed. These peaks are interpreted as M H +, [MNH4] + and M ' H +, [M'NH4] + with M = 202, the molecular weight of dodecyl hydroperoxide and M' = 216, the molecular weight of oxododecyl hydroperoxide. The detection of two pairs of peaks in each case is probably due to isomers. Identity of L C / M S and H P L C peaks cannot be entirely assured due to the different modes of elution and modes of detection; in H P L C detection is made at atmospheric pressure and under low pressure in case of L C / M S .
Abundance
Ion
203.00
(202.70
to
H P L C analyses of samples from the Diesel engine show many peaks, but among them peaks A, C and D in Fig. 9 appear clearly. Another peak B, which, unlike A, C and D, exists only with tests on the engine, appears in the same area (see Fig. 9). The results of the probe samples are illustrated in Figs. 10 and 11. Figure 10 (for a distance from the jet axis of 3.4 mm) illustrates the increase and decrease of peroxides with the exhaust gas temperature increase AT, having a maximum at ~ 10 K. Figure 11 shows that the maximum concentrations (for a given exhaust gases temperature increase AT = 6 K) seem to be located on the jet's edge, where the flame appears; this is in agreement with previous rapid cinematography visualization by Fluzin [10], which showed the first appearance of ignition in an area situated between 3 and 6 mm from the jet's axis, depending on the running conditions. The observed coincidence between the maximum of the hydroperoxides and the localization of the onset of ignition at the edge of the fuel jet, leads us to the conclusion that these hydroperoxides play a significant role in this ignition starting.
Formation of Heavy Products After several hours running the engine without any ignition, the unburned dodecane collected in the exhaust is slightly yellow; also there is a
203.70)
3000
2000
i000
Time-->
2.00
Abundance
Ion
4.00
6.00
220.00
219.70
8.00
to
i0.00
220.70)
3000
2000
i000
' ' ' Time-->
I .... 2.00
I 4.00
6.00
8.00
I0.00
Fig. 7. LC/MS analysis of hydroperoxides recoveredfrom the flowsystem; columnLichrosorb Merck RP18, 200 x 4.6 mm, 7 /zm, eluent acetonitrile/water 65/35, ammonium acetate thermospray; dodecylhydroperoxide.
DODECANE IGNITION IN A DIESEL ENGINE Abundance
6
0
Ion
0
217.00(216.70to 217.70) 0 ~
213 IPeroxidesl Arbitrary unit
4000
2000 +
Time--> Abundance
2.00
4.00
Ion
6.00
_~-~\
\
8.00 i0.00
234.00(233.70 tO 234.70) o
e peak D
24000 00060]~I 00 AT(K )
Fig. 10. Peroxides concentration versus temperature in-
i--i, , 4.00 Time--> ....2.00
crease at the exhaust pipe; the sampling probe is located at
6.00
8.00 I0.00
Fig. 8. L C / M S analysis of hydroperoxides recovered from the flow system; column Lichrosorb, 7 ~ m , eluent acetonitrile/water 6 5 / 3 5 , flow 0.7 m L / m n , time in mn, a m m o nium acetate thermospray; oxo-dodecyl hydroperoxide.
3.4 m m from the fuel jet axis.
brown viscous phase, insoluble in dodecane. In the cylinder itself, small amounts of a tar-like, burnt deposit were observed. It is likely that these solid particles make a major contribution to the laser beam attenuation, measured in this engine during the ignition delay [11].
exhaust, and that amount of H 2 rises exponentially when AT is increasing. This is seen in Fig. 12, where the quantity of H 2 (in arbitrary units) is plotted versus AT. The highest value observed, close to ignition, corresponds to about 10 ppm of H 2. It should be noticed that samples collected from the exhaust pipe show hydrogen concentrations lower than those collected through the probe.
Formation of Molecular Hydrogen
MODELING
Gaseous samples taken at the point where the amounts of hydroperoxide were highest, have clearly shown that hydrogen exists as soon as a temperature increase of 3 K is reached at the
Basic Postulates of the Kinetic Model The regime prior to ignition in a diesel engine is characterized by significant heterogeneities
IPeroxidesl
~A
i
Arbitrary unit
I.L
9.46 10.48
B
11.54
13.90
/
, S .,6
A peakD peakC peak
o
Fig. 9. H P L C c h r o m a t o g r a m of hydroperoxides and products from the diesel engine; column Lichrosorb Merck RP18, 200 × 4.6 m m , 7 /~m, eluent acetonitrile/water 65/35, U V detection at 220 nm.
fuel j e t axis
fuel j e t edge
d(mm)
Fig. 11. Peroxides concentration versus distance of the
sampling probe from the fuel jet axis; the temperature increase at the exhaust pipe is AT = 6 K.
214
K. SAHETCHIAN ET AL, ]H21 / / / / / / / / / /
A r b i t r a r y unit
/autoignition / / /
/* o
I
•
5
io
aT(K )
Fig. 12. Concentration of molecular hydrogen versus the temperature increase AT.
regarding the fuel phase (liquid or gas), the fuel mixture (rich or lean), the temperature and the pressure. In the constant volume combustion chamber, the same phenomena arise. At any time and any location in the chamber, the concentration of reactants and radicals, the temperature and pressure, the generated heat and the mass transfer of the chemical species, all depend both on chemical processes and turbulence. A detailed mechanism, including every possible elementary reaction, would be time consuming in a multidimensional computational code and uncertain because several reactions are questionable. The classical models of diesel ignition are based on very simplified chemistry, including one or several global reactions [1, 2, 4, 12, 13]. The reduced mechanism proposed in this work is based on a chemical kinetic model consisting of elementary reactions, emphasizing those preceding ignition, mainly the isomerization of R O 2 radicals. This model, initially developed to predict ignition data in a CFR engine [3], is modified to describe properly the phenomena occurring in the combustion chamber of a diesel engine or an oxidation chamber. Modifications are necessary because of the structure of the fuel and of the heterogeneous nature of diesel combustion. To predict various phenomena in
the combustion process, such as exothermicity and the formation of solid particles, reactions are added, especially global ones. It should be pointed out that these global reactions are not real ones and are introduced to keep a reduced mechanism, even though fuel consumption, solid particle formation and reaction exothermicity are maintained. Ignition delay includes temperature heterogeneities from 500 up to 2000 K or more; therefore, the simulation of this process needs more than one branching and one termination reaction. This is the reason why the proposed model is based on the assumption of three types of branching reactions, each corresponding to a determined temperature range in which a major role is devoted to different species. Table 1 shows the species involved for the different temperature levels. A similar approach has already been taken by Westbrook et al. [14] and Morley [15]. In the previous study [3], the formation of hydroperoxides by isomerization reactions was shown to be important up to 1000 K. When the temperature increases, the importance of these reactions decreases. This could be explained by the following mechanism (see Table 2), in which the R radical can react in different ways: R + 0 2 --~ R O 2 , R+O 2~P+HO
(2) 2,
(11)
R --* decomposition products.
(26)
Reaction 2 and reaction 11 have a low activation energy and are not sensitive to temperature. The R O 2 radical reacts in: R + 02,
(3)
RO 2 --* ROOH'.
(4)
RO 2 ~
TABLE 1 Species Involved in Branching Reactions for Different Temperature Ranges Involved Species
T (K)
RO2, RO2H HO2, H 2 0 2 H
300 < T < 900 700 < T < 1100 800 < T
DODECANE
IGNITION
IN A DIESEL
ENGINE
215
TABLE 2 Chemical Reactions and Kinetic Constants for Dodecane Oxidation (Units: mol, cm, s, K, caD, where R designates an alkyl radical, P an intermediary compound, and N 2 is considered as an inert species No.
Reaction
EA/R
z
ai
Ref.
R H q-
2 3
0 2 -+ R + HO 2 R -k 0 2 - + R O 2 RO 2 ~ R + 0 2
3.16 × 1013 5.64 X 1012 2.50 × 1013
24000 0 13700
0 0 0
1-1 1-1 1
[3] [3] [3, 18]
4 5
R O 2 --+ ROOH" RO 2 + P ~ RO2H + R
7.90 X 1011 2.00 X 1013
8700 5100
0 0
1 1-1
13] [3]
ROOH" ~ R O 2 ROOH" -~ P + O H ROOH" + O 2 ~ O O R O O H OOROOH ~ RO2H + OH RO2H ~ OH + R R + 0 2 ~ P + HO 2 HO 2 + HO 2 ~ H20 2 + 0 2 HO 2 + P ~ H20 2 + R HO 2 + RH ~ H20 2 + R H O 2 + H O 2 + N 2 --* H 2 0 2 + O 2 + N 2 H 2 0 2 + N 2 --~ 2 O H + N 2 O H 4- H 2 --* H 2 0 4- H OH + RH ~ R + H20 2 O H 4- C ~ C O 2 4- H 2 OH + P ~ R + H20 H + 0 2 -~ O H + O H + 0 2 + N 2 ~ HO 2 + N 2 l H + C ~ C + H 2 + gH 2 H + H + N2 ~ H e + N2 H 4- O H 4- N 2 --+ H 2 0 4- N 2 R --+ 12 C + 12 H 2 4- H R + 18 0 2 --+ 12 C O 2 4- 12 H 2 0 + H O + C ~ CO O + R H --+ O H + R O 4- H 2 --~ O H + H C O + H O 2 ~ CO 2 + O H CO + O H -~ CO 2 + H
7.50 × 109
2.45 1.08 1.99 8.00 1.50 4.20 5.03 9.62 7.68 1.50 1.02 3.75 3.00 1.13 1.99 7.70 5.00 3.00 1.40 1.00 6.00 1.30 6.02 5.10 1.51 1.45
5500 15500 0 8500 21000 3000 603 6160 8000 0 22700 1660 435 0 435 8460 0 0 0 0 14700 6500 0 2270 3160 11900 0
0 0 0 0 0 0 0 0 0 0 0 1.6 2 0 2 0 --0.8 0 0 -2 0 0 0 0 2.67 0 0
1 1 1-1 1 1 1-1 2 1-1 1-1 2-1 1-1 1-1 1-1 1-1 1-I 1-1 1-1-1 1-1 1-1-1 1-1-1 1 1-0 1-1 1-1 1-1 1-1 1-1
1
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
An increase
of temperature
of the importance i.e.,
A
reaction
3.
ROOH"
radicals
ROOH"
~
entails an increase
of the inverse Reactions
of reaction
introduced
N 1014
X × X × N x × x X × N
1012 10 It 1015 1011 1014
1013 1013 1012 1017 108
107 × 1012
× 108 × 1014 × 1017
× × )< ×
101° 1015 1023 1013
× 106
× 1011 x 10 u × 104
× 1014 x 10 u
branching
follows
2, for
are
UO 2 ÷ RH H20
(6)
R O 2,
--* H 2 0
2 + M ~
above
2 + R,
(14)
+ M,
(16)
2OH
700 K, where
For high pressures, ROOH"
---, P + O H ,
ROOH"
+
Reactions ture.
0
2 --+
OOROOH.
in the
engine
the
increase
M is a n i n e r t s p e c i e s . the reaction
H + 0 2 --*
(7)
OH
(8)
8 0 0 K . A t 8 0 0 K a n d a p r e s s u r e o f 15 b a r , t h e r a t e o f t h e b r a n c h i n g r e a c t i o n 21 is a b o u t 1 %
6 and 7 are likelier at high tempera-
However,
[3] [3] [3] [3] [3] [3] [3] [3] [3] [3] [3] [19] this work this work [19] [19] [19] this work [20] [19] this work this work this work [21] [19] [22] [23]
of
÷ O ( 2 1 ) is c o n s i d e r e d
to play a part above
of that of HO 2 formation in reaction 22. During the delay, the importance o f r e a c t i o n 21 increases
with temperature.
Under
these
con-
pressure entails larger O2 concentrations, which partially offset the effect of tempera-
ditions,
ture.
delay, reactions are assumed to be so slow, that t h e p r o c e s s is g o v e r n e d by chemistry, turbu-
ROOH
° is t h u s
t i o n s 11 a n d 2 6 b e c o m e
stabilized. more
When
reac-
important,
chain
tion
we consider
21 is n o t
that
negligible.
the
branching
During
the
reacignition
216 lence being created only by disturbance due to the spray. It has been observed that, in the diesel engine, ignition first occurs at the spray's periphery [16, 17]. After ignition, the difficult problem concerning the simultaneous representation of turbulence and chemistry still remains. Chemical Mechanism
The proposed mechanism reproducing ignition in an oxidation chamber is described with 32 reactions, whose rates are given by the generic Arrhenius equation:
where A is the rate constant's preexponential factor, C i is a concentration, E A the activation energy, R the universal gas constant, T the temperature, cei is a partial order, generally, the stoichiometric coefficient, and z a constant. In Table 2, all the chemical reactions are given with their kinetic parameters. In this mechanism, the first 16 reactions come from the previous mechanism of ignition [3]. These reactions are sufficient to describe oxidation during the ignition delay of an homogeneous mixture. They contain R O 2 and H O 2 radicals, as well as peroxides. Their rate constants are derived considering the length of the linear molecule, especially for reactions 2, 4-11, and 14. For diesel conditions, the mixture is heterogeneous and other reactions (17-32) have been introduced to reproduce the evolution of different variables. The heavy products experimentally observed probably contain atoms of C, H and O. To simplify all solid species containing C, H and O or C and H or only C atoms these are all symbolized by C. Likewise molecular H 2 is supposed to be created at the same time as solid C, whereas during the ignition delay H 2 is probably formed through R radicals reacting to form alcenes. These heavy products have been introduced to reproduce their possible effect on the ignition delay, which can interfere with the effect of the peroxides. This is the reason why kinetic parameters have been estimated for reactions 19, 23, 26, and 27.
K. S A H E T C H I A N E T AL. In the mechanism previously proposed [3], the following transformation of R radicals into smaller non isomerizable alkyl radicals is a termination reaction: R --, R' + P.
(a)
In this work, a higher hydrocarbon fuel is used and reaction a is not considered as a termination step, R' being still isomerizable, because of the number of carbon atoms. Furthermore, the observation of heavy products leads us to introduce a termination reaction between a radical and solid C. Several studies have been made of reactions between soot and H, O H [24, 25, 26] or O [24, 25, 27]. These reactions are considered here to be heterogeneous. The oxidation of C by O atoms has a rate constant estimated from previous work [24, 25, 27] to have a mean reaction probability of 0.23. The oxidation of long hydrocarbon chains is very complex, because of the variety of products, so that the global reactions 19 and 26-28 are introduced to the mechanism. Reaction 19 is also an heterogeneous termination step. Some reactions are not elementary processes. Two global reactions explain hydrocarbon consumption, viz 26 and 27. The global reaction 26 is the degradation without oxidation of the hydrocarbon into species noted as C atoms and molecular hydrogen, whereas in reaction 27, the whole oxidation is introduced, with formation of H 2 0 and CO 2. This global reaction 27 is highly exothermic and does not lead to an increase in the active centers, the H radical taking the place of the R radical. The formation and oxidation of particles in diesel combustion are very complex. To describe them, a simplification is introduced by means of C atoms, the mass of particles being equivalent to the global mass of C atoms. These C atoms appear in two types of reaction: the oxidation reactions 19 and 28 with CO formation and the heterogeneous termination reactions 19, 23, and 28. Concerning hydrogen, reactions 17, 21, 22, 24, 25, and 30 are branching or termination reactions. The oxidation of CO appears in reactions 31 and 32. In order to limit the number of species and reactions, a few simplifications have been introduced. In
D O D E C A N E I G N I T I O N IN A D I E S E L E N G I N E reaction 9, the hydroperoxide R O 2 H appears instead of a ketohydroperoxide O R O 2 H. This assumption was made because the two decomposition rates are very close and radicals appear in these reactions. It is validated by R R K M studies [28]. In reaction 10, R corresponds to the RO radical which is assumed to be decomposed [29]. In reactions 5, 13, and 20, the radical created from the intermediate compound is assumed to be R. The global reaction 27 is first order with respect to the radical R and zeroth order with respect to oxygen. Reaction 19 is first order with respect to O H and C. Computational Code
The numerical three-dimensional, time-dependent, code KIVA-II [30] uses the Arbitrary Lagrangian Eulerian method through a finite volume grid to solve the conservation equations for the fluids. KIVA was first developed to describe combustion in a reciprocating engine, so this code is designed to accept chemical kinetic, multistep, models in a constant volume combustion chamber. The computation is pseudo-3-D. The calculation is not stochastic, but the distribution of the droplets at the output of the injector is. The spray is assumed to be composed of several "particles," each of them being formed by a set of droplets which have the same physical and chemical properties. KIVA lI incorporates break-up, collisioncoalescence and evaporation of the droplets. The turbulent effects due to the fuel spray are taken into account, and the turbulent gas flow is computed with the k-e compressible model. The thermodynamical data of the unknown species, especially all radicals, are added to the inner program. They were calculated by means of the NIST data base [31]. For the axisymmettic geometry of the combustion chamber, the
217
numerical solution of the set of equations is computed only for a fraction of the chamber, defined by an angular sector of 0.5 degree (one cell), its radius and height being equal to those of the combustion chamber (respectively 20 and 100 cells). As pointed out above, during the ignition delay, the chemical time scale is assumed to be small enough for the density fluctuations for the species to be neglected, when modeling the mean reaction rate [32]. However, once ignition itself is achieved, the rates of change of temperature and chemical rates increase strongly. Thus, turbulent combustion takes place; but such a model does not exist in the original KIVA II and must be developed. Thus, even if the chemical time scale is small due to the high pressure, we make the assumption that it remains large compared to the turbulence time scale, i.e., the Damk6hler number is small [32]. It is worth noting that for some years, the flamelet concept [33, 34] seems to be a powerful tool to describe the interaction between chemistry and turbulence. However for diesel engines, few models are available and taking into account more than one Arrhenius equation in the existing modeling [16, 32] does not appear to be easy. C O M P U T E D RESULTS AND DISCUSSION Validation
Five experiments in the oxidation chamber were chosen, as explained above to validate the mechanism (see Table 3), in such a way that the calculation would lead to a better understanding of the combustion process. The experimental delay of each test (No. 1 to 5) is an average value obtained for the same operating conditions (temperature and pressure). The rate constants in the mechanism have been
TABLE 3
Comparison Between the Values of Ignition Delay Measured in the Bomb and Computed by KIVA with the Kinetic Model (Each Experimental Datum Is an Average Value) Experiment Initial pressure (bar) Initial temperature (K) Calculated delay (ms) Mean measured delay (ms)
1 15 719 3.3 3.5
2 15 741 2.3 2.6
3 20 715 3.4 3.2
4 20 740 2.4 2.2
5 25 760 1.7 1.5
218 chosen to simulate ignition with agreement on average for the five experiments. Table 3 presents a typical comparison of the measured and computed delays for different temperatures and pressures. The chemical kinetic model reproduces experimental values with good agreement. Indeed, the average of the five relative errors is smaller than 8% for each experiment, but the relative error is of the same order as the measurement error. Before fuel injection the fluid is at rest and the temperature field is homogeneous. The delays, validated by pressure measurements, give our mechanism complete coherence. However, it should be noted that self-ignition delay results from a physical delay, including mechanical and thermal phenomena (atomization, heating, evaporation... ) described by KIVA II and a chemical delay, described by the kinetics. The spray atomization model was validated using measurements on a PDA (Particle Dynamic Analyser) [35]. Taking into account this result, the good agreement between experimental and calculated total delays shows that the chemical mechanism suitably describes the real process.
K. SAHETCHIAN ET AL. t,0 Normalized mass fractions & temperature
0,8"
!
! 0,6'
0,4'
I .....
RO2H
.........
H202
- -
H Temperature
*
'
/
0,2
$
V
] -
.," ..... ,..'j
0,0-: :.: -" "-
,
.-'
time (ms)
.
.
I
2
(a)
+/
1,0
Normalized mass fractions & temperature
l'i,
i 4i
0,8
i
O,6 .....
RO2
.........
HO2 OH
0,4
3
i |
io
!l s
;
• ~
r,
Temperature
•
0,2 j ..,-__'~....... } 0,0
:
:
.m
m
time (ms)
t
(b) Fig. 13. Calculated time-resolved evolution of temperature and mass fractions of: (a): ROmH, H202, H; (b): ROm, HOm, OH; Tmax = 930 K. (Experiment No. 5:P0 = 25 bar; TO = 760 K). The normalized temperature is defined by T * = ( T - T o ) / ( T m a x - T o ) , where T O is the initial temperature and T the mean instantaneous thermodynamical temperature in the chamber defined by:
Simulation Results T =
Figure 13 reproduces the time-resolved evolution of the mean temperature and species concentration in the whole bomb volume for experiment No. 5. During the delay, an increase in the concentration of RO 2 and R O z H is observed while the temperature increase is still insignificant; then the hydroperoxide concentration increases rapidly and reaches a maximum at a time of 1.70 ms. Afterwards, the concentrations of RO 2 and R O z H decrease continuously but are still important. At 1.75 ms, maxima in the concentrations of H O 2 and H 2 0 2 appear, followed by a fall within the flame, while the mean temperature and amounts of H and OH both rise. The histories of T and [OH] are identical. The two-dimensional representation of the results in Fig. 14 shows that these radicals and peroxides are concentrated near the spray's periphery. In this region, a particular point exists where heat generation is particularly important. This "hot spot" corresponds to the
Tin i \i:1
ni, ]/i=l
where T/ and n i are, respectively, the temperature and the number of moles in the cell i, and N the total number of cells in the chamber.
highest radical accumulation and to the start of ignition. Figure 15 shows the time-resolved evolution of this "hot spot." Before ignition, a temperature rise AT of about 100-150 degrees can be locally observed. This can be linked with experimental results obtained in the motored diesel engine without autoignition, showing a mean temperature increase of about 12 degrees in the exhaust gases for the highest air intake temperature. This region, changing from one simulation to another, corresponds approximatively to the radial and axial limits of the liquid spray. It shows good agreement with high-speed cinematography of the reactive spray, presently under development. The first pressure increase has been well predicted and
DODECANE
IGNITION IN A DIESEL ENGINE
Fresh Air (T0=760,0K)
AutoignitionPoint(Tmax=900,5K)
,
Nozzle
Two-PhaseMedium(Train=486,7K)
r e d u c e d c h e m i c a l kinetic m e c h a n i s m o f 32 reactions is p r o p o s e d to i n t e r p r e t t h e o b s e r v a tions. T h e c h e m i c a l m e c h a n i s m correctly p r e dicts ignition delay, for d i f f e r e n t t e m p e r a t u r e s a n d pressures. T e m p o r a l e v o l u t i o n s o f the conc e n t r a t i o n s o f d i f f e r e n t species w e r e o b t a i n e d . A v e r a g e t i m e - r e s o l v e d values show m a x i m u m c o n c e n t r a t i o n s for R O 2 H , t h e n for H 2 0 2 a n d the H atom. A rapid increase of OH concent r a t i o n a n d t e m p e r a t u r e c o r r e s p o n d s to the a p p e a r a n c e o f ignition. T h e m o d e l shows t h a t c h e m i c a l r e a c t i o n s o c c u r d u r i n g t h e ignition d e l a y at t h e b o u n d a r y o f the spray. T h e s e t r a n s f o r m a t i o n s could b e t h e s o u r c e o f p o l l u t a n t s likely to play a p a r t in the c o m b u s t i o n process.
[
Fig. 14. Calculated isotherms in the chamber at 1.70 ms. This instant corresponds exactly to the numerical delay. t h e spatial d i s t r i b u t i o n o f the species confirms that the flame, in the first stage, is p r e m i x e d in n a t u r e . It p r o p a g a t e s in a r e g i o n w h e r e mixing o f fuel a n d o x i d i z e r b e c o m e s b e t t e r , i.e., in the o u t l i n e o f t h e g a s e o u s spray, T h e t h r e e - d i m e n sional h i s t o r i e s p r e s e n t several m a x i m a at the s a m e time. CONCLUSION In this work, e x p e r i m e n t s w e r e p e r f o r m e d on the ignition o f a d o d e c a n e s p r a y u n d e r conditions close to t h o s e existing in a diesel engine. Significant c h e m i c a l c h a n g e s have b e e n shown to o c c u r d u r i n g the ignition delay. M e n t i o n was especially m a d e o f t h e significant h e a t release, a t e m p e r a t u r e i n c r e a s e o f the exhaust gases a n d t h e f o r m a t i o n o f h y d r o p e r o x i d e s . A
"2200 Hottesttemperaturein the chamber(K) 1900I600-
219
/
/
Aut°igniti°idela~
1300" 1000" , AT 700 . . . . . . . . . ~. . . . 1,0 1,1 1.2 1,3 1,4 1,5 1,6
REFERENCES 1. Westbrook, C. K., and Dryer, F. L., Prog. Ener. Combust. Sci. 1:1 (1984). 2. Zellat, M., Rolland, Th., and Poplow, F., SAE Paper 900254 (1990). 3. Blin-Simiand, N., Rigny, R., Viossat, V., Circan, S., and Sahetchian, K., Combust. Sci. Technol. 88:329 (1993). 4. Kong, S. C., and Reitz, R. D., ASME 93, ICE 22, 6. 5. Blin-Simiand, N., Rigny-Gautrand, R., Sahetchian, K., and Brun, M., Entropie 17:115 (1993). 6. Aligrot C., Th~se de Doctorat, Ecole Centrale de Lyon (1994). 7. Sahetchian, K., Rigny, R., and Blin, N., Combust. Sci. Technol. 60:117 (1988). 8. Sahetchian, K., Rigny, R., and Circan, S., Combust. Flame 85:511 (1991). 9. Sahetchian, K., Heiss, A., and Rigny, R., Can. J. Chem. 60:2896 (1982). 10. Fluzin, G., Th~se de doctorat, Ecole Centrale de Lyon (1990). 11. Ayachi, N., Laurent, P., and Brun, M., Entropie 16l:45 (1991). 12. Theobald, M. A., Ph.D. thesis, Department of Mechanical Engineering, M.I.T. 1986. 13. Miiller, U. C., Peters, N., and Linan, A., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, p. 777. 14. Westbrook, C. K., and Pitz, W. J., SAE Paper SAlE 872107 (1987). 15. Morley, C., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p. 911. 16. Baritaud, T., Henriot, S., N6raud, E., and Veynante, D., Colloque du Groupement Scientifique Moteur,
1,7
time (ms) 1.8 1.9 2,0
Fig. 15. Time-resolved evolution of the "hot spot" (Experiment No. 5:P0 = 25 bar; To = 760 K).
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Received 20 June 1994; accepted 20 March 1995.