Journal of Loss Prevention in the Process

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ica

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ario

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ignition, possible explanations for the phenomena observed experimentally have been suggested. The results of the study indicate that

established to study ammability of fuel-rich n-buta-

(2005) observed the inuence of the forced ignition delaytime (IDT) on the explosion pressure ratio and maximumrate of pressure rise. The explosion pressure ratio was

Starting from about 1220 s, the test mixture self-ignited.

termination. Thus, in the experiments of Pekalski et al.(2005), several phenomena occurred simultaneously. Theseare mixing, forced ignition followed by ame propagation,

ARTICLE IN PRESSpreame oxidation, heat transfer, and natural convection.To understand the relationship between characteristic

times and scales of the phenomena, multidimensional CFD

0950-4230/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jlp.2007.03.008

Corresponding author. Tel.: +7095 9397228.E-mail address: smfrol@online.ru (S.M. Frolov).neoxygen mixtures at elevated temperatures and pres-sures. A presumably nonammable n-butaneoxygenmixture with the equivalence ratio of 23 was prepared byfast injection of oxygen from the heated and pressurizedcanister to the heated explosion vessel lled with n-butane.The temperature of the setup was kept at a constant valueof 500K. The pressure in the vessel after completion ofoxygen injection was 4.1 bar. The mixture was ignited inthe vessel center with different delay times after completionof oxygen injection. Using this method, Pekalski et al.

Preliminary multidimensional computational uid dy-namics (CFD) simulations of the mixing process of high-and low-pressure air in the 20-l vessel have been performedby Smetanyuk, Skripnik, Frolov, and Pasman (2005) andFrolov, Basevich, Smetanyuk, Belyaev, and Pasman(2006). It has been shown that the thermochemicalconditions at the ignition site in the experiments ofPekalski et al. (2005) could be affected in the long run bythermal stratication of the matter in the vessel and by theresidual oxygen coming from the manifold after injectionr 2007 Elsevier Ltd. All rights reserved.

Keywords: Standard 20-l explosion vessel; Ignition; Autoignition; Butaneoxygen mixture

1. Introduction

Physical and chemical processes in a standard spherical20-l explosion vessel are very complicated. In the experi-ments of Pekalski, Terli, Zevenbergen, Lemkowitz, andPasman (2005), a new mixture preparation method was

shown to increase from 1.3 at an IDT of 20 s to a maximumof 1.6 at an IDT of 40 s. After the maximum, the explosionpressure ratio dropped with increasing the IDT, reached aminimum of 1.27 at an IDT of 360 s and rose again to asecond maximum of 1.4 at an IDT of 1020 s. The maximumrate of pressure rise roughly followed a similar trend.seemingly inammable mixtures can become hazardous depending on the mixture preparation procedure and forced ignition timing.Modeling of n-butane ignition, coin the 2

S.M. Frolova,, V.Ya. Basevicha, V.A. SaSemenov Institute of ChembDelft University of Techn

Received 26 November 2006; received in revise

Abstract

The objective of the study reported herein is to simulate v

n-butaneoxygen mixture preparation, ignition, preame oxidation

mathematical models. Based on the computational uid dynamics (

ignition kernel, as well as on the analysis of the detailed reaction mIndustries 20 (2007) 562569

bustion, and preame oxidationl vessel

etanyuka, A.A. Belyaeva, H.J. Pasmanb

l Physics, Moscow, Russia

y, Delft, The Netherlands

orm 10 March 2007; accepted 11 March 2007

us physical and chemical phenomena accompanying fuel-rich

nd combustion in the standard 20-l explosion vessel, by applying

D) simulations of the mixing process and natural convection of the

anism of n-butane oxidation, laminar ame propagation, and self-

www.elsevier.com/locate/jlp

simulations of mixture preparation in the vessel, ignition,and ame kernel buoyancy have been performed togetherwith detailed calculations of ame velocity and structure inthe fuel-rich n-butaneoxygen mixtures at elevated tem-perature (500K) and pressure (4.1 bar), relevant to theexperiments of Pekalski et al. (2005).

2. Thermodynamics

The test mixture of Pekalski et al. (2005) contained 78%C4H10 and 22% O2, which corresponded to the equivalenceratio of F 23, i.e., it was extremely fuel rich. The meanmolecular mass of the test mixture was 52.3767 kg/kmol.Tables 1 and 2 show the results of thermodynamic

suspended and deposited soot particles, buoyancy effects,and heat loss. The effects produced by these imperfectionsare discussed below.

3. Mixture preparation

To understand the mixing dynamics of oxygen andn-butane in the 20-l explosion vessel, 2D RANS-basedsimulations were performed using AVL SWIFT code(2002). In the calculations, the ow dynamics in theoxygen canister, in the manifold connecting the canisterand the vessel, and in the vessel itself were simulated. Aftervalve closing, only the dynamics of the ow in the vesseland in the manifold section downstream from the valve was

ARTICLE IN PRESS

ies

Gas

H2

32.1

ies mole fractions at constant-volume combustion (UV-problem) of the test

Gas

H2

S.M. Frolov et al. / Journal of Loss Prevention in the Process Industries 20 (2007) 562569 563Table 2

Thermodynamic temperature, pressure, density, molecular mass, and spec

mixture at p0 4.1 bar and T0 500K

T (K) p (bar) r (kg/m3) m (kg/kmol)calculations for the test mixture at p0 4.1 bar andT0 500K using the thermochemical code TDS ofVictorov (2002). Presented in the tables are the thermo-dynamic temperature T, density r, pressure p, molecularmass m, and species mole fractions at constant-pressure(HP-problem) and constant-volume (UV-problem) com-bustion of the test mixture at p0 4.1 bar and T0 500K.According to Table 1, if the test mixture would be

ammable, the combustion temperature in the constant-pressure ame could be equal to 911.63K. According toTable 2, constant-volume combustion of the test mixturewould result in the maximal thermodynamic temperatureand pressure equal to 1049.42K and 25.47 bar, respec-tively, i.e., the thermodynamic pressure ratio could bepmax/p0 6.21. The equilibrium combustion productscould contain much (about 39%) soot and 61% gaseousspecies, most of which are hydrogen (about 30%) andmethane (about 22%). The maximal pressure ratiosmeasured by Pekalski et al. (2005) were considerably less(1.6) than the thermodynamic pressure ratio (6.21), whichwas the indication of incomplete oxidation of the testmixture. Among possible reasons of incomplete oxidationcould be imperfect mixing of n-butane with oxygen andame quenching caused by radical recombination on

Table 1

Thermodynamic temperature, pressure, density, molecular mass, and spec

mixture at p0 4.1 bar and T0 500K

T (K) P (bar) r (kg/m3) m (kg/kmol)

911.63 4.1 0.9508 10.3611049.42 25.47 5.2341 10.792 30.1eous species Soot

CH4 H2O CO CO2considered with due regard for the gravity force effects.The geometrical dimensions of the 20-l vessel, 0.6 l oxygencanister and connecting manifolds were taken close to thedesign drawings. Contrary to similar CFD simulations ofFrolov et al. (2006) without massive ignition electrodes, theignition electrodes were added to the computational modelreported herein. Three massive ignition electrodes, pro-truding from the upper ange of the vessel to the vesselcenter, were modeled by a hollow cylinder and anembodied coaxial cylindrical rod. All rigid walls of thecomputational volume were kept at constant temperatureof 500K, like in the experiments of Pekalski et al. (2005).The initial temperatures of oxygen and n-butane were alsotaken equal to 500K to t the experimental conditions ofPekalski et al. (2005). The turbulence model used in thesimulations was the standard ke model, which was alsoused for modeling species and energy transport inBoussinesq approximation via effective Schmidt andPrandtl numbers. The duration of oxygen injection wasvaried from 10 to 12ms depending on the requiredexperimental value of the nal pressure of the mixture inthe 20-l vessel.It was shown by Frolov et al. (2006) that oxygen

injection in the experiments of Pekalski et al. (2005) could

mole fractions at constant-pressure combustion (HP-problem) of the test

eous species Soot

CH4 H2O CO CO2

6 19.43 6.12 1.42 0.58 40.281 22.31 5.61 2.40 0.53 39.03

lead to a considerable mixture stratication in terms ofvelocity, temperature and composition. The starting shockwave caused by a high initial pressure ratio (21 bar/0.4 bar 52.5) between the canister and the vessel wasshown to be rather strong and resulted in high local owvelocities, exceeding 700m/s in the jet core and attaining400m/s in the near-wall regions of the vessel.Fig. 1 compares the predicted turbulent velocity in the

vessel center with the root-mean-squared values of twopulsating velocity components measured by Pekalski(2004) using laser doppler anemometry with the computa-tional results of Frolov et al. (2006). Note that in theliterature there are other measurements of turbulence decayin the 20-l vessel, e.g. Dahoe, Cant, and Scarlett (2001). Itis seen that at t40.1 s, the predicted and measured resultscorrelate reasonably well with each other, and theturbulence in the experiments of Pekalski (2004) andPekalski et al. (2005) was isotropic at least at t40.04 s. Atthe initial injection stage, a large discrepancy between the

However, shortly after injection termination, the oxygenmass fraction and temperature attained the values of 0.117and 500K close to the corresponding mean values.Nevertheless, starting from t 34 s, one can observe thedeection of the local oxygen mass fraction from the meanvalue in Fig. 2a (shown by arrow). The growth of theoxygen mass fraction in the vessel center continued tillt 5 s, when it attained a value of 0.14, and then turned todecrease gradually to the mean value of 0.117 at aboutt 100 s.The present results appeared to be somewhat different

from the computational results of Frolov et al. (2006)without electrodes. Fig. 3 compares the present results withthe calculations of Frolov et al. (2006) in terms of thepredicted oxygen mass fraction and temperature in thevessel center. Clearly, the values of oxygen mass fractionand temperature at the ignition site during oxygen injectiondiffer considerably. However, after injection termination,the effect of electrodes is not much pronounced.The reason for the elevated values of the oxygen mass

fraction in the vessel center at 3oto100 s is evident fromFig. 4 showing the calculated distributions of the oxygen

ARTICLE IN PRESS

Fig. 2. Calculated time histories of (a) oxygen mass fraction and (b)

temperature in the 20-l vessel. Solid curves correspond to local

instantaneous values at the ignition site (vessel center). Dashed curves

correspond to mean values. Initial oxygen pressure in the canister is

29.67 bar. Initial n-butane pressure in the vessel is 3.2 bar. Injection

duration is 0.01 s.

S.M. Frolov et al. / Journal of Loss Prevention564predicted and measured data is observed, which could beexplained by the failure of the standard ke model tosimulate turbulence decay in the high-speed compressibleow. The data relevant to Fig. 1 show that the ow in thevessel can be treated as laminar at a time exceedingconsiderably 1 s.Solid curves in Fig. 2 show the predicted time histories of

oxygen mass fraction (Fig. 2a) and temperature (Fig. 2b) inthe vessel center (ignition site) up to t 100 s. For the sakeof comparison, the time histories of mean oxygen massfraction and mean temperature in the vessel are shown bythe dashed curves in the same gures. In the computationalruns relevant to Fig. 2, the oxygen injection periodterminated at about t 0.01 s. During injection, theoxygen mass fraction and temperature at the ignition siteattained the values of 0.87 and 410K, respectively.

0.01 0.1 1 10

0.1

1

10

without electrodes

with electrodes

u`

.m/s

t.s

Fig. 1. Comparison of predicted and measured (Pekalski et al., 2005)

root-mean-squared velocity uctuations u0 in the vessel center. Initialoxygen pressure is 21 bar. Initial n-butane pressure is 0.4 bar. Injectionduration is 0.0117 s. The solid line corresponds to the calculations of

Frolov et al. (2006). The dashed curve corresponds to present calculations.in the Process Industries 20 (2007) 562569mass fraction (left hemisphere) and temperature (righthemisphere) at time (a) 3; (b) 10; (c) 20; and (d) 100 s after

ARTICLE IN PRESSionS.M. Frolov et al. / Journal of Loss Preventoxygen injection termination. It is seen that at t 3 s themixing in the bulk of the vessel had been alreadycompleted. However, at this time there was still a portionof oxygen stagnated in the manifold at the instant of valveclosing. This residual oxygen spread slowly along theinjection axis. At time of about 3 s, the mass fraction ofoxygen started to increase in the vessel center due to arrivalof the residual oxygen from the manifold. Moreover,contrary to the calculations of Frolov et al. (2006) withoutelectrodes, the present calculations indicated that someinjected oxygen was accumulated in the space between theelectrodes. It is seen from Fig. 4 that even at t 100 s, theoxygen mass fraction in the space between the electrodeswas considerably higher than the mean value. Of course,the coaxial electrodes in the present calculations served as arough approximation of the complex 3D conguration ofthe realistic electrodes in the 20-l vessel. However, theadditional calculations made with ring-shaped electrodes

assuming adiabatic walls and neglecting radiation heat

Fig. 3. Calculated time histories of oxygen mass fraction (a) and

temperature (b) in the center of the 20-l vessel. Solid curves correspond

to the present calculations taking the effect of ignition electrodes into

account. Dashed curves correspond to the calculations of Frolov et al.

(2006) without electrodes. Initial oxygen pressure in the canister was

29.67 bar. Initial n-butane pressure in the vessel was 3.2 bar. Injection

duration was 0.01 s.loss. It is seen from Fig. 5a that the test mixture withF 23 exhibited a laminar burning velocity un of 3.4 cm/sat T0 500K and p0 4.1 bar, which was by two orders ofmagnitude less than the value of un for the stoichiometricn-butaneoxygen mixture at similar initial conditions(458 cm/s).Fig. 5b shows the calculated temperature prole in the

laminar ame at F 23. The temperature in the ameincreased from T0 500K to about 896K. The predictedame temperature was somewhat less than the thermo-dynamic value of 911K (see Table 1). The arisingdiscrepancy was evidently caused by the lack of thermo-dynamic equilibrium at short distances behind the amefront shown in Fig. 5b.The results of calculations presented in this sectionallowing oxygen to escape through the gaps have alsodemonstrated the effect of oxygen accumulation in thespace between the electrodes, presumably due to effectivestagnation of the oxygen jet in this blocked ow region.These ndings are very important for understanding theresults of the experiments of Pekalski et al. (2005) withforced ignition of excessively fuel rich n-butaneoxygenmixture.Thus, the CFD simulation of the mixing process in the

20-l vessel provided a possible explanation for the localmaximum of explosion pressure at about t 40 s. Thiscould be a result of ignition of a less fuel-rich (andtherefore more reactive) n-butaneoxygen mixture than thetest mixture with F 23.

4. Laminar ame propagation in test mixture

To estimate the reactivity of the test mixture in termsof the laminar burning velocity and the self-ignitiondelay, a detailed reaction mechanism of n-butane oxida-tion developed and validated recently by Basevich,Belayev, and Frolov (2007) was used. The mechanismcontained 288 reversible elementary reactions and 54species and was capable of simulating reasonably wellboth low- and high-temperature self-ignition of n-butane.At low temperatures and high pressures, this mechanismprovided the parametric domain with the negativetemperature coefcient of the reaction rate correlatingwell with the experimental data of Minetti and Sochet(1994). It is worth mentioning, however, that thismechanism did not include reactions responsible for sootformation.Fig. 5a shows the predicted laminar burning velocity in

the n-butaneoxygen mixture as a function of theequivalence ratio F under initial pressure and temperatureconditions relevant to the experiments of Pekalski et al.(2005). The laminar burning velocity was calculated usingthe laminar ame code of Belyaev and Posvianskii (1985)

in the Process Industries 20 (2007) 562569 565indicate that the test mixture in the experiments of Pekalskiet al. (2005) was ammable indeed.

ARTICLE IN PRESSonS.M. Frolov et al. / Journal of Loss Preventi5665. Self-ignition of test mixture

As follows from Tables 1 and 2 oxidation andcombustion of the test mixture should be accompaniedby considerable soot formation. Under the extremely fuel-rich experimental conditions of Pekalski et al. (2005), thecontribution of chain termination reactions at sootparticles deposited on the vessel walls and suspended inthe mixture could be signicant. In the experiments ofPekalski et al. (2005), the existence of soot deposits on thevessel walls could be caused by incomplete air-blastremoval of particles formed in previous tests. As for theorigin of suspended soot particles in the vessel volume, theycould be blown in the volume from the vessel walls by astrong starting shock wave (see Section 2), or could form inthe mixture during the induction period according to a low-temperature mechanism of soot formation (Bohm, Hesse,et al., 1988).To model test mixture self-ignition at the conditions

relevant to the experiments of Pekalski et al. (2005), thedetailed reaction mechanism of n-butane oxidation(Basevich, Belayev, & Frolov, 2007) was used. Due to thelack of reliable information on kinetics of low-temperatureheterogeneous reactions on the surface of soot particles, it

Fig. 4. Calculated isolines of oxygen mass fraction (left hemisphere) and temp

oxygen injection termination. Initial oxygen pressure was 29.67 bar. Initial n-bin the Process Industries 20 (2007) 562569was assumed that the most important chain terminationreactions were represented by two overall, quasi-volu-metric, monomolecular reactions (Kondratiev & Nikitin,1974):

C4H9O2H! C4H10 O2 (1)

H2O2 ! H2O 0:5O2 (2)The effective activation energies E1 and E2 of reactions

(1) and (2) were taken zero, while the correspondingpreexponential factors were taken equal to k1 k2 k 80 s1. Fig. 6a shows the results of calculations for thetest mixture. The heat transfer coefcient in the calcula-tions was taken equal to 0.615W/m2K. The arrow showsthe experimentally measured self-ignition delays byPekalski et al. (2005).It is seen from Fig. 6a that the predicted and measured

ignition delays correlate well with each other. Note that theassumed value of k 80 s1 seemed quite reasonable forthe problem under study. Note also that the increase of theheat transfer coefcient by the order of magnitude did notexert a signicant effect on the computational results.The calculations revealed the existence of a cool-ame

stage during test mixture oxidation. The temperature curve

erature (right hemisphere) at time (a) 3 s; (b) 10; (c) 20; and (d) 100 s after

utane pressure was 3.2 bar. Injection duration was 0.01 s.

ARTICLE IN PRESSionS.M. Frolov et al. / Journal of Loss Preventin Fig. 6a exhibits a staged behavior with the rst stageattributed to cool ame and the second to hot explosion.Fig. 6b shows the corresponding time histories of butylhydroperoxide mole fraction in the test mixture, whichexhibits two stages of peroxide accumulation and decom-position at 8001100 s and at 12001340 s, correspondingto the cool ame and hot explosion stages.It is worth noting that the origin of cool ame (tE1000 s)

coincided well with the second maximum of the explosionpressure in the experiments of Pekalski et al. (2005). At thecool-ame stage of hydrocarbon oxidation, the mixturereactivity is known to increase considerably (Sokolik,1960). Therefore, the second maximum at the explosionpressure curve could be explained by faster amepropagation in the test mixture at its ignition during thecool-ame conversion.

6. Effect of buoyancy

Slow burning mixtures are known to exhibit a stronginuence of buoyancy effects (Sokolik, 1960). Fig. 7 showsthe results of CFD simulations of upward natural convection

Fig. 5. (a) Predicted laminar burning velocities of n-C4H10O2 mixtures as

a function of equivalence ratio F at T0 500K and p0 4.1 bar; (b)calculated temperature prole in the n-C4H10O2 laminar ame at F 23at T0 500K and p0 4.1 bar.in the Process Industries 20 (2007) 562569 567of the hot ignition kernel in the gravity eld. In this example,the mixture was ignited 10 s after termination of oxygeninjection. These calculations were aimed at estimating thecharacteristic times taken for the kernel to reach the upperwall of the vessel and to cool down to the wall temperature.Combustion reactions in these calculations were notactivated for the sake of simplicity.It follows from Fig. 7 that the entire process of ame

kernel dissipation took 12 s. If one takes into account thatin the actual experiments of Pekalski et al. (2005) the amekernel moved upward along the massive electrodes, nowonder that combustion was incomplete and the maximalexplosion pressure did not exceed a value of about 1.6instead of the thermodynamic value of 6.21. Nevertheless,the present CFD simulations indicated that the ame kernelmoved upwards in the region with the elevated local oxygenconcentration as compared to its mean value in the vessel.

7. Discussion and conclusions

Experimental studies by Pekalski et al. (2005) ofconstant-volume combustion of the extremely fuel-rich

Fig. 6. (a) Predicted temperature history in the 78% C4H10 +22% O2 test

mixture in the course of self-ignition at T0 500K and p0 4.1 bar(arrow shows the experimental self-ignition delay); (b) predicted time

history of butyl hydroperoxide mole fraction in the test mixture.

ARTICLE IN PRESSonS.M. Frolov et al. / Journal of Loss Preventi56878% C4H10+22% O2 mixture (an equivalence ratio of 23)in the standard 20-l vessel at elevated initial temperature(500K) and pressure (4.1 bar) resulted in some unexpectedphenomena, which needed explanation. Contrary to astandard testing procedure, Pekalski et al. (2005) prepareda fueloxygen mixture by fast injection of oxygen from thepressurized canister to the explosion vessel lled with fuel.The mixture was ignited in the vessel center after differentdelay times (from several seconds to 1400 s) after comple-tion of oxygen injection. It was found by Pekalski et al.(2005) that the forced IDT affected considerably theexplosion pressure and the maximum rate of pressure rise.The explosion pressure ratio was shown to increase withincreasing IDT to a maximum at an IDT of 40 s. After themaximum, the explosion pressure ratio dropped withincreasing the IDT, reached a minimum at an IDT of360 s and rose again to a second maximum at an IDT of1020 s. Starting from about 1220 s, the test mixtureself-ignited.Based on the CFD simulations of the mixing process and

natural convection of the ignition kernel, as well as on theanalysis of the detailed reaction mechanism of n-butane

Fig. 7. Calculated elds of oxygen mass fraction (left hemisphere) and tempera

after termination of oxygen injection: (a) ignition completion; (b) 0.3 s; (c) 1; a

Initial n-butane pressure in the vessel is 3.2 bar. Injection duration is 0.01 s. Tin the Process Industries 20 (2007) 562569oxidation, laminar ame propagation, and self-ignition,possible explanations for the phenomena observed byPekalski et al. (2005) can be suggested.The test mixture was found to be ammable. At the

initial conditions used, the mixture exhibited the laminarburning velocity of about 3.4 cm/s and the thermodynamictemperature of combustion products of 911.63K. Self-ignition of the test mixture exhibited a two-stage behaviorwith a cool ame and hot explosion arising at about 1000and 1300 s, respectively, after oxygen injection termination.Since the burning velocity of the test mixture was very low,its combustion in the vessel after forced ignition was highlyaffected by buoyancy. According to the estimations, thebuoyant ame kernel could dissipate completely during12 s after ignition due to the contact with massiveelectrodes and the upper wall of the vessel. As a result,combustion of the test mixture in the experiments ofPekalski et al. (2005) was always incomplete: a maximalexplosion pressure ratio was only 1.6 instead of thethermodynamic value of 6.21. Thus, the behavior of theexplosion pressure ratio in the experiments of Pekalskiet al. (2005) could be explained primarily by the variation

ture (right hemisphere) after ignition of test mixture in the vessel center 10 s

nd (d) 2 s after ignition. Initial oxygen pressure in the canister is 29.67 bar.

he interval between the isolines is uniform.

implication, further experimental and computationalstudies are required.

Pasman, H. J. (2005). Inuence of the ignition delay time on the

Moscow: USSR Academy of Sciences Publication.

ARTICLE IN PRESSS.M. Frolov et al. / Journal of Loss Prevention in the Process Industries 20 (2007) 562569 569quenching in 12 s after ignition, one could expect that ahigher explosion pressure ratio be attained at a higherburning velocity.The mixture preparation procedure used by Pekalski

et al. (2005) implied a possibility of obtaining excessive(as compared to the mean value) oxygen concentration atthe ignition site and in the space between the electrodes at atime between 3 and 100 s after injection termination.A higher oxygen concentration in a fuel-rich mixtureimplied a higher burning velocity (see Fig. 5a). This couldbe a reason for the rst maximum at the explosion pressurecurve, obtained by Pekalski et al. (2005) at an IDT of about40 s. Further temporal relaxation of the oxygen massfraction in the vessel center and in the space between theelectrodes to the mean value resulted in decreasing theburning velocity and the maximal explosion pressure tothe minimal value at an IDT of about 360400 s.The subsequent increase in the maximal explosion

pressure with the IDT exceeding 360400 s could beexplained by the growing inuence of preame reactionsleading to formation of various intermediate combustionproducts including alkyl peroxides. As mixture reactivity isknown to increase due to these processes, the laminarburning velocity in the preconditioned test mixture shouldalso increase. Thus, the second maximum of the explosionpressure arising in the experiments of Pekalski et al. (2005)at an IDT of 1000 s could be explained by faster amepropagation in the test mixture at its forced ignition duringthe cool-ame conversion. Further decrease of the maximalexplosion pressure at an IDT exceeding 1000 s could beexplained by lower reactivity and exothermicity of the testmixture. As a matter of fact, cool-ame conversion resultsin releasing of up to 710% of available chemical energy inthe test mixture. Therefore, both the combustion tempera-ture and the laminar burning velocity of the mixture passedthrough cool-ame oxidation should decrease. Forcedignition of such a mixture then should result in a lowerexplosion pressure.Self-ignition of the test mixture at time exceeding 1220 s

could be expected to result in the third prominentmaximum of the explosion pressure ratio, caused byvolumetric hot explosion (see Fig. 6a). However, thecorresponding experimental data of Pekalski et al. (2005)do not seem to have it. It could be expected that att 12201380 s hot explosion in the experiments occurredlocally in exothermic centers and therefore the resultantexplosion pressure was still lower than at forced igni-tion. This implication is substantiated by the highestmaximum rates of pressure rise relevant to self-ignitiondetected in the experiments. Nevertheless, to verify thisSWIFT manual 3.1 (2002). AVL AST, AVL List GmbH, Graz, Austria.

Victorov, S. B. (2002). The effect of Al2O3 phase transitions on detonation

properties of aluminized explosives. In: Proceedings of the 12th

international detonation symposium.explosion parameters of hydrocarbon-air-oxygen mixtures at elevated

pressure and temperature. Combustion Institute Proceedings, 30,

19331939.

Smetanyuk, V. A., Skripnik, A. A., Frolov, S. M., & Pasman, H. J. (2005).

Mixing dynamics in a standard 20-liter explosion vessel. In G. D. Roy,

S. M. Frolov, & A. M. Starik (Eds.), Nonequilibrium processes:

Combustion and detonation, Vol. 1 (pp. 3743). Moscow: Torus Press.

Sokolik, A. S. (1960). Self-ignition, flame, and detonation in gases.In general, the results of the study clearly indicate thatapparently inammable mixtures can nevertheless becomehazardous depending on the mixture preparation proce-dure and forced ignition timing.

Acknowledgments

The work was partly supported by the Russian-DutchResearch Cooperation Project no. 046.016.012 and Rus-sian Foundation for Basic Research Projects 05-08-50115aand 05-08-33411a.

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Modeling of n-butane ignition, combustion, and preflame oxidation in the 20-l vesselIntroductionThermodynamicsMixture preparationLaminar flame propagation in test mixtureSelf-ignition of test mixtureEffect of buoyancyDiscussion and conclusionsAcknowledgmentsReferences