Journal of Physics D: Applied Physics J. Phys. D: Appl. Phys. 48 (2015) 015201 (16pp)
doi:10.1088/0022-3727/48/1/015201
Discharge effect on the negative temperature coefficient behaviour and multistage ignition in C3H8-air mixture E A Filimonova Joint Institute for High Temperatures of Russian Academy of Sciences, Izhorskaya st.,13, bld.2, 125412 Moscow, Russia E-mail:
[email protected] Received 3 August 2014, revised 19 September 2014 Accepted for publication 29 September 2014 Published 3 December 2014 Abstract
It has been first shown by modelling that the discharge does not suppress but stimulates the development and increases the intensity of cool flame. It sharply reduces the delay time of cool flame ignition (isothermal stage) by more than 2 orders of magnitude and diminishes the total induction time by more than 1 order of magnitude due to radicals resulting from dissociation of fuel and oxygen molecules by electron impact. Under the discharge impact, the negative temperature coefficient (NTC) of the overall oxidation rate of the combustible mixture disappears or its maximum is largely decreased at the temperature range 640–760 K. A diminution of NTC maximum depends on the specific deposited energy and is provided by a replacement of the chemical kinetic mechanism at the isothermal stage in the NTC region. The relation between the specific input energy, concentrations of primary radicals resulting from a discharge and key component C3H7O2 which determines the NTC behaviour has been obtained. Keywords: plasma-assisted combustion, discharge impact, multistage ignition, negative temperature coefficient, chemical kinetics (Some figures may appear in colour only in the online journal)
1. Introduction
surface dielectric barrier discharge (SDBD), streamer corona or spark discharges, are investigated to find the best igniter and stabilizer for the combustion process. The common property for the discharges considered [3–5] is a non-uniformity filling by discharge filaments (channels) of volume, in contrast to the spark plug which gives a small hot area. A reduction of ignition delay time with SDBD or corona discharge was also demonstrated; however, the results had a qualitative character. The feasibility of igniting a very lean CH4:air mixture with a millisecond corona discharge in contrast to the spark plug was shown in [3]. In the work [5] an elimination of the negative temperature coefficient (NTC) behaviour of the induction time has been observed in C4H10:O2:Ar stoichiometric mixture under the plasma-assisted ignition in the rapid compression machine (RCM). Unfortunately, it is difficult to say what the pressure gauge registers: either a result of autoignition of volume due to the change of chemical reactivity
To provide a high efficiency and low toxic emission operation of internal combustion engines, low temperature combustion strategies have been considered in the last decade. Because the spark ignition (SI) engine is the least fuel efficient and gives a high level NOx emission, it is considered more effective to use a homogenous charge compression ignition (HCCI) engine with a stratificated reactivity of premixed mixture in the chamber [1]. The multiple ignition control in conditions of low intake temperatures in diluted (or lean) fuel-air mixtures could be supplied with plasma-assisted techniques [2]. Only a few papers, for example [3–5], are devoted to the experimental study of discharge influence on the ignition in closed volume, simulating the conditions of HCCI and SI with higher pressures (2–40 bar) and low temperatures (500– 950 K). In such papers, several kinds of discharge, namely, 0022-3727/15/015201+16$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
E A Filimonova
J. Phys. D: Appl. Phys. 48 (2015) 015201
added to the mixture in real combustible mixtures to avoid engine knock. It has recently been shown by modeling [21] that the discharge action may reduce the appearance of NTC and moreover, NTC may disappear. In the given work, the influence of the discharge operation on the chemical kinetics responsible for NTC has been investigated. Currently, there are no papers where the modelling of discharge impact on the negative temperature coefficient region might be presented. As a rule, the ignition delay time of a combustible mixture decreases when an initial temperature increases. In the NTC region, the induction time increases with the growth of the initial temperature (see figure 1). The reactions which control the NTC behaviour are the following:
of the mixture, or a result of the flame propagation from the igniter. Another important problem in the plasma-assisted ignition (PAI) of the combustible mixture is how to perform a discharge occupying a rather large volume under high pressure (5–40 bars). Investigations of new kinds of corona discharges at pressure values above 1 bar have been carried out in the works [6, 7]. Evidence of non-equilibrium plasmas to exert strong influence on the combustion process has been presented in numerous experiments and numerical modelling. One of the main effects of the discharge operation is the production of numerous radicals and excited particles for the acceleration of the chemical process. The remarkable reduction of ignition delay time and shift of the ignition temperature at ~200º in the lower temperature range in CxH2x + 2:O2:Ar stoichiometric mixtures (x = 1–5) was observed in experiments [8, 9] at the temperatures ~ 1500 K and pressures ~ 0.5 bar under the action of a high-voltage nanosecond discharge. In simulation, the key role of O atom in the decreasing of induction time was shown. A shortening of the induction zone length in a supersonic nonpremixed H2-air flow due to the discharge production of singlet oxygen molecules and O atoms was obtained in the modelling [10]. The role of several excited species in the ignition of premixed H2-O2 mixtures in different conditions was analyzed in the review [11]. Time-resolved measurements of particle concentration distributions such as N, O atoms in air [12] and OH, H in Ar-H2-O2 mixture [13] and OH in a very lean hydrocarbon-air mixture (equivalence ratio φ = 0.1 [2]) have been obtained experimentally in the plasma afterglow. However, it looks as if there are no experiments simultaneously measuring active particle concentrations and induction time. In some experiments on the plasma-assisted combustion (PAC) at low initial temperatures, the blue colour light emission before the inflammation in closed volume [14, 15], the fast propagation flame (200 m s−1 and more [16]) and the almost instantaneous ignition of the whole volume [14, 16] have been observed. These phenomena are characteristic features of a low temperature oxidation and low temperature inflammation in conditions of combustion in a spark ignition engine and diesel engine with homogeneous charge (for example, natural gas with air [17]). The visible appearance of this oxidation is the fluorescence of formaldehyde (CH2O*) and formyl radical (CHO*) with the wave-length of the blue spectral region. In the work [18] it was suggested that the multistage ignition in closed discharge systems may be responsible for the phenomena mentioned. Multistage ignition includes the cool flame (Т~550–750 К), blue flame (Т~800–1000 К) and hot flame (Т > 1000 K) stages, and at definite conditions, also a negative temperature coefficient (NTC) behaviour of oxidation rate. Every stage is characterized by the peak increase of OH density and stepped release of fuel mixture chemical enthalpy [19]. One of the last observations of cool and hot diffusive flames in n-C7H16: O2 has been established in the work [20]. Negative temperature coefficient behaviour of the overall rate of the chemical process is one of the important peculiarities of hydrocarbon–oxygen (air) mixture. NTC is relevant to the problem of the engine knock. Antidetonants are
R +O2 ↔ RO2
(1)
RO2 + C3H8 → ROOH + C3H7
(2)
ROOH → RO + OH + ΔE ,
(3)
where R = n-C3H7, i-C3H7, CH3, C2H5, CH3CO, ΔE is a sharp released energy via the organic peroxide decomposition. In propane-air mixture the main radicals affecting NTC are i-C3H7, n-C3H7, CH3 radicals with corresponding alkyl peroxy radicals i-C3H7O2, n-C3H7O2 and CH3O2. These peroxy radicals are stable in the NTC region. The primary radicals in this mixture are i-C3H7, n-C3H7 radicals. The CH3 radicals and others appear later (see figure 2). Based on reactions (1)–(3), one radical R produces three radicals: RO, OH and C3H7. This reaction sequence is chain branching. With the increase in temperature, the equilibrium in reaction (1) is shifted to the left, RO2 concentration decreases and the overall rate of chain branching (or the oxidation rate of mixture) decreases and ΔE is very small. The NTC region is reached. If the equilibrium in reaction R + O2 ↔ RO2 were to forcibly shift to the right, the NTC region will decrease and, moreover, disappear. Plasmas have selectivity in the initiation of required reactions. By means of discharge plasma it is possible to increase the production rate of peroxy radicals and their concentrations by such an amount that RO2 decomposition (1) in the NTC region will be insignificant to decrease the chain branching noticeably. The description of the first experiments on the cool and blue flames initiation by an external source may be found in the books [17, 22]. For example, in the works of Townend [23, 24] (cited in [22]) the cool and blue flames appeared in the closed tube under the impact of heated wire or a ceramic element with controllable temperature in the mixture of ether or alkanes with oxygen (air). A cool flame moved from the heated source at the end of the tube to the opposite end of the tube, a blue flame appeared behind the cool flame (in the products of cool flame) and overtook it. These flames arose at lower temperatures than in the self-ignition case. The velocity of propagation flames is changed in the range of 10–30 cm s−1. The same heating but at higher pressure created a hot flame with the velocity of 200 cm s−1. An electrical spark also resulted in the hot flame. On the basis of gas composition after the passage of cool and blue flames, the authors concluded that the main content of cool flame is an incomplete oxidation of 2
E A Filimonova
J. Phys. D: Appl. Phys. 48 (2015) 015201
mixture, a system of 700 elementary reactions (sum of forward and reverse reactions) and 103 components has been created. Two forms of propyl radicals (n-C3H7 and iso-C3H7) are taken into account. Using this system, the auto-ignition of CH4-air (or CH4-air-steam) mixture at T0 = 700 K and P = 50 bar was performed for the induction time of ~ 4s [31]. This value is very close to the calculated results obtained with the KINTECH mechanism [32] under the same conditions. The basic set of particles and reactions of our chemical kinetic database has been used earlier for the description of the formation and removal of toxic impurities in the automobile exhaust gases [29, 30, 33, 34], for the simulation of physical and chemical processes in the chemical compression reactor [35], in the study of ignition of fuel-air counterflow jets by electrical discharge [36] and other tasks. To solve the low temperature inflammation problem, a basic chemical kinetic scheme was supplemented with new reactions and components taken from several sources, including [19, 27, 28]. The important reactions for the low-temperature oxidation and ignition in considered conditions are given in the Appendix, together with some results of tests on the ignition delay time. There are some approaches to building a chemical kinetic scheme dependent on an assigned task. First of all, this is a detailed mechanism of oxidation and combustion containing hundreds of components and thousands of reactions. For example, the full n-decane mechanism includes 715 particles and 3872 reactions [37] which was a computer-generated mechanism. The measuring of mole fractions of the reactants and reaction products in the oxidation process help to verify the kinetic mechanisms [38]. Other methods use the nonextensive principle of construction of the kinetic mechanism, (the so-called optimal), which suggests that observed distinctions in the behaviour of alkanes are related mainly to a difference in rate constants of the same type of key reactions, rather than originating from a variety of reactions due to the increase in the number of reagents [19, 39]. A sufficiently compact detailed mechanism for small hydrocarbons was developed by Konnov [40, 41] and includes 1200 reactions and 127 components. For several applications, it is necessary to create a short kinetic scheme [42] and include it in the computational fluid dynamics (CFD) code. As a rule, short schemes catch some properties of the combustion process rather well–for example, an ignition delay time in chosen conditions. However, to investigate the ignition and combustion in unusual conditions, such as the forced ignition by an external energy source (discharge), the optimal kinetic mechanism is needed. There are two opinions concerning the description of a low temperature ignition (T0 = 550–800 K) of a hydrocarbon–oxidizer mixture. For example, in the work [42] the description of inflammation of C3H8-air mixture includes the second addition of the O2 molecule to the isomerization form C3H6OOH of propylperoxy radical C3H7O2, that is C3H6OOH + O2 → OC3H5OOH + OH. A second O2 addition becomes likely only due to a free valency in the middle of the radical C3H6OOH. The authors [19, 27, 39, 43] hold a different opinion. They deduce that for the lowest alkanes CH4, C2H6, C3H8 the isomerization process (C3H7O2 → C3H6OOH) is rather difficult because of hindrance inside the restructuring
initial hydrocarbon with a formation of aldehydes and organic hydroperoxides (ROOH). The addition of ozone to the combustible mixture with methane, propane, butane, butylenes and other hydrocarbons [22], eliminated the induction time and significantly reduced the critical pressure at the low pressure limit; for example, in contrast to self-ignition case at T0 = 733 K and P0 = 200 Torr, the addition of 15% O3 to C4H8:O2 mixture at T0 = 295 K and P0 = 30 Torr resulted in an ignition. The authors noted the ozone action was associated with its decomposition and formation of atomic oxygen and then, formation of hydroperoxides (ROOH). The intensity of O3 effect increases with the decrease in temperature. The photochemical impact (illumination in Schumann–Runge spectral region), resulting in the oxygen dissociation and addition of ozone, led to the identical products of oxidation reaction and the similar kinetic dependence of hydroperoxide formation: linear dependence on the initial hydrocarbon concentration and no dependence on the oxygen concentration [22]. The discharge effect on the oxidation of n-C4H10:O2 jet was studied in the experiments of Badin [25] (cited in [17]) at P > 1, then [17] ψ t
640
Figure 6. Dependence of induction time on initial temperature and equivalence ratio. Solid lines are auto-ignition, dashed lines are with discharge impact at W = 0.005 eV/molecule. P0 = 5.5 bar.
Figure 5. Induction time dependence on initial temperature and specific input energy. P0 = 5.5 bar, φ = 1.66. Symbols are experiments [47]. ‘1’ is W = 0, ‘2’ is W = 0.005,’3’ is W = 0.01, ‘4’ is W = 0.02, ‘5’ is W = 0.05 eV/molecule.
10
C3H7O2
1
C3H7
600
650
700
750
Initial temperature, K
0.1 800
Figure 7. Concentrations and induction time for auto-ignition case (solid line) and under discharge impact (dashed line), W = 0.005 eV/ molecule, P0 = 5.5 bar, φ = 1.66. [C3H7O2]max = [i-C3H7O2]max + [n-C3H7O2]max, [C3H7]max = [i-C3H7]max + [n-C3H7]max.
The discharge effect on the negative temperature coefficient is illustrated in figure 5. As can be seen, even a small value of specific input energy W = 0.005 eV/molecule noticeably reduces the maximum of the NTC in the rich mixture. At W ≈ 0.02 eV/molecule the NTC disappeared. In figure 6 the differences in discharge impact for three equivalence ratios are shown at the deposited energy W = 0.005 eV/molecule. Note, that at φ = 0.7 (lean mixture) the NTC is not observed now. The worse a mixture ignites, the more a discharge influences it. This fact was noticed in several works relevant to the plasma-assisted ignition, for example in [3]. In figure 7 the induction time and total concentrations [C3H7O2]max = [i-C3H7O2]max + [n-C3H7O2]max and correlated with them the concentrations [C3H7]max = [i-C3H7]max + [n-C3H7]max are presented. The concentrations were taken in the maximum value for the corresponding initial temperature. The difference in the behaviour and amounts of concentrations in the auto-ignition case and under discharge assistance are clearly expressed. Due to the initial active particles (O and C3H7), the concentration of propyle peroxy radicals C3H7O2 increased by almost two orders of magnitude and the C3H8 oxidation rate also grew (reactions (11), (11a)). The large value of C3H7O2 radicals shifted the equilibrium in reaction R+O2 ↔ RO2 (R = C3H7) to the right; their maximum
concentrations almost do not depend on the initial temperature. The maximum of the NTC diminishes at W = 0.005 eV/ molecule and moreover, disappears at the higher deposited energy (see figure 5). Disappearance of the NTC under the discharge impact is provided by a replacement of a chemical kinetic mechanism at the isothermal stage, as is demonstrated in figure 8 at W = 0.01 eV/ molecule at T0 = 710 K. The isothermal stage (auto-ignition, dashed line) reduces and a cool flame stage (discharge action, solid line) appears due to switching on a low temperature branching mechanism (10)–(12a). Due to a high C3H7O2 concentration (figure 7) and then, [C3H7OOH], because of a partially released enthalpy under C3H7OOH decomposition (12) and (12a), the elevation of temperature achieves ~60º. The hot flame stage begins at T~770 K which corresponds to the end of the NTC region (see figure 1), where the oxidation rate with respect to the high temperature branching mechanism (25)– (28) is appreciably more than that of T0 = 710 K. Induction time τid reduces from 2.594s to 0.139s by a factor of 18.7. One could say that due to the discharge impact we move the system in another temperature range where the branching takes place 8
E A Filimonova
J. Phys. D: Appl. Phys. 48 (2015) 015201
685 K 10 700
1E-4
τind, s
1E-6 1E-7
n-C3H7O2
1E-8 1E-9
1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 0.01 0.1
Time, s
1
1
760
620 K 650 685 700 760
0.1
1E-10
600
φ = 0.7
620 650
1E-5
1000
800
(a)
1E-3
C3H7OOH
Mole fraction
Temperature, K
1200
1E-11
0.01
Figure 8. Dependence of temperature, propyl peroxy radical and propylperoxide concentrations on time. Solid line is with discharge impact, W = 0.01 eV/molecule, dashed line is auto-ignition case. T0 = 710 K. P0 = 5.5 bar, φ = 1.66. The behaviour of i-C3H7O2 is similar to the n-C3H7O2 one.
(b)
0.00
10 620 730 660
1
τind, s
under the same high temperature mechanism but at higher temperatures and thus faster. 4.4 Relations between the specific input energy and the disappearance of the negative temperature coefficient
0.02
0.03
0.04
W, eV/molecule
700 K
(c) 1
0.05
φ=1
770
0.1
0.01
As it has been shown above (figures 5 and 6), the induction time and diminution of the NTC maximum depend on the specific deposited energy W, equivalence ratio φ and initial temperature of mixture T0. On the basis of applied modelling, it was possible to find the correlation between W and key value [C3H7O2]max (figure 7) to predict the minimal required value W to eliminate the NTC for the different φ. The dependence of the induction time on the deposited energy at φ = 0.7, 1, 1.66 is presented for different initial temperatures in figure 9. The temperature range is considered where a non-monotony of induction time is observed, i.e. T0 = 620–780 K. The NTC region disappears or is less expressed at W = 0.005 eV/molecule. Before this W value, a mixed behaviour of induction time curves is clearly seen in dependence on T0, for example, at φ = 1 τid(T0 = 730 K) > τid(T0 = 660 K). With larger W, the curves array in the right order: the induction time reduces with the increase of the initial temperature: τid(T0 = 730 K)
