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Combined Effect of Ignition Energy and Initial Turbulence on the Explosion Behavior of Lean Gas/Dust-Air Mixtures† Almerinda Di Benedetto,‡ Anita Garcia-Agreda,‡ Paola Russo,*,§ and Roberto Sanchirico‡ ‡
Istituto di Ricerche sulla Combustione (IRC), Consiglio Nazionale delle Ricerche (CNR), Napoli, Italy Dipartimento di Ingegneria Industriale, Universita di Salerno, Fisciano (SA), Italy
§
ABSTRACT: Explosions of hybrid mixtures of methane and nicotinic acid are investigated near the lower-flammability-limit conditions. The effect on the maximum pressure and deflagration index of the ignition energy and, then, of the ignition source in combination with the turbulence is analyzed. In correspondence of limit conditions for pure methane and pure nicotinic acid, the variation of both the ignition energy and the turbulence was found to affect the behavior of the explosion. It was observed that the deflagration index is determined to be independent from the ignition energy, even though the dependence on the turbulence still remains.
1. INTRODUCTION Among all the factors affecting the explosion behavior of fuelair mixtures, turbulence is generally recognized to have the primary role. Especially, in dust explosion testing, dust clouds are usually formed by means of a pneumatic dispersion system. During dust dispersion, intensive turbulence is induced in the dustair mixture; this is the so-called “dust-dispersion-induced turbulence”1 (also known as “initial turbulence”2 or “preignition turbulence”3). Preignition turbulence is required for suspending the dust cloud, but its intensity decays quickly over time. Then, the dust cloud is ignited at a defined time (tv) from the beginning of dust dispersion and, hence, at a given initial level of turbulence. Typically, the ignition delay time (tv) is used as an indirect measurement of the preignition turbulence level. However, it is only a relative or arbitrary measure of the turbulence for a given system1 since the turbulence level changes with the air pressure used for dust dispersion, vessel volume4 and apparatus geometry.3 Several works deal with quantifying the effect of tv on the dustair and dust/gasair mixtures explosion parameters, such as maximum pressure and deflagration index1,310 in different apparatuses, such as 20-L4,8 and 1-m 3 spheres,4,6 a scaled-up version of the classical Hartmann tube,5,7 a spherical bomb with a volume of 0.065 m3,11 a cylindrical 1-m3 vessel,11 and a 4-m3 vessel.3 Experiments performed at decreasing ignition delay times (tv) (i.e., increasing the preignition turbulence) show that the maximum explosion pressure and the deflagration index significantly increase for both dustair1,39 and dust/gasair10 mixtures. It has also been found that turbulence affects the ignitability of dustair mixtures and, more precisely, the minimum ignition energy value.3 This means that, by increasing the turbulence level (decreasing tv), the minimum ignition energy value increases for both gaseous mixtures12 and dustair mixtures.13,14 This trend was elucidated by De Soete,15 who studied the influence of turbulence on the ignition energy of methaneair and propaneair mixtures. He explained the increase in ignition energy under turbulent flow conditions to be the result of the r 2011 American Chemical Society
effect of the increased turbulent diffusivity, leading to an increase of the flame-front thickness and then an increase in the volume in which the ignition energy is released, and of the enhanced heat dissipation, which determines a decrease in the fraction of energy available for ignition. Accordingly, Ballal and Lefevbre12 underlined that, as a consequence of higher turbulence in the kernel of hot gas generated by the ignition energy, the rate of heat generation is lower than the rate of heat loss, up to eventually reducing the ability of self-propagation. Furthermore, it has been shown that, upon increasing the turbulence level by using explosion devices that use fans to generate the desired turbulence level, the time required for the ignition of methaneair mixtures decreases.11 All these studies allow one to conclude that the turbulence plays a major role in affecting the ignitability of both gasair and dustair mixtures. On the other hand, the ignition energy has also been found to have a primary role on the explosion behavior of gasair and dustair mixtures. Several works are devoted to study the effect of different ignition sources (electrical spark or chemical igniters) and different energy values on the gasair,16 dustair,16,17 and dust/ gasair mixtures18 explosion parameters, such as maximum pressure and deflagration index. Furthermore, researchers have investigated the role of ignition energy in affecting the turbulence level.6 They carried out explosion experiments on cornstarch dustair mixtures at a fixed value of the preignition turbulence level by varying the ignition energy. They found that, upon decreasing the ignition energy, both the explosion pressure starts to rise later (i.e., the induction time increases) and the maximum rate of pressure rise decreases. This behavior was explained by considering that, upon decreasing Special Issue: Russo Issue Received: July 29, 2011 Accepted: November 22, 2011 Revised: November 14, 2011 Published: November 22, 2011 7663
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Figure 1. Schematic view of the Siwek 20-L spherical vessels for the determination of dust explosion parameters. Figure 2. Temporal evolution of the explosion pressure.
the ignition energy, and then increasing the induction time, the preignition turbulence level decreases and then dust sedimentation occurs, reducing the strength of the explosion. To our knowledge, none of the available literature takes into account the mutual interaction between preignition turbulence and ignition energy on the deflagration index of dustair and/or dust/gasair mixtures. The objective of the present work is to fill this gap by quantifying the combined role of preignition turbulence and ignition energy on the ignitability and explosion features of dustair, gasair, and dust/gasair mixtures. To this end, we performed measurements of the pressure history of methane/nicotinic acidair mixtures in the 20-L Siwek bomb, at changing the dust and gas concentrations, the ignition source (chemical igniters or electric spark), and the ignition delay time (tv). In a previous paper, Proust et al.,19 by determining the explosion indexes of several different dusts in a standard 20-L sphere, showed that many phenomena affect the deflagration indexes, such as the actuation time of the igniters, the time of the onset of combustion, and also the preheating of the mixture by the ignition source. They suggest caution when applying the data measured in the 20-L sphere to the sizing of the venting. Conversely, they found out that the data coming from the 1-m3 sphere are not significantly affected by these phenomena. In this paper, we quantify the relevant role of the interplay between the ignition energy and the turbulence intensity in affecting the deflagration index. In agreement with Proust et al.,19 it comes out that care must be taken when using the deflagration index values measured mainly for less-reactive mixtures and low ignition energies.
2. EXPERIMENTAL SECTION The explosion experiments were performed in the standard 20-L sphere apparatus manufactured by Adolf K€uhner AG (CH), with a rebound nozzle introduced by Siwek.20 Some modifications (Figure 1) were made to be able to perform experiments on hybrid mixtures. All the details regarding the apparatus used and the experimental method followed have been reported in a previous work.21 The tests were performed in accordance with ASTM Method E1226.22
The explosion tests were carried out on hybrid mixtures of methane and nicotinic acid (supplied by SigmaAldrich). The methane concentration was fixed to the limit value of 6 vol %, whereas the concentration of nicotinic acid was varied in the range of 3060 g/m3. Tests were performed using as ignition source both chemical igniters of 500, 1000, and 10 000 J and an electric spark of lower energy capable of supplying 15 kV and 30 mA, as produced also by K€uhner (KSEP 320). The effect of the turbulence on the behavior of the explosion was studied by varying the value of the ignition delay time in the range of 60500 ms. Experiments were carried out in triplicate: the standard deviations of our data are for maximum pressure less than or equal to (5% and for deflagration index less than or equal to (20%. From the explosion experiments, the pressuretime history was obtained (Figure 2). The explosivity parameters—the maximum explosion pressure (PEX) and the maximum rate of pressure rise (dP/dt)— were extrapolated from this pressure versus time series of data. The evolution of the pressure may be described by also taking into account some characteristic times (Figure 2): • td, defined as the time delay of the outlet valve (that must be in the range of 3050 ms); • tv, defined as the ignition delay time (that is, the time between the beginning of the air blast and the moment of ignition); • t2, which is defined as the induction time (that is, the time difference between the activation of the ignition and the intersection of the inflection point of this curve with the 0 bar line); and • t1 is the time of the combustion. The attention in this work will be focused on the tv and t2 times, and on a combination of the two. Table 1 shows a list of the tests performed and the relevant experimental conditions. Tests on different mixtures were carried out by changing the delay time and using different types of ignition source (chemical igniters and electric spark) in order to elucidate the effect of turbulence on the explosion behavior of the mixtures analyzed. Tests were then performed at different delay times using chemical igniters of different energy, for quantifying the effect of turbulence on the induction time. Furthermore, the effect of ignition energy at constant preignition turbulence was also investigated. 7664
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Table 1. Experimental Conditions of Explosion Tests test type
nicotinic acid
methane %
[g/m3]
[v/v]
Ei [J]
tv [ms]
turbulence
60
10000
60
turbulence
60
10000
120 60
turbulence
6
10000
turbulence
6
10000
90
turbulence
6
10000
120
turbulence
30
6
10000
60
turbulence turbulence
30 30
6 6
10000 10000
90 120
turbulence
60
6
10000
90
turbulence
60
6
10000
120
turbulence
6
electric spark
60
turbulence
6
electric spark
120
turbulence
6
electric spark
250
turbulence
30
6
electric spark
60
turbulence
30
6
electric spark
120
turbulence
30
6
electric spark
250
turbulence
60
6
electric spark
60
turbulence
60
6
electric spark
120
induction time
30
6
10000
60, 90, 120
induction time
30
6
1000
60, 90, 120
induction time
30
6
500
60, 90, 120 60500
induction time
30
6
electric spark
induction time
60
6
10000
60500
induction time
60
6
1000
60500
induction time
60
6
500
60500
induction time
60
6
electric spark
60, 120
energy
6
10000
60
energy
6
1000
60
energy
6
electric spark
60 60
energy
30
6
10000
energy
30
6
1000
60
energy
30
6
500
60
energy
30
6
electric spark
60
energy
60
6
10000
60
energy
60
6
1000
60
energy
60
6
500
60
energy
60
6
electric spark
60
3. RESULTS AND DISCUSSION 3.1. Effect of Turbulence. We studied the effect of preignition turbulence at a fixed value of the ignition energy (10 kJ, by chemical igniters). Preignition turbulence level was varied by changing the ignition delay time (tv) for the nicotinic acidair, methaneair, and methane/nicotinic acidair mixtures. In Figure 3, the pressure time histories are shown for nicotinic acid (60 g/m3)air and methaneair mixtures at different values of the ignition delay time (tv = 60, 90, and 120 ms). In the figure, the point at which the ignition is triggered is also indicated with a vertical dashed line. For the nicotinic acidair mixture, it was
found that, upon increasing the ignition delay time, the maximum pressure and the slope of pressuretime curve decreases. This behavior may be addressed to the particle sedimentation phenomenon, which is more evident at high tv, when the preignition turbulence level is low. Conversely, in the case of the methaneair mixture, it seems that the maximum pressure and the maximum slope of the temporal pressure trend are almost unaffected by tv in the cases of 60 and 90 ms; and a decrease of both parameters, smaller than those in the case of the dustair mixture, is observed at 120 ms. The observed decrease in the maximum slope of the pressure-time curve as the delay time increases is probably due to the occurrence of unmixing and stratification phenomena in the gas mixture. By adding different amounts of nicotinic acid to the methane air mixture (6 vol %), effects of the preignition turbulence level on the maximum pressure and maximum slope of the temporal pressure trend similar to those in the case of a pure gasair mixture were found (Figure 4). This behavior suggests that gas combustion is the leading factor of the explosion process. With the objective of showing the effect on the explosion process of the different ignition sources, several tests were performed using different ignition sources: chemical igniters and electric spark. In Figures 3 and 4, the results of the test performed using 10 kJ chemical igniters are shown. In Figures 5 and 6, the results obtained as gathered using electric sparks are reported. These figures highlight the fact that, in the case of an electric spark and for nicotinic acid concentrations of 30 and 60 g/m3 (lean limit), the nicotinic acid does not explode. In the case of electric spark, deflagration index values lower than those with chemical igniters were measured for both homogeneous and hybrid mixtures. From the pressure trends of Figures 36, it appears that the pressure starts to increase at times greater than the ignition delay time. This behavior is due to the fact that there is an induction time (t2) required by the reactive mixture to produce a kernel able to allow self-propagation. In order to quantify the effect of the preignition turbulence on the induction time (t2), we computed t2 as a function of the ignition delay time at fixed values of the ignition energy and mixture composition. Figure 7 shows the results for the mixtures of 6 vol % methane and, respectively, 30 and 60 g/m3 of nicotinic acid. The induction time increases as the ignition delay time increases and, hence, as the preignition turbulence level decreases. This result is in agreement with the previous findings of Leuckel et al.11 and De Soete.15 This effect could be ascribed to the fact that, upon increasing the turbulence level, the heat and mass diffusivities increase, thus accelerating the propagation of heat and radicals from the kernel toward the unburnt mixture; hence, the induction time decreases. It is worth noting that, upon increasing the ignition energy, the effect of turbulence is less significant, because of the prevailing role of the overdriving effect of the flame propagation by the released energy. 3.2. Effect of Ignition Energy. The role played by the ignition energy has been studied at a fixed value of the preignition turbulence (i.e., tv = 60 ms). The results are shown in Figure 8 for the methaneair mixture and in Figures 9 and 10 for the methane/nicotinic acidair mixture at different dust concentrations (60 and 30 g/m3, respectively). As previously mentioned, both chemical igniters (E = 10 kJ and 1000 J) and electric spark are used to perform the tests. 7665
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Figure 3. Pressure versus time at different tv values for nicotinic acidair (left) and methaneair (right) mixtures. E = 10 kJ, chemical igniters.
Figure 4. Pressure versus time at different values of tv for the methane/nicotinic acidair mixture. (Left) Nicotinic acid (C = 60 g/m3) and 6 vol % methane; (right) nicotinic acid (C = 30 g/m3) and 6 vol % methane. (Conditions: E = 10 kJ, chemical igniters.)
In the case of chemical igniters, the induction times of the methaneair mixture are ∼15 ms and ∼110 ms, respectively, for the 10-kJ and 1-kJ chemical igniters (see Figure 8). Conversely, when using the electric spark, the induction time (t2) is significantly higher (∼660 ms); therefore, in certain cases, the systems result not explosive. According to the literature data, it is worth noting that the peak pressure and the deflagration index are both decreasing when the ignition energy is decreased. With regard to the nicotinic acidair mixture, it is important to underline that the concentration values that have been investigated are near the lean limit and the mixture is ignited only when the concentration is 60 g/m3 and the 10-kJ chemical igniters are used (tv = 60120 ms) (see Figure 3, left). In Figure 9, the same results are shown, as obtained for the methane/nicotinic acid (60 g/m3)air mixture. The induction time (t2) is ∼15 ms when 10-kJ chemical igniters are used as the ignition source and its value increases upon decreasing the ignition energy (20, 25, and 35 ms, respectively, for the 1-kJ chemical igniter, the 500-J chemical igniter, and the electric spark). The maximum pressure remains approximately constant and the deflagration index decreases upon decreasing the ignition energy. In Figure 10, the maximum pressure and the deflagration index of methane/nicotinic acid (30 g/m3)air mixture decrease upon decreasing the ignition energy; this is probably due to the lower concentration of the dust, which is lower than the MEC for all ignition sources. It should be stressed that, when using the
Figure 5. Pressure versus time at different tv values for a methaneair mixture, with an electric spark as the ignition source.
electric spark and adopting the same tv value, the ignition delay is higher than that found in the other cases when chemical igniters are used and also for hybrid mixtures. It is worth noting that the overall time of the tests for methane and for hybrid mixture is not the same; this can be explained since the presence of the dust gives an effective contribution to the explosion. In summary, the results here reported have shown that: • the preignition turbulence affects the induction time, which increases with increasing tv, because of its detrimental effect 7666
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Figure 6. Pressure versus time at different tv values for the methane/nicotinic acidair mixture. (Left) Nicotinic acid (C = 60 g/m3) and 6 vol % methane; (right) nicotinic acid (C = 30 g/m3) and 6 vol % methane. (An electric spark was used as the ignition source in both mixture tests.)
Figure 7. Induction time (t2) as a function of the ignition delay time (tv) and various ignition energies for the methane/nicotinic acidair mixtures. (Left) Nicotinic acid (C = 60 g/m3) and 6 vol % methane; (right) nicotinic acid (C = 30 g/m3) and 6 vol % methane.
on mass and heat diffusivity and, hence, on flame propagation. This effect is minor in the presence of dust where the occurrence of dust sedimentation has a primary role in the decrease in the severity of the explosion. • the ignition energy directly influences the induction time and indirectly influences the preignition turbulence level. 3.3. Combining Turbulence and Ignition Energy. From the above-discussed results, it turns out that (i) the ignition energy mainly affects the induction time (t2) of the mixture and (ii) the delay of the induction time leads to variation in the preignition turbulence. As a consequence, we may assume that the deflagration index is influenced by the ignition energy due to an indirect effect: the lower the ignition energy, the higher the induction time, and, hence, the lower the turbulence level and then the deflagration index. We then calculated the here-defined explosion delay time (tex) as the key parameter affecting the real preignition turbulence level and then the deflagration index: tex ¼ tv þ t2
ð1Þ
We plotted all the deflagration index data obtained by varying both tv (ignition delay time) and t2 (induction time) as a function of tex: for each mixture considered, all the data overlap and lay on the same curve, regardless of the energy used in the explosion (Figure 11).
Figure 8. Pressure versus time at different values of the ignition energy for the methaneair mixture. (Conditions: methane, 6 vol %; tv = 60 ms.)
These results lead one to consider tex as the major parameter that should be considered as a measure of the preignition turbulence level. Dahoe et al.23 measured the velocity root mean square (v0 rms) in the 20-L sphere, as a function of time. They derived the following correlation of v0 rms: n 0 vrms t ¼ ð2Þ 00 t0 vrms 7667
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Figure 9. Pressure versus time at different values of the ignition energy for the methane/nicotinic acidair mixture. (Conditions: nicotinic acid (C = 60 g/m3), 6 vol % methane, tv = 60 ms.)
Figure 11. Deflagration index as a function of tex at different values of the methane and nicotinic acid concentration.
formula for evaluating St is given as follows:10 2 !α 3 0 v St ¼ S1 41 þ β rms 5 S1
ð5Þ
where Sl is the laminar burning velocity. The value of the parameter α is dependent on the formula used for the evaluation of the turbulence burning velocity. As previously shown,10 different equations can be adopted to calculate the turbulent burning velocity. By substituting the equation of Dahoe et al.23 (eq 2), we get the correlation for KG,St, as a function of the time tex: !α 0 0 KG, St vrms tex n 3 α ¼ 1 þ β ð6Þ ° Sl t0 KG, St Figure 10. Pressure versus time at different ignition energy values for the methane/nicotinic acidair mixture. (Conditions: nicotinic acid (C = 30 g/m3), 6 vol % methane, tv = 60 ms.) 0 0 is the value of the velocity root mean square at time where vrms t = t0. Dahoe et al.23 found that, when using the rebound nozzle as a dispersion system, the fitting of the data up to t = 200 ms leads to determine the value of n to be 1.61 ( 0.03. Moreover, after t = 200 ms, they also found a decrease of the slope of the trend of v0 rms as a function of time. At times of >200 ms, no regression data were given by Dahoe et al.23 However, from the data reported by them,23 it is possible to derive the following relationship: 0
vrms ¼ 0 v0rms
0:34 t t0
ð3Þ
Concerning the deflagration index, in a previous paper,10 we showed that it is possible to assume the following equation: ° KG, St ¼ f ðKG, St , St Þ
ð4Þ
where K°G,St is the deflagration index evaluated under quiescent conditions and St is the turbulent burning velocity. A general
We evaluated the parameters n and α by fitting the curves of Figure 11. The results are given in Table 2. For the methane/ nicotinic acidair mixtures, we found that, at times tex 200 ms), the exponent n 3 α is significantly lower and, for the methane/ nicotinic acidair mixtures, it is equal to 0.7 or 0.75. From this value, it turns out that α ≈ 2. Conversely, for the methane air mixtures (n 3 α = 0.34 or 0.37), the α value is lower than or equal to 1. Different values of the parameter α, relative to the turbulence level, are due to the variation of the turbulent combustion regime, which changes when the v0 rms/Sl value changes.25 It is worth noting that, at high values of the turbulence and low ignition delay time, the exponent α is almost equal for the methaneair and the methane/nicotinic acidair mixtures. Conversely, at higher values of the ignition delay time and low values of the preignition turbulence level, the exponent α is higher for the methane/nicotinic acidair mixtures. This result may be addressed to the role of the particle sedimentation, which occurs at high ignition delay time and/or low turbulence level. Under these conditions, particle sedimentation plays a more significant detrimental effect on the turbulence level. 7668
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’ REFERENCES
Table 2. Results of the Fitting from Experimental Tests methane (vol %)
nicotinic acid (g/m3)
tex (ms)
n3α
n
α
6
0
60200
0.98
1.6
0.6
7.3
0
60200
0.98
1.6
0.6
6
60
60200
1
1.6
0.6
6
30
60200
1.1
1.6
0.7
6
0
>200
0.34
0.34
1.0
7.3
0
>200
0.37
0.34
1.1
6
60
>200
0.75
0.34
2.2
6
30
>200
0.7
0.34
2.1
4. CONCLUSIONS Measurements of the pressure history of methane/nicotinic acidair mixtures are performed in the 20-L Siwek bomb, by changing the dust and gas concentration (near the lower flammability limit conditions), the ignition source (chemical igniters or electric spark), and the ignition delay time (tv). In this way, the effect on the maximum pressure and deflagration index of the ignition energy and, then, of the ignition source, in combination with the turbulence, is analyzed. It was observed that the deflagration index results are independent of the ignition energy, even though the dependence on turbulence still remains. The combined role of preignition turbulence and ignition energy on the ignitability and explosion features of dust/gas-air mixtures is quantified by means of the definition of a new parameter: the explosion delay time (t ex ). From the obtained results, the violence of the explosion of an hybrid mixture decreases as t ex increases independent of the ignition energy level. Moreover, it was observed that the experimental results are in accordance with theoretical correlations when considering, as the ignition delay time, the explosion delay time tex, and assuming values of 200 ms, the particle sedimentation plays a significant role in affecting the influence of the turbulence level on the violence of the explosion, inducing an higher decrease of deflagration index than that expected without sedimentation. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes †
This work is originated in the high-pressure laboratory at the Institute of Research on Combustion (CNR), where Prof. Gennaro Russo has been director for more than 10 years. The authors wish to acknowledge Prof. Russo for his scientific support and his encouragement in the development of the high-pressure laboratory and of the group working on dust explosions.
’ ACKNOWLEDGMENT The authors wish to acknowledge Mr. Andrea Bizzarro for his technical support.
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