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Determination of turbulent burning velocities of dust air mixtures with the open tube method
ARTICLE IN PRESS
Journal of Loss Prevention in the Process Industries 20 (2007) 470–476 www.elsevier.com/locate/jlp
Determination of turbulent burning velocities of dust air mixtures with the open tube method H. Schneidera,, Ch. Proustb a
Fraunhofer-Institut Chemische Technologie, Joseph von Fraunhoferstr. 7, 76327 Pfinztal, Germany b INERIS, Parc Technologique ALATA, P.O. Box 2, 60550 Verneuil-en-Halatte, France Received 6 November 2006; received in revised form 24 April 2007; accepted 26 April 2007
Abstract Correlating turbulent burning velocity to turbulence intensity and basic flame parameters-like laminar burning velocity for dust air mixtures is not only a scientific challenge but also of practical importance for the modelling of dust flame propagation in industrial facilities and choice of adequate safety strategy. The open tube method has been implemented to measure laminar and turbulent burning velocities at laboratory scale for turbulence intensities in the range of a few m/s. Special care has been given to the experimental technique so that a direct access to the desired parameters was possible minimising interpretation difficulties. In particular, the flame is propagating freely, the flame velocity is directly accessible by visualisation and the turbulence intensity is measured at the flame front during flame propagation with special aerodynamic probes. In the present paper, those achievements are briefly recalled. In addition, a complete set of experiments for diametrically opposed dusts, starch and aluminium, has been performed and is presented hereafter. The experimental data, measured for potato dust air mixtures seem to be in accordance with the Bray Gu¨lder model in the range of 1.5 m/sou0 o3.5 m/s. For a further confirmation, the measurement range has been extended to lower levels of turbulence of u0 o1.5 m/s. This could be achieved by changing the mode of preparation of the dust air mixture. In former tests, the particles have been injected into the tube from a pressurised dust reservoir; for the lower turbulence range, the particles have been inserted into the tube from above by means of a sieve–riddler system, and the turbulence generated from the pressurised gas reservoir as before. For higher levels of turbulence, aluminium air mixtures have been investigated using the particle injection mode with pressurised dust reservoir. Due to high burning rates much higher flame speeds than for potato dusts of up to 23 m/s have been obtained. r 2007 Elsevier Ltd. All rights reserved. Keywords: Dust explosion; Turbulent burning velocity; Turbulence intensity
1. Introduction Efforts are devoted to the development of numerical codes to help with the safe design of industrial facilities handling explosible dusts (Hansen, Skjold, & Arntzen, 2004; Proust, 2005). Although several routes may be followed to achieve this aim, one of the most important tasks, whatever these codes are, is to be able to correlate the local aerodynamics of the dust air mixture in the process, especially the turbulence and the flame speed. Only a few attempts to quantify this relationship have been done Corresponding author. Tel.: +49 721 464 0380; fax: +49 721 464 0111.
E-mail addresses: [email protected] (H. Schneider), [email protected] (C. Proust). 0950-4230/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jlp.2007.04.035
so far with dust air mixtures (Tezok, Kauffman, Sichel, & Nicholls, 1985) and most of these data have been obtained in a very indirect way. Experiments have been done in closed bombs. The turbulence intensity has been measured during pretests without dusts with LDA or hot wire techniques, assuming it will be the same with the presence of dust particles and during flame propagation. Very often the flame speed is deduced and extrapolated from the pressure signals with a number of assumptions. In an attempt to overcome these difficulties, a new concept of experimental system has been devised and used to produce new data at laboratory scale from zero turbulence level to a few m/s, that is from laminar flame to moderate turbulence as typically observed during closed bomb experiments.
ARTICLE IN PRESS H. Schneider, C. Proust / Journal of Loss Prevention in the Process Industries 20 (2007) 470–476
Nomenclature
hi
st sl u0 L Z0 UF AF A
pr n s Ds
turbulent burning velocity (m/s) laminar burning velocity (m/s) turbulence intensity (m/s) integral length scale of turbulence (m) flame thickness (m) flame velocity in lab coordinate system (m/s) area of flame front (m2) projection of AF on cross section of the channel (m2) P1, P2, P3 overpressures in dust reservoir, channel and location of dust injection (bar) m mass on sieve–riddler system (starch) or mass injected into channel (Al)(g) Some basic principles about turbulence and turbulent combustion are firstly recalled to point out the possible links between the main parameters. The experimental setup is presented as a second step whereas the results are depicted in the last part of this paper. 2. Background Rather recent findings (Proust, 2006a, b) demonstrated that the main lines of the explosion processes may be very similar in gas and dust atmospheres. Laminar, cellular and turbulent flame regimes have been identified, with the laminar flame appearing as a smooth well-defined front. As far as the laminar flame is concerned, there are at least three main steps for the class of hydrocarbon-based particles. The reactants ahead of the flame are preheated by conduction up to being pyrolysed after which the combustion occurs in gaseous phase. The relevant parameters of this basic combustion regime are the consumption rate of the flame front, called the ‘‘laminar burning velocity’’ sl, and the flame thickness, Z0, which may be deduced from sl via the thermal diffusivity of the medium. A laminar flow becomes turbulent as soon as the lowspeed layers of the flow are rolling up with the higher speed layers to produce eddies which appear and vanish rapidly. Such eddies are the ‘‘turbulence’’ of the flow. It is generally admitted that such structures bear a chaotic behaviour and need to be studied with statistics (Hinze, 1975). The relevant theories introduce the notion of ‘‘turbulent cascade’’ according to which the initial eddies are destroyed in smaller and smaller structures until dissipation by molecular diffusion. In practise, this cascade is often seen as an intrinsic process, independent from the mean flow. Because of this, it is then sufficient to know the characteristics of the primary eddies, those directly issued from the mean flow, to characterise the turbulent flowfield. These characteristics are the scale of the largest eddies (L ¼ ‘‘integral scale of turbulence’’) and their peripheral velocity (u0 ¼ ‘‘r.m.s. of the velocity fluctuations’’).
s(fit) tv ti
471
height of ignitor above open end of channel (cm) starting pressure in dust reservoir (bar) relative motor speed of sieve–riddler system flame speed in centre of channel average flame speed derived from incremental flame speeds flame speed derived from maxima of parabola fits time of valve trigger, starting gas injection, with respect to start of motor (s) time of ignitor trigger with respect to start of motor (starch) or with respect to valve opening, starting injection of Al dust in (s)
The parameter u0 is in principle a space averaged variable and L is the area under the curve giving the evolution of the correlation coefficient of the velocity signals around a reference point. It is assumed that the situation may be comparable with dust clouds (Tezok et al., 1985) except that the particles are pushed around the turbulent eddies (Proust, 2006b). The propagation of flames in turbulent gaseous mixtures has been studied for a long time (Borghi & Destriaux, 1998). A ‘‘turbulent burning velocity’’, st, has been defined in a somewhat similar way as the laminar burning velocity. It can be viewed as the local flame consumption rate once the turbulent fluctuations have been smoothed out and should be representative of the speed at which the flame progresses against the mean flow. Depending on the scale ratios between L and the flame thickness Z0, and between u0 and sl, different flame propagation regimes (Proust, 2006b) may be identified. In most safety applications, the combustion should proceed locally in laminar flames, the main effect of the turbulence being to convolute the front around eddies. Since the flame remains locally laminar, the scales of the laminar flame are relevant parameters like Z0 and L. It is then tempting trying to correlate those parameters. A number of attempts have resulted from such considerations and a popular one has been advocated by Bray (Bray, 1990) and Gu¨lder (Gu¨lder, 1990) and gives 0 st u L ¼ 0:6 þ 1. (1) Z0 sl sl It is not clearly known if the basis of the analysis of the turbulent regimes for gases, and further the Bray–Gu¨lder model, can be fully transfered to dust flames, since the intrinsic mechanisms have not received such attention yet. A first favourable clue is to be found in the laminar flame propagation mechanism which seems similar at least for hydrocarbon dusts. The second may be found into the aspect of such flames for small levels of turbulence (Proust, 2006b; Proust & Veysierre, 1988) where the eddies of the
ARTICLE IN PRESS 472
H. Schneider, C. Proust / Journal of Loss Prevention in the Process Industries 20 (2007) 470–476
flow appear clearly and seem to convolute the flame. Because of this, it has been proposed to use the Bray–Gu¨lder model as a reference although many aspects are not clear even concerning the fundamentals of the underlying theory. Since a direct access to the quantities st, sl, Z0, u0 and L is required, original techniques have been used. 3. Test apparatus 3.1. Principle To measure the turbulent burning velocities, the classical ‘‘tube method’’ has been used (Andrews & Bradley, 1972). Consider a vertical tube, filled with an explosible mixture, closed on top and open below, where the ignition source is located. The flame is then propagating freely without being pushed by the expansion of the burnt products which are vented through the open end. Due to buoyancy forces for instance, the flame front may not be flat, but more or less curved (Pelce, 1985). To obtain the burning velocity, the application of the mass conservation law to a control volume is used (Andrews & Bradley, 1972). If UF is the incident reactant velocity in the control volume (flame velocity in the laboratory co-ordinates), AF the flame front area and A the projection of AF on the cross section of the tube, it follows sl ¼ U F A=AF .
(2)
To implement the tube method, simple-video recording can be used to find UF and estimate AF as derived from the propagation of the flame front contours. 3.2. Technical details The test setup working on this principle is shown in Fig. 1 (Schneider, 2006; Schneider & Proust, 2005). It consists of a vertical channel, 1.8 m long, with a square cross section (30 cm 30 cm), transparent on the front side. If a ‘‘high’’ turbulence intensity is required (e.g. 41.5 m/s), the dust can be injected from a 5 l pressurised dust reservoir through small small tubes (8 mm inner diameter), running in parallel to the axis of the channel along its corners. Each tube is perforated with small holes (1 mm diameter at a distance of 40 mm each), the openings of which are facing to the centre of the channel. Jets emerging from the nozzles collide in the centre and particles are subsequently mixed and distributed thoroughly within the channel. The flow rate and consequently the turbulence level can be varied by changing the pressure in the reservoir. The nominal dust concentration is the ratio between the amount of dust injected and the volume of the setup. For lower turbulence levels (typically smaller than 1.5 m/s), another dust injection mode is required because the former leads to unreproducible amounts of injected dust and heterogeneities of the cloud. To overcome this difficulty, a sieve riddler system is mounted on top of the channel, thus
Fig. 1. Sketch of test apparatus.
introducing the dust from above; after some time delay, pressurised air out of the 5 l reservoir is injected via the perforated tubes into the channel, thus generating turbulence within the channel already completely filled with dust. The dust concentration was estimated from the measured feeding rate of dust (100 g/s) and the measured velocity of the falling dust cloud (1.8 m/s), which results in a concentration of about 620 g/m3. The experimental parameters are given along with the results in Tables 1 and 2. For the ignition of the dusts a pyrotechnic ignitor is used, located between 0.1 and 0.4 m above the lower end of the channel. The ignition energy is 100 or 5000 J depending on the nature of the dust.
3.3. Measurement techniques One great difficulty is to measure the turbulence in a burning dust air mixture. Standard laboratory equipments and especially LDA and hot wire anemometry are not applicable. INERIS has been working for years on the development of alternative techniques (Proust, 2004). A Pitot tube technique based on a very refined concept of Mc Caffrey gauges (Mc Caffrey & Heskestad, 1976) has been implemented (Fig. 2). The sensor head is a short tube (length 2 cm, diameter 1 cm) with a solid wall in the middle and the differential pressure is measured on both sides with a precise FURNACE CONTROL 720 Pa transducer. The aeraulics of the system had to be refined to have sufficient dynamics. Extensive testing in a reference jet proved that such transducers are able to detect eddies as small as 2 cm with
ARTICLE IN PRESS H. Schneider, C. Proust / Journal of Loss Prevention in the Process Industries 20 (2007) 470–476
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Table 1 Experimental data of tests with potato dust (Swely gel) Test no.
hi (cm)
pr (bar)
n
s (m/s)
Ds (m/s)
st (m/s)
tv (s)
ti (s)
500 500
40 20
22 23
1 1
19 19 26 15 10 13 13 10 11 11 26 26 50
1 1 1 1 1 1 1 1 2 2 2 2 2
1.4170.10 1.4270.24 1.2870.21 1.2170.24 1.5470.16 1.1770.15 0.7670.15 1.4170.17 0.9270.30 0.8170.16 0.9970.16 1.0270.17 0.8370.17 1.7270.19 1.9870.38 3.2270.60
3.8 3.8
20 20 20 20 20 50 30 30 20 20 20 20 20
2.2170,14 1.8970.28 2.0770.35 1.4570.29 2.0370.23 1.7270.21 0.9670.22 1.6170.21 1.0770.34 1.0770.18 1.7870.30 1.2370.18 1.2770.24 2.2570.26 2.3970.50 3.7070.61
3 3
500 500 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000
2.1870.04 1.8470.09 1.8670.14 1.5370.07 1.9870.05 1.5670.06 1.0370.07 1.6670.05 1.0870.13 1.0170.03 1.7270.09 1.2470.07 1.1670.13 2.1870.22 2.2470.13 3.6970.22
3 2 1 1 1 1 1 1 2 2 2 2 2
3.8 3.0 4.0 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5
m (g)
11j 12j 12j 13j 15j 3s 5s 6s 8s 9s 10s 15s 16s 17s 18s 19s
Table 2 Experimental data of tests with aluminium dusta Test no.
pr (bar) s (m/s)
s (fit) (m/s)
Ds (m/s)
st (m/s)
ti (s)
25 27 33 29 30 37 31 35 36
50 50 50 80 80 80 100 100 100
10.170.3 8.370.3 8.970.5 17.971.3 14.270.6 15.070.4 22.071.2 16.670.9 16.771.0
11.572.4 10.672.5 8.871.9 22.375.8 16.874.5 14.072.7 22.875.3 15.172.4 20.574.8
8.971.6 7.471.5 5.671.2 15.773.9 14.173.6 9.771.9 13.973.0 12.471.9 14.673.5
1.4 1.4 1.4 1.2 1.2 1.2 1.2 1.2 1.2
a
10.370.3 8.170.3 8.670.4 18.171.3 14.070.7 15.070.5 23.071.0 16.070.7 16.771.0
In each test 250 g of Al dust is injected.
Fig. 2. Pitot tubes with sensor and head.
a peripheral velocity of 0.2 m/s. Details of this original technique will be published in the near future. Several of these devices have been used simultaneously (Fig. 3) in order to derive both u0 and L by cross correlation of the signals (see Schneider & Proust, 2005 for further details). A number of tests have been done showing typical values of u0 between 1 and 3.5 m/s (depending on the injection device) with an integral scale of turbulence of the
order of 3–5 cm (see Schneider & Proust, 2005, Fig. 9). Very interestingly, in the range investigated, the influence of the concentration of particles on the turbulence is not significant so that injecting air only or air with dust from the reservoir does not change the turbulence intensity nor the scale. The pressure in the reservoir has been found to be the leading parameter. The qualitative reason for this may be that the turbulence field inside the tube may depend directly from the injection velocity which is a direct function of the injection pressure. A calibration curve is shown in Fig. 4, obtained from data recorded during flame propagation just ahead of the flame front. It can be estimated that u0 is known within an accuracy of 720%. Measurements of u0 have been obtained only in the upper range of u0 . Further checkings are required in the lower range. The observation of the flame shape and trajectory has been achieved by means of a high-speed digital video camera, type Phantom V5 from Vision Research, New Jersey. Flame contours are extracted from the pictures (Schneider, 2006). From a single picture, seven samples are taken along the flame front and its correspondend (x/y)coordinates stored (y being along the axis of the channel). The flame contours can be nicely fitted with parabolas. The same points can be used to derive the flame trajectory and then the flame speed. The raw points taken in the centre of the channel or the maxima of the parabola fits are used to obtain the trajectory. There is only a minor difference between both of them (see Tables). It can be inferred that the flame speed and the burning velocity are known within an accuracy of 710% and 715%, respectively. Three pressure transducers are used to record the pressure within the dust reservoir (P1), the pressure within the channel (P2) and the ‘‘injection’’ pressure (P3) at the point where the particles are injected into the channel (Fig. 3).
ARTICLE IN PRESS H. Schneider, C. Proust / Journal of Loss Prevention in the Process Industries 20 (2007) 470–476
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Fig. 3. Time control and data acquisition system with test apparatus.
Turbulence intensity (m/s)
4
Table 3 Particle size distributions (in mm)
3.5 3 2.5 2
D(0.1) D(0.5) D(0.9) Sauter mean
y = 0.061x R2 = 0.8004
1.5
Swely gel
Aluminium
11.1 33.2 76.9 17.3
3.7 6.5 10.9 5.8
1 0.5 0 0
10
20 30 40 50 Overpressure in the reservoir (bar)
60
Fig. 4. Calibration curve: turbulence intensity vs. starting pressure in reservoir.
2006b). It is believed that the premixed gaseous flame model may not be valid for all kinds of dust particles, aluminium being an example explained elsewhere (Proust, 2006b). The burning parameters have not yet been estimated. Some data characterising the particle size distributions of the particles used in the tests are listed in Table 3.
4. Results 4.2. Flame propagation and pressure effects 4.1. Dusts A set of different starches have been tested so far (Schneider, 2006; Schneider & Proust, 2005). They seem to give rather similar results. A number of points have been obtained with Microlys potato starch but less with Swely gel. The present additional tests have been performed with Swely gel potato starch. It is known that for such dusts, the basic flame propagation mechanism is driven by heat conduction like in premixed gaseous flames (Proust,
A linear propagation of the flame can be observed with some oscillations superimposed (Fig. 5). These oscillations are more or less severe in different tests. The oscillations are linked with internal pressure oscillations (P2 in Fig. 6), which are specifically severe with aluminium dust (730 mbar) with a frequency of 50 Hz during flame propagation, corresponding to the natural frequency of the tube. This suggests an acoustic instability since the frequency of the oscillations of the flame trajectory and
ARTICLE IN PRESS H. Schneider, C. Proust / Journal of Loss Prevention in the Process Industries 20 (2007) 470–476
475
5 4.5 4 St (m/s)
3.5 3 2.5 2 1,5
pneumatic inj. of dust riddler inj. of dust laminar flame "Gülder correlation"
1 0.5 0
0
0.5
1
1.5 2 2.5 3 Turbulence intensity (m/s)
3.5
4
4.5
Fig. 7. Correlation between turbulent burning velocity and turbulence intensity for Swely gel dust. Fig. 5. Flame propagation in the centre of the channel.
20 18 16 St (m/s)
14 12 10 8 6 4 2 0
0
1
2
3 4 5 6 Turbulence intensity (m/s)
7
8
Fig. 8. Turbulent burning velocity vs. turbulence intensity for Al dust (experiment).
even of the apparent flame area is the same (Schneider & Proust, 2005).
Fig. 8 for aluminium. In Fig. 7 the ‘‘Gu¨lder correlation’’ starts for zero turbulence at st ¼ 0.2 m, which is the laminar burning velocity for potato starch as measured by (Proust, 2006a).
4.3. Turbulent burning velocities
5. Conclusion and perspectives
The flame front is not flat so that expression (2) should be used to extract the turbulent burning velocity. This relation is applied to the data with A ¼ p (0.15 cm)2 and AF ¼ area of the paraboloid surface derived from the parabola fits. Since the flame area may vary between two frames (and the flame speed accordingly), expression (2) is processed (Schneider, 2006) for each picture taking as a flame speed the average displacement of the tip of the flame divided by the time elapsed between two successive frames (Ds in Tables 1 and 2). Doing so, the acoustic vibration is merely cancelled and st appears nearly constant during the propagation (Schneider & Proust, 2005, Figs. 14 and 15). The turbulence intensity can be derived from the calibration curve of Fig. 4. The turbulent burning velocities are shown in Fig. 7 for Swely gel potato starch and in
The new data obtained with the sieve–riddler device are fully coherent with the data from former measurements with direct injection of dust. The curve is now much more complete and available data seem sufficiently accurate to compare with the Gu¨lder correlation at least at lab scale. Apparently the correlation overestimates the experimental data a bit although giving the same trend. The situation for aluminium dust is more difficult to assess because of the lack of fundamental data, but there is a trend for an increase of turbulent burning velocity with turbulence intensity, too. As expected, burning velocities of aluminium dust air mixtures are much higher than those of starch air mixtures. It is intended to continue the successful cooperation between the companies involved, doing more experiments
Fig. 6. Pressure records of transducers P1, P2, P3 and flame propagation.
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with use of turbulence sensors for in situ measurement of turbulence in front of the flame for starch dusts and aluminium dusts as well. References Andrews, G. E., & Bradley, D. (1972). Determination of burning velocities: A critical review. Combustion and Flame, 18, 133–153. Borghi, R., & Destriaux, M. (1998). Combustion and flame-chemical and physical principles, Edition, TECHNIP. Bray, K. (1990). Studies of turbulent burning velocities. In Proceedings of the royal society of London (Vol. A431, pp. 315–325). Gu¨lder, O¨. L. (1990). Turbulent premixed flame propagation models for different combustion regimes. In Comptes-rendus du 22nd symosium (International) on combustion. Hansen, O. R., Skjold, T. & Arntzen, B. J. (2004). DESC—a CFD-tool for dust explosions. In International European safety Management group (ESMG)-symposium, Nu¨rnberg. Hinze, J. O. (1975). Turbulence (2nd ed.). New-York: Mc Graw-Hill company. Mc Caffrey, B. J., & Heskestad, G. (1976). A robust bidirectional lowvelocity probe for flame and fire application. Combustion and Flame, 26, 125. Pelce, P. (1985). Effect of gravity on the propagation of flames in tubes. Journal of Physique, 46, 503–510.
Proust, Ch. (2004). Formation, inflammation, combustion des atmospheres explosives (ATEX) et effets associe´s. Me´moire d’HdR pre´sente´ a` l’Institut National Polytechnique de Lorraine, 12 Fe´v 2004. Proust, Ch. (2005). The usefulness of phenomenological tools to simulat the consequences of dust explosions: The experience of EFFEX. In Presented at the ESMG meeting, 11–14 October, 2005, Nu¨rnberg. Proust, Ch. (2006a). Flame propagation and combustion in some dust air mixtures. Journal of Loss Prevention in Process Industries, 19, 89–100. Proust, Ch. (2006b). A few fundamental aspects about ignition and flame propagation in dust clouds. Journal of Loss Prevention in Process Industries, in press, doi:10.1016/j.jlp.2005.06.035. Proust, Ch., & Veysierre, B. (1988). New experimental apparatus for studying the propagation of dust-air flames. Progress in astronautics and aeronautics series (Vol. 113, pp. 43–61). Schneider, H. (2006). Measurement of turbulent burning velocities by means of the open tube method. Journal of Loss Prevention in Process Industries, 19, 130–134. Schneider, H., & Proust, Ch. (2005). Laminar and turbulent burning velocities of dust clouds. In International ESMG symposium on process safety and industrial explosion protection, Nu¨rnberg, Germany. Tezok, F. I., Kauffman, C. W., Sichel, M., & Nicholls, J. A. (1985). Turbulent burning velocity measurement for dust/air mixtures in a constant volume spherical bomb. In 10th ICDERS, Berkeley.
Journal of Loss Prevention in the Process Industries 20 (2007) 470–476 www.elsevier.com/locate/jlp
Determination of turbulent burning velocities of dust air mixtures with the open tube method H. Schneidera,, Ch. Proustb a
Fraunhofer-Institut Chemische Technologie, Joseph von Fraunhoferstr. 7, 76327 Pfinztal, Germany b INERIS, Parc Technologique ALATA, P.O. Box 2, 60550 Verneuil-en-Halatte, France Received 6 November 2006; received in revised form 24 April 2007; accepted 26 April 2007
Abstract Correlating turbulent burning velocity to turbulence intensity and basic flame parameters-like laminar burning velocity for dust air mixtures is not only a scientific challenge but also of practical importance for the modelling of dust flame propagation in industrial facilities and choice of adequate safety strategy. The open tube method has been implemented to measure laminar and turbulent burning velocities at laboratory scale for turbulence intensities in the range of a few m/s. Special care has been given to the experimental technique so that a direct access to the desired parameters was possible minimising interpretation difficulties. In particular, the flame is propagating freely, the flame velocity is directly accessible by visualisation and the turbulence intensity is measured at the flame front during flame propagation with special aerodynamic probes. In the present paper, those achievements are briefly recalled. In addition, a complete set of experiments for diametrically opposed dusts, starch and aluminium, has been performed and is presented hereafter. The experimental data, measured for potato dust air mixtures seem to be in accordance with the Bray Gu¨lder model in the range of 1.5 m/sou0 o3.5 m/s. For a further confirmation, the measurement range has been extended to lower levels of turbulence of u0 o1.5 m/s. This could be achieved by changing the mode of preparation of the dust air mixture. In former tests, the particles have been injected into the tube from a pressurised dust reservoir; for the lower turbulence range, the particles have been inserted into the tube from above by means of a sieve–riddler system, and the turbulence generated from the pressurised gas reservoir as before. For higher levels of turbulence, aluminium air mixtures have been investigated using the particle injection mode with pressurised dust reservoir. Due to high burning rates much higher flame speeds than for potato dusts of up to 23 m/s have been obtained. r 2007 Elsevier Ltd. All rights reserved. Keywords: Dust explosion; Turbulent burning velocity; Turbulence intensity
1. Introduction Efforts are devoted to the development of numerical codes to help with the safe design of industrial facilities handling explosible dusts (Hansen, Skjold, & Arntzen, 2004; Proust, 2005). Although several routes may be followed to achieve this aim, one of the most important tasks, whatever these codes are, is to be able to correlate the local aerodynamics of the dust air mixture in the process, especially the turbulence and the flame speed. Only a few attempts to quantify this relationship have been done Corresponding author. Tel.: +49 721 464 0380; fax: +49 721 464 0111.
E-mail addresses: [email protected] (H. Schneider), [email protected] (C. Proust). 0950-4230/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jlp.2007.04.035
so far with dust air mixtures (Tezok, Kauffman, Sichel, & Nicholls, 1985) and most of these data have been obtained in a very indirect way. Experiments have been done in closed bombs. The turbulence intensity has been measured during pretests without dusts with LDA or hot wire techniques, assuming it will be the same with the presence of dust particles and during flame propagation. Very often the flame speed is deduced and extrapolated from the pressure signals with a number of assumptions. In an attempt to overcome these difficulties, a new concept of experimental system has been devised and used to produce new data at laboratory scale from zero turbulence level to a few m/s, that is from laminar flame to moderate turbulence as typically observed during closed bomb experiments.
ARTICLE IN PRESS H. Schneider, C. Proust / Journal of Loss Prevention in the Process Industries 20 (2007) 470–476
Nomenclature
hi
st sl u0 L Z0 UF AF A
pr n s Ds
turbulent burning velocity (m/s) laminar burning velocity (m/s) turbulence intensity (m/s) integral length scale of turbulence (m) flame thickness (m) flame velocity in lab coordinate system (m/s) area of flame front (m2) projection of AF on cross section of the channel (m2) P1, P2, P3 overpressures in dust reservoir, channel and location of dust injection (bar) m mass on sieve–riddler system (starch) or mass injected into channel (Al)(g) Some basic principles about turbulence and turbulent combustion are firstly recalled to point out the possible links between the main parameters. The experimental setup is presented as a second step whereas the results are depicted in the last part of this paper. 2. Background Rather recent findings (Proust, 2006a, b) demonstrated that the main lines of the explosion processes may be very similar in gas and dust atmospheres. Laminar, cellular and turbulent flame regimes have been identified, with the laminar flame appearing as a smooth well-defined front. As far as the laminar flame is concerned, there are at least three main steps for the class of hydrocarbon-based particles. The reactants ahead of the flame are preheated by conduction up to being pyrolysed after which the combustion occurs in gaseous phase. The relevant parameters of this basic combustion regime are the consumption rate of the flame front, called the ‘‘laminar burning velocity’’ sl, and the flame thickness, Z0, which may be deduced from sl via the thermal diffusivity of the medium. A laminar flow becomes turbulent as soon as the lowspeed layers of the flow are rolling up with the higher speed layers to produce eddies which appear and vanish rapidly. Such eddies are the ‘‘turbulence’’ of the flow. It is generally admitted that such structures bear a chaotic behaviour and need to be studied with statistics (Hinze, 1975). The relevant theories introduce the notion of ‘‘turbulent cascade’’ according to which the initial eddies are destroyed in smaller and smaller structures until dissipation by molecular diffusion. In practise, this cascade is often seen as an intrinsic process, independent from the mean flow. Because of this, it is then sufficient to know the characteristics of the primary eddies, those directly issued from the mean flow, to characterise the turbulent flowfield. These characteristics are the scale of the largest eddies (L ¼ ‘‘integral scale of turbulence’’) and their peripheral velocity (u0 ¼ ‘‘r.m.s. of the velocity fluctuations’’).
s(fit) tv ti
471
height of ignitor above open end of channel (cm) starting pressure in dust reservoir (bar) relative motor speed of sieve–riddler system flame speed in centre of channel average flame speed derived from incremental flame speeds flame speed derived from maxima of parabola fits time of valve trigger, starting gas injection, with respect to start of motor (s) time of ignitor trigger with respect to start of motor (starch) or with respect to valve opening, starting injection of Al dust in (s)
The parameter u0 is in principle a space averaged variable and L is the area under the curve giving the evolution of the correlation coefficient of the velocity signals around a reference point. It is assumed that the situation may be comparable with dust clouds (Tezok et al., 1985) except that the particles are pushed around the turbulent eddies (Proust, 2006b). The propagation of flames in turbulent gaseous mixtures has been studied for a long time (Borghi & Destriaux, 1998). A ‘‘turbulent burning velocity’’, st, has been defined in a somewhat similar way as the laminar burning velocity. It can be viewed as the local flame consumption rate once the turbulent fluctuations have been smoothed out and should be representative of the speed at which the flame progresses against the mean flow. Depending on the scale ratios between L and the flame thickness Z0, and between u0 and sl, different flame propagation regimes (Proust, 2006b) may be identified. In most safety applications, the combustion should proceed locally in laminar flames, the main effect of the turbulence being to convolute the front around eddies. Since the flame remains locally laminar, the scales of the laminar flame are relevant parameters like Z0 and L. It is then tempting trying to correlate those parameters. A number of attempts have resulted from such considerations and a popular one has been advocated by Bray (Bray, 1990) and Gu¨lder (Gu¨lder, 1990) and gives 0 st u L ¼ 0:6 þ 1. (1) Z0 sl sl It is not clearly known if the basis of the analysis of the turbulent regimes for gases, and further the Bray–Gu¨lder model, can be fully transfered to dust flames, since the intrinsic mechanisms have not received such attention yet. A first favourable clue is to be found in the laminar flame propagation mechanism which seems similar at least for hydrocarbon dusts. The second may be found into the aspect of such flames for small levels of turbulence (Proust, 2006b; Proust & Veysierre, 1988) where the eddies of the
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flow appear clearly and seem to convolute the flame. Because of this, it has been proposed to use the Bray–Gu¨lder model as a reference although many aspects are not clear even concerning the fundamentals of the underlying theory. Since a direct access to the quantities st, sl, Z0, u0 and L is required, original techniques have been used. 3. Test apparatus 3.1. Principle To measure the turbulent burning velocities, the classical ‘‘tube method’’ has been used (Andrews & Bradley, 1972). Consider a vertical tube, filled with an explosible mixture, closed on top and open below, where the ignition source is located. The flame is then propagating freely without being pushed by the expansion of the burnt products which are vented through the open end. Due to buoyancy forces for instance, the flame front may not be flat, but more or less curved (Pelce, 1985). To obtain the burning velocity, the application of the mass conservation law to a control volume is used (Andrews & Bradley, 1972). If UF is the incident reactant velocity in the control volume (flame velocity in the laboratory co-ordinates), AF the flame front area and A the projection of AF on the cross section of the tube, it follows sl ¼ U F A=AF .
(2)
To implement the tube method, simple-video recording can be used to find UF and estimate AF as derived from the propagation of the flame front contours. 3.2. Technical details The test setup working on this principle is shown in Fig. 1 (Schneider, 2006; Schneider & Proust, 2005). It consists of a vertical channel, 1.8 m long, with a square cross section (30 cm 30 cm), transparent on the front side. If a ‘‘high’’ turbulence intensity is required (e.g. 41.5 m/s), the dust can be injected from a 5 l pressurised dust reservoir through small small tubes (8 mm inner diameter), running in parallel to the axis of the channel along its corners. Each tube is perforated with small holes (1 mm diameter at a distance of 40 mm each), the openings of which are facing to the centre of the channel. Jets emerging from the nozzles collide in the centre and particles are subsequently mixed and distributed thoroughly within the channel. The flow rate and consequently the turbulence level can be varied by changing the pressure in the reservoir. The nominal dust concentration is the ratio between the amount of dust injected and the volume of the setup. For lower turbulence levels (typically smaller than 1.5 m/s), another dust injection mode is required because the former leads to unreproducible amounts of injected dust and heterogeneities of the cloud. To overcome this difficulty, a sieve riddler system is mounted on top of the channel, thus
Fig. 1. Sketch of test apparatus.
introducing the dust from above; after some time delay, pressurised air out of the 5 l reservoir is injected via the perforated tubes into the channel, thus generating turbulence within the channel already completely filled with dust. The dust concentration was estimated from the measured feeding rate of dust (100 g/s) and the measured velocity of the falling dust cloud (1.8 m/s), which results in a concentration of about 620 g/m3. The experimental parameters are given along with the results in Tables 1 and 2. For the ignition of the dusts a pyrotechnic ignitor is used, located between 0.1 and 0.4 m above the lower end of the channel. The ignition energy is 100 or 5000 J depending on the nature of the dust.
3.3. Measurement techniques One great difficulty is to measure the turbulence in a burning dust air mixture. Standard laboratory equipments and especially LDA and hot wire anemometry are not applicable. INERIS has been working for years on the development of alternative techniques (Proust, 2004). A Pitot tube technique based on a very refined concept of Mc Caffrey gauges (Mc Caffrey & Heskestad, 1976) has been implemented (Fig. 2). The sensor head is a short tube (length 2 cm, diameter 1 cm) with a solid wall in the middle and the differential pressure is measured on both sides with a precise FURNACE CONTROL 720 Pa transducer. The aeraulics of the system had to be refined to have sufficient dynamics. Extensive testing in a reference jet proved that such transducers are able to detect eddies as small as 2 cm with
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Table 1 Experimental data of tests with potato dust (Swely gel) Test no.
hi (cm)
pr (bar)
n
s (m/s)
Ds (m/s)
st (m/s)
tv (s)
ti (s)
500 500
40 20
22 23
1 1
19 19 26 15 10 13 13 10 11 11 26 26 50
1 1 1 1 1 1 1 1 2 2 2 2 2
1.4170.10 1.4270.24 1.2870.21 1.2170.24 1.5470.16 1.1770.15 0.7670.15 1.4170.17 0.9270.30 0.8170.16 0.9970.16 1.0270.17 0.8370.17 1.7270.19 1.9870.38 3.2270.60
3.8 3.8
20 20 20 20 20 50 30 30 20 20 20 20 20
2.2170,14 1.8970.28 2.0770.35 1.4570.29 2.0370.23 1.7270.21 0.9670.22 1.6170.21 1.0770.34 1.0770.18 1.7870.30 1.2370.18 1.2770.24 2.2570.26 2.3970.50 3.7070.61
3 3
500 500 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000
2.1870.04 1.8470.09 1.8670.14 1.5370.07 1.9870.05 1.5670.06 1.0370.07 1.6670.05 1.0870.13 1.0170.03 1.7270.09 1.2470.07 1.1670.13 2.1870.22 2.2470.13 3.6970.22
3 2 1 1 1 1 1 1 2 2 2 2 2
3.8 3.0 4.0 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5
m (g)
11j 12j 12j 13j 15j 3s 5s 6s 8s 9s 10s 15s 16s 17s 18s 19s
Table 2 Experimental data of tests with aluminium dusta Test no.
pr (bar) s (m/s)
s (fit) (m/s)
Ds (m/s)
st (m/s)
ti (s)
25 27 33 29 30 37 31 35 36
50 50 50 80 80 80 100 100 100
10.170.3 8.370.3 8.970.5 17.971.3 14.270.6 15.070.4 22.071.2 16.670.9 16.771.0
11.572.4 10.672.5 8.871.9 22.375.8 16.874.5 14.072.7 22.875.3 15.172.4 20.574.8
8.971.6 7.471.5 5.671.2 15.773.9 14.173.6 9.771.9 13.973.0 12.471.9 14.673.5
1.4 1.4 1.4 1.2 1.2 1.2 1.2 1.2 1.2
a
10.370.3 8.170.3 8.670.4 18.171.3 14.070.7 15.070.5 23.071.0 16.070.7 16.771.0
In each test 250 g of Al dust is injected.
Fig. 2. Pitot tubes with sensor and head.
a peripheral velocity of 0.2 m/s. Details of this original technique will be published in the near future. Several of these devices have been used simultaneously (Fig. 3) in order to derive both u0 and L by cross correlation of the signals (see Schneider & Proust, 2005 for further details). A number of tests have been done showing typical values of u0 between 1 and 3.5 m/s (depending on the injection device) with an integral scale of turbulence of the
order of 3–5 cm (see Schneider & Proust, 2005, Fig. 9). Very interestingly, in the range investigated, the influence of the concentration of particles on the turbulence is not significant so that injecting air only or air with dust from the reservoir does not change the turbulence intensity nor the scale. The pressure in the reservoir has been found to be the leading parameter. The qualitative reason for this may be that the turbulence field inside the tube may depend directly from the injection velocity which is a direct function of the injection pressure. A calibration curve is shown in Fig. 4, obtained from data recorded during flame propagation just ahead of the flame front. It can be estimated that u0 is known within an accuracy of 720%. Measurements of u0 have been obtained only in the upper range of u0 . Further checkings are required in the lower range. The observation of the flame shape and trajectory has been achieved by means of a high-speed digital video camera, type Phantom V5 from Vision Research, New Jersey. Flame contours are extracted from the pictures (Schneider, 2006). From a single picture, seven samples are taken along the flame front and its correspondend (x/y)coordinates stored (y being along the axis of the channel). The flame contours can be nicely fitted with parabolas. The same points can be used to derive the flame trajectory and then the flame speed. The raw points taken in the centre of the channel or the maxima of the parabola fits are used to obtain the trajectory. There is only a minor difference between both of them (see Tables). It can be inferred that the flame speed and the burning velocity are known within an accuracy of 710% and 715%, respectively. Three pressure transducers are used to record the pressure within the dust reservoir (P1), the pressure within the channel (P2) and the ‘‘injection’’ pressure (P3) at the point where the particles are injected into the channel (Fig. 3).
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Fig. 3. Time control and data acquisition system with test apparatus.
Turbulence intensity (m/s)
4
Table 3 Particle size distributions (in mm)
3.5 3 2.5 2
D(0.1) D(0.5) D(0.9) Sauter mean
y = 0.061x R2 = 0.8004
1.5
Swely gel
Aluminium
11.1 33.2 76.9 17.3
3.7 6.5 10.9 5.8
1 0.5 0 0
10
20 30 40 50 Overpressure in the reservoir (bar)
60
Fig. 4. Calibration curve: turbulence intensity vs. starting pressure in reservoir.
2006b). It is believed that the premixed gaseous flame model may not be valid for all kinds of dust particles, aluminium being an example explained elsewhere (Proust, 2006b). The burning parameters have not yet been estimated. Some data characterising the particle size distributions of the particles used in the tests are listed in Table 3.
4. Results 4.2. Flame propagation and pressure effects 4.1. Dusts A set of different starches have been tested so far (Schneider, 2006; Schneider & Proust, 2005). They seem to give rather similar results. A number of points have been obtained with Microlys potato starch but less with Swely gel. The present additional tests have been performed with Swely gel potato starch. It is known that for such dusts, the basic flame propagation mechanism is driven by heat conduction like in premixed gaseous flames (Proust,
A linear propagation of the flame can be observed with some oscillations superimposed (Fig. 5). These oscillations are more or less severe in different tests. The oscillations are linked with internal pressure oscillations (P2 in Fig. 6), which are specifically severe with aluminium dust (730 mbar) with a frequency of 50 Hz during flame propagation, corresponding to the natural frequency of the tube. This suggests an acoustic instability since the frequency of the oscillations of the flame trajectory and
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5 4.5 4 St (m/s)
3.5 3 2.5 2 1,5
pneumatic inj. of dust riddler inj. of dust laminar flame "Gülder correlation"
1 0.5 0
0
0.5
1
1.5 2 2.5 3 Turbulence intensity (m/s)
3.5
4
4.5
Fig. 7. Correlation between turbulent burning velocity and turbulence intensity for Swely gel dust. Fig. 5. Flame propagation in the centre of the channel.
20 18 16 St (m/s)
14 12 10 8 6 4 2 0
0
1
2
3 4 5 6 Turbulence intensity (m/s)
7
8
Fig. 8. Turbulent burning velocity vs. turbulence intensity for Al dust (experiment).
even of the apparent flame area is the same (Schneider & Proust, 2005).
Fig. 8 for aluminium. In Fig. 7 the ‘‘Gu¨lder correlation’’ starts for zero turbulence at st ¼ 0.2 m, which is the laminar burning velocity for potato starch as measured by (Proust, 2006a).
4.3. Turbulent burning velocities
5. Conclusion and perspectives
The flame front is not flat so that expression (2) should be used to extract the turbulent burning velocity. This relation is applied to the data with A ¼ p (0.15 cm)2 and AF ¼ area of the paraboloid surface derived from the parabola fits. Since the flame area may vary between two frames (and the flame speed accordingly), expression (2) is processed (Schneider, 2006) for each picture taking as a flame speed the average displacement of the tip of the flame divided by the time elapsed between two successive frames (Ds in Tables 1 and 2). Doing so, the acoustic vibration is merely cancelled and st appears nearly constant during the propagation (Schneider & Proust, 2005, Figs. 14 and 15). The turbulence intensity can be derived from the calibration curve of Fig. 4. The turbulent burning velocities are shown in Fig. 7 for Swely gel potato starch and in
The new data obtained with the sieve–riddler device are fully coherent with the data from former measurements with direct injection of dust. The curve is now much more complete and available data seem sufficiently accurate to compare with the Gu¨lder correlation at least at lab scale. Apparently the correlation overestimates the experimental data a bit although giving the same trend. The situation for aluminium dust is more difficult to assess because of the lack of fundamental data, but there is a trend for an increase of turbulent burning velocity with turbulence intensity, too. As expected, burning velocities of aluminium dust air mixtures are much higher than those of starch air mixtures. It is intended to continue the successful cooperation between the companies involved, doing more experiments
Fig. 6. Pressure records of transducers P1, P2, P3 and flame propagation.
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with use of turbulence sensors for in situ measurement of turbulence in front of the flame for starch dusts and aluminium dusts as well. References Andrews, G. E., & Bradley, D. (1972). Determination of burning velocities: A critical review. Combustion and Flame, 18, 133–153. Borghi, R., & Destriaux, M. (1998). Combustion and flame-chemical and physical principles, Edition, TECHNIP. Bray, K. (1990). Studies of turbulent burning velocities. In Proceedings of the royal society of London (Vol. A431, pp. 315–325). Gu¨lder, O¨. L. (1990). Turbulent premixed flame propagation models for different combustion regimes. In Comptes-rendus du 22nd symosium (International) on combustion. Hansen, O. R., Skjold, T. & Arntzen, B. J. (2004). DESC—a CFD-tool for dust explosions. In International European safety Management group (ESMG)-symposium, Nu¨rnberg. Hinze, J. O. (1975). Turbulence (2nd ed.). New-York: Mc Graw-Hill company. Mc Caffrey, B. J., & Heskestad, G. (1976). A robust bidirectional lowvelocity probe for flame and fire application. Combustion and Flame, 26, 125. Pelce, P. (1985). Effect of gravity on the propagation of flames in tubes. Journal of Physique, 46, 503–510.
Proust, Ch. (2004). Formation, inflammation, combustion des atmospheres explosives (ATEX) et effets associe´s. Me´moire d’HdR pre´sente´ a` l’Institut National Polytechnique de Lorraine, 12 Fe´v 2004. Proust, Ch. (2005). The usefulness of phenomenological tools to simulat the consequences of dust explosions: The experience of EFFEX. In Presented at the ESMG meeting, 11–14 October, 2005, Nu¨rnberg. Proust, Ch. (2006a). Flame propagation and combustion in some dust air mixtures. Journal of Loss Prevention in Process Industries, 19, 89–100. Proust, Ch. (2006b). A few fundamental aspects about ignition and flame propagation in dust clouds. Journal of Loss Prevention in Process Industries, in press, doi:10.1016/j.jlp.2005.06.035. Proust, Ch., & Veysierre, B. (1988). New experimental apparatus for studying the propagation of dust-air flames. Progress in astronautics and aeronautics series (Vol. 113, pp. 43–61). Schneider, H. (2006). Measurement of turbulent burning velocities by means of the open tube method. Journal of Loss Prevention in Process Industries, 19, 130–134. Schneider, H., & Proust, Ch. (2005). Laminar and turbulent burning velocities of dust clouds. In International ESMG symposium on process safety and industrial explosion protection, Nu¨rnberg, Germany. Tezok, F. I., Kauffman, C. W., Sichel, M., & Nicholls, J. A. (1985). Turbulent burning velocity measurement for dust/air mixtures in a constant volume spherical bomb. In 10th ICDERS, Berkeley.