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Suppression of premixed flames with inert particles
Journal of Loss Prevention in the Process Industries 35 (2015) 46e51
Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Suppression of premixed flames with inert particles Sreenivasan Ranganathan a, Minkyu Lee a, V'yacheslav Akkerman b, Ali S. Rangwala a, * a b
Department of Fire Protection Engineering, Worcester Polytechnic Institute, 100 Institute Rd., Worcester, MA 01609, USA Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506-6106, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 September 2014 Received in revised form 9 March 2015 Accepted 9 March 2015 Available online 17 March 2015
Dispersal of inert particles on a flame front is one of the techniques employed to suppress explosions. The current study investigates the influence of micron-sized (75e90 mm) inert (sand) particles on the laminar burning velocity of methane-air premixtures of different equivalence ratios (0.9e1.2) and reactant temperatures (297, 350, 400 K) using a Bunsen-burner type experimental apparatus. When an inert particle interacts with the flame zone, it extracts energy from the flame, thereby acting like a heat sink and hence reducing the flame temperature. Results show that for sand particle size in the range of 75 e90 mm, a concentration of 380e520 g/m3 is necessary for extinction of a methane-air flame at ambient temperature. An increase in reactant temperature reduces the heat-sink effect necessitating a higher concentration of sand to extinguish the flame. A mathematical model is developed to generalize the results and make them applicable to a wide range of parameters. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Burning velocity Heat sink Premixed flame Inert Sand
1. Introduction 1.1. Background and scope of work Prevention of explosions by using inert gases such as nitrogen is well known and is one of the most commonly used methods. The usage of chemically inert dusts such as sand, rock dust, limestone is another way to protect against explosions (Dewitte et al. 1964; Harris et al., 2010) and the practice of rock-dusting in coal mines has been adopted since very old days (Haswell Colliery explosion and Faraday and Lyell report (Faraday and Lyell) in 1845). It is important to scrutinize the influence of particle interaction on the rate at which a flame propagates to evaluate the hazardousness of any explosion. Fundamentally, this requires an investigation of the impact of inert particles on the laminar burning velocity of a gas-air premixture. The present study is focused on the experimental investigation of the effect of different concentrations of sand particles on the laminar burning velocity of premixed methane-air flames for a range of equivalence ratios and reactant temperatures. 1.2. Prior studies relevant to the topic Prior studies on suppression of flames using dust particles are
* Corresponding author. E-mail address: [email protected] (A.S. Rangwala). http://dx.doi.org/10.1016/j.jlp.2015.03.009 0950-4230/© 2015 Elsevier Ltd. All rights reserved.
divided into that devoted to inert and chemically reacting particles. The suppression mechanisms of the two types of particles are different (Chelliah, 2007; Fleming, 1999; Rumminger and Linteris, 2000). Specifically, while, inert particles in a reacting gas flow can suppress the flame through cooling, reacting particles produce inert gasses, which locally dilute the fuel or oxidizer levels thereby suppressing the flame. For example, chemically reacting particles such as sodium bicarbonate, potassium chromate and metal salts are found to decompose and produce CO2 to extinguish the flame (Birchall, 1970; Linteris et al., 2008; Mitani and Niioka, 1984; Rosser et al., 1963; Trees and Seshadri, 1997) whereas the thermal inhibitors like silica, alumina etc. reduce the flame temperature significantly (Amyotte, 2006; Andac et al., 2000; Dong et al., 2005; Harris et al., 2010; Mitani, 1981). As explained in Amyotte (2006) (Amyotte), the chemical inhibitors terminate the chain branching reactions by capturing the free radicals thereby inhibiting the chain reactions. An extensive literature review about the influence of solid inert-particles in mitigating and preventing explosions can be found in (Amyotte, 2006; Kosinski, 2008; Qiao et al., 2005). The current study is focused on suppression mechanisms using inert particles and hence the remainder of the literature review is focused on understanding the controlling parameters related to this type of suppression. In general, when a particle enters the flame zone, it absorbs some energy from the flame, thereby acting like a heat sink. The extinction mechanism is controlled mainly by thermal energy balance, where in the particle size, concentration and thermal
S. Ranganathan et al. / Journal of Loss Prevention in the Process Industries 35 (2015) 46e51
Nomenclature B Cp Cs c E k nproduct ns n_ s n_ air Q_ 00 q_ R Su Tb Tf 00 Tf
Frequency factor characterizing rate of gas phase oxidation of gaseous fuel Heat capacity of air Heat capacity of solid particle Concentration Activation energy characterizing the gas phase reaction Thermal conductivity Number of moles of products Number of particles Number of particles per unit volume per unit time passing through the flame Number of moles of air per unit time Heat release rate Heat flux absorbed by particles Universal gas constant Laminar burning velocity Flame temperature based on gaseous mixture and unburnt gas temperature Flame temperature Reduced flame temperature due to heat sink effect of particles
properties such as k, r, Cp are the important controlling parameters. Dewitte et al. (1964) (Dewitte et al.) conducted one of the earliest experimental studies on the inhibition and extinction of premixed flames by different dust particles based on the variation of the mean flame temperature and flame propagation velocity with respect to the concentration of dust particles. Specifically, a limiting value of the mean kinetic flame temperature (1500e1600 K) below which flame cannot self-sustain was identified and subsequently used to predict the critical dust concentration for the thermal inhibitors (Dewitte et al., 1964). Mitani (Mitani, 1981) developed a flame inhibition theory for ‘thermal’ inhibitors alone, based on two non-dimensional parameters e one related to the heat capacity of the particle; and the other related to the rate of heat of absorption by dust particle. On a large scale, the ability of the inert particles to suppress an explosion was investigated, experimentally and computationally, by Dong et al. 2005 (Dong et al.). An increase in the particle cloud density and the decrease in the particle size, facilitates explosion suppression because of an increase in the inhibition surface area of a contact with the gaseous flame front (Dong et al., 2005). The current study is a step towards improving our understanding of suppression of flames by inert particles, using an experimental platform with a simple flow geometry which can be conveniently reproduced in the laboratory. The influence of different concentrations of inert particles (sand particles of diameter 75e90 mm) are investigated using a laminar Bunsen burner type burner capable of handling dusty flows (Xie et al., 2012)). A mathematical model is also developed to generalize the experimental results and make them applicable to a wide range of parameters.
2. Experimental set-up and procedure
Tu U V_
Temperature of unburned gas Average flow velocity at burner nozzle
V_ CH4 V Ze
Volumetric flow rate of methane Volume Zeldovich number
air
Volumetric flow rate of air
Greek Symbols a Flame half cone angle ε ¼1/Ze, expansion parameter r Density of the solidegas mixture rs Density of the particle f Gaseous mixture equivalence ratio Subscripts a Ambient condition f Flame g Gas phase s Solid particle u Conditions in the controlled reactant temperature condition. s Solid particle
Air supplied to the burner is preheated using a gas heater and the inert particles are fed using a screw feeder, which is pre-calibrated for different feed rates ensuring continuous supply of particles to the burner (Fig. 2). The methane-air gas flow is controlled through a mass flow controller and the particles used in the experiments are in the size range of 75e90 mm. To ensure that the desired reactant temperature is achieved, a K-type thermocouple is located at the exit of the burner to measure the temperature of the reactants both before and after the experiments. During the experiments, the thermocouple is removed. The difference between the temperatures measured before and after the experiment, as compared to the mean desired temperature, did not exceed 3 K. For image acquisition, the presence of particles in the flame makes shadowgraph technique a mandatory requirement to clearly capture the flame edges (Fig. 3). The significance of this can also be referred from the previous studies of Xie et al. (Xie et al., 2012), Lee et al. (Lee et al., 2014), and Rockwell & Rangwala (Rockwell and Rangwala, 2013). Since the above mentioned literature goes over the accuracy of the flow controllers and the specifications with respect to shadowgraph system used in detail, for brevity these details are not repeated here. From a processed shadowgraph image of the flame zone as shown in Fig. 2d, the cone half angle a is measured for each sloping line detected and the values are averaged. The sloping line is detected from the middle portion of the Bunsen flame cone, in order to eliminate the stretch effects due to curvature at the flame tip and flame region close to the exit of the burner. These are the two regions which are susceptible to stretch effects in a Bunsen flame, due to the maximum local burning velocity at the flame tip and the minimum local burning velocity as a result of heat loss to the burner rim (Law, 2006). This half cone angle a, measured, is employed to calculate the laminar burning velocity using:
Su ¼ U$sinðaÞ; The experimental set-up used in this study, shown in Fig. 1. Specifically, an insulated steel tube of 1 cm diameter with inlets from methane, air and particle feeder has been used as the burner.
47
(1)
where U is the unburned gaseous mixture velocity at the exit of the burner, which is obtained by U ¼ Ua ðTu =Ta Þ. Though the total
48
S. Ranganathan et al. / Journal of Loss Prevention in the Process Industries 35 (2015) 46e51
Fig. 1. Schematic of Experimental set-up.
sand particle entrained methane-air premixed flame is adopted from previous studies of Xie et al. (Xie et al., 2012) and Lee et al. (Lee et al., 2014). Fig. 4 illustrates the interaction of a sand particle with the flame region. Particles are assumed to travel along a streamline of a mixture flow field. When a sand particle passes through the flame zone, it absorbs heat from the flame and thus behaves as a heat sink whereby reducing the flame temperature. This effect has been considered in the following section to develop an expression for such a reduced flame temperature. 3.1. Heat sink effect of inert particles Fig. 2. Different dust concentrations issuing from the burner exit.
volume flow rate of the mixture at ambient temperature is maintained the same, at a higher temperature, the unburned mixture velocity and volumetric flow rate will increase due to the decrease in the overall density. 3. Mathematical model Mathematical model to predict the laminar burning velocity of
In order to estimate the heat absorbed by the inert particles, first, the heat released from a flame without any particles is calculated. The chemical reaction for the combustion process reads:
f f CH4 þ ðO2 þ 3:76N2 Þ0 CO2 þ fH2 O þ 3:76N2 þ 2ð1 fÞO2 : 2 2 (2) The heat consumed to raise the temperature for f=2 moles of P methane or 4.76 mol of air, is given by,½ðTb Tu Þ Cp $nproduct . With the assumption that all the heat released is used to raise the
Fig. 3. Flame images: (a) dustless flame, (b) dusty flame, (c) shadowgraph, and (d) processed image.
S. Ranganathan et al. / Journal of Loss Prevention in the Process Industries 35 (2015) 46e51
49
Fig. 4. Illustration of an inert particle interacting with flame.
temperature of the mixture, the heat release rate of the methaneair premixed flame for a given flow of air and the equivalence ratio is calculated as
i n_ h X air : Cp $nproduct Q_ ¼ ðTb Tu Þ 4:76
(3)
Here nproduct is the number of moles of the products that depends on the equivalence ratio f. Assuming that the flame with particles releases the same amount of heat while it is also influenced by the temperature rise of particles, a new flame temperature can be estimated using the energy conservation below:
Q_ ¼
h
00
Tf Tu
X
Cp $nproduct
i n_ 00 air þ n_ s Cs Tf Tu : 4:76
(4) 00
Rearranging Eq. (4), we express the new flame temperature, Tf , in the form 00
Tf ¼
Q_
n_air P Cp $nproduct 4:76
þ n_ s Cs
þ Tu ;
(5)
where n_ s is the number of particles per unit volume per unit time passing through the flame, and it is calculated by:
n_ s ¼ V_ air þ V_ CH4 ns rs Vparticle :
(6)
The new flame temperature estimated after accounting for the heat sink effect is plotted in Fig. 5, and it indicates a continous decrease in the flame temperature with the addition of sand for all the conditions tested. As expected, the higher reactant temperature and stoichiometric gaseous mixture condition results in maximum flame temperature. The laminar burning velocity, Su, is then calculated using the expression from Seshadri et al. (Seshadri et al., 1992):
"
2Bku ε2 E Su ¼ exp 00 rg Cg RTf
!#1=2 ;
(7)
where
00 E Tf Tu rair V_ air þ rCH4 V_ CH4 1 rg ¼ ; ε ¼ ; Ze ¼ : 00 Ze RTf 2 V_ air þ V_ CH4
(8)
The valuesB ¼ 3:5 106 =ðmole$sÞ and E ¼ 110:5 kJ=mole in Eq. (7) are chosen to match the calculated laminar burning velocity of
Fig. 5. New flame temperature, for different sand concentration for equivalence ratios f ¼ 0.9, f ¼ 1.0 and f ¼ 1.2; Tu ¼ 297 K, 350 K, and 400 K.
no dust conditions with that in experiments performed without dusts. 4. Results and discussion Experimentally obtained laminar burning velocities of sandmethane-air flame are shown in Fig. 6. It can be observed that irrespective of the equivalence ratio or the sand particles concentration, the laminar burning velocity increases with the reactant temperature. Indeed, the increase in the reactant temperature is accompanied by the increase in the adiabatic flame temperature, flow velocity and flame cone angle. Further, the increase in the reactant temperature reduces the density of the gas, which results in a higher flow velocity. However, since a sand particle is inert, its addition in any amount only results in the decrease in the flame temperature. Therefore, a decreasing trend in laminar burning velocity is observed with increasing the sand-dust concentration. In terms of percentage decrease in the laminar burning velocity values, the experimental results indicate that the addition of first 50 g/m3 sand creates a decrease in the burning velocity value of around 5% as compared to the no-sand burning velocity values from all the cases tested. Further addition of sand, till the concentrations of 100 g/m3, results in approximately 2.6% decrease in the burning velocity as compared to the 50 g/m3 case; while more addition of sand, till 150 g/m3, provides 2.9% more reduction of the burning velocity as compared to the 100 g/m3 case. These are the average values from all the conditions tested for respective sand additions. Table 1 presents the percentage decrease in the burning velocity values for all the conditions considered in the study. Further, the comparison between the experimental and mathematical model results (Fig. 6) show reasonable agreement, which indicates that the present model is able to predict the trend of the variation of a burning velocity well. As the concentration of sand particle increases, a critical value of Su is reached whereby flame propagation is no longer possible. This value for the particle size of 75e90 mm, used in present study, can be obtained by extrapolating the mathematical model result. The upper flammability limit of methane is about 15% in volume (Law, 2006). Corresponding laminar burning velocity observed from experiment is 0.13 m/s. Using this burning velocity as the limiting value in the mathematical model, the critical sand concentrations for flame extinctions are predicted by creating a linear fit for all the conditions. Table 2 reports these sand concentrations for different
50
S. Ranganathan et al. / Journal of Loss Prevention in the Process Industries 35 (2015) 46e51
Fig. 6. Experimental results and mathematical model of laminar burning velocity at different sand concentration for equivalence ratios: (a) f ¼ 0.9, (b) f ¼ 1.0 and (c) f ¼ 1.2.
Table 1 Percentage decrease in experimental value of laminar burning velocity due to increase in dust concentration. Percentage decrease in Su with sand addition Tu (K)
cs at ambient (g/m3)
f ¼ 0.9
f ¼ 1.0
f ¼ 1.2
297
0e50 50e100 100e150 0e50 50e100 100e150 0e50 50e100 100e150
5.23 3.54 3.26 5.44 3.12 6.23 3.31 1.61 1.89
6.98 3.25 2.53 6.23 2.13 2.18 2.58 3.43 3.53
6.62 3.10 3.46 6.56 1.8 1.58 2.69 1.21 2.18
350
400
cases considered at which the burning velocity reaches the limiting value of 0.13 m/s. The critical sand concentration for flame extinction was checked using the current experimental set-up. It is observed that around a concentration range of 380e420 g/m3, for the flame condition of f ¼ 0.9 at ambient temperature, the flame became unstable and a few attempts showed the flame extinction as well. For the same condition the predicted value of cs is around 386 g/m3 (Table 2).
Table 2 Predicted critical sand concentration of sand required for flame extinction. Predicted critical concentration of sand in g/m3 required for flame extinction
f
Tu (K) ¼ 297
Tu (K) ¼ 350
Tu (K) ¼ 400
0.9 1.0 1.2
386.3 520.5 396.3
578.5 585.2 597.8
620.2 630.3 646
The results shown in Table 2 indicate the effect of the increase in Tu: it leads to the increase in the critical concentration of sand dust necessary for the flame extinction. This effect is due to two main reasons. Firstly, an increase in reactant temperature causes the flame temperature to increase thereby causing the threshold of the heat-sink effect necessary to quench the flame to increase. Secondly, for the same concentration of sand particles, at a higher reactant temperature, the residence time of the particle in the flame zone will be lower due to the higher propagation velocity. This results in a lower heat loss from the flame zone to the particles at these conditions. Hence to produce the adequate heat sink effect to reduce the burning velocity to the critical value, at higher reactant temperatures, larger concentration of sand particles are required.
S. Ranganathan et al. / Journal of Loss Prevention in the Process Industries 35 (2015) 46e51
5. Conclusions In the present study, the influence of sand particles (75e90 mm) on the laminar burning velocity of premixed methane-air flames has been investigated for different concentrations of particles at different reactant temperatures. The experiments are conducted using a Bunsen burner set-up which can allow the constant flow of particles with the gaseous reactant mixture. The results indicate a considerable reduction in the burning velocity (in the range of 1.2e7%) with the addition of every 50 g/m3 sand particles, due to the heat loss from the flame zone with the addition of particles. A mathematical model, developed based on this heat sink effect, has been employed to predict the burning velocity values, with good qualitative agreement between the predicted quantities and the experimental results. Validation of predicted critical concentration of the sand required for flame extinction indicates that the predicted model value is within the range observed from experiments. Thus, the model is used to predict the critical sand concentration value at which the flame can no longer self-sustain based on the critical burning velocity value for all the cases considered. It is identified that at higher reactant temperatures, larger concentration of sand is required to inhibit the flame. Acknowledgements During the course of this study, Lee and Rangwala were supported by the National Science Foundation (NSF) Award #0846764, while Ranganathan and Akkerman were supported by the Alpha Foundation for the Improvement of Mine Safety and Health. References Amyotte, P.R., 2006. Solid inertants and their use in dust explosion prevention and mitigation. J. Loss Prev. Process Ind. 19 (2), 161e173. Andac, M.G., Egolfopoulos, F.N., Cambell, C.S., Lauvergne, R., 2000. Effects of inert dust clouds on the extinction of strained, laminar flames at normal- and microgravity. Proc. Combust. Inst. 28, 2921e2929. Birchall, J., 1970. On the mechanism of flame inhibition by alkali metal salts.
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Combust. Flame 14 (1), 85e95. Chelliah, H., 2007. Flame inhibition/suppression by water mist: droplet size/surface area, flame structure, and flow residence time effects. Proc. Combust. Inst. 31 (2), 2711e2719. Dewitte, M., Vrebosch, J., Van Tiggelen, A., 1964. Inhibition and extinction of premixed flames by dust particles. Combust. Flame 8 (4), 257e266. Dong, G., Fan, B., Xie, B., Ye, J., 2005. Experimental investigation and numerical validation of explosion suppression by inert particles in large-scale duct. Proc. Combust. Inst. 30 (2), 2361e2368. Faraday, M., Lyell, C., 1845. Report on the explosion at the Haswell Collieries, and on the means of preventing similar accidents. Philos. Mag. 26, 16e35. Fleming, J.W., 1999. Chemica Fire Suppressants: How can we replace Halon?. Paper Presented at the Proc. Fall Technical Meeting of the Eastern States Combustion Institute, Raleigh, NC. Harris, M.L., Weiss, E.S., Man, C., Harteis, S.P., Goodman, G.V., Sapko, M.J., 2010. Rock dust considerations in underground coal mines. In: Proceedings of the US/North American Mine Ventilation Symposium, vol. 13, pp. 267e271 (Sudbury, Ontario, Canada). Kosinski, P., 2008. Numerical investigation of explosion suppression by inert particles in straight ducts. J. Hazard. Mater. 154 (1), 981e991. Law, C.K., 2006. Combustion Physics. Cambridge University Press. Lee, M., Ranganathan, S., Rangwala, A.S., 2014. Influence of the reactant temperature on particle entrained laminar methaneeair premixed flames. Proc. Combust. Inst. 35. Linteris, G.T., Rumminger, M.D., Babushok, V.I., 2008. Catalytic inhibition of laminar flames by transition metal compounds. Prog. Energy Combust. Sci. 34 (3), 288e329. Mitani, T., 1981. A flame inhibition theory by inert dust and spray. Combust. flame 43, 243e253. Mitani, T., Niioka, T., 1984. Extinction phenomenon of premixed flames with alkali metal compounds. Combust. Flame 55 (1), 13e21. Qiao, L., Kim, C., Faeth, G., 2005. Suppression effects of diluents on laminar premixed hydrogen/oxygen/nitrogen flames. Combust. Flame 143 (1), 79e96. Rockwell, S.R., Rangwala, A.S., 2013. Influence of coal dust on premixed turbulent methaneeair flames. Combust. Flame 160 (3), 635e640. Rosser Jr., W., Inami, S., Wise, H., 1963. The effect of metal salts on premixed hydrocarbondair flames. Combust. Flame 7, 107e119. Rumminger, M.D., Linteris, G.T., 2000. The role of particles in the inhibition of premixed flames by iron pentacarbonyl. Combust. Flame 123 (1), 82e94. Seshadri, K., Berlad, A.L., Tangirla, V., 1992. The structure of premixed particle-cloud flames. Combust. Flame 89, 333e342. Trees, D., Seshadri, K., 1997. Experimental studies of flame extinction by sodium bicarbonate (NaHCO3) powder. Combust. Sci. Technol. 122 (1e6), 215e230. Xie, Y., Raghavan, V., Rangwala, A.S., 2012. Study of interaction of entrained coal dust particles in lean methane-air premixed flames. Combust. Flame 159, 2449e2456.
Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Suppression of premixed flames with inert particles Sreenivasan Ranganathan a, Minkyu Lee a, V'yacheslav Akkerman b, Ali S. Rangwala a, * a b
Department of Fire Protection Engineering, Worcester Polytechnic Institute, 100 Institute Rd., Worcester, MA 01609, USA Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506-6106, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 September 2014 Received in revised form 9 March 2015 Accepted 9 March 2015 Available online 17 March 2015
Dispersal of inert particles on a flame front is one of the techniques employed to suppress explosions. The current study investigates the influence of micron-sized (75e90 mm) inert (sand) particles on the laminar burning velocity of methane-air premixtures of different equivalence ratios (0.9e1.2) and reactant temperatures (297, 350, 400 K) using a Bunsen-burner type experimental apparatus. When an inert particle interacts with the flame zone, it extracts energy from the flame, thereby acting like a heat sink and hence reducing the flame temperature. Results show that for sand particle size in the range of 75 e90 mm, a concentration of 380e520 g/m3 is necessary for extinction of a methane-air flame at ambient temperature. An increase in reactant temperature reduces the heat-sink effect necessitating a higher concentration of sand to extinguish the flame. A mathematical model is developed to generalize the results and make them applicable to a wide range of parameters. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Burning velocity Heat sink Premixed flame Inert Sand
1. Introduction 1.1. Background and scope of work Prevention of explosions by using inert gases such as nitrogen is well known and is one of the most commonly used methods. The usage of chemically inert dusts such as sand, rock dust, limestone is another way to protect against explosions (Dewitte et al. 1964; Harris et al., 2010) and the practice of rock-dusting in coal mines has been adopted since very old days (Haswell Colliery explosion and Faraday and Lyell report (Faraday and Lyell) in 1845). It is important to scrutinize the influence of particle interaction on the rate at which a flame propagates to evaluate the hazardousness of any explosion. Fundamentally, this requires an investigation of the impact of inert particles on the laminar burning velocity of a gas-air premixture. The present study is focused on the experimental investigation of the effect of different concentrations of sand particles on the laminar burning velocity of premixed methane-air flames for a range of equivalence ratios and reactant temperatures. 1.2. Prior studies relevant to the topic Prior studies on suppression of flames using dust particles are
* Corresponding author. E-mail address: [email protected] (A.S. Rangwala). http://dx.doi.org/10.1016/j.jlp.2015.03.009 0950-4230/© 2015 Elsevier Ltd. All rights reserved.
divided into that devoted to inert and chemically reacting particles. The suppression mechanisms of the two types of particles are different (Chelliah, 2007; Fleming, 1999; Rumminger and Linteris, 2000). Specifically, while, inert particles in a reacting gas flow can suppress the flame through cooling, reacting particles produce inert gasses, which locally dilute the fuel or oxidizer levels thereby suppressing the flame. For example, chemically reacting particles such as sodium bicarbonate, potassium chromate and metal salts are found to decompose and produce CO2 to extinguish the flame (Birchall, 1970; Linteris et al., 2008; Mitani and Niioka, 1984; Rosser et al., 1963; Trees and Seshadri, 1997) whereas the thermal inhibitors like silica, alumina etc. reduce the flame temperature significantly (Amyotte, 2006; Andac et al., 2000; Dong et al., 2005; Harris et al., 2010; Mitani, 1981). As explained in Amyotte (2006) (Amyotte), the chemical inhibitors terminate the chain branching reactions by capturing the free radicals thereby inhibiting the chain reactions. An extensive literature review about the influence of solid inert-particles in mitigating and preventing explosions can be found in (Amyotte, 2006; Kosinski, 2008; Qiao et al., 2005). The current study is focused on suppression mechanisms using inert particles and hence the remainder of the literature review is focused on understanding the controlling parameters related to this type of suppression. In general, when a particle enters the flame zone, it absorbs some energy from the flame, thereby acting like a heat sink. The extinction mechanism is controlled mainly by thermal energy balance, where in the particle size, concentration and thermal
S. Ranganathan et al. / Journal of Loss Prevention in the Process Industries 35 (2015) 46e51
Nomenclature B Cp Cs c E k nproduct ns n_ s n_ air Q_ 00 q_ R Su Tb Tf 00 Tf
Frequency factor characterizing rate of gas phase oxidation of gaseous fuel Heat capacity of air Heat capacity of solid particle Concentration Activation energy characterizing the gas phase reaction Thermal conductivity Number of moles of products Number of particles Number of particles per unit volume per unit time passing through the flame Number of moles of air per unit time Heat release rate Heat flux absorbed by particles Universal gas constant Laminar burning velocity Flame temperature based on gaseous mixture and unburnt gas temperature Flame temperature Reduced flame temperature due to heat sink effect of particles
properties such as k, r, Cp are the important controlling parameters. Dewitte et al. (1964) (Dewitte et al.) conducted one of the earliest experimental studies on the inhibition and extinction of premixed flames by different dust particles based on the variation of the mean flame temperature and flame propagation velocity with respect to the concentration of dust particles. Specifically, a limiting value of the mean kinetic flame temperature (1500e1600 K) below which flame cannot self-sustain was identified and subsequently used to predict the critical dust concentration for the thermal inhibitors (Dewitte et al., 1964). Mitani (Mitani, 1981) developed a flame inhibition theory for ‘thermal’ inhibitors alone, based on two non-dimensional parameters e one related to the heat capacity of the particle; and the other related to the rate of heat of absorption by dust particle. On a large scale, the ability of the inert particles to suppress an explosion was investigated, experimentally and computationally, by Dong et al. 2005 (Dong et al.). An increase in the particle cloud density and the decrease in the particle size, facilitates explosion suppression because of an increase in the inhibition surface area of a contact with the gaseous flame front (Dong et al., 2005). The current study is a step towards improving our understanding of suppression of flames by inert particles, using an experimental platform with a simple flow geometry which can be conveniently reproduced in the laboratory. The influence of different concentrations of inert particles (sand particles of diameter 75e90 mm) are investigated using a laminar Bunsen burner type burner capable of handling dusty flows (Xie et al., 2012)). A mathematical model is also developed to generalize the experimental results and make them applicable to a wide range of parameters.
2. Experimental set-up and procedure
Tu U V_
Temperature of unburned gas Average flow velocity at burner nozzle
V_ CH4 V Ze
Volumetric flow rate of methane Volume Zeldovich number
air
Volumetric flow rate of air
Greek Symbols a Flame half cone angle ε ¼1/Ze, expansion parameter r Density of the solidegas mixture rs Density of the particle f Gaseous mixture equivalence ratio Subscripts a Ambient condition f Flame g Gas phase s Solid particle u Conditions in the controlled reactant temperature condition. s Solid particle
Air supplied to the burner is preheated using a gas heater and the inert particles are fed using a screw feeder, which is pre-calibrated for different feed rates ensuring continuous supply of particles to the burner (Fig. 2). The methane-air gas flow is controlled through a mass flow controller and the particles used in the experiments are in the size range of 75e90 mm. To ensure that the desired reactant temperature is achieved, a K-type thermocouple is located at the exit of the burner to measure the temperature of the reactants both before and after the experiments. During the experiments, the thermocouple is removed. The difference between the temperatures measured before and after the experiment, as compared to the mean desired temperature, did not exceed 3 K. For image acquisition, the presence of particles in the flame makes shadowgraph technique a mandatory requirement to clearly capture the flame edges (Fig. 3). The significance of this can also be referred from the previous studies of Xie et al. (Xie et al., 2012), Lee et al. (Lee et al., 2014), and Rockwell & Rangwala (Rockwell and Rangwala, 2013). Since the above mentioned literature goes over the accuracy of the flow controllers and the specifications with respect to shadowgraph system used in detail, for brevity these details are not repeated here. From a processed shadowgraph image of the flame zone as shown in Fig. 2d, the cone half angle a is measured for each sloping line detected and the values are averaged. The sloping line is detected from the middle portion of the Bunsen flame cone, in order to eliminate the stretch effects due to curvature at the flame tip and flame region close to the exit of the burner. These are the two regions which are susceptible to stretch effects in a Bunsen flame, due to the maximum local burning velocity at the flame tip and the minimum local burning velocity as a result of heat loss to the burner rim (Law, 2006). This half cone angle a, measured, is employed to calculate the laminar burning velocity using:
Su ¼ U$sinðaÞ; The experimental set-up used in this study, shown in Fig. 1. Specifically, an insulated steel tube of 1 cm diameter with inlets from methane, air and particle feeder has been used as the burner.
47
(1)
where U is the unburned gaseous mixture velocity at the exit of the burner, which is obtained by U ¼ Ua ðTu =Ta Þ. Though the total
48
S. Ranganathan et al. / Journal of Loss Prevention in the Process Industries 35 (2015) 46e51
Fig. 1. Schematic of Experimental set-up.
sand particle entrained methane-air premixed flame is adopted from previous studies of Xie et al. (Xie et al., 2012) and Lee et al. (Lee et al., 2014). Fig. 4 illustrates the interaction of a sand particle with the flame region. Particles are assumed to travel along a streamline of a mixture flow field. When a sand particle passes through the flame zone, it absorbs heat from the flame and thus behaves as a heat sink whereby reducing the flame temperature. This effect has been considered in the following section to develop an expression for such a reduced flame temperature. 3.1. Heat sink effect of inert particles Fig. 2. Different dust concentrations issuing from the burner exit.
volume flow rate of the mixture at ambient temperature is maintained the same, at a higher temperature, the unburned mixture velocity and volumetric flow rate will increase due to the decrease in the overall density. 3. Mathematical model Mathematical model to predict the laminar burning velocity of
In order to estimate the heat absorbed by the inert particles, first, the heat released from a flame without any particles is calculated. The chemical reaction for the combustion process reads:
f f CH4 þ ðO2 þ 3:76N2 Þ0 CO2 þ fH2 O þ 3:76N2 þ 2ð1 fÞO2 : 2 2 (2) The heat consumed to raise the temperature for f=2 moles of P methane or 4.76 mol of air, is given by,½ðTb Tu Þ Cp $nproduct . With the assumption that all the heat released is used to raise the
Fig. 3. Flame images: (a) dustless flame, (b) dusty flame, (c) shadowgraph, and (d) processed image.
S. Ranganathan et al. / Journal of Loss Prevention in the Process Industries 35 (2015) 46e51
49
Fig. 4. Illustration of an inert particle interacting with flame.
temperature of the mixture, the heat release rate of the methaneair premixed flame for a given flow of air and the equivalence ratio is calculated as
i n_ h X air : Cp $nproduct Q_ ¼ ðTb Tu Þ 4:76
(3)
Here nproduct is the number of moles of the products that depends on the equivalence ratio f. Assuming that the flame with particles releases the same amount of heat while it is also influenced by the temperature rise of particles, a new flame temperature can be estimated using the energy conservation below:
Q_ ¼
h
00
Tf Tu
X
Cp $nproduct
i n_ 00 air þ n_ s Cs Tf Tu : 4:76
(4) 00
Rearranging Eq. (4), we express the new flame temperature, Tf , in the form 00
Tf ¼
Q_
n_air P Cp $nproduct 4:76
þ n_ s Cs
þ Tu ;
(5)
where n_ s is the number of particles per unit volume per unit time passing through the flame, and it is calculated by:
n_ s ¼ V_ air þ V_ CH4 ns rs Vparticle :
(6)
The new flame temperature estimated after accounting for the heat sink effect is plotted in Fig. 5, and it indicates a continous decrease in the flame temperature with the addition of sand for all the conditions tested. As expected, the higher reactant temperature and stoichiometric gaseous mixture condition results in maximum flame temperature. The laminar burning velocity, Su, is then calculated using the expression from Seshadri et al. (Seshadri et al., 1992):
"
2Bku ε2 E Su ¼ exp 00 rg Cg RTf
!#1=2 ;
(7)
where
00 E Tf Tu rair V_ air þ rCH4 V_ CH4 1 rg ¼ ; ε ¼ ; Ze ¼ : 00 Ze RTf 2 V_ air þ V_ CH4
(8)
The valuesB ¼ 3:5 106 =ðmole$sÞ and E ¼ 110:5 kJ=mole in Eq. (7) are chosen to match the calculated laminar burning velocity of
Fig. 5. New flame temperature, for different sand concentration for equivalence ratios f ¼ 0.9, f ¼ 1.0 and f ¼ 1.2; Tu ¼ 297 K, 350 K, and 400 K.
no dust conditions with that in experiments performed without dusts. 4. Results and discussion Experimentally obtained laminar burning velocities of sandmethane-air flame are shown in Fig. 6. It can be observed that irrespective of the equivalence ratio or the sand particles concentration, the laminar burning velocity increases with the reactant temperature. Indeed, the increase in the reactant temperature is accompanied by the increase in the adiabatic flame temperature, flow velocity and flame cone angle. Further, the increase in the reactant temperature reduces the density of the gas, which results in a higher flow velocity. However, since a sand particle is inert, its addition in any amount only results in the decrease in the flame temperature. Therefore, a decreasing trend in laminar burning velocity is observed with increasing the sand-dust concentration. In terms of percentage decrease in the laminar burning velocity values, the experimental results indicate that the addition of first 50 g/m3 sand creates a decrease in the burning velocity value of around 5% as compared to the no-sand burning velocity values from all the cases tested. Further addition of sand, till the concentrations of 100 g/m3, results in approximately 2.6% decrease in the burning velocity as compared to the 50 g/m3 case; while more addition of sand, till 150 g/m3, provides 2.9% more reduction of the burning velocity as compared to the 100 g/m3 case. These are the average values from all the conditions tested for respective sand additions. Table 1 presents the percentage decrease in the burning velocity values for all the conditions considered in the study. Further, the comparison between the experimental and mathematical model results (Fig. 6) show reasonable agreement, which indicates that the present model is able to predict the trend of the variation of a burning velocity well. As the concentration of sand particle increases, a critical value of Su is reached whereby flame propagation is no longer possible. This value for the particle size of 75e90 mm, used in present study, can be obtained by extrapolating the mathematical model result. The upper flammability limit of methane is about 15% in volume (Law, 2006). Corresponding laminar burning velocity observed from experiment is 0.13 m/s. Using this burning velocity as the limiting value in the mathematical model, the critical sand concentrations for flame extinctions are predicted by creating a linear fit for all the conditions. Table 2 reports these sand concentrations for different
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Fig. 6. Experimental results and mathematical model of laminar burning velocity at different sand concentration for equivalence ratios: (a) f ¼ 0.9, (b) f ¼ 1.0 and (c) f ¼ 1.2.
Table 1 Percentage decrease in experimental value of laminar burning velocity due to increase in dust concentration. Percentage decrease in Su with sand addition Tu (K)
cs at ambient (g/m3)
f ¼ 0.9
f ¼ 1.0
f ¼ 1.2
297
0e50 50e100 100e150 0e50 50e100 100e150 0e50 50e100 100e150
5.23 3.54 3.26 5.44 3.12 6.23 3.31 1.61 1.89
6.98 3.25 2.53 6.23 2.13 2.18 2.58 3.43 3.53
6.62 3.10 3.46 6.56 1.8 1.58 2.69 1.21 2.18
350
400
cases considered at which the burning velocity reaches the limiting value of 0.13 m/s. The critical sand concentration for flame extinction was checked using the current experimental set-up. It is observed that around a concentration range of 380e420 g/m3, for the flame condition of f ¼ 0.9 at ambient temperature, the flame became unstable and a few attempts showed the flame extinction as well. For the same condition the predicted value of cs is around 386 g/m3 (Table 2).
Table 2 Predicted critical sand concentration of sand required for flame extinction. Predicted critical concentration of sand in g/m3 required for flame extinction
f
Tu (K) ¼ 297
Tu (K) ¼ 350
Tu (K) ¼ 400
0.9 1.0 1.2
386.3 520.5 396.3
578.5 585.2 597.8
620.2 630.3 646
The results shown in Table 2 indicate the effect of the increase in Tu: it leads to the increase in the critical concentration of sand dust necessary for the flame extinction. This effect is due to two main reasons. Firstly, an increase in reactant temperature causes the flame temperature to increase thereby causing the threshold of the heat-sink effect necessary to quench the flame to increase. Secondly, for the same concentration of sand particles, at a higher reactant temperature, the residence time of the particle in the flame zone will be lower due to the higher propagation velocity. This results in a lower heat loss from the flame zone to the particles at these conditions. Hence to produce the adequate heat sink effect to reduce the burning velocity to the critical value, at higher reactant temperatures, larger concentration of sand particles are required.
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5. Conclusions In the present study, the influence of sand particles (75e90 mm) on the laminar burning velocity of premixed methane-air flames has been investigated for different concentrations of particles at different reactant temperatures. The experiments are conducted using a Bunsen burner set-up which can allow the constant flow of particles with the gaseous reactant mixture. The results indicate a considerable reduction in the burning velocity (in the range of 1.2e7%) with the addition of every 50 g/m3 sand particles, due to the heat loss from the flame zone with the addition of particles. A mathematical model, developed based on this heat sink effect, has been employed to predict the burning velocity values, with good qualitative agreement between the predicted quantities and the experimental results. Validation of predicted critical concentration of the sand required for flame extinction indicates that the predicted model value is within the range observed from experiments. Thus, the model is used to predict the critical sand concentration value at which the flame can no longer self-sustain based on the critical burning velocity value for all the cases considered. It is identified that at higher reactant temperatures, larger concentration of sand is required to inhibit the flame. Acknowledgements During the course of this study, Lee and Rangwala were supported by the National Science Foundation (NSF) Award #0846764, while Ranganathan and Akkerman were supported by the Alpha Foundation for the Improvement of Mine Safety and Health. References Amyotte, P.R., 2006. Solid inertants and their use in dust explosion prevention and mitigation. J. Loss Prev. Process Ind. 19 (2), 161e173. Andac, M.G., Egolfopoulos, F.N., Cambell, C.S., Lauvergne, R., 2000. Effects of inert dust clouds on the extinction of strained, laminar flames at normal- and microgravity. Proc. Combust. Inst. 28, 2921e2929. Birchall, J., 1970. On the mechanism of flame inhibition by alkali metal salts.
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