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Flame acceleration due to water droplets action
ARTICLE IN PRESS Journal of Loss Prevention in the Process Industries 21 (2008) 472– 477
Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Flame acceleration due to water droplets action Marian Gieras Warsaw University of Technology, 00-665 Warsaw, Poland
a r t i c l e in f o
a b s t r a c t
Article history: Received 22 October 2007 Received in revised form 13 March 2008 Accepted 14 March 2008
Results of experimental and numerical studies on the influence of water droplets on premixed gaseous flame propagation are presented. Experimental data were obtained in the vertical standard tube of 1.2 m length and of 0.05 0.05 m2 cross-section. The research was focused on the mechanism of a flame acceleration caused by water droplets action. For better understanding of these processes, the neutral sand particles were also used. Pictures documenting flame acceleration caused by water droplets are presented. The simulations of a flame acceleration process resulting from the presence of neutral particles were made by using FLUENT 6.0 computer code. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Hexane–air mixture Ignition Explosion Upper explosion limit
1. Introduction Fire and explosion hazard have forced people to look for effective suppression systems, as well as effective extinguishing media. One of the oldest known fire suppressant is water. Water appears to be very attractive as an extinguishing agent because of its relatively low cost and large availability. Moreover, water causes relatively low pollution to the environment. It was stated, on the ground of experimental and theoretical investigation (Heskestad, 2002, 2003; Jones, & Thomas, 1993; Parra, Castro, Mendez, Villfrula, & Rodriguez, 2004; Rasbash, & Rogowski, 1956; Rasbash, Rogowski, & Stark, 1960; Schewille, & Leuptow, 2006), that the effectiveness of flame suppression by means of water can be significantly improved by using suitable water sprays in suppression of some forms of combustion. For example, finer spray can cause the most rapid extinction of fires of volatile fuels (alcohol, benzol), whereas the coarser sprays were more suitable to fire extinction than less volatile fuels (oil). Experiments have shown that, if the suppression systems are not capable of delivering the required quantity of water to stop combustion, then the explosion could be even more violent than without the action of the device (Proust, 1996). It is believed that the local increase of turbulence induced by the injection of the extinguishing agent, might be responsible for this dramatic effect. On the one hand, the same quantities of a water spray can often accelerate relatively slow flames, but on the other they can sometimes cause suppression of detonation (Jones, & Thomas,
Tel.: +48 22 234 52 22; fax: +48 22 825 05 65.
E-mail address: [email protected] 0950-4230/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jlp.2008.03.004
1993; Thomas, Edwards, & Edwards, 1990; Thomas, Jones, & Edwards, 1991). The main objective of this work is to show the mechanism of a flame acceleration caused by water droplets (moving relatively slowly in relation to the premixed flame) during their interaction with the propagating flame. Good understanding of this mechanism can be helpful in the development of fire protection systems for industry and can lead to improved numerical modelling.
2. Apparatus and experimental details Studies of water droplets–gaseous flame interaction were carried out in the research stand presented in Fig. 1. The main part of it is the standard vertical tube, 1.2 m. Long and of 0.05 0.05 m2 square cross-section (according to Jarosinski, Strehlow, & Azarbazin, 1982). The upper end of the tube is closed and the lower end is opened. Gas supply system was located at the top of the tube. Methane–air mixture was prepared in 40 l cylinder by the method of partial pressures with the concentration accuracy of 70.1%. Technical methane (99% CH4) was used as a gaseous fuel. The studies of the mechanism of interaction between water droplets and propagating gaseous flame were conducted for two cases. In the first one, there was applied a special system of ducting and fixing of quartz-glass needle, on which the single water droplet was hanged. This system was placed in the central part of the tube. Glass window of dimension 0.05 0.05 m2 were also located in the central part of the tube for visualization of the process directly or by means of Michelson interferometer. The diameter of glass needles was 0.1 and 2 mm, however the
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Fig. 2. Flame front disturbance caused by a water droplet hanged on a quartz-glass needle (graphically illustrated on the left side of the picture). Methane concentration in mixture—5.5%; droplet diameter—3 mm.
Fig. 1. Picture of the research stand with standard vertical tube, 1.2 m long. 1—electric motor; 2—spark generator; 3—control system; 4—vertical combustion chamber; 5—starting switch; 6—plate of Michelsen interferometer; 7—collimator; 8—video camera. Fig. 3. Direct pictures of flame front disturbance caused by a free falling water droplet in the methane–air mixture. 5.5% CH4; droplet diameter—4 mm.
diameter of a small ball at the end of the needle (which helps to fix the water droplet on the glass needle) was 0.2 and 0.3 mm, respectively. In the second one, a special generator (consisting of pressurized tank with water, equipped with special pipe nozzles in three configurations: 1 1 ¼ 1, 5 5 ¼ 25 and 6 6 ¼ 36 twodimensional systems) of water droplet of different sizes was placed at the top of the tube. An oblong window of dimension 0.05 0.6 m2 was installed in the central part of the duct. Water flow rates and water droplet sizes could be changed by changing of pressure in the tank (up to 10 bar), quantity and diameter of a pipe nozzle. In the second option of the research stand, the combustion chamber was equipped with a special vibrating dust feeder, which made it possible to fill up the tube with the dust cloud. Lycopodium powder with mean diameter equal to 10 mm was used as dust fuel. The same vibrating dust feeder was also used to generate a sand cloud. Concentrations of dust as well as sand were changed by changing of vibration frequency and changing of special vibrating screens. More detailed information about the research stands are presented by Gieras (2000, 2001). During the experimental tests, the following parameters: water droplet diameter and velocity, water droplets mass flow rate as well as the concentration of methane in the mixture, were changed. Ignition was initiated by the electric spark (energy 0.1 J) located 10 cm from the lower end of the tube. Ignition occurred just after the tube was fully filled in with the combustible gaseous or lycopodium–air mixture. All tests were recorded by a video camera. Flame velocity was measured on the basis of pictures taken by the video camera and on the basis of three microthermocouples located at the wall of the combustion tube.
3. Experimental results and discussion It was observed that collision of single water droplets with the flame propagating in a methane–air mixture causes the clear visible disturbance of the flame surface in a form of hollows, craters or creases. The size of the disturbance increases along with increase of droplet diameter and velocity in relation to the flame. An example of a penetration process of a slow propagating flame by a single water droplet can be observed in Figs. 2 and 3.
Fig. 4. Direct pictures of flame front disturbance caused by a water droplet hanged on a quartz-glass needle. The lycopodium–air mixture. Concentration of lycopodium—45 g/m3; droplet diameter—3 mm.
Fig. 5. Graphically illustrated process of increasing of flame front surface caused by water droplets.
A disturbance of flame front can be also observed in dusty flames (Fig. 4), but in the case of flames due to an irregular flame front surface, the disturbance is not so clearly visible as in the case of gaseous flames. A number of water droplets can cause a radical change in a flame shape which can lead to a rapid increase of the flame surface (Fig. 5). In general, it can be stated, that aerodynamic effects produced by the moving (in relation to the flame) droplets lead to the turbulization of the flammable mixture and high disturbance and corrugation of the flame front shape. They cause an increase of the flame surface and flame thickness. These effects can play more important role than effects of cooling and dilution of the combustion zone. In such situation, a sudden acceleration of flame propagation is obtained. As an example of these processes, the change of a gaseous flame structure and propagation velocity due to an action of water droplets for two different concentrations is
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Fig. 7. Flame propagation process in methane–air mixture (6.5% CH4). (a) Nondisturbed, (b) disturbed by free falling water droplets of mean diameter equal to 0.25 mm. And water mass flow equal to 1.0 g/s. Camera speed 50 fr/s.
Fig. 6. Streak pictures of the flame propagating in the methane–air mixture disturbed by free falling water droplets. 7.5% CH4. Water droplet diameter—dE3.2 mm. (a) Water droplets concentration—0.1 kg/m3. (b) Water droplets concentration—0.7 kg/m3.
shown in Fig. 6. It is seen that for lower concentration of water droplets equal to 0.1 kg/m3 the flame front is almost smooth, but for higher concentration of water droplets equal to 0.7 kg/m3 the flame front becomes strongly irregular and flame propagation velocity increases more than two times. Comparison of the disturbed and non-disturbed flame propagation process (by water droplets with diameter of about 0.25 mm) in methane–air mixture (6.5% CH4) is shown in Fig. 7. It can be observed, that the flames are totally different in shape as far as the flame front and colour of the flame are concerned. The change of a flame colour is probably partially obtained as a result of local flame cooling effect caused by small water droplets due to water evaporating processes. Fig. 8 shows the change of velocity of the flame propagating in the lean methane–air mixture (5.6% CH4) as a function of concentration of (free falling) water droplets as well as sand particles for two different diameters of droplet and sand particle. Sand particles were used as chemically neutral particles, which have the coefficient of heat conductivity comparable to that one of water droplets. It is seen that the velocity of the flame propagation increases along with an increase of the water and sand concentration. For the same concentration, the flame velocity increases more for smaller water droplets and sand particles than for bigger ones, even though aerodynamic effects produced by a single small droplet or a particle is weaker than that produced by a single big droplet or a particle. It is probably due to the fact, that the total surface of all smaller droplets or particles is greater than that of bigger ones and, as a result, they cause higher intensity of turbulence of the mixture. It should be also noted that for a similar diameter of water droplets and sand particles a higher flame velocity is obtained for water droplets than for sand particles. This is because in an elementary volume of mixture
Fig. 8. Influence of water droplets and sand particles concentration on flame propagation velocity in methane–air mixture for two different sizes of droplets and sand particles.
there are more water droplets than sand particles due to a lower density of water than of sand. For this reason, the total surface of all water droplets is higher than that of sand particles.
4. Numerical calculation and discussion For the purpose of theoretical confirmation and better understanding of the above-mentioned results, a numerical simulation
ARTICLE IN PRESS M. Gieras / Journal of Loss Prevention in the Process Industries 21 (2008) 472–477
with Fluent 6.0 computer code was made. For the simulation, the turbulent premixed combustion model, based on work by Zimont (2000), Zimont, Polifke, Bettelini, and Weisenstein (1998) and Zimont and Lipatnikov (1995) (involves the solution of a transport equation for the reaction progress variable) was used. The closure of this equation is based on the definition of the turbulent flame speed uT. The model is based on the ‘‘flamelet’’ theory, where the 0 turbulence length scale is computed from, l ¼ CDu 3/e0 (where CD is 0 constant value, u is RMS velocity and e is the turbulent dissipation rate). The Reynolds-Averaged Navier–Stokes (RANS) method was used for solving of the problem of turbulent premixed flames. According to many authors, e.g. Poinsot and Veynante (2005) and Pope (1990) the Large Eddy Simulation (LES) method is not so good for calculation of turbulent premixed flames due to the fact that a real thickness of those flames is generally less than 1 mm, and it is usually smaller than the size of a calculation grid—but just in that small zone many fundamental chemical and physical processes occur. A physical model was developed and the maximum size of a spatial grid (for which satisfactory results of flames propagation velocity in a comparison to the experimental ones were obtained) was determined as equal to 4 4 4 mm3 (Fig. 9.). The examples of results of numerical calculations are presented in Figs. 10 and 11. Fig. 11 shows that the flame front velocity increases along with the increasing of the sand mass flow rate, and higher velocity is obtained for smaller sand particles. It should be noted that similar results (in quality) were obtained from the experiments. Comparison of the numerical results with experimental ones is shown in Figs. 12 and 13. It is seen, that for relatively big sand particles with the diameter in the range of 0.5–1.0 mm, an agreement between numerical and experimentally obtained results is quite good (Fig. 12), but for smaller sand particles with diameter in the range of 0.09–0.125 mm the agreement is only good for a low sand concentrations (Fig. 13). In this case, the continued increase of the sand concentration (for small sand particles) causes the radical increase of numerically obtained values of flame velocity, but the experimentally obtained results remains on the constant level. Probably, in experiments more and more important role (in a relation to the flame acceleration process) is played by flame cooling and extinguishing processes caused by small sand particles—but in a numerical simulations, the mechanism of the flame quenching by small sand particles is not presented correctly. Fig. 14 shows the results of a numerical experiment of the influence of particle diameters on gaseous flame propagation
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Fig. 10. The change of flame velocity along the combustion chamber for several longitudinal cross sections (6.5% methane–air mixture turbulized by free falling sand particles with diameter of 0.09–0.125 mm and the flow rate 0.46 g/s). Ignition source at the point–0.6 m upper end of combustion chamber at the point 0.6 m.
Fig. 11. Flame front velocity in methane–air mixture (6.5% CH4) as a function of sand particles concentration.
velocity. It is seen that for the same number of sand particles, higher flame acceleration is caused by bigger particles than by smaller ones—but in this case concentration of bigger particles is higher.
5. Discussion
Fig. 9. Physical model.
It was recognized (e. g. Thomas & Brenton, 1993), that in general there may be two sources of turbulence generated by water droplets: a small-scale turbulence in the wake of droplets and a large-scale turbulence caused by bulk flow of water droplets in the chamber. Water droplets can cause a disturbance of mixture ahead of a flame front, and they also cause a disturbance of the flame surface directly (Figs. 2–4 and 7). According to Wasowski and Blaub (1987), the turbulence generated in the wake of a particle does not occur until Reynolds
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Fig. 12. Flame front velocity in methane–air mixture (6.5% CH4) as a function of sand particles.
Fig. 14. Influence of particles diameter on gaseous flame propagation velocity; numerical calculations.
Fig. 13. Flame front velocity in methane–air mixture (6.5% CH4) as a function of sand particles mass flow.
numbers is higher than Re ¼ 400–500. They also found that the volume of the wake at Reynolds number of about 1000 is of the same order as the particle volume. The Reynolds numbers for free falling water droplets and for relatively slowly propagating flames, which were tested in the present experiments, (for water droplet diameter d ¼ 0.1–4 mm) vary from approximately 20 to about 1200. The above results mean that only relatively big water droplets (d41.5 mm) can cause an acceleration of flame propagation due to the turbulence generated in the wake. But from conducted experiments (Fig. 8), it follows, that for the same water
Fig. 15. Turbulent propagation velocity to laminar propagation velocity ratio as a function of delay time between the moment of switching off the water droplet generation and that of mixture ignition.
droplets flow rate the higher flame acceleration is obtained for water flow with smaller mean droplets than for that with larger droplets (it refers also to sand particles, Figs. 8 and 11). It means
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that the turbulence generated in the wake of a droplet is not the basic mechanism of the flame acceleration. In Fig. 15, the experimental results (were obtained in the test tube, Fig. 1) are presented where the generation of the water droplets was switched off before ignition of flammable mixture was initiated. It can be seen, that action of water droplets causes mixture turbulence, which affects flame propagation up to the delay time of 9 s (i.e. between the moment of switching off the water droplets generation and that of mixture ignition). From formula (presented by Hinze, 1975) for isotropic turbulence: L ¼ ð2pntÞ1=2 where L is the integral length scale of turbulence, n the kinematic viscosity and t the decay time of isotropic turbulence, it can be calculated, that the integral scale (macroscale) of turbulence for results of the tests presented in Fig. 15 varies from 22 to 29 mm. The integral scale (macroscale) of a turbulence is comparable with a length of side of tube cross-section. These results show that the large-scale flame turbulence is caused by the bulk flow of water and depends strongly on total surface of water droplets.
6. Conclusions The obtained results allow for formulating the following statements:
Aerodynamic effects produced by moving water droplets lead
to the turbulization of the flammable mixture and of the flame front. As a result, sudden acceleration of flame propagation can be obtained. The large-scale turbulence caused by the bulk flow of water droplets is mainly responsible for acceleration of the flame propagation. For the same concentration, the higher acceleration can be generated by smaller water droplets than by bigger ones—due to higher total surface of small droplets than of bigger ones. The turbulent premixed combustion model, based on work by Zimont et al. (1998) (which is used in FLUENT 6.0) is good enough for simulation of flame acceleration processes caused by neutral particles.
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References Gieras, M. (2000). Interaction of water droplets with propagating gaseous flames. Archivum Combustionis, 20(1–2), 49–61. Gieras, M. (2001). Interaction of water sprays with gaseous flames. Archivum Combustionis, 21(4), 245–260. Hinze, J. O. (1975). Turbulence. McGraw-Hill. Heskestad, G. (2002). Scaling the interaction of water sprays and flames. Fire Safety Journal, 37(6), 535–548. Heskestad, G. (2003). Extinction of gas and liquid pool fires with water sprays. Fire Safety Journal, 38(4), 301–317. Jarosinski, J., Strehlow, R., & Azarbazin, A. (1982). The mechanism of lean limit extinguishment of an upward and downward propagating flame in a standard flammability tube. In 19th Symposium (International) on Combustion (pp. 1549–1557). The Combustion Institute. Jones, A., & Thomas, G. O. (1993). The action of water sprays on fires and explosions: A review of experimental work. Transactions of the Institution of Chemical Engineers, 71(Part B), 1–4. Parra, T., Castro, F., Mendez, C., Villfrula, J., & Rodriguez, M. (2004). Extinction of premixed methane–air flames by water mist. Fire Safety Journal, 39(0), 581–600. Poinsot, T., & Veynante, D. (2005). Theoretical and numerical combustion. R.T. Edwards, Inc. Pope, S. (1990). Computation of numerical combustion—progress and challenge. In: Symposium (international) on combustion, 23rd, Orleans, France, 22–27 July (pp. 591–612). Proust, C. H. (1996). Flame– water droplets interactions. Paper presented at seventh colloquium of dust explosion. Bergen: Norway. Rasbash, D. J., & Rogowski, Z. W. (1956). Extinction of fires in liquids by cooling with water sprays. Combustion and Flame, 1. Rasbash, D. J., Rogowski, Z. W., & Stark, G. W. (1960). Mechanism of extinction of liquid fires using water sprays. Combustion and Flame, 4. Schewille, J. A., & Leuptow, R. M. (2006). Effect of a fire plume on suppression spray droplet motion. Atomization and Sprays, 16(5), 563–577. Thomas, G. O., & Brenton, J. R. (1993). A study of turbulent flame acceleration in water sprays. In Paper presented at the 14th international colloquium on the dynamics of explosions and reactive system. Portugal: Coimbra. Thomas, G. O., Edwards, M. J., & Edwards, D. H. (1990). Studies of detonation quenching by water sprays. Combustion Science and Technology, 71. Thomas, G. O., Jones, A., & d Edwards, M. J. (1991). Influence of water sprays on explosion development in fuel–air mixtures. Combustion Science and Technology, 80. Wasowski, T., & Blaub, E. (1987). Wake phenomena behind solid and fluids particles. Chemie– Ingenieur Technik, 7, 544–555. Zimont, V. (2000). Gas premixed combustion at high turbulence. Turbulent flame closure model combustion model, experimental. Thermal and Fluid Science, 21, 179–186. Zimont, V., Polifke, W., Bettelini, M., & Weisenstein, W. (1998). An efficient computational model for premixed turbulent combustion at high Reynolds numbers based on a turbulent flame speed closure. Journal of Gas Turbines Power, 120, 526–532. Zimont, V., & Lipatnikov, A. (1995). A numerical model of premixed turbulent combustion of gases. Chemical Physics Report, 14(7), 993–1025.
Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Flame acceleration due to water droplets action Marian Gieras Warsaw University of Technology, 00-665 Warsaw, Poland
a r t i c l e in f o
a b s t r a c t
Article history: Received 22 October 2007 Received in revised form 13 March 2008 Accepted 14 March 2008
Results of experimental and numerical studies on the influence of water droplets on premixed gaseous flame propagation are presented. Experimental data were obtained in the vertical standard tube of 1.2 m length and of 0.05 0.05 m2 cross-section. The research was focused on the mechanism of a flame acceleration caused by water droplets action. For better understanding of these processes, the neutral sand particles were also used. Pictures documenting flame acceleration caused by water droplets are presented. The simulations of a flame acceleration process resulting from the presence of neutral particles were made by using FLUENT 6.0 computer code. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Hexane–air mixture Ignition Explosion Upper explosion limit
1. Introduction Fire and explosion hazard have forced people to look for effective suppression systems, as well as effective extinguishing media. One of the oldest known fire suppressant is water. Water appears to be very attractive as an extinguishing agent because of its relatively low cost and large availability. Moreover, water causes relatively low pollution to the environment. It was stated, on the ground of experimental and theoretical investigation (Heskestad, 2002, 2003; Jones, & Thomas, 1993; Parra, Castro, Mendez, Villfrula, & Rodriguez, 2004; Rasbash, & Rogowski, 1956; Rasbash, Rogowski, & Stark, 1960; Schewille, & Leuptow, 2006), that the effectiveness of flame suppression by means of water can be significantly improved by using suitable water sprays in suppression of some forms of combustion. For example, finer spray can cause the most rapid extinction of fires of volatile fuels (alcohol, benzol), whereas the coarser sprays were more suitable to fire extinction than less volatile fuels (oil). Experiments have shown that, if the suppression systems are not capable of delivering the required quantity of water to stop combustion, then the explosion could be even more violent than without the action of the device (Proust, 1996). It is believed that the local increase of turbulence induced by the injection of the extinguishing agent, might be responsible for this dramatic effect. On the one hand, the same quantities of a water spray can often accelerate relatively slow flames, but on the other they can sometimes cause suppression of detonation (Jones, & Thomas,
Tel.: +48 22 234 52 22; fax: +48 22 825 05 65.
E-mail address: [email protected] 0950-4230/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jlp.2008.03.004
1993; Thomas, Edwards, & Edwards, 1990; Thomas, Jones, & Edwards, 1991). The main objective of this work is to show the mechanism of a flame acceleration caused by water droplets (moving relatively slowly in relation to the premixed flame) during their interaction with the propagating flame. Good understanding of this mechanism can be helpful in the development of fire protection systems for industry and can lead to improved numerical modelling.
2. Apparatus and experimental details Studies of water droplets–gaseous flame interaction were carried out in the research stand presented in Fig. 1. The main part of it is the standard vertical tube, 1.2 m. Long and of 0.05 0.05 m2 square cross-section (according to Jarosinski, Strehlow, & Azarbazin, 1982). The upper end of the tube is closed and the lower end is opened. Gas supply system was located at the top of the tube. Methane–air mixture was prepared in 40 l cylinder by the method of partial pressures with the concentration accuracy of 70.1%. Technical methane (99% CH4) was used as a gaseous fuel. The studies of the mechanism of interaction between water droplets and propagating gaseous flame were conducted for two cases. In the first one, there was applied a special system of ducting and fixing of quartz-glass needle, on which the single water droplet was hanged. This system was placed in the central part of the tube. Glass window of dimension 0.05 0.05 m2 were also located in the central part of the tube for visualization of the process directly or by means of Michelson interferometer. The diameter of glass needles was 0.1 and 2 mm, however the
ARTICLE IN PRESS M. Gieras / Journal of Loss Prevention in the Process Industries 21 (2008) 472–477
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Fig. 2. Flame front disturbance caused by a water droplet hanged on a quartz-glass needle (graphically illustrated on the left side of the picture). Methane concentration in mixture—5.5%; droplet diameter—3 mm.
Fig. 1. Picture of the research stand with standard vertical tube, 1.2 m long. 1—electric motor; 2—spark generator; 3—control system; 4—vertical combustion chamber; 5—starting switch; 6—plate of Michelsen interferometer; 7—collimator; 8—video camera. Fig. 3. Direct pictures of flame front disturbance caused by a free falling water droplet in the methane–air mixture. 5.5% CH4; droplet diameter—4 mm.
diameter of a small ball at the end of the needle (which helps to fix the water droplet on the glass needle) was 0.2 and 0.3 mm, respectively. In the second one, a special generator (consisting of pressurized tank with water, equipped with special pipe nozzles in three configurations: 1 1 ¼ 1, 5 5 ¼ 25 and 6 6 ¼ 36 twodimensional systems) of water droplet of different sizes was placed at the top of the tube. An oblong window of dimension 0.05 0.6 m2 was installed in the central part of the duct. Water flow rates and water droplet sizes could be changed by changing of pressure in the tank (up to 10 bar), quantity and diameter of a pipe nozzle. In the second option of the research stand, the combustion chamber was equipped with a special vibrating dust feeder, which made it possible to fill up the tube with the dust cloud. Lycopodium powder with mean diameter equal to 10 mm was used as dust fuel. The same vibrating dust feeder was also used to generate a sand cloud. Concentrations of dust as well as sand were changed by changing of vibration frequency and changing of special vibrating screens. More detailed information about the research stands are presented by Gieras (2000, 2001). During the experimental tests, the following parameters: water droplet diameter and velocity, water droplets mass flow rate as well as the concentration of methane in the mixture, were changed. Ignition was initiated by the electric spark (energy 0.1 J) located 10 cm from the lower end of the tube. Ignition occurred just after the tube was fully filled in with the combustible gaseous or lycopodium–air mixture. All tests were recorded by a video camera. Flame velocity was measured on the basis of pictures taken by the video camera and on the basis of three microthermocouples located at the wall of the combustion tube.
3. Experimental results and discussion It was observed that collision of single water droplets with the flame propagating in a methane–air mixture causes the clear visible disturbance of the flame surface in a form of hollows, craters or creases. The size of the disturbance increases along with increase of droplet diameter and velocity in relation to the flame. An example of a penetration process of a slow propagating flame by a single water droplet can be observed in Figs. 2 and 3.
Fig. 4. Direct pictures of flame front disturbance caused by a water droplet hanged on a quartz-glass needle. The lycopodium–air mixture. Concentration of lycopodium—45 g/m3; droplet diameter—3 mm.
Fig. 5. Graphically illustrated process of increasing of flame front surface caused by water droplets.
A disturbance of flame front can be also observed in dusty flames (Fig. 4), but in the case of flames due to an irregular flame front surface, the disturbance is not so clearly visible as in the case of gaseous flames. A number of water droplets can cause a radical change in a flame shape which can lead to a rapid increase of the flame surface (Fig. 5). In general, it can be stated, that aerodynamic effects produced by the moving (in relation to the flame) droplets lead to the turbulization of the flammable mixture and high disturbance and corrugation of the flame front shape. They cause an increase of the flame surface and flame thickness. These effects can play more important role than effects of cooling and dilution of the combustion zone. In such situation, a sudden acceleration of flame propagation is obtained. As an example of these processes, the change of a gaseous flame structure and propagation velocity due to an action of water droplets for two different concentrations is
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Fig. 7. Flame propagation process in methane–air mixture (6.5% CH4). (a) Nondisturbed, (b) disturbed by free falling water droplets of mean diameter equal to 0.25 mm. And water mass flow equal to 1.0 g/s. Camera speed 50 fr/s.
Fig. 6. Streak pictures of the flame propagating in the methane–air mixture disturbed by free falling water droplets. 7.5% CH4. Water droplet diameter—dE3.2 mm. (a) Water droplets concentration—0.1 kg/m3. (b) Water droplets concentration—0.7 kg/m3.
shown in Fig. 6. It is seen that for lower concentration of water droplets equal to 0.1 kg/m3 the flame front is almost smooth, but for higher concentration of water droplets equal to 0.7 kg/m3 the flame front becomes strongly irregular and flame propagation velocity increases more than two times. Comparison of the disturbed and non-disturbed flame propagation process (by water droplets with diameter of about 0.25 mm) in methane–air mixture (6.5% CH4) is shown in Fig. 7. It can be observed, that the flames are totally different in shape as far as the flame front and colour of the flame are concerned. The change of a flame colour is probably partially obtained as a result of local flame cooling effect caused by small water droplets due to water evaporating processes. Fig. 8 shows the change of velocity of the flame propagating in the lean methane–air mixture (5.6% CH4) as a function of concentration of (free falling) water droplets as well as sand particles for two different diameters of droplet and sand particle. Sand particles were used as chemically neutral particles, which have the coefficient of heat conductivity comparable to that one of water droplets. It is seen that the velocity of the flame propagation increases along with an increase of the water and sand concentration. For the same concentration, the flame velocity increases more for smaller water droplets and sand particles than for bigger ones, even though aerodynamic effects produced by a single small droplet or a particle is weaker than that produced by a single big droplet or a particle. It is probably due to the fact, that the total surface of all smaller droplets or particles is greater than that of bigger ones and, as a result, they cause higher intensity of turbulence of the mixture. It should be also noted that for a similar diameter of water droplets and sand particles a higher flame velocity is obtained for water droplets than for sand particles. This is because in an elementary volume of mixture
Fig. 8. Influence of water droplets and sand particles concentration on flame propagation velocity in methane–air mixture for two different sizes of droplets and sand particles.
there are more water droplets than sand particles due to a lower density of water than of sand. For this reason, the total surface of all water droplets is higher than that of sand particles.
4. Numerical calculation and discussion For the purpose of theoretical confirmation and better understanding of the above-mentioned results, a numerical simulation
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with Fluent 6.0 computer code was made. For the simulation, the turbulent premixed combustion model, based on work by Zimont (2000), Zimont, Polifke, Bettelini, and Weisenstein (1998) and Zimont and Lipatnikov (1995) (involves the solution of a transport equation for the reaction progress variable) was used. The closure of this equation is based on the definition of the turbulent flame speed uT. The model is based on the ‘‘flamelet’’ theory, where the 0 turbulence length scale is computed from, l ¼ CDu 3/e0 (where CD is 0 constant value, u is RMS velocity and e is the turbulent dissipation rate). The Reynolds-Averaged Navier–Stokes (RANS) method was used for solving of the problem of turbulent premixed flames. According to many authors, e.g. Poinsot and Veynante (2005) and Pope (1990) the Large Eddy Simulation (LES) method is not so good for calculation of turbulent premixed flames due to the fact that a real thickness of those flames is generally less than 1 mm, and it is usually smaller than the size of a calculation grid—but just in that small zone many fundamental chemical and physical processes occur. A physical model was developed and the maximum size of a spatial grid (for which satisfactory results of flames propagation velocity in a comparison to the experimental ones were obtained) was determined as equal to 4 4 4 mm3 (Fig. 9.). The examples of results of numerical calculations are presented in Figs. 10 and 11. Fig. 11 shows that the flame front velocity increases along with the increasing of the sand mass flow rate, and higher velocity is obtained for smaller sand particles. It should be noted that similar results (in quality) were obtained from the experiments. Comparison of the numerical results with experimental ones is shown in Figs. 12 and 13. It is seen, that for relatively big sand particles with the diameter in the range of 0.5–1.0 mm, an agreement between numerical and experimentally obtained results is quite good (Fig. 12), but for smaller sand particles with diameter in the range of 0.09–0.125 mm the agreement is only good for a low sand concentrations (Fig. 13). In this case, the continued increase of the sand concentration (for small sand particles) causes the radical increase of numerically obtained values of flame velocity, but the experimentally obtained results remains on the constant level. Probably, in experiments more and more important role (in a relation to the flame acceleration process) is played by flame cooling and extinguishing processes caused by small sand particles—but in a numerical simulations, the mechanism of the flame quenching by small sand particles is not presented correctly. Fig. 14 shows the results of a numerical experiment of the influence of particle diameters on gaseous flame propagation
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Fig. 10. The change of flame velocity along the combustion chamber for several longitudinal cross sections (6.5% methane–air mixture turbulized by free falling sand particles with diameter of 0.09–0.125 mm and the flow rate 0.46 g/s). Ignition source at the point–0.6 m upper end of combustion chamber at the point 0.6 m.
Fig. 11. Flame front velocity in methane–air mixture (6.5% CH4) as a function of sand particles concentration.
velocity. It is seen that for the same number of sand particles, higher flame acceleration is caused by bigger particles than by smaller ones—but in this case concentration of bigger particles is higher.
5. Discussion
Fig. 9. Physical model.
It was recognized (e. g. Thomas & Brenton, 1993), that in general there may be two sources of turbulence generated by water droplets: a small-scale turbulence in the wake of droplets and a large-scale turbulence caused by bulk flow of water droplets in the chamber. Water droplets can cause a disturbance of mixture ahead of a flame front, and they also cause a disturbance of the flame surface directly (Figs. 2–4 and 7). According to Wasowski and Blaub (1987), the turbulence generated in the wake of a particle does not occur until Reynolds
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Fig. 12. Flame front velocity in methane–air mixture (6.5% CH4) as a function of sand particles.
Fig. 14. Influence of particles diameter on gaseous flame propagation velocity; numerical calculations.
Fig. 13. Flame front velocity in methane–air mixture (6.5% CH4) as a function of sand particles mass flow.
numbers is higher than Re ¼ 400–500. They also found that the volume of the wake at Reynolds number of about 1000 is of the same order as the particle volume. The Reynolds numbers for free falling water droplets and for relatively slowly propagating flames, which were tested in the present experiments, (for water droplet diameter d ¼ 0.1–4 mm) vary from approximately 20 to about 1200. The above results mean that only relatively big water droplets (d41.5 mm) can cause an acceleration of flame propagation due to the turbulence generated in the wake. But from conducted experiments (Fig. 8), it follows, that for the same water
Fig. 15. Turbulent propagation velocity to laminar propagation velocity ratio as a function of delay time between the moment of switching off the water droplet generation and that of mixture ignition.
droplets flow rate the higher flame acceleration is obtained for water flow with smaller mean droplets than for that with larger droplets (it refers also to sand particles, Figs. 8 and 11). It means
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that the turbulence generated in the wake of a droplet is not the basic mechanism of the flame acceleration. In Fig. 15, the experimental results (were obtained in the test tube, Fig. 1) are presented where the generation of the water droplets was switched off before ignition of flammable mixture was initiated. It can be seen, that action of water droplets causes mixture turbulence, which affects flame propagation up to the delay time of 9 s (i.e. between the moment of switching off the water droplets generation and that of mixture ignition). From formula (presented by Hinze, 1975) for isotropic turbulence: L ¼ ð2pntÞ1=2 where L is the integral length scale of turbulence, n the kinematic viscosity and t the decay time of isotropic turbulence, it can be calculated, that the integral scale (macroscale) of turbulence for results of the tests presented in Fig. 15 varies from 22 to 29 mm. The integral scale (macroscale) of a turbulence is comparable with a length of side of tube cross-section. These results show that the large-scale flame turbulence is caused by the bulk flow of water and depends strongly on total surface of water droplets.
6. Conclusions The obtained results allow for formulating the following statements:
Aerodynamic effects produced by moving water droplets lead
to the turbulization of the flammable mixture and of the flame front. As a result, sudden acceleration of flame propagation can be obtained. The large-scale turbulence caused by the bulk flow of water droplets is mainly responsible for acceleration of the flame propagation. For the same concentration, the higher acceleration can be generated by smaller water droplets than by bigger ones—due to higher total surface of small droplets than of bigger ones. The turbulent premixed combustion model, based on work by Zimont et al. (1998) (which is used in FLUENT 6.0) is good enough for simulation of flame acceleration processes caused by neutral particles.
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