- Email: [email protected]
Smoke dissipation by solid particles and charged water spray in enclosed spaces
ARTICLE IN PRESS Fire Safety Journal 44 (2009) 668–671
Contents lists available at ScienceDirect
Fire Safety Journal journal homepage: www.elsevier.com/locate/firesaf
Technical note
Smoke dissipation by solid particles and charged water spray in enclosed spaces R.G. Maghirang , E.B. Razote Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS 66506, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 14 August 2007 Received in revised form 1 December 2008 Accepted 14 December 2008 Available online 30 January 2009
This research investigated the effectiveness of various particles in clearing smoke in enclosed spaces. Metal oxide nanostructured particles, conventional particles (i.e., calcium hydroxide, sodium bicarbonate), or water (electrostatically charged or uncharged) were sprayed into an enclosed experimental chamber filled with combustion smoke. Improvement in visible light transmission through the chamber served as a measure of the effectiveness of the material in clearing smoke. Results showed that the negatively charged water spray was most effective in clearing smoke and improving visibility in the chamber. The smoke treated with charged water spray dissipated approximately 15 times faster than the untreated smoke. Also, compared with the solid particulate materials, the charged water spray resulted in a 3–12-fold increase in the speed of smoke dissipation. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Nanostructured particles Water spray Smoke clearing Combustion smoke
1. Introduction Some components of smoke from fires and military obscurants are considered toxic. In addition, smoke results in dramatic impairment in visibility [1], which is a problem in fire fighting applications. Techniques to rapidly dissipate smoke generated by fires in enclosed spaces should be developed. Smoke is composed of fine particles suspended in air. Like other aerosols, the concentration and physical properties of smoke change with time. These changes can be a result of external forces or physical and chemical processes that serve to change the size and/or properties of the smoke particles [2]. Such processes include coagulation, condensation, evaporation, adsorption, absorption, and chemical reaction. Enhancing at least one of these processes can enhance the clearing of smoke particles in the air. In enclosed spaces and if dilution with outside air is not feasible, enhancing coagulation may be one of the most promising methods of smoke dissipation. Recent research has shown the potential of spraying scavenging particles to enhance the rate of smoke dissipation in enclosed rooms [3]. The said study considered solid particles, including conventional particulate materials and aggregates of metal oxide nanostructured particles, which have demonstrated effectiveness for the inactivation and destruction of a wide variety of contaminants [4,5]. It should be noted that the said research used an aerosol-type smoke (i.e., glycol smoke), the properties of which could be different
Corresponding author. Tel.: +1785 532 2908; fax: +1785 532 5825.
E-mail address: [email protected] (R.G. Maghirang). 0379-7112/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.firesaf.2008.12.003
from those of more complex smokes, including combustion smoke. Water droplets (charged and uncharged) could also be used as smoke clearing agents. Charged water sprays have been investigated for airborne particle removal [1,6,7]. Almuhanna et al. [7], for example, observed that charged water sprays were effective in reducing dust in enclosed spaces. The effectiveness of scavenging particles, including solid particles and charged water sprays, in clearing combustion smoke and improving visibility in enclosed spaces needs to be established. This study was conducted to establish the effectiveness of solid particles and charged water spray in dissipating combustion smoke from enclosed spaces.
2. Materials and methods 2.1. Description of the experimental chamber and instruments Experiments were conducted in an enclosed experimental chamber measuring 3.8 m 2.3 m 2.3 m (Fig. 1). The chamber was equipped with a transmissometer and smoke generators. The transmissometer, connected to a datalogger, was employed for measuring changes in light transmission through the chamber [3]. It has a spectral response in the visible region (400–700 nm wavelength). Measurement is achieved by directing a beam of visible light across the chamber (path length of 3.8 m) onto the light detector, whose output varies with changes in light transmission. A reading of 100% indicates complete obscuration (i.e., no transmission of light from the source to the detector) while a reading of 0% indicates 100% light transmission. Smoke
ARTICLE IN PRESS R.G. Maghirang, E.B. Razote / Fire Safety Journal 44 (2009) 668–671
was generated by burning moistened paper (newspaper) inside the combustion container of smoke generators. The smoke has an aerodynamic geometric mean diameter (GMD), based on mass or volume, of 1.2 mm and geometric standard deviation (GSD) of 2.0 (Table 1). The GMD and GSD values were obtained by using an 8stage cascade impactor [3], which was operated at a sampling flow rate of 2 L/min for 1–2 min. 2.2. Scavenging particles Three types of nanostructured particles (NA TiO2, NA MgO, and NA MgO plus) and two conventional powders (fire-extinguisher grade NaHCO3, Ca(OH)2) were considered (Table 1). The nanostructured materials were manufactured by NanoScale Corporation, through proprietary processes and were marketed as NanoActiveTM particles, herein referred to as NA. These particles were considered based on their relative effectiveness for glycol smoke [3]. Table 1 summarizes the aerodynamic size distributions of the particles when dispersed into the chamber, as measured with 8-stage cascade impactor [3], which was operated at a sampling flow rate of 2 L/min for 1–2 min after particle dispersion. Uncharged and charged water sprays were also considered. Charged water spray was generated using a commercially Mixing Fan Smoke from Smoke Generator
Transmissometer
2.3 m
Scavenging Particles
669
available electrostatic spraying system (Electrostatic Spraying Systems, Inc., Watkinsville, GA, USA), which has been developed for chemical application [7]. The spraying system uses air-assisted induction charging to generate charged droplets. In this process, air and water enter separately at the rear of the nozzle. The air moves at near-sonic speed through the nozzle and impacts the water at the tip, causing atomization of the spray. As the spray is atomized, the water droplets pass an electrode inside the nozzle (voltage of 1000 VDC). Electrons are induced onto the water stream and the droplets leave the nozzle with a high negative charge. The mean charging level for the charged water spray was 6.5 mC/kg (S.D. ¼ 0.9 mC/kg), as measured with a dynamic Faraday-cup sampler [8]. The spraying system was operated at a liquid flow rate of 120 mL/min (water tank pressure of 15 psig). The primary droplet size was 30–40 mm, as reported by the manufacturer. In this study, only one nozzle was used; however, for practical application and depending on the size of the airspace, more than one nozzle will have to be used. There were two sets of experiments for the charged water spray. The first set compared the effectiveness of the charged water spray to that of uncharged water spray and also untreated smoke; there were three replicates for each treatment. The second set of experiments evaluated the effect of spray duration on the effectiveness of both the charged and uncharged water sprays. In this set of experiments, the spraying system was operated at spray durations of 1, 5, and 20 min. For the uncharged water spray experiments, the number of replicates were 2, 3, and 2 for spray durations of 1, 5, and 20 min, respectively. For the charged water spray experiments, on the other hand, the corresponding numbers of replicates were 4, 3, and 3.
1.2 m 0.61 m
2.2. Experimental procedure 3.8 m
Scavenging Particles
2.3 m 1.5 m
1.5 m
Smoke from Smoke Generator 3.8 m Fig. 1. Schematic diagram of the experimental setup: (a) plan view and (b) elevation. Not drawn to scale.
For each experiment, the chamber was prepared by cleaning the surfaces (walls and floor) and running its high-efficiency particulate air filtration system for approximately 30 min. Smoke was introduced into the chamber until there is complete obscuration or light transmission is at or near 0%. While the smoke was being introduced, two mixing fans inside the chamber were operated for approximately 2 min to disperse the smoke inside the chamber. As soon as light transmission reaches 0%, the mixing fans were turned off. Scavenging particles were then immediately sprayed into the chamber through a pressurized canister (for solid particles) or spray nozzle (for water sprays). Light transmission through the chamber was monitored with the transmissometer. From the transmissometer data, the times to reach light transmission of 10% (t10) and 20% (t20) through the
Table 1 Summary of experiments.1 Treatment
Untreated smoke Smoke treated with Smoke treated with Smoke treated with Smoke treated with Smoke treated with Smoke treated with Smoke treated with 1 2 3 4
NA MgO plus NA MgO NA TiO2 NaHCO3 Ca(OH)2 uncharged water spray4 negatively charged water spray4
Number of replicates
Actual mass deployed2 (g) mean (S.D.)
Aerodynamic size distribution GMD (GSD)3
t10 Mean (S.D.)
t20 Mean (S.D.)
3 2 2 3 2 2 3 3
– 78.0 16.6 9.6 57.2 48.0
1.2 13.9 6.5 7.5 7.7 6.6
122 63 96 47 72 86 75 8
184 127 152 88 110 145 129 29.5
(2.7) (7.3) (5.3) (6.6) (2.4)
(2.0) (2.4) (4.9) (3.6) (4.0) (3.0)
Column means followed by the same letter are not significantly different at the 5% level of significance. Nominal mass deployed for the solid particles was 100 g. Geometric mean diameter (GMD) in mm and geometric standard deviation (GSD). Continuous spraying for five minutes.
(7.0)a (1.4)de (0.9)b (17.5)e (5.4)cd (2.9)bc (9.0)cd (1.0)f
(13.7)a (11.7)bc (17.5)ab (40.4) c (6.1)bc (6.0)ab (20.0)bc (14.0)f
ARTICLE IN PRESS R.G. Maghirang, E.B. Razote / Fire Safety Journal 44 (2009) 668–671
chamber were determined. Small values of t10 and t20 relative to untreated smoke indicate effective smoke clearing. The 10% and 20% levels were chosen in this study because, at these levels, one can begin to ‘‘see through’’ the smoke. The t10 and t20 values were analyzed by using least-squares means procedure of SAS (Version 9.1, SAS Institute, Inc., Cary, NC) [9].
3. Results and discussion Table 1 summarizes the mean t10 and t20 values for the untreated smoke and smoke treated with various solid scavenging particles. For untreated smoke, the mean t10 and t20 values were 122 and 184 min, respectively. Smoke dissipates naturally because of gravitational settling and evaporation; gravitational settling may be enhanced by collision and coagulation of smoke particles, forming particles that are larger and would settle out faster. The large t10 and t20 values for the untreated smoke are likely due to the relatively small size of the smoke particles (Table 1). Because of the small size, most of the smoke particles have very low settling velocity and would therefore stay airborne for relatively long periods of time. For example, the settling velocity for 1.2 mm particles is approximately 0.0048 cm/s. By spraying solid particles into the smoke-filled chamber, the mean t10 values were reduced significantly, ranging from 47 min for smoke treated with NA TiO2 to 96 min for smoke treated with NA MgO. The mean t20 values were also reduced significantly, ranging from 88 min for NA TiO2 to 152 min for NA MgO. The significant reduction in the mean t10 and t20 values is due to capture of smoke particles by the solid particles. Note that a large fraction of the solid particles is larger in size than the smoke particles (Table 1) and would therefore settle out much faster than the smoke particles. It should be noted, however, that while spraying solid particles significantly reduced the mean t10 and t20 values, the resulting values are too long (over 40 min), indicating that they would not be of much benefit in actual settings, e.g., in fire fighting applications. The solid particles differed in terms of their effectiveness in dissipating smoke (Table 1). This could be due to differences in particulate loading (which depends on mass deployed), size distribution, and possibly their affinity to smoke particles. The mass deployed ranged from 9.6 g for NA TiO2 to 78.0 g for NA MgO plus (Table 1). Surprisingly, even with the least mass deployed, NA TiO2 appeared to be the best of the solid particles. Reasons for TiO2 being the most effective of the solid particles are not clear from the experiments. Table 1 also shows the mean t10 and t20 values for smoke treated with uncharged water spray and smoke treated with charged water spray. Similar to the solid particles, both the uncharged and charged water sprays resulted in significant reduction in the mean t10 and t20 values compared with the untreated smoke. The uncharged water spray was generally similar to the solid particles in terms of mean t10 and t20 values. The negatively charged water spray, on the other hand, was significantly more effective than the solid particles and the uncharged water spray. As indicated by the mean t10 values, the smoke treated with charged water spray dissipated approximately 15 times faster than the untreated smoke. Also, based on the mean t10 values, the charged water spray resulted in 3–12-fold increase in the speed of smoke dissipation compared with solid particles and uncharged water spray. The mechanisms for particle removal of water droplets (charged or uncharged) are relatively well understood [10]. When an uncharged water droplet approaches a cloud of smoke particles with a relative velocity, it may directly collide with some of the smoke particles (i.e., impaction), barely touch some of the smoke particles (i.e., interception), or entirely miss some particles [2,10]. The relative effect of the mechanisms
Time to reach 10% light transmission, min
670
100 Smoke treated with uncharged water spray Smoke treated with charged water spray
80
60
40
20
0 1
5 Water spray duration, min
20
Fig. 2. Effect of spray duration on the mean t10 values for the combustion smoke treated with uncharged water spray and combustion smoke treated with charged water spray. Error bars represent one standard deviation.
of interaction between the droplet and the particles depends upon the size of the particles. The primary collection mechanism for large particles is impaction, while that for sub-micrometer particles is Brownian diffusion [2,10]. When the water droplets are highly charged, electrostatic force between the droplets and the smoke particles would enhance the collection mechanisms, resulting in improvement in overall collection efficiency of the water droplets. Fig. 2 summarizes the mean t10 values for the water sprays (i.e., uncharged and negatively charged particles) as a function of spray duration. For both the uncharged and charged water sprays, the mean t10 values generally decreased with increasing spray duration. Similar trend was observed for the mean t20 values (data not shown). The decrease in the mean t10 and t20 values is expected because increasing the spray duration increases the number of water droplets in the air. With greater number of water droplets, collisions (and collection) of smoke particles by the water droplets would be enhanced. For the charged water spray, the 5- and 20-min spray durations did not significantly differ in the mean t10 and t20 values. The lack of significant difference between the 5- and 20-min spray durations could indicate that for the test conditions in this study, spray duration of about 5 min would be close to the optimum.
4. Conclusions The effectiveness of solid particles and electrostatically charged water spray in enhancing smoke dissipation in enclosed spaces was evaluated under controlled laboratory conditions. The following conclusions were drawn from this research:
Charged water sprays significantly enhanced the dissipation
rate of smoke in the enclosed experimental room. Longer spray duration improved the dissipation rate of smoke. Spraying either solid particles or uncharged water spray also enhanced smoke dissipation; however, the improvement appeared to be quite limited and of little practical use.
While the charged-water spray resulted in significant improvements in the rate of smoke dissipation, there is a need to still reduce the time it takes to produce acceptable visibility levels. Further work will be conducted to improve the effectiveness of
ARTICLE IN PRESS R.G. Maghirang, E.B. Razote / Fire Safety Journal 44 (2009) 668–671
the charged water spray. Factors including the concentration of charged water spray, number of nozzles, size of water droplets, among others, will be considered. In addition, more detailed characterization of smoke and charged water spray will also be conducted. Numerical research based on aerosol kinetics will also be pursued.
Acknowledgements This work was partly funded through the award of a contract from the United States Marine Corps Systems Command to M2 Technologies, Inc. The authors acknowledge the assistance of Darrell Oard, Emad Almuhanna, Tyler Pjesky, Andrew Rosander, Matt Woerman, and Bala Kakumanu. References [1] X.D. Xiang, I. Colbeck, Charged water drops and smoke dissipation, Fire Safety J. 28 (1997) 227–232.
671
[2] W.C. Hinds, Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, second ed., John Wiley & Sons, Inc., New York, 1999. [3] R. Yadav, R.G. Maghirang, L.E. Erickson, B. Kakumanu, S.G. Castro, Laboratory evaluation of the effectiveness of nanostructured and conventional particles in clearing smoke in confined spaces, Fire Safety J 43 (1) (2008) 36–41. [4] S.P. Decker, J. Klabunde, A. Khaleel, K.J. Klabunde, Catalyzed destructive adsorption of environmental toxins with nanocrystalline metal oxides, Environ. Sci. Technol. 36 (4) (2002) 762–768. [5] O.B. Koper, J. Klabunde, G.L. Marchin, K.J. Klabunde, P. Stoimenov, L. Bohra, Nanoscale powders and formulations with biocidal activity toward spores and vegetative cells of Bacillus species, viruses, and toxins, Curr. Microbiol. 44 (1) (2002) 49–55. [6] J. McCoy, L. Meicher, Evaluation of charged water sprays for dust control. US Bureau of Mines, Report no. 818, 1983. [7] E. Almuhanna, R.G. Maghirang, J.P. Murphy, L.E. Erickson, Effectiveness of electrostatically-charged water spray in reducing dust concentration in enclosed spaces, Trans. ASABE 51 (1) (2008) 279–286. [8] E. Almuhanna, Dust control in livestock buildings with electrostaticallycharged water spray, Ph.D. dissertation, Kansas State University, Manhattan, Kansas, 2007. [9] SAS. SAS/STAT User’s Guide, SAS Institute, Inc., Cary, NC, 1990. [10] C.V. Mathai, Charged fog technology. Part II: Prototype tests of a new charged fog generator for fugitive emission control, J. Air Pollut. Control Assoc. 33 (8) (1983) 756–759.
Contents lists available at ScienceDirect
Fire Safety Journal journal homepage: www.elsevier.com/locate/firesaf
Technical note
Smoke dissipation by solid particles and charged water spray in enclosed spaces R.G. Maghirang , E.B. Razote Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS 66506, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 14 August 2007 Received in revised form 1 December 2008 Accepted 14 December 2008 Available online 30 January 2009
This research investigated the effectiveness of various particles in clearing smoke in enclosed spaces. Metal oxide nanostructured particles, conventional particles (i.e., calcium hydroxide, sodium bicarbonate), or water (electrostatically charged or uncharged) were sprayed into an enclosed experimental chamber filled with combustion smoke. Improvement in visible light transmission through the chamber served as a measure of the effectiveness of the material in clearing smoke. Results showed that the negatively charged water spray was most effective in clearing smoke and improving visibility in the chamber. The smoke treated with charged water spray dissipated approximately 15 times faster than the untreated smoke. Also, compared with the solid particulate materials, the charged water spray resulted in a 3–12-fold increase in the speed of smoke dissipation. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Nanostructured particles Water spray Smoke clearing Combustion smoke
1. Introduction Some components of smoke from fires and military obscurants are considered toxic. In addition, smoke results in dramatic impairment in visibility [1], which is a problem in fire fighting applications. Techniques to rapidly dissipate smoke generated by fires in enclosed spaces should be developed. Smoke is composed of fine particles suspended in air. Like other aerosols, the concentration and physical properties of smoke change with time. These changes can be a result of external forces or physical and chemical processes that serve to change the size and/or properties of the smoke particles [2]. Such processes include coagulation, condensation, evaporation, adsorption, absorption, and chemical reaction. Enhancing at least one of these processes can enhance the clearing of smoke particles in the air. In enclosed spaces and if dilution with outside air is not feasible, enhancing coagulation may be one of the most promising methods of smoke dissipation. Recent research has shown the potential of spraying scavenging particles to enhance the rate of smoke dissipation in enclosed rooms [3]. The said study considered solid particles, including conventional particulate materials and aggregates of metal oxide nanostructured particles, which have demonstrated effectiveness for the inactivation and destruction of a wide variety of contaminants [4,5]. It should be noted that the said research used an aerosol-type smoke (i.e., glycol smoke), the properties of which could be different
Corresponding author. Tel.: +1785 532 2908; fax: +1785 532 5825.
E-mail address: [email protected] (R.G. Maghirang). 0379-7112/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.firesaf.2008.12.003
from those of more complex smokes, including combustion smoke. Water droplets (charged and uncharged) could also be used as smoke clearing agents. Charged water sprays have been investigated for airborne particle removal [1,6,7]. Almuhanna et al. [7], for example, observed that charged water sprays were effective in reducing dust in enclosed spaces. The effectiveness of scavenging particles, including solid particles and charged water sprays, in clearing combustion smoke and improving visibility in enclosed spaces needs to be established. This study was conducted to establish the effectiveness of solid particles and charged water spray in dissipating combustion smoke from enclosed spaces.
2. Materials and methods 2.1. Description of the experimental chamber and instruments Experiments were conducted in an enclosed experimental chamber measuring 3.8 m 2.3 m 2.3 m (Fig. 1). The chamber was equipped with a transmissometer and smoke generators. The transmissometer, connected to a datalogger, was employed for measuring changes in light transmission through the chamber [3]. It has a spectral response in the visible region (400–700 nm wavelength). Measurement is achieved by directing a beam of visible light across the chamber (path length of 3.8 m) onto the light detector, whose output varies with changes in light transmission. A reading of 100% indicates complete obscuration (i.e., no transmission of light from the source to the detector) while a reading of 0% indicates 100% light transmission. Smoke
ARTICLE IN PRESS R.G. Maghirang, E.B. Razote / Fire Safety Journal 44 (2009) 668–671
was generated by burning moistened paper (newspaper) inside the combustion container of smoke generators. The smoke has an aerodynamic geometric mean diameter (GMD), based on mass or volume, of 1.2 mm and geometric standard deviation (GSD) of 2.0 (Table 1). The GMD and GSD values were obtained by using an 8stage cascade impactor [3], which was operated at a sampling flow rate of 2 L/min for 1–2 min. 2.2. Scavenging particles Three types of nanostructured particles (NA TiO2, NA MgO, and NA MgO plus) and two conventional powders (fire-extinguisher grade NaHCO3, Ca(OH)2) were considered (Table 1). The nanostructured materials were manufactured by NanoScale Corporation, through proprietary processes and were marketed as NanoActiveTM particles, herein referred to as NA. These particles were considered based on their relative effectiveness for glycol smoke [3]. Table 1 summarizes the aerodynamic size distributions of the particles when dispersed into the chamber, as measured with 8-stage cascade impactor [3], which was operated at a sampling flow rate of 2 L/min for 1–2 min after particle dispersion. Uncharged and charged water sprays were also considered. Charged water spray was generated using a commercially Mixing Fan Smoke from Smoke Generator
Transmissometer
2.3 m
Scavenging Particles
669
available electrostatic spraying system (Electrostatic Spraying Systems, Inc., Watkinsville, GA, USA), which has been developed for chemical application [7]. The spraying system uses air-assisted induction charging to generate charged droplets. In this process, air and water enter separately at the rear of the nozzle. The air moves at near-sonic speed through the nozzle and impacts the water at the tip, causing atomization of the spray. As the spray is atomized, the water droplets pass an electrode inside the nozzle (voltage of 1000 VDC). Electrons are induced onto the water stream and the droplets leave the nozzle with a high negative charge. The mean charging level for the charged water spray was 6.5 mC/kg (S.D. ¼ 0.9 mC/kg), as measured with a dynamic Faraday-cup sampler [8]. The spraying system was operated at a liquid flow rate of 120 mL/min (water tank pressure of 15 psig). The primary droplet size was 30–40 mm, as reported by the manufacturer. In this study, only one nozzle was used; however, for practical application and depending on the size of the airspace, more than one nozzle will have to be used. There were two sets of experiments for the charged water spray. The first set compared the effectiveness of the charged water spray to that of uncharged water spray and also untreated smoke; there were three replicates for each treatment. The second set of experiments evaluated the effect of spray duration on the effectiveness of both the charged and uncharged water sprays. In this set of experiments, the spraying system was operated at spray durations of 1, 5, and 20 min. For the uncharged water spray experiments, the number of replicates were 2, 3, and 2 for spray durations of 1, 5, and 20 min, respectively. For the charged water spray experiments, on the other hand, the corresponding numbers of replicates were 4, 3, and 3.
1.2 m 0.61 m
2.2. Experimental procedure 3.8 m
Scavenging Particles
2.3 m 1.5 m
1.5 m
Smoke from Smoke Generator 3.8 m Fig. 1. Schematic diagram of the experimental setup: (a) plan view and (b) elevation. Not drawn to scale.
For each experiment, the chamber was prepared by cleaning the surfaces (walls and floor) and running its high-efficiency particulate air filtration system for approximately 30 min. Smoke was introduced into the chamber until there is complete obscuration or light transmission is at or near 0%. While the smoke was being introduced, two mixing fans inside the chamber were operated for approximately 2 min to disperse the smoke inside the chamber. As soon as light transmission reaches 0%, the mixing fans were turned off. Scavenging particles were then immediately sprayed into the chamber through a pressurized canister (for solid particles) or spray nozzle (for water sprays). Light transmission through the chamber was monitored with the transmissometer. From the transmissometer data, the times to reach light transmission of 10% (t10) and 20% (t20) through the
Table 1 Summary of experiments.1 Treatment
Untreated smoke Smoke treated with Smoke treated with Smoke treated with Smoke treated with Smoke treated with Smoke treated with Smoke treated with 1 2 3 4
NA MgO plus NA MgO NA TiO2 NaHCO3 Ca(OH)2 uncharged water spray4 negatively charged water spray4
Number of replicates
Actual mass deployed2 (g) mean (S.D.)
Aerodynamic size distribution GMD (GSD)3
t10 Mean (S.D.)
t20 Mean (S.D.)
3 2 2 3 2 2 3 3
– 78.0 16.6 9.6 57.2 48.0
1.2 13.9 6.5 7.5 7.7 6.6
122 63 96 47 72 86 75 8
184 127 152 88 110 145 129 29.5
(2.7) (7.3) (5.3) (6.6) (2.4)
(2.0) (2.4) (4.9) (3.6) (4.0) (3.0)
Column means followed by the same letter are not significantly different at the 5% level of significance. Nominal mass deployed for the solid particles was 100 g. Geometric mean diameter (GMD) in mm and geometric standard deviation (GSD). Continuous spraying for five minutes.
(7.0)a (1.4)de (0.9)b (17.5)e (5.4)cd (2.9)bc (9.0)cd (1.0)f
(13.7)a (11.7)bc (17.5)ab (40.4) c (6.1)bc (6.0)ab (20.0)bc (14.0)f
ARTICLE IN PRESS R.G. Maghirang, E.B. Razote / Fire Safety Journal 44 (2009) 668–671
chamber were determined. Small values of t10 and t20 relative to untreated smoke indicate effective smoke clearing. The 10% and 20% levels were chosen in this study because, at these levels, one can begin to ‘‘see through’’ the smoke. The t10 and t20 values were analyzed by using least-squares means procedure of SAS (Version 9.1, SAS Institute, Inc., Cary, NC) [9].
3. Results and discussion Table 1 summarizes the mean t10 and t20 values for the untreated smoke and smoke treated with various solid scavenging particles. For untreated smoke, the mean t10 and t20 values were 122 and 184 min, respectively. Smoke dissipates naturally because of gravitational settling and evaporation; gravitational settling may be enhanced by collision and coagulation of smoke particles, forming particles that are larger and would settle out faster. The large t10 and t20 values for the untreated smoke are likely due to the relatively small size of the smoke particles (Table 1). Because of the small size, most of the smoke particles have very low settling velocity and would therefore stay airborne for relatively long periods of time. For example, the settling velocity for 1.2 mm particles is approximately 0.0048 cm/s. By spraying solid particles into the smoke-filled chamber, the mean t10 values were reduced significantly, ranging from 47 min for smoke treated with NA TiO2 to 96 min for smoke treated with NA MgO. The mean t20 values were also reduced significantly, ranging from 88 min for NA TiO2 to 152 min for NA MgO. The significant reduction in the mean t10 and t20 values is due to capture of smoke particles by the solid particles. Note that a large fraction of the solid particles is larger in size than the smoke particles (Table 1) and would therefore settle out much faster than the smoke particles. It should be noted, however, that while spraying solid particles significantly reduced the mean t10 and t20 values, the resulting values are too long (over 40 min), indicating that they would not be of much benefit in actual settings, e.g., in fire fighting applications. The solid particles differed in terms of their effectiveness in dissipating smoke (Table 1). This could be due to differences in particulate loading (which depends on mass deployed), size distribution, and possibly their affinity to smoke particles. The mass deployed ranged from 9.6 g for NA TiO2 to 78.0 g for NA MgO plus (Table 1). Surprisingly, even with the least mass deployed, NA TiO2 appeared to be the best of the solid particles. Reasons for TiO2 being the most effective of the solid particles are not clear from the experiments. Table 1 also shows the mean t10 and t20 values for smoke treated with uncharged water spray and smoke treated with charged water spray. Similar to the solid particles, both the uncharged and charged water sprays resulted in significant reduction in the mean t10 and t20 values compared with the untreated smoke. The uncharged water spray was generally similar to the solid particles in terms of mean t10 and t20 values. The negatively charged water spray, on the other hand, was significantly more effective than the solid particles and the uncharged water spray. As indicated by the mean t10 values, the smoke treated with charged water spray dissipated approximately 15 times faster than the untreated smoke. Also, based on the mean t10 values, the charged water spray resulted in 3–12-fold increase in the speed of smoke dissipation compared with solid particles and uncharged water spray. The mechanisms for particle removal of water droplets (charged or uncharged) are relatively well understood [10]. When an uncharged water droplet approaches a cloud of smoke particles with a relative velocity, it may directly collide with some of the smoke particles (i.e., impaction), barely touch some of the smoke particles (i.e., interception), or entirely miss some particles [2,10]. The relative effect of the mechanisms
Time to reach 10% light transmission, min
670
100 Smoke treated with uncharged water spray Smoke treated with charged water spray
80
60
40
20
0 1
5 Water spray duration, min
20
Fig. 2. Effect of spray duration on the mean t10 values for the combustion smoke treated with uncharged water spray and combustion smoke treated with charged water spray. Error bars represent one standard deviation.
of interaction between the droplet and the particles depends upon the size of the particles. The primary collection mechanism for large particles is impaction, while that for sub-micrometer particles is Brownian diffusion [2,10]. When the water droplets are highly charged, electrostatic force between the droplets and the smoke particles would enhance the collection mechanisms, resulting in improvement in overall collection efficiency of the water droplets. Fig. 2 summarizes the mean t10 values for the water sprays (i.e., uncharged and negatively charged particles) as a function of spray duration. For both the uncharged and charged water sprays, the mean t10 values generally decreased with increasing spray duration. Similar trend was observed for the mean t20 values (data not shown). The decrease in the mean t10 and t20 values is expected because increasing the spray duration increases the number of water droplets in the air. With greater number of water droplets, collisions (and collection) of smoke particles by the water droplets would be enhanced. For the charged water spray, the 5- and 20-min spray durations did not significantly differ in the mean t10 and t20 values. The lack of significant difference between the 5- and 20-min spray durations could indicate that for the test conditions in this study, spray duration of about 5 min would be close to the optimum.
4. Conclusions The effectiveness of solid particles and electrostatically charged water spray in enhancing smoke dissipation in enclosed spaces was evaluated under controlled laboratory conditions. The following conclusions were drawn from this research:
Charged water sprays significantly enhanced the dissipation
rate of smoke in the enclosed experimental room. Longer spray duration improved the dissipation rate of smoke. Spraying either solid particles or uncharged water spray also enhanced smoke dissipation; however, the improvement appeared to be quite limited and of little practical use.
While the charged-water spray resulted in significant improvements in the rate of smoke dissipation, there is a need to still reduce the time it takes to produce acceptable visibility levels. Further work will be conducted to improve the effectiveness of
ARTICLE IN PRESS R.G. Maghirang, E.B. Razote / Fire Safety Journal 44 (2009) 668–671
the charged water spray. Factors including the concentration of charged water spray, number of nozzles, size of water droplets, among others, will be considered. In addition, more detailed characterization of smoke and charged water spray will also be conducted. Numerical research based on aerosol kinetics will also be pursued.
Acknowledgements This work was partly funded through the award of a contract from the United States Marine Corps Systems Command to M2 Technologies, Inc. The authors acknowledge the assistance of Darrell Oard, Emad Almuhanna, Tyler Pjesky, Andrew Rosander, Matt Woerman, and Bala Kakumanu. References [1] X.D. Xiang, I. Colbeck, Charged water drops and smoke dissipation, Fire Safety J. 28 (1997) 227–232.
671
[2] W.C. Hinds, Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, second ed., John Wiley & Sons, Inc., New York, 1999. [3] R. Yadav, R.G. Maghirang, L.E. Erickson, B. Kakumanu, S.G. Castro, Laboratory evaluation of the effectiveness of nanostructured and conventional particles in clearing smoke in confined spaces, Fire Safety J 43 (1) (2008) 36–41. [4] S.P. Decker, J. Klabunde, A. Khaleel, K.J. Klabunde, Catalyzed destructive adsorption of environmental toxins with nanocrystalline metal oxides, Environ. Sci. Technol. 36 (4) (2002) 762–768. [5] O.B. Koper, J. Klabunde, G.L. Marchin, K.J. Klabunde, P. Stoimenov, L. Bohra, Nanoscale powders and formulations with biocidal activity toward spores and vegetative cells of Bacillus species, viruses, and toxins, Curr. Microbiol. 44 (1) (2002) 49–55. [6] J. McCoy, L. Meicher, Evaluation of charged water sprays for dust control. US Bureau of Mines, Report no. 818, 1983. [7] E. Almuhanna, R.G. Maghirang, J.P. Murphy, L.E. Erickson, Effectiveness of electrostatically-charged water spray in reducing dust concentration in enclosed spaces, Trans. ASABE 51 (1) (2008) 279–286. [8] E. Almuhanna, Dust control in livestock buildings with electrostaticallycharged water spray, Ph.D. dissertation, Kansas State University, Manhattan, Kansas, 2007. [9] SAS. SAS/STAT User’s Guide, SAS Institute, Inc., Cary, NC, 1990. [10] C.V. Mathai, Charged fog technology. Part II: Prototype tests of a new charged fog generator for fugitive emission control, J. Air Pollut. Control Assoc. 33 (8) (1983) 756–759.