Scholarly article on topic 'Experimental Study on Fire Extinguishing of Water Mist with a Newly Prepared Multi-component Additive'

Experimental Study on Fire Extinguishing of Water Mist with a Newly Prepared Multi-component Additive Academic research paper on "Earth and related environmental sciences"

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Procedia Engineering
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{"Water mist" / "Multi-component additive" / "Wood crib fire" / "Oil pool fire" / "Fire suppression performance"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Binbin Wu, Guangxuan Liao

Abstract In order to enhance the fire suppression performance of water mist, a new multi-component additive of water mist is produced through physics process for the typical fire (oil pool fires and wood crib fires). To study the effectiveness of the water mist adding this new multi- component additive in fire suppression, a laboratory-scale suppression system was built and a series of tests were conducted in this system. The experimental result reveals that the fire suppression effectiveness of water mist is improved greatly by adding the multi-component additives, Along with increasing the amount of multi-component additives, the fire suppression time of oil pool fire drops rapidly, then increases slightly at some points; but the time of wood crib fire declines gently, then tends to become stable. Finally, the suppression mechanism is discussed so as to find out the deep relationship between the multi-component additive and suppression effectiveness.

Academic research paper on topic "Experimental Study on Fire Extinguishing of Water Mist with a Newly Prepared Multi-component Additive"

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ScienceDirect Procedia

Engineering

Procedia Engineering 62 (2013) 317 - 323 =

www.elsevier.com/locate/procedia

The 9th Asia-Oceania Symposium on Fire Science and Technology

Experimental study on fire extinguishing of water mist with a newly prepared multi-component additive

Binbin Wu*, Guangxuan Liao

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, China

Abstract

In order to enhance the fire suppression performance of water mist, a new multi-component additive of water mist is produced through physics process for the typical fire (oil pool fires and wood crib fires). To study the effectiveness of the water mist adding this new multi-component additive in fire suppression, a laboratory-scale suppression system was built and a series of tests were conducted in this system. The experimental result reveals that the fire suppression effectiveness of water mist is improved greatly by adding the multi-component additives, Along with increasing the amount of multi-component additives, the fire suppression time of oil pool fire drops rapidly, then increases slightly at some points; but the time of wood crib fire declines gently, then tends to become stable. Finally, the suppression mechanism is discussed so as to find out the deep relationship between the multi-component additive and suppression effectiveness. © 2013 InternationalAssociation forFireSafetyScience. Published byElsevierLtd. AllRights Reserved Selection and peer-review under responsibility of the Asian-Oceania Association of Fire Science and Technology

Keywords: Water mist; Multi-component additive; Wood crib fire; Oil pool fire; Fire suppression performance

1. Introduction

The chemical substances which have the fire suppression function can improve the fire suppression performance of water mist, as an aqueous solution, the chemical substances make the physical and chemical fire extinguishing function work together to form a totally internal function. The impact of chemicals in water mist on fire suppression performance mainly lies in the following four factors. The first one is the viscosity of water. The greater the viscosity, the more conducive to adhere to the surface, but the greater viscosity will increase the resistance of pipeline transportation and the difficulty of spraying. In contrast, the smaller the viscosity the more conducive to the transmission and spray, but decreased adhesion. The second one is the amount of additives. In the same fire suppression performance premise, additive dosage should be as little as possible. The third one is the preventing thermal radiation performance of the liquid membrane formed after the injection. When the object is burned, it itself and the surrounding ones will be heated up by strong thermal radiation. When the water mist with additives is sprayed to the object, liquid membrane will form on the surface, and at the same time, it can keep out part of thermal radiation, delay the temperature of the object and prevent the fire from spreading. The above abilities of liquid membrane depend on the thickness, evaporation speed and the spectral characteristics of the additives in it, especially the infrared spectral characteristics. The last one is the fluidity of water. Fire extinguishing agent gets through the pump and pipeline transportation, and then is sprayed to the surface. In this process, the shear rates are very different. If the extinguishing agent has thixotropy, with increasing shear rate, the viscosity would decrease, namely when the extinguishing agent transporting in the pump and pipeline, the viscosity is small, also with small resistance. Once the extinguishing agent is sprayed to the surface, the shear rate will decrease, and the viscosity and adhesion will be enhanced. So the thixotropy of the extinguishing agent would be good for fire suppression [1-3].

* Corresponding author. Tel.: +86 551 6360 6461. E-mail address: wubb@mail.ustc.edu.cn.

1877-7058 © 2013 International Association for Fire Safety Science. Published by Elsevier Ltd. All Rights Reserved Selection and peer-review under responsibility of the Asian-Oceania Association of Fire Science and Technology doi: 10.1016/j.proeng.2013.08.070

At present, the environmental protection, fire suppression performance and economy of water mist have been widely recognized, and extensive application of water mist is also validated. So, the study of the additives for water mist and further demonstration of the fire suppression performance of the new additives have broad prospects.

2. Experimental apparatus

In order to analyze and evaluate the effect of the multi-component additive on fire suppression performance, an experimental platform of water mist was designed according to different sources (oil pool fire and wood crib fire), then a series of experiments were carried out in this system under conditions of different additive concentrations [4].

The whole experiment system included three parts: combustion system, fire extinguisher system and data acquisition system, which is shown in Fig. 1. The nozzle was installed 2 m above the fuel surface, with an adjustable height. A suitable low pressure water mist nozzle for this experiment was designed by our group through contrast analysis, which is shown in Fig. 2. A fixed pressure from 0.35 MPa to 0.7 MPa could achieve the perfect atomization effect. The flow rate was 420 mL/min. For oil pool fire suppression, in each case, about 200 mL of diesel oil was used as fuel and 10 mL of alcohol was used for ignition. For wood crib fires, the wood crib was approximately 200 mm x 200 mm x 120 mm, consisting of four layers of fir lumber. The wood crib was centered over an alcohol fuel pan. K-type thermo couples were arrayed along the centerline of the flame to measure the local temperatures, while a digital video camera was used not only for filming the dynamic process of fire suppression, but also for determining the fire extinguishing time.

the temperature acquisition module

smoke components anaiysis instrument

Fig. 2. The low pressure water mist nozzle used in the experiment.

The multi-component additive used in these experiments was developed by our group. The formulation of this multi-component additive is shown in Table 1. The new multi-component additive of water mist includes four kinds of additives material. The first one is a material which can increase chemical extinguishing effects of water mist. This kind of material includes thermal sensitive material and chemicals which can interrupt the burning chain reaction. The material contains

metal ions and can improve entrainment fire suppression performance effectively. The second one is a hydrocarbon surfactant. This material, when it is added into water mist, can improve the insulation of oxygen and reduce the thermal radiation effect of flame. At the same time, after injection on the burning surface, it can form a layer of water film which can exclude oxygen and reduce heat radiation. This is the inheritance of the ability of fire suppression characteristics of the foam additives, and because it seldom produces foam, it can greatly reduce the pollution of the scene of the fire. The third one is fluorine surfactants. The main function of fluorine surfactants is to reduce the surface tension, improve the flow characteristics and the atomization effect of water mist. The last is solvent. The role of the solvent is to make the above three kinds of material and water form homogeneous liquid, at the same time, it can improve entrainment antifreeze ability, reduce the melting point of water mist which has the great significance to improve the temperature range for water mist. Every component is non-toxic and noncorrosive, and the multi-component additive is usable as an environmentally friendly fire suppression agent for water mist.

Table 1. The formulation of multi-component additives

Chemical composition Function Mass percent / %

fluorine surfactant reduce the surface tension 6

organic solution make solution stable and uniform, decreasing interfacial tension 10

surfactant reduce the oil/water interface tension 3

inorganic salt improve fire extinguishing effects 20

firm bubble agent, antifreeze agent , etc 6

water 55

The configuration steps are divided into several steps. Pour 6% fluorine surfactant into the 10% organic solvents, stir until completely dissolved. At the same time, add 20% inorganic salt in 55% water. Then mix the above two kinds of solution in together. Then slowly pour 3% surfactants and 6% firm bubble agent and antifreeze agent into the above solution by stirring in slow speed. Last stir the solution to form clear solution.

The experiments could be divided primarily into seven steps:

1. Prepare a certain concentration of extinguishing media which contains multi-component additive;

2. Close control valves of the pressure system, open the inlet valve, store fire extinguishing agent medium into water tank, and then close the inlet valve and release valve;

3. Set the position of the fire, distance of height between water mist nozzle and fire, etc;

4. Set the type and size of the fire;

5. Open the valve of nitrogen tank, adjust pressure regulating valve of reducing valve, set up the pressure driver of water mist system;

6. Kindle the fire, make oil pool fire or wood crib fire in a stable state when the burning;

7. Open control valves of water, water mist begin to work to extinguish fire. Fire extinguishing time is T.

3. Results and discussion

3.1. Diesel oil pool fire

Fire extinguishing process will experience three stages. First, water mist intensifies the flow field disturbances, and increases the flame instantly; Second, more droplets fall into fire plume flow and reduce the flame temperature by absorbing heat, which make the motion resistance of droplets reduced, then the flame is gradually covered by water mist; Finally, water droplets get to the fuel surface through flame area, reduce the temperature of fuel surface and decrease the evaporation of flammable gas, eventually leading to the flame extinguishment. The third stage is the most time consuming.

In order to investigate the effects of the new multi-component additive on fire suppression and to determine the optimization value of the additive, a series of experiments were conducted wherein the additive concentration varied from 1% to 5%. From Table 2 it can be seen that with the increase of multi-component additives content, the fire extinguishing time fell rapidly, and then became to be stable.

Table 2. Fire extinguishing time with different multi-component additive concentrations

Figures 3 and 4 respectively provide video pictures of diesel oil fire suppression process and the temperature before and after the injection of 3% multi-component additive. As can be seen from the figures, water mist droplets get to the fuel surface, not only dilute liquid fuel, but also cover the fuel surface with a liquid membrane rapidly, which greatly shorten the time of third stage mentioned before, also extremely shorten the whole fire extinguishing time.

Fig. 3. Diesel oil fire behavior before and after the injection of different multi-component additive concentrations. (a) 1% multi-component additive; (b) 2% multi-component additive; (c) 3% multi-component additive.

the oil surface 10cm above the oil surface 20cm above the oil surface 30cm above the oil surface 40cm above the oil surface

Start water mist

inside the wood crib the wood crib surface 10cm above the wood crib surface 20cm above the wood crib surface 30cm above the wood cribsurface 40cm above the wood crib surface

0 20 40 60 80 100 120 140 160 180 200 Time / s

Time / s

Fig. 4. The temperature of diesel oil fire before and after the injection of 3% multi-component additive.

Fig. 5. The temperature of wood crib fire before and after the injection of no multi-component additive.

3.2. Wood crib fire

Figures 5 and 6 respectively show the temperature of wood crib fire before and after the injection of no multi-component additive and 6% multi-component additive. As can be seen from the figures, after opening the water mist, when the wood crib fire was burning completely, the flame temperature descended immediately and was suppressed effectively, meanwhile the flame of wood crib surface burning was put out quickly. This influence mentioned above is slight whether the multi-component additive is added, but the additive accelerates the process of internal flame suppression, shorten the whole fire suppression time as shown in Fig. 7. With the increase of additives content, the fire suppression time decreases gradually, and then becomes to be stable.

Start water misl

Start water mist

inside the wood crib the wood crib surface 10cm above the wood crib surface 20cm above the wood crib surface 30cm above the wood cribsurface 40cm above the wood crib surface

1% multi-component additive 3% multi-component additive 6% multi-component additive

Fig. 6. The temperature of wood crib fire before and after the injection of 6% multi-component additive.

Fig. 7. The inside temperature of wood crib after the injection of different multi-component additive concentrations.

Start water mist

1% multi-component additive 2% multi-component additive 3% multi-component additive

Time / s

Time / s

Time / s

Fig. 8. The temperature of fire after the injection of different multi-component additive concentrations.

3.3. Suppression mechanism

Physical and chemical fire suppression mechanisms are involved in the water mist with the new multi-component additive.

Flame cooling and oxygen displacement by steam are considered to be the dominant physical mechanisms of extinguishment, and fine water mist droplets are more efficient at absorbing heat. However, extinguishing open space fire with water mist depends on many parameters, such as spray momentum and spray angle as well as the distance between the fire and the water mist nozzle. On the one hand, the multi-composition additive makes the chemical and physical properties of water be changed, and enhances the physical fire suppression. Take the fluorine surfactant as an example. The main function of fluorine surfactants is to reduce the surface tension, improve the flow characteristics and the atomization effect of water mist. The lower the surface tension of water mist, the faster the water mist evaporate and take away heat, which make the temperature of the fire be reduced quickly, as shown in Fig. 8. At the same time, some larger water droplets of water mist can get to the surface of the burning oil or wood crib, and take away heat rapidly by vaporizing, so as to make the fuel surface and inside be cooled, as shown in Fig. 7.

On the other hand, some additives enhance the chemical fire suppression. A large number of documents pointed out that

the spread of fire comes from the rapid response between key groups (hydrogen and oxygen atom, hydroxide radicals) and fuel molecules [5]. The multi-composition additive can work out the cation and anion, which is very active chemically. These cation and anion react with free radical induced by combustion reactions to break chemical-looping combustion, and thus inhibit the burning to achieve the purpose of fire suppression. Chain reactions include:

RH+O2-> H + 2O+- (1)

O~ +H -+OH~ (2)

2OH"->H 2O (3)

The last step is a strong exothermic reaction, which has much more quantity of heat than thermal absorbent of burnable decomposition in the first step. Meanwhile, O- is decomposed again to make burning continues.

For the inorganic salts in the multi-composition additive, the main component is sodium carbonate, sodium bicarbonate, etc. They extinguish fire mainly through the following several steps:

2NaHCO3 = Na2CO3+ CO2 T +H2O T (4)

Na2CO3 = Na2O + CO2 T (5)

Na2O = Nafit (6)

The above three reactions exist simultaneously in the injection process of water mist. The fine droplets of water mist stay in the flame for only a little while, and the contact area of the droplets to the flame is sufficiently enough to complete the above three reactions. The gasification Na2O can be reacted with steam in the flame to make sodium hydroxide.

Na2O + H 2O = 2NaOH (7)

Then the gasification sodium hydroxide react with the free radical induced by the combustion reactions to break chemical-looping combustion:

NaOH + H+ = Na+ + H 2O (8)

NaOH + OH - =NaO~ + H2O (9)

The product of the above two reactions combine with the radical OH-and H+ to elimination free radical and generate sodium hydroxide.

Na++OH~=NaOH (10)

NaO~ +H+ = NaOH (11)

4. Conclusions

The results of a series of experiments on fire suppression show that the newly prepared multi-component additive for water mist can improve the fire suppression performance of diesel pool fire and wood crib fire. For diesel pool fire and wood crib fire, the optimized concentration values are respectively about 3% and 6%.

The new multi-component additive for water mist is composite material, and each component can affect the fire suppression mechanism of water mist. The inorganic salts in the multi-composition additives can enhance the chemical extinguishing effects, and the fluorine surfactant in the multi-composition additives can enhance the physical fire suppression.

References

[1] Edwards, M., Watkins, S., Glockling, J., 1999. Low Pressure Water Mist, Fine Water Spray, Water Source and Additives; Evaluation for the Royal Navy. Proceedings of the 8th International Fire Science and Engineering Conference. Interflam'99, Edinburgh, Scotland, 1, pp. 639-650.

[2] Kim, A K., 2000. "Fire Suppression Performance of Water Mist Systems with Additives," Proceedings of Safety Science and Technology International Symposium. Beijing, pp. 561-56.

[3] NFPA 18. Standard Oil Water Additives for Fire Control and Vapor Mitigation.

[4] Cong, B. H., Mao, T., Liao, G. X., 2004. Experimental Investigation on Fire Suppression Effectiveness for Pool Fires by Water Mist Containing NaCl Additive. Journal of Thermal Science and Technology 3(1), p. 65.

[5] Ewing, C. T, Faith, F. R., Hughes, J. T. Carhart, H. W., 1989. Flame Extinguishment Properties of Dry Chemicals: Extinction Concentrations for Small Diffusion Pan Fires. Fire Technology 25(2), p.133.