The Experimental Study and Simplified Model of. Water Mist Absorbing Heat Radiation

Transcription

1 Advanced Materials Research Submitted: ISSN: , Vols , pp Accepted: doi: / Online: Trans Tech Publications, Switzerland The Experimental Study and Simplified Model of Water Mist Absorbing Heat Radiation Chunyi Lu 1, a Yonggang Yu 2, b 1 College of Urban Construction and Safety Engineering, Nanjing Tech University, Nanjing , China; 2. School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing , China a lu.chuny@gmail.com, b yyg801@njust.edu.cn Keywords: Water mist Thermal radiation Absorb Spray pressure Heat flux measurements Abstract: The small-scale experiments of water mist extinguishing cardboard fire were carried out. Thermocouples and heat radiation sensor were used to measure the fire temperature and radiation heat flux of the burning area. The variation characteristics of the combustion field radiation heat flux under different spray pressure were studied. The results show that: the spray pressure has a significant influence on the extinguishing process. Based on the experiments, The simplified model of water mist absorbing heat radiation has been established, the results of this model agree well with the results of the experiments, and is good for predicting the effect of water mist particle diameter. Introduction Water mist has some advantages of no environmental pollution, high extinguishing efficiency, non-toxic to humans, less-disruptive to object protection, which is considered as the main substitute product of halogenated series extinguishant. Water mist fire suppression technology has been rapid development in the 1990s. Many European countries have been carried out on basic experiment and applied research for many years, such as water mist for fire protection of historic buildings and museums[1], water-mist fire suppression systems for the telecommunication and utility industries[2], characterization of a water mist based on digital particle images[3] and so on. At the same time they have already developed a variety types of water mist fire suppression systems, and will be widely used in related fields and places. The development and testing of water mist fire suppression systems are began to research in the late 1990s in our country, BinYao et al[4] studied experimental study on interaction of water mists with fires in a confined space, Jiang-hong Liu et al[5] observed a quasi-steady-state model of water mist fire suppression within a confined space and the infrared thermographic of interaction between water mist and wood crib flame are researched. The high-temperature gas products are generated when the fire broke out. As for most carbon hydrogen fuel, flaming combustion will produce the high temperature carbon black, blackbody radiation intensity of approximately the same temperature is emited, and have a great impact on the spread of fire evacuation and safety of personnel[7]. Previous studies have shown that water mist has a strong attenuation on flame radiation and has good application prospects in terms of blocking thermal radiation. This paper reports the small-scale experiments of interaction between water mist and cardboard flame. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-05/03/16,21:46:10)

2 dbadwatermisdwatermisadvanced Materials Research Vols Experimental Device and Testing System Fig.1 are the diagram of experiment platform, which is divided into three parts, water mist spraying system, burning platform and sample, data collection and processing system. Water mist nozzle is a two-fluid nozzle, the nozzle hole diameter of 0.4 mm, which can be at the same time to puff of water mist in six directions, the conical spray vertex angle of 140 is formed. Heat flow radiation sensor is the WR heat flux sensor (circular foil heat flux sensors), the range is from 0 to 5w/cm 2. Eight mm 3 cardboard is used in burning sample, four cascading together, as shown in fig.2, placed in a iron burning plate (at the bottom of the inner diameter of 130 mm, evenly covered by 4ml of alcohol). Two way nickel chrome-nickel silicon thermocouple temperature measurement system is adopted in the experiment. Fire temperature of 9cm place from the ground is measured in one way, the upper board is inserted in another way. Heat radiation sensors is placed beside the water mist nozzle (which can be considered to be the fire center right above), measuring surface and the nozzle at the same height, 40cm off the ground. While experiment, the data acquisition system processing the thermocouples and heat flow radiation sensor signal, at the same time with a digital camera to record the whole process. Fig.1 A block diagram of the experimental platform Fig.2 Cardboard stacked form Experimental Results and Discussion of Water Mist Effects on the Cardboard Flame Thermal Radiation ta 1.0 A0.8 te/ W/cm T/ 0 C t/s t/s A-Fire temperature Fig. 3 Temperature-time curve B-Cardboard surface temperature Fig. 4 Heat density-time curve

3 888 Applied Power and Energy Technology II The experiments have been made under different conditions of the spray air pressure, the water pressure (gauge pressure) remains unchanged 2.25bar, alcohol flame is used to ignite the cardboard, water mist is began to spray in the 50s. The spray air pressure (gauge pressure) is 0.8bar, the whole process of temperature and radiation heat flux changes with time is shown in fig. 3 and fig. 4. Seen from fig.3, cardboard surface temperature curve is relatively stable, changing slowly. Because the temperature changes only affected by combustion heat release and heat conduction. And there are large fluctuations in the fire temperature, which is due to natural convection disturbances. Swirl is continue formed outside the flame zone and flame is fluctuated which leading to the flame plume. Therefore, the measured fire temperature is also fluctuated, but its average temperature is still higher than the cardboard surface temperature. The measured fire temperature is lower between 0s ~ 40s. The alcohol burning is more intense and flame is high in the stage, the inner flame temperature is actually measured by thermocouple and is relatively low. Both temperature decrease, but the fire temperature drops more intense after water mist is applied. Seen from fig.4, after ten seconds, radiation flux density reaches a higher value, fluctuating in a position, but the overall trend is small. After about 40s, the radiation heat flux values rise rapidly, and dynamically change in a certain range after achieving a more stable value. Radiant heat flux values decrease rapidly after water mist is applied. After about 9s, heat flux value is close to zero bar 0.8bar 1.5bar E(w/cm 2 ) t(s) Fig.5 Changing trends of radiant heat flux after water mist is applied After water mist is applied, radiant heat flux trends under different spray pressure are shown in fig.5. The downward trend of 0.8 bar curve is more steep than 0.5 bar curve. The mist particle diameter becomes smaller because of the increased pressure, thus accelerating evaporation and the rate of heat absorption. Also, water vapor concentration increases with the gas pressure increasing, the effect of diluting oxygen is more obvious, thereby decreasing the burning rate of cardboard and reducing combustion heat release. However, when the spray pressure is 1.5 bar, the extinguishing time becomes longer, the descent rate of radiant heat flux is slower, which seems to conflict with the previous analysis. Through careful analysis: I consider that air is used as atomizing gas in the experiment, and it contains a lot of oxygen which strengthen the combustion of cardboard, it has a greater effect on the decrease of radiation. It will not come to this situation if using nitrogen as the atomizing gas.

4 Advanced Materials Research Vols Simplified Model and Numerical Calculation Mathematical Model of Water Mist Absorbing Thermal Radiation In order to establish the mathematical model, the following assumptions are made: a) Water mist particle size is the same under the same spray pressure. b) The influence of water mist on radiation is mainly evaporation heat absorption, the vaporizing oxygen barrier effect etc. are converted to the reduce of the blackness coefficient. c) Heat conduction, convection and radiation are weakened because of endothermic evaporation of water mist, wherein the effect of radiation can be represented by the radiation absorption coefficients. d) The flame shape is cylinder which bottom area equals to the burning site area, height equals to the height of the flame. After the water mist passing through the fire, the diameter can be represented by D 2 law: d 2 d Kt (1) = 2 0 8λ cp 1 K = ln 1 + ( T T0 ) ρ r cp 1 H If diameter droplet is calculated, Then the reduced mass can be calculated : m = ρ π ( d0 d ) (3) 6 Then absorbing heat of a small water droplets can be calculated: ( T 100) + c (100 )] q = m H (4) [ + c p1 p2 T0 In the formula, C p1 is the specific heat of water vapor, C p2 is the specific heat of water, H is the latent heat of vaporization of water. According to equivalent cylinder representing the volume fraction of the entire water mist volume fraction (β) and particle number of spray water mist (n) in the unit time, the total evaporation heat absorbed of water mist in this period of time can be worked out: Q = qnβt (5) The total evaporation heat absorption will convert to the reduce of radiation through radiation absorption coefficient φ, radiation heat flux reduction can be calculated: ϕ Q E = (6) 1 ta f According to assumption d) and the empirical formula 8, the size of the fire cylinder can be estimated: bottom diameter is 13cm, height is 19cm. By calculating the angular coefficient, the relationship between radiant heat flux receiving by heat radiation sensor and cylinder of fire can be get: E = 0.087E (7) 2 1 So after water mist is applied, radiant heat flux receiving by heat radiation sensor is: E = 0.087( εσt E ) (8) (2)

5 890 Applied Power and Energy Technology II The Comparison of Theoretical and Experimental Results The downward trend of heat flux through heat flux sensor can be get by mathematical models and iterative calculation, absorption coefficient of evaporation to radiation is obtained by fitting experimental: φ= 0.41 The effects of endothermic evaporation of water mist on the thermal radiation of the flame are be drawn and compared with measured values. The specific relationship is shown in fig.6, theoretical curves agree well with the measured curve. This indicates that the theoretical model accurately simulate the experimental process, and the impact of water mist on solid flame heat radiation is basically reflected. Because the various confounding factors of experimental procedures are not considered, such as air convection conditions, ventilation, injection angle and fog flux changes over time when water mist is applied, unevenness of mist particles, etc, so the theoretical curve is much smoother than the experimental curve. A - Measured value A 150 µm, B 100 µm, C 50 µm B - Theoretical value Fig.7 The comparison chart of water mist Fig. 6 The comparison chart between the absorbing heat radiation theoretical and measured values of water mist under different sizes conditions absorbing thermal radiation (pressure 0.8 bar) Theoretical Predictions According to the mathematical model, the influence rule of the water mist on thermal radiation of flame under different spray pressure conditions (changing the mist particle diameter) can be predict, as shown in fig.7. Combustion effects of compressed oxygen on flame are not consider. Comparison of the three curves shows that the smaller the particle diameter of water mist, the faster the decline of radiation and the shorter the extinguishing time. The radiation flux density is reduced to almost zero when the particle size is 50µm after applying water mist about 6.3s (as shown 56.3s in fig.7, radiation reduced to almost zero when the particle size is 100µm after spraying about 7s. Radiant heat flux is also larger, indicating that it will be some time to fully extinguish the fire when the particle size is 150µm after spraying about 8s. Because the effect of evaporation endothermic is only considered in the model, when particle diameter of water mist is small and fire temperature is higher, the applied water mist completely evaporated is equal to the absorbed heat. Therefore, curves 50µm and 100µm overlap in a long period of time.

6 Advanced Materials Research Vols Conclusion 1) The small-scale simulation experiments show heat radiation of solid combustibles flame can be effectively suppressed by water mist. The size of spray air pressure has a significant influence on the heat radiation. 2) On the experimental basis, a mathematical model of water mist absorbing thermal radiation is established, calculated and measured values are in good agreement, the model can also be used to predict the impact of water mist particle diameter. References [1] T. Log, P.c. Brookes. Water mist for fire protection of historic buildings and museums[j]. Museum management and curatorship Vol.14(1995), p.336 [2] J. R. Mawhinney. Water-mist fire suppression systems for the telecommunication and utility industries[a].proceedings: Halon Alternatives Technical Working Conference[C].Albuquerque 1994 [3] X. Wang,X. Wu, G. Liao, Y. Wei, J. Qin. Characterization of a water mist based on digital particle images[j]. Experiments in Fluids,Vol. 33(2002), p.587 [4] B.Yao, G.X. Liao, W.C.Fan, Experimental study of the interaction of water mist and fire in confined space[j]. Fire Sci. Vol. 6(1997), p.49 [5] J. H. Liu, G. X. Liao, L. P. De, J. Qin, W. C. Fan. The infrared thermography research on interaction of water Mist and solid wood crib fire[j]. Infrared Technol. Vol. 23(2001), p.38 [6] J. H. Liu, G. X. Liao, L. P. De, Q. Lu. The quasi-steady-state model of water mist fire extinguishing in confined space[j]. Combus. Sci. and Technol. Vol. 4(2003), p.380 [7] J. H. Zhu, J. C. Zhu. Calculation of Characteristic Parameters of Pool Fire and Assessment of Its Thermal Radiation Hazard[J]. Chin Safety Sci. J. Vol. 13(2003), p.25 [8] F.H. Jiang, Q. Zhong. Estimation of flame radiation and temperature of polymer burning[j]. Fire Sci. Vol. 1(1994), p.18

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