EXTINGUISHMENT OF HORIZONTAL WOOD SLABS FIRE BY A WATER SPRAY

Transcription

1 ILASS Americas Conference on Liquid Atomization and Spray Systems, Chicago, IL, May 2007 EXTINGUISHMENT OF HORIZONTAL WOOD SLABS FIRE BY A WATER SPRAY T. Poespowati * Department of Chemical Engineering The Institute of National Technology Jl. Bend. Sigura-gura 2, Malang - Indonesia Abstract Experiments were carried out on the extinguishment of burning wood slabs of Western Red cedar, Radiata pine, and River Red gum using a modified mass loss cone calorimeter equipped with a water spray system. The slabs are subjected to an electrical radiant heater to enhance the burning rate. The water is applied as a uniform spray from a single nozzle of 1/8 BLM 4-90 o -angle Delavan full cone with a round spray pattern. The time taken to extinguish the fire under suppressive action is determined as functions of irradiance heat flux, type of wood (i.e. porosity), and flow rate of water application. The effectiveness of water in suppressing the fire is determined to be primarily thermal effects, i.e. an evaporation time and a recovery time. Behavior of second ignition or re-ignition time after extinguishment process shows that overall re-ignition time increases as the water application time is increased. Other results indicated that average evaporation rate is a function of the external heat flux and type of sample (i.e. porosity). Overall re-ignition time decreases as the increases oh external heat flux and sample porosity significantly influenced the evaporation and recovery times. Evaporation time of water layer on the surface sample decreased with an increase of porosity. On the other hand, recovery time of water evaporation in the sample structure increased with sample porosity. * Corresponding author. Tel: , ext. 112; fax: address: poespowati@yahoo.com.au

2 Introduction Study of fire suppression has attracted a great deal of attention over the years because of its importance in reducing the damage caused by accidental fires. Fire suppression by means of water has been practiced for many types of fires for centuries since water is inexpensive, easy to find, has a wide range of absorbing capacity, posses a relatively high latent heat of vaporization, and is non-toxic. Generally, water is delivered to the source of fire in the form of fine droplets, which are produced by a nozzle or sprinkler system. It must be highlighted that the mechanism of fire suppression by water greatly depends on the size of water droplets used to extinguish the fire. If water droplets are bigger than 100 ìm they can penetrate through the fire plume and reach the actual burning object. In this case the extinguishment is by cooling down the solid fuel. However, if water is applied as a flow of mist particles with average size of less that ìm, then its momentum is not high enough to penetrate into the fire plume. The extinguishment in this case is achieved by cooling the gas phase (i.e. plume) surrounding the burning solid fuel and, thereby, indirectly increasing the heat losses from the burning fuel. The interaction of mists particles with a flame in a confined space is generally more complex than the effect of the droplets on the solid fuel. Typically, more smoke and toxic gas would be produced during or after the application of water mist (Yao et al., 1999). Also, it must be noted that the water spray with solid cone pattern and finer water droplet size is more effective in extinguishing fires than the one with hollow cone pattern and coarse water droplet size. To suppress a fire, the water spray flow rate has to be more than a certain critical value. However, if the flow rate is too high the fire suppression efficiency will not greatly increased. It only leads to increased operational cost because of the excess water consumption (Hua et al., 2002). Minimum rate of water application is approximately between mg.cm -2.s -1 (Tamanini, 1976). Rasbash et al. (1960) studied the mechanisms of extinction of liquid fires (alcohol, petrol, benzole, kerosene, gas oil, and transformer oil) with water sprays. Effects of the fuel nature, the time between the onset of ignition and the water application time, the spray properties, and the direction of water application on the ease of extinction were carefully investigated. When water was applied in perpendicular direction across the fire, the extinction was found to be more effective. The extinction time was found to decrease as a result of increasing the rate of water applied and decreasing the size of water droplets. Magee and Reitz (197) conducted a series of experiments on samples of four types of thermally thick plastic namely: Polymethyl methacrylate, Polyoxymethylene, Polystyrene, and Polyethylene. They subjected the samples to an external radiant heater in both horizontal and vertical orientations. In each case the resulting flame was extinguished by applying a uniform water spray. These authors studied the effect of external radiant heat flux on extinction time and steadystate rate of burning. They reported that both extinction time and the steady-state burning rate will linearly increase if the radiant heat flux is increased. Hietaniemi et al. (1999) also investigated the influence of water suppression on burning nylon 66, polypropylene, 4-chloro-3-nitrobenzoic acid, tetramethylthiuram monosulphide, and chlorobenzene. Ventilation controlled cone calorimeter was employed in this study. Effect of water application on ignition moke, and chemical emission such as CO 2, CO, NO, and NO 2 was also investigated. These researchers found application of water to the fuel object prior to flaming ignition significantly increases the ignition delay time. The general study of a liquid spray propelled towards a heated surface can be divided into two subprocesses: the behaviour of the droplets prior to impacting on the heated surface, and the dynamics of the drops after they hit the surface (González and Black, 1997). In a study on liquid spray cooling of a heated surface, the lowest surface temperature possible for the existence of spray evaporative cooling is determined experimentally to be a linear function of the impinging spray mass flux. The initiation of the Leidenfrost state provides the upper surface temperature bound for spray evaporative cooling (Grissom and Wierum, 1981). The heat transfer of impacting spray on a hot surface is composed of three mechanisms, namely, the droplet-wall impacting heat transfer, the air convective heat transfer, and the thermal radiative heat transfer. When the surface temperature is not extremely high, the first two heat transfer mechanisms are considered relatively important. However, these two mechanisms are complex. The droplet-wall impacting heat transfer is usually dependent upon the impacting dynamics of the droplets (Choi and Yao, 1987). Unoki (198) in his experiment of fire extinction by three water sprinkler systems studied the extinguishment of ceder cribs. In his experiments the distance between sprinkler and the top of cedar crib was set to 3. m. The moisture content of cedar wood was 9-12%. Unoki allowed the cribs to lose 30% of their weight to combustion before activating the sprinkler systems. This author considered the burning rates from the ignition to water discharge to be almost constant at 60, 90, and 130 kcal.s -1. Unoki (198) examined the interaction between various water sprays and the buoyant fire plume and the cooling effect of water. He

3 found that droplets with radii of 200 ìm perfectly follow the Newton s law and can easily penetrate into the plume. In 1986, Takahashi conducted a combined experimental and theoretical study on the extinction of Japanese cedar wood crib by water that applied from a glass-capillary. The water applied by two different methods: from top to bottom and from bottom upwards of the crib. Similar to other relevant studies Takahashi reported that the rate of critical water application was dependent on the burning degree and the method of water application. He also reported that in general the critical water application rate for bottom application case was higher than that of the top application case. Moghtaderi et al. (1997) conducted a series of small-scale experiments to investigate the effect of water spray on re-ignition characteristics of wood and PMMA in a cone calorimeter. For extinction purposes, the authors used a set of water spray system consists of a full cone nozzle with an effective angle of 90 o. The distance between nozzle tip and the surface of solid fuel was set to 7 cm, and the range of water flow rates were maintained between kg.m -2.s -1. They stated that the higher the water flow rate, the shorter the time of extinguishment. Moreover, Novozhilov et al. (1997) carried out a series of extinguishment experiments (using sprinklers) on Radiata pine slats to validate their computational fluid dynamics fire suppression model. Prior to burning process, all samples with 10-0% moisture content were air-dried in 60 o C until their moisture content reduced to about 2%. Burning rate was measured by CO 2 analysis. Water application was initiated on the burning samples 0 s after ignition and the extinguishment achieved in about 2. min. Different water flow rates were investigated and it was shown that the extinguishment time was weakly influenced by the rate of water application. This paper gives information on the fire suppression performance of burning wood slabs using water spray and investigated the effectiveness of water application on the further ignition or re-ignition of the samples that was influenced by the evaporation time and recovery time. Other parameters: external heat flux and wood type (i.e. porosity) also discussed. Experimental The experimental apparatus used in this study was a modified version of the FTT mass loss cone calorimeter with the addition of a water spray system for extinguishing of test samples during re-ignition experiments, mounting of a thermocouple assembly on the sample holder for measuring the temperature distribution within the fuel sample, and several auxiliary components for protecting the load cell against water and collection of runoff water. Figure 1 shows the photograph of system which in particular demonstrates the set-up when an experiment was in progress and the shutter gate was in the open position. It must be pointed out that in our studies we were mainly concern with the spontaneous ignition of samples. Therefore the spark igniter was not used. Also the flux level was calibrated by using a heat flux meter (Schmidt-Boelter type) that was placed at the centre line and underneath the cone heater at a distance of 2 cm. One of the switch buttons in cone controller adjusts temperature level, and the measurement of heat flux level related with temperature adjustment was provided in NIST standard cone calorimeter. Figure 1. Photograph of the experimental set-up Extinguishment of test samples was achieved by the introduction of water droplets through the water spray system that consisted of a nozzle and a set of valves, piping and fittings. The photograph of the water spraying during operation is shows in Figure 2. Tap water was used in all experiments and sprayed on the samples through a 1/8 BLM 4-90 o -angle Delavan full cone nozzle with a round spray pattern, the flow rate of water was typically 10 ml.min -1. To facilitate the water application and to minimise the blockage of the radiant heat by the nozzle, the distance between the nozzle tip was kept at 11 cm. During each test the nozzle flow was held constant by maintaining a constant water pressure. No attempt was made to measure the drop size distribution. However, according to information supplied by the manufacturer, at an operating pressure of 300 kpa the volume median diameter of the drops was 80 µm. The experimental procedure consisted of the following steps. At first, the sample was instrumented with thermocouples and then placed in the sample holder assembly. The radiant heater was then set to the desired value while the shutter gate was closed. Upon reaching the target heat flux the sample was exposed to the external radiation by placing the sample holder under the radiant

4 heater and opening the shutter gate. Once a stable flame appeared at the surface of the samples, the water spray system was turned on in order to simulate the extinguishment phase of the process. At the same time measurement of the re-ignition time was started. The water spray was left on for a selected period of time (, 10 or 1 s), which was much longer than the time required for extinguishment. The water spray was then turned off and the sample was kept under the external radiation until it regained the second flame. The time delay corresponds to this was judged as the re-ignition delay time. The samples (Western Red cedar, Radiata pine, and River Red gum) were oven dried prior to each test in order to eliminate the effect of moisture content. The experimental procedures outlined above were systematically repeated for various combinations of radiant heat flux (between kw.m -2 ), water application periods (, 10 or 1 s) and pre-burn. Water Figure 2. The photograph of the water spraying during operation. A thermocouple on the surface of the sample was used for measurements of temperature distribution within the test samples. For this study, stainless steel, sheathed, ungrounded junction chromel-alumel thermocouples (0.003 inch out side diameter) were used. The type of thermocouple used for this purpose was K-type made for the temperature range of o C. Temperature histories were recorded throughout each experiment at 1 s intervals using a digital data acquisition system. Diagram of temperature history versus time was programmed by the LabVIEW- language, which is a graphical language with two basic concepts, namely block diagram and front panel. Results and Discussions A typical virgin wood sample is made of solid cells strongly bounded but with evident cavities (slits), which seem to continue along the grain direction. After being exposed to external heat and formation of the char layer, the natural porosity of the material allows the volatile release and no major morphological changes occur within the virgin layer. However, the cell structure practically does not exist within the char layer. The cross sectional image of the sample suggests that the sample have larger cavities with thinner cell walls in its char layer. The summary of the measured re-ignition times in triplicate tests for various samples against the heat flux levels between 40 and 60 kw.m -2 as a function of the water application time is presented in Table 1. The following observations can be made from the results summarised in the table: For each sample, as the heat flux is decreased the re-ignition time is increased. For samples of lower porosity the-re-ignition time was longer. The re-ignition time is increased as a result of the increase in the water application time. Type of wood Western red cedar % porosity Radiata pine % porosity River red Gum % porosity Water application Re-ignition 60 kw.m -2 0 kw.m kw.m Table 1. Summary of measured re-ignition times of wood samples.

5 Effect of the Water Application Time The effect of different water application times on the surface temperature profile is shown in Figure 3 for Radiata pine exposed to the heat flux level of 40 kw.m - 2. Clearly, for longer water application times the reignition delay time is longer than those correspond to the shorter application times. This is simply due to the fact that for longer application times a larger quantity of water will accumulate in the sample and on its top surface and, as such, more time is needed to evaporate the water. Similar behaviour was observed for samples of other wood species although their exact values of reignition delay time were different. result, takes longer time to evaporate. Similar behaviours were observed for other samples. The amount of water droplets has directly correlation with the mass of the sample. The mass of the sample increases as a result of water application, and after water droplets have completely evaporated the mass of the sample returns to its initial value since there is no mass loss due to chemical or thermal reactions. Therefore, the overall re-ignition time increases as the water application time is increased simply because there is more water to be evaporated. The results indicate that the average evaporation rate is a function of the external heat flux and sample thickness. Temperature, C First ignition Second ignition s of WAT s of WAT 1s of WAT Figure 3. Surface temperature versus time for Radiata Pine exposed to a heat flux level of 40 kw.m -2 at different time of water time application. The influence of water application time on the mass loss behaviour of Western Red cedar under the heat flux level of 40 kw.m -2 is illustrated in Figure 4. In this figure the weight of the sample has been tarred off. Thus, a zero mass refers to the initial weight of the sample. As Figure 4 shows, as soon as the heat-up phase begins and material starts to pyrolyze the mass of wood sample gradually decrease. After water application the sample starts to gain weight as a result of water applied. Once the water application is stopped and evaporation begins, weight loss is observed again. But in the greater part of this phase the material temperature is not high enough to cause pyrolysis. Therefore, the observed weight loss is primarily due to evaporation of water. However, once the water has completely evaporated and sample weight returns to a value similar to that prior to the application of water, then the material weight loss begins again. The points corresponding to this phenomenon have been marked on Figure 4. Clearly, the overall re-ignition time increases as the water application time is increased simply because there is more water to be evaporated. The higher the water application time, more water spreads over the surface and penetrate into the interior of the sample and, as a Mass, g s of WAT 10s of WAT 1s of WAT End of the evaporation and recovery time Figure 4. Mass loss of Western Red cedar exposed to a heat flux level of 40 kw.m -2. Effect of external heat flux Figure illustrates the result of the surface temperature history of a typical Radiata pine sample as a function of external heat flux. This particular graph corresponds to a 10 s water application time. As can be seen from Figure, the first spontaneous ignition can be identified by the first sudden jump in the temperature measurement. The temperature drop after the first ignition and the appearance of the flame, is essentially due to the cooling effect of the water droplets of which some penetrate into the sample, some form a thin layer on the sample surface, and some runoff from the edges. Once the application of water is stopped, evaporation of water begins from the top layer. This stage can be identified by a relatively low temperature plateau region on the graphs shown in Figure. In this stage, the incident energy due to the external heat flux is essentially consumed to evaporate the water rather than to increase the surface temperature. At the end of water layer evaporation stage more and more energy gets to the sample itself and as a result its temperature starts to raise allowing pyrolysis to proceed again. As soon as enough pyrolysis products (i.e. volatiles)

6 accumulate in the boundary layer surrounding the sample surface, the second ignition, which is referred to here as re-ignition, takes place. This usually corresponds to a surface temperature close to that just prior to the application of water (see Figure ). Again, the second ignition is identified by the sudden jump in the temperature measurement. The re-ignition time is considered as the interval between the time at which the water spray is applied and the time at which the second ignition is achieved. The influence of the heat flux on re-ignition time is quite significant. As illustrated, the re-ignition delay time decreases with the increase in the external heat flux. The re-ignition time for Radiata pine under the heat flux levels of 40, 0, and 60 kw.m -2 were 90 s, 74 s, and 6 s, respectively. It was also found that the higher the external heat flux, the lower the delay time corresponds to the first ignition time. Similar results were obtained for the other two wood species (Western red cedar, % initial porosity, and River red gum, % initial porosity). Temperature, o C First ignition Second ignition kw.m -2 0 kw.m kw.m -2 Figure. Surface temperature versus time for Radiata Pine exposed to different level of heat flux, with 10s of water time application. Effect of the Wood Type (i.e. Porosity) Figure 6 depicts the influence of wood types (implicitly the influence of wood sample porosity) on the re-ignition behaviour, the evaporation time, and the recovery time. These results were obtained at an external heat flux of 0 kw.m -2 and the water application time of 1 s. The values of the first ignition delay econd ignition delay time (i.e. re-ignition delay), evaporation time, and recovery time of the same types of wood extracted from Figure 4 are tabulated in Table 2. Temperature, Cedar Gum A Pine A A : Evaporation time B : Recovery time Cedar Pine Gum Figure 6. The influence of wood porosity on the reignition behaviour, evaporation time, and recovery time under 0 kw.m -2 of heat flux and 1s of water application. From Figure 6 and Table 2, it can be concluded that evaporation time for more porous wood sample was shorter than the evaporation time of less porous ones. Inversely, the more porous wood sample, took more time to recover. This is very much similar to what was observed also for surrogate samples. The phenomenon can be explained in terms of the characteristic of the porous structure. With the same water application time, the sample with a higher porosity level will absorb more water into its structure and, hence, a less quantity of water will stay on the sample surface. Therefore, less time is required to evaporate the water layer on the sample surface. On the other hand, it will require more time to evaporate water content in the sample structure. B B A B Wood type 1 st ignition 2 nd ignition Re-ignition Evaporation Recovery Western Red cedar Radiata pine River Red gum Table 2. First Ignition econd ignition time, evaporation time, and recovery time of wood samples subjected to a heat flux of 0 kw.m -2 and 1s water application.

7 Conclusions The results of experimental and theoretical investigations of the re-ignition characteristics of wood samples are presented in this chapter. Particularly, reignition characteristic of wood is greatly affected by wood porosity. The following conclusions can be made regarding the impact of material porosity and other parameters such as water application time and irradiance heat flux on the re-ignition phenomenon. Sample porosity significantly influenced the evaporation and recovery times. Evaporation time of water layer on the surface sample decreased with an increase of porosity. On the other hand, recovery time of water evaporation in the sample structure increased with sample porosity. This phenomenon agrees with the results obtained, and also agrees with the experimental result that has been done by Abu Zaid (1988). The re-ignition time of wood-based materials, decreases linearly with the increase of the external heat flux, and increases with the increase of water application time. References 1. Abu-Zaid, M.D., Effect of Water on Ignition of Cellulosic Materials, A Dissertation, Michigan State University, East Lansing, Michigan, Choi, K.J. and Yao, S.C., Mechanisms of film boiling heat transfer of normally impacting spray. International Journal Heat Mass Transfer, Vol. 30, No. 2, (1987). 3. González, J.E. and Black, W.Z., Study of Droplet Sprays Prior to Impact on a Heated Horizontal Surface. Journal of Heat Transfer, Vol. 119, (1997). 4. Grissom, W.M. and Wierum, F.A., Liquid Spray Cooling of A Heated Surface. Int. J. Heat Mass Transfer, 24, (1981).. Hietaniemi, J., Kallonen, R., and Mikkola, E., Burning Characteristics of Selected Substances: Influence of Suppression with Water. Fire and Material Journal, 23, (1999). 6. Hua, J., Kumar, K., Khoo, B.C., and Xue, H., A Numerical study of the interaction of water spray with a fire plume, Fire Safety Journal, 37, (2002). 7. Magee, R.S. and Reitz, R.D., Extinguishment of Radiation Augmented Plastic Fires by Water Sprays. Fifteenth International Symposium on Combustion, (197). 8. Moghtaderi, B., Novozhilov, V., and Kent, J.H., Effect of Water Spray on Re-Ignition Characteristics of Solid Fuels. Fire Safety Science- Proceedings of the Fifth International Symposium, p (1997). 9. Novozhilov, V., Harvie, D.J.E., and Kent, J.H., A Computational Fluid Dynamics Study of Wood Fire Extinguishment by Water Sprinkler. Fire Safety Journal, 29, (1997). 10. Poespowati, T., A Fundamental Study on Reignition Characteristics of Wood-Based Materials, PhD Thesis, Chemical Engineering Department, the University of Newcastle, Rasbash, D.J., Rogowski, Z.W., and Stark, G.W.V., Mechanisms of Extinction of Liquid Fires with Water Sprays. Combustion and Flame, 4, (1960). 12. Takahashi, S., Experiments and Theory in the Extinction of a Wood Crib. Fire Safety Science- Proceedings of the First International Symposium, (1986). 13. Tamanini, F., A Study of the Extinguishment of Vertical Wood Slabs in Self-sustained Burning by Water Spray Application. Combustion Science and technology, 14, 1-1, (1976). 14. Unoki, J., Fire Extinguishing Time by Sprinkler. Fire Safety Science-Proceeding of the First International Symposium, (198). 1. Yao, B., Fan, W., and Liao, G., Interaction of Water Mists With a Diffusion Flame in a Confined Space, Fire Safety Journal, 33, (1999).