Smoldering Propagation Characteristics of Flexible Polyurethane Foam under Different Air Flow Rates Zhou Y., Fei Y.*, Hu S. Q. Nanjing Tech University, College of Fire Safety Science and Engineering & Institute of Fire Science and Engineering, Nanjing, Jiangsu, China *Corresponding author email: yfei@njtech.edu.cn ABSTRACT The smoldering characteristics of flexible polyurethane foam (FPU), including smoldering spread, smoldering temperature, and high temperature duration, were studied and analyzed under four different air flow rates, namely 2, 3, 5 and 7 L/min, with mesoscale smoldering simulation device to investigate the smoldering combustion performance of the material. Air was forced in the direction of smolder propagation under conditions that produce approximately one-dimensional forward smolder propagation. Results show that there was an approximately linear relationship between the average smoldering rate and airflow rates. The smoldering velocity along the foam remains almost constant during the propagation process. When air flow rate varies from 2 to 5 L/min, smoldering temperature of FPU increases with the increase of air flow rate. The maximum temperature is 400 under air rate of 5 L/min, a peak temperature above 400 lies in 5-7 L/min. The duration of high temperature in intermediate location is higher than those at two end locations, FPU is consumed more and strongly at both ends. KEYWORDS: Flexible polyurethane foam, smoldering, mesoscale smoldering simulation device, smolder propagation. INTRODUCTION Smoldering combustion is a non-flaming, self-sustaining, exothermic, propagating, surface heterogeneous reaction in the porous combustible materials [1]. One-dimensional smoldering has two distinctive modes, forward and opposed smoldering. The forward smoldering is defined as the oxidizer flow moves in the same direction as the reaction zone, while the opposed smoldering moves in the opposite direction. Examples of materials that can induce smoldering combustion are cotton, cigarettes, wood chips, dusts, polyurethane foams and coal. Smolder produces a large amount of toxic gases and leads to flaming fire which makes great damage. Smoldering combustion contains complex physical and chemical processes. Domestic and foreign scholars have done a great amount of experimental simulations to explore the propagation of smoldering combustion. Torero [2] found that as flow velocity was increased, there was a transition from a single exothermic oxidation (smolder) reaction to an oxidizing smolder reaction preceded by an endothermic pyrolysis reaction. Lei Peng [3] conducted experimental and theoretical studies on smoldering propagation of the flexible polyurethane foam with controlled air supply to show that increasing air supply both enhanced the oxidation reaction and increased the heat loss. Leach [4] present a one-dimensional transient model of forward smoldering to explain that at low gas velocities the smolder process was oxygen limited and oxidation frequency factor had little effects on smolder velocity. However, at higher inlet gas velocities, the smolder process becomes kinetically limited and the smolder velocity is therefore highly dependent on the fuel oxidizing frequency factor. Zhou et al. [5] have measured smoldering wave temperature, taken two-step smoldering chemical reaction model, established one-dimensional smoldering diffusion and propagation integral model Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8), pp. 45-51 Edited by Chao J., Liu N. A., Molkov V., Sunderland P., Tamanini F. and Torero J. Published by USTC Press ISBN:978-7-312-04104-4 DOI:10.20285/c.sklfs.8thISFEH.005 45
Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) and set up the mathematical expression of stable transmission speed under conditions of natural and forced convection. Torero and Fernandez-Pello [2] used air an oxidizer and forced it in the direction of a forward smolder propagation of polyurethane foam, and calculated the smolder propagation velocity as a function of the air flow velocity. Tse et al. [6] proved that char was not totally consumed by the flowing oxidizer and had a different reaction level from that of the virgin foam in air supply or temperature requirements for its oxidation. Previous literature show that air supply rate plays a crucial role in the smoldering propagation. In this paper, smoldering combustion of polyurethane foam with controlled air supply rates is examined to further understand the characteristics of smoldering propagation including smoldering rates, smoldering temperatures and duration of high temperature. EXPERIMENTAL SETUP The mesoscale experiment configuration whose interior dimensions are 0.60 m 0.25 m 0.25 m was shown in Fig. 1. Temperature Collecting System (thermocouple tree) Smoldering Generator Gas Analyzer Data Acquisition and Processing System Weighing System Gas Inflow Control System Figure 1. Mesoscale smoldering simulation device. The smolder experiments were conducted in a horizontal stainless steel container. The porous material used in the experiments was open-cell and non-flame-retardant flexible polyurethane foam with a density of 30 kg/m 3.The cross section area and length of the foam sample was 0.25 m 0.25 m and 0.60 m. Insulation material was filled tightly between the container walls and the sample to minimize heat losses and prevent preferential flow of gas through edges of the sample. An electrically heater was used as an ignitor and placed at one side of the sample to heat up the foam and initiate the combustion. The ignitor was connected to an AC transformer whose voltage can be adjusted. The ignition power was 8 kw and applied for 350. The temperatures of the foam during smoldering processes were measured by three rows of thermocouples (stainless steel sheath) which were shown in Fig. 2 and the distance between two adjacent thermocouples was 10 cm. In this paper, it s assumed that propagation of the smolder is one-dimensional. We chose the thermocouples which were embedded along the center line of the foam to analyze. The gas flow rate was measured and controlled with a flow meter. The thermocouples were linked to A/D convertor and recorded into a computer. 46
Part II Fire A B C D E A1 B1 C1 D1 E1 A2 B2 C2 D2 E2 Polyurethane foam A3 B3 C3 D3 E3 Figure 2. Reactor and thermocouple arrangement scheme. EXPERIMENTAL RESULTS Analyses of the temperature profiles along the foam sample and under different air flow rates show that the characteristics of the smolder reaction vary depending on the magnitude of the air flow. Experiments of forward smolder propagation were conducted in forced air flow under four different flow rates, namely, 2, 3, 5 and 7 L/min. Temperature profiles were recorded to characterize the reaction zone. The influence of air flow rate on smoldering propagation rate Based on these temperature profiles, the average smoldering spread V was determined from the time which elapsed between the charring front that could pass through the two thermocouples (A and E) with an interval of 44 cm. The charring front was adjudged to have passed when bulk temperature of foam T reached 200 since it was confirmed to be constant [7]. Table 1. The average smoldering rate of FPUF under different airflow rates. Density (kg/m 3 ) Ignition power (kw) Airflow rate (L/min) Smoldering rate (mm/s) 30 8 2 0.038 30 8 3 0.043 30 8 5 0.057 30 8 7 0.076 Fig. 3 shows the average smoldering propagation velocity along the foam sample in several air fluxes. As can be seen, the average smoldering rate increased rapidly with the increase of air flow rate in almost a linear relationship. The results agree very well with those of Torero and Fernandez-Pello [2]. The smolder reaction was determined by the temperature value and was due to a balance between the heat generated by the exothermic oxidation of the fuel and the transfer of heat to the foam ahead of the reaction. The increase of airflow rate will bring enough oxygen and promote the oxidation of bubbled exothermic reaction. The heat of exothermic oxidation is more than that of transfer to keep 47
Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) propagation. What s more, airflow rate increase will also take more pyrolysis volatiles including a large number of water vapor, carbon dioxide and other products of combustion away, therefore oxygen can spread to the burning surface more easily. As a result, smoldering propagation will speed up. 0.08 The average smoldering rate (mm/s) 0.07 0.06 0.05 0.04 2 3 4 5 6 7 Airflow rate (L/min) Airflow rate (L/min) Figure 3. The average smoldering rate of FPUF under different airflow rates. The smoldering rate at different location was determined from the time which elapsed between the charring front that passed through the two adjacent thermocouples (A and B, B and C, C and D, D and E) with an interval of 11 cm. The charring front was judged when T reached 200 as mentioned above. Fig. 4 shows the smoldering propagation velocity along the foam sample in several oxygen fluxes at different locations of FPUF. The smolder velocity along the foam remains almost constant during the smoldering process. The experimental measurements results agree very well with the computational ones of Lei Peng [3]. 0.10 The The smoldering smoldering rate(mm/s) rate (mm/s) 0.09 0.08 0.07 0.06 0.05 0.04 2 3 5 7 2L/min 3L/min 5L/min 7L/min 0.03 A-B B-C C-D D-E Lacation Location Figure 4. The smoldering rate of FPUF at different locations. 48
Part II Fire The influence of air flow rate on highest temperature The smoldering temperature at different positions in the various airflow rates are shown in Fig. 5. This smoldering temperature was determined from the highest temperature of the central thermocouple (A2- E2). As can be seen from Fig. 5, when the airflow rates range from 2 to 5 L/min, the smoldering temperatures show the same variation trend, that is to say, they decrease quickly and then increase slowly. A reason for this change is due to both side of the foam have more oxidation, leading to more intense oxidation reaction and more heat release. Another reason is the smoke generated in front of the foam concentrating on the middle, meanwhile the smoke produced in the tail exhausting easily, which leads to a lower oxygen concentration in the middle area and heat release of oxidation reaction. But when flow rate increases to 7 L/min, the smoldering temperatures at different locations show obviously different trend with others. Overall, the smoldering temperature at this rate are less than those at other rates. This is because when the airflow increases further the cooling effect of the gas also gradually strengthens and takes away a lot of heat. When airflow rate was 7 L/min, smoldering temperature became higher along smoldering propagation direction and finally a flaming occurred. 420 Smoldering temperature ( ) 400 380 360 340 2 2L/min 3 3L/min 5 5L/min 7 7L/min 320 A B C D E The location of the thermocouples Figure 5. The smoldering temperature at different locations. Fig. 6 shows the average smoldering temperature under different flow rates. As can be seen, when the airflow rates range from 2 to 5 L/min, the average smoldering temperature keeps rising and a maximum temperature of 400 occurs at 5 L/min. At 7 L/min, the temperature decreased further; therefore, the peak temperature appears to be between 5 and 7 L/min. The influence of air flow rate on high temperature duration Fig. 7 shows the influence of flow rate on smoldering high temperature duration. Here high temperature duration is defined as the duration of the smoldering temperature higher than 300 at different locations. To some extent, high temperature duration is a good indicator to learn smoldering degree of reaction materials. Choosing 300 is due to the maximum mass loss rate of polyurethane foam in air atmosphere corresponding to the temperature at 300 [8], and the polyurethane foam shows most violent reaction near the temperature. 49
Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) The average smoldering temperature ( ) 400 390 380 370 360 350 2 3 4 5 6 7 Airflow rate (L/min) Figure 6. The average smoldering temperature under different flow rates. High temperature duration (s) 4000 3500 3000 2500 2000 1500 1000 500 2 L/min 3 L/min 5 L/min 7 L/min 0 A B C D E The location of the thermocouples Figure 7. The smoldering high temperature duration at different locations. Combined with Figs. 4 and 7, it can be seen that general smoldering temperature inside the reaction container is low and high temperature lasts longer, while on both ends of the temperature is high and the temperature duration is short. On the one hand, on both ends of the pyrolysis gases are much easier to exchange with external environment, oxygen concentration is relatively high and smoldering combustion can be proceed more fully, so it can reach higher temperature. On the other hand, at both ends heat can transfer outside faster, so it is more difficult to maintain high temperature for a long time. Smoke concentrates in the middle of the container, which results in low oxygen concentrations and smoldering temperatures. While the heat is hard to lose heat to the surrounding by convection and radiation. So high temperature duration is longer. This is the reason why smoldering is hard to detect and usually has a long incubation period Fig. 8 shows the photograph of lowest layer material after smoldering. As shown in Fig. 8, due to buoyancy effect, the heat flow will hardly flow through the lowest layer of polyurethane foam, oxygen is thus difficult to reach the bottom, the bottom layer of polyurethane foam is generally not involved in smoldering and most of material remains relatively complete structure. Also 50
Part II Fire known from Fig. 3, the duration of high temperature maintains for a long duration at A, C, E positions and a relatively short duration at B and D position. Combined with Figs. 7 and 8, it can be seen that the duration of high temperature is kept longer and the consumption of FPU is more in intermediate part of smoldering container. Figure 8. The photograph of lowest layer material after smoldering. CONCLUSION 1. When gas velocity was varied from 2 to 7 L/min, there was an approximately linear relationship between the average smoldering rate and airflow rates. 2. When inlet gas velocity was at 2, 3 and 5 L/min, the smoldering temperature shows the same variation trend in which temperature decreases quickly and then increase slowly at different locations. The peak temperature should appear between 5 and 7 L/min and higher than 400. 3. The duration of high temperature in intermediate location is higher than those at two end locations, and FPU is consumed more and strongly at both ends. REFERENCES 1. Ohlemiller, T. J. Modeling of Smoldering Combustion Propagation, Progress in Energy and Combustion Science, 11(4): 277-310, 1985. 2. Torero, J. L., and Fernandez-Pello, A. C. Forward Smolder of Polyurethane Foam in a Forced Air Flow, Combustion and Flame, 106(1-2): 89-109, 1996. 3. Peng, L., and Lu, C. Smoldering Combustion of Horizontally Oriented Polyurethane Foam with Controlled Air Supply, In: Gottuk, D. T. and Lattimer, B. Y. (Eds.), Fire Safety Science-Proceedings of the Eighth International Symposium, pp. 693-704, 2005. 4. Leach, S. V., and Reiin, G. Kinetic and Fuel Property Effects on Forward Smoldering Combustion, Combustion and Flame, 120(3): 346-358, 2000. 5. Peng, L., Zou, Y. H., and Zhou, J. J. A Model of Transition from Smoldering to Flaming with Natural Diffusion, Fire Safety Science, 13(1): 28-34, 2004. 6. Tse, S. D., Fernandez-Pello, A. C., and Miyasaka, K. Proceedings of the Combustion Institute, 26: 1505-1513, 1996. 7. Suzuki, T., and Sucahyo, B. Polyurethane Foam Smoldering Supported by External Heating, In: Kashiwagi, T. (Ed.), Fire Safety Science Proceedings of the Fourth International Symposium, pp. 397-408, 1994. 8. Mostashari, S. M., Zanjanchi, M. A., and Baghi, O. Burning of a Cotton Fabric Impregnated by Synthetic Zinc Carbonate Hydroxide as a Flame Retardant, Combustion, Explosion, and Shock Waves, 41(4): 426-429, 2005. 51