Experimental and scale up study of the flame spread over the PMMA sheets

THERMAL SCIENCE: Vol. 13 (2009), No. 1, pp. 79-88 79 EXPERIMENTAL AND SCALE UP STUDY OF THE FLAME SPREAD OVER THE PMMA SHEETS by Mojtaba MAMOURIAN, Javad A. ESFAHANI, and Mohammad B. AYANI Original scientific paper UDC: 662.612.5:66.011 BIBLID: 0354-9836, 13 (2009), 1, 79-88 DOI: 10.2298/TSCI0901079M To explore the flame spread mechanisms over the solid fuel sheets, downward flame spread over vertical polymethylmethacrylate sheets with thicknesses from 1.75 to 5.75 mm have been examined in the quiescent environment. The dependence of the flame spread rate on the thickness of sheets is obtained by one-dimensional heat transfer model. An equation for the flame spread rate based on the thermal properties and the thickness of the sheet by scale up method is derived from this model. During combustion, temperature within the gas and solid phases is measured by a fine thermocouple. The pyrolysis temperature, the length of the pyrolysis zone, the length of the preheating zone, and the flame temperature are determined from the experimental data. Mathematical analysis has yielded realistic results. This model provides a useful formula to predict the rate of flame spread over any thin solid fuel. Key words: flame spread, pilot ignition, polymethylmethacrylate, scale up, solid fuel Introduction Polymers are used in nearly every commercial buildings, residential house, transportation vehicle, etc. Thus the majority of polymer containing end products (cables, carpets, furniture, …) must pass some type of regulatory test to help assure public safety from fire. To minimize their hazards, the burning behaviors and combustion mechanism should be understood. Polymethylmethacrylate (PMMA) is a transparent material and has excellent corrosion resistance. These advantages make it so popular and widely used in building, industry, and the general consumer products market [1]. Therefore, attention is restricted to PMMA, whose properties are simpler and better understood than those of most other polymeric materials. Flame spread over the surface of polymeric material is one of the problems in fire researching. Many mathematical and experimental models have been constructed to describe the process of flame spread over a solid fuel. The controlling mechanism of flame spread appears to differ with the surrounding conditions, such as the oxygen concentration [2], or the direction of the gas flow velocity relative to the direction of the flame spread [3, 4]. The flame spread rate depends on the rate of heat transfer from the flame into the preheat region (unburned fuel). The estimation of the heat transfer not only through gas phase but also through solid phase is important for further understanding. Gas phase conductive/convective heat transfer from flame to the solid fuel is the dominate path for downward flame spread [1, 5]. 80 Mamourian, M., Esfahani, J. A., Ayani, M. B.: Experimental and Scale up Study of the ... In order to estimate the rate of heat transfer, one needs to know the detail temperature profiles in the gas and solid phases. Esfahani et al. [6], and Esfahani [7] determined the history of temperature in the solid and gas phases of the PMMA sample by a numerical model. Fernandes-Pello et al. [8, 9], Hirano et al. [10], and Krishnamurthy et al. [11] measured the histories of surface and interior temperature of PMMA for horizontal flame spread by using of thermocouples. In the present work, the relation between the flame spread rate and the thicknesses over the thin solid sheets is studied by order of magnitude analysis and investigates the effects of the type of the heat flux on the flame spread rates. The temperature histories were obtained from chart recording of the fine thermocouple output. The lengths of the pyrolysis zone and the preheating zone were extracted from the temperature histories. Analytical results show a good agreement with the experimental results. Physical model A sample of thin solid sheet fuel (PMMA) is burned with a slit burner from its top surface and is held in the quiescent environment at a fixed temperature T4. The sample is assumed to be very large in width and length so that, a one-dimensional model is appropriate for spreading behavior. The lengths of the sample are considered without expansion during combustion. The schematic of the physical problem is shown in fig. 1. The reaction zone can be divided into three major parts: the initial (preheated zone), thermal decomposition, and combustion zone. In the initial zone, preheating occurs mainly due to absorption of thermal energy and energy transferred through this region by conduction. The thermal decomposition zone, where the rapid thermal de- composition occurs, is due to the convection heat flux from combustion product to the sample. The diffusion flame is formed over this zone which is called combustion zone. For the flame propFigure 1. The schematic view of the combustion agation, the most important processes take process in the solid sheet place in the thermal decomposition zone. The solid fuel situated ahead of the flame edge is heated from the ambient temperature to the pyrolysis temperature, Tp. When the temperature of the sample rises, bubbles form and the pyrolysis occurs. The pyrolysis temperature of most polymers is between 180 and 400 ºC [4]. When the temperature of the layer exceeds to a pyrolysis temperature, the intensity of gasification is enough to form a diffusion flame. The combustion process occurs as long as the gaseous volatiles are intensively delivered into the reaction zone. Experimental setup A schematic of experimental apparatus is shown in fig. 2, according to ASTM 1356. Experiments were carried out under the normal atmospheric conditions, T4 = 300 K, P4 = 90 kPa. Sample PMMA sheets are made from various thicknesses of 1.75 to 5.75 mm. The sheets are made by Acrylic Enterprise Co., Ltd. in Taiwan. The dimensions of the sample sheets are 150 mm high and 40 mm wide, set up vertically and ignites at the top edge by a pilot flame. A 25 mm wire diameter THERMAL SCIENCE: Vol. 13 (2009), No. 1, pp. 79-88 81 chromel-alumel thermocouple was used to measure the history of the temperature in solid and gas phases. The thermocouple is pressed into a hole which is drilled in the middle of the sample, about 40 mm under the top edge. At every 1 s, the recorded data by thermocouple is entered to a computer. For each thickness, the test is repeated 3 times to minimize experimental errors. Experimental results Figure 2. A schematic of the apparatus The temperature distribution for various thicknesses of the sample is shown in fig. 3. It shows that the solid temperature increases gradually (A-B), then it sharply increases (B-C) and reaches to a peak point (C), the pyrolysis temperature about 390 ºC, then decreases slightly (C-D) and after (D), it jumps rapidly to a higher level (E). The release volatile of flammable gases moves to the outer atmosphere and mixes with air and abs (...truncated)