Experimental study and heat transfer analysis of downward flame spread over PMMA under the effect of wall spacing
01 Oct 2019-Journal of Thermal Analysis and Calorimetry (Springer International Publishing)-Vol. 138, Iss: 2, pp 1711-1722
TL;DR: In this paper, the effects of wall spacing on the downward flame spread over polymethyl methacrylate (PMMA) were studied, and the authors found that the average flame height decreases with the increase in distances.
Abstract: To study the effects of spacing on the downward flame spread over polymethyl methacrylate (PMMA), an experiment was conducted by pure PMMA (with 200 mm height, 50 mm width, and 1 mm thickness) with spacings of 7 mm, 10 mm, 13 mm, 16 mm, 19 mm, 22 mm, and 25 mm to observe the flame height, pyrolysis spread rate of fuels, and heat feedback from the wall. The heat feedback received by PMMA was used to analyze the influencing mechanism of wall spacing on flame spread. The results are as follows: (1) The average flame height decreases with the increase in distances (
$$\delta$$
). This decrease in average flame height cycles through two stages: a fast drop stage and a slow drop stage. (2) The average pyrolysis spread rate first increases with the increase in distance, and a maximum pyrolysis spread rate occurred in the 13 mm spacing scenario. Then, the average pyrolysis spread rate decreases monotonously when the distance between wall and sample exceeds 13 mm. (3) The heat flux received by the sample consists of both heat flux from the flame and heat feedback from the wall. With the increase in distance, the heat feedback from the wall follows a downward trend, while the heat flux from the flame first increases and then remains constant. Because of the effects of heat flux from flame and heat feedback from the wall, the heat flux received by the sample first increases and then decreases with the increase in distance.
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Abstract: Experimental study on flame spread and heat transfer characteristics of discrete polymethyl methacrylate (PMMA) plate array with different spacings (L) was conducted in this work. For larger L, flame height in the middle of PMMA plate is larger than those on the sides. Flame merging phenomenon is observed when 6 mm ≤ L ≤ 10 mm. A heat transfer model for upward flame spread over PMMA plate array is established. The model predicts that flame spread rate drops as the spacing rises, consistent with experimental results. And through the flame spread rate prediction equation, it is found that the predicted value is very consistent with the experimental value, and the error does not exceed 10%. The experimental results also indicate that influence of spacing on flame spread rate is slight for 8 mm ≤ L ≤ 10 mm, while the influence is significant for other ranges of L. The experimental results indicate that average flame height first rises and then drops with the increase in the spacing. A mathematical model is built to predict the average flame height. The changing trend of predicted results versus spacing agrees with the experimental results. It is found from the temperature field that smaller spacing corresponds to larger high-temperature zone and pyrolysis area. By observing the surface temperature of PMMA and calculating the theoretical value of radiative heat flux on the surface of preheating zone, it is found that the view factor decreases and radiative heat flux decreases with the increase in spacing, which is consistent with the variation trend of flame spread rate.
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Abstract: As thermoplastic materials are widely used in buildings, the fire hazards of thermoplastic materials are increasingly becoming a central issue in fire safety research due to their unique pyrolysis and melting mechanisms. In this paper, the features and common types of thermoplastic materials are introduced first. Then, the combustion behavior of thermoplastic materials is theoretically analyzed based on the empirical formulas and heat balance equations, such as the pyrolysis kinetics, ignition time, melting and dripping, flame, burning rate and mass loss rate, temperature and heat flow, gas products, and influencing factors. The influencing factors basically include the sample properties (width, incline angle, and thickness, etc.), the façade structure (sidewalls, curtain wall, etc.), the ambient conditions (altitude, pressure, and gravity, etc.), and the flame retardant treatment. Similarly, this study also illustrates the vertical and horizontal flame spread behavior of the thermoplastic materials and the influencing factors. The utilized methods include the experimental methods, the analytical methodologies, and the approaches for numerical simulation. Finally, the problems encountered at this stage and worthy of further study in the future are presented.
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