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Prediction of Upward Flame Spread Over Polymers

Isaac T. Leventon

Sponsor: 
Federal Aviation Administration

 

With their expanding use in a variety of applications across a wide range of environments, especially with the continuing drive towards lighter and cheaper materials, synthetic polymers pose an increasing fire safety risk. Unfortunately, despite these mounting challenges, widely practiced test standards, which are used to indicate some level of fire resistance, are often severely limited in their ability to predict material behavior outside of test conditions and, to further complicate matters, conflicting assessments often arise between different tests. One aspect of flammability that is of particular interest is upward, concurrent flame spread over a material’s surface – a process that is almost always present in the early stages of a fire and that is a key determinant of its initial growth.

Here, the existing understanding of flame spread dynamics is enhanced through an extensive study of the heat transfer from flames spreading vertically upwards across 5 cm wide, 20 cm tall samples of Poly(methyl methacrylate) (PMMA). These experiments have provided highly spatially resolved measurements of flame to surface heat flux and material burning rate at the critical length scale of interest, with a level of accuracy and detail unmatched by previous empirical or computational studies. Using these measurements, an analytical expression was developed that describes a flame’s heat feedback profile (both in the continuous flame region and in the thermal plume above) solely as a function of material burning rate [1]. In a recent work, the flame model has been generalized through extensive testing of seven common materials – five pure polymers and two composites – which present distinctly different burning behaviors including melting, dripping, charring, and heavy soot formation (Fig. 1) [2].

 

Figure 1. Representative behavior of flames supported by seven common commodity plastics:
(from left to right) PMMA, POM, PP, HIPS, ABS, PBT, FRP.

 

This wall flame model has been coupled with a powerful solid phase pyrolysis solver, ThermaKin2D, which computes the transient rate of gaseous fuel production of constituents of a pyrolyzing solid in response to an external heat flux, based on fundamental physical and chemical properties [3]. ThermaKin has been successfully applied to the simulation of the combustion of non-charring [4] as well as charring polymers [5] in a cone calorimetry-type scenario. In both cases, the model was parameterized using mg-scale and g-scale property measurement techniques.

Together, these works capture the two fundamental controlling mechanisms of upward flame spread – gas phase flame heat transfer and solid phase material degradation.  The new unified model was employed to predict vertical burning and upward flame spread on 4 and 17.5 cm tall samples of PMMA with a reasonable computational cost and accuracy beyond that of current models.  As seen in Video 1, model predictions – including time to ignition and initial, peak, and rate of rise of sample mass loss rate – were found to closely match experimental results [6].

This work demonstrates the potential of using small scale measurements for assessment of flame spread dynamics through modeling and provides a framework for quantitative prediction of material behavior in response to a wide range of fire-like scenarios – two extremely useful capabilities for development of new, flame resistant materials. Ongoing research is directed towards expanding the application of this flame spread model to predict material behavior in the UL-94 and Room Corner Tests and understanding the mechanisms by which flame retardants affect polymer flammability. Future work is aimed at quantifying processes that significantly impact material burning behavior such as polymer melt flow and soot deposition (Fig. 2).

 

Video 1. ThermaKin2D simulation of upward flame spread. Predicted in-depth temperature (left) and flame heat flux (center) profiles are plotted alongside experimental and model-predicted material burning rate (right) as a 0.175m tall and 5 × 10-3 m thick solid material supports upward flame spread. The material is ignited at its base by a small propane burner, which is applied for 125 s.

 

Figure 2. Auto-suppression of upward flame spread over HIPS by soot deposition. Times shown here indicate time after sample ignition.

 

  1. Leventon, I.T. and Stoliarov, S.I., Evolution of Flame to Surface Heat Flux During Upward Flame Spread on Poly(methyl methacrylate). Proceedings of the Combustion Institute 34: 2523-2530. doi: 10.1016/j.proci.2012.06.051. (2013).
  2. Korver, K.T. A Generalized Model for Wall Flame Heat Flux During Upward Flame Spread on Polymers. Department of Fire Protection Engineering, University of Marylan, College Park. Masters Thesis 2015.
  3. Stoliarov, S. I., Leventon, I. T. and Lyon, R. E., Two-dimensional Model of Burning for Pyrolyzable Solids. Fire and Materials 38: 391-408. doi: 10.1002/fam.2187 (2013).
  4. Stoliarov, S. I., Crowley, S., Lyon, R. E. and Linteris, G. T., Prediction of the Burning Rates of Non-Charring Polymers. Combustion and Flame 156: 1068-1083 (2009).
  5. Stoliarov, S. I., Crowley, S., Walters, R. N. and Lyon, R. E., Prediction of the Burning Rates of Charring Polymers. Combustion and Flame 157: 2024-2034 (2010).
  6. Leventon, I. T. and Stoliarov, S. I., A Flame Spread Simulation Based on a Comprehensive Solid Pyrolysis Model Coupled with a Detailed Empirical Flame Structure Representation. Combustion and Flame doi:10.1016/j.combustflame.2015.07.025 (2015).

A detailed description of ThermaKin2D – the mathematics of the model, a series of verification exercises, a demonstration of some of its capabilities, and reference input and output files – is provided in a Federal Aviation Administration technote, which can be accessed at: www.fire.tc.faa.gov/pdf/TC12-59.pdf. Although we cannot provide technical support for the program, ThermaKin2D is available upon request. A Graphical User Interface (GUI) is currently in development for ThermaKin2D. An overview of this GUI and its capabilities is provided in the project poster at the top of this page.

Isaac Leventon

Isaac Leventon is a doctoral candidate of the A. James Clark School of Engineering. In December 2015, he will begin a NIST/NRC Postdoctoral Research Apprenticeship with the Flammability Reduction group at the National Institute of Standards and Technology (NIST). His current research is in the area of upward flame spread over polymeric materials, focusing on the coupled feedback system between flame to surface heat transfer and material burning rate.  He also maintains a strong interest in developing student involvement and interest in science and engineering, mentoring and teaching at both the high school and university levels. He is the current leader of the new pre-college program, An Introduction to Math and Physics through Fire Dynamics.

Student Phone: 
301-910-6062