vertical flame spread experiments and modeling

Corner fires are more hazardous than open wall fires due to the restricted geometry. The geometrical constraints cause corner flames to have longer flame lengths and more hazardous due to increased radiative feedback between the two adjacent walls. Thus, corner fires on combustible material serve as a severe test for the materials before they are employed in commercial applications. Developing prediction capability of fire hazard in such scenarios is important to assist material development, screening, and testing.

Propane flame used to initiate flame spread over PMMA.

Poly(Methyl Methacrylate) (PMMA) is probably the most widely used polymer in fire research. It’s application in commercial industry is enormous as it behaves as a relatively cheap, transparent and lighter substitute for glass. But due to it’s combustible nature, it has been utilized to study fire spread scenarios such as mass burning rate, incident flame heat flux in small test setups which replicate laminar flames.

PMMA exhibits thermal degradation that is non-charring, non-swelling, and primarily emanates gases having chemical composition of it’s monomer. This simplifies the pyrolysis model development issues and several research groups have developed pyrolysis model for PMMA thermal degradation. This advantage of having a resolved condensed phase decomposition scheme, PMMA is an ideal material for understanding the fundamental dynamics of flame spread in corner scenarios.

This research project is focused on studying flame spread in corner fire over PMMA so as to experimentally measure crucial parameters that primarily govern the flame spread phenomenon, namely heat release rate (HRR) via oxygen consumption calorimetry, well-resolved flame heat fluxes during the flame spread process, and emissions at 900 nm narrowband to observe emissions from soot (a primary emitter at this wavelength). The wealth of collected data can be used for validation of sub-models used in Computational Fluid Dynamics (CFD) models. The objective also involves developing an empirical flame feedback model and developing a semi-empirical flame spread model to predict HRR evolution during the flame spread process.

Check out more information in the full publication (Chaudhari et al., 2021).

A new experimental setup was developed to study turbulent, buoyancy-driven flame spread on a corner wall. Two 146 \texttimes 50 \texttimes 0.58 cm panels of cast black poly(methyl methacrylate) (PMMA), whose pyrolysis properties were fully characterized in a separate study, were ignited using a triangular propane burner. The corner wall assembly was placed into a well-ventilated enclosure and heat release rate (HRR) and flame heat flux to the corner wall were measured in 7 repeated experiments. The HRR measurement was designed to achieve a fast response time (13 s) and was correlated with the flame heat flux, which was obtained using water-cooled heat flux gauges positioned in 28 locations distributed over the PMMA surface. Simultaneously, a monochromatic, 900 nm, video of the spreading flame was recorded and analyzed to determine the evolution of soot radiation intensity and flame geometry. Using collected data, an empirical model relating spatially resolved heat feedback from the flame to the solid fuel surface and HRR was developed and integrated with a detailed pyrolysis model. Simulations were conducted in an uncoupled and coupled mode. The uncoupled simulations, where the flame heat feedback was effectively prescribed to match the experimental measurements, revealed the importance of the knowledge of the split between the convective and radiative portions of the flame heat flux. The coupled simulations, where the flame heat feedback was computed from the simulated HRR, revealed that the coupling amplifies even relatively small uncertainties in the model parameters to produce large errors in the HRR predictions.

References

2021