Flammability characteristics at applied heat flux levels up to 200 kW/m²

Abstract

SUMMARYThis paper documents the first of the two interrelated studies that were conducted to more fundamentally understand the scalability of flame heat flux, the motivation being that it has been reported that flame heat flux back to the burning surface in bench-scale experiments is not the same as for large-scale fires. The key aspect was the use of real scale applied heat flux up to 200 kW/m 2 which is well beyond that typically considered in contemporary testing. The main conclusions are that decomposition kinetics needs to be included in the study of ignition and the energy balance for steady burning is too simplistic to represent the physics occurring.An unexpected non-linear trend is observed in the typical plotting methods currently used in fire protection engineering for ignition and mass loss flux data for several materials tested and this nonlinearity is a true material response. Using measured temperature profiles in the condensed phase shows that viewing ignition as an inert material process is inaccurate at predicting the surface temperature at higher heat fluxes. The steady burning temperature profiles appear to be invariant with applied heat flux. This possible inaccuracy was investigated by obtaining the heat of gasification via the 'typical technique' using the mass loss flux data and comparing it to the commonly considered 'fundamental' value obtained from differential scanning calorimetry measurements. This comparison suggests that the 'typical technique' energy balance is too simplified to represent the physics occurring for any range of applied heat flux. Observed bubbling and melting phenomena provide a possible direction of study.

The effects of gas-phase and in-depth radiation absorption on ignition and steady burning rate of PMMA

“…Figure 8 shows results from the model with the parameter set given in Table 1 with the experimental data of Beaulieu [2], [3], Kashiwagi [1] and the coated PMMA data of Jiang et al [4]. The obvious point to note is the model confirms that in-depth absorption plays an important role in ignition at high heat flux.…”

“…Beaulieu [2], [3], presented empirical ignition results that are representative of a broader super-set discussed by Bal & Rein [5]. Beaulieu's results were obtained using 25 mm thick specimens in the AFM apparatus where an infra-red lamp is used as the heat source.…”

“…Beaulieu [2], [3], was unable to estimate attenuation for a pyrolysing PMMA sample but instead used a propylene burner with an embedded heat flux gauge. She first measured the heat flux from the propylene flame only and then varied the output from the apparatus' external source and noted the new heat flux readings.…”

“…Kashiwagi's observations of ignition of PMMA by CO 2 laser [1] show that there is significant absorption of radiation by gaseous MMA in the plume above the degrading sample and the role of gas-phase absorption in ignition and steady burning rate is unclear. Measurements of quasi-steady mass flux (the approximately steady mass loss rate per unit surface area due to out-gassing of volatiles for an initially thick sample, also referred to as "burning rate") as external heat flux is varied indicate a nonlinear trend at high heat flux [2] - [4]. This is unexpected from simple ablation theory, which predicts that steady mass flux should be proportional to the ratio of net heat flux at the exposed surface to heat of gasification [7] - [11].…”

Evaluation of FDS V.4: Upward Flame Spread

“…Figure 10 represents the inverse of the mass loss rate vs. applied heat flux for PMMA. The mass loss flux data from Lee's experiment and model is plotted with the values from Beaulieu [33], Tewarson [10], and Hopkins and Quintiere [34]. As can be seen in Figure 9 and Figure The thermal properties for calcium silicate and steel (metal sheet) are used as presented in FDS V.4 database.…”