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On the Influence of the Sample Absorptivity when Studying the Thermal Degradation of Materials

The change in absorptivity during the degradation process of materials is discussed, and its influence as one of the involved parameters in the degradation models is studied. Three materials with very different behaviors are used for the demonstration of its role: a carbon composite material, which is opaque, almost grey, a plywood slab, which is opaque and spectral-dependent and a clear PMMA slab, which is semitransparent. Data are analyzed for virgin and degraded materials at different steps of thermal degradation. It is seen that absorptivity and emissivity often reach high values in the range of 0.90–0.95 with a near-grey behavior after significant thermal aggression, but depending on the materials of interest, some significant evolution may be first observed, especially during the early stages of the degradation. Supplementary inaccuracy can come from the heterogeneity of the incident flux on the slab. As a whole, discrepancies up to 20% can be observed on the absorbed flux depending on the degradation time, mainly because of the spectral variations of the absorption and up to 10% more, depending on the position on the slab. Simple models with a constant and unique value of absorptivity may then lead to inaccuracies in the evaluation of the radiative flux absorption, with possible consequences on the pyrolysis analysis, especially for properties related to the early step of the degradation process, like the time to ignition, for example.

Radiative flux emitted by a burning PMMA slab

The degradation of a PMMA sample has been studied based on experimental results obtained for the radiation emission by a burning slab. Observations of the infrared emission perpendicular to the plate, in the range where the optically thin flame is weakly emitting, indicate a plate temperature close to 680 K which is an indication on the surface temperature during the degradation process. Observations from the side allow a flame characterization without the plate emission superimposition. This is a promising way for evaluating data regarding the flame characteristics: temperature, gaz concentration and soot volumetric fraction.

Determination of Woody Fuel Flame Properties by Means of Emission Spectroscopy Using a Genetic Algorithm

Because radiation from flames is often the dominant mechanism for wildfire spread, detailed information on flame properties is required. The proposed procedure combines a spectrally resolved radiation model for simulating the line-of-sight infrared emission intensity and spectroscopy data and uses a genetic algorithm (GA) to determine a set of flame properties, allowing optimal agreement between model and outdoor experiments. GA calibration and sensitivity analysis were conducted using well-defined reference flames. The combined GA/radiation model was used with emission data to estimate the effective properties of flames from the combustion of woody fuel beds from 0.5 to 4 m in thickness. Experimental results show that the contribution of soot particles to flame emission increases with flame thickness. The GA was found to be robust and efficient in providing relevant flame properties from line-of-sight intensities on the infrared spectrum of radiation.

Study of a V-shape flame based on IR spectroscopy and IR imaging

Applicability of an IR imaging/spectroscopy diagnostic was tested on a laboratory- scale flame. For this purpose, measurements were carried out on a V-shape flame developed along a wall, with the aim of evaluating the wall temperature and of identifying the flame properties (temperature and species concentrations). Infrared measurements with a multiband camera and a spectrometer were post-processed and compared, in particular, with thermocouple measurements carried out for the wall temperature. Simple evaluation involving a correction for the emissivity showed a quite good agreement when assessed against experimental data. An attempt to reconstruct a flame emission spectrum was also carried out, expecting a possible inverse identification of the flame properties. The method showed a promising behaviour on synthetic data built with a radiative transfer model for gas and wall radiation. However, the spectrum reconstruction method is not yet accurate enough to allow an identification of the flame properties in full confidence when applied to actual experimental data. First tests showed a correct qualitative behaviour, but model refinements are required at least for the flame radiation, before getting accurate flame properties.

Effect of a coupled thermomechanical loading on the residual mechanical strength and on the surface temperature of wound carbon/epoxy composite

Wound composite structures such as hyperbaric hydrogen tanks may experience accidental situations, for example in case of a fire. The FCH-JU project FireComp aims at better characterizing the conditions that need to be achieved in order to avoid a failure of a composite pressure vessel. This research program involves specific experiments to improve the understanding of loss of strength of composite high-pressure vessels in fire conditions. The present study investigates the effect of a coupled thermomechanical loading (cone calorimeter exposure and, simultaneously, mechanical stress) on the residual strength of a composite material. A specific device combining a cone and a four-point bending bench has been designed. The influence of the coupled aggression is addressed by comparing the temperature on the front and the rear sides, the mass loss, and the residual tensile strength of a set of samples subjected to a heat flux only and a set subjected to a heat flux and a four-point bending. The results do not exhibit a clear effect of the mechanical load: the thermomechanical properties of both sets of samples are similar.