In-depth Absorption Research Articles

Effect of Bubbling on Ignition of PMMA Slab: Change in Thermo-Physical and Thermo-Radiative Properties

In semi-transparent polymers, ignition is not only dependent on conductive thermal transfer into the material but also on in-depth absorption of the radiation. The aim of this work was to investigate the influence of bubbling on the thermo-physical and thermo-radiative properties of PMMA and how it may affect its ignition. PMMA plates of varying thickness were exposed to the heat flux of two radiative sources with different emission spectra. Exposure was stopped after different periods of time to study bubbling kinetics and bubble size distribution by optical microscopy. Front and back surface temperatures of samples were recorded during heat exposure. The results indicate that the bubble size distribution is closely related to the temperature gradient within the sample. Steep thermal gradients lead to small-sized bubbles underneath the exposed surface, while weak thermal gradients generate a wider size distribution with in-depth bubbling. All thermo-physical quantities k, ρ and Cp were shown to decrease with increasing bubbling degree. Likewise, it was highlighted that bubbling modifies the thermo-radiative properties of PMMA, especially in the near-infrared range. Transmittance decreases while absorbance increases with a bubbling degree. The increase in the absorption coefficient was attributed to multiple scattering by bubbles that expand the pathway of radiation into the materials. It was concluded that changes in both the thermo-physical and thermo-radiative properties with bubbling were likely to account for the delay in ignition observed when using the near-infrared heating source.

Influence of detailed in-depth radiation modeling on the computational predictions of liquid pool burning rates

Since thermal radiation substantially contributes to the flame heat feedback, a significant impact is anticipated for the in-depth radiation absorption in pool fires. This research focuses on investigating the influence of a detailed in-depth radiation absorption model on the accurate prediction of the flammability characteristics (i.e., ignition time and burning rate) of liquids in using computational fluid dynamics. First, we determined the radiative properties of hydrogenated tetrapropylene (TPH) (i.e., fuel used in the OECD PRISME project) using UV–Vis–NIR and FTIR spectroscopy and derived an expression for the depth-dependent absorption coefficient and average surface reflectivity. Then, a new approach was introduced for the modeling of in-depth radiation absorption. Subsequently, simulations were conducted for three test cases: heptane evaporation, pool fire, and spill fire scenarios involving both heptane and TPH. Results indicated mild effects of in-depth absorption modeling on heptane evaporation, negligible changes in heptane pool fires, and significant alterations in spill fires compared to the predictions with gray calculations. For all the studied cases except heptane pool fire, the ignition times decreased significantly. Sensitivity analysis revealed that the assumed source temperature had negligible influence on the mass loss rate predictions. The importance of the liquid’s Nusselt number (Nul) calculation varied between the studied scenarios.

Controlling Electronic Events Through Rational Structural Design in Subphthalocyanine-Corrole Dyads: Synthesis, Characterization, and Photophysical Properties.

Porphyrinoids are considered perfect candidates for their incorporation into electron donor-acceptor (D-A) arrays due to their remarkable optoelectronic properties and low reorganization energies. For the first time, a series of subphthalocyanine (SubPc) and corrole (Cor) were covalently connected through a short-range linkage. SubPc axial substitution strategies were employed, which allowed the synthesis of the target molecules in decent yields. In this context, a qualitative synthetic approach was performed to reverse the expected direction of the different electronic events. Consequently, in-depth absorption, fluorescence, and electrochemical assays enabled the study of electronic and photophysical properties. Charge separation was observed in cases of electron-donating Cors, whereas a quantitative energy transfer from the Cor to the SubPc was detected in the case of electron accepting Cors.

Defect measurement limits using flash thermography with application to additive manufacturing

This paper assesses the possibility of measuring small, shallow defects in low thermal diffusivity materials with existing pulse thermography techniques. Defects like these are commonly introduced in additive manufacturing (AM), and their presence can cause inconsistencies in the mechanical properties of the final part. An axisymmetric, numerical model was created to simulate the heat transfer within these low thermal diffusivity structures during flash thermography. Deviations from the ideal conditions commonly used in flash thermography models such as conduction across the flaw and in-depth absorption of the incident pulse were included in the model and their effects on defect measurability were investigated. These nonideal conditions (in addition to free convection) introduce depth measurement errors of >10% even for large aspect ratio defects (such as delaminations). The figures provided quantify the amount of error associated with these individual parameters and can be used to determine if a given thermography system is ideal. The simulation results also demonstrate that traditional 1D thermography models may be used to approximate the depth of a defect with negligible error if the defect has an aspect ratio greater than 6. Smaller defects may also be measured with minimal error. If flash thermography were performed during AM to identify the layers where defects most likely reside, the measurement limits would be even less restrictive.

Laser in-depth heating thermal wave detection of adhesive debonding in glass curtain walls: Numerical simulation and experiment

Glass falling from hidden frame-supported glass curtain walls due to adhesive debonding threaten people life and property security. A novel laser in-depth heating thermal wave detection method is proposed for adhesive debonding in glass curtain walls and the temperature field is reconstruct to analyzed the damage detection capability by numerical simulation procedure. Firstly, the experimental procedure is introduced. Secondly, the principles of in-depth absorption and equivalent heat conduction are introduced. Finally, the temperature fields of structure adhesive, well-bonded and debonding structure were analyzed, and temperature differences of in-depth heating and surface heating were compared. The simulated and experimental results are consistent and verified the effectiveness and superiority of laser in-depth heating thermal wave detection on adhesive debonding in glass curtain wall, which can provide reliable guidance for practical application on efficient detection and getting a balance between sufficient heating energy and prevention of overheating.

Pyrolysis and piloted ignition of thermally thick PMMA exposed to constant thermal radiation in cross forced airflow

Impact of cross forced airflow on pyrolysis and piloted ignition of PMMA (Poly methyl methacrylate) under constant heat flux is experimentally, analytically and numerically addressed in this study. Experimental setup consisting of a heating and blowing units was employed to provide the target constant heat flux and airflow velocity. Surface temperature and ignition time were measured in tests to examine the combined effect of forced airflow and heat flux. No ignition was observed when the heat flux and air velocity were 11.53 kW/m2 and 1.43 m/s combined, indicating a critical airflow velocity dominates ignition. An analytical model and a solid phase numerical model were developed to predict and validate the experimental measurements. Both surface and in-depth absorption of thermal radiation were considered in the models. The results showed both the critical temperature and ignition time increase with airflow velocity. Radiation reducing was found to be enhanced by increasing airflow velocity. Both the analytically and numerically predicted results employing either pure surface or in-depth absorption of thermal radiation matched the experimental measurements at acceptable levels, but the agreement between the calculations of in-depth absorption and measurements was better.

Ignition risk analysis of common building material cylindrical PMMA exposed to an external irradiation with in-depth absorption

The Poly methyl methacrylate (PMMA) material with a cylindrical shape are widely used for the building components. Under external irradiation, it may be ignited by a pilot ignition. Previous studies focused more on a flat surface sample, while for a cylindrical configuration, the heat flux distribution, solid-phase temperature profile and ignition time establishes different aspects induced by the cylinder curvature. Therefore, this study investigated the ignition behaviors of the cylindrical PMMA samples with various radius exposed to an external irradiation by experimental and numerical methods. Firstly, decomposition gases of PMMA samples under the irradiation were observed to evidently accumulate on the top surface of the cylinders, after which an incipient ignition occurred in conjunction with blue flames, quickly followed by sustained ignition. With varied cylinder radius, the characteristic parameters of solid-phase temperature profile and thermal penetration depth changed, which are dependent upon sample radius. While for thermal-thick slabs, these parameters have no changes to be independent of the sample thickness. Furthermore, two absorption heat flux coefficients were introduced to analyze the in-depth heat absorption by considering the effects of angle and sample radius. Finally, the critical mass flux and ignition time were given. The cylindrical samples were easier to be ignited than slab samples. With smaller sample radius, the ignition time is established to be shorter. With increasing sample radius, the critical mass flux is lower. This investigation will provide a tractable way to obtain a comprehensive evaluation of fire risk of the commonly used cylindrical components in construction and buildings.

Ignition of polymers under exponential heat flux considering both surface and in-depth absorptions

Analytical models addressing ignition of solids under constant heat flux have been developed in previous studies utilizing surface or in-depth absorption or combination of them. When encountering time-dependent heat flux, the majority of the studies focused on polynomial heat flux and surface absorption assumption. However, in-depth absorption also should be taken into account under time-dependent heat flux in analytical models especially for infrared translucent solids. In this work, an analytical model aiming at revealing the ignition mechanism of translucent polymers under exponential time-increasing heat flux is established considering both surface and in-depth absorptions. Critical temperature is employed as ignition criterion. Four typical non-charring polymers, polymethyl methacrylate (PMMA), polyoxymethylene (POM), polyamide 6 (PA 6) and polypropylene (PP), are utilized as the reference materials, and a numerical solver is employed to validate the analytical model. The results show that the developed analytical model provides accurate predictions of surface temperature and ignition time. Surface heat loss by convection and reradiation has little effect on surface temperature, ignition time and critical energy, but it affects the ignition heat flux greatly. Thermal penetration depth differs from the one under constant heat flux, and it gets smaller as the surface heat loss is considered. The ignition time, thermal penetration depth and critical energy decrease as the heat flux increasing rate gets larger. Meanwhile, the ignition heat flux for in-depth absorption is higher than that for surface absorption, and both increase with heat flux increasing rate. Furthermore, the linearity between ignition time and the squared critical energy, proposed in constant and linear heat flux scenarios, is also found valid under this exponential heat flux condition.

From Cells to Residues: Flame-Retarded Rigid Polyurethane Foams

ABSTRACT Rigid polyurethane foams (RPUFs) exhibit short times to ignition as well as rapid flame spread and are therefore considered to be hazardous materials. This paper focuses on the fire phenomena of RPUFs, which were investigated through a multimethodological approach. Water-blown polyurethane (PUR) foams without flame retardants (FRs) as well as water-blown PUR foams containing triethyl phosphate as a gas phase-active FR were examined. The aim of this study is to clarify the influence of the FR on the fire phenomena during combustion of the foams. Additionally, materials’ densities were varied to range from 30 to 100 kg/m3. Thermophysical properties were studied by means of thermogravimetry; fire behavior and flammability were investigated via cone calorimeter and limiting oxygen index, respectively. During the cone calorimeter test, the temperature development inside the burning specimens was monitored with thermocouples, and cross sections of quenched specimens were examined visually, giving insight into the morphological changes during combustion. The present paper delivers a comprehensive study, illuminating phenomena occurring during foam combustion and the influence of a FR active in the gas phase. The superior fire performance of flame-retarded PUR foams was found to be based on flame inhibition, and on increased char yield leading to a more effective protective layer. It was proven that in-depth absorption of radiation is a significant factor for estimation of time to ignition. Cross sections investigated with the electron scanning microscope exhibited a pyrolysis front with an intact foam structure underneath. The measurement of temperature development inside burning specimens implied a shift of burning behavior towards that of non-cellular materials with rising foam density.

Improved Pyrolysis Model of Polymer Materials under Windy Conditions

The polymer materials are widely used in various occasions, of which polymethyl methacrylate(PMMA) is used in architectural transparent roofs, telephone booths, stairs, and their fire risk is getting more and more attention. The ignition conditions （critical surface temperatures and critical mass flow rates） of polymer materials have extensively researched, but their internal heat transfer studies have been relatively rare, especially in wind environments. Considering that internal heat transfer research is important for the heat transfer of multi-layer materials, the study of heat transfer inside materials is also worthy of attention. In this work, we found that in-depth absorption theory is more suitable for solid pyrolysis models under wind conditions. An absorption coefficient depends with temperature are assumed, and it fits better than the constant absorption coefficient. In addition, this work made some improvements to the ignition model by employing the theory of in-depth absorption.

Fire Phenomena of Rigid Polyurethane Foams.

Rigid polyurethane foams (RPUFs) typically exhibit low thermal inertia, resulting in short ignition times and rapid flame spread. In this study, the fire phenomena of RPUFs were investigated using a multi-methodological approach to gain detailed insight into the fire behaviour of pentane- and water-blown polyurethane (PUR) as well as pentane-blown polyisocyanurate polyurethane (PIR) foams with densities ranging from 30 to 100 kg/m3. Thermophysical properties were studied using thermogravimetry (TG); flammability and fire behaviour were investigated by means of the limiting oxygen index (LOI) and a cone calorimeter. Temperature development in burning cone calorimeter specimens was monitored with thermocouples inside the foam samples and visual investigation of quenched specimens’ cross sections gave insight into the morphological changes during burning. A comprehensive investigation is presented, illuminating the processes taking place during foam combustion. Cone calorimeter tests revealed that in-depth absorption of radiation is a significant factor in estimating the time to ignition. Cross sections examined with an electron scanning microscope (SEM) revealed a pyrolysis front with an intact foam structure underneath, and temperature measurement inside burning specimens indicated that, as foam density increased, their burning behaviour shifted towards that of solid materials. The superior fire performance of PIR foams was found to be based on the cellular structure, which is retained in the residue to some extent.

Analytical prediction of pyrolysis and ignition time of translucent fuel considering both time-dependent heat flux and in-depth absorption

This contribution reports an approximate analytical model to predict transient mass flux and ignition time of translucent fuel, black poly(methyl methacrylate) (PMMA), subjected to a time-dependent incident heat flux, atb, where t is time and a and b are constants. The model can be easily extended to other non-charring translucent solids. The model takes into account in-depth absorption of thermal radiation in the condensed phase, which is typically ignored in the analytical formulations. Both critical temperature and critical mass flux were employed as the ignition criteria to examine their effects on the predictions. The model was validated using exact numerical solutions and experimental data, and compared with earlier analytical models based on the assumption of surface absorption. Linear and quadratic heat fluxes were considered for validation and discussion. The results show that surface absorption accelerates the pyrolysis process and leads to higher mass flux and shorter ignition time with respect to the in-depth absorption case. The discrepancy between the predicted transient mass fluxes of these two absorption modes increases with increasing a. The ignition heat flux increases with increasing a and decreases with increasing b for both surface and in-depth absorption cases. However, the critical energy is independent of heat flux in in-depth absorption scenario. Furthermore, parametric studies of in-depth absorption coefficient and critical mass flux were conducted to investigate their effects on the quality of the model predictions. Also, the equivalent ignition temperature was calculated and compared with the experimental values. It is expected that the developed model will find its use in performance-based design applications.

Piloted ignition delay times on optically thin PMMA cylinders

The theory to predict ignition of solid fuels exposed to incident radiant heat fluxes has permitted to obtain simple correlations of the ignition delay time with the incident heat flux which are useful in practical engineering applications. However, the theory was developed under the assumption that radiation does not penetrate into the solid phase. In the case of semi-transparent solids, where the penetration of radiation plays an important role in the heating and subsequent ignition of the fuel, the predictions of the classical ignition theory are not applicable. A new theory for the piloted ignition of optically thin cylindrical fuels has been developed. The theory uses an integral method and an approximation of the radiative transfer equation within the solid to predict the heating of an inert solid. An exact and an approximate analytical solution are obtained. The predictions are compared with piloted ignition experiments of clear PMMA cylinders. The results indicate that for opticallythin media, the heating and ignition are not sensible to the thermal conductivity of the solid, they are highly dependent on the in-depth absorption coefficient. Using the approximate solution, the correlation 1/tig∝q˙inc″ was established. This correlation is adequate for engineering applications, and allows the estimation of effective properties of the solid fuel. The form of the correlation that was obtained is due to the integral method used in the solution of the heat equation, and does not imply that the semi-transparent solid behaves like a thermally thin material. The approximate solution presented in this article constitutes a useful tool for pencil-and-paper calculations and is an advancement in the understanding of solid-phase ignition processes.

Analytical prediction of heat transfer and ignition time of solids exposed to time-dependent thermal radiation

An analytical model was developed in this study to predict the heat transfer and ignition time of solids subjected to time-dependent thermal radiation (HF=atb). Both surface and in-depth absorptions, corresponding to opaque and translucent materials, were considered in the model and critical temperature was employed. The predictions of the new model fit well with the experimental and numerical results. The results show that for surface absorption the ignition time to the power of −(b+0.5) is proportional to a, and the reciprocal of square root of ignition time is linearly correlated with ignition HF. Furthermore, a critical ignition HF was found to represent the lower limit of ignition HF range, which is different with the critical HF at constant HF. While for in-depth absorption, the ignition time to the power of −(b+1) and −1 were linearly proportional to a/(b+1) and ignition HF, respectively. For translucent solids, the analytical model cannot be applied to constant HF but can provide relatively high accuracy in predicting ignition time under variable HF. Also, the effect of in-depth absorption coefficient on ignition time were addressed, and it was found that this important parameter exerts its influence on ignition process following the similar mechanism with that of constant HF.

Effect of radiation absorption modes on ignition time of translucent polymers subjected to time-dependent heat flux

Majority of previous solid ignition models, including numerical and analytical ones, considered only surface absorption of incident heat flux for simplification. However, the influence of in-depth absorption on pyrolysis and subsequent ignition cannot be ignored for infrared translucent polymers. This work addresses this problem and focuses on time-dependent heat flux to establish an analytical model for ignition behaviors prediction by means of theoretical analysis. Ignition temperature was utilized as the ignition criterion, and both surface and in-depth absorption scenarios were considered. Thermally thick polymethyl methacrylate and polyamide 6 were selected as reference materials to verify the reliability and applicability of the proposed model by comparing the analysis results with experimental data as well as numerical simulations. A method for determining the approximation parameters of the theoretical analysis was presented to derive the relationship between ignition time and the coefficients in heat flux expressions. The results show that the higher surface temperature owing to surface absorption accelerates the pyrolysis rate and results in a shorter ignition time, while in-depth absorption affects the ignition time inversely. The effect of surface heat loss was also evaluated quantitatively through both analytical and numerical models. The uncertainty of the proposed model is mainly caused by the selection of the approximation parameters. Nevertheless, it provides an alternative approach to estimate the ignition time of translucent polymers besides numerical simulation.

Approximate analytical solutions for temperature based transient mass flux and ignition time of a translucent solid at high radiant heat flux considering in-depth absorption

Most studies, employing ignition temperature as the ignition criterion, utilized surface absorption of radiant incident heat flux in analytical models when investigating the ignition mechanism of solid combustibles. However, in-depth absorption exerts its influence on ignition time significantly for translucent solid, especially at high radiant heat flux. In this work, we extend the previous researches from surface absorption to in-depth absorption to develop an approximate analytical ignition model using critical mass flux instead of critical temperature. An approximation methodology is proposed during derivation to study the in-depth absorption scenario. The comparison among this model, available experimental data of black PMMA in the literature and previous numerical simulations indicates that the proposed model provides relatively high accuracy in predicting ignition time. Furthermore, the pure surface absorption circumstance is also reexamined and compared with the classical ignition theory. The results show that surface absorption hypothesis accelerates the total mass flux, which consequently shortens the ignition time. However, in-depth absorption assumption eliminates the heat accumulation on surface and results in good prediction for ignition time at high heat flux. For in-depth absorption, the absorption coefficient affects the heat penetration depth and temperature distribution in this layer which consequently affects the thermal degradation reaction rate, mass flux and finally ignition time. Meanwhile, the ignition time considering both surface and in-depth absorption is discussed, and the relationship with pure surface and in-depth absorption conditions is obtained.

A numerical study of thermal degradation of polymers: Surface and in-depth absorption

A one-dimension numerical model is developed in this study to investigate numerically the absorption effect, including surface and in-depth absorption, on thermal degradation process that occurs in polymer gasification. Surface and in-depth absorption hypotheses, most commonly used in literatures and their effect on the simulation results are discussed. Three polymers are selected to conduct the simulation: Poly(methyl Methacrylate) (clear PMMA), High Impact Polystyrene (HIPS) and Poly(acrylonitrile Butadiene Styrene) (ABS). The availability of the developed model is verified by the published experimental data. The result indicates that large temperature gradient exists in the heat penetration layer for surface absorption, and in-depth absorption leads to a relatively uniform temperature distribution in this layer. Top surface temperature increases with increasing heat flux (HF) and higher value is observed for surface absorption than that of in-depth absorption. The model overestimates the mass loss rate of ABS at low HF due to the negligence of existence of a thin char layer generated on the surface of sample which cracks immediately after generation under high HF. Good agreement between the numerical and experimental results of polymers suggests that both assumptions are acceptable in modeling bench scale tests.

Development of an anaerobic pyrolysis model for fire retardant cable sheathing materials

Wire and cable coverings are potentially a major cause of fire in buildings and other installations. As they need to breach fire walls and are frequently located in vertical ducting, they have significant potential to increase the fire hazard. It is therefore important to understand the ignition and burning characteristics of cables by developing a model capable of predicting their burning behaviour for a range of scenarios. The fire performance of electrical cables is usually dominated by the fire performance of the sheathing materials. The complexity of the problem increases when cable sheathing incorporates fire retardants. One-dimensional pyrolysis models have been constructed for cable sheathing materials, based on milligram-scale and bench-scale test data by comparing the performance of three different software tools (ThermaKin, Comsol Multiphysics and FDS, version 6.0.1). Thermogravimetric analysis and differential scanning calorimetry were conducted on powdered cable coatings to determine the thermal degradation mechanism, the enthalpy of decomposition reactions, and the heat capacities of all apparent species. The emissivity and the in-depth absorption coefficient were determined using reflectance and transmittance measurements, with dispersive and non-dispersive spectrometers and integrating spheres. Bench-scale tests were conducted with a mass loss calorimeter flushed with nitrogen on samples in a horizontal orientation, for comparison with the pyrolysis model of non-flaming decomposition at an external heat flux of 50 kW m−2. The parameters determined through analysis of the milligram-scale data were used to construct a pyrolysis model that predicted the total mass loss from calorimeter tests in anaerobic conditions. A condensed phase pyrolysis model that accurately predicts in-depth temperature profiles of a solid fuel, and the mass flux of volatiles evolved during degradation of the fuel, is an essential component of a comprehensive fire model, which when coupled to a computational fluid dynamics code can be used to predict the burning processes in a fire scenario. Pyrolysis models vary considerably in complexity based on the assumptions incorporated into the development of the model.

Pool fires – An empirical correlation

This study presents the experimental procedure and results of highly controlled pool fire tests, in a quiescent environment, designed to accurately measure the fuel burning rate and, consequently for sufficiently large pools, the thermal radiation flux back to the fuel surface. Steps were taken to minimize the effects of in-depth absorption of flame radiation, circulation within the liquid, and changes in fuel composition due to distillation of more volatile fuel components. With these precautions, focus is placed on gas phase phenomena controlling the heat release rate per unit pool area. The primary variables considered are: pool diameter, heat of gasification, flame sootiness as characterized by the inverse of the fuel smoke-point flame height, and, to a lesser extent, absorption of flame radiation by the fuel vapors just above the liquid surface. Results reported herein agree well with literature values for experiments conducted under similarly controlled conditions. A simple empirical formula is developed based primarily on heat of gasification and smoke point and is shown to correlate the mass burning rate within 9%, on average, of the experimental data.

Experimental study of radiative heat transfer in a translucent fuel sample exposed to different spectral sources

Radiative heat transfer to a solid is a key mechanism in fire dynamics, and in-depth absorption is especially of importance for translucent fuels. The sample-heater interaction for radiative heat transfer is experimentally investigated in this study with two different heaters (electric resistance and tungsten lamp) using clear PolyMethylMethAcrylate (PMMA) samples from two different formulations (Plexiglass and Lucite). First, the significant effects of the heater type and operating temperature on the radiative heat transfer are revealed with broadband measurements of transmittance on samples of different thicknesses. Then, the attenuation coefficient in Beer–Lambert’s law has been calculated from detailed spectral measurements over the full wavelength range encountered in real fires. The measurements present large spectral heterogeneity. These experimental results and calculation of in-depth absorption are used to explain the reason behind the apparent variation of the fuel absorbance with the sample thickness observed in past studies. The measurement of the spectral intensity emitted by the heaters verifies that the common assumption of blackbody behavior is correct for the electric resistance, whereas the tungsten lamp does not even behave as a greybody. This investigation proofs the necessity of a multi-band radiation model to calculate accurately the fire radiative heat transfer which affects directly the in-depth temperature profiles and hence the pyrolysis process for translucent fuel.