Constant Incident Heat Flux Research Articles

Experimental study on combustion and flame spread characteristics in horizontal arrays of discrete fuels

Abstract The combustion and flame spread behaviors of discrete fuels often depend on the distribution of the fuel arrays, including the fuel bed width and the spacing of the fuel arrays. In this study, scenarios with six different numbers of columns (n, 1–11 with an interval of 2) and nine different spacings (S, 1–9 mm) were designed. By varying n and S, 54 groups of experimental scenarios over thermally thick birch rod arrays were studied to determine the effects of fuel bed width and spacing. The experimental results showed that the critical spacing for multi-column arrays was larger than that for single-column arrays. Moreover, flame extinction was found when n > 1 and S = 1 mm . The value of S had a limited influence on the mass loss rate when S > 2 mm , but restrained the mass loss rate when S ≤ 2 mm . A piecewise correlation between a newly defined dimensionless mass loss rate and the exposed area ratio was proposed. Moreover, a prediction model of the mass loss rate was developed, which agreed reasonably well with the experimental data. With increasing spacing, the flame height showed three stages: increasing, relatively stable, and decreasing, which were dominated by the balance between air entrainment and radiant heat feedback. The three processes were further described by a piecewise correlation between the dimensionless flame height and fuel bed width. Based on the assumption of a constant incident heat flux, a prediction model of the global flame spread rate was built, which presented better predicted results than the previous models. In addition, it was found that the newly defined dimensionless global flame spread rate linearly increased with increasing porosity.

Conjugate simulation of solar honeycomb receiver for high temperature heat absorption at constant incident heat flux

Conjugate simulation of solar honeycomb receiver for high temperature heat absorption at constant incident heat flux

Optimization of fire-related properties in layered structures

In present paper, fire-related properties of materials with multilayered structure are obtained through reverse heat transfer analysis. For this purpose, one-dimensional heat transfer equation including pyrolysis is analyzed to simulate the fire phenomenon of multi-layered solid materials. The fire-related properties can be obtained by optimizing the objective function expressing the difference between the measured values and the results (surface temperature and mass loss rate under constant incident heat flux) obtained by analyzing the considered heat transfer equations. Repulsive particle swarm optimization is used as the optimization technique. The optimized properties of single-, two- and three-layered materials obtained with the assumed properties were compared to the assumed for various incident heat fluxes. Using a cone calorimeter, the surface temperature and mass loss rate were measured over time with three constant heat fluxes applied to three layers of sandwich type material. The fire-related properties of each layer of the sandwich type material were calculated using the measured results. The technique considered in this study can be applied for determining the fire characteristics of multilayered materials is suitable for engineering the necessary properties to predict fire propagation in solid material.

Computer Tomography Based Structure Characterization of Expanded Intumescent Coatings for Fire Protection

Intumescent coatings are used to protect structural steel from fires. In this study, morphological characteristics of expanded and charred intumescent coatings, which is a key factor of its thermal insulating mechanisms, have been studied using a computer tomography (CT)-based analysis method. Specimens of the charred materials are prepared by a laboratory-scale mass-loss cone heating method for measuring thermal insulation performance. Three types of coatings are exposed to three constant incident heat fluxes (25 kW/m2, 50 kW/m2, and 75 kW/m2) for 2 h. The micro-CT scanning technology is applied on the charred materials to generate 3D reconstructed images with 20 µm resolution by ImageJ software. By using an image processing technique, the central region of each scanned 3D model is divided into three horizontal layers at different depths: top, middle and bottom. The morphological structure varies significantly with the type of coatings, the incident heat flux, and the depth in the char, but the median value of pore size (i.e. equivalent diameter) for all tested samples varies within a narrow range (0.352 mm to 0.424 mm). The qualitative morphological observations as well as the quantitative pore properties (i.e., 3D porosity, size distributions, and sphericity) are obtained. The present non-destructive structure analyses method is useful to assist in new formulation developments and to advance numerical modeling.

Experimental study on the onset of swelling for thin intumescent coatings

Intumescent coatings (also known as reactive coatings) are widely used to protect load-bearing steel structural elements during fire. Upon heating, these initially thin coatings swell to form a low density, highly insulating foamed char that insulates steel substrate. Within the context of modern fire safety engineering design of steel structural systems, thermal insulation during fire may be key for assuring that steel does not reach ‘critical’ temperatures that may yield local structural instability and/or progressive failure (i.e. global instability).Traditionally, the fire performance of intumescent coatings is typically based on compliance (and to some extent their performance) to the standard fire resistance test in furnace, where single coated elements are exposed to the standard temperature-time curve. Numerous research studies have emphasised the influence of heating conditions on the onset of swelling and the overall effectiveness of intumescence. Moreover, some studies have shown that slow growing fires or low heating regimes may cause an incomplete swelling of the intumescent coating, or even melting or delamination. Consequently, the onset of swelling of is key for assuring its effectiveness in providing thermal insulation to the steel substrate during fire. Past researchers have shown that the onset of swelling occurs for temperatures in the range of 200-300°C. There are limited research studies that have look at the effects of varied, non-standard heating regimes on the onset or effectiveness of swelling of thin intumescent coatings.The study presented herein investigates the onset of swelling for a commercially available thin intumescent coating exposed to a wide range of heating regimes. Experiments are performed by using radiant panels and controlling incident radiant heat flux at the exposed surface of coated steel samples. This allows for the direct and precise control of the thermal boundary conditions at the exposed surface of tested samples. The onset of swelling criteria were defined in two ways: (1) visual observation of swelling during heating or (2) time-history of the protected steel temperature.The outcomes of this work aim at yielding a sound understanding of the true effectiveness of thin intumescent coatings. Experimental results mainly show that onset of swelling is directly influenced by the heating regime and it occurs for test samples heated at constant incident radiant heat flux above 20 kW/m2. The onset of swelling for the tested thin intumescent coating occurs for steel temperatures between 150 and 300°C and ‘mean coating temperature’ between 350 and 450°C. However, the steel temperature at onset of swelling is inversely proportional to the amount of constant incident heat flux at the exposed surface of tested samples, while the ‘mean coating temperature’ is directly proportional. As a conclusion, the time-history of net heat flux at the exposed surface of tested samples is not the only parameter that governs the onset of swelling for thin intumescent coatings.

Structural response of cross-laminated timber compression elements exposed to fire

A set of novel structural fire tests on axially loaded cross-laminated timber (CLT) compression elements (walls), locally exposed to thermal radiation sufficient to cause sustained flaming combustion, are presented and discussed. Test specimens were subjected to a sustained compressive load, equivalent to 10% or 20% of their nominal ambient axial compressive capacity. The walls were then locally exposed to a nominal constant incident heat flux of 50 kW/m2 over their mid height area until failure occurred. The axial and lateral deformations of the walls were measured and compared against predictions calculated using a finite Bernoulli beam element analysis, to shed light on the fundamental mechanics and needs for rational structural design of CLT compression elements in fire. For the walls tested herein, failure at both ambient and elevated temperature was due to global buckling. At high temperature failure results from excessive lateral deflections and second order flexural effects due to reductions the walls' effective cross-section and flexural rigidity, as well as a shift of the effective neutral axis in bending during fire. Measured average one-dimensional charring rates ranged between 0.82 and 1.0 mm/min in these tests. As expected, the lamellae configuration greatly influenced the walls' deformation responses and times to failure; with 3-ply walls failing earlier than those with 5-plies. The walls' deformation response during heating suggests that, if a conventional reduced cross section method (RCSM), zero strength layer analysis were undertaken, the required zero strength layer depths would range between 15.2 mm and 21.8 mm. Deflection paths further suggest that the concept of a zero strength layer is inadequate for properly capturing the mechanical response of fire-exposed CLT compression elements.

A methodology for the estimation of ignition delay times in forest fire modelling

A methodology for the estimation of ignition times on solid materials is presented. It is based on the observation that the time to ignition is proportional to the squared time integral of the incident heat flux. This relationship can be readily demonstrated for the classical solutions for time to ignition which consider constant incident heat fluxes. Thus, ignition times can be calculated only with the knowledge of the incident heat flux on the sample (gas-phase calculations) and experimental ignition results. This method is particularly useful when modelling wildfire propagation, where grid resolution is larger than some solid objects. Analytical solutions of the transient conduction problem for constant and ramping incident heat fluxes are discussed. The proportionality between the time to ignition and the squared integral of the incident heat flux was verified experimentally for constant and time-varying incident heat fluxes, showing that the proposed methodology can be applied to both cases. The methodology is outlined and an example is presented.

Thermal behavior and Toxic Emissions of Flame Retarded Timber in Cone Calorimeter Tests

The thermal behavior and toxic emissions of timber products common in industrial buildings in Northern Greece treated or not with flame retardants are investigated. Eight species of wood treated or not with three typical intumescent flame retardants were subjected to constant incident heat fluxes of 35, 50, 65 and 80 kW/m2 in a cone calorimeter linked to the FTIr analyzer. The test results presented in this paper cover the following characteristics: (i) time to ignition, (ii) heat release rate (hrr), (iii) average (300 s) hrr, (iv) effective heat of combustion (mJ/kg), (v) smoke production and (vi) toxic species emissions. The main findings of the experimental analysis are that under the influence of the flame retardants: there was either ‘no ignition’ of the samples or a considerable ignition delay (at lower irradiances) compared with untreated samples. Thermal emissions significantly decrease in terms of the values of ‘peak hrr’ and ‘First 300 s mean hrr’ (by a factor varied from 2 to 5 up to type flame retardant used). In the cases of flame retarded samples, where there was ‘no ignition’ or a considerable ignition delay of the samples there were similar or less toxic emissions compared with the bare samples. Nh 3 was an exception, since both flame retardants contained ammonium in their chemical composition, which was released during the intumescent action of the samples. based on the results of this study, the application of intumescent flame retardants on wooden surfaces is proposed for the cases where ‘no ignition’ or considerable ignition delay occurred. This could be a safe and cost effective approach in reducing fire losses in industrial buildings.

Flammability of oil‐based painted gypsum wallboard subjected to fire heat fluxes

AbstractThe flammability of painted gypsum wallboard (GWB) exposed to fire heat fluxes is investigated. GWB samples coated with multiple layers of alkyd/oil‐based paint are subjected to constant incident heat fluxes of 35, 50 and 75 kW/m2 in the Cone Calorimeter for periods of 5, 10 and 15 min. A number of coats of alkyd/oil‐based interior semi‐gloss enamel paint, including 1, 2, 4, 6 and 8 coats, are applied over a single coat of oil‐based primer to the exposed surface of 16 mm (5/8 in.) thick type X GWB. Unpainted type X GWB is also evaluated under the same exposure conditions. The potential for upward flame spread based on the Cone Calorimeter results is evaluated. The occurrence of paint ‘blistering’ is observed to have a significant effect on the time to ignition and consequently on the potential for upward flame spread. Further work is needed to evaluate the conditions under which ‘blistering’ will occur and its effects on the potential for surface flame spread on painted gypsum wallboard. Copyright © 2004 John Wiley & Sons, Ltd.

The Effect Of Paint On The Ignition Resistance Of Plywood And Chipboard

In this work the influence of paint loading on the time to ignition is investigated for chipboard and plywood. Unpainted and painted specimens, 100-mm-square, 18-mmthick, were subjected to constant incident heat fluxes of 35, 50 and 65 kWm in a cone calorimeter. The time to ignition, mass loss rate and heat release rate are reported as functions of time and of the initial dried mass of paint on the sample (as opposed to number of coats reported in previous work by others). It was found that addition of a small amount of paint (up to 2 g or 200 g/m equivalent to 2 coats) increased the ignition time by a maximum factor of approximately two. However, addition of even more paint (up to about 4.5 g or 450 g/m equivalent to 5 coats) actually reduced the ignition time by up to a factor of approximately 7 relative to the unpainted material. A critical paint loading appears to exist (which decreases with increasing external heat flux), at which the physical mechanism controlling the ignition process changes. A mathematical model with simple kinetics was used in order to interpret the results and understand the ignition behaviour.

Flammability And Dehydration Of Painted Gypsum Wallboard Subjected To Fire Heat Fluxes

The flammability and dehydration of painted gypsum wallboard (GWB) exposed to fire heat fluxes are investigated. Painted GWB samples are subjected to constant incident heat fluxes ranging from 25 to 75 kWlm2 for periods ranging from 5 to 15 minutes in the Cone Calorimeter. A number of coats of latex interior paint, including 0, 2, 4, 6 and 8 coats, are applied over a single coat of latex primer to the exposed surface of 15.9-mm (518-in.) thick type X GWB. A model is used to evaluate the potential for flame spread based on the Cone Calorimeter results. A two-step dehydration model based on a finite difference formulation is described for GWB. Experimental results indicate a distinct dehydration front can be observed by visual inspection; further analysis is needed to determine the composition of the GWB on each side of this front.