Radiative ignition mechanism of solid fuels

Transmittance of external radiation from a CO₂ laser through a boundary layer of decomposition products over a vertical sample surface is measured during the ignition period. The results indicate that there is significant absorption of the external radiation for PMMA, and a lesser but still not negligible amount, for red oak. An increase in gas phase temperature over surface temperature is observed over much of the ignition interval. Using the experimentally measured incident flux at the sample surface, surface temperature history was calculated from a model that included re-radiation and convection losses from the surface, endothermic decomposition and conduction into the material. The results confirm the significant effect of gas phase absorption on surface temperature. Steady-state-derived surface regression rate expression was used for PMMA in this model. The results raise questions about the validity of such data for the dynamic heating conditions during the ignition period. Further studies needed to understand the radiative ignition mechanism are identified.     

Piloted ignition times,critical heat fluxes and mass loss rates at reduced oxygen atmospheres

Ignition, pyrolysis and burning of materials in reduced oxygen atmospheres occur when recirculating combustion gases are mixed with the air flowing into an enclosure. Still the incoming air can be sufficient for the complete combustion of the pyrolysis gases. Thus, for the prediction of fires in enclosures it is essential to understand the ignition and burning of materials in a reduced oxygen atmosphere even when plenty of oxidizer is available for complete combustion. Previous work employing gaseous fuels has shown that under these conditions, but before extinction, burning of gaseous fuels issuing from a nozzle is complete but radiation from the flames decreases owing to the reduction of their temperature. Complementary to that work, piloted ignition of solids is investigated here at reduced oxygen concentrations by measuring the ignition times and mass loss rates of the solid at ignition.These results were obtained in a cone calorimeter modified to supply air at reduced oxygen concentrations. Two types of plywood, normal and fire retardant 4 mm thick were examined at three imposed heat fluxes 25, 35 and 50 kW/m² and at oxygen concentrations of 21%, 18% and 15% by volume. Because heating at these heat fluxes and material thickness corresponds to intermediate thermal conditions (i.e. neither thin nor thick), novel analytical solutions are developed to analyze the data and extract the thermal and ignition properties of the material. The same novel solutions can be applied to modeling concurrent or countercurrent flame spread. Moreover, a theory for piloted ignition explains why the ignition times and mass pyrolysis rates are weakly dependent on reduced oxygen concentrations.     

A porous media model for sprinkler wetting: Ceramic fiberboard validation

A one-dimensional porous media model has been developed to investigate water based fire suppression. The model is for heat and mass transfer in porous materials subjected to external water sprays and radiant heating. In the model, heat transfer inside the material occurs by conduction, convection, and phase change. Mass transfer occurs by gas phase diffusion and convection in the liquid and gas phases. Convective mass fluxes are driven by pressure gradients according to Darcy's Law. Boundary conditions that are appropriate for a range of cases are presented. Experiments were conducted to validate the model and investigate two scenarios relevant to water based suppression: spray wetting and radiant heating. Both of these scenarios represent a novel application of a porous media model. Ceramic fiberboard samples were used as a test material. For the wetting tests, the model is shown to be able to reasonably predict the rate of water absorption into the samples. Radiant heating tests were conducted in the cone calorimeter with pre-wetted samples. For the heating tests, the model is shown to reasonably predict the drying behavior that would directly precede an ignition event.     

Pyrolysis and spontaneous ignition of wood under transient irradiation: Experiments and a-priori predictions

Wood is a material widely used in the built environment, but its flammability and response to fire are a disadvantage. Therefore, it is essential to have substantial knowledge of the behavior of wood undergoing external heating such as in a fire. The majority of studies in the literature use constant irradiation. Although this assumption simplifies both modelling and experimental endeavors, it is important to assess the behavior of materials under more comprehensive heating scenarios which might challenge the validity of solid-phase ignition criteria developed previously. These criteria are evaluated here for the spontaneous ignition under transient irradiation by combining experimental measurements and a-priori predictions from a model of heat transfer and pyrolysis. We have applied a two-step transient irradiation in the cone calorimeter in the form of a growth curve followed by constant irradiation. We use white spruce samples of size 100×100 mm and thickness of 38 mm . We measure the temperature at different depths and the mass loss. A one dimensional model written in the open source code Gpyro is used to predict the pyrolysis behavior. The model has a chemical scheme in which the components of wood (hemicellulose, cellulose, lignin) become active, then decompose in two competing reactions: one reaction to char and gas, and one reaction to tar. The kinetic parameters, as well as the thermal properties of the wood and char are taken from the literature, while ρ and moisture content are measured experimentally. A priori predictions of the temperature, made prior to the experiments, show excellent agreement with the measurements, being within the experimental uncertainty range. The mass loss rate (MLR) predictions are qualitatively similar to the measurements, but there is a large uncertainty in the measurements. For a-posteriori simulations, certain parameters are changed after having access to the measurements to improve the simulations. Also, we perform an evaluation of the solid phase ignition criteria used in the literature, and find that neither criteria is a consistent indicator of ignition. These results help understand the spontaneous ignition of wood subjected to transient irradiation and identify strengths and gaps in the topic.     

Material fire properties and predictions for thermoplastics

Ignition and burning rate data are developed for nylon 6/6, polyethylene, polypropylene and black polycast PMMA in a cone calorimeter heating assembly. The objective is to examine a testing protocol that leads to the prediction of ignition and burning rate for thermoplastics from cone calorimeter data. The procedure consists of determining material properties, i.e. thermal inertia, specific heat, thermal conductivity, ignition temperature, heat of gasification and flame heat flux from cone data, and utilizing these properties in a model to predict the time to ignition and transient burning rate. The procedure is based on the incident flame heat flux being constant in the cone calorimeter which occurs for flames above the top of the cone heater. A constant net flame heat flux of approximately 20 kW/m² for nylon 6/6, 19 kW/m² for polyethylene, 11 kW/mP² for polypropylene and 28 kW/m² for black PMMA is obtained for irradiation levels ranging from 0 to 90 kW/m². The burning rate model is shown to yield good accuracy in comparison to measured transient burning in the cone assembly.     

Onset of smoldering in cotton: Effects of density

The ignition temperature for smoldering in cotton has been determined for several densities. Experiments and consistent theoretical calculations show that an increase in density leads to decrease in ignition temperature. Experimentally it is found that repeated heating of a cotton sample results in a higher ignition temperature than for samples heated once, as a result of partial decomposition during heating. For the lowest density investigated, smoldering occurs but is not self-sustained. The smoldering front moves through the cotton with a velocity independent of the density. The mass loss rate due to smoldering increases with density. It can be concluded that density affects both the ignition temperatures and the mass loss rates in a systematic way.     

燃料间距对PMMA竖向火蔓延行为的影响

以PMMA（聚甲基丙烯酸甲酯）板（8 cm×10 cm×1 cm）为实验材料,开展不同燃料间距（0~10 cm）的竖向火蔓延实验,分析火焰高度、燃烧速率、热解前锋位置及点燃滞后时间的变化规律。结果发现,随着燃料间距增加,固体表面的净火焰高度与燃烧速率均呈现先增大后减小的趋势。此外,火焰在相邻固体燃料表面的蔓延过程中出现“点燃滞后”现象,点燃滞后时间随着燃料间距的增加呈指数级增长。     

An experimental evaluation of critical surface temperature as a criterion for piloted ignition of solid fuels

With a view to developing a simple engineering method for the prediction of piloted ignition, the validity of the critical surface temperature criterion for piloted ignition is examined experimentally for seven thermoplastic materials. The results indicate that the surface temperature at piloted ignition for each material studied varies by ±15 K or less. As such, the surface temperature criterion appears to be suitable for engineering calculations.

Analysis of time-surface temperature histories shows that the radiant heat source temperature has a significant effect on the material heating over the range of source temperatures utilized (700–1050 K). The variations in material heating are of sufficient magnitude to cause changes in ignition times by a factor of two or more for different heat sources present in typical fire scenarios.

Our current level of understanding of piloted ignition is shown to be insufficient to support extrapolation procedures to determine the minimum incident radiant flux required for piloted ignition. An experimental approach to determination of the minimum radiant flux required for piloted ignition is demonstrated to be feasible.     

电缆燃烧典型火源模拟方法综述

针对电缆火灾问题,综述了国内外电缆火灾模拟试验及电缆燃烧测试中使用的火源模拟方法,并对比探讨了多种火源模拟方法的点火时间、火源温度、火源面积、火源功率、可扩展性、可移动性以及可持续使用性等方面的差异：燃气喷灯法和辐射加热法（锥形量热计）所需点火时间短;电加热法火源温度高,操作简便易移动;燃油点火法的点火面积大,可扩展性强;电弧引燃法的温度最高、可控性强。以期为电缆火灾模拟及防火提供参考。     

Smolder ignition of polyurethane foam: effect of oxygen concentration

Experiments have been conducted to study the ignition of both forward and opposed smolder of a high void fraction, flexible, polyurethane foam in a forced oxidizer flow. Tests are conducted in a small scale, vertically oriented, combustion chamber with supporting instrumentation. An electrically heated Nichrome wire heater placed between two porous ceramic disks, one of which is in complete contact with the foam surface, is used to supply the necessary power to ignite and sustain a smolder reaction. The gaseous oxidizer, metered via mass flow controllers, is forced through the foam and heater. A constant power is applied to the igniter for a given period of time and the resulting smolder is monitored to determine if smolder is sustained without the assistance of the heater, in which case smolder ignition is considered achieved. Reaction zone temperature and smolder propagation velocities are obtained from the temperature histories of thermocouples embedded at predetermined positions in the foam with junctions placed along the fuel centerline. Tests are conducted with oxygen mass fractions ranging from 0.109 to 1.0 at a velocity of 0.1 mm/s during the ignition period, and 0.7 or 3.0 mm/s during the self-sustained propagation period. The results show a well-defined smolder ignition regime primarily determined by two parameters: igniter heat flux, and the time the igniter is powered. These two parameters determine a minimum igniter/foam temperature, and a minimum depth of smolder propagation (char), which are conditions required for ignition to occur. The former is needed to establish a strong smolder reaction, and the latter to reduce heat losses from the incipient smolder reaction to the surrounding environment. The ignition regime is shifted to shorter times for a given igniter heat flux with increasing oxygen mass fraction. A model based on concepts similar to those developed to describe the ignition of solid fuels has been developed that describes well the experimental ignition results.     

Smoldering initiation in cellulosics under prolonged low-level heating

Scant field information and the difficulty of conducting long-term research experiments on the low-level heating of cellulosics until ignition occurs have led to the advancement of an incipient smoldering ignition predictive model.The Laboratory is maintained at Madison, Wisconsin in cooperation with the University of Wisconsin.     

Coupled analytical approach to predict piloted flaming ignition of non-charring polymers

Classical thermal theory of piloted ignition is extended by coupling the heat balance at the exposed sample surface and the finite-rate pyrolysis in the material volume. Approximate analytical solutions for the sample temperature are obtained for an arbitrary sample thickness, with the external radiative heating, surface re-radiation, heat of gasification, and the convective heat flux corrected for blowing taken into account. The volatile mass flux is evaluated by integrating the pyrolysis rate throughout the layer, with the assumption of high activation energy limit. Critical mass flux of combustible volatiles is used as the ignition criterion. This enables the ignition temperature to be evaluated instead of being pre-assumed as is done in the classical thermal theory. Coupled analytical approach proposed in this work is verified by comparisons to the numerical solution obtained by the Pyropolis model for the same problem setup. This approach has also been validated by comparisons to published experimental data (ignition temperatures and times to ignition) for three non-charring thermoplastics: polymethylmethacrylate, polyethylene and polypropylene.     

Self-ignition of natural fuels: Can wildfires of carbon-rich soil start by self-heating?

Carbon-rich soils, like histosols or gelisols, cover more than 3% of the Earth's land surface, and store roughly three times more carbon than the Earth's forests. Carbon-rich soils are reactive porous materials, prone to smouldering combustion if the inert and moisture contents are low enough. An example of soil combustion happens in peatlands, where smouldering wildfires are common in both boreal and tropical regions. This work focuses on understanding soil ignition by self-heating, which is due to spontaneous exothermic reactions in the presence of oxygen under certain thermal conditions. We investigate the effect of soil inorganic content by creating under controlled conditions soil samples with inorganic content (IC) ranging from 3% to 86% of dry weight: we use sand as a surrogate of inorganic matter and peat as a surrogate of organic matter. This range is very wide and covers all IC values of known carbon-rich soils on Earth. The experimental results show that self-heating ignition in different soil types is possible, even with the 86% inorganic content, but the tendency to ignite decreases quickly with increasing IC. We report a clear increase in ambient temperature required for ignition as the IC increases. Combining results from 39 thermostatically-controlled oven experiments, totalling 401 h of heating time, with the Frank-Kamenetskii theory of ignition, the lumped chemical kinetic and thermal parameters are determined. We then use these parameters to upscale the laboratory experiments to soil layers of different thicknesses for a range of ambient temperatures ranging from 0 °C to 40 °C. The analysis predicts the critical soil layer thicknesses in nature for self-ignition at various possible environmental temperatures. For example, at 40 °C a soil layer of 3% inorganic content can be ignited through self-heating if it is thicker than 8.8 m, but at 86% IC the layer has to be 1.8 km thick, which is impossible to find in nature. We estimate that the critical IC for a ambient temperature of 40 °C and soil thickness of 50 m is 68%. Because those are extreme values of temperature and thickness, no self-heating ignition of soil can be expected above the 68% threshold of inorganic content. This is the first in-depth experimental quantification of soil self-heating and shows that indeed it is possible that wildfires are initiated by self-heating in some soil types and conditions.     

Calculation of the thermal response of buildings by the total thermal time constant method

This paper describes the theoretical development and experimental proof of the total thermal time constant (TTTC) method for calculation of the thermal response of buildings. The output is obtained in the form of time sequences of temperature, under given time-variation of internal heating load, or in the form of time dependent heating (or cooling) loads, under given patterns of internal temperature variation in time.TTTC method considers ventilation conditions, internal heating, metabolic heat production, cooling, solar radiation absorption on, and longwave i.r. radiation loss from, the external surfaces, solar radiation penetration through windows and the external air temperature and humidity variations in time. The main feature of this method is that each component of the building is represented here, as a heat transfer path, only by two easy to calculate numbers: the thermal resistance and the TTTC (this includes thermal resistances and heat capacities and their relative position in the heat transfer path, including partitions and ceilings) [1–3]. The two parameters characterize the influence of the element on the thermal response of a building as a whole.Experimental demonstration of the accuracy of the TTTC method in computing the thermal response of buildings is presented and compared with measured temperature time patterns both in models and actual buildings under various external conditions. The method is useful not only for the thermal design of buildings and the selection of building materials, but also for the design of passive methods of climatization, e.g. by the use of solar radiation for heating, and conversely, the cooling of a structure by longwave radiation loss (to the outer space through the atmosphere) and by ventilation. Thermal storage and insulation properties are also considered.     

Solar powered absorption air conditioning

The design of buildings to provide a suitable thermal environment is discussed and the reasons for artificial heating or cooling introduced. The problem of sizing a solar-powered cooling plant is investigated. An iterative method of estimating heat flow and resultant temperatures in buildings subject to fluctuating heat loads is described. A model is developed to allow investigation of the performance of a solar collector and thermal storage system and some of the basic relationships between performance and physical parameters are determined.

An iterative method of predicting the cooling output from a lithium bromide-water absorption refrigeration plant having variable heat input is described.

The design of a solar collector/thermal storage) absorption cooler system, its performance on a particular building and its fine tuning are examined.     

高压击穿电弧作用下SMC绝缘隔板引燃特性

绝缘隔板是高压开关柜的重要部件,采用高压起弧实验平台,针对5,10 mm两种厚度的SMC绝缘隔板开展了电弧引燃与燃烧实验,研究了质量损失率、余焰时间、温度曲线等特征量随电弧作用时间的变化规律。结果表明,高压作用下,两电极首先绕SMC竖直板形成倒“U”型夹持加热电弧;持续电弧作用下的烧蚀过程可以分为起弧上扬、夹持引燃、侧面弧降、余焰燃烧四个阶段。10 mm样品余焰的产生时间较晚,且需要更长的加热时间达形成显著自维持燃烧,烧蚀结束后样品炭化烧蚀度较低,可见,SMC绝缘隔板增厚,有利于其结构完整性和防火隔火能力的提升。由样品不同区域的表面温度曲线,弧根对该区域对样品表面的加热强度反而较弱,即样品表面的温升受弧柱距离和自持火焰的双重作用,包含多个互相影响的热过程。研究结果可用于高压开关柜绝缘隔板的科学选型与优化布设,为开关柜防火安全性能的提升提供科学依据。     

Modelling thermal degradation of polymers using single-step first-order kinetics

A theoretical framework for characterising single-step Arrhenius degradation kinetics in terms of a characteristic temperature and temperature range is developed. It is demonstrated that for the purposes of practical calculation, the reaction order may be assumed to be unity and also that a first-order approximation to an nth-order TG curve remains a good approximation over an order-of-magnitude variation in heating rate. This fact implies that when the pyrolysis of much larger samples of material is modelled, the error involved in using first-order kinetics is small. The equivalent first-order approximation is then applied to a global in-depth model of polymer degradation in order to predict mass loss rates in bench-scale experiments such as the cone calorimeter test. The mass loss rate curves obtained from the equivalent first-order approximation are found to compare well with the full nth-order model. Finally, an estimate of the average or steady mass loss rate is developed which fully accounts for the interaction between the degradation kinetics, the external heat flux, the heat losses and the latent heat of vaporisation.     

Sensitivity of the total heat loss coefficient determined by the energy signature approach to different time periods and gained energy

In order to identify buildings that have energy saving potential there is a need for further development of robust methods for evaluation of energy performance as well as reliable key energy indicators. To be able to evaluate a large database of buildings, the evaluation has to be founded on available data, since an in-depth analysis of each building would require large measurement efforts in terms of both parameters and time. In practice, data are usually available for consumed energy, water, and so on, namely consumption that the tenants or property holder has to pay for. In order to evaluate the energy saving potential and energy management, interesting key energy indicators are the total heat loss coefficient K_tot (W/K), the indoor temperature (T_i), and the utilisation of the available heat (solar radiation and electricity primarily used for purposes other than heating). The total heat loss coefficient, K_tot, is a measure of the heat lost through the building's envelope, whereas T_i and the gained energy reflect the user's behaviour and efficiency of the control system.In this study, a linear regression approach (energy signature) has been used to analyse data for 2003-2006 for nine fairly new multifamily buildings located in the Stockholm area, Sweden. The buildings are heated by district heating and the electricity used is for household equipment and the buildings’ technical systems. The data consist of monthly energy used for heating and outdoor temperature together with annual water use, and for some buildings data for household electricity are also available. For domestic hot water and electricity, monthly distributions have been assumed based on data from previous studies and energy companies. The impact on K_tot and T_i of the time period and assumed values for the utilised energy are investigated.The results show that the obtained value of K_tot is rather insensitive to the time period and utilised energy if the analysis is limited to October-March, the period of the year when the solar radiation in Sweden yields a minor contribution to heating. The results for the total heat loss coefficient were also compared to the calculations performed in the design stage; it was found that K_tot was on average 20% larger and that the contribution to heating from solar radiation was substantially lower than predicted. For the indoor temperature, however, the utilised energy had a large impact.With access to an estimate of K_tot and T_i, an improved evaluation of the energy performance may be achieved in the Swedish real estate market. At present the measure commonly used, despite the fact that monthly data is available, is the annual use of energy for space heating per square metre of area to let.     

Extinction of surface stabilized gaseous diffusion flames: Part I Simplified numerical model and implications for solid fuels in fires

In this work a previously proposed empirical and analytical criterion for extinction is numerically extended and validated for varying fuel dilution, oxidant dilution, strain rate, and surface temperature. The output of this work is presented in two parts: the current Part I uses simple kinetics and constant thermal transfer properties and Part II uses detailed kinetics, varying thermal transfer properties, flame radiation feedback and flame suppression agents in order to demonstrate that conclusions from the simplified model are still valid. In addition this work goes beyond the concept of critical flame temperature or mass flux for extinction by including the influence of slow chemical kinetics through the Damkohler number which becomes even more important for commonly used fire retarded materials.Extinction of flames on solid fuels is modeled by decoupling the pyrolysis chemistry from the gas-phase combustion chemistry using the flame energy feedback versus pyrolysis rate curves and an energy balance at the surface. This approach has the advantage of identifying and deducing key materials properties for solid and gaseous phase from experiments. Simulations are performed in a planar stagnation-point flow diffusion flame configuration using one-step Arrhenius chemical kinetics and a simplified transport model with Lewis number equal to unity. Only quasi-steady conditions are considered for the gaseous phase even if the pyrolysis rate of the solid is transient because the response time for the solid phenomena is, in general, much larger than the response (diffusion) time for the gaseous phenomena.It is found that at high pyrolysis rates and low straining rates (infinitely fast kinetics regime) there is no leakage of oxygen to the surface of solid fuel. However, as the solid fuel extinction is approached, oxygen leakage occurs because the effective air to fuel mass stoichiometric ratio becomes less than one owing to fuel dilution near the surface. At high straining rates, solid combustion cannot be sustained at any pyrolysis rate. In the infinitely fast kinetics regime, an appropriate scaling has been developed which collapses the convective heat flux curves onto a single one. In general, the critical pyrolysis fuel mass flux exhibits a universal behavior for variation of various model parameters when plotted versus a modified Damköhler number, and becomes constant when the latter is sufficiently high. Comparison with experiments is discussed, and the implications of the criterion for characterizing ignition flammability properties of solid fuels are also discussed.     

Ventilation measurement using spot sampling of sulphur hexafluoride on a solid adsorbent

A technique for measuring the ventilation rate using spot sampling of sulphur hexafluoride (SF₆) on a solid adsorbent for subsequent thermal desorption and gas chromatographic analysis has been developed. This paper describes the mathematical model used, the analytical details of the technique, as well as results from a validation test using an experimental chamber. The mathematical model assumes an exponential decay of the tracer gas concentration. The precision of the analytical procedure was estimated to be better than 9% whilst the error of the measured ventilation rate of the test chamber was 5%.