Pyrolysis Front Research Articles

Simulation of ablation in composites via an interface tracking method

This work is an extension of a previous work where the two-dimensional computational simulation of the ablative process in composites for rocket thermal protection systems was presented. The numerical method employs an interface tracking method to simulate the moving front problem that appears in ablation. Two moving fronts were considered, the char layer formation and the pyrolysis front. A complementary model that considers temperature dependence for the reaction rate in the pyrolysis is also tested. The results are compared with some experimental data for quartz-phenolic and carbon-phenolic resin samples exposed to a plasma jet, showing a much better agreement than the traditional one-front mode, allowing a more accurate dimensioning of rocket thermal protection systems.

Prediction of Three-Dimensional Downward Flame Spread Characteristics over Poly(methyl methacrylate) Slabs in Different Pressure Environments.

The present study is aimed at predicting downward flame spread characteristics over poly(methyl methacrylate) (PMMA) with different sample dimensions in different pressure environments. Three-dimensional (3-D) downward flame spread experiments on free PMMA slabs were conducted at five locations with different altitudes, which provide different pressures. Pressure effects on the flame spread rate, profile of pyrolysis front and flame height were analyzed at all altitudes. The flame spread rate in the steady-state stage was calculated based on the balance on the fuel surface and fuel properties. Results show that flame spread rate increases exponentially with pressure, and the exponent of pressure further shows an increasing trend with the thickness of the sample. The angle of the pyrolysis front emerged on sample residue in the width direction, which indicates a steady-burning stage, varies clearly with sample thicknesses and ambient pressures. A global non-dimensional equation was proposed to predict the variation tendency of the angle of the pyrolysis front with pressure and was found to fit well with the measured results. In addition, the dependence of average flame height on mass burning rate, sample dimension and pressure was proposed based on laminar diffusion flame theory. The fitted exponent of experimental data is 1.11, which is close to the theoretical value.

Upward flame spread over discrete fuels

Upward flame spread over discrete fuels has been analyzed through experiments on vertical arrays of alternating lengths of PMMA and inert insulation board. By manipulating the lengths of the PMMA fuel and the insulation, trends relating flame spread to fuel loading were assessed.The peak upward flame spread rate was observed in non-homogeneous fuel arrays where the fraction of exposed area consisting of fuel (i.e., the fuel coverage f) was below unity. A maximum flame spread rate was observed for f=0.67, possibly due to a delayed thickening of the boundary layer or increased air entrainment. In arrays with f≤0.5, the flame spread rate decreased; in fact, deceleration of the pyrolysis front was observed. This behavior indicates that a homogeneous fuel bed approximation, which might be applicable when f is near unity, would be highly unsuitable for arrays with low fuel coverage. Trends for the mass fluxes and flame heights were also assessed, and it was noted that the mass loss rate per burning area was negatively correlated with f. This provides further support to the hypothesis that decreased thickening of the boundary layer, which would lower the flame standoff distance, plays a causal role.A method for approximation of the fuel spread rate was also proposed. This estimate requires reasonable estimates for the homogeneous flame spread rate and the lowest fuel coverage value that sustains spread.

Experimental study of vertically upward flame spread over polymethyl methacrylate slabs at different altitudes

SummaryExperiments of vertically upward flame spread over polymethyl methacrylate slabs were conducted in Hefei (with an altitude of 29.8 m) and Lhasa (with an altitude of 3658.0 m). Measurements were taken for the flame heights, the flame heat flux to the fuel surface and the flame spread rate. Two regions were identified for the dependence of the flame height on the heat release rate per unit width . When is less than 22 kW/m, the flame height scales as while it scales as , when is greater than 22 kW/m. The flame heights in Lhasa are approximately 1.34 and 1.25 times, respectively, of those in Hefei for these two regions. The flame heat flux to the fuel surface decreases significantly from the pyrolysis front to the flame tip, whereas it decreases slowly above the flame tip. In both regions, it can be correlated reasonably well with (x − xp)/(xf − xp) using the form of . The flame heat flux to the fuel surface in Lhasa is approximately 0.75 times of that in Hefei. The flame spread process can be divided into three stages, which correspond to a flow region of laminar, transitional, and turbulent, respectively. The transition to a turbulent flow is delayed in Lhasa compared with Hefei. The flame spread rate in Lhasa is about half of that in Hefei because of the lower flame heat flux caused by the lower ambient pressure. Copyright © 2015 John Wiley & Sons, Ltd.

Radiative pyrolysis of wet wood under intermediate heat flux: Experiments and modelling

An experimental study of thermally thick biomass (beech wood spheres) pyrolysis under high radiative heat flux was performed. The influence of sample diameter (5–20mm), incident heat flux (60–180kW/m2) and initial moisture content (1–50wt%) was studied. Char yields and temperature histories were monitored. Initial moisture content impact was highlighted. Indeed, steam coming from the sample core drying can gasify the external char layer, reducing therefore the char yield and increasing syngas production. This study was supported by a 2D unsteady numerical model of biomass degradation (mass, momentum and heat conservation coupled with Broido-Shafizadeh reaction scheme). This model gave more insight about phenomena occurring inside the degrading sample. It revealed that a pyrolysis front follows up a drying one. Therefore, steam is forced out of the sample through a high temperature char layer, making char steam gasification chemically possible.

SIMULATION OF ABLATION IN A SOUNDING ROCKET TPS VIA AN INTERFACE TRACKING METHOD WITH TWO MOVING FRONTS

In this work, an interface tracking method is employed to simulate the ablative process in the region near the stagnation point of the VSB-30 sounding rocket Thermal Protection System (TPS). The ablation model considers the presence of two-fronts, the char-melting and the pyrolysis fronts. The results for the proposed model are compared to the ones obtained from the traditional one-front model, which supposes a constant ablation temperature. Results show that the one-front model overestimates the ablation period, mass loss and the internal temperature after the flight. The increase in the accuracy with this model shall provide a better dimensioning of the TPS, reducing its weight and cost.

Experimental study on flame spread behavior along poly(methyl methacrylate) corner walls at different altitudes

The effects of altitude and intersection angle on the flame spread behavior and pyrolysis front characteristics along corner walls were experimentally studied. The experiments were conducted using mock corners made of poly(methyl methacrylate) slabs with intersection angles varying from 60° to 120° at two altitudes of 29.8 and 3658.0 m. Measurements were taken for the upward and lateral flame spread rates, the flame heights, the flame heat flux to the fuel surface, and the mass loss rates of the tested slabs. An “M”-shaped pyrolysis front was observed for all the intersection angles examined at both altitudes. The upward flame spread rate increases with decreasing intersection angles while no obvious trend was found for lateral spread rate when the intersection angle is varied. The high altitude leads to a spread rate about half of that at the lower altitude both for upward and lateral spread irrespective of the intersection angle. The measured mass loss rates are significantly higher than twice of that over a flat vertical slab.

Experimental and quantitative assessment of flame retardancy by the shielding effect in layered silicate epoxy nanocomposites

A quantitative experimental assessment of flame retardancy by the heat shielding in epoxy layered silicate nanocomposite (EP/TPPMMT) is presented. Online heat flux measurements and temperature monitoring within the specimen are performed during the burning in the cone calorimeter. For EP the surface layer equals a pyrolysis front. The reradiation by the hot surface corresponds to the fourth power of the pyrolysis temperature. The surface reradiation (around 10kWm−2) is thus fairly invariable over burning time and different external heat fluxes. Further, the thermal feedback of the flame is approximated to 20kWm−2 for both EP and EP/TPPMMT and invariable over different irradiations. Thus the net heat fluxes transformed to the fuel release rate within the pyrolysis front of EP are increased to 45–80kWm−2 when irradiations of 35–70kWm−2 are applied. For a residue-forming EP/TPPMMT the surface temperature and thus the reradiation (42–68kWm−2) crucially increases compared to EP and with increasing irradiation. The net heat fluxes are reduced to 13–22kWm−2 accordingly. This quantitative assessment of the heat shielding in EP/TPPMMT goes along with proportional and consistent improvement in the fire performance, such as the pyrolysis front velocity, the heat release rate (HRR) characteristics such as averaged and quasi-steady-state HRR and the peak HRR (PHRR). The heat shielding is proven to be the only major flame retardancy effect occurring in nanocomposites based on non-charring polymers.

Upward flame spread over corrugated cardboard

As part of a study of the combustion of boxes of commodities, rates of upward flame spread during early-stage burning were observed during experiments on wide samples of corrugated cardboard. The rate of spread of the flame front, defined by the burning pyrolysis region, was determined by visually averaging the pyrolysis front position across the fuel surface. The resulting best fit produced a power-law progression of the pyrolysis front, x p = At n , where x p is the average height of the pyrolysis front at time t, n = 3/2, and A is a constant. This result corresponds to a slower acceleration than was obtained in previous measurements and theories (e.g. n = 2), an observation which suggests that development of an alternative description of the upward flame spread rate over wide, inhomogeneous materials may be worth studying for applications such as warehouse fires. Based upon the experimental results and overall conservation principles it is hypothesized that the non-homogeneity of the cardboard helped to reduce the acceleration of the upward spread rates by physically disrupting flow in the boundary layer close to the vertical surface and thereby modifying heating rates of the solid fuel above the pyrolysis region. As a result of this phenomena, a distinct difference was observed between scalings of peak flame heights, or maximum “flame tip” measurements and the average location of the flame. The results yield alternative scalings that may be better applicable to some situations encountered in practice in warehouse fires.

Modeling Kinetics of Cd, Pb, and Zn Vaporization during Municipal Solid Waste Bed Incineration

The fate of heavy metals contained in municipal solid waste is of major concern in the incineration process. This study is the first attempt to analyze the kinetic behavior of Cd, Pb, and Zn in a waste bed burning on the grate of an incinerator. The vaporization rates of the three heavy metals were derived from laboratory experiments. The kinetic law was then introduced into Garbed-ss, a mathematical model of the on-grate incineration of a refuse bed. Results show that the calculated vaporization of the three metals is fast, proceeds to nearly full completion, occurs at the pyrolysis front, and is indeed controlled by the thermal degradation of the waste. Alhough in agreement with thermodynamic calculations, the vaporization of lead and zinc seems to be overestimated. The model would be improved by accounting for the initial physical and chemical speciation of the heavy metals in the waste feed.

Limiting Length, Steady Spread, and Nongrowing Flames in Concurrent Flow Over Solids

A detailed two-dimensional transient model has been formulated and numerically solved for concurrent flames over thick and thin solids in low-speed forced flows. The processes of flame growth leading to steady states are numerically simulated. For a thick solid, the steady state is a nongrowing stationary flame with a limiting length. For a thin solid, the steady state is a spreading flame with a constant spread rate and a constant flame length. The reason for a nongrowing limiting flame for the thick solid is the balance between the flame heat feedback and the surface radiative heat loss at the pyrolysis front, as first suggested by Honda and Ronney. The reason for achieving a steady spread for thin solids is the balance between the solid burnout rate and the flame tip advancing rate. Detailed transient flame and thermal profiles are presented to illustrate the different flame growth features between the thick- and thin-solid fuel samples.

Application of a simple enthalpy‐based pyrolysis model in numerical simulations of pyrolysis of charring materials

AbstractA new, simple pyrolysis model for charring materials is applied to several numerical and experimental test cases with variable externally imposed heat fluxes. The model is based on enthalpy. A piecewise linear temperature field representation is adopted, in combination with an estimate for the pyrolysis front position. Chemical kinetics are not accounted for: the pyrolysis process takes place in an infinitely thin front, at the ‘pyrolysis temperature’. The evolution in time of pyrolysis gases mass flow rates and surface temperatures is discussed. The presented model is able to reproduce numerical reference results, which were obtained with the more complex moving mesh model. It performs better than the integral model. We illustrate good agreement with numerical reference results for variable thickness and boundary conditions. This reveals that the model provides good results for the entire range of thermally thin and thermally thick materials. It also shows that possible interruption of the pyrolysis process, due to excessive heat losses, is automatically predicted with the present approach. Finally, an experimental test case is considered. Copyright © 2009 John Wiley & Sons, Ltd.

An embedded upward flame spread model using 2D direct numerical simulations

A fully coupled 2D fluid–solid direct numerical simulation (DNS) approach is used to simulate co-flow flame spread over poly(methyl methacrylate) (PMMA) at different angles of inclination. Comparison of simulations and experimental measurements are conducted over a range of flame spread rates. Results show that the heat flux to the preheating region varies considerably in time — contradicting often employed assumptions used in established flame spread theories. Accounting for the time dependent behavior is essential in accurate predictions of flame spread, however, a universal characterization in terms of easily defined parameters is not found. Alternatively, a reaction progress variable based embedded flame model is developed using mixture fraction, total enthalpy and surface temperature. State maps of the gas-phase properties and surface heat flux are constructed and stored in pre-computed lookup tables. The resulting model provides a computationally efficient and a local formulation to determine the flame heat flux to the surface resulting in excellent agreement to DNS and experiments for predictions of flame spread rate and position of the pyrolysis front.

Width effects on the early stage of upward flame spread over PMMA slabs: Experimental observations

Experiments were carried out to study the effects of sample width on the early upward flame spread, when typically the heat release rate per unit width ( Q ˙ ′ ) is less than 30 kW/m. Samples of clear poly-methyl-methacrylate ( PMMA) were 0.4 m high and 0.03 m thick, and width values of 0.025, 0.050, 0.1, 0.15, and 0.2 m were tested. A new processing method, based on wavelet decomposition, was developed to detect the pyrolysis front location from video recordings of the bubbling surface. Different propagation behaviours are observed depending on the slab width, w. For w⩾0.1 m, flame height, rate of spread ( ros), and Q ˙ ′ are found to be width-independent. The flame seems to remain laminar during the entire spreading process and flame height measurements are found to be in good agreement with available literature data. For the narrower slabs (0.025 and 0.05 m), a transition from laminar to turbulent flame is observed, the value of Q ˙ ′ at which the transition occurs decreasing with the width of the slab. Moreover the spread rate decreases with the slab width as a result of reduction in flame height due to three-dimensional aerodynamic effects. For w=0.05 m, flame height decreases whereas Q ˙ ′ is not affected. This may be related to the lateral entrainment of air above the pyrolysis region. For w=0.025 m, three-dimensional effects extend to the burning region. A significant decrease in the heat release rate per unit width is then observed, which in turn leads to a reduction in flame height. This configuration, where the flame tip is very close to the pyrolysis front, is of particular interest for fire safety since it corresponds to near-critical conditions for flame spread.

Carbon emissions from smouldering peat in shallow and strong fronts

A series of experiments of shallow and strong smouldering fronts in boreal peat have been conducted under laboratory conditions to study the CO and CO 2 emissions. Peat samples of 100 mm by 100 mm in cross section and 50 mm in depth were smouldered in the cone calorimeter apparatus. Two laboratory variables, moisture content and the external heat flux, are varied over a wide range of values to establish different burning rates and front thicknesses. This provides a novel framework to study smouldering dynamics by varying the controlling mechanisms and providing burning conditions that otherwise cannot be obtained. Measurements of the burning rate and gas flow, yields and ratio for CO and CO 2 are reported at steady state burning conditions. Average mass flow rates per area of smouldering front are reported here for the first time to be 0.27 g/s/m 2 for CO and 0.65 g/s/m 2 for CO 2. This CO 2 mass flux is about 3000 times larger that the natural decomposition flux from peatlands. The CO yield in dry base is 17% g/g and the CO 2 yield 42% g/g. The CO and CO 2 total yield is of 59% g/g, and the CO to CO 2 ratio was on average 0.43. The results indicate that peat with high moisture content smoulders producing larger CO 2 yield but the same CO yield compared to dryer peat. This suggests that smouldering of biomass at lower moisture contents develops wider pyrolysis fronts that release a larger fraction of other carbon-containing gas species.

Determination of pyrolysis temperature for charring materials

An energy and mass balanced method of determining the pyrolysis temperature is proposed. The concept is to find the pyrolysis temperature that consumes the same amount of energy to produce the same amount of mass when using the pyrolysis front model as when using finite rate kinetics models for the entire charring process. The resulting pyrolysis temperature has the form of pyrolysis rate weighted average temperature. Comparisons between finite rate kinetics and pyrolysis front models for various boundary conditions, geometries, heats of decomposition, kinetic parameters and assumptions used in the literature were made to assess the proposed method. Models using energy and mass balanced pyrolysis temperature show good agreement with finite rate models and the experiments. Extensive numerical studies on various factors influencing the charring material pyrolysis show that heat flux, sample size, heat of decomposition and kinetic parameters are the most important factors for determining an appropriate pyrolysis temperature. Thermal conductivity, specific heat and density have a lesser effect on the pyrolysis temperature. For practical application, a non-dimensional correlation is developed to determine the appropriate pyrolysis temperature without solving the problem by using finite rate models. With this correlation the energy and mass balanced pyrolysis temperature can be determined with a standard deviation of 7.6 K. These predictions are validated by comparison with measurements of wood cylinder pyrolysis. A good agreement suggests that simpler pyrolysis front models yield practically useful and accurate results given an appropriate pyrolysis temperature.

Flame Spread Analysis Using a Variable B-Number

Flame spread over solid materials is commonly described in the literature by a two-dimensional reactive boundary layer solution first formulated by Emmons (1956). In the recent past, experimental measurements associated with material flammability testing (e.g. NASA-STD-6001) are compared with the Emmons solution, as modified by Pagni and Shih, and found in disagreement. In the classical solution the B-number (Spalding mass transfer number) appears as one of the boundary conditions, and has been traditionally assumed a constant for mathematical simplicity. However, experimental results show that the B- number is not a constant, but varies with time and space. A simple modification to the standard measurement procedure is described, which allows calculation of the B- number from the flame stand-off distance. Measurements of flame spread on solid fuels are performed to determine the flame and pyrolysis front spread rates, as a function of time, and the flame stand-off distance as a function of space and time. The classical combustion problem is revisited to model flame spread over a combustible surface, including three-dimensional effects. An experimentally- obtained B-number, variable in time and space, is used to model flame propagation, showing good agreement with current and previous experimental data obtained from the literature

Numerical analysis of the heating process in upward flame spread over thick PMMA slabs

A detailed analysis of the unburned material heat-up during upward flame spread over small slabs of PMMA is provided using a numerical model. The two-dimensional time-dependent Favre-averaged Navier–Stokes equations coupled with sub-models for turbulence, combustion, soot formation, and radiation are solved for the gas phase. The modelling of condensed phase processes is based on the one-dimensional heat conduction equation and pyrolysis is treated as a phase change using the latent heat approach. Pyrolysis heights, x p, up to 0.6 m are considered. Model results agree with data on the pyrolysis front trajectory and heat release rate per unit width, Q ˙ ′ , as a function of x p from experiments in which the lateral air entrainment is expected to have no significant effect on flame spread. It is shown that the average luminous flame height, x fl, defined by the height at which the total wall heat flux is equal to 10 kW/m 2, correlates with Q ˙ ′ 1.25 for Q ˙ ′ < 20 kW / m , whereas it correlates with Q ˙ ′ 2 / 3 for Q ˙ ′ 20 kW / m , as deduced from experiments. Appropriate scaling of the total wall heat flux distribution is found in terms of ξ = ( x - x p ) / ( x fl - x p ) . This scaling allows us to locate the continuous flame region between ξ=0 and 0.4 and the plume region above ξ=1.6, the intermittent flame region being located between the two. Both continuous and intermittent flames are found to play an important role in the process of heating the unburned solid fuel whereas the plume contribution is negligible.

Numerical simulation of compressible gas flow and heat transfer in a microchannel surrounded by solid media

In this study, a numerical model is developed to investigate the coupled compressible gas flow and heat transfer in a microchannel surrounded by solid media. To accommodate the varying flow cross-section, the compressible gas flow model is established in a non-orthogonal curvilinear coordinate system. An iterative numerical procedure is employed to solve the coupled heat transfer and gas flow equations. The computer code for the compressible gas flow is first validated against two test problems, and then extended by including the heat conduction in the solid media. The effect of the inlet Mach number on the Nusselt number is examined. It is found that the pressure difference from the pyrolysis front to the heated surface is induced essentially by the gas addition from the channel wall, instead from the pyrolysis front. The necessity of accounting for the gas compressibility is clearly demonstrated when severe heating is applied. The pressure distribution obtained along the channel axial direction is useful for further structural analysis of composite materials.

Behavior of Flame Spread Downward over Thick Wood Sheets and Heat Transfer Analysis

To investigate the flame spreading mechanism over charring materials under natural convection, wood sheets from 0.9 to 3 mm thickness have been chosen. The variation of mean flame spread velocity V with thickness δ is measured. Typical flame shapes over the wood surfaces are recorded using CCD. The temperature fields in the gas phase are examined by a set of fine thermocouples. On the basis of the results, the relative contributions of local heat transfer modes ahead of the pyrolysis front have been evaluated. It is found that heat conduction through solid phase increases notably with δ. A flame spreading model coupled with a pyrolysis structure is proposed.