Thin Solid Fuels Research Articles

Analytical Model for the Downward Flame Spread over a Thermally Thin Fuel into an Opposed Flow

We derive a new analytical expression for the flame front speed in the downward combustion of thin solid fuels based on a model that consists of a preheating region plus a pyrolyzing one. The solid phase is modeled by means of a simple but physically realistic behavior of the main variables in these two regions, whereas the gas phase follows classical reaction–diffusion–convection equations for the fuel mass fraction, oxygen mass fraction, and gas temperature. Both fuel and oxygen mass fractions are expressed in terms of the gas temperature by using a simplified vertical flame model. This approach allows us to reduce the combustion model into a single reaction–diffusion–convection equation of a single variable (gas temperature) for both regions. Matching conditions at the flame leading edge gives us an analytical expression for the burning rate that depends on the kinetics parameters of the combustion reaction and the opposed flow. In contrast with de Ris's classical formula, our analytical expression predicts (1) extinction limits at low values of oxygen concentration and at large values of opposed flow (blowoff), (2) a decrease of the burning rate as gravity increases, and (3) a finite value of the burning rate as the solid thickness tends to zero. A comparison with experimental data is also performed with reasonable results near the extinction limits.

Numerical Modeling of Limiting Oxygen Index Apparatus for Film Type Fuels

A detailed three-dimensional numerical model is used to compute the flow pattern and the flame behavior of thin solid fuels in a rectangular column that resembles a standard Limiting Oxygen Index (LOI) device. The model includes full Navier-Stokes equations for mixed buoyant-forced flow and finite rate combustion and pyrolysis reactions so that the sample LOI can be computed to study the effect of feeding flow rate, sample width and gravity levels. In addition to the above parameters, the sample location in the column and the column cross-sectional area are also investigated on their effect on the ambient air entrainment from the top.

Effect of gas phase heat sink on suppression of downward flame spread over thin solid fuels

A two-dimensional numerical model of opposed flow flame spread over thin solid fuel is formulated and modeled to study effect of gas phase heat sink (a wire-mesh placed parallel to the fuel surface) on the flame spread rate and flame extinction. The work focuses on the performance of the wire-mesh in normal gravity environment of 21 % oxygen concentration. The simulations were carried out for various mesh parameters (wire diameter, ‘\( \overline{d}_{wr} \)’ and number of wires per unit length, ‘N’) and mesh distance perpendicular to fuel surface (Ymesh). Experiments were carried out for wire mesh with \( \overline{d}_{wr} = 0.035\,{\text{cm}} \) and N = 5 and were found to complement numerical predictions. Both simulations and experiments show that wire mesh is effective in reducing flame spread rate when placed at distance less than flame width (which is about 0.50 cm). Mesh wire diameter is determined to not have major influence on heat transfer. However, smaller wire diameter is preferred on account better aerodynamics and for increasing heat transfer surface area (here prescribed by parameter ‘N’). Computations show that meshes with more number of wires per unit length are more effective heat sinks. However, the flame extinction occurs only if mesh is located very close to the fuel surface (of the order of 0.15 cm) and at this distance flame suppression is relatively insensitive to number of wires per unit length.

Gravity modulation study on opposed flame spread over thin solid fuels

In this work a numerical investigation has been carried out to study the effect of g-jitter on zero-gravity (0ge) opposed flow spreading flame over thin solid fuels. For comparison simulations have also been carried out for normal gravity (1ge) downward spreading flames. G-jitter is emulated by gravity modulation of sinusoidal (Agesin(2πt/Tge)) gravity perturbation (g-perturbation) of a particular time-period (Tge) and amplitude (Age) over a selected base gravity level (0ge or 1ge). The response of flames at 0ge base gravity and at 1ge base gravity was different to the imposed g-perturbation. While at 0ge the mean and the amplitude of the oscillatory flame spread rate (FSR) magnified with increase in the time period of g-perturbation, interestingly for the 1ge flame a maximum mean FSR and oscillation amplitude occurs at certain perturbation time period. Further, at very small perturbation time-periods (Tge) the FSR at 1ge was lower than the steady state FSR. The amplitude of oscillatory FSR increased with increase in perturbation amplitude (Age). However, the 0ge flame which is little affected (compared to 1ge flame) at small perturbation amplitude (Age) is affected severely at large perturbation amplitude (Age). Both the gas phase and fuel pyrolysis (or fuel response) follow perturbation signal with a lag but fuel pyrolysis is more sluggish and lags behind gas phase. The phase lag between fuel pyrolysis and gas increases at smaller time-periods (Tge) and tends to enhance the effect of external perturbation whereas at larger time-periods (Tge) this lag inhibits the effect of external perturbation.

On the role of radiation and dimensionality in predicting flow opposed flame spread over thin fuels

In this work a flame-spread model is formulated in three dimensions to simulate opposed flow flame spread over thin solid fuels. The flame-spread model is coupled to a three-dimensional gas radiation model. The experiments [1] on downward spread and zero gravity quiescent spread over finite width thin fuel are simulated by flame-spread models in both two and three dimensions to assess the role of radiation and effect of dimensionality on the prediction of the flame-spread phenomena. It is observed that while radiation plays only a minor role in normal gravity downward spread, in zero gravity quiescent spread surface radiation loss holds the key to correct prediction of low oxygen flame spread rate and quenching limit. The present three-dimensional simulations show that even in zero gravity gas radiation affects flame spread rate only moderately (as much as 20% at 100% oxygen) as the heat feedback effect exceeds the radiation loss effect only moderately. However, the two-dimensional model with the gas radiation model badly over-predicts the zero gravity flame spread rate due to under estimation of gas radiation loss to the ambient surrounding. The two-dimensional model was also found to be inadequate for predicting the zero gravity flame attributes, like the flame length and the flame width, correctly. The need for a three-dimensional model was found to be indispensable for consistently describing the zero gravity flame-spread experiments [1] (including flame spread rate and flame size) especially at high oxygen levels (>30%). On the other hand it was observed that for the normal gravity downward flame spread for oxygen levels up to 60%, the two-dimensional model was sufficient to predict flame spread rate and flame size reasonably well. Gas radiation is seen to increase the three-dimensional effect especially at elevated oxygen levels (>30% for zero gravity and >60% for normal gravity flames).

Experimental Study of the Effects of Side-Edge Burning in the Downward Flame Spread of Thin Solid Fuels

A comparison between the downward flame spread rate for thermally thin samples with one or two inhibited edges is done in multiple situations. The effects of atmospheric composition as well as the width and thickness of a cellulosic-type fuel are tested experimentally. We have found that the normal velocity to the inclined flame front in a side-edge burning is very similar to the downward flame front speed when the sample is inhibited by both edges. Also, the effect of locating a sidewall close to the free edge of the sample is investigated. All these results may be important in order to validate or refute possible models of downward flame spread that take into account side effects.

Effect of Gas Phase Heat Sink on Suppression of Opposed Flow Flame Spread over Thin Solid Fuels in Microgravity Environment

A two-dimensional numerical model of opposed flow flame spread over thin solid fuel is formulated and modeled to study the effect of gas phase heat sink (a wire mesh placed parallel to the fuel surface) on the flame-spread rate and flame extinction. The work focuses on the performance of the wire mesh in microgravity environment at an oxygen concentration 21%. The simulations were carried out for various mesh parameters (wire diameter, “” and number of wires per unit length, “”) and mesh distance perpendicular to fuel surface “”. Simulations show that wire mesh is effective in reducing flame-spread rate when placed at distance less than flame width (which is about 1 cm). Mesh wire diameter is determined not to have major influence on heat transfer. However, smaller wire diameter is preferred for better aerodynamics and for increasing heat transfer surface area (here prescribed by parameter “”). Flame suppression exhibits stronger dependence on number of wires per unit length; however, it is relatively insensitive to number of wires per unit length beyond certain value (here 20 cm−1).

Analytical expressions for the flame front speed in the downward combustion of thin solid fuels and comparison to experiments

We derive analytical expressions for the propagation speed of downward combustion fronts of thin solid fuels with a background flow initially at rest. The classical combustion model for thin solid fuels that consists of five coupled reaction-convection-diffusion equations is here reduced into a single equationwith the gas temperature as the single variable. For doing so we apply a two-zone combustion model that divides the system into a preheating region and a pyrolyzing region. The speed of the combustion front is obtained after matching the temperature and its derivative at the location that separates both regions. We also derive a simplified version of this analytical expression expected to be valid for a wide range of cases. Flame front velocities predicted by our analytical expressions agree well with experimental data found in the literature for a large variety of cases and substantially improve the results obtained from a previous well-known analytical expression.

Thin Film Low Temperature Solid Oxide Fuel Cell (LTSOFC) by Reactive Spray Deposition Technology (RSDT)

The present work describes the effect on the performance of a SOFC when a Gd0.2Ce0.8O1.9 (GDC) layer has been introduced as diffusion barrier layer between the yttria stabilized zirconia (YSZ) electrolyte and the La0.6Sr0.4Co1-xFexO3-δ (LSCF) cathode of an anode supported cell with a Ni-YSZ anode. The dense, thin and fully crystalline GDC films were directly applied by RSDT, without any post-deposition heating or sintering steps. The quality of the film and performance of the cell prepared by Reactive Spray Deposition Technology (RSDT) was compared to a GDC blocking layer deposited by screen printing (SP) and then sintered at 1000 oC. By applying RSDT to deposit the GDC- barrier layer onto the YSZ electrolyte a lower ohmic resistance has been obtained for the RSDT deposited cell vs. the SP cell. The lower resistance can be attributed to the well crystallized, thin and dense GDC layer deposited at 900 oC.

Thin Film Low Temperature Solid Oxide Fuel Cell (LTSOFC) by Reactive Spray Deposition Technology (RSDT)

not Available.

A Computational Study on Opposed Flow Flame Spread Over Thin Solid Fuels with Side-Edge Burning

A steady-state flame spread model has been used to study the effect of side-edge burning on flame spread over thin solid fuel strips of finite width. Simulations have been carried out for fuel strips with both inhibited (by metallic strips) and uninhibited side edges. The effect inhibition on both normal- and microgravity flame spread along with several intermediate gravity levels has been investigated. Such a study is important for understanding the physiochemical processes controlling the flame spread in low gravity where human experience is limited. Although simulations have shown an overall increase in spread rate for uninhibited cases for both normal- and microgravity flames, some effects such as flame spread variation with external imposed velocity and flame extinction limits show different behavior for microgravity and normal gravity flames. The heat and mass transport processes in the flame have been discussed in detail to explain the observed trends.

Opposed Flow Flame Spread Over Thin Solid Fuels: A 2D Numerical Study on Flame-Spread Transition from Normal Gravity to Zero Gravity and Vice Versa

An unsteady two dimensional numerical model of flame spread is formulated to study the transient flame spread over thin solid fuels. The work focuses on the study of transient flame spread from a steady normal gravity flame to micro gravity flame and vice versa. The gas-phase is described by two-dimensional governing equations comprising of full Navier-Stokes equations for laminar flow along with the conservation equations of mass, energy and species. The specie equations are for fuel vapor, oxygen, carbon dioxide and water vapor. A one-step, second-order finite rate Arrhenius reaction between fuel vapor and oxygen is assumed. The thin solid fuel model comprises of equations of continuity and energy in the one-dimension parallel to the solid fuel surface along with a solid fuel pyrolysis law. The solid fuel considered here is an aerodynamically and thermally thin cellulosic material. The solid is assumed to burn ideally i.e. it vaporizes to form fuel vapors without melting. The radiation transport is mode...

Pattern formation in flame spread over thin solid fuels

The fingering char pattern emerging on the surface of thin cellulosic sheets burning against an oxidizing wind is discussed. Employing collocation-based averaging, the assumption of diffusive-thermal equilibrium, the strong temperature dependence of the reaction rate, and the strong disparity between the densities of the solid and gaseous phases, an elementary two-dimensional free-interface model for the flame spread is formulated. It is shown that the pattern-forming dynamics is functionally akin to the well-studied cellular instability occurring in low Lewis number premixed gas flames.

The Opposed-Flow Flame Spread over Thin Solid Fuel in Finite Space under Normal and Microgravity

The Opposed-Flow Flame Spread over Thin Solid Fuel in Finite Space under Normal and Microgravity

Ni-YSZ Cermet Supported Thin Ceria-Based Electrolyte Solid Oxide Fuel Cell for Reduced Temperature Operation

not Available.

Ni-YSZ Cermet Supported Thin Ceria-Based Electrolyte Solid Oxide Fuel Cell for Reduced Temperature (500–600°C) Operation

A Ni-YSZ cermet supported thin ceria-based electrolyte solid oxide fuel cell based on tape casting, screen-printing and co-firing technologies has been fabricated and characterized. Studies were made of the anode substrate, paste solvent and binder, powder loading, co-firing profiles as well as cathode composition and firing conditions. Interaction between YSZ and SDC was investigated by EDX analysis. Cell performance of different electrolyte thicknesses and cathode firing conditions at low temperatures are also presented.

E152 バイオマス燃料の燃焼過程に関する実験的研究

Combustion processes contain chemical reactions with heat release and hydrodynamic instability. Especially, solid combustion is much more complicated than gas combustion. Therefore, the physico-chemical mechanism has not been clarified fully. More recently, there have been a few investigations on thin solid fuels without flame. The biomass fuel which is energy which I can revitalize for fossil fuel attracts attention. We think that solid fuel including biomass fuel gives a combustion form bug influence because heterogeneousness is strong. In the present study, we used a biomaterial sheet of wood system as fuel and examined the influence that heterogeneousness of fuel gave a combustion form.

Effects of Velocity Boundary Layer on Opposed Flame Spread over a Thermally Thin Solid Fuel in Microgravity.

Upstream (opposed) flame spread over a thermally thin cellulosic sheet in various imposed flow speed and ambient oxygen concentration under microgravity environment is studied numerically. Two-dimensional time-dependent phenomena, from spontaneous ignition to the flame spread in a fully-developed velocity boundary layer formed over a flat cellulosic sheet, are simulated with a well-developed numerical model. The model predicts that the upstream flame spread in the boundary layer is essentially time-dependent phenomena, and the unsteadiness is pronounced when the imposed flow speed is very slow/high and lower oxygen concentration. This unsteady flame spread behavior is due to the boundary layer effect ; when the flame spreads to upstream (i. e. thin boundary layer zone), local flow speed at the flame front is not constant but increased. The local flow speed has excellent correlation with the local flame spread rate. Plots of the flame temperature vs. the local flame spread rate show their linear relationship for all conditions considered in the present study, indicating that classical deRis's theory still be valid for present unsteady (quasi-steady) flame spread mode.

Flame spread over thin solid fuels in partially premixed atmospheres

Experiments on flame spread rates ( S f ) over thin solid fuels in oxidizing atmospheres to which gaseous fuel is added reveal a large increase in S f for some gaseous fuels. The most to least effective fuels tested are, in order, H 2, CO, hydrocarbons, and NH 3. From these experiments it is inferred that, unlike nonpremixed flame spread, the finite-rate kinetics of the gaseous fuel have a dominant influence on S f even far from extinction conditions. A simple model developed to account for the effect of the gaseous fuel on S f , while still retaining the framework of deRis's model of S f for infinite-rate kinetics of the solid fuel vapors, shows good qualitative and fair quantitative agreement with experimental data. Furthermore, it is shown that for some fuels, e.g., CO, the gaseous products of partial combustion produce a more combustible environment than the fresh atmosphere; the practical implications of this finding for modeling fires in enclosures are discussed.

Flame spread over thin solid fuels in partially premixed atmospheres

Flame spread over thin solid fuels in partially premixed atmospheres