Experimental study of burning rates of cardboard box fires near sea level and at high altitude

To study the difference of solid fuel fire characteristics at different altitudes, two series of fire experiments of cardboard boxes filled with shredded office paper were conducted separately in Lhasa (altitude: 3650 m; air pressure: 65 kPa) and Hefei (altitude: 24 m, air pressure: 100.8 kPa), using a specially designed igniter. The measured parameters in the experiments include mass loss and flame axis temperature. Fuel load quantity and configuration were varied in the experiments. The results of the study indicate a likelihood that an ignition will result in smoldering fire at the high altitude. There is also likelihood that incipient phase may occur before the onset of full flaming combustion regardless of the altitude. The fuel mass loss fraction of flaming fires was found to follow a simple form of correlation with time when normalized over the half fuel consumption time, which was found to be inversely proportional to four-third power of the ambient pressure.     

Piloted ignition and extinction for solid fuels

Piloted ignition of solid fuels is investigated by simulating the transport and chemical reaction in a counter-flow arrangement where a known fuel (methane) is supplied through a porous burner and the power and the location of the igniter are varied. The porous burner arrangement simulates a pyrolyzing solid fuel at constant temperature by separating the gas phase from the solid conduction and pyrolysis phenomena. An Arrhenius one-step global reaction and a simplified transport model with Lewis number equal to one were used in the simulation. Only quasi-steady conditions are considered for the gas phase in this work because the response time for the solid phenomena is, in general, much larger than the response diffusion time for the gaseous phenomena. The relation of piloted ignition to extinction is also investigated. The effect of Damköhler number on ignition and extinction and the effect of the igniter on ignition are presented through a characteristic S curve obtained by plotting the evolving maximum temperature as a function of fuel mass flux. Based on the S-shaped curve (representing the maximum temperature in the system versus the mass flux of fuel), the relationship between the piloted ignition and extinction turning points and mass fluxes has been demonstrated in this paper. The piloted ignition turning point gradually approaches the extinction turning point with increasing Damköhler number and also with increasing power of the igniter. The ignition mass flux is found to depend basically on three parameters, Damköhler number, the location of the igniter and the power of the igniter all expressed in dimensionless forms.     

Ignition behavior of pure and blended methyl octanoate,n-nonane,and methylcyclohexane

Excited-state species profiles and ignition delay times were obtained under dilute conditions (99% Ar) using a heated shock tube for methyl octanoate (C₉H₁₈O₂), n-nonane (n-C₉H₂₀), and methylcyclohexane (MCH) over a broad range of temperature and equivalence ratio (? = 0.5, 1.0, 2.0) at pressures near 1 and 10 atm. Measurements were then extended to include two ternary blends of the fuels using 5% and 20% (vol.) of the methyl ester under stoichiometric conditions. Using three independently validated chemical kinetics mechanisms, a model was compiled to assess the influence of methyl ester concentration on ignition delay times of the ternary blends. Under near-atmospheric pressure, ignition delay times were of the following order for the pure fuels: methyl octanoate < n-nonane < methylcyclohexane. Experimental results indicate that the ignition behavior of the higher-order methyl ester approaches that of the higher-order linear alkane with increased pressure regardless of equivalence ratio. Methyl octanoate also displayed significantly lower pressure dependence relative to the linear alkane and the cycloalkane species. Both of these results are supported by model calculations. Blending of methyl octanoate with n-nonane and methylcyclohexane impacted ignition delay time results more strongly at 1.5 atm, yet had only a small effect near 10 atm for temperatures above 1400 K. The study provides the first shock-tube data for a ternary blend of a linear alkane, a cycloalkane, and a methyl ester. Ignition delay time measurements for the C₉:0 methyl ester were also measured for the first time.     

Evolution of flame to surface heat flux during upward flame spread on poly(methyl methacrylate)

The heat feedback profile across 5 cm wide and 15 cm tall samples of poly(methyl methacrylate) was studied from ignition until total sample involvement as a flame spread vertically upward. Incident heat flux to a water-cooled gauge was measured at 1 cm intervals. At 6–15 cm above the bottom edge of the flame, the maximum heat flux value was found to be on the order of 35 kW m^?2. Lower in the sample, 2–5 cm above the flame bottom, where the flame is thinner and thus closer to the sample’s surface, the maximum heat flux is slightly higher, about 40 kW m^?2. Using these results and finely resolved measurements of sample burning rate recorded throughout the length of experiments, an analytical model that accurately predicts the measured heat flux profile along the vertical dimension of samples solely as a function of the burning rate was developed. Coupling this model with an accurate pyrolysis solver, which predicts material burning rate based on incident heat flux, is expected to enable highly accurate simulations of the flame spread dynamics.     

Oxygen permeation characteristics of zirconia with surface modification

Yttria-stabilized zirconia (YSZ) can be used as an oxygen-permeating membrane at elevated temperature (> 1400 °C) due to its chemical and mechanical stability. It was previously shown that the oxygen transport through YSZ membrane in reducing oxygen partial pressure (P_O2) was highly influenced by the surface-exchange kinetics that can be improved by porous surface coating layers such as YSZ, GDC (Gd-doped ceria) or YSZ–GDC mixture [H.J. Park, G.M. Choi, J. Eur. Ceram. Soc. 25 (2005) 2577]. However, the increased oxygen flux was still lower than that estimated assuming bulk-diffusion limit and rapidly decreased with time due to the sintering of coating layers and the reaction between bulk YSZ and coating layers. In this study, the oxygen fluxes through YSZ with LaCrO₃, GDC + LaCrO₃ (bilayer), LaCrO₃ + 5 wt.% GDC (mixture), or LaCr_0.7Co_0.3O₃ coatings were measured under controlled P_O2 gradient (permeate-side P_O2: ∼ 3 × 10^− 12 atm, feed-side P_O2: 2 × 10^− 10–2 × 10^− 8 atm) at 1600 °C. The oxygen flux drastically increased with these coatings. The highest increase in oxygen flux was shown with GDC + LaCrO₃ (bilayer) coating and was maintained for a long time. The presence of highly catalytic Ce ions while maintaining porous structure in the coating layer may explain the observation. The prevention of formation of resistive layer due to ceria coating may also be partly responsible for the observation.     

Experimental study of minimum ignition energy of lean H2-N2O mixtures

Ignition energies for short duration (<50 ns) spark discharges were measured for undiluted and nitrogen-diluted H₂-N₂O mixtures of equivalence ratios ? = 0.15 and 0.2, dilution of 0% and 20% N₂, and initial pressures of 15–25 kPa. The ignition events were analyzed using statistical tools and the probability of ignition versus spark energy density (spark energy divided by the spark length) was obtained. The simple cylindrical ignition kernel model was compared against the results from the present study. Initial pressure has a significant effect on the width of the probability distribution, ranging from a broad (P = 15 kPa) to a narrow (P = 25 kPa) probability distribution indicating that the statistical variation of median spark energy density increases as initial pressure of the mixture decreases. A change in the equivalence ratio from 0.15 to 0.2 had a small effect on the median spark energy density. The addition of 20% N₂ dilution caused a significant increase in the median spark energy density when compared to no dilution. The extrapolation of the present results to atmospheric pressure, stoichiometric H₂-N₂O indicates that the electrostatic discharge ignition hazards are comparable to or greater than H₂-O₂ mixtures.     

The diffusion flame structure of an ammonium perchlorate based composite propellant at elevated pressures

Examination of the surface behavior and flame structure of a bimodal ammonium perchlorate (AP) composite propellant at elevated pressure was performed using high speed (5 kHz) planar laser-induced fluorescence (PLIF) from 1 to 12 atm and visible surface imaging spanning 1–20 atm. The dynamics of the combustion of single, coarse AP crystals were resolved using these techniques. It was found that the ignition delay time for individual AP crystals contributed significant to the particle lifetime only at pressures below about 6 atm. In situ AP crystal burning rates were found to be higher than rates reported for pure AP deflagration studies. The flame structure was studied by exciting OH molecules in the gas phase. Two types of diffusion flames were observed above the composite propellant: jet-like flames and v-shaped, inverted, overventilated, flames (IOF) lifted off the surface. While jet-like diffusion flames have been imaged at low pressures and simulated by models, the lifted IOFs have not been previously reported or predicted. The causes for the observed flame structures are explained by drawing on an understanding of the surface topography and disparities in the burning rates of the fuel and oxidizer.     

Experimental and modeling study on the pyrolysis and oxidation of iso-octane

High pressure iso-octane shock tube experiments were conducted to assist in the development of a Jet A surrogate kinetic model. Jet A is a kerosene based jet fuel composed of hundreds of hydrocarbons consisting of paraffins, olefins, aromatics and naphthenes. In the formulation of the surrogate mixture, iso-octane represents the branched paraffin class of hydrocarbons present in aviation fuels like Jet A. The experimental work on iso-octane was performed in a heated high pressure single pulse shock tube. The mole fractions of the stable species were determined using gas chromatography and mass spectroscopy. Experimental data on iso-octane oxidation and pyrolysis were obtained for temperatures from 835 to 1757 K, pressures from 21 to 65 atm, reactions times from 1.11 to 3.66 ms, and equivalence ratios from 0.52 to 1.68, and ∞. Iso-octane oxidation showed that the fuel decays through thermally driven oxygen free decomposition at conditions studied. This observation prompted an experimental and modeling study of iso-octane pyrolysis using an iso-octane sub-model taken from a recently published n-decane/iso-octane/toluene surrogate model. The revised iso-octane sub-model showed improvements in predicting intermediate species profiles from pyrolytic experiments and oxidation experiments. The modifications to the iso-octane sub-model also contributed to better agreement in predicting the formation of carbon monoxide and carbon dioxide when compared to the recently published 1st Generation Surrogate model and a recently published iso-octane oxidation model. Model improvements were also seen in predicting species profiles from flow reactor oxidation experiments and ignition delay times at temperatures above 1000 K at both 10 and 50 atm.     

A study on the ignition characteristics for dust explosion of industrial wastes

Paying attention to accidents related to recycling, the authors investigated factors related to the dust explosions. The minimum explosive concentrations were 30–40 g/m³ (polyurethane and plastic dusts) and 70 g/m³ (toner); the explosibility was reduced by about 40% when combustible gas (cyclopentane) existed together with the dust; ignition temperature of a dust cloud was around 500 °C; and the minimum ignition energy was about 11 mJ (polyurethane). Ignition energy was the minimum when the spark duration was about 0.2 ms or more; the explosion probability was the maximum when the spark gap was 5–10 mm; and the ignition energy was the minimum when the electrode tip angle was around 30 °C (polyurethane). From these findings, the dusts produced in the recycling process can be easily evaluated so as to know whether dust explosion is likely to occur.     

Effects of NO2 addition on hydrogen ignition behind reflected shock waves

Ignition delay time measurements of H₂/O₂/NO₂ mixtures diluted in Ar have been measured in a shock tube behind reflected shock waves. Three different NO₂ concentrations have been studied (100, 400 and 1600 ppm) at three pressure conditions (around 1.5, 13, and 30 atm) and for various H₂–O₂ equivalence ratios for the 100 ppm NO₂ case. Results were compared to some recent ignition delay time measurements of H₂/O₂ mixtures. A strong dependence of the ignition delay time on the pressure and the NO₂ concentration was observed, whereas the variation in the equivalence ratio did not exhibit any appreciable effect on the delay time. A mechanism combining recent H₂/O₂ chemistry and a recent high-pressure NOx sub-mechanism with an updated reaction rate for H₂ + NO₂ ? HONO + H was found to represent correctly the experimental trends over the entire range of conditions. A chemical analysis was conducted using this mechanism to interpret the experimental results. Ignition delay time data with NO₂ and other NOx species as additives or impurities are rare, and the present study provides such data over a relatively wide pressure range.     

Phase stability and oxygen non-stoichiometry of SrCo0.8Fe0.2O3−δ measured by in situ neutron diffraction

The phase stability, oxygen stoichiometry and expansion properties of SrCo_0.8Fe_0.2O_3−δ (SCF) were determined by in situ neutron diffraction between 873 and 1173 K and oxygen partial pressures of 5 × 10^− 4 to 1 atm. At a pO₂ of 1 atm, SCF adopts a cubic perovskite structure, space group Pm3¯m, across the whole temperature range investigated. At a pO₂ of 10^− 1 atm, a two-phase region exists below 922 K, where the cubic perovskite phase coexists with a vacancy ordered brownmillerite phase, Sr₂Co_1.6Fe_0.4O₅, space group Icmm. A pure brownmillerite phase is present at pO₂ of 10^− 2 and 5 × 10^− 4 atm below 1020 K. Above 1020 K, the brownmillerite phase transforms to cubic perovskite through a two-phase region with no brownmillerite structure observed above 1064 K. Large distortion of the BO₆ (B = Co, Fe) octahedra is present in the brownmillerite structure with apical bond lengths of 2.2974(4) Å and equatorial bond lengths of 1.9737(3) Å at 1021 K and a pO₂ of 10^− 2 atm. SCF is highly oxygen deficient with a maximum oxygen stoichiometry, 3 − δ, measured in this study of 2.58(2) at 873 K and a pO₂ of 1 atm and a minimum of 2.33(2) at 1173 K and a pO₂ of 5 × 10^− 4 atm. Significant differences in lattice volume and expansion behavior between the brownmillerite and cubic perovskite phases suggest potential difficulties in thermal cycling of SrCo_0.8Fe_0.2O_3−δ membranes.     

Ion and electron conduction in SrFe1−xScxO3−δ

The oxygen ion and electron transport in SrFe_1−xSc_xO_3−δ  (x = 0.1–0.3) system at 700–950 °C were studied analyzing the total conductivity dependencies on the oxygen partial pressure, pO₂. The conductivity measurements were performed both under reducing conditions (10^− 19 ≤ pO₂ ≤ 10^− 8 atm) comprising the electron-hole equilibrium point, and in oxidizing atmospheres (10^− 5 ≤ pO₂ ≤ 0.5 atm) which are characterized by extensive variations of the oxygen content studied by coulometric titration technique. The incorporation of 10% Sc³⁺ cations into the iron sublattice suppresses transition of the cubic perovskite phase into vacancy-ordered brownmillerite, thus improving ion conduction at temperatures below 850 °C. When scandium content increases, the ion conductivity becomes considerably lower. The hole mobility is thermally-activated and varies in the range of 0.001 to 0.05 cm² V^− 1 s^− 1, increasing with oxygen concentration and decreasing on Sc doping.     

Transport properties and structural stability of tetragonal CeNbO4+δ

The electrical properties of CeNbO_4+δ have been investigated at 1073–1223 K in the oxygen partial pressure range 10^− 17 to 0.36 atm. The conductivity and Seebeck coefficient behaviour indicates that, at oxygen chemical potentials close to atmospheric, tetragonal CeNbO_4+δ possesses a mixed ionic and p-type electronic conductivity. The ion transference numbers under the p(O₂) gradient of 0.93/0.21 atm, measured by the modified e.m.f. technique, are close to 0.4 decreasing in more reducing environments. The variations of partial ionic and electronic conductivities can be described in terms of the oxygen intercalation into the scheelite-type lattice, which results in increasing concentrations of both dominant charge carriers, oxygen interstitials and holes, when p(O₂) increases. Reduction leads to p(O₂)-independent electrical properties, followed by a drastic decrease in the conductivity at oxygen pressures below 10^− 15–10^− 9 atm due to a reversible transition into the monoclinic phase. Contrary to the zircon-type CeVO_4±δ, no traces of the parent binary oxides were detected in the reduced cerium niobate.     

A new way to apply ultrasound in cross-flow ultrafiltration: Application to colloidal suspensions

A new coupling of ultrasound device with membrane process has been developed in order to enhance cross-flow ultrafiltration of colloidal suspensions usually involved in several industrial applications included bio and agro industries, water and sludge treatment. In order to reduce mass transfer resistances induced by fouling and concentration polarization, which both are main limitations in membrane separation process continuous ultrasound is applied with the help of a vibrating blade (20 kHz) located in the feed channel all over the membrane surface (8 mm between membrane surface and the blade). Hydrodynamic aspects were also taking into account by the control of the rectangular geometry of the feed channel.Three colloidal suspensions with different kinds of colloidal interaction (attractive, repulsive) were chosen to evaluate the effect of their physico-chemical properties on the filtration.For a 90 W power (20.5 W cm⁻²) and a continuous flow rate, permeation fluxes are increased for each studied colloidal suspension, without damaging the membrane. The results show that the flux increase depends on the initial structural properties of filtered dispersion in terms of colloidal interaction and spatial organizations.For instance, a Montmorillonite Wyoming–Na clay suspension was filtered at 1.5 × 10⁵ Pa transmembrane pressure. Its permeation flux is increased by a factor 7.1, from 13.6 L m⁻² h⁻¹ without ultrasound to 97 L m⁻² h⁻¹ with ultrasound.     

Numerical simulation of ultrasonic enhancement on mass transfer in liquid–solid reaction by a new computational model

Mass transfer coefficient is an important parameter in the process of mass transfer. It can reflect the degree of enhancement of mass transfer process in liquid–solid reaction and in non-reactive systems like dissolution and leaching, and further verify the issues by experiments in the reaction process. In the present paper, a new computational model quantitatively solving ultrasonic enhancement on mass transfer coefficient in liquid–solid reaction is established, and the mass transfer coefficient on silicon surface with a transducer at frequencies of 40 kHz, 60 kHz, 80 kHz and 100 kHz has been numerically simulated. The simulation results indicate that mass transfer coefficient increases with the increasing of ultrasound power, and the maximum value of mass transfer coefficient is 1.467 × 10⁻⁴ m/s at 60 kHz and the minimum is 1.310 × 10⁻⁴ m/s at 80 kHz in the condition when ultrasound power is 50 W (the mass transfer coefficient is 2.384 × 10⁻⁵ m/s without ultrasound). The extrinsic factors such as temperature and transducer diameter and distance between reactor and ultrasound source also influence the mass transfer coefficient on silicon surface. Mass transfer coefficient increases with the increasing temperature, with the decreasing distance between silicon and central position, with the decreasing of transducer diameter, and with the decreasing of distance between reactor and ultrasound source at the same ultrasonic power and frequency. The simulation results indicate that the computational model can quantitatively solve the ultrasonic enhancement on mass transfer coefficient.     

An experimental and modeling study of the shock tube ignition of a mixture of n-heptane and n-propylbenzene as a surrogate for a large alkyl benzene

Alkyl aromatics are an important chemical class in gasoline, jet and diesel fuels. In the present work, an n-propylbenzene and n-heptane mixture is studied as a possible surrogate for large alkyl benzenes contained in diesel fuels. To evaluate it as a surrogate, ignition delay times have been measured in a heated high pressure shock tube (HPST) for a mixture of 57% n-propylbenzene/43% n-heptane in air (≈21% O₂, ≈79% N₂) at equivalence ratios of 0.29, 0.49, 0.98 and 1.95 and compressed pressures of 1, 10 and 30 atm over a temperature range of 1000–1600 K. The effects of reflected-shock pressure and equivalence ratio on ignition delay time were determined and common trends highlighted. A combined n-propylbenzene and n-heptane reaction mechanism was assembled and simulations of the shock tube experiments were carried out. The simulation results showed very good agreement with the experimental data for ignition delay times. Sensitivity and reaction pathway analyses have been performed to reveal the important reactions responsible for fuel oxidation under the shock tube conditions studied. It was found that at 1000 K, the main consumption pathways for n-propylbenzene are abstraction reactions on the alkyl chain, with particular selectivity to the allylic site. In comparison at 1500 K, the unimolecular decomposition of the fuel is the main consumption pathway.     

Hydrogen permeation of tungsten phosphate glass thin films

Thin films of tungsten phosphate glasses were deposited on a Pd substrate by a pulsed laser deposition method and the flux of hydrogen passed thorough the glass film was measured with a conventional gas permeation technique in the temperature range 300–500 °C. The glass film deposited at low oxygen pressure was inappropriate for hydrogen permeation because of reduction of W ions due to oxygen deficiency. The membrane used in the hydrogen permeation experiment was a 3-layered membrane and consisted of Pd film (~ 20 nm), the glass film (≤ 300 nm) and the Pd substrate (250 µm). When the pressure difference of hydrogen and thickness of the glass layer were respectively 0.2 MPa and ~ 100 nm, the permeation rate through the membrane was 2.0 × 10^? 6 mol cm^? 2 s^? 1 at 500 °C. It was confirmed that the protonic and electronic mixed conducting glass thin film show high hydrogen permeation rate.     

Structure and conductivity investigations of alkaline earth substituted uranium oxide,U1 − xMxO2 ± δ (M = Mg,Ca, Sr) for solid oxide fuel cell applications

Alkaline earth substituted UO₂ (U_1 − xM_xO_2 ± δ; M = Mg, Ca, Sr; 0.1 ≤ x ≤ 0.525) with fluorite structure was synthesized in reducing atmosphere. Structure and conductivity properties of U_1 − xM_xO_2 ± δ fluorites were investigated for possible application in solid oxide fuel cells (SOFC). At room temperature and ambient atmosphere the materials are stable; however they decompose at an oxygen partial pressure p_O2 > 10^− 4 atm and temperatures higher than 600 °C. The total conductivity measured for the best conducting U_1 − xM_xO_2 ± δ material with M = Ca and x = 0.177 is as high as 3 S/cm at p_O2 < 10^− 4 atm at 600 °C. The relatively low ionic transference number (t_i∼0.02) is disadvantageous for potential use as electrolyte material for SOFC applications. The high conductivity and possible depolarization effects suggest potential use as anode materials in SOFC.     

Impact of ultrasound on solid–liquid extraction of phenolic compounds from maritime pine sawdust waste. Kinetics,optimization and large scale experiments

Maritime pine sawdust, a by-product from industry of wood transformation, has been investigated as a potential source of polyphenols which were extracted by ultrasound-assisted maceration (UAM). UAM was optimized for enhancing extraction efficiency of polyphenols and reducing time-consuming. In a first time, a preliminary study was carried out to optimize the solid/liquid ratio (6 g of dry material per mL) and the particle size (0.26 cm²) by conventional maceration (CVM). Under these conditions, the optimum conditions for polyphenols extraction by UAM, obtained by response surface methodology, were 0.67 W/cm² for the ultrasonic intensity (UI), 40 °C for the processing temperature (T) and 43 min for the sonication time (t). UAM was compared with CVM, the results showed that the quantity of polyphenols was improved by 40% (342.4 and 233.5 mg of catechin equivalent per 100 g of dry basis, respectively for UAM and CVM). A multistage cross-current extraction procedure allowed evaluating the real impact of UAM on the solid–liquid extraction enhancement. The potential industrialization of this procedure was implemented through a transition from a lab sonicated reactor (3 L) to a large scale one with 30 L volume.     

High-pressure shock-tube investigation of the impact of 3-pentanone on the ignition properties of primary reference fuels

Ignition-delay times for pure 3-pentanone, 3-pentanone/iso-octane (10/90% by volume) and 3-pentanone/n-Heptane mixtures (10/90% by volume) have been determined in a high-pressure shock tube under engine-relevant conditions (p₅ = 10, 20, and 40 bar) for equivalence ratios ? = 0.5 and 1.0 and over a wide temperature range 690 K < T₅ < 1270 K. The results were compared to ignition delay times of primary reference fuels under identical conditions. A detailed kinetics model is proposed for the ignition of all fuel mixtures. The model predicts well the ignition delay times for pure 3-pentanone for a wide range of pressure and temperature and equivalence ratios in argon dilution as well as in air. Ignition delay times for 3-pentanone-doped mixtures, especially in the low-temperature range are overpredicted by approx. a factor of 0.5 (at 800 K, 40 bar, ? = 1.0) by the calculation but the model still reproduces the overall trend of the experimental data. For lean conditions, 10% 3-pentanone reduces the reactivity of n-Heptane below 1000 K while for stoichiometric conditions it does not alter the ignition delay by more than 11% at 850 K and 20 bar. In iso-octane the effect is inverse, leading to acceleration of the main ignition. Based on the model, the influence of 3-pentanone on the main heat release in a n-Heptane-fueled HCCI engine cycle is simulated.