A Comparative Study of Near-Limit Flame Spread Over a Thick Solid in Space- and Ground-Based Experiments

Microgravity experiments have been performed aboard the SJ-10 satellite of China to investigate flame spread behaviors over a thick PMMA in low-velocity opposed flow. Two variables are considered: opposed-flow velocity in a range of 0 to 9 cm/s, and ambient oxygen concentration in a range of 25% to 50%. It is found that, when the flow velocity is reduced, the initial extended flame may breaks into separate flamelets after a dynamic transition process. This is the first observation of the flamelets spreading over a thick solid fuel in microgravity. Flame and flamelet propagate with a steady spread rate, which increases with the increasing flow velocity and oxygen concentration. A flammability map using oxygen concentration and flow velocity as coordinates is established, which delineates the uniform regime, the flamelet regime, and extinguished regime. The flammability boundary was extended to lower oxygen concentrations and lower flow velocities by the flamelet regime. The microgravity results are compared with the counterparts in ground-based narrow channel apparatus (NCA) experiments. Results showed that although the NCA tests overestimate the flame spread rate and flammable area, also exhibit differences in detailed flamelet formation process, flame and flamelet behaviors agree well with that in microgravity in a qualitative manner.     

Comparison and evaluation of methods for the determination of flammability limits, applied to methane/hydrogen/air mixtures

Different methods, both experimental and numerical, to determine the flammability limits are compared and evaluated, exemplified by a determination of the flammability limits of methane/hydrogen/air mixtures for hydrogen fuel molar fractions of 0, 0.2, 0.4 and 0.6, at atmospheric pressure and ambient temperature. Two different experimental methods are used. The first method uses a glass tube with visual observation of the flame, whereas the second method uses a closed spherical vessel with a pressure rise criterion to determine whether flame propagation has occurred. In addition to these experiments, the flammability limits are determined numerically. Unsteady planar and spherically expanding flames are calculated with a one-dimensional flame code with the inclusion of radiation heat loss in the optically thin limit. Comparison of the experimental results with the results of the planar flame calculations shows large differences, especially for lean mixtures. These differences increase with increasing hydrogen content in the fuel. Better agreement with the experimental results is found for the spherically expanding flame calculations. A limiting burning velocity of 5 cm/s is found to predict the upper flammability limit determined with the tube method very well, whereas the limiting flame temperature approach was found to give poorer agreement. Further analysis indicates that the neglect of flame front instabilities is the probable cause of the large differences between experimental and numerical results at the lower flammability limit.     

Flame Spread and Extinction Over a Thick Solid Fuel in Low-Velocity Opposed and Concurrent Flows

Flame spread and extinction phenomena over a thick PMMA in purely opposed and concurrent flows are investigated by conducting systematical experiments in a narrow channel apparatus. The present tests focus on low-velocity flow regime and hence complement experimental data previously reported for high and moderate velocity regimes. In the flow velocity range tested, the opposed flame is found to spread much faster than the concurrent flame at a given flow velocity. The measured spread rates for opposed and concurrent flames can be correlated by corresponding theoretical models of flame spread, indicating that existing models capture the main mechanisms controlling the flame spread. In low-velocity gas flows, however, the experimental results are observed to deviate from theoretical predictions. This may be attributed to the neglect of radiative heat loss in the theoretical models, whereas radiation becomes important for low-intensity flame spread. Flammability limits using oxygen concentration and flow velocity as coordinates are presented for both opposed and concurrent flame spread configurations. It is found that concurrent spread has a wider flammable range than opposed case. Beyond the flammability boundary of opposed spread, there is an additional flammable area for concurrent spread, where the spreading flame is sustainable in concurrent mode only. The lowest oxygen concentration allowing concurrent flame spread in forced flow is estimated to be approximately 14 % O₂, substantially below that for opposed spread (18.5 % O₂).     

Test method for evaluating fabric flammability and predicted skin burn injury in microgravity

To address fire safety concerns associated with the use of flammable fabrics during space travel, an apparatus was designed to be flown on low-gravity parabolic aircraft flights in order to assess the flammability of cotton and 50% cotton/50% polyester fabrics, and the resulting skin burn injury that would occur if these fabrics were to ignite. The apparatus, modelled after a standard fabric flammability test, was also used on the ground for experiments under earth’s gravity. Variables examined in the tests include gravity level, fabric type, air gap size, and orientation of the fabric. Flame spread rates, heat fluxes, and skin burn predictions determined from test results were compared under the two gravity levels. The orientation of the fabric had a large effect on flame spread rates, heat fluxes and predicted skin burn times for tests conducted under earth’s gravity. Flame spread rates and heat fluxes were highest when the fabric was held in the vertical orientation, which resulted in the lowest predicted times to produce skin burns. Flame spread rates and heat fluxes were considerably lower in microgravity than under earth’s gravity, which resulted in longer predicted times to produce skin burns.     

Toward generalization of calculations of the stationary velocity of diffusion flame propagation over the surface of a fuel

The results of numerical investigations of the laws governing the spreading of a two-dimensional diffusion flame over thermally thin layers of a cellulose material using the algorithm of calculation of stationary velocity of flame propagation, which is based on the principle of minimum entropy production, are presented. The obtained dependences of the velocity of flame propagation on thermophysical parameters (thickness of the fuel, oxygen concentration and pressure of the ambient, velocity of the blowing flow) agree quantitatively with the well-known experimental data. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 3, pp. 103–111, May–June, 2007.     

Similarity solution of fuel mass transfer,port mass flux coupling in hybrid propulsion

Boundary layer combustion is the primary mechanism of hot gas generation in hybrid rockets. The idea of a hybrid rocket is to store the oxidizer as a liquid and the fuel as a solid, producing a design that is less susceptible to chemical explosion than conventional solid and bipropellant liquid designs. The fuel is contained within the rocket combustion chamber in the form of a cylinder, with a circular channel called a port hollowed out along its axis. Upon ignition, a diffusion flame forms over the fuel surface along the length of the port. The combustion is sustained by heat transfer from the flame to the solid fuel causing continuous fuel vaporization until the oxidizer flow is turned off. Theory shows that the fuel mass transfer rate is proportional to the mass flux averaged across the port. The mass flow rate increases with axial distance, leading to coupling between the local fuel regression rate and the local mass flux. For proper design, accurate expressions are needed for both the time-dependent oxidizer-to-fuel ratio at the end of the port and the time at which all the fuel is consumed. As the fuel is depleted, the flame approaches the motor case, at which point the burn must be terminated. The purpose of this paper is to present a similarity solution for the coupled problem and a brief discussion of current practice.     

Calculation of the upper flammability limit of methane/air mixtures at elevated pressures and temperatures

Four different numerical methods to calculate the upper flammability limit of methane/air mixtures at initial pressures up to 10 bar and initial temperatures up to 200 degrees C are evaluated by comparison with experimental data. Planar freely propagating flames are calculated with the inclusion of a radiation heat loss term in the energy conservation equation to numerically obtain flammability limits. Three different reaction mechanisms are used in these calculations. At atmospheric pressure, the results of these calculations are satisfactory. At elevated pressures, however, large discrepancies are found. The spherically expanding flame calculations only show a marginal improvement compared with the planar flame calculations. On the other hand, the application of a limiting burning velocity with a pressure dependence Su,lim approximately p(-1/2) is found to predict the pressure dependence of the upper flammability limit very well, whereas the application of a constant limiting flame temperature is found to slightly underestimate the temperature dependence of the upper flammability limit.     

Experimental study of pre-ignition phenomena of self igniting fuel droplets observed by a new diagnostic method

Interaction effects with special attention on staged ignition phenomena between two self igniting n-decane fuel spheres were studied in microgravity conditions and compared to single droplet results. All combustion experiments were done at the 4.7s Drop Tower Bremen and were performed with a fixed droplet size of 1.5mm, varying mid spacings and ambient temperatures and pressures up to 15bar. The work was done to support the development of numerical models for fuel spray auto ignition. A new technique, based upon chemiluminescence was applied to visualize staged ignition phenomena. chemically excited formaldehyde (CH2O*) was used as a excellent natural tracer for low temperature reactions. According to the experimental requirements an intensified camera (ICCD) concept was developed, so that the real weak chemiluminescence signals become detectable with a sufficient quantum of efficiency. For diagnostics evaluation data derived from chemiluminescence monitoring were compared with those from planar laser induced fluorescence (LIF) of formaldehyde excited at 352.2nm by means of an XeF-Excimer Laser.     

Fabrication–microstructure–performance relationships of reversible solid oxide fuel cell electrodes–review

Abstract

A reversible solid oxide fuel cell system can act as an energy storage device by storing energy in the form of hydrogen and heat, buffering intermittent supplies of renewable electricity such as tidal and wave generation. The most widely used electrodes for the cell are lanthanum strontium manganate–yttria stabilised zirconia and Ni–yttria stabilised zirconia. Their microstructure depends on the fabrication techniques, and determines their performance. The concept and efficiency of reversible solid oxide fuel cells are explained, along with cell geometry and microstructure. Electrode fabrication techniques such as screen printing, dip coating and extrusion are compared according to their advantages and disadvantages, and fuel cell system commercialisation is discussed. Modern techniques used to evaluate microstructure such as three-dimensional computer reconstruction from dual beam focused ion beam–scanning electron microscopy or X-ray computed tomography, and computer modelling are compared. Reversible cell electrode performance is measured using alternating current impedance on symmetrical and three electrode cells, and current/voltage curves on whole cells. Fuel cells and electrolysis cells have been studied extensively, but more work needs to be done to achieve a high performance, durable reversible cell and commercialise a system.     

Model of large pool fires

A two zone entrainment model of pool fires is proposed to depict the fluid flow and flame properties of the fire. Consisting of combustion and plume zones, it provides a consistent scheme for developing non-dimensional scaling parameters for correlating and extrapolating pool fire visible flame length, flame tilt, surface emissive power, and fuel evaporation rate. The model is extended to include grey gas thermal radiation from soot particles in the flame zone, accounting for emission and absorption in both optically thin and thick regions. A model of convective heat transfer from the combustion zone to the liquid fuel pool, and from a water substrate to cryogenic fuel pools spreading on water, provides evaporation rates for both adiabatic and non-adiabatic fires. The model is tested against field measurements of large scale pool fires, principally of LNG, and is generally in agreement with experimental values of all variables.     

纳米二氧化硅对阻燃中密度纤维板性能的影响研究

以纳米二氧化硅作为改性剂,采用正交实验优化阻燃中密度纤维板的热压工艺,探讨了纳米二氧化硅加入量对板材物理力学性能和阻燃性能的影响。结果表明,适量添加纳米二氧化硅能提高阻燃中密度纤维板的内结合强度、弹性模量,略微降低阻燃性能。本实验的优化工艺按其对实验的影响由大到小排列为:脲醛胶施加量10%(质量分数,下同),纳米二氧化硅施加量4%,热压温度(160±5)℃,阻燃剂施加量13%。     

Generation of isothermal spreading data and interfacial energy data for liquid reactive metals on ceramic substrates: The copper-titanium/alumina system

The sessile drop technique is frequently used to evaluate the wettability and spreadability of liquid metals on ceramic substrates. In this study, the spreading kinetics of copper-20 wt% titanium alloys on polycrystalline alumina were evaluated based on measurements of spreading radius versus time. The process of spreading was monitored by anin situ video recording system. The tests were performed using three different initial metal configurations. It was found that conventional sessile drop testing configurations cannot be used to generate isothermal spreading kinetics data because of significant spreading during the heat-up cycle from the solidus temperature to the test temperature. An improved sessile drop technique was developed which eliminated the non-isothermal experience by introducing the liquid copper to the solid titanium/alumina at the desired testing temperature. Using this technique, only a few seconds of data were lost (while the liquid copper dissolved the solid titanium). Because very limited interfacial energy data exist for the copper-titanium/alumina system, especially at higher titanium concentrations, the equilibrium contact angle, the solid-liquid interfacial energy, and the work of adhesion from 1000 to 1300 °C are also presented.     

Evaluation of lower flammability limits of fuel-air-diluent mixtures using calculated adiabatic flame temperatures

The lower flammability limit (LFL) of a fuel is the minimum composition in air over which a flame can propagate. Calculated adiabatic flame temperatures (CAFT) are a powerful tool to estimate the LFL of gas mixtures. Different CAFT values are used for the estimation of LFL. SuperChems is used by industry to perform flammability calculations under different initial conditions which depends on the selection of a threshold temperature. In this work, the CAFT at the LFL is suggested for mixtures of fuel-air and fuel-air-diluents. These CAFT can be used as the threshold values in SuperChems to calculate the LFL. This paper discusses an approach to evaluate the LFL in the presence of diluents such as N2 and CO2 by an algebraic method and by the application of SuperChems using CAFT as the basis of the calculations. The CAFT for different paraffinic and unsaturated hydrocarbons are presented as well as an average value per family of chemicals.     

Flame retardance and thermal stability of wool fabric treated by boron containing silica sols

In order to improve flame retardance and thermal stability of wool fabric, a series of boron doped silica sols were prepared from a tetraethyl silicate inorganic precursor and applied to wool fabric as a flame retardant finish through the sol–gel process. Boric acid, zinc borate and ammonium borate were used as flame retardant additives. The effect of the boron containing flame retardant doped sol coatings on flammability, combustion behavior and thermal property of treated wool fabric was investigated via limited oxygen index (LOI) test, vertical burning test, thermogravimetric (TG) analysis, micro-calorimeter combustion (MCC) and smoke density test. It was found that the coatings on wool fabric acted as a heat insulation barrier, increasing the carbon residue and slowing down the spread of flame when burning, thus improving the flame retardance and thermal stability of the treated fabric. Meanwhile, the smoke density result indicated that the NH₄HB₄O₇ doped silica sol treated fabric had good smoke suppression property. Furthermore, the result of mechanical properties showed that there was no damage on tensile strength and air permeation of treated wool fabric.     

Mathematical simulation of containment of molten fuel in the vessel of fast reactor under conditions of heavy accident: Results of calculation using the BRUT computer codes

The developed numerical model is realized in the form of BRUT computer codes. Individual blocks of the computer codes are verified. The BRUT computer codes are used for performing numerical investigation of containment of molten fuel in the vessel of a fast reactor under conditions of destruction of 36 fuel assemblies (FA) and of the core melting. Information is given in the present paper about the method of solution of the problem and about the special features of the method.     

Flame Spread and Group-Combustion Excitation in Randomly Distributed Droplet Clouds with Low-Volatility Fuel near the Excitation Limit: a Percolation Approach Based on Flame-Spread Characteristics in Microgravity

Stable operation of liquid-fueled combustors requires the group combustion of fuel spray. Our study employs a percolation approach to describe unsteady group-combustion excitation based on findings obtained from microgravity experiments on the flame spread of fuel droplets. We focus on droplet clouds distributed randomly in three-dimensional square lattices with a low-volatility fuel, such as n-decane in room-temperature air, where the pre-vaporization effect is negligible. We also focus on the flame spread in dilute droplet clouds near the group-combustion-excitation limit, where the droplet interactive effect is assumed negligible. The results show that the occurrence probability of group combustion sharply decreases with the increase in mean droplet spacing around a specific value, which is termed the critical mean droplet spacing. If the lattice size is at smallest about ten times as large as the flame-spread limit distance, the flame-spread characteristics are similar to those over an infinitely large cluster. The number density of unburned droplets remaining after completion of burning attained maximum around the critical mean droplet spacing. Therefore, the critical mean droplet spacing is a good index for stable combustion and unburned hydrocarbon. In the critical condition, the flame spreads through complicated paths, and thus the characteristic time scale of flame spread over droplet clouds has a very large value. The overall flame-spread rate of randomly distributed droplet clouds is almost the same as the flame-spread rate of a linear droplet array except over the flame-spread limit.     

Numerical analysis of cement calciner fuel efficiency and pollutant emissions

Efficient mixing of pulverized fuel and limestone particles inside cement calciners is important due to the reason that the calcination process directly affects the final fuel consumption. The focus of this paper is on the numerical analysis of cement calciner’s operating conditions and pollutant emissions. The paper analyzes the influence of different amounts of fuel, mass flow of the tertiary air and the adiabatic wall condition on the decomposition rate of limestone particles, burnout rate of coal particles, and pollutant emissions of a newly designed cement calciner. Numerical models of calcination process and pulverized coal combustion were developed and implemented into a commercial computational fluid dynamics code, which was then used for the analysis. This code was used to simulate turbulent flow field, interaction of particles with the gas phase, temperature field, and concentrations of the reactants and products, by solving the set of conservation equations for mass, momentum, and enthalpy that govern these processes. A three-dimensional geometry of a real industrial cement calciner was used for numerical simulations. The results gained by these numerical simulations can be used for the optimization of cement calciner’s operating conditions, and for the reducing of its pollutant emissions.     

A mechanistic approach of the sintering of nuclear fuel ceramics

The CEA and the COGEMA, as part of their effort to model the different stages of the MOX fuel fabrication process, have specifically worked, on the sintering stage. A physical mechanistic model of MOX fuel sintering is proposed, as well as the numerical schemes that will lead to the achievement of the corresponding software tool. The model takes into account surface diffusion, grain boundary diffusion, volume diffusion, exchanges between solid and gas, as well as the mechanical strains of grains. The scale at which concentrations and strains are taken into account is smaller than grain size (0.1 μm for green pellets and 10 μm for sintered ones). The numerical resolution schemes of this problem have been conceived, and are also presented. They mainly consist of relaxation procedures to uncouple partial derivative mass conservation equations, flux balances along interfaces, non-linear potential equality conditions at interfaces, and Navier–Lamé equations over each grain. A code (SALAMMBO) is under construction, based on the latter; once validated by experimental and parametric tests, this code will describe Pu distribution, grain and pore size distribution, and local density of the pellets as functions of the sintering conditions, thus enabling further developments of the fabrication process such as the obtention of advanced microstructures of improved MOX fuels.     

Modeling the impact of a molten metal droplet on a solid surface using variable interfacial thermal contact resistance

An analytical model of the true area of contact between molten metal and a rough, solid surface has been used to calculate thermal contact resistance and to predict how it changes with surface roughness, substrate thermal properties and contact pressure. This analytical model was incorporated into a three-dimensional, time-dependent numerical model of free-surface flows and heat transfer. It was used to simulate impact, spreading and solidification of molten metal droplets on a solid surface while calculating contact resistance distributions at the liquid–solid interface. Simulations were done of the impact of 4 mm diameter molten aluminum alloy droplets and 50 μm diameter plasma sprayed nickel particles on steel plates. Predicted splat shapes were compared with photographs taken in experiments and simulated substrate temperature variation during droplet impact was compared with measurements.