Stable and critical heat- and mass-transfer regimes of a traveling carbon particle

We consider the effect of the relative velocity of a carbon particle on the specific features of the time dependences of the temperature and dimater of the particle at given temperatures of air and the wall of a reaction device and at a specified oxygen concentration in the medium. The stable and critical values of the temperature and combustion rate of the particle versus the initial particle diameter at its given velocity and the relative velocity of a particle of given size are studied. The effect of the diameter and velocity of the particle on the critical values of its initial temperature, which determine ignition inside hysteresis loops, is analyzed. The dependences of the critical particle diameter at which the thermal regime changes (ignition and extinction) on the relative particle velocity are derived. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 1, pp. 25–30, January–February, 1998.     

Composite Propellant Ignition and Extinction by CO2 Laser at Subatmospheric Pressure

Aspects of the heating, ignition and extinction of an AP/HTPB(86/14) composite propellant, induced by means of a CO₂ laser energy pulse, are investigated in the subatmospheric pressure range to determine the influence of the operating conditions on these processes. Measurements of the ignition delay time and ignition temperature for any operating condition have been done by microthermocouples able to record the temperature history of the irradiated surface and of the gas phase. Determinations of the boundaries location, defining the propellant behavior after the external energy removal, by the go-nogo techniques, have permitted to evaluate the minimum exposure time which assures to avoid the propellant extinction after the propellant ignition. Combustion transients have been studied to determine the burning propellant response to the laser energy pulse and to define the pulse features which yields the propellant extinction for different working pressure. Laser Doppler Velocimetry(LDV)technique, able to measure the velocity of the gas coming out from the burning propellant surface, was used to follow the whole combustion transient giving a precise picture of the phenomenon. Comparisons between data obtained studying the ignition and extinction by laser energy pulse phenomena show that the operating pressure has opposite effects on the propellant response to the external energy input having the same characteristics.     

Unsteady auto-ignition of hydrogen in a perfectly stirred reactor with oscillating residence times

The ignition characteristics of a hydrogen–air mixture in a perfectly stirred reactor (PSR) with an oscillating residence time were investigated numerically. An unsteady numerical algorithm was developed and solved using a stiff-equation solver in order to investigate the unsteady auto-ignition behavior of the fuel/air mixture. The amplitude, frequency, and phase of the residence time oscillations were varied, and the effects on the simulated ignition behavior were recorded. Under small amplitude oscillations of the residence time, once ignited, the temperature in the reactor varied following the phase of the oscillations. Under larger amplitude variations, periodic ignition, and extinction events were observed. A critical frequency was observed, where the ignition delay time became significantly large than at the other frequencies. The existence of this critical frequency was found to depend on the phase of the residence time oscillation, and only occurred when the phase was such that the residence time decreased from the initial conditions. Ignition did not occur for frequencies of the oscillation in the residence time beyond 2.84 kHz, regardless of the phase. The physics of ignition delay for the case where the oscillatory residence time decreased initially could be clarified by investigating the time variation of characteristic chemical times of important reactions to ignition.At low frequencies of the residence time oscillation, similar behavior to that of the steady state was observed. However, the ignition delay time was found to be significantly different at high frequencies, especially for larger amplitude fluctuations in the residence time. Combustion of the fuel/air mixture could be sustained at shorter residence times under the oscillating residence time conditions than under the steady-state conditions. The reaction could not be sustained at high frequencies, and a pulsed-mode flame was observed, where the period of the ignition and extinction events was the same as the period of the oscillations in the residence time.The concentration of free radicals was found to increase with time prior to ignition, and the H radical concentration saturated at a maximum at the ignition time, indicating that the H radical concentration is a good indicator of ignition time under oscillating residence times.     

Velocity and Temperature Gradient Boundary Layer Formation During a Continuum Drawing in a Polymer Melt Chamber

This paper presents a boundary layer model for the velocity and temperature gradient on the introduction of a continuum (wire, strip, tube, etc.) in a melt bath of polymer positioned before the plasto-hydrodynamic pressure unit prior to drawing or coating. Results are presented for the boundary layer thicknesses of velocity and temperature at different wire velocities. Also, the friction factor and Nusselt number have been given for polymer melt at different continuum velocities. The velocity and temperature gradient boundary layer thickness are also discussed in relation to the quality of hydrodynamic coating of wire.     

Influence of radiation on the critical heat-and mass-transfer conditions in parallel reactions at the surface of a particle

Analytical expressions are derived for the critical diameters bounding the region of high-temperature heat and mass transfer in parallel reactions at the surface of a particle. It is found that radiation with a large initial particle diameter leads to increase in the critical diameter of ignition with increase in the initial particle temperature. Formulas relating the combustion temperature and the particle diameter are derived. The influence of the prevailing conditions on the critical particle diameter at which spontaneous (induced) extinction and ignition occur is investigated in detail. An expression is obtained for the limiting gas temperature and oxidant content below which particle combustion is impossible. Degeneracy of the critical parameters of heterogeneous particle ignition and damping is investigated.State University, Odessa 270074. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 4, pp. 63–74, July–August, 1994.     

Combustion characteristics and flame stability at the microscale: a CFD study of premixed methane/air mixtures

A two-dimensional elliptic, computational fluid dynamics (CFD) model of a microburner is solved to study the effects of microburner dimensions, conductivity and thickness of wall materials, external heat losses, and operating conditions on combustion characteristics and flame stability. We have found that the wall conductivity and thickness are very important as they determine the upstream heat transfer, which is necessary for flame ignition and stability, and the material's integrity by controlling the existence of hot spots. Two modes of flame extinction occur: a spatially global type for large wall thermal conductivities and/or low flow velocities and blowout. It is shown that there exists a narrow range of flow velocities that permit sustained combustion within a microburner. Large transverse and axial gradients are observed even at these small scales under certain conditions. Periodic oscillations are observed near extinction in cases of high heat loss. Engineering maps that delineate flame stability, extinction, and blowout are constructed. Design recommendations are finally made.     

Hot‐Wire Ignition of AN‐Based Emulsions

Emulsions based on ammonium nitrate (AN) and water locally ignited by a heat source do not undergo sustained combustion when the pressure is lower than some threshold value usually called the Minimum Burning Pressure (MBP). This concept is now being used by some manufacturers as a basis of safety. However, before a technique to reliably measure MBP values can be designed, one must have a better understanding of the ignition mechanism. Clearly, this is required to avoid under ignitions which could lead to the erroneous interpretation of failures to ignite as failures to propagate. In the present work, facilities to prepare and characterize emulsions were implemented at the Canadian Explosives Research Laboratory. A calibrated hot‐wire ignition system operated in a high‐pressure vessel was also built. The system was used to study the ignition characteristics of five emulsion formulations as a function of pressure and ignition source current. It was found that these mixtures exhibit complicated pre‐ignition stages and that the appearance of endotherms when the pressure is lowered below some threshold value correlates with the MBP. Thermal conductivity measurements using this hot‐wire system are also reported.     

Exhaustion of a silane jet into a space

Issues of explosion and fire safety of silane during its transportation and storage are intimately related to its possible catastrophic leak off from tanks if they become cracked. It turned out that silane self-ignition is possible at some leak off velocities. The interest of the aerospace industry to problems of ignition and combustion of silane combined with other fuels should be also noted. Experimental investigations of such problems, which are rather expensive and laborconsuming, provide primary information for the development and verification of mathematical models of physical and chemical processes, capable of predicting the characteristics of silane mixing, ignition, and combustion induced by its exhaustion. Recent experiments of Prof. Chen and his colleagues from Taiwan allowed them to determine the critical length of the silane jet (distance from the tube exit where ignition of exhausting silane occurred) and the time of silane ignition. In the present work, an attempt is made to simulate these phenomena within the framework of the concept of the critical concentration of silane with studying the dynamics of silane leak off from a tube.     

Autothermal operation of an adiabatic simulated counter current reactor

Periodic operation of several adiabatic fixed beds connected in series is an alternative to conventional steady-state fixed-bed operation or transient reverse flow reactor operation, e.g. to process lean waste gases or to perform efficiently slightly exothermic equilibrium limited reactions. This work explains the periodic operation of a multi-bed reactor network using both a simplified model assuming a true counter current of the phases involved and a more detailed dynamic model. An appropriate expression for relating the switching times and solid phase velocities is applied. The shapes of the thermal waves are studied in a range of relevant switching times. Results of a stability analysis considering both models reveal hysteresis for ignition and extinction of the reactor network. The accompanying regions of multiplicity provide insight in the potential of performing in such reactor networks exothermal reactions for certain ranges of the adiabatic temperature rise.     

Regimes of combustion of a premixed mixture of gases in a heated microchannel with the wall temperature smoothly increasing in the downstream direction

Specific features of the unsteady flame in a microchannel with a controlled wall temperature are theoretically studied within the framework of a one-dimensional diffusion-thermal model. The case with the channel wall temperature increasing in the gas flow direction and the channel size being smaller than the critical value determined on the basis of the ambient temperature is considered. Depending on the flow rate of the combustible mixture of gases through the channel, either flame stabilization or alternation of flame repetitive extinction/ignition is possible. The influence of the characteristic length of channel wall heating on the domains of existence of various combustion modes is studied for the first time. The theoretical study shows that there exists a critical value of the temperature gradient in the channel walls, below which the regime of flame repetitive extinction/ignition is no longer observed. At small values of the temperature gradient, a hysteresis phenomenon is found, which is associated with different changes in the flame position in the cases with increasing and decreasing flow rates of the gas.     

Spectral Acquisition and Calibration Techniques for the Measurement of Radiative Flux Incident upon Propellant

A technique for the absolute calibration of a time‐resolved spectrographic system has been developed at QinetiQ, specifically designed to be relevant to spectral acquisition from within the interior of translucent gun‐propellant samples. The technique has shown itself to be particularly useful in the realm of propellant ignition as it allows for the precise determination of the moment that propellant combustion processes begin, as well as measuring the incident radiative flux leading up to ignition. Scope exists to extend its use for high‐pressure measurements of the incident radiative flux during both conventional propellant burn and high‐powered electrothermal‐chemical (ETC) discharges. This paper sets out to describe both the technique and some of the pitfalls encountered during the development of the technique. The use of this technique in some of the experimental work performed at QinetiQ, including the results of measurements that compare the incident radiative flux with propellant ignition during both ETC discharges and conventional gunpowder burn, have been published separately; references for this experimental work are given in this paper.     

Studies on polymer ignition and development of a relative hazard ranking method

The argument is presented that a primary determinant for polymer ignition is the rate of heat input during thermal decomposition. An experimental protocol is described in which a conventional thermogravimetric analyzer is used to monitor the behavior of polymeric solids brought to ignition by exposure to preheated air. The data obtained are used to establish the minimum heating rate required for ignition and the initial decomposition temperature at that critical rate. These two properties are then combined to give a numerical ranking value which seems to be a reasonable indicator of a material's ignitability hazard potential. Data for a variety of polymers and mixed systems are presented.     

Effects of molecular weight distribution and branching on rheological properties of polyolefin melts

The objective of this work was to determine the relationships among molecular and melt parameters of polyolefins. The polyolefins studied are polypropylene, poly-1-butene, poly-1-hexene, poly-1-dodecene, these have regularly spaced short-chain branches. Conclusions from previous work, as well as some new data, on polyethylene are given. As the molecular weight increases, the critical shear rate decreases but the melt viscosity and non-Newtonian ratio increase. As the molecular weight distribution broadens, the critical shear rate decreases, whereas the normal forces and the non-Newtonian ratio increase. Increasing the number of short-chain branches increases the energy of activation and the melt viscosity but decreases the non-Newtonian ratio. As the length of the short-chain branches increases, the non-Newtonian ratio increases, but the melt viscosity, critical shear rate, and energy of activation decrease. Increasing the number of long-chain branches decreases the non-Newtonian ratio, but the normal forces and the melt viscosity increase. Such information allows the polymer chemist to design a polyolefin molecule having the critical melt properties required for a given production technique.     

高氯酸铵含量对高铝富燃料推进剂中重要组分高氯酸铵/铝 一次燃烧性能和铝粉团聚的影响

采用CO2激光点火装置联合高速摄影系统及扫描电子显微镜等凝聚相燃烧产物分析技术,研究了高氯酸铵(AP)含量对高Al富燃料推进剂中重要组分AP/Al一次燃烧过程中燃烧现象、引燃时间、燃烧扩散时间、燃尽时间、燃烧效率、颗粒团聚及凝聚相燃烧产物的表面形貌、粒径及其分布的影响。结果表明,各AP/Al混合粉体的燃烧过程均可分为表面引燃、燃烧扩散和火焰熄灭3个阶段,但各样品在不同燃烧阶段的燃烧现象存在明显差异。AP含量由10wt%增至30wt%,样品燃烧剧烈程度增强,燃烧过程中固相颗粒的溅射现象越加明显;在火焰熄灭阶段,各样品燃烧由以停留在样品燃面处的燃烧为主逐渐变为以溅射颗粒的燃烧为主,且随反应进行,燃面已燃固相颗粒最先熄灭,各样品表面引燃时间、燃烧扩散时间、燃烧持续时间均缩短,即燃烧反应速率逐渐加快。在AP/Al混合物中,铝粉的燃烧效率、凝聚相燃烧产物粒度及其团聚程度随AP含量增加而增加。     

MPP/PER/APP阻燃PP的阻燃及热裂解行为

采用聚磷酸蜜胺(MPP)／季戊四醇(PER)／聚磷酸铵(APP)三元膨胀型阻燃剂(IFR)阻燃聚丙烯(PP),测定了阻燃PP的极限氧指数(LOI)、UL94V阻燃性及热稳定性,以傅里叶变换红外光谱(FTIR)分析了阻燃PP的热分解残余物。以锥形量热仪(CONE)测定了阻燃PP的诸多与火灾有关的阻燃参数,包括释热速度、质量损失速度、总释热量、有效燃烧热、比消光面积及引燃时间等,以光电子能谱(XPS)测定了阻燃PP残炭表面的元素组成及XPS曲线拟合数据,还以扫描电镜(SEM)观测了阻燃PP残炭的形态。     

Critical diameter and transverse waves of powder combustion

A critical review is given of the problem of powder extinction with a decrease in the sample diameter due to heat release. The results of our experimental studies of the critical combustion diameter are presented. A comparison of the experimental data on the critical diameter as a function of burning rate is shown to be the most informative. These functions follow a power law with an exponent of −1.15 to −1.17. The relations between the sizes of the cells (hot spots) formed by the set of transverse waves on the burning surface and the burning rate follow the same law. The sizes of the cells are 2.1–2.3 times smaller than the critical combustion diameter. It is also found that the transverse wave decays if its curvature exceeds a critical value. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 2, pp. 72–78, March–April, 2007.     

Study of polymethyl methacrylate ignition and extinction on exposure to a high-power laser pulse

Experimental results of polymethyl methacrylate ignition and extinction on exposure to laser radiation with power density q=200–1000 W/cm ² are presented. For q>220 W/cm², gasphase ignition is shown to occur in the center of the circular-vortex leading edge far from the surface of a sample. For q220 W/cm ², ignition occurs simultaneously on two sites above the surface. The action of laser radiation with q>220 W/cm ² on polymethyl methacrylate after its gas-phase ignition at room temperature and atmospheric pressure does not ensure a steady combustion regime — instead, extinction is observed.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 5, pp. 30–33, September–October, 1996.     

Fire-hazard assessment of extended-chain polyethylene and aramid composites by cone calorimetry

A cone calorimeter is used to determine the fire performance of polymer composite materials containing combustible reinforcing fibres in addition to combustible matrix resins. Extended-chain polyethylene and aramid fibre-reinforced composites containing epoxy, vinylester and phenolic matrix resins are examined at various cone irradiances. Values for time to ignition, rate of heat release, effective heat of combustion, smoke density and evolved carbon monoxide and carbon dioxide are reported for the reinforcements, matrix resins and composites. The reinforcements have a significant effect on the fire-hazard properties of the composite materials. For the epoxy and vinylester composites, times to ignition reflect those of the component of higher ignitability. This was not the case for the aramid-reinforced phenolic composite, in which the resin surface layer hinders combustion of the fabric reinforcement. Resin and reinforcement contributions to the composite rate of heat release behaviour as a function of time are generally discernible.     

The persistence of burning of textiles in different oxygen environments and the determination of the extinction oxygen index

The influence that oxygen has on the burning behaviour of cotton single- and multilayered fabries has been determined by recording persistence of burning times as a function of oxygen concentration. For a given fabric subjected to a specified igniter application time, an extinction oxygen index parameter, EOI, may be determined. EOI is defined as the oxygen concentration expressed as a volume fraction in an oxygen-nitrogen mixture, necessary to give a persistence of burning time equal to zero. For a given fabric, EOI is found to decrease as the igniter application time increases and two limiting EOI values may be observed, one corresponding to zero inginter application conditions and one for igniter application times greater than 10 s. At any given ignition time, the extinction oxygen index increases linearly with fabric area density and decreases linearly with the logarithm of the respective air permeability. Similar results are observed for nylon 6.6 and polyester single- and multilayered fabrics. The validity of the extinction oxygen index is discussed with reference to other published works.     

Do open flame ignition resistance treatments for cellulosic and cellulosic blend fabrics also reduce cigarette ignitions?

Mattresses/bedding and upholstered furniture are subject to ignition by cigarettes (smoulder) and open flames leading to injuries, fatalities and property damage. There are mandatory and voluntary cigarette ignition standards in the USA for mattresses (16 CFR 1632) and upholstered furniture (UFAC voluntary standards) as well as open flame ignition standards in California (TB 117) and the UK (BS 5852). Open flame ignition standards are being considered/developed for these products. Some suggest that fire retardant (FR) treatments to prevent/reduce open flame ignitions also reduce cigarette ignitions. Some reports suggest that the smoulder ignition propensity of some cellulosic fabrics can be affected adversely by open flame ignition resistance treatments. Ignitions caused by cigarettes and open flames result from different types of combustion that are retarded by different mechanisms. Flaming combustion is a gas phase reaction and occurs when heat causes degradation of the polymer releasing volatile products that undergo rapid oxidation in the air, whereas smouldering combustion is a direct oxidation of either the polymer or its char. The results of textile/fibre industry studies with FR treated upholstery fabrics and a critical review of the available published literature indicate that cigarette ignition propensity of cellulose fabrics is complicated and affected by many factors and that smoulder ignition resistance of these fabrics can be affected adversely by open flame ignition resistance treatments. Copyright © 2004 John Wiley & Sons, Ltd.