An ignition criterion for combustible solids integrating surface temperature and heating rate

Surface ignition temperature has been widely used as an ignition criterion for the piloted ignition of common combustible solids. However, experimental observations have shown that the surface temperature of a solid at ignition varies with external heat flux. In addition, if the external heat flux is smaller than the critical heat flux for ignition, the solid will not ignite while the actual surface temperature may be higher than the defined surface ignition temperature. To overcome these limitations and maintain the simplicity of the surface ignition temperature criterion, a new ignition criterion integrating heating rate and surface temperature is proposed, developed, and validated. Predictions based on the new criterion compare well with experimental results on piloted ignition of a thermoplastic material (black PMMA), a thermoset composite material (E‐glass fiber reinforced polyester composite) and a cellulosic material (Red Oak) subjected to different heat flux levels. Potential factors affecting the accuracy and predictive capability of the new heating rate‐related ignition temperature criterion are discussed. The method and associated procedures to construct the heating rate‐related temperature ignition criterion can be used to obtain the same ignition criterion for other combustible solids. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of charge stock composition on the kinetic parameters in catalytic cracking

It is shown that the addition of wax to a wax free neutral distillate has no synergistic effect on the initial rate constant for cracking. The initial rate constant is in fact found to be a linear function of the mole fraction of wax in the feed. At the same time it is found that the activation energy for the cracking of pure wax is ? 48 kcal/inole and is some 4 kcal/mole lower than the activation energy for the cracking of the wax free aromatics and cyclic aliphatics. We are therefore led to believe that the initial rate constant for the catalytic cracking of any mitxure of compounds is simply the sum of the individual rate constants weighted by their respective mole fractions. It is also shown that the introduction of wax into the wax free feed results in an increased apparent order of the overall reaction and in more rapid decay of catalyst activity.     

New formula for calculating time to ignition of semi‐infinite solids

An analytical closed form formula is presented for explicitly calculating time to reach ignition temperature of semi‐infinite solids exposed to constant incident radiation and gas temperature as for example in the cone calorimeter. The non‐linear boundary condition due to the emitted radiation from the surface being proportional to the surface temperature raised to the fourth power according to the Stephan–Boltzmann law is accurately considered. The formula works for a wide range of the parameter values like the thermal inertia of the solid, the emissivity of the exposed surface and the convective heat transfer coefficient. They are all assumed constant. The new formula contains a single constant coefficient, which has been derived by comparing results obtained by accurate numerical finite element simulations using two different codes, comsol and TASEF , as well as calculations based on a Duhamel superposition scheme. Thus, the formula can be classified as semi‐empirical. It offers a simple approximate solution of a non‐linear problem that requires cumbersome numerical calculation methods to obtain more exact results. Any exact analytical solution is not available. The new method is carefully verified by comparisons with numerical solutions. However, as it is an analysis of well‐defined theoretical methods, any validation and comparisons with test data are not required and has therefore not been made.In comparison with other similar approximation formulas found in the literature, the accuracy as well as simplicity of applying the new formula is outstanding. Copyright © 2015 John Wiley & Sons, Ltd.     

Analysis of catalyst particle strength by impact testing: The effect of manufacturing process parameters on the particle strength

The mechanical strength of porous alumina catalyst carrier beads, used in the reforming units with continuous catalytic regeneration, was measured by impact testing. With this testing method particle strength can be measured at higher strain rates than the traditional crushing test method, hence providing a better simulation of pneumatic conveying and chute flow conditions, and also a large number of particles can be tested quickly. This is important for particles with a brittle failure mode such as the alumina particles used in this work as a wide distribution of mechanical strength usually prevails. Extensive impact testing was carried out first with an industrial sample, in order to understand the failure mechanism of this type of particles and to develop a methodology for analysing the extent of breakage by impact. Then the method was used to analyse the effect of a number of process parameters, such as filler, macroporosity and drying procedure on the particle strength with the aim of optimising the manufacturing process. The impact test results were then used to test the model of breakage behaviour of particulate solids proposed by Vogel and Peukert [Vogel and Peukert, Breakage behaviour of different materials—construction of a mastercurve for the breakage probability. Powder Technol., 129 (2003) pp. 101-110].     

Evaluation of kinetic parameters of coal ignition

A method is presented of evaluating the kinetic parameters for the ignition reaction of coal based on the Semenov's thermal ignition theory adopted to coal particle ignition. The evaluated apparent activation energy, E, for bituminous coal, anthracite and bituminous coal char are in the range 46–103 kJ mol^?1. A verification was made by comparing the measured induction periods for the coal particles with those calculated numerically. The oxidation rate is controlled by diffusion into pores.     

Effects of valve events on the engine efficiency in a homogeneous charge compression ignition engine fueled by dimethyl ether

Combustion characteristics of a homogeneous charge compression ignition (HCCI) engine were investigated with regard to the residual gas, i.e. internal exhaust gas recirculation (IEGR), by changing the intake and exhaust maximum opening points (MOP) and the exhaust cam lifts. Three different exhaust camshafts were used and had 2.5 mm, 4.0 mm and 8.4 mm exhaust valve lift. In-cylinder gas was sampled at the intake valve immediately before ignition to measure the IEGR rate. The heat release, fuel conversion efficiency and combustion efficiency were calculated using the in-cylinder pressure and composition of exhaust gases to examine the combustion features of the HCCI engine. The negative valve overlap (NVO) was increased as exhaust valve lift was reduced. Longer NVO made an increased IEGR through exhaust gas trapping. The IEGR rate was increased as the exhaust valve timing advanced while it was affected more by exhaust valve timing than by intake valve timing. Combustion phase was advanced by lower exhaust valve lift and early exhaust and intake MOP. It was because of higher amount of IEGR gas and effective compression ratio. The fuel conversion efficiency with higher exhaust valve lift was higher than that with lower exhaust valve lift. The late exhaust and intake MOP made the fuel conversion efficiency improve.     

How shrinkage of igniter-tablet residue affects ignition parameters

Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 27, No. 5, pp. 41–44, September–October, 1991.     

Viscoelastic model of impact ignition of solid explosives

A physicomathematical model is developed for the nonisothermal deformation of a disk, which is made of an incompressible, elastoplastic material, during an impact. The model takes into account the formation in the radial flow of viscous boundary layers at the contact boundaries on the striker and the anvil. In spite of their small thickness and the short duration, the boundary layers serve as effective concentrators of the dissipated impact energy and play a decisive role in the processes of thermal softening of the material and of the disk disintegration. The model is used to compute the critical conditions and the principal characteristics for the mechanical initiation of solid explosive charges. The good agreement between the theoretical and experimental data on the impact sensitivity of the explosive lends support to our concepts of the nature of explosion initiation using mechanical effects, on which the model was founded.Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 2, pp. 78–86, March–April, 1993.     

Effect of spark discharge parameters on the ignition of gas mixtures

The influence of the duration and spatial extent of a heat-releasing spark discharge on the critical conditions of spark ignition of a reactive gas is considered. A region insensitive to the effect of these parameters and, thus, suitable for work with the instantaneous source model is defined. Tomsk. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 5, pp. 3–7, September–October, 1993.     

Effect of the parameters of a dispersed material on the minimum energy for spark ignition

An engineering relationship between the minimum energy for ignition of a spark source and the parameters of a dust cloud is proposed on the basis of an analysis of a theoretical heat diffusion model for spark ignition of a gaseous suspension.Tomsk. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 2, pp. 9–12, March–April, 1993.     

On Zhurkov's approach to the strength of solids

Two modifications to the Zhurkov kinetic concept of static (creep) rupture is discussed: (a) solids where the common pole τ₀ is much larger than that associated with a vibrational frequency of bonded atoms in the solid and (b) materials where a common pole does not occur. Both cases are discussed within the Zhurkov framework. The activation thermodynamics and the association with transition state theory are revealed.     

Experimental investigation on impact ignition of Al/PTFE reactive materials

Reactive materials are a new class of energetic materials that extremely and efficiently release energy under the influence of high impact loading. An impact tester is used in the present study to explore the impact ignition characteristics of Al/PTFE reactive materials and the impact ignition pressure of Al/PTFE reaction materials under different conditions. The experimental result shows that the critical ignition pressure is approximately 1.44 GPa. Meanwhile, it also shows that the material compactness has a much less pronounced effect on the impact ignition pressure for this reactive material if the loading time scales are of the order of several milliseconds. Plastic work and viscous heat both play a significant role in impact ignition. Finally, it is shown that impact ignition ensures a higher energy release rate than surface ignition.     

Correlation of heterogeneity parameters for adsorption of single gases and gas mixtures on solids

Adsorption isotherms of single hydrocarbons on polystyrene and activated carbon adsorbents are described by an equation, which is an extension of Langmuir isotherm for a quasi-Gaussian energy distribution function. A simple analog of this equation is used to represent adsorption of binary hydrocarbon mixtures on these adsorbents. It is shown that this equation gives a good representation of mixed-gas adsorption isotherms; moreover, the heterogeneity parameters, including the dispersion of the adsorption energy, extracted from mixed-gas isotherms correlate with those obtained from suitable single-gas isotherms. Some suggestions concerning prediction of mixed-gas adsorption equilibria are presented.     

抗冲ABS树脂的研究

介绍了采用物理共混改性提高ABS树脂的抗冲击性的方法,确定了抗冲击改性剂的种类和用量,并研究了改性剂对ABS的抗冲性能、耐热性能和熔融流动性能的影响。     

Numerical simulation of cavitation in the conical-spray nozzle for diesel premixed charge compression ignition engines

The conical-spray injector is capable of achieving lean mixture with high homogeneity in the cylinder for diesel Premixed charge compression ignition (PCCI) engine with advanced injection timing. To better understand the cavitating flow inside the conical-spray injector, numerical simulations have been conducted by using a mixture multiphase model and a full cavitation model in this study. The results indicate that the cavitation evolution significantly affects the liquid sheet thickness and velocity at nozzle exit, which further change the spray angle and droplet Sauter mean diameter (SMD) dramatically. Based on the cavitation distribution inside the nozzle, the cavitating flow inside the conical-spray nozzle can be classified into four regimes with no cavitation, cavitation inception at inlet, developing cavitation at nozzle exit and super cavitation respectively. The extension of cavitation to nozzle exit in the super cavitation regime significantly improves the fuel atomization by increasing the injection velocity and decreasing the thickness of the liquid sheet. A cavitation map for the conical-spray injector has been developed by sweeping the ambient pressure and injection pressure simultaneously. It is found that the phenomenon of super cavitation only occurs in a narrow region where ambient pressure is very low. Therefore, the start of injection timing should be kept well before top dead center (TDC) to ensure the occurrence of super cavitation inside the nozzle in order to provide more homogeneous fuel/air mixture for diesel PCCI engines.     

硬质橡胶抗冲击强度测定方法的改进

提出了对硬质橡胶抗冲击强度测定方法的改进建议,包括将摆锤能量单位改为J,冲击强度单位改为J@m-3,摆锤规格由3种改为10种,并以大量试验数据说明抗冲击强度的试验结果取值改为取测定结果的中位数不影响试验结果的准确性,并可简化计算操作.