Premixed hydrogen-air flame front dynamics in channels with central and peripheral ignition

Experimental and analytical study of burning hydrogen-air mixtures with 12, 13, and 15 vol% hydrogen concentrations in channels with central and peripheral ignition was performed. Flame propagation speeds were determined by shadow and infrared high-speed imaging in the transverse and longitudinal directions, respectively. It was found that the increase in the flame front speed during the peripheral ignition reaches up to 1.7 times compared to the central ignition depending on mixture content. The pressure growth rate was examined in a closed channel. It was estimated that the time to reach a maximum pressure is 1.1 times less in the case or peripheral ignition than the central one. An analytical model was formed to describe the dynamics of the flame front in both cases. The model of a “reversed finger-flame” generated by a peripheral ignition was presented. The obtained results could be used in designing hydrogen-fueled combustible engines with the reduced knock-effect.     

Effects of acoustic excitation on pinch-off flame structure and NOx emissions in H2/CH4 flame

The flame structure and exhaust emissions of pinch-off flames, in which flames are separated under acoustic excitation conditions, were studied. Flame structure analysis was performed using OH1 chemiluminescence measurements, and exhaust emissions analysis was performed with a gas analyzer. Structure analysis of the pinch-off flame for acoustic excitation was performed basis the OH1 images, and various structures were confirmed according to the forcing frequency and velocity perturbation intensity. To analyze the correlation between the flame structure and NO_x emission, the flame residence time and emission index of NO_x (EINO_x) were analyzed according to the Strouhal number. The flame residence time and EINO_x decreased as the velocity perturbation intensity increased; analyzing the NO_x emission characteristics is limited when based only on the flame residence time. 1/2-power scaling was followed by EINO_x analysis and the Strouhal number was normalized to the flame residence time.     

Numerical investigation on the effects of reaction orders on the flame propagation dynamic behaviors for premixed gas in a closed tube

In this paper, the premixed flame propagation in a closed tube is surveyed using Computational Fluid Dynamics. The propagation characteristics of premixed flame are obtained coupling a single-step reaction mechanism with a laminar flame model. Three single-step reaction mechanisms are established with different reaction orders for hydrocarbon fuels. This study is to establish a wider range of reaction mechanisms and represent actual experimental conditions better. The numerical simulation results demonstrate that reaction orders can affect the tulip flame development. As the flame spreads, the tulip flame fronts become wrinkled. When the reaction order is 2, there are more wrinkles in the flame front and the degree of wrinkles is more obvious. Reaction orders also affect the flame tip velocity and the flame skirt velocity. The main reason is that laminar flame speeds are significantly different. When the reaction orders are 1.5 and 2, laminar flame speeds are mainly affected by temperature, which respectively increase by about 25% and 75%. When the reaction order is 1, the pressure is crucial for the variation of laminar flame speed. The laminar flame speed decreases by about 33%.     

二硫代焦磷酸酯微胶囊阻燃PVA纤维的制备及性能

采用原位聚合的方法制备了以新戊二醇二硫代焦磷酸酯(DDPS)为核、三聚氰胺甲醛树脂(MF)为壳的阻燃微胶囊(MDDPS),分别将DDPS和MDDPS无卤阻燃剂与聚乙烯醇(PVA)共混纺丝,制备了无卤阻燃PVA纤维;表征了MDDPS的结构及形貌,研究了纯PVA纤维、DDPS/PVA纤维、MDDPS/PVA纤维的力学性能、阻燃性能、热稳定性、燃烧前后的形貌。结果表明:微胶囊MDDPS的平均粒径增大至7.82μm,仍能满足共混纺丝要求;DDPS及MDDPS的加入,PVA纤维力学性能下降;MDDPS/PVA纤维阻燃性能较好,极限氧指数达31.3%,燃烧时膨胀较为明显,且600℃时残炭率达到19.1%,炭层较为致密,有气泡产生,释放不燃性气体阻止了燃烧。     

膨胀型阻燃剂/聚乙烯的制备与性能研究

聚乙烯(PE)被广泛应用于电子、电器、化工、机械等领域。聚乙烯在燃烧时会产生熔融滴落现象,燃烧的滴落物可使其它可燃物着火,加速了火灾的蔓延和扩大。为满足防火和环境安全的要求,常常需要对聚乙烯采取阻燃处理。首先合成了双环笼状磷酸酯1-氧基磷杂-4-羟甲基-2,6,7-三氧杂双环[2.2.2]辛烷(PEPA)。然后将合成的膨胀型阻燃剂PEPA均匀的掺杂在PE中,形成了膨胀型阻燃剂/PE体系。之后研究了PEPA的含量对PE阻燃性能的影响。实验表明,随着PEPA的含量增加,阻燃等级达到FV-0,拉伸强度下降。     

Flammability and thermal properties of epoxy/glass/MWNT Composites

The aim f this work is to study the effect of nanotubes on flammability properties of epoxy/glass composites. Multiwalled carbon nanotubes (MWNT) and its functionalized derivative (amino functionalized nanotubes) were incorporated into epoxy resin. To disperse MWNTs in the epoxy resin, different ways were employed. Microscopic observations showed that, the best dispersion state was gained by using ultrasonication method and high shear flow simultaneously. Thermal resistance of cured epoxy resins containing various amounts of nanotubes (0.25–0.7 wt %), was investigated by thermo gravimetric analysis (TGA). Introducing MWNTs and amino‐MWNTs to samples increased the initial thermal decomposition temperature for about 32 and 37°C, respectively. LOI measurements of composite samples showed an increase up to 32. Cone calorimetry test was carried out on epoxy/glass and epoxy/glass containing 0.5% MWNT. The results showed that, introducing 0.5% MWNTs decreases maximum average rate of heat emission for about 26%. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39849.     

Fire‐retardant hybrid thermosetting resins from unsaturated polyesters and polysilazanes

This introduces an organic–inorganic thermosetting hybrid resin system based on unsaturated polyester and polysilazanes. It shows the chemical modification of unsaturated polyester structures by end capping to enable the combination of both components. In general, halogen‐free unsaturated polyesters are not fire‐retardant and have to be equipped with additives. Fillers and intumescent additives are preponderantly used in today's fire‐retardant formulations. In contrast to these fire‐retardants, polysilazanes act as ceramizing agents. Polysilazanes are suitable fire‐retardants for resin transfer molding due to their low viscosity. Both burning behavior and glass transition temperature (T_g) are investigated as important application properties. In contrast to state‐of‐the‐art fire‐retardant formulations polysilazane‐based thermosetting hybrid resins burn with high intensity and fast extinction. Therefore, total heat and smoke emission is decreased. The formation of ceramic structures during burning results in high residual mechanical properties and a low mass loss. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40375.     

Flame retardancy and mechanical properties of polyamide 6 with melamine polyphosphate and ionic liquid surfactant‐treated montmorillonite

The mechanical properties and inflammability of polyamide 6 (PA6) nanocomposites incorporated with Montmorillonite organoclay (MMT) modified with thermal stable ionic liquid surfactants were investigated. The compatibility between ionic liquid‐treated MMT and PA6 matrix was improved and the intercalation morphology was achieved, which resulted in the increaseof tensile modulus. However, the addition of organo‐MMTs alone did not improve the inflammability of the PA6 nanocomposite, because of strong melt‐dripping behavior of PA6 matrix. Addition of auxiliary melamine polyphosphate (MPP) intumescent flame retardant to the nanocomposite prevented the melt dripping and enhanced inflammability performance. The enhanced inflammability of PA6/organoclay/MPP nanocomposites was attributed to the synergistic effect between imidazolium or phosphonium organo‐MMTs and intumescent flame retardant MPP. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40648.     

Biobased hyperbranched shape‐memory polyurethanes: Effect of different vegetable oils

Hyperbranched polyurethanes were synthesized from poly(ε‐caprolactone) diol as a macroglycol, butanediol as a chain extender, a monoglyceride of a vegetable oil (Mesua ferrea, castor, and sunflower oils separately) as a biobased chain extender, triethanolamine as a multifunctional moiety, and toluene diisocyanate by a prepolymerization technique with the A₂ + B₃ approach. The structure of the synthesized hyperbranched polyurethanes was characterized by ¹H‐NMR and X‐ray diffraction studies. M. ferrea L. seed‐oil‐based polyurethane showed the highest thermal stability, whereas the castor‐oil‐based one showed the lowest. However, the castor‐oil‐based polyurethane exhibited the highest tensile strength compared to the other vegetable‐oil‐based polyurethanes. All of the vegetable‐oil‐based polyurethanes showed good shape fixity, although the castor‐oil‐based polyurethane showed the highest shape recovery. Thus, the characteristics of the vegetable oil had a prominent role in the control of the ultimate properties, including the shape‐memory behaviors, of the hyperbranched polyurethanes. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39579.