微波辐射下CaSO4生成反应过程中的热点特性

在微波辐射化学反应过程中，常常会因为存在热点而导致爆炸或反应物烧毁等现象发生。本文以微波辐射下的硫酸钙生成反应为例，利用有限元方法求解Maxwell方程组、流体力学方程以及热传导方程，研究了微波辐射反应过程中热点的变化情况，并用实验验证了计算结果的正确性。研究结果表明：在加热过程中，随反应的进行，热点的位置是在不断变化的，且热点常出现在物质分相界面附近。     

微波辐射下CaSO4生成反应过程中的热点特性

在微波辐射化学反应过程中,常常会因为存在热点而导致爆炸或反应物烧毁等现象发生。本文以微波辐射下的硫酸钙生成反应为例,利用有限元方法求解Maxwell方程组、流体力学方程以及热传导方程,研究了微波辐射反应过程中热点的变化情况,并用实验验证了计算结果的正确性。研究结果表明：在加热过程中,随反应的进行,热点的位置是在不断变化的,且热点常出现在物质分相界面附近。     

Modellbasierte Untersuchung des Wärmeübergangs in einer induktiv beheizten Wirbelschicht

The aim of the presented study is to develop a new technology for a highly efficient heating of fluidized beds. The energy input in fluidized beds can be performed by convective indirect heating of the fluidizing medium, or by induction by means of electrically conductive inert particles (such as iron hollow balls), in which energy is transferred via an induction field. On the surface of these particles, the heat is released directly and without contact into the fluidized bed. The impact of parameters like bed mass, air velocity and supplied induction power on the inductive heating was systematically investigated. A simplified heat transfer model was developed. With this model a good agreement with the measured values for heating and cooling can be achieved.     

Stationary and traveling hot spots in the catalytic combustion of hydrogen in monoliths

In the course of catalytic combustion of hydrogen (1-5% H₂ in air) in monolith reactors, strongly localized stationary and traveling hot spots arise in response to a sudden and persistent rise of gas flow velocity. Such hot spots may occur, e.g. in a catalytic converter following the acceleration of a car or in a catalytic combustor as a result of a load increase. This phenomenon is illustrated by simulations using a two-phase reactor model. The temperature overshoot of the adiabatic limit is typically of the order of the adiabatic temperature rise itself.The following mechanism underlies this behavior. Light fuel is supplied to the catalytic wall by fast diffusion (in the direction perpendicular to flow), while the heat released by reaction is removed from the wall by the slower, mixture-averaged heat conduction. This leads to accumulation of heat at the catalytic surface that eventually saturates at high temperatures. The hot spots may exhibit intricate dynamics, propagating downstream or upstream, or they may remain stationary. The direction of propagation depends on the relative strength of convective downstream and conductive upstream contributions to the overall displacement of reaction fronts. Generally, the hot spot tends to drift downstream at low flow velocities, remain stationary at intermediate flow velocities, and drift upstream at high flow velocities.     

Comparative analysis of physical mechanisms of detonation initiation in HMX and in a low-sensitive explosive (TATB)

Based on the concept of hot spots, the basic physical factors determining the macrokinetics of chemical reactions in heterogeneous crystalline explosives are estimated. The macrokinetics in plasticized TATB is shown to be basically determined by the velocity of combustion propagation from microscopic reaction spots (hot spots), whereas the macrokinetics in plasticized HMX is determined by the density of hot spots. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 89‐99, January–February, 2009.     

Control of microwave-generated hot spots. Part IV. Control of hot spots on a heterogeneous microwave-absorber catalyst surface by a hybrid internal/external heating method

A problem with microwave-absorbing heterogeneous catalysts (MAHCs) in non-polar solvents is resolved with a novel approach that also uses an external heating bath in combination with microwave heating. In non-polar solvents, the microwave radiation dielectrically and selectively heats only the catalyst resulting in the frequent occurrence of hot spots under these conditions. However, such hot spots can be controlled through a combined (hybrid) internal/external heating methodology (MAHS). Moreover, high temperatures can be maintained with significant energy saving. The potential benefit of MAHS has been examined by carrying out the synthesis of 4-methylbiphenyl using the Suzuki–Miyaura coupling reaction in toluene solvent in the presence of palladium catalytic particles supported on activated carbon particulates (Pd/AC). The hybrid internal/external heating method saved 65% of microwave energy and increased the chemical yield of 4-methylbiphenyl nearly twofold in comparison with a conventional microwave heating method.     

Electric Spark and ESD Sensitivity of Reactive Solids (primary or secondary explosive,propellant, pyrotechnics). Part II: Energy transfer mechanisms and comprehensive study on E50

The energy transfer mechanism from the electric spark to the reactive solid sample in powder form has been determined. Only a small portion of the solid is heated to the explosion critical temperature through a resistive heating by electric current. In fact, hot spots develop at the thinnest part of the solid, i.e. at intergrain contact points. We studied the evolution of thermal diffusivity and of sensitivity to electric spark as a function of powdered HMX granulometry. It allowed us to specify the respective importance of the grain size and shape in this evolution.     

Gelation of methylcellulose hydrogels under isothermal conditions

The current work deals with the gelation of methylcellulose (MC) in aqueous solutions under isothermal conditions. The isothermal gelation was monitored during two consecutive heating and cooling cycles. The gelation was observed to occur early during the second cycle and progress at a higher rate. Micelle‐like structures were found to form in MC solutions during the first gelation cycle. These were largely responsible for the accelerated gelation during the second heating cycle. The influence from the state of water was examined by studying the gelation phenomenon for MC using either cold or hot DI water as solvent. The possible mechanism involved is discussed. A gel indexing method was established to provide a quantitative measure for the state of gelation achievable using different MC concentrations with either cold or hot water solvent. Stabilization kinetics for the gel under isothermal conditions was described using the Malkin and Kulichikhin model. The kinetics parameters were determined. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Analysis of catalytic reaction systems under microwaves to save energy

Effects of microwaves on catalytic reaction systems are analyzed theoretically in this work. Use of microwaves is encouraged to save energy. The effects of microwave heating are analyzed theoretically by assuming that the catalyst pellet is homogeneous. The temperature and concentration profiles within the catalyst pellet were obtained by numerical simulations for the cases of microwave heating and conventional heating. In the modeling the catalyst pellet is regarded as a continuum. When a chemical reaction was conducted in a heterogeneous medium with microwave heating, the reaction rate and the yield were found to be increased compared to conventional heating under the same reaction conditions. This is due to hot spots generated by selective heating of the catalyst pellet, resulting in an increased reaction rate.     

Modeling of a metal monolith catalytic reactor for methane steam reforming-combustion coupling

A novel metal monolith reactor for coupling methane steam reforming with catalytic combustion is proposed in this work, the metal monolith is used as a co-current heat exchanger and the catalysts are deposited on channel walls of the monolith. The transport and reaction performances of the reactor are numerically studied utilizing heterogeneous model based on the whole reactor. The influence of the operating conditions like feed gas velocity, temperature and composition are predicted to be significant and they must be carefully adjusted in order to avoid hot spots or insufficient methane conversion. To improve reactor performance, several different channel arrangements and catalyst distribution modes in the monolith are designed and simulated. It is demonstrated that reasonable reactor configuration, structure parameters and catalyst distribution can considerably enhance heat transfer and increase the methane conversion, resulting in a compact and intensified unit.     

Nonuniform Heating in Zinc Oxide Varistors Studied by Infrared Imaging and Computer Simulation

State-of-the-art infrared (IR) thermal imaging was used to monitor the heating of ZnO varistors by electrical transients. On a macroscopic scale (e.g., 10 mm), heating in large varistor blocks (i.e., diameter of 42 mm) was found to be the greatest near the block edges and to be approximately radially symmetric in blocks fabricated at a low aspect ratio. In blocks fabricated at a higher aspect ratio, the heating was less symmetric, presumably because uniform properties are more difficult to achieve. Nonuniform heating in large blocks can be attributed to processing-induced variations in the electrical properties of the blocks. On an intermediate size scale (e.g., 1 mm), the heating in small varistor disks (e.g., diameter of 10 mm) was observed to be most intense along localized electrical paths. The high electrical conductivity of these paths originates from the statistical fluctuations in properties that inevitably occur in polycrystalline materials. On a microscopic scale (e.g., 10 μm), the heating in thin varistor slices (e.g., thickness of 100 μm) was observed to be localized in strings of tiny hot spots. The hot spots occur at the grain boundaries in a conducting path, where the potential is decreased across Schottky-type barriers and the heat is generated. The experimentally observed heating is interpreted by applying transport theory and using computer simulations. It is shown that, on the scale of the grain size, the heat transfer is too fast to permit temperature differences that could cause a varistor failure. Current localization and nonuniform heating on an intermediate size scale can have a microstructural origin (e.g., statistical fluctuations of grain sizes and grain-boundary properties). However, these are shown to be significant only in small varistors, whereas destructive failures (puncture and cracking) of large varistor blocks can be caused only by nonuniform heating on a macroscopic scale.     

微波有机合成及在混凝土减水剂制备中的应用研究进展

微波辐射作为反应活化的新型能源,已成为有机化学中普遍应用的先进技术。本文从过热、热点和选择性加热以及高极化场下的非热效应4方面阐述微波有机合成机理,结合国内外应用案例进行理论验证。重点从反应动力学角度分析了微波辐射对提高混凝土减水剂品质上的作用效果,总结了近几年微波辅助合成减水剂的研究进展,旨在推动微波技术在水泥混凝土外加剂领域中的实际应用。最后,提出微波辐射通过在电磁场作用下极性分子的快速振动实现,其热效应对体系起深层加热作用,非热效应改变反应动力学,但对作用的反应体系具有"选择性",应着重分析。微波辅助合成减水剂具有显著优势,可增大反应速率常数且存在非热效应,应加深研究及进一步推广。     

Numerical investigation of the error on flow measurements due to exhaust gas heating and cooling in the SBI‐configuration

The possible flow measurement error due to heating or cooling of exhaust gases in the Single‐Burning‐Item (SBI) test is estimated from numerical experiments. It is illustrated that there is no one‐to‐one correspondence between the velocity profile shape and the instantaneous Reynolds number, due to the time‐dependent temperature and density profile evolution in the exhaust gas pipe. A non‐ambiguous relation is found between the velocity profile shape and an ‘effective’ Reynolds number, based on the turbulent viscosity. Maximum variations of the velocity correction factors, relating the mean velocity to the velocity on the pipe axis, are found to be in the order of 2% for limiting circumstances for the SBI test. The primary effect is caused by instantaneous Reynolds number variations. The effect of heating or cooling of the flow by the hot or cold pipe is noticeable, too. The statements are proved to be valid independent of the computational grid, the turbulence model and the time steps taken to obtain the numerical solutions. Copyright © 2006 John Wiley & Sons, Ltd.     

Direct observation of the mesophase in coal

Thin sections of vitrinite from Pittsburgh seam, Hazard No. 4, Illinois No. 6, and Lower Kittanning coking coals were ion-thinned to electron transparency and examined in a Philips EM300 transmission electron microscope. A heating stage was used in the microscope to observe their transformation to semicoke. Before heating, the vitrinite samples showed little structure, except for the Hazard No. 4, which contained pre-existing dark spots. Upon heating, small dark spots began to appear, growing in size and number with increasing temperature. These spots were identified as liquid crystals (mesophase). As heating continued, the mesophase in the Pittsburgh seam vitrinite coalesced into irregular units about 300 nm in diameter. In the Hazard No. 4 vitrinite, coalescence also occurred but at a slower rate. In the lllinois No. 6 vitrinite, only scattered spots of mesophase appeared. They grew to about 100 nm but did not coalesce. In the Lower Kittanning vitrinite, the mesophase formed at a higher temperature and grew at a slower rate than in the other samples. After one hour at the maximum stage temperature, small rods appeared, and their growth was easily observed with the electron microscope. Electron diffraction patterns obtained while the specimens of the four coals were hot consisted of diffuse rings. The observations in the electron microscope at elevated temperatures confirm the existence of the mesophase in the coal-to-coke transformation. The relation between the observed mesophase formation and the coking properties of the coals is discussed.     

Effects of Microwave Dielectric Heating on Heterogeneous Catalysis

The effect of microwave dielectric heating on both endothermic and exothermic reactions was investigated. Apparent equilibrium shifts for both reactions were observed which were attributed to the formation of spatial hot spots in the catalyst bed. The possible location of remarkable temperature gradients was examined experimentally and theoretically.     

硅基介孔材料研究进展

硅基介孔材料作为一种新兴的固体多孔材料,不仅具有孔道大小均一、排列有序、孔径可以在2-50 nm内连续调节等重要特征,而且还具有较大的孔容、高的比表面积、孔道表面可进行物理吸附或化学修饰及较好的水热稳定性等特点.其在吸附分离、催化、生物材料、药物缓释等方面,以及光、电、磁等领域都具有广阔的前景,是目前材料领域的研究热点...     

Carbon Dioxide Reforming of Methane with Pt Catalysts Using Microwave Dielectric Heating

Microwave heating was applied to the catalytic reforming reaction of methane with carbon dioxide over platinum catalysts. It was found that CO₂ and CH₄ conversions and the product selectivity (H₂/CO) were generally higher under microwave conditions than that obtained with conventional heating at the same measured temperature. The effect of microwave heating was attributed to the formation of hot spots with higher temperature than that measured in the bulk catalyst bed.     

Gas self-ignition in a plane vortex chamber

This paper describes the numerical modeling of gas flow in a plane vortex chamber by using the Navier–Stokes equations. The model is based on the laws of conservation of mass, momentum, and energy for nonstationary two-dimensional compressible gas flow in the case of axial symmetry with a tangential component of the gas velocity. The processes of viscosity, thermal conductivity, and turbulence are accounted for. It is shown that the transition of the kinetic energy of gas into thermal energy as a result of transfer processes leads to the formation of hot spots in the boundary layers near the walls of the chamber. The gas temperature at these hot spots can exceed the gas combustion temperature, while the gas remains rather cold in the neighboring regions. This could be the reason for the cold gas self-ignition observed in the experiments. The turbulence of the flow and the processes of mixing and diffusion of the components make a significant contribution to the capacity of gas self-ignition.     

Structural and physicochemical changes in RDX under vibration

The kinetics and velocity of solid-state RDX decomposition under the action of vibration were determined experimentally as functions of vibration parameters and the initial temperature of the substance. Changes in the macro- and microstructure of the crystals were studied using x-ray diffraction and x-ray phase analyses and microscopy. The change in the chemical composition after vibration was studied by IR spectroscopy. It is concluded that the mechanism and kinetics of RDX decomposition under vibration differ significantly from those of thermal decomposition. This is associated with the mechanochemical activation of potential active centers at the sites of formation and multiplication of dislocations, with the fatigue mechanism of crack initiation under cyclic loads, and with the appearance of heating spots during breakup of particles. Samara State Technical University, Samara 443010. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 5, pp. 103–110, September–October, 1997.     

Analysis of pulsed microwave processing of polymer slabs supported with ceramic plates

A detailed theoretical analysis has been carried out to study the role of ceramic supports (alumina and SiC) and pulsed microwave heating of polymer (Natural Rubber, NR, and Nylon 66) slabs due to various distributions of microwave incidences. Ceramic plates are typical representations as they withstand high temperature without any deformation. It is found that ceramic plates influence the heating processes significantly and local hot spots within samples are governed by specific type of ceramic plates for various sample thicknesses and distributions of microwave incidence (one side or both sides). Optimized pulsing of microwave incidence has been employed to minimize the thermal runaway or hot spots in order to achieve uniform temperature distribution and pulsing is introduced based on two parameters: setpoint (ΔT_S) and on-off constraint (T^′). Detailed spatial distributions of power and temperature are illustrated for a few representative length scales to demonstrate the role of local maxima in power and temperature on heating rate as well as thermal runaway with or without pulsing. Pulsing ratio (PR) has been defined as PR=t_off/t_p, where t_off is power-off time and t_p is the total processing time such that smaller PR denotes large processing rates. It is found that one side incidence gives smaller values of PR for both the ceramic plates whereas SiC plate may be suitable for both sides incidence with large sample thicknesses of NR samples. It is also found that larger values of setpoints also minimize PR. The setpoints along with the on-off constraint play critical role to select the heating strategy as a function of ceramic plates and types of incidence. Pulsing may not be important for smaller thicknesses of Nylon samples and SiC or alumina plates may be recommended for processing larger thicknesses of Nylon samples in presence of pulsing. Current study recommends the efficient microwave heating methodologies for polymer processing attached with ceramic plates by means of optimized pulsing for the first time.