Kinetics of collisional ionization of alkali metal atoms and recombination of electrons with alkali metal ions in flames

The rate constant k₂ has been measured in flames of H₂ + O₂ + N₂ for the ionization of alkali metal atoms M in collisions with flame species X in

$M+X\to M^{+}+e^{?}+X$

. Results are compared with every previously reported measurement of k₂ in similar flames for each alkali metal. It is concluded that

$k_2=(9.9\pm 2.7)\times {10}^{?9}{T}^{1/2}exp?(?V/RT)$

, ml molecule ^?1 sec^?1, where V is the ionization potential of the alkali metal atom and T the temperature. k₂ does not depend on flame composition and is the same for each alkali to a good approximation.The third-order recombination rate constant k_?2 for the reverse of (II) can be written as

$k_{?2}=(4.1\pm 1.1)\times {10}^{?24}{T}^{?1}$

, ml² molecule^?2 sec^?1. There is no significant dependence of k_?2 on flame composition or on which alkali metal is considered. These facts enable the rates of the forward and reverse processes in (II) to be estimated in flames generally.     

Effects of hydrogen addition and Carbone dioxide dilution on the velocity field in non reacting and reacting flows

The evolution of anti-pollution standards and the optimization of combustion efficiency push the development of new fuels with high energy efficiency. It is necessary to develop new alternative fuel to improve the efficiency of conventional systems, reduce emissions (NO_x, SO_x, soot particles) and recover for its materials. A new fuel called bio-hythane, a mixture of natural gas up to 20% hydrogen and up to 50% Carbone dioxide, from the recovery of the waste from households and agriculture, via suitable digesters provides a source of renewable energy and usable, is a very interesting solution to improve emission standards and optimization of the combustion chambers.     

Open space for the physisorption of H2: Cointercalation of graphite with Li,Ti metal atoms and ethylene molecules

Based on first-principles plane-wave calculations, we explored the method with the ethylene molecules and Ti, Li atoms intercalated into the graphite to open space for the physisorption of hydrogen. And our simulation indicated that the interlayer distance of the graphene is close to the optimal physisorption of hydrogen with this method. From our computation, we got that the type of 3 × 3 supercell has the lowest converge energy and is energetically favorable. The energy barrier of changing the type of 2 × 2 supercell to 3 × 3 supercell is high.     

Tunable kinetics of nanoaluminum and microaluminum powders reacting with water to produce hydrogen

This paper reports on the kinetics and reaction processes of 40‐nm and 1‐μm aluminum powders with water to produce hydrogen at atmospheric pressure. This reaction produces aluminum hydroxide with irregular morphologies as by‐products. It was found that the nucleation and growth of the aluminum hydroxides affect the kinetics of the reaction and thus the hydrogen production. The heat release in isothermal microcalorimetry and hydrogen production in a nonisothermal batch reactor were used to determine the rate‐determining steps of the reaction mechanism and the corresponding activation energies. Model and model‐free methods have been implemented to describe the reaction sequence between aluminum particle and water while the phase of newly produced aluminum hydroxide in the system plays an important role. The reaction of nanoaluminum particles and water, being more sensitive to temperature, goes to completion to produce bayerite, Al (OH)₃ at 30°C, and boehmite, AlOOH at 50°C, whereas the microaluminum particles do not react completely and produce only bayerite at 30°C and also low‐amount boehmite at 50°C. Nevertheless, these processes exhibit two distinct and sequential stages: a kinetically controlled stage with the apparent activation energy (E_a) of 100 to 110 kJ/mol, where nucleation and growth are limited by the chemical reactions on the surface of aluminum, and a diffusion controlled stage with E_a of 44 kJ/mol for the 40‐nm Al/water reaction and 86 kJ/mol for the 1‐μm Al/water reaction, where growth is limited by the mass diffusion through the aluminum hydroxide by‐products. The separation of these two stages is more obvious under isothermal conditions. For nonisothermal conditions, two stages are overlapped, and the one with a lower E_a dominates.     

Interaction of a moving shock wave with a two-phase reacting medium

The flow field, that develops when a moving shock wave hits a two-phase medium of gas and particles, has a practical application to industrial accidents such as explosions at coal mine and in grain elevator and furthermore to solid propellant combustion in rocket engine. Therefore, a successful prediction of the thermo-fluid mechanical characteristics development of gas and particles is very crucial and imperative for the successful design and operation of rocket nozzles and energy conversion systems. This paper describes an interaction phenomenon when a moving shock wave hits a two-phase medium of gas and particles with/without chemical reaction. A particle-laden gas is considered to be located along a ramp so that numerical integration is accomplished from the tip of ramp for a finite period. For the numerical solution, a fully conservative unsteady implicit second order time-accurate sub-iteration method and the second order Total Variation Diminishing scheme are used with the finite volume method for gas phase. For particle phase, the Monotonic Upstream Schemes for Conservation Laws as well as the solution of the Riemann problem for the particle motion equations is also used together with the schemes above. Transient development of thermo-fluid mechanical characteristics is calculated and discussed by changing the particle mass density and particle specific heat. For the case of the reacting particle-laden gas flow, a carbon particle-laden oxygen gas is considered to be located along a ramp. The results are discussed by comparison with the cases of the pure gas and the inert particle-laden gas. Major results reveal that when the particle mass density is smaller, there is a stronger interaction between two phases so that the velocity and temperature differences between two phases more rapidly decrease. When the particle specific heat is varied, only a thermal effect is observed while the other effects are minor. The case with reacting particles yields significantly different results due to chemical reaction such that the gas density does not monotonously but rapidly decrease due to the slip line in the relaxation zone, while the pressure and temperature become higher in comparison with the non-reacting case. But the dynamic variation would be only secondary to the thermal one.     

Immersed particles of a multiphase reacting flow with chemical decomposition in the stokes flow regime

This paper examines the transient chemical decomposition of solid particles in a multiphase flow consisting of molten and gaseous phases at high temperature and low Reynolds number. The mass, energy, momentum and chemical reaction equations are solved for a particle immersed in a viscous flow of uniform properties. Numerical solutions are obtained and the results are validated with experimental data of samples involving the chemical decomposition of the mixture of CuO and CuCl₂ particles in a molten salt. The analysis provides new insight into the implications of the rate of reaction vs. heat and mass transfer in a complex gas–solid–molten salt multiphase reacting system, as well as the impact on hydrogen yield in the thermochemical copper–chlorine (Cu–Cl) cycle of hydrogen generation.     

反应炉富氧燃烧特性研究

针对常规燃烧和富氧燃烧两种工况,对反应炉辐射室中的燃烧过程进行了数值模拟,富氧燃烧时空气中氧气摩尔分数为0.34%。模拟结果与实际测量浓度相近,验证了计算模型的有效性。研究结果表明:富氧燃烧能满足管内化学反应对温度控制的要求,与理论混合比燃烧相比,采用富氧燃烧节约燃料12.66%,NOX排放量有所增加。在维持温度场均匀的前提下,提出了燃烧器优化配置方案。     

Silicon photovoltaic cells with clusters of nickel atoms

This article considers the technology of fabricating clusters of nickel atoms in a silicon crystalline lattice with controlled parameters. Silicon solar cells with clusters of nickel atoms have been fabricated and their parameters determined. It has been established that the content of nickel atoms in the lattice makes it possible to considerably expand the spectral sensitivity region of silicon photovoltaic cells to the IR range up to 4 μm.     

Critical behaviour in chemically reacting systems: I—Difficulties with the Semenov theory

The Semenov theory of thermal explosion without fuel consumption cannot be derived correctly from the exact equations describing the problem, i.e. the equations with fuel consumption. The limiting process used to derive the former from the latter (ε → 0, B → ∞) is shown to involve an infinite stretching of the dimensionless time coordinate. A correct limiting process is given which provides different equations from the Semenov theory.     

Direct numerical simulation of auto-ignition of a hydrogen vortex ring reacting with hot air

Direct numerical simulation (DNS) is used to study chemically reacting, laminar vortex rings. A novel, all-Mach number algorithm developed by Doom et al. [J. Doom, Y. Hou, K. Mahesh, J. Comput. Phys. 226 (2007) 1136–1151] is used. The chemical mechanism is a nine species, nineteen reaction mechanism for H₂/air combustion proposed by Mueller et al. [M.A. Mueller, T.J. Kim, R.A. Yetter, F.L. Dryer, Int. J. Chem. Kinet. 31 (1999) 113–125]. Diluted H₂ at ambient temperature (300 K) is injected into hot air. The simulations study the effect of fuel/air ratios, oxidizer temperature, Lewis number and stroke ratio (ratio of piston stroke length to diameter). Results show that auto-ignition occurs in fuel lean, high temperature regions with low scalar dissipation at a ‘most reactive’ mixture fraction, ${\zeta }_{MR}$ (Mastorakos et al. [E. Mastorakos, T.A. Baritaud, T.J. Poinsot, Combust. Flame 109 (1997) 198–223]). Subsequent evolution of the flame is not predicted by ${\zeta }_{MR}$; a most reactive temperature $T_{MR}$ is defined and shown to predict both the initial auto-ignition as well as subsequent evolution. For stroke ratios less than the formation number, ignition in general occurs behind the vortex ring and propagates into the core. At higher oxidizer temperatures, ignition is almost instantaneous and occurs along the entire interface between fuel and oxidizer. For stroke ratios greater than the formation number, ignition initially occurs behind the leading vortex ring, then occurs along the length of the trailing column and propagates toward the ring. Lewis number is seen to affect both the initial ignition as well as subsequent flame evolution significantly. Non-uniform Lewis number simulations provide faster ignition and burnout time but a lower maximum temperature. The fuel rich reacting vortex ring provides the highest maximum temperature and the higher oxidizer temperature provides the fastest ignition time. The fuel lean reacting vortex ring has little effect on the flow and behaves similar to a non-reacting vortex ring.     

Influence of jet-to-crossflow pressure ratio on nonreacting and reacting processes in a scramjet combustor with backward-facing steps

The jet-to-crossflow pressure ratio has a large impact on the combustion mode transition in the scramjet engine, and this information needs to be explored comprehensively. The effect of the jet-to-crossflow pressure ratio on the mixing and combustion processes in a backward-facing step combustor has been investigated numerically, and two similar cases have been utilized to validate the numerical approaches employed. The obtained results show that the wall pressure distribution for the nonreacting flow field has been predicted well, and the peak pressures are all a bit underestimated. However, the predicted wall pressure distribution for the reacting flow field does not match well with the experimental data, and it is overestimated. When the hydrogen is injected only from the bottom wall of the combustor, the mixing efficiency decreases with the increase of the jet-to-crossflow pressure ratio irrespective of the nonreacting or reacting flow field. When the hydrogen is injected simultaneously from the top and bottom walls, the separation shock wave is pushed forward to the entrance of the combustor, and it varies from an oblique one to a normal one. This means that the jet-to-crossflow pressure ratio has a great impact on the combustion mode transition for the scramjet engine, and the stable ramjet/scramjet mode transition can be obtained by controlling the fuel injection scheme.     

Recent advances in the measurement of strongly radiating, turbulent reacting flows

Recent advances in diagnostic methods are providing new capacity for detailed measurement of turbulent, reacting flows that are strongly radiating. Radiation becomes increasingly significant in flames containing soot and/or fine particles, and also increases with physical size. Therefore many flames of practical significance are strongly radiating. Under these conditions, the coupling between the turbulence, chemistry and radiative heat transfer processes is significant, making it necessary to obtain simultaneous measurement of controlling parameters. These environments are also particularly challenging for laser-based measurements, since soot and other particles increase the interferences to the signal and the attenuation of the beam. The paper reviews the influence of physical scale and of the properties of the medium on approaches to perform measurements in such strongly radiating flows. It then reviews the recent advances in techniques to measure temperature, mixture fraction, soot volume fraction, velocity, particle number density and the scattered, absorbed and transmitted components of radiation propagation through particle laden systems. Finally it also considers remaining challenges to diagnostic techniques under such conditions.     

A 1D model for the description of mixing-controlled reacting diesel sprays

The paper reports an investigation on the transient evolution of diesel flames in terms of fuel-air mixing, spray penetration and combustion rate. A one-dimensional (1D) spray model, which was previously validated for inert diesel sprays, is extended to reacting conditions. The main assumptions of the model are the mixing-controlled hypothesis and the validity of self-similarity for conservative properties. Validation is achieved by comparing model predictions with both CFD gas jet simulations and experimental diesel spray measurements. The 1D model provides valuable insight into the evolution of the flow within the spray (momentum and mass fluxes, tip penetration, etc.) when shifting from inert to reacting conditions. Results show that the transient diesel flame evolution is mainly governed by two combustion-induced effects, namely the reduction in local density and the increase in flame radial width.     

煤焦水蒸气催化气化动力学分析

利用固定床反应器研究了K、Ca、Ni和Fe金属对600～900℃内煤焦水蒸气气化的催化效果,分析了适用于原煤焦、脱灰煤焦和添加K、Ca、Ni和Fe金属后的煤焦水蒸气气化动力学模型。     

Effects of heat transfer on the sorption kinetics of complex hydride reacting systems

In this work, the effect of powder bed size on the absorption and desorption kinetics of NaAlH₄ catalyzed with TiCl₄ was studied experimentally. For this purpose, volumetric titration measurements were performed using cells of different diameters. The temperature was measured during the process at different positions inside the hydride bed, providing detailed information about the influence of heat conduction. Experimental results show that, under the applied conditions up to a critical size, larger diameters can lead to faster kinetics for the first and second absorption reactions. At larger cell diameters, however, temperatures up to 200 °C were measured during the first absorption step in the hydride bed. This leads to a significant delay in the start of the second absorption step, reducing the overall rate of the process. Reasons for the observed behaviour are discussed and measures for optimization are proposed.     

Kinetics of woodchips char gasification with steam and carbon dioxide

Kinetics of woodchips char gasification has been examined. Steam and CO₂ were used as the gasifying agents. Differences and similarities between kinetics of steam gasification and CO₂ gasification have been discussed. Comparison was conducted in terms of gasification duration, evolution of reaction rate with time and/or conversion, and effect of partial pressure on reaction rate. Reactor temperature was maintained at 900 °C. Partial pressure of gasifying agents varied from 1.5 bars to 0.6 bars in intervals of 0.3 bars. Steam and CO₂ flow rates were chosen so that both gasifying agents had equal amount of oxygen content. CO₂ gasification lasted for about 60 min while steam gasification lasted for about 22 min. The average reaction rate for steam gasification was almost twice that of CO₂. Both reaction rate curves showed a peak value at certain degree of conversion. For steam gasification, the reaction rate peak was found to be at a degree of conversion of about 0.3. However, for CO₂ gasification the reaction rate peak was found to be at a conversion degree of about 0.1. Reaction rates have been fitted using the random pore model (RPM). Average structural parameter, ψ for steam gasification and CO₂ gasification was determined to be 9 and 2.1, respectively. Average rate constant at 900 °C was 0.065 min⁻¹ for steam gasification and 0.031 min⁻¹ for CO₂ gasification. Change in partial pressure of gasifying agents did not affect the reaction rate for both steam and CO₂ gasification.     

Photosensitization of wide bandgap semiconductors with antenna molecules

Polynuclear metal complexes, supporting efficient intramolecular energy transfer processes, can be used to increase the light harvesting efficiency of sensitized wide bandgap semiconductors. Experimental studies are discussed to emphasize: (i) how structural changes at the molecular level may affect the performances of photoelectrochemical cells based on antenna-sensitizer molecular assemblies, (ii) the availability of fast time-resolved resonance Raman and infrared spectroscopies for monitoring intercomponent energy transfer processes, and (iii) the possibility to design extended antenna units acting as molecular conduits for long-range energy transfer.     

An experimental analysis on the evolution of the transient tip penetration in reacting Diesel sprays

Schlieren imaging has helped deeply characterize the behavior of Diesel spray when injected into an oxygen-free ambient. However, when considering the transient penetration of the reacting spray after autoignition, i.e. the Diesel flame, few studies have been found in literature. Differences among optical setups as well as among experimental conditions have not allowed clear conclusions to be drawn on this issue. Furthermore, soot radiation may have a strong effect on the image quality, which cannot be neglected.     

EXAFS and SAXS studies of ZrCo alloy doped with Hf,Sc and Ti atoms

The local structures of ZrCo alloy doped with Hf, Sc and Ti atoms before and after hydrogenation were studied by extended X-ray absorption fine structure (EXAFS) technique. For all the samples, the length of the Co–Zr bond increased and the coordination numbers of Co reduced after hydrogenation. As the Hf, Sc and Ti atoms came into the ZrCo alloy, the lengths of the Co–Zr bond varied. The ingoing Ti atoms had shortened the Co–Zr bond lengths from 2.74 to 2.62 Å in the pre-hydrogenation, whereas all of the doped atoms had no obvious effect in the post-hydrogenation. Also the size distribution of the nanoparticles in the as-cast and hydrogenated Zr–Co alloy was investigated by small-angle X-ray scattering (SAXS) technique.     

中国典型无烟煤焦水蒸气汽化活性及动力学研究

以水蒸气为汽化剂,采用热重方法,在920～1050℃条件下,研究了6种典型中国无烟煤焦的水蒸气汽化特性．结果表明,6种无烟煤焦水蒸气汽化反应的活性与无烟煤的煤化程度相对应,即煤化程度越大,无烟煤与水蒸气反应的活性越小．无烟煤焦汽化结果用缩核反应模型描述较为理想,其活化能值在213～250kJ／mol范围;活化能与指前因子存在着动力学补偿效应．