Complex Physicochemical Phenomena Research Articles

A diffusion-kinetic model for pulverized-coal combustion and heat-and-mass transfer in a gas stream

A diffusion-kinetic model for pulverized-coal combustion and heat-and-mass transfer in a gas stream is proposed, and the results of numerical simulation of the burnout dynamics of Kansk-Achinsk coals in the pulverized state at different treatment conditions and different model parameters are presented. The mathematical model describes the dynamics of thermochemical conversion of solid organic fuels with allowance for complex physicochemical phenomena of heat-and-mass exchange between coal particles and the gaseous environment.

Comprehensive experimental and theoretical study of fluid flow and heat transfer in a microscopic evaporating meniscus in a miniature heat exchanger

The complex physicochemical phenomena occurring in the contact line region of an evaporating meniscus are described using a unique combination of high-resolution experimental data and three complementary models. The following were used: (1) high-resolution experimental liquid profile data (thickness, slope, curvature and curvature gradient) to obtain the pressure gradient in the evaporating pentane meniscus in a vertical constrained vapor bubble (VCVB); (2) macroscopic outside surface temperature profile data; (3) a finite element model to obtain the two-dimensional heat conduction profile in the solid substrate wall (macro-model) and the solid–liquid interfacial temperature profile in the evaporating meniscus region; (4) a continuum fluid-dynamics model (micro-model) to obtain the liquid–vapor interfacial temperature, mass flow rate, Marangoni stresses, and evaporative heat flux profiles along the length of the evaporating meniscus; and (5) the Kelvin–Clapeyron model to obtain the vapor temperature profile (liquid–vapor interfacial temperature jump) in the evaporating meniscus region. The retarded dispersion constant and high-resolution thickness, slope, curvature and curvature gradient profiles were obtained from the experimental reflectivity profiles. There was a substantial increase in the measured curvature in the transition region, where the evaporation rate and flux are a maximum. To obtain numerical closure between the three complementary models, the continuum fluid dynamics model (micro-model) required slip at the solid–liquid interface to support the observed high mass flow rates in the evaporating pentane meniscus. Mass flow rates due to Marangoni stresses, capillary pressure and disjoining pressure are compared. Depending on the liquid thickness, Marangoni stresses can either enhance or hinder fluid flow towards the contact line for the evaporating pure pentane meniscus. Due to the high heat removal rate by the evaporating pentane meniscus in the transition region, dips in the vapor, liquid–vapor and solid–liquid interface temperature were obtained. The results demonstrate and describe the sensitivity and complexity of the phase change process in micro-regions.

Pétroarchéologie du silex : un retour aux sources

Petroarchaeology of flint artefacts – a return to the source. Crystallizations and various stigmata on the surfaces of prehistoric stone artefacts are the result of complex physicochemical and mechanical phenomena. Once decoded, they allow us to assign the object to a genetic–stratigraphic position, also to a post-genetic palaeogeographic site location and provide a story relative to different places of residence. This petroarchaeological method, based on the optimization of the observation techniques commonly used in petrography, mineralogy, micropalaeontology and morphoscopy at different scales, is a simple tool for the identification and sourcing of lithic raw materials, whose only limits are those determined by experience and collections available for reference. To cite this article: P. Fernandes, J.-P. Raynal, C. R. Palevol 5 (2006) .

Plasma generators for re-entry simulation

The qualification of thermal protection systems (TPS) and numerical design tools for re-entry vehicles and space probes requires the ability to understand and duplicate the prevailing complex physico-chemical phenomena, including thermal and chemical nonequilibrium near the surface of a body that enters the atmosphere of the Earth or another celestial body. At the Institut fur Raumfahrtsysteme of the University of Stuttgart, four plasma wind tunnels (PWK1-4) are in operation to simulate the thermal, aerodynamic, and chemical loads on the surface of a space vehicle. Three different plasma sources have been developed for this purpose: 1) a magnetoplasmadynamic generator for the simulation of the highenthalpy and low-pressure environment during the first phase of re-entry, 2) a thermal arcjet device for the follow-on flight path at moderate specific enthalpies and higher stagnation pressures, and 3) an inductively heated generator for basic materials experiments over a wide range of specific enthalpies and pressures. Special efforts were made to avoid electrode erosion to preclude impairing the erosion and catalytic behavior of TPS materials. A detailed description of these plasma generators and an overview of the simulation regions and operation areas of the plasma wind tunnels are presented.

Analysis and Modeling of In Situ Boron‐Doped Polysilicon Deposition by LPCVD

The low pressure chemical vapor deposition of in situ boron‐doped polysilicon films from a mixture of and was investigated in a tubular horizontal hot‐wall reactor. Only when isothermal conditions and no distributed injection scheme are used, this process presents the following features: (i) the presence of the boron doping gas enhances the deposition rate of polysilicon; and (ii) a sharp drop in the polysilicon growth rate and boron concentration along the load is generally observed which may be detrimental to the reactor yield. Based on a combined approach of modeling and experimental results, an understanding of the complex physicochemical phenomena involved is presented. In particular, the accelerating effect of boron and the slowing down effect of hydrochloric acid, one of the products of chemical reactions, are demonstrated. The global model developed, based on a continuously stirred tank reactor approach for mass balances, has proven to be a valuable tool for determining the effects of process parameters, and specially the influence of the ratio on deposition rate. An adequate solution for solving the practical problem of depositing boron‐doped polysilicon on a large number of wafers with a satisfying uniformity is proposed, which requires a reactant injection scheme and temperature ramping.

CONVECTION AND CHEMISTRY EFFECTS IN CVD - A 3-D ANALYSIS FOR SILICON DEPOSITION

The computational fluid dynamics code FLUENT has been adopted to simulate the entire rectangular-channel-like (3-D) geometry of an experimental CVD reactor designed for Si deposition. The code incorporated the effects of both homogeneous (gas phase) and heterogeneous (surface) chemistry with finite reaction rates of important species existing in silane dissociation. The experiments were designed to elucidate the effects of gravitationally-induced buoyancy-driven convection flows on the quality of the grown Si films. This goal is accomplished by contrasting the results obtained from a carrier gas mixture of H2/Ar with the ones obtained from the same molar mixture ratio of H2/He, without any accompanying change in the chemistry. Computationally, these cases are simulated in the terrestrial gravitational field and in the absence of gravity. The numerical results compare favorably with experiments. Powerful computational tools provide invaluable insights into the complex physicochemical phenomena taking place in CVD reactors. Such information is essential for the improved design and optimization of future CVD reactors.

Deactivation phenomena by site poisoning and pore blockage: The effect of catalyst size, pore size, and pore size distribution

The problem of catalyst deactivation by active site poisoning and pore blockage is analyzed. The effect of catalyst size, average pore size, and pore size distribution on the phenomenon of deactivation is investigated for two simple pore structure models, i.e., the “single pore” and “parallel bundle of pores” models. It is shown that the overall catalytic behavior and performance strongly depend on the catalyst's physical properties, such as its size, pore size, and pore size distribution. The mathematical models studied here are admittedly only oversimplified analogs of the complex physicochemical phenomena occurring during realistic industrial processes. The main qualitative features, however, of the overall catalytic behavior predicted here are the result of basic and strongly counteracting, underlying physicochemical processes. As such, the types of catalytic behavior described are not strongly dependent on the particular kinetic and diffusion models employed but are closely associated with macromolecular catalytic reaction systems that deactivate by simultaneous active site coverage and pore blockage.