Thermal Diffusion and Associated Stress Field due to High Speed Source

This paper presents a simple model of the thermal transport caused by a source moving at high speeds along an infinite half-space. The steady-state solution is obtained and examined to determine the effects of source velocity on the effective thermal penetration depth. The temperature field produced for a particular set of boundary conditions is then expanded to indicate how an abrupt temperature application can create a stress wave response incorporating the relationships included in a well-known hydrocode.     

Water-to-Air Mass Transfer of VOCs: Laboratory-Scale Air Sparging System

Nonequilibrium air-water mass transfer experiments for six volatile organic compounds (VOCs) were conducted using a bench-scale air sparging system. VOCs used were carbon tetrachloride, trichloroethylene, tetrachloroethylene, chloroform, dichloromethane, and toluene. The average particle size of the porous media used ranged from 0.278 to 1.71 mm. The air-water mass transfer coefficients were estimated by fitting the experimental data to a lumped parameter model. The model assumed that the saturated porous media under air sparging conditions consisted of two zones. In the “mass transfer” zone, VOCs were directly impacted by the flow of air in the air channels, while in the “bulk water” zone, VOCs were not directly affected by the air flow in the air channels. The estimated air-water mass transfer coefficients (KGa) were found to increase for higher injected air flow rates and for larger mean particle sizes of porous media but were inversely proportional to the Henry's law constant of the VOCs. An empirical correlation was developed by correlating the Sherwood number with the Péclet number, the Henry's law constant, and the mean particle size of porous media. The estimated fraction (F) of the total volume of the porous media directly affected by air channels was between 5 and 20% for fine sand, indicating that a small fraction of the porous media was affected by the advecting air stream.     

Chlorinated Ethene Reduction by Cast Iron: Sorption and Mass Transfer

Tetrachloroethylene (PCE) and trichloroethylene (TCE) exhibited significant nonlinear sorption to nonreactive sites when exposed to four cast irons. Cast iron is a reactive material that promotes reductive dechlorination and has recently been used for in-situ remediation of chlorinated solvent contaminated ground water. Comparisons between PCE sorption to cast iron, graphite, and iron-containing minerals indicate that nonreactive sorption is due to exposed graphite inclusions in the cast iron. Sorption of the homologous series of chloroethenes to a cast iron adheres to Traube's rule; thus, the extent of sorption is related primarily to compound hydrophobicity. An analytical model incorporating rate-limited sorption∕desorption to nonreactive sites was used to assess sorption nonequilibrium. Effective sorption and desorption rate coefficients determined how significant mass transfer limitations to nonreactive sorption sites exist for PCE and not for TCE. The nonreactive sorption observed indicates that flow-through cast iron treatment systems will exhibit significant delayed attainment of steady-state conditions for chlorinated ethenes, particularly PCE and TCE.     

Periodic Diffusional Mass Transfer near Sediment/Water Interface: Theory

Time-variable (periodic) flow over a lake bed, and the associated boundary layer development, have the potential to control or at least influence rates of mass transfer across the sediment/water interface. An analysis for instantaneous and time averaged flux of a material across the sediment/water interface for infinite supply in the water and infinite sink in the sediment is presented. The water flow above the interface is characterized by the shear velocity (U?) which is a periodic function of time with a maximum amplitude of (U?0) as may be typical of an internal seiche (internal standing wave) motion in a density stratified lake. The relationship between the shear velocity on the lake bed and the wind shear on the lake surface is illustrated for an extremely simplified two-layered lake of constant depth. For a less restrictive analysis, shear velocities on a lake bed have to be obtained either from field measurements or from a three-dimensional lake circulation model driven by atmospheric forcing including wind. Smaller and wind-sheltered lakes will have lower (U?0) and periodicities (T). The response of the diffusive boundary layer was related to the period of the periodic motion (T), Schmidt number (Sc), and shear velocity (U?). The vertical diffusive flux at the sediment/water interface was expressed by a Sherwood number (Sh), either instantaneous or time averaged. The mean Sherwood number (Shave) varies with shear velocity of the wave motion over the sediment bed, Schmidt number (Sc) and the period (T) due to the response of the diffusive boundary layer to the time variable water velocity. Effective diffusive boundary layers develop only at low shear velocities. Where they do, maximum and minimum boundary layer thickness depends on all three independent variables (T, Sc, and U?0). The diffusive boundary layer strongly affects sediment/water mass transfer, i.e., Sherwood numbers. Mass transfer averaged over a period can be substantially less than that produced by steady-state flow at the same U?0 and Sc. At Sc = 500, typical for dissolved oxygen, the mass transfer ratio can be reduced to 60% of steady state, depending on the internal wave period (T).     

Diffusion of hydrogen in titanium alloys due to composition,temperature, and stress gradients

A theoretical and experimental study was conducted to determine the combined effects on interstitial diffusion in metals of gradients in interstitial concentration, in solvent composition, in stress, and in temperature. The theory consolidates relationships, some of which have been previously published. It is based on macroscopic irreversible thermodynamics, and is applicable to anisotropic or Isotropic materials. Experiments were conducted and literature analyzed to determine the numerical quantities required to predict the change in hydrogen distribution with time for pure and alloy titanium, where the solvent gradient, stress gradient, and temperature gradient are constant with time. The diffusion driving forces for solute gradient, solvent gradient, and stress gradient are related to the effect of each factor on hydrogen activity. In addition, the material property which determines the diffusion driving force of a stress gradient for anisotropic material is a matter tensor analogous to the scalar partial molal volume used for isotropic material. The experiments, conducted with commercially pure titanium and titanium alloy, 6A1-4V, consist of measuring the effect of alloy additions and stress on the hydrogen activity in solid solution and the dilatation effect of hydrogen. Stress states tested were tension, compression, and torsion. The measurements were made by exposing titanium and the alloy to hydrogen at temperatures from 600° to 800° C, and measuring the equilibrium hydrogen gas pressure at various solid solution hydrogen contents. Both materials were tested with and without stress. Tension decreased the activity, compression increased it, and torsion had no effect. This is consistent with the stress effect theory of Li, Oriani, and Darken. The stress effect corresponds to an apparent partial molal volume of 1.7 to 2.2 cm³/mol, depending on the alloy and hydrogen content.     

Mass Transfer Correlation Coefficients for Two-Phase Systems: A General Review for Liquid-Liquid

A general review of the mass transfer correlation coefficients available in the literature was done. The emphasis was for liquid-liquid phases and the main mathematical forms of the correlations accessible are reported. In general, there is a necessity of general models. More work has to be done in this area in order to establish more general models that permit reporting only their parameters for a given system and/or equipment.     

Physical parameters in the use of MRI tomography. Limitation due to physiologic reactions and guidelines. Radiation Protection Commission

PURPOSE: The standards and regulations concerning the protection of patients and operator staff within the context of MRI are compiled. Resulting consequences regarding physical parameters are evaluated. MATERIAL AND METHODS: The static magnetic field, heating effects caused by RF-fields and acoustical noise are outlined. The actual boundaries of these parameters are compared against the relevant published standards. Peripheral stimulation limits due to pulsed gradient fields have been determined in a new clinical study. RESULTS: Many parameters recommended for the normal operating mode are already exceeded during routine MRI. Referring to our clinical study, we found that limits recommended in the MRI relevant standards are unnecessarily conservative and can actually be doubled. CONCLUSIONS: The applicable national and international standards and regulations show (at least partly) that serious differences in the definition of terms and values exist. The application of these standards would be much easier if they were made uniform. The values defined in the MR-specific standards should be adapted to actual knowledge concerning patients' safety.     

Mass Transfer in Slag Refining of Silicon with Mechanical Stirring: Transient Interfacial Phenomena

Experiments have been carried out to study the rates of mass transfer between liquid silicon and CaO-SiO₂ slag with impeller stirring at 1823 K (1550 °C). The occurrence of transient interfacial phenomena related to the mass transfer of calcium has been observed; the evidence suggests that the reduction of calcium oxide at the interface leads to a rapid, temporary drop in the apparent interfacial tension. At low apparent interfacial tension, mechanical agitation facilitates the dispersion of metal into the slag phase, which dramatically increases the interfacial area; here, it has been estimated to increase by at least one order of magnitude. As the reaction rate slows down, the apparent interfacial tension increases and the metal recoalesces. The incidental transfer of calcium very likely promotes the transfer of boron by increasing the interfacial area. Mechanical mixing appears to be an extremely effective means to increase the reaction rate of boron extraction and could feasibly be implemented in the industrial slag refining of silicon to improve reaction rates.     

Editorial: Opportunistic infections due to Aspergillus

Changes in plasma thyroxine (T4) concentrations were followed in 27 fetal sheep after surgical implantation of catheters. Fourteen days were required before stable concentrations of T4 were achieved, whether surgery was performed between 90 and 96 days or 109 and 120 days gestation. Twenty-three fetuses were followed to birth, and during the last four days the T4 concentrations showed no change in 11 fetuses and a significant decrease in the other 12. Birth occurred between 142 and 157 days gestation in both groups. There was a significant rise in T4 concentration during labour in all 23 fetuses. There were large differences among the plasma T4 concentrations of individual ewes which were not related to ambient temperature.     

Breaking Antidunes: Cyclic Behavior due to Hysteresis

The cyclic behavior of breaking antidunes (growth, breaking of surface wave, obliteration) is investigated by use of a numerical model. The model includes the transition between supercritical and transcritical flow. As the antidune grows the flow becomes transcritical and a hydraulic jump is formed upstream of the antidune crest. The relation between growth of the antidune and supercritical flow and between decay and transcritical flow is shown, and the significance of hysteresis in the flow conditions is investigated.     

Design Model for Selective Catalytic Reduction Process: Solution of Convective Diffusion and NOx Reaction

A simple model for the design of selective catalytic reduction (SCR) process is presented. The two-dimensional mass transfer equation shown in Bai and Chwu (1997) is simplified and the analytic solution is obtained. The analytic solution is verified by the solution of the numerical two-dimensional model as well as published experimental data. Then it is applied for the design of the SCR reactor. Effects of operating temperature, inlet molar ratio of NH3∕NO, channel pitch, and gas flow rate on the SCR reactor volume are evaluated based on a designed NO removal efficiency. The NH3 slip problem of a given inlet ratio of NH3∕NO also is discussed. It is shown that the injection ratio of NH3∕NO must be slightly higher than the desired NO removal efficiency so that the reactor volume can be reduced and the problem of NH3 slip is not a major concern.     

Molecular genetics: from bench to bedside

Pancreatic ductal adenocarcinoma is characterized by a high rate of activating mutations involving codon 12 of the K-ras protooncogene. As a means of ras-targeted intervention, the effects of enhanced Krev-1 gene expression on the growth and tumorigenicity of the hamster pancreatic adenocarcinoma cell line PC-1 were evaluated. Overexpression of the Krev-1 gene product resulted in morphologic reversion to a less transformed phenotype, as well as retarded growth kinetics and diminished potential for anchorage-independent growth. Among six transfected cell lines, the magnitude of these changes correlated with the degree of Krev-1 overexpression as assessed by Western blot. When PC-1 cells overexpressing high levels of the Krev-1 gene product were assessed for tumorigenicity in syngeneic animals, an increased latency to tumor growth and a decreased tumor size were noted. The results confirm that overexpression of the Krev-1 gene may provide a useful strategy for ras-targeted intervention in this disease.