Coalbed methane reservoir dynamic prediction model by combination of material balance equation and crossflow-diffusion

The proposed dynamic prediction model is based on the occurrence and migration law of coalbed methane and on the pseudo-steady state diffusion model. The model integrated multi-phase flow theory and material balance equation. It is established by considering not only desorption and diffusion but also crossflow. Moreover, the calculation results show the rationality of the method used. The research analyzes the effects of crossflow factor and diffusion coefficient on development effectiveness. The effect of crossflow factor is obvious in the late production stage but not in the early stage. The diffusion coefficient affects gas production during the exploitation stage.     

扩散过滤燃烧的数值模拟研究

通过二维双温模型,使用甲烷详细的化学反应机理,设计了甲烷与氧气扩散燃烧器模型,并与实验值进行对比验证了其有效性。分析了小球直径,燃料质量分数等对火焰高度的影响。结果表明:火焰高度随着入口流速和甲烷质量分数的增大而增大;随着小球直径、固体导热系数、分子扩散系数和质量弥散系数的增大而减小。与预混气体过滤燃烧相比,扩散过滤燃烧的反应区域的气体和固体的温度分布是不同的,二者之间的最大温差达到188 K。     

Hydrogen permeation under high pressure conditions and the destruction of exposed polyethylene-property of polymeric materials for high-pressure hydrogen devices (2)-

Aiming to elucidate physical property affecting to hydrogen gas permeability of polymer materials used for liner materials of storage tanks or hoses and sealants under high-pressure environment, as model materials with different free volume fraction, five types of polyethylene were evaluated using two methods. A convenient non-steady state measurement of thermal desorption analysis (TDA), and steady-state high-pressure hydrogen gas permeation test (HPHP) were used both under up to 90 MPa of practical pressure. The limit of TDA method of evaluation for the specimens suffering fracture during decompression process after hydrogen exposure was found. Permeability coefficient decreased with the decrease of diffusion coefficient under higher pressure condition. Specific volume and degree of crystallinity under hydrostatic environment were measured. The results showed that the shrinkage in free volume caused by hydrostatic effects of the applied hydrogen gas pressure decreases diffusion coefficient, resulting in the decrease of permeability coefficient with the pressure rise.     

Modeling lithium diffusion in nickel composite graphite

A simple theoretical model is presented to simulate the galvanostatic discharge behavior of the Ni-composite graphite electrode. The discharge profiles predicted by using a constant diffusion coefficient (CDC) and by a varied diffusion coefficient (VDC) are compared in this paper. The results show that, the VDC model can be simplified to the CDC for discharge rates less than 2C for a 5 μm particle. Also, an approximate analytical solution is presented for VDC model, which is found to be valid for discharge rates up to 6C. Exchange current and diffusion coefficient for the lithium-diffusion are predicted.     

A simple moisture transfer model for drying of sliced foods

The mathematical formulation of mass transfer in drying processes is often based on the nonlinear unsteady diffusion equation. In general, numerical simulations are required to solve these equations. Very often, however, indirect and simplified methods neglecting fundamentals of the processes are used. In this work, a new mathematical model approach for the mass transfer occurring during drying of sliced foods is proposed. The model considers fundamentals of the drying process and takes internal resistance to moisture transfer into account. The parameters in the formulation have physical meaning and permit giving clear view of the moisture depletion process occurring during drying. The proposed model has an analytical solution and allows finding effective diffusion coefficient accurately. The verification of the model is made with basic drying experiments performed for chili red peppers sliced in slab form. The results reveal that there is nearly perfect match between the drying curves obtained by the model and the experiments.     

A simplified model for bi-component droplet heating and evaporation

A simplified model for bi-component droplet heating and evaporation is developed and applied for the analysis of the observed average droplet temperatures in a monodisperse spray. The model takes into account all key processes, which take place during this heating and evaporation, including the distribution of temperature and diffusion of liquid species inside the droplet and the effects of the non-unity activity coefficient (ideal and non-ideal models). The effects of recirculation in the moving droplets on heat and mass diffusion within them are taken into account using the effective thermal conductivity and the effective diffusivity models. The previously obtained analytical solution of the transient heat conduction equation inside droplets is incorporated in the numerical code alongside the original analytical solution of the species diffusion equation inside droplets. The predicted time evolution of the average temperatures is shown to be reasonably close to the measured one, especially in the case of pure acetone and acetone-rich mixture droplets. It is shown that the temperatures predicted by the simplified model and the earlier reported vortex model are reasonably close. Also, the temperatures predicted by the ideal and non-ideal models differ by not more than several degrees. This can justify the application of the simplified model with the activity coefficient equal to 1 for the interpretation of the time evolution of temperatures measured with errors more than several degrees.     

Numerical simulation model for predicting air quality along urban main roads: First report,development of atmospheric diffusion model

A simulation model for predicting air quality along urban main roads is being studied. The objective of the model is to predict the effects on air quality of various road-related parameters such as configurations of roads and surrounding buildings as well as traffic flow, chemical reactions, and other related phenomena. In this paper, the development of an atmospheric diffusion model, which will be the platform of the whole simulation model, is reported. A new concentration diffusion coefficient model is proposed, in which the effect of vehicle wind is taken into account. By comparing with field experiments in which the diffusing tracer gas concentration was measured in a real street canyon, the validity of the simulation model is verified. Also, the new diffusion coefficient model is found to be capable of improving predictive accuracy of air quality around a street canyon. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(7): 483–496, 1998     

Experimental and Numerical Studies on the Diffusion of CO2 from Oil to Water

Injecting CO2 into oil reservoirs can improve the oil recovery,meanwhile achieve CO2 storage.The diffusion of CO2 in oil-water systems has a substantial impact on this process.The interface significantly affects the mass transfer of CO2 between oil and water phase.In this paper,based on the determination of the CO2 diffusion coefficient in oil or water phases,the diffusion processes of CO2 from oil to water were experimentally investigated under different pressures.A numerical method was proposed to calculate the pressure drop and the diffusion coefficient in the process of CO2 diffusion from oil to water.The experimental results indicated that the CO2 diffusion coefficient in oil or water increased rapidly with pressure up to the critical pressure of CO2 and gradually slowed down thereafter.The CO2 diffusion from oil to water was much slower than that in oil or water.The diffusion coefficient of CO2 from oil to water was one magnitude lower than that in the single liquid phase of oil or water,and the effect of pressure was not significant.Based on the diffusion coefficient of CO2 in a single liquid phase and the proposed numerical method,the pressure drop and the numerical diffusion coefficient in the process of CO2 diffusion from oil to water were calculated.The relative errors between the experimental and numerical results were within 9%.Therefore,the numerical method proposed herein can be used to predict the diffusion process of CO2 from oil to water and the diffusion coefficient associated with this process.     

Evaluation of hydrogen permeation through standalone thermally sprayed coatings of AISI 316L stainless steel

This research evaluates hydrogen permeation and its diffusion characteristics through standalone thermally sprayed coatings of AISI 316L stainless steel. The effects of various charging currents and other parameters on hydrogen diffusion coefficient were scrutinized using electrochemical hydrogen permeation tests. Hydrogen permeation through the thermally sprayed coatings displayed anomalous behavior such that a maximum pinnacle was observed in the permeation curves, attributed to heavily trapped hydrogen atoms in the delayed surface cracks. Therefore, new diffusion parameters were defined for modeling of the anomalous permeation curves. The fitted diffusion parameters were consistently identified, and hence, the model perfectly explained experimental data. The results showed that the increase in charging current caused fast activation and development of surface cracks. The measured diffusion coefficient of hydrogen in the stainless steel thermally sprayed coating was relatively high because the microstructure of the coating contained some ferritic phases and dense dendritic structure, which configure fast diffusion paths.     

Analysis of hydrogen diffusion coefficient during hydrogen permeation through pure niobium

The hydrogen solubility and the hydrogen permeability of pure niobium at high temperature are investigated in order to analyze the hydrogen diffusion coefficient during the hydrogen permeation. It is shown that the hydrogen dissolution reaction into niobium metal does not follow the Sieverts' law at the practical hydrogen permeation pressures. The hydrogen diffusion coefficient during the hydrogen permeation through pure niobium at high temperature is evaluated from the linear relationship between the normalized hydrogen flux, J·d, and the hydrogen concentration difference, ΔC. It is found that the hydrogen diffusion coefficient under the practical condition is much lower than the reported values measured for dilute hydrogen solid solutions. Surprisingly, the hydrogen diffusion is found to be faster in Pd–Ag alloy with fcc crystal structure than in pure niobium with bcc crystal structure at 773 K during the hydrogen permeation.     

The kinetic behaviour of ion transport in WO3 based films produced by sputter and sol–gel deposition:: Part I. The simulation model

This paper develops a new simulation model for charge injection and extraction in electrochromic WO₃ films under conditions of constant current during the charge injection process. The model is applied to the constant current coloration and bleaching of electrochromic films, and values for the diffusion coefficient and other model parameters have been obtained by fitting the model to experimental data. Application of the model to coloration and bleaching of electrochromic devices is discussed, in particular the use of the model in the design of electrochromic switching schemes.     

Determination of moisture diffusion coefficient in transformer paper using thermogravimetric analysis

This paper presents the methodology used and the results obtained in the determination of moisture diffusion coefficient of non-impregnated transformer insulating paper. In order to establish the diffusion coefficient, drying curves of paper samples were obtained by means of thermogravimetric experiments. The diffusion coefficient parameters were found by applying an optimization process based on genetic algorithms. The error function between measured and simulated curves was determined, and the parameters achieving the best correspondence between measured and estimated values were obtained. As a result, a new equation for the diffusion coefficient of non-impregnated insulating paper is proposed, depending on average moisture concentration, temperature and insulation thickness. The proposed coefficient was validated through experimental cases finding a good agreement between the experimental drying curves and those obtained by simulation using the diffusion coefficient. The proposed diffusion coefficient can be used for the determination of the time required to dry power transformers in factory.     

预裂混凝土氯离子扩散系数计算模型及验证

为探讨水胶比、裂缝宽度、裂缝深度等因素对预裂混凝土内氯离子扩散的综合影响,采用快速氯离子迁移系数法获取多因素影响下的氯离子扩散系数,并考虑水胶比、时间衰减和裂缝等因素,建立了氯离子扩散系数计算模型。结果表明,氯离子扩散系数随水胶比增大而增大,在0.05~0.50mm裂缝宽度范围内与裂缝宽度成正相关,裂缝处氯离子渗透深度与裂缝深度成正比;氯离子扩散系数计算模型拟合结果与试验结果均吻合较好。     

Effect of plastic deformation at room temperature on hydrogen diffusion of S30408

Understanding the influence of plastic deformation on diffusion is critical for hydrogen embrittlement (HE) study. In this work, thermal desorption spectroscope (TDS), slow strain rate test (SSRT), feritscope, transmission electron microscope (TEM) and TDS model were used to study the relation between plastic deformation and hydrogen diffusion, aiming at unambiguously elucidating the effect of plastic deformation on hydrogen diffusion of austenitic stainless steel, S30408. An effective method was developed to deduce apparent hydrogen diffusion coefficient of austenitic stainless steel in this paper. Results indicate apparent hydrogen diffusion coefficient decreases firstly and then increases with increasing plastic deformation at room temperature. Hydrogen diffusion effected by plastic deformation is a complicated process which is suggested to be divided into two processes controlled by dislocation and strain-induced martensite, respectively, and the transition point is about 20% strain demonstrated by experiments in this case.     

Modeling of inhomogeneous compression effects of porous GDL on transport phenomena and performance in PEM fuel cells

A comprehensive, three‐dimensional model of a proton exchange membrane (PEM) fuel cell based on a steady state code has been developed. The model is validated and further be applied to investigate the effects of various porosity of the gas diffusion layer (GDL) below channel land areas, on thermal diffusivity, temperature distribution, oxygen diffusion coefficient, oxygen concentration, activation loss and local current density. The porosity variation of the GDL is caused by the clamping force during assembling, in terms of various compression ratios, that is, 0%, 10%, 20%, 30% and 40%. The simulation results show that the higher compression ratio on the GDL leads to lower porosity, and this is helpful for the heat removal from the cell. The compression effects of the GDL below the land areas have a contrary impact on the oxygen diffusion coefficient, oxygen concentration, cathode activation loss, local current density and cell performance. Generally, a lower porosity leads to a smaller oxygen diffusion coefficient, a less uniform oxygen concentration, a higher activation loss, a smaller local current density and worse cell performance. In order to have a better cell performance, the clamping force on the cell should be as low as possible but ensure gas sealing. Copyright © 2016 John Wiley & Sons, Ltd.     

Multiscale insights into hydrogen charging behavior in metal hydride reactor packed with porous ZrCo particles

The primary purpose of this work is to develop a novel model for comprehensively investigating the hydrogen storage performance under the framework of diffusion of hydrogen atoms through hydride layer. The proposed model is constructed upon perfectly mathematical-physical equations, by taking into account complicated multi-scale and multi-physics coupling actions. Importantly, three-dimensional numerical simulations are performed to explore the coupling effects of micro diffusion, mesoscopic permeation, and macroscopic fluid flow. An analytical approach accounting for the characteristics of reaction bed, particle, and crystal grain is presented as well. In addition, a parametric analysis is conducted to reveal that the hydride particle dimension, particle porosity, grain size, and diffusion coefficient of reacted layer have a significant effect on overall hydrogen storage performance, highlighting that grain size and hydrogen diffusion coefficient are vital factors that need to be considered for material preparation and design.     

Steam gasification of char from wood chips fast pyrolysis: Development of a semi-empirical model for a fluidized bed reactor application

This study, performed in the context of GAYA project, focuses on the development of a simple predictive model about steam gasification of char from woodchips fast pyrolysis. A semi-empirical model was developed through experiments in a macro thermogravimetric analyzer which owns the peculiar ability of fast heating, as well as to deal with macro-size particles and higher mass loads compared to conventional TGA. The experimental results show that gasification is controlled by chemical kinetics and internal transfer phenomena. During gasification, char particles can be considered as isothermal in a given range of temperatures and particle sizes, more likely for low values. The gasification model was based on the effectiveness factor, which involves the chemical kinetics and diffusion rate. The chemical kinetics were expressed by a classical Arrhenius law, whereas empirical expressions from mathematical fitting of the experimental data were established for the diffusion coefficient and surface function. The diffusion coefficient from this work is suspected to probably include supplementary rate limiting phenomena, apart from steam porous diffusion, such as H₂ inhibition and/or the decrease of temperature within char particles because of the endothermic character of gasification. The model globally predicts with accuracy the gasification rate in typical operating conditions of a fluidized bed reactor. For its simplicity and reliability, this approach can be used for the modelling of char gasification in the conditions of interest.     

Statistical treatment of scalar transfer in isotropic turbulence

It has recently been known that local high-gradient regions of an advected scalar, such as temperature or mass contaminant, in a turbulent state of fluid form thin sheets, randomly oriented and moving around with turbulent motion. Here is presented a joint multifractal model for velocity and scalar dissipations in isotropic turbulence which can predict the statistical distributions of the worms, vorticity-concentrated regions, as well as the above-described scalar-gradient sheets. This model allows us to derive turbulent diffusion coefficient in isotropic turbulence scaled by Reynolds number and Prandtl number, which predominates far over molecular diffusion coefficient.     

Effective diffusion coefficient of methylene blue in agar gel

The effective diffusion coefficient of dyes in polymers is generally highly dependent on the concentration of the diffused substance. This fact has been taken into account in proposing the equations of mass transfer and has made it possible to obtain experimentally the effective diffusion coefficient of methylene blue in agar gel. Densitometric techniques were used to quantify concentration profiles under nonstationary state conditions. The dependence of the diffusion coefficient on the concentration was interpreted theoretically and the comparison of experimental distributions of concentration with numerical solutions of the differential equations allowed to obtain the corresponding parameters. The influence of the dye concentration and of the temperature on the diffusion coefficient and position of the colored front was analyzed and equilibrium concentrations in the interphase have been determined by spectrophotometric techniques. The apparent activation energy for the diffusion of the dye in the gel was quantified and the results obtained were applied to the development of a time-temperature indicator integrator for frozen foods.