Papers by Keyword: Effective Diffusion Coefficient

Abstract: To study the behavior of materials with special properties, such as micro and nanograin structure, it is necessary to know how the size and the form of grain influences on the effective properties of the material. In particular, for materials with fine-dispersed structure characterized by high mass transfer rate, which could be due to several reasons. To study this kind of materials is necessary to build mathematical models taking into account the peculiarities that arise from the transition to the micro structure of the macrostructure. This paper presents a method of calculating the effective diffusion coefficient, which takes into account the influence of the size and form of grains. This method could be useful for the construction of multilayer models of mass transfer. On the example of hexagonal polycrystalline material shown that the dependence of the effective diffusion coefficient of the angle at the grain boundary acquires nonlinear character with the increase of grain boundary layer.

Abstract: The present paper focuses on calculation of the effective diffusion coefficient of a matrix composite with spherical inclusions. We address the problem of the effective diffusion coefficient dependence on the size of the inhomogeneities. In this work, the basic idea of replacing an inhomogeneous inclusion by an equivalent homogeneous one is formulated. The diffusivity contribution tensor, that characterizes the inclusion’s contribution to the overall process of diffusion, is derived in the course of analysis. It is shown that the effect of the interphase reduces the “apparent” volume fraction of inclusions. The thickness of the interphase zone is identified as the parameter of dominant importance among all the characteristics of the interphase.

Abstract: To explore the effect of concrete cracks on chloride erosion, different dry-wet cycle immersion experiment for bending cracking component is designed. The chloride content in concrete is tested by using the method of RCT in simulated marine environment. It is showed that the chloride convection diffusion zone depth in the cracking component is between 15mm and 25mm. The chloride erosion process is accelerated by macro-cracks. According to experiment result, a new chloride time-dependent diffusion model is put forward, considering crack width and effective diffusion coefficient. It is found that the chloride concentration is increased with the increase of crack width, approaching to the saturated concentration.

Abstract: The most important mechanism of deterioration occurring in concrete in the cold regions are chloride ingress and freeze-thaw cycles. In this paper, the process of chloride ingress into concrete exposed to freeze-thaw cycles is experimentally researched. From the experimental results, it appears that freeze-thaw cycles make the effective diffusion coefficient become bigger. As w/c ratio increasing and fly ash content decreasing, the effective diffusion coefficient increases. The concentration of salt solution has little influence on the effective diffusion coefficient of concrete.

Abstract: Through the measured effective diffusion coefficients of Dagang vacuum residue supercritical fluid extraction and fractionation (SFEF) fractions in FCC catalysts and SiO₂ model catalysts, the relation between pore size of catalyst and effective diffusion coefficient was researched and the restricted diffusion factor was calculated. The restricted diffusion factor in FCC catalysts is less than 1 and it is 1~2 times larger in catalyst with polystyrene (PS) template than in conventional FCC catalyst without template, indicating that the diffusion of SFEF fractions in the two FCC catalysts is restricted by the pore. When the average molecular diameter is less than 1.8 nm, the diffusion of SFEF fractions in SiO₂ model catalyst which average pore diameter larger than 5.6 nm is unrestricted. The diffusion is restricted in the catalyst pores of less than 8 nm for SFEF fractions which diameter more than 1.8 nm. The tortuosity factor of SiO₂ model catalyst is obtained to be 2.87, within the range of empirical value. The effective diffusion coefficient of the SFEF fractions in SiO₂ model catalyst is two orders of magnitude larger than that in FCC catalyst with the same average pore diameter. This indicate that besides the ratio of molecular diameter to the pore diameter λ, the effective diffusion coefficient is also closely related to the pore structure of catalyst. Because SiO₂ model catalyst has uniform pore size, the diffusion coefficient can be precisely correlated with pore size of catalyst, so it is a good model material for catalyst internal diffusion investigation.

Abstract: Based on the pore size distribution simulation and the derivation of the effective diffusion coefficient, a mathematical model describing the mass transfer and chemical reaction in a cylindrical gas sensitive porous medium is formulated. The reaction features and the effective utilization of the gas sensitive porous medium are analyzed under different pore size distribution. The results indicate that, an increase in porosity or in the most probably pore diameter leads to a higher mass transfer rate of the target gas, while the surface chemical reaction ability decreases. Depending on the calculated value of the Thiele number, the variation of the gas sensitivity and the effective surface utilization can be analyzed. The gas sensitivity and the effective utilization coefficient of the gas sensitive porous medium can be controlled by adjusting pore structure parameters. This will be useful for the pore structure design of gas sensor and choosing the most reasonable operation conditions.

Abstract: Since the adverse factors such as deficient penetration and long reaction time have restricted the complete microwave-used drying of municipal sludge, the microwave-induced drying was considered which has advantages in such aspects. The investigation of the microwave-induced drying to uncover the mechanism has great meaning for its development and usage. The experiments indicated that temperature was the decisive factor affecting the drying rate. The microwave-induced sludge reached the highest drying rate at the moisture rate of 40%, with a 20% grade promotion compared with that of the original one. The molecular fracture caused by microwave radiation had obviously accelerated the drying process and the drying rate was rising in proportion to the microwave radiation dose. The diffusion coefficient of microwave-induced sludge was obviously enlarged more than 2 times grade. In this research, model Weibull proved to fit the experiment of thin-layer drying for municipal sludge best compared with the other ones.

Abstract: Spherical sponge iron (SSI) with high activity and intension could be prepared through direct reduction by hydrogen. To optimize the reduction technology, kinetic model of SSI reduction was established. The total reaction rate changing with reduction index R was deduced which describing the total reaction rate with effective diffusion coefficient De and chemical reaction rate constant k. According to the weight loss curve of SSI reduction, De and k were calculated. The total reactive rate increased with the increasing of temperature because both De and k increased with the increasing of reaction temperature. Compared De with k, it concluded that SSI reduced by hydrogen was controlled by the chemical reaction, combination of the chemical reaction and the internal diffusion, the internal diffusion when the temperature was lower than T3, from T3 to T4, over T5, respectively.

Abstract: Microsegregation induced inhomogeneity of the coarsening of cuboidal γ’(Ni3(Al,Ti)) precipitates was studied in a single crystal nickel-based superalloy CMSX-4 at temperatures ranging from 850 to 1000 °C and for an ageing time up to 5000 h. Experimental results showed a significant statistical difference in the size of the γ’ precipitates between the dendrites and interdendritic region. The achieved results are analyzed and discussed from the point of chemical heterogeneity, activation energy for coarsening, time exponent, effective diffusion coefficient and γ/γ’ interfacial energy which control coarsening kinetics of the cuboidal γ’ precipitates within the dendrites and interdendritic region.

Abstract: Fuel cell is one of the promising candidates for mobile power source. In the fuel cell, the design considering micro structure is considered to be indispensable for developing a highperformance cell. In this work, the effect of the anode electrode structural design of the DMFC (direct methanol fuel cell) on the cell performance was investigated by three-dimensional numerical simulation. The influence of diffusion layer thickness in the electrode on the mass fraction of methanol and carbon dioxide in the reaction layer is examined for various operation conditions. As a result, we clarified that the micro structure change in the electrode significantly affected the mass fraction of the species in reaction layer in the fuel cell.