Papers by Keyword: Mass Diffusion

Abstract: With the mass diffusion module of ABAQUS, the moisture diffusion behavior of recycled aggregate concrete material (RAC) was investigated in this work. The simulation was carried out under the standard drying conditions (Ambient humidity: H=60%; Temperature: T=20°C). The results indicate that: with the dry age grows, the water evaporation begins from the RAC’s surface first, and the water concentration reaches the equilibrium value rapidly; then the evaporation process gradually extends to the interior, but the process is quit slowly. In moisture diffusion process, the water concentration takes on gradient distribution. the calculated value of water concentration matches well with the experiment results, and can provide a theoretical base to the caculation of drying shrinkage stress of RAC.

Abstract: Diffusion and partition coefficients are key parameters for the characterization of building materials as sources and sinks of air pollutants indoors. A simple and rapid experimental method for direct measurement of the diffusion coefficient (D) was presented in a previous article [1]: the modified dry cup method. This method was based upon two existent methodologies, the passive sampling on Tenax TA and the dry cup method, proposed by Haghighat et al. [2]. In the present article, some improvements were made in the experimental set-up in order to obtain more precise results. The previous cup was modified by increasing the number of Tenax tubes, connected to the bottom of the cup, from one to five. The results obtained with both cups were compared. Measurements with single compounds and with three compounds simultaneously were compared in order to evaluate the influence of competition between molecules of different compounds.

Abstract: The Lattice Monte Carlo (LMC) method recently developed by the authors is an unusually powerful and flexible method in which a given phenomenological thermal or mass transport problem is mapped onto a fine-grained lattice which is then analyzed with discrete random walk methods. We provide an overview of the LMC method. For mass diffusion we highlight the addressing of diffusion with reversible reaction. For thermal transport we highlight a calculation of the effective thermal conductivity of sintered hollow sphere structures making use of CT scans of actual material as well as the determination of temperature profiles in phase-change composites.

Abstract: Diffusion and partition coefficients are two key parameters of importance for the characterization of building materials as sources and sinks of air pollutants indoors. The number of coefficients for each pair ‘chemical compound – material surface’ available in the literature are still scarce and with a high discrepancy in the results obtained by different methods, even for the same method [1]. The objectives of this study were (1) to develop a simple and rapid experimental method for directly measuring the diffusion coefficient (D), and (2) to perform determinations with both the new and the cup method comparing the results obtained. The new method is based on two existent methodologies, the passive sampling on Tenax TA and the dry cup method, proposed by Haghighat et al. [1] and also developed by this research team [2]. The experiments were carried for three different building materials (cork based material, gypsum board and PVC) and three different compounds (cyclohexane, n-octane and m-xylene). The discussion of the results obtained for D by the two methods is presented in this paper. Observed differences were not of the same order of magnitude for the pairs compound/material studied, varying from the order of 101 in the case of PVC to 102 in the gypsum and in the cork. However some facts were the same: the gypsum board presents the higher values of D and PVC the lower values of D for the three compounds studied in both methods. One of the causes of these marked differences could be high difference of the concentration of the compound in the air between the methods, saturation value in cup method (dozens g/m3) and low values typical from indoor environments (few mg/m3) in the proposed method.

Abstract: An analytical procedure is developed to design and predict the behavior of a pressure vessel. If a pressure vessel contains hydrogen, it is difficult to predict what will happen in the future. In this study, this is accounted for and the stress intensity factor for mode-Ι is calculated because the main factor controlling mass diffusion, as a driving force, is related to the stress in this mode. Also, it is known that the stress intensity factor depends upon concentration. The main challenge in hydrogen embrittlement is the prediction of crack growth and the estimation of lifetime for a pressure vessel. This paper investigates the effect of hydrogen diffusion upon crack in a pressure vessel by using numerical finite-element simulations. The fracture behavior of the alloy as related to hydrogen embrittlement was also studied. The computational simulations involved sequentially-coupled stress and mass-diffusion concentrations at the crack tip. Although there have been various previous works in this area, most of them have been experimental estimates of hydrogen diffusion. In this paper, we calculate the stress intensity factor by using the finite-element method (FEM) and use mathematical analysis simultaneously. The analytical method alone could not be used because the mass diffusion has special characteristics. That is, the treatment of diffusion is different at each step. We conducted finite-element modeling simulations of the intergranular fracture of alloy X-750 due to hydrogen embrittlement. Sequentially coupled stress and mass diffusion determinations were carried out in order to determine crack tip stresses and hydrogen diffusivity in the crack-tip region. Good qualitative agreement between the FEM modeling and the analysis was observed.

Abstract: By bombarding a target with stable ion beams it is possible to produce nuclear radioactive species. The EXCYT facility at the Istituto Nazionale di Fisica Nucleare - Laboratori Nazionali del Sud (INFN-LNS), in Catania, is based on a K-800 superconducting cyclotron injecting high intensity beam (up to 500 W) in a Target Ion Source (TIS) assembly, and on a 15 MV Tandem for post-accelerating the Radioactive Ion Beams (RIB). Radioactive species are produced in a solid porous target where, by diffusion, they will reach the target surface. After effusion through the target container, radioactive atoms are ionized, extracted at low energy, and changed in charge (if needed). Two stages of isobaric mass separators will clean the resulting radioactive beam from contaminants, thus the RIB is injected into the Tandem for the post-acceleration. TIS is operating at high temperature (up to 2700 K) under vacuum (10-4 mbar). The production of 8Li radioactive beam was performed by injecting a 13C4+ primary beam of 45 MeV/u on a porous graphite target. In this work we present a study to optimize the target geometry in order to increase the production of the RIB. Our first RIB was 8Li which mean life time is 1.212 s. The generation density of 8Li inside the solid target is calculated by EPAX II code. A simple diffusion model which takes into account either the nuclei generation and their decay, was computer simulated to estimate the efficacious diffusion coefficient of 8Li in porous graphite which resulted to be in the range of 10-6 - 10-5 cm2/s. Since the porous nature of the target, many diffusion mechanisms are active, our assumption, supported by experimental evidence, is that at such high temperature multi-grain diffusion prevails. Diffusion length inside the target of 8Li, before its decay, resulted to be sensible smaller than the target size, therefore we modified its geometry enhancing the RIB production by a factor of 3-4.