Molecular Dynamics Study of Hydrogen on Alkali-Earth Metal Cations Exchanged X Zeolites

The self-diffusion of hydrogen in Ca2+-, Mg2+- and Ba2+-exchanged X zeolites (Mg46X, Ca46X, and Ba46X) has been studied by molecular dynamics (MD) simulations for various temperatures and loadings. The results indicate that in the temperature range of 77–298 K and the loading range of 1–80 molecules/cell, the self-diffusion coefficients are found to range from 1.2×10-9 m2·s−1 to 2.3×10-7 m2·s−1 which are in good agreement with the experimental values from the quasielastic neutron scattering (QENS) and pulse field gradients nuclear magnetic resonance (PFG NMR) measurements. The self-diffusion coefficients decrease with loading due to packing of sorbate-sorbate molecules which causes frequent collusion among hydrogen molecules in pores and increases with increasing temperature because increasing the kinetic energy of the gas molecules enlarges the mean free path of gas molecule. The mechanism of diffusion of hydrogen molecules in these zeolites is transition diffusion. Knudsen diffusion occurs at low loading and the molecular bulk diffusion occurs at higher loading. For given temperature and loading, the self-diffusion coefficients decrease in the order Ba46X<Mg46X<Ca46X, due to the different sizes and locations of the divalent cations. Moreover, the effect of concentration of molecular hydrogen on self-diffusion coefficient also is analyzed using radial distribution function (RDF)

Development of braided rope seals for hypersonic engine applications: Flow modeling

A new type of engine seal is being developed to meet the needs of advanced hypersonic engines. A seal braided of emerging high temperature ceramic fibers comprised of a sheath-core construction was selected for study based on its low leakage rates. Flexible, low-leakage, high temperature seals are required to seal the movable engine panels of advanced ramjet-scramjet engines either preventing potentially dangerous leakage into backside engine cavities or limiting the purge coolant flow rates through the seals. To predict the leakage through these flexible, porous seal structures new analytical flow models are required. Two such models based on the Kozeny-Carman equations are developed herein and are compared to experimental leakage measurements for simulated pressure and seal gap conditions. The models developed allow prediction of the gas leakage rate as a function of fiber diameter, fiber packing density, gas properties, and pressure drop across the seal. The first model treats the seal as a homogeneous fiber bed. The second model divides the seal into two homogeneous fiber beds identified as the core and the sheath of the seal. Flow resistances of each of the main seal elements are combined to determine the total flow resistance. Comparisons between measured leakage rates and model predictions for seal structures covering a wide range of braid architectures show good agreement. Within the experimental range, the second model provides a prediction within 6 to 13 percent of the flow for many of the cases examined. Areas where future model refinements are required are identified

The Mechanism of Theory-based HIV Behavioral Intervention on Condom Use among Rural-to-urban Migrants in China: The Mediating Roles of HIV Knowledge and Condom Use Self-efficacy

Previous studies have suggested HIV knowledge and self-efficacy are important cognitive factors that might influence condom use behaviors. However, data were limited regarding their mediating effects on condom use during behavioral interventions. This study examined the mechanistic roles of these two factors on the effect of a community-based intervention aiming to increase condom use behaviors and intention among young rural-to-urban migrants in China. Data were derived from a community-based HIV behavioral intervention trial among 639 young sexually active rural-to-urban migrants in Beijing, China. Path analyses were used to examine the direct and indirect effects of the intervention program on condom use behaviors and intention over a 12-month follow-up. HIV knowledge and condom use self-efficacy at 6-month follow-up served as mediators in models. Path analyses revealed that intervention program increased condom use behaviors at 12 months through the increase of HIV knowledge at 6 months. Likewise, the intervention program increased condom use intention through the increases of HIV knowledge and condom use self-efficacy. The results suggested HIV knowledge played an important mediating role on the effect of the intervention program on condom use behaviors and intention. Additionally, condom use self-efficacy played an important role in increasing condom use intention. To increase condom use behaviors and intention among migrants, future studies are warranted that focus on improving HIV knowledge and helping migrants overcome cognitive barriers of condom use. Other efforts targeting structural and environmental barriers, such as limited healthcare access due to household registration status, are also needed to increase HIV protective behaviors

Dynamic model of field modulated magnetic gear system

Considering the special working principle and the structure of field modulation magnetic gear (FMMG), the magnetic coupling stiffnesses (MCS) are calculated and analyzed by finite element method. A magnetic coupling dynamic model is presented and the dynamic differential equations are founded. The natural frequencies and mode shapes of FMMG system are investigated. The effects of main design parameters on the natural frequencies are discussed. The results show that MCS are much smaller than the meshing stiffnesses of mechanical gears and are affected greatly by system parameters. Six modes of FMMG system have distinctive features, and six natural frequencies change differently as main parameters increase

Molecular Simulation of Hydrogen Storage in Ion-Exchanged X Zeolites

Grand Canonical Monte Carlo (GCMC) method was employed to simulate the adsorption properties of molecular hydrogen on ion-exchanged X zeolites at 100–293 K and pressures up to 10 MPa in this paper. The effect of cation type, temperature, and pressure on hydrogen adsorption capacity, heat of adsorption, adsorption sites, and adsorption potential energy of ion-exchanged X zeolites was analyzed. The results indicate that the hydrogen adsorption capacity increases with the decrease in temperatures and the increase in pressures and decreases in the order of KX<LiX<CaX. The isosteric heat of adsorption for all the three zeolites decreases appreciably with the increase in hydrogen adsorption capacity. The hydrogen adsorption sites in the three zeolites were determined by the simulated distribution of hydrogen adsorption energy and the factors that influence their variations were discussed. Adsorption temperature has an important effect on the distribution of hydrogen molecules in zeolite pores