Asymptotic preserving and uniformly unconditionally stable finite difference schemes for kinetic transport equations

In this paper, uniformly unconditionally stable first and second order finite difference schemes are developed for kinetic transport equations in the diffusive scaling. We first derive an approximate evolution equation for the macroscopic density, from the formal solution of the distribution function, which is then discretized by following characteristics for the transport part with a backward finite difference semi-Lagrangian approach, while the diffusive part is discretized implicitly. After the macroscopic density is available, the distribution function can be efficiently solved even with a fully implicit time discretization, since all discrete velocities are decoupled, resulting in a low-dimensional linear system from spatial discretizations at each discrete velocity. Both first and second order discretizations in space and in time are considered. The resulting schemes can be shown to be asymptotic preserving (AP) in the diffusive limit. Uniformly unconditional stabilities are verified from a Fourier analysis based on eigenvalues of corresponding amplification matrices. Numerical experiments, including high dimensional problems, have demonstrated the corresponding orders of accuracy both in space and in time, uniform stability, AP property, and good performances of our proposed approach

A NUMERICAL STUDY OF LINK AND PATH DURATIONS IN MOBILE AD HOC NETWORKS

A theoretical analysis has shown that under a set of assumptions, the distribution of path duration can be well approximated by an exponential distribution when the path hop count is sufficiently large. The goal of this thesis is two folds: Using NS-2 simulations to (i) Investigate how fast the path distributional convergence takes place, and how quickly the inverse of the expected duration of a path converges to the sum of the inverses of the expected durations of the links along the path, and (ii) Validate the conditions under which the distributional convergence is established. Simulation results show that the convergence of path duration distribution takes place quickly (for path hop count larger than 6) for all eight scenarios. However, the ratio of the inverse of the expected path duration to the sum of the inverses of the expected link durations along the path does not get close to one for path hop count less than 12

Performance Analysis of a Polygeneration System for Methanol Production and Power Generation with Solar-biomass Thermal Gasification

AbstractBy using the cotton stalk as the feedstock, a polygeneration system for generating methanol and power with solar thermal gasification of biomass is proposed in this work. The endothermic reaction of biomass gasification is driven by the high temperature solar thermal energy with the range of 800∼1200°C. The flat-plate solar collector and the parabolic trough solar steam generator are used to preheat biomass and generate steam as gasification agent, respectively. The thermodynamic performance of the polygeneration system is investigated. The compressed syngas, produced by the biomass gasification, is used to produce methanol via the synthesis reactor. The un-reacted gas is used for power generation through a combine cycle power unit. The results indicate that the methanol output rate and the output power in steady operation condition is 41.56kg/s and 524.88 MW, respectively, and the maximum total exergy efficiency is 49.50% when the solar gasification temperature is 900°C. Furthermore, the highest exergy efficiency of the optimized scheme by recycling partial un-reacted syngas for methanol production reaches to 50.69%. The above studies provide a feasible way to exploit the abundant solar energy and biomass in the Western China

Advances in Polar Materials for Lithium-Sulfur Batteries

Lithium-sulfur batteries are regarded as promising candidates for energy storage devices due to their high theoretical energy density. Various approaches are proposed to break through the obstacles that are preventing Li-S batteries from realizing practical application. Recently, the importance of the strong chemical interaction between polar materials and polysulfides is recognized by researchers to improve the performance of Li-S batteries, especially with respect to the shuttle effect. Polar materials, unlike nonpolar materials, exhibit strong interactions with polysulfides without any modification or doping because of their intrinsic polarity, absorbing the polar polysulfides and thus suppressing the notorious shuttle effect. The recent advances on polar materials for Li-S batteries are reviewed here, especially the chemical polar-polar interaction effects toward immobilizing dissolved polysulfides, and the relationship between the intrinsic properties of the polar materials and the electrochemical performance of the Li-S batteries are discussed. Polar materials, including polar inorganics in the cathode and polar organics as binder for the Li-S batteries are respectively described. Finally, future directions and prospects for the polar materials used in Li-S batteries are also proposed

Harvesting Plasmonic Excitations in Graphene for Tunable Terahertz/Infrared Metamaterials

In this chapter, we focus on the development on tunable terahertz/infrared metamaterials enabled with plasmonic excitations in graphene micro-/nanostructures. We aimed the issue that high loss in the plasmonic excitations of graphene limits the performance of graphene’s ability in manipulating light. We show the enhancement of light-graphene interactions by employing plasmonic metamaterial design for proper plasmonic excitations, and coherent modulation on optical fields to further increase the bonding of light field for boosted plasmonic excitations. The enhanced plasmonic excitations in graphene provide the possibility of practical applications for terahertz and infrared band graphene photonics and optoelectronics