Yangyu Guo

Lattice Boltzmann modeling of phonon transport

A novel lattice Boltzmann scheme is proposed for phonon transport based on the phonon Boltzmann equation. Through the Chapman-Enskog expansion, the phonon lattice Boltzmann equation under the gray relaxation time approximation recovers the classical Fouriers law in the diffusive limit. The numerical parameters in the lattice Boltzmann model are therefore rigorously correlated to the bulk material properties. The new scheme does not only eliminate the fictitious phonon speed in the diagonal direction of a square lattice system in the previous lattice Boltzmann models, but also displays very robust performances in predicting both temperature and heat flux distributions consistent with analytical solutions for diverse numerical cases, including steady-state and transient, macroscale and microscale, one-dimensional and multi-dimensional phonon heat transport. This method may provide a powerful numerical tool for deep studies of nonlinear and nonlocal heat transports in nanosystems.

Microstructure Effects on Effective Gas Diffusion Coefficient of Nanoporous Materials

In this work, we develop a numerical framework for gas diffusion in nanoporous materials including a random generation-growth algorithm for microstructure reconstruction and a multiple-relaxation-time lattice Boltzmann method for solution of diffusion equation with Knudsen effects carefully considered. The Knudsen diffusion is accurately captured by a local diffusion coefficient computed based on a corrected Bosanquet-type formula with the local pore size determined by the largest sphere method. A robust validation of the new framework is demonstrated by predicting the effective gas diffusion coefficient of microporous layer and catalyst layer in fuel cell, which shows good agreement with several recent experimental measurements. Then, a detailed investigation is made of the influence on effective gas Knudsen diffusivity by many important microstructure factors including morphology category, size effect, structure anisotropy, and layering structure effect. A widely applicable Bosanquet-type empirical relation at the Darcy scale is found between the normalized effective gas diffusion coefficient and the average Knudsen number. The present work will promote the understanding and modeling of gas diffusion in nanoporous materials and also provide an efficient platform for the optimization design of nanoporous systems.

Thermodynamic Extremum Principles for Nonequilibrium Stationary State in Heat Conduction

Minimum entropy production principle (MEPP) is an important variational principle for the evolution of systems to nonequilibrium stationary state. However, its restricted validity in the domain of Onsager’s linear theory requires an inverse temperature square-dependent thermal conductivity for heat conduction problems. A previous derivative principle of MEPP still limits to constant thermal conductivity case. Therefore, the present work aims to generalize the MEPP to remove these nonphysical limitations. A new dissipation potential is proposed, the minimum of which thus corresponds to the stationary state with no restriction on thermal conductivity. We give both rigorous theoretical verification of the new extremum principle and systematic numerical demonstration through 1D transient heat conduction with different kinds of temperature dependence of the thermal conductivity. The results show that the new principle remains always valid while MEPP and its derivative principle fail beyond their scopes of validity. The present work promotes a clear understanding of the existing thermodynamic extremum principles and proposes a new one for stationary state in nonlinear heat transport. [DOI: 10.1115/1.4036086]

Electric controlled spin and valley transport of massive electrons in graphene with spin-orbit coupling

In this work, we have explored the influence of an external electric field on the spin and valley transport of massive electrons in a graphene system with spin-orbit coupling. Both the strength and width of the spin- and valley-polarization are greatly dependent on the external electric field. As the external electric field increases, the spin/valley polarization can be enhanced, even up to 100%. In addition, the presence of a gap resulting from the interplay of massive electrons and spin-orbit coupling can occur in the direction of the spin polarization being changed. Without the gap, spin-down electrons can be filtered at the low-energy Fermi level. However, with the gap, the effect is just the opposite; spin-up electrons are filtered. These findings may open an avenue for the electric control of valley and spin transport in graphene-based electronic devices.