The Knudsen pump (KP) is a kind of micro-pump that can form thermally induced flows induced by temperature fields in rarefied gas environments. It has the advantages of having no moving parts, simple structure, easy construction and extension, a wide range of energy sources, and low energy consumption. With the development of Micro/Nano Electro Mechanical Systems (MEMS/NEMS), extensive studies have been conducted on KPs, and the applications of KPs have widened. In order to obtain efficient flow fields in KPs, it is necessary to adopt modern computational methods for simulation and analysis. In many circumstances, the simulation and experimental results have good agreement. However, there seems to be no comprehensive review on KPs at present. In this paper, KPs are first defined and classified according to the flow mechanisms of the thermally induced flows. Then, the three aspects of configurations, performance, and applications of KPs in the current state of research are reviewed and analyzed. Finally, the current problems of KP are discussed, and some suggestions are provided for future research and applications.

Aiming at improving the food quality during microwave process, this article mainly focused on the numerical simulation of shape effect, which was evaluated by microwave power absorption capability and temperature distribution uniformity in a single sample heated in a domestic microwave oven. This article only took the electromagnetic field and heat conduction in solid into consideration. The Maxwell equations were used to calculate the distribution of microwave electromagnetic field distribution in the microwave cavity and samples; then the electromagnetic energy was coupled as the heat source in the heat conduction process in samples. Quantitatively, the power absorption capability and temperature distribution uniformity were, respectively, described by power absorption efficiency (PAE) and the statistical variation of coefficient (COV). In addition, we defined the comprehensive evaluation coefficient (CEC) to describe the usability of a specific sample. In accordance with volume or the wave numbers and penetration numbers in the radial and axial directions of samples, they can be classified into different groups. And according to the PAE, COV, and CEC value and the specific need of microwave process, an optimal sample shape and orientation could be decided.

A Knudsen pump operates under the thermal transpiration effect or the thermal edge effect on the micro-scale. Due to the uneven temperature distribution of the walls in the channel axis direction or the constant temperature of the tips on the walls, directional thermally-induced flow is generated. In this paper the Direct Simulation Monte Carlo (DSMC) method is applied for N₂–O₂ gas mixtures in the ratios of 4:1, 1:1, and 1:4 with different Knudsen numbers in a classic rectangular Knudsen pump to study the flow characteristics of the gas mixtures in the pump. The results show that the changing in the gas physical properties does not affect the distribution of the velocity field, temperature fields, or other fields in the Knudsen pump. The thermal creep effect is related to the molecular mass of the gas. Even in N₂ and O₂ gas mixtures with similar molecular masses, N₂ can be also found to have a stronger thermal creep effect. Moreover, the lighter molecular weight gas (N₂) can effectively promote the motion of the heavier gas (O₂).

This paper systematically reviews the mathematical modeling based on the computational fluid dynamics (CFD) method of equilibrium and nonequilibrium hypersonic flows. First, some physicochemical phenomena in hypersonic flows (e.g., vibrational energy excitation and chemical reactions) and the flow characteristics at various altitudes (e.g., thermochemical equilibrium, chemical nonequilibrium, and thermochemical nonequilibrium) are reviewed. Second, the judgment rules of whether the CFD method can be applied to hypersonic flows are summarized for accurate numerical calculations. This study focuses on the related numerical models and calculation processes of the CFD method in a thermochemical equilibrium flow and two nonequilibrium flows. For the thermochemical equilibrium flow, the governing equations, chemical composition calculation methods, and related research on the thermodynamic and transport properties of air are reviewed. For the nonequilibrium flows, the governing equations that include one-, two-, and three-temperature models are reviewed. The one-temperature model is applied to a chemical nonequilibrium flow, whereas the two- and three-temperature models are applied to a thermochemical nonequilibrium flow. The associated calculations and numerical models of the thermodynamic and transport properties, chemical reaction sources, and energy transfers between different energy modes of the three models are presented in detail. Finally, the corresponding numerical models of two special wall boundary conditions commonly used in hypersonic flows (i.e., slip boundary conditions and catalytic walls) and related research, are reviewed.

This paper proposed a one-dimensional (1D) meta-beam through embedding re-entrant hexagons on a 1D single beam. Then a two-dimensional (2D) square resonator lattice with re-entrant hexagons was designed via replacing the ribs with the 1D meta-beams. Based on Bloch’s theorem and finite element method, the dispersions and bandgap properties of 1D meta-beam and 2D lattice with re-entrant hexagon were analyzed. The numerical results indicated that the bandgap property of both structures was improved via embedding re-entrant hexagons. For the 2D square resonator lattice with re-entrant hexagon, the total bandgap width was 34.3% higher than the 2D lattice without re-entrant hexagon. Then a parametric study was implemented, and the results showed the re-entrant ratio, rib thickness and the number of re-entrant hexagons had strong effects on bandgap distribution, especially the re-entrant hexagons, which enhanced the bandgap property in the low-frequency region significantly. Through calculating the contours of group velocity, the anisotropy of the 2D square resonator lattice with re-entrant hexagons was investigated, and the results indicated that there was pronounced caustic at specific parameter sets which reflects energy focus. To verify the bandgap property in the realistic application, the specimen of 2D square resonator lattice with re-entrant hexagons was prepared through additive manufacturing technology, and a low amplitude harmonic test was performed. The experimental result demonstrated the bandgap property of the 2D square resonator lattice with re-entrant hexagons. This paper may introduce a method to enhance the bandgap property of lattice metamaterial.