Kangbin Lei

Numerical simulation of the cone–jet formation and current generation in electrostatic spray—modeling as regards space charged droplet effect

A physical model of the electric field induced by charged droplets taking account of the effect of space charged droplet emitted from the tip of cone–jet to the external electric field is proposed. Combining this model with the fluid flow equations and charge conservation equation, the evolution of the cone–jet is simulated. The diameter of droplets emitted from the cone–jet tip and current on cone–jet are predicted at various applied voltages and flow rates. The calculated droplet diameter agrees well with experimental measurement. For low conductivity liquid, the droplet diameter decreases with the increment of applied voltage, but decreases with the reduction of flow rate. The simulation result also indicates that the current on the cone–jet increases linearly with the applied voltage. The electric field induced by charged droplets results in the decrease of the cone angle and the presence of space charged droplets has a non-negligible effect on the operation parameters.

An inter-particle contact area and time restoration for softening treatment in thermal discrete element modeling

The thermal discrete element method (TDEM), a coupling of the discrete element model (DEM) with the inter-particle heat transfer models, has been applied to the simulation of the particulate systems with heat transfer. Additionally, small normal spring stiffness coefficient has often been adopted instead of the real value in the softening treatment to improve the calculation efficiency. However, present research has indicated that the heat transfer thus simulated is exaggerated even though such softening treatment has almost no effect on the movement of particles. In this letter, we propose a restoration method to restore the inter-particle contact time and contact area in the real heat transfer process even when particle stiffness is softened by several orders. This restoration method keeps the merit of the softening treatment in the DEM simulation of the particulate systems with heat transfer and avoids the unreasonable heat transfer calculations using the softening treatment method.

Numerical simulation of contact heat transfer between particles by TDEM

Heat transfer in particulate systems plays an important role in a range of industrial processes, such as drying, heating, cooling, and so on. However, the inter‐particle heat transfer mechanism is comparatively less understood. Due to the multi‐scale complexity of the system and the limitations of measurement techniques, the details of the processes are difficult to investigate experimentally. DEM is a powerful tool for us to obtain insight into the dynamics of particulate system. To extend the classical DEM to the Thermal Discrete Element Method (TDEM) for particulate system with heat transfer, the effect of softening treatment for TDEM was analyzed, which results in the change of some important micro‐behaviors of inter‐particle, such as impact time and contact area etc, and the unrealistic heat transfer between particles. Two coefficients, time restoration coefficient Crt and area restoration coefficient Cra, were derived to simulate the contact heat transfer between particles more realistically even if...

Simulation of Eccentric-Shaft Journal Microbearing by DSMC

Many micromachines use rotating shafts and other moving parts which carry a load and need fluid bearings for support. Most of them operate with air or water as the lubricating fluid. The present study analyzes air microbearing represented as an eccentric cylinder rotating in a stationary housing. The fluid mechanics and operating characteristics of microbearing are different from their larger cousins. The small length-scale may invalidate the continuum approximation in Navier-Stokes equations, and slip flow, rarefaction, compressibility and other unconventional effects may have to be taken into account. Surface effects dominate in small devices due to a high surface-volume ratio. In this study, two-dimensional eccentric-shaft journal microbearings with different eccentricities are simulated by direct simulation Monte Carlo (DSMC) code incorporated with a Volume-CAD software. The diffuse reflection model and Cercignani-Lampis-Lord (CLL) model are applied to model the molecule-surface interaction by considering the accommodation coefficients of shaft wall and housing wall separately. The distribution of mean free path in the flow field indicates that the continuum model may break down and it is necessary to carry our molecular modeling. Calculation results show that at high eccentricity and high accommodation coefficient on the housing wall (ACO) the flow may develop a recirculation region. However, the accommodation coefficient on shaft wall (ACI) does not have any effect on the occurrence of recirculation and the size of recirculation zone. There is antisymmetry of the pressure about a vertical axis, which produces a pressure force on the shaft wall. The influence of ACI to isobars is larger than that of ACO. The shear stress profile on shaft wall is big at low ACI. At the region of short clearance between the shaft wall and housing wall, it is also influenced by the surface condition of housing wall and may even change its direction at low ACO. The pressure profile is reduced in amplitude as the ACI increases, but it is enhanced a little with the increase of ACO. The ACO has great impact on the viscous force in the case of big eccentricity. With the increase of ACI, the viscous force decreases. The pressure force is high at large eccentricity. The influence of ACO to pressure force is insignificant, but the pressure force fall is enormous when ACI increases, especially for large eccentricity. The total force decreases markedly at high eccentricity when ACI increases. The ACO almost has no impact on the total force. The torque increases with ACO, but decreases with ACI. The eccentricity also has great impact on the torque, and the microbeaing may have large torque at high eccentricity. The method developed in this paper would be very useful for designing and evaluating journal microbearing.Copyright

Solid particle distribution in particle-laden turbulent channel flows

Large Eddy Simulation (LES) of fully developed particle-laden turbulent channel flows were performed at Reynolds number 180 and 644, respectively. The effects of Stokes numbers, turbulent flows SGS fluctuations and inter-particle collisions on the distributions of solid particles were discussed. It was verified that, regardless of the Reynolds number of fluid flows, the maximum departure from randomness is when the ratio of the particles aerodynamic response time to the Kolmogorov time scale of flow was approximately one. In addition, the simulation results showed that the degrees of particle preferential concentration are selectively associated with the SGS fluctuations of fluid turbulence.

Effect of boundary conditions on the performances of gas-lubricated micro journal bearing

As significant components of micromechanics, gas-lubricated microbearings are more prevalent for their special advantages than other types. The fluid dynamics of the microbearing is different from their larger cousins due to the noncontinuum effect and surface-dominated effect, which may make the Navier-Stokes equations invalid. In this paper, by considering the accommodation coefficients on journal (? i) and that on bearing (? o) separately, the microbearings with different bearing numbers under the assumption of large L/D (length to diameter) are simulated using direct simulation Monte Carlo (DSMC) program incorporated with a Volume-CAD software. The diffuse reflection model and Cercignani-Lampis-Lord (CLL) model are applied to model the molecule-surface interaction. The flow field characteristics, as well as the performances of gas-lubricated journal bearings including load-carrying capacity, attitude angle and bearing drag are obtained. The results reveal that ?i and ?o have different effects to flow field characteristics and bearing performances. The bearing number has significantly impact on the bearing performances. The method developed in this paper would be very useful for designing and evaluating the gas-lubricated journal microbearing.

Cylindrical Couette Flow of a Rarefied Gas From Macro- to Micro-Scales

The cylindrical Couette flow of a rarefied gas from macro- to micro-scales, in the case where the inner cylinder is rotating whereas the outer cylinder is at rest, is extensively investigated by direct simulation Monte Carlo (DSMC) code incorporated with a Volume-CAD software. The generalized soft sphere (GSS) model is applied to an intermolecular collision calculation. The diffuse reflection model and Cercignani-Lampis-Lord (CLL) model are used to model the molecule-surface interaction by considering the accommodation coefficients on inner cylinder (ACI hereafter) and outer cylinder (ACO hereafter) separately. The contents in this paper include following three aspects: I the flow field characteristics and force and torque on inner cylinder for eccentric Couette flow between different scales with same non-dimensional parameters (accommodation coefficients, eccentricity-clearance ratio, Knudsen number and Reynolds number) are compared; the flow field characteristics for different scales are same; with the increase of the scale, the total force on the inner cylinder increases slightly, while the torque is proportional to the scale; II the velocity profiles in concentric Couette flow under different non-dimensional parameters are studied; the result shows that the phenomenon of inverted velocity profile in the concentric Couette flow is only induced by a smooth outer cylinder; the non-dimensional tangential velocity, as well as its gradient is high at low Reynolds number; the Knudsen number has great impact on the tangential velocity profile, and the velocity profile may not be inverted in the case of low Knudsen number; III the flow field characteristics in eccentric Couette flow under different non-dimensional parameters are obtained; the recirculation zone may not appear when Knudsen number is high; the position of its center may be different depending on Reynolds number; with the increase of Reynolds number, the compressibility effect becomes important; stratified distribution of the density becomes obvious at low Knudsen number.Copyright