Junbo Xu

The aggregation and diffusion of asphaltenes studied by GPU-accelerated dissipative particle dynamics

The heavy crude oil consists of thousands of compounds and much of them have large molecular weights and complex structures. Studying the aggregation and diffusion behavior of asphaltenes can facilitate the understanding of the heavy crude oil. In previous studies, the fused aromatic rings were treated as rigid bodies so that dissipative particle dynamics (DPD) integrated with the quaternion method can be used to study asphaltene systems. In this work, DPD integrated with the quaternion method is implemented on graphics processing units (GPUs). Compared with the serial program, tens of times speedup can be achieved when simulations performed on a single GPU. Using multiple GPUs can provide faster computation speed and more storage space for simulations of significant large systems. By using large systems, simulations of the asphaltene toluene system at extremely dilute concentrations can be performed. The determined diffusion coefficients of asphaltenes are similar to that in experimental studies. At last, the aggregation behavior of asphaltenes in heptane was investigated, and the simulation results agreed with the modified Yen model. Monomers, nanoaggregates and clusters were observed from the simulations at different concentrations

Integration of rotational algorithms into dissipative particle dynamics: modeling polyaromatic hydrocarbons on the meso-scale

Heavy crude oil consists of thousands of compounds, a significant fraction of which have fairly large molecular weights and complex structures. Our work aims at constructing a meso-scale platform to explore this complex fluid in terms of microstructure, phase behavior, stability and rheology. In the present study, we focus on the treatment of the structures of fused aromatic rings as rigid body fragments in fractions such as asphaltenes and resins. To derive the rotational motion of rigid bodies in a non-conservative force field, we conduct a comparison of three rigid body rotational algorithms integrated into a standard dissipative particle dynamics (DPD) simulation. The simulation results confirm the superiority of the Quaternion method. To ease any doubt concerning the introduction of rigid bodies into DPD, the performance of the Quaternion method was tested carefully. Finally, the aggregation dynamics of asphaltene in very diluted toluene was investigated. The nanoaggregates are found to experience forming, breaking up and reforming. The sizes of the asphaltene monomer and nanoaggregate are identified. The diffusion coefficient of diluted asphaltene in toluene is similar to that found experimentally. All these results verify the rotational algorithm and encourage us to extend this platform to study the rheological and colloidal characteristics of heavy crude oils in the future.

Accelerating dissipative particle dynamics with multiple GPUs

Dissipative particle dynamics (DPD) simulation is implemented on multiple GPUs by using NVIDIAs Compute Unified Device Architecture (CUDA) in this paper. Data communication between each GPU is executed based on the POSIX thread. Compared with the single-GPU implementation, this implementation can provide faster computation speed and more storage space to perform simulations on a significant larger system. In benchmark, the performance of GPUs is compared with that of Material Studio running on a single CPU core. We can achieve more than 90x speedup by using three C2050 GPUs to perform simulations on an 80 * 80 * 80 system. This implementation is applied to the study on the dispersancy of lubricant succinimide dispersants. A series of simulations are performed on lubricant-soot-dispersant systems to study the impact factors including concentration and interaction with lubricant on the dispersancy, and the simulation results are agreed with the study in our present work

Apparent hydrodynamic slip induced by density inhomogeneities at fluid-solid interfaces

This study demonstrates that even when the no-slip condition is satisfied on the surface of a solid wall, apparent hydrodynamic slip can be clearly seen owing to a continuous variation of viscosity associated with density inhomogeneity near the wall. The relationship between the apparent slip length and the local fluid properties, such as viscosity and/or density, has been established theoretically. The apparent slip length depends on the flow type and three cases are considered: shear-driven flow, body force-driven flow, and flow driven by external force acting on adsorbed solutes. Particle-based simulations have been performed and the consistency between our theory and the simulation has been verified.

Effect of different O2/N2 flow rate on the size and yield of ZnO nanostructures

The effect of O2/N2 flow rate on the size and yield of ZnO nanostructures was studied in a horizontal tube furnace via a thermal evaporation process. When the total flow rate of N2 and O2 is 400 sccm, the height/diameter ratio of the ZnO nanonails decreases with the increasing of O2 flow rate and the yield firstly increases then decreases. X-ray diffraction patterns indicate all ZnO nanonails grow preferentially along the [0001] direction. Room-temperature photoluminescence spectra show that the intensity ratio of UV to green emission increases with increasing O2/N2 flow rate. The CFD simulation results indicate that the controlled growth of ZnO nanonails can be achieved by adjusting the O2/N2 flow rate.

Effect of the cooling condition on the morphology and photoluminescence properties of ZnO nanostructures

The cooling condition has a significant effect on the morphology and optical properties of ZnO nanostructures. In this study, four types of nail-shaped nanostructures were prepared by thermal evaporation of Zn powders under different cooling conditions, including natural cooling, vacuum cooling, rapid cooling and vacuum & rapid cooling. XRD results suggest that our samples are highly crystallized wurtzite ZnO with preferable c-orientation. XPS indicated vacuum and/or rapid cooling could reduce the oxygen vacancies in the surface of ZnO samples. PL spectra showed that the intensity ratio of UV to green emission was increased under vacuum and/or rapid cooling conditions, which was in agreement with the XPS results. We can control the tip morphology of ZnO nanostructures and oxygen vacancies on the surface by adjusting cooling conditions for different applications.

Dissipative particle dynamics simulation of molecule self-diffusion under cylindrical confinement

The self-diffusion coefficient of molecules of one bead or flexible chain type under cylindrical confinement with strong exclusion interface was simulated by dissipative particle dynamics, and the anisotropy of molecular self-diffusion was analyzed in detail. The simulation results show that the self-diffusion coefficients of molecules of one bead or flexible chain type first decreased and then increased with the increase of the cylinder radius, and the cylinder radius corresponding to the minimum self-diffusion coefficient decreased with the increase of molecule length. The variation range of the self-diffusion coefficient of flexible chain molecules was amplified with the increase of the molecule length, due to the difference of their extension between the confinement and the bulk state. With the increase of the cylinder radius, the radial self-diffusion coefficient increased and approximately has an exponential relationship with the cylinder radius; the axial self-diffusion coefficient first decreased and then gradually converged to the bulk self-diffusion coefficient.

Dissipative particle dynamics simulation on the rheological properties of heavy crude oil

The rheological properties of heavy crude oil have a significant impact on the production, refining and transportation. In this paper, dissipative particle dynamics (DPD) simulations were performed to study the effects of the addition of light crude oil and emulsification on the rheological properties of heavy crude oil. The simulation results reflected that the addition of light crude oil reduced the viscosity effectively. The shear thinning behaviour of crude oil mixtures were becoming less distinct as the increase of the mass fraction of light crude oil. According to the statistics, the shear had an influence on the aggregation and spatial orientation of asphaltene molecules. In addition, the relationship between the viscosity and the oil mass fraction was investigated in the simulations of emulsion systems. The viscosity increased with the oil mass fraction slowly in oil-in-water emulsions. When the oil mass fraction was higher than 50%, the increase became much faster since systems had been converted into water-in-oil emulsions. The equilibrated morphologies of emulsion systems were shown to illustrate the phase inversion. The surfactant-like feature of asphaltenes was also studied in the simulations.

Effect of surfactants on the deformation of single droplet in shear flow studied by dissipative particle dynamics

ABSTRACT The behaviour of a single droplet in shear flow is a fundamental problem in immiscible liquid–liquid multiphase fluid systems. In this article, the deformation and inclination angle of single droplet covered with surfactants in shear flow at moderate Reynolds number, when both the inertial effects and interfacial tension are the key governing factors, were simulated by dissipative particle dynamics (DPD). Weber number We was adopted to indicate the force state of the droplet and a linear relationship between the deformation parameter D and We was found when Reynolds number Re is about 1–10, which is similar to the relation of D and Capillary number Ca when Re ≪ 1. When the surfactant concentration is lower than the critical micelle concentration (CMC), the distribution of surfactants, the droplet inclination angle θ and the droplet deformation parameter D were investigated at different surfactant density at interface ds and shear rate . When the droplet size is close to the characteristic size of surfactant molecules, phase interfaces of water in oil (W/O) and oil in water (O/W) systems have different microstructures, which result in differences in the surfactant distribution, the droplet inclination angle and deformation of the two systems.

Fabrication of micro-structural ZnO crystal and the concentration fields of the CVD reactor

Arrayed ZnO crystals were fabricated with Zn as raw material by thermal chemical vapor deposition in a tubular reactor. The characterizations indicate the product grown at upstream is the well-arrayed ZnO submicro-column with good crystal properties, that at downstream is well-arrayed nanoneedle with more defects. In combination of mass field analysis with Fluent, it is revealed that the concentration fields at the upstream and downstream positions of Zn source are not identical even in an identical temperature field. It is the diffusion that drives Zn vapor move to upstream position where the O2 is rich, and the amount diffuse to upstream is rare due to the transportation of working gas. Most of O2 is consumed when passes the Zn source, leading to more lean O2 but rich Zn at the downstream position of Zn source. Our results support that lower concentration of Zn and O2 is in favor of the formation of perfect crystal as happened at upstream, but in such condition that more Zn and rare O2, defects of O vacancy is liable to form during ZnO growth.