HS-PP-SS Coupled Simulation And Its Applications In Chemical Engineering

Flow and transportation at nano-/micro-scales is ubiquitous in chemical engineering such as at the interface of different phases in the multilevel pores of catalytic particles and in micro-chemical engineering systems. With the ever increasing demand of chemical engineering for more precise design and control of the processes and equipment, deeper understanding of its behaviors and mechanism is becoming more and more important. However, comprehensive investigation of the dynamic processes at nano-/micro-scales is still difficult in experiments, and traditional simulation methods based on the continuum hypothesis gradually become invalid with the increase of the system’s characteristic Knudsen number (Kn). At such scales, the fluids show more molecular discrete nature, so discrete particle methods have received more and more attention in recent years. However, the contradiction between the computing speed and accuracy has not been solved satisfactorily. In this thesis, a set of coupled models and algorithms combining the merits of different discrete particle methods are developed which can effectively and precisely simulate flow, diffusion and reaction processes at nano-/micro-scales or other systems with high Knudsen numbers. The main contents of the thesis are summarized as the following:In chapter 1, the commonly used discrete particle models, such as soft spheres (SS), hard spheres (HS), pseudo particles (PP) and direct simulation of Monte Carlo (DSMC), are briefly introduced including their advantages and disadvantages and previous work about these methods, based on which the main ideas of the thesis are proposed:The complex process near the interface or in high density regions could be simulated with SS (or their combinations) while the flow and diffusion process far from the boundary or in low density regions could be simulated with HS (or their combinations). PP with some merits of HS and SS could bridge the two models and provide local approximations in the parallelization of HS simulation.In chapter 2, the coupling between the event-driven HS simulation and the time-driven pseudo-particle modeling (PPM) is optimized and validated with pipe flow simulation. Rigorous mapping between HS and PP properties is also provided. It is proven that the optimized method can carry out parallel simulations at large scales, which can enhance the scalability of HS simulation and improve the efficiency of PPM for rarefied gas. Non-equilibrium diffusion in nano-scale channels and in complex porous media is also simulated demonstrating its potential applications to micro-chemical engineering and catalyst development. The method is also applied to supersonic flows to reveal its prospect in the aerospace field.In chapter 3, SS simulation is improved in algorithm and optimized for parallelization. A multilevel-skin neighbor-list algorithm is developed, which can enhance the searching efficiency substantially by sorting and tracing the particles of the skin domain with a simple prediction procedure. A complete set of optimization schemes including parallelization using multi-thread and vectorization techniques, multi-core and many-core hybrid parallel computing, overlapping of data caching, transferring and computing is developed to build a set of high performance parallel programs which can be implemented at large scales.In chapter 4, HS-PP-SS simulation method is finally built. A universal coupling scheme for different models is provided which supports arbitrary interfaces under the condition of a layer of PP outside SS regions where particle properties are adjusted based on theoretically derived rules. Several types of boundary conditions, such as simple geometry structure or fixed particles are supported and can be used in combination, facilitating the practical application of the method. The method is validated in classical pipe flow simulation.In chapter 5, HS-PP-SS coupled method is applied to study the influence of interface structure on the coupling of the diffusion and reaction processes in gas-solid interfacial reactions under idealized reaction scenarios. It is found that, under given reaction conditions, an optimal interface structure exists for the highest overall reaction rate, reflecting the best coordination of the diffusion and reaction processes. By establishing more realistic and precise reaction and interface models, the coupled method will be useful for analyzing the mechanism of catalytic reactions and optimization of the catalyst and catalytic particle structures.Chapter 6 summaries the main conclusions and innovations of the thesis and provides prospects on future studies.