Existence of optimal delay-dependent control for finite-horizon continuous-time Markov decision process

This paper intends to study the optimal control problem for the continuous-time Markov decision process with denumerable states and compact action space. The admissible controls depend not only on the current state of the jumping process but also on its history. By the compactification method, we show the existence of an optimal delay-dependent control under some explicit conditions, and further establish the dynamic programming principle. Moreover, we show that the value function is the unique viscosity solution of certain Hamilton-Jacobi-Bellman equation which does not depend on the delay-dependent control policies. Consequently, under our explicit conditions, there is no impact on the value function to make decision depending on or not on the history of the jumping process.Comment: 22 page

Molecular Dynamics Simulation of Macromolecules Using Graphics Processing Unit

Molecular dynamics (MD) simulation is a powerful computational tool to study the behavior of macromolecular systems. But many simulations of this field are limited in spatial or temporal scale by the available computational resource. In recent years, graphics processing unit (GPU) provides unprecedented computational power for scientific applications. Many MD algorithms suit with the multithread nature of GPU. In this paper, MD algorithms for macromolecular systems that run entirely on GPU are presented. Compared to the MD simulation with free software GROMACS on a single CPU core, our codes achieve about 10 times speed-up on a single GPU. For validation, we have performed MD simulations of polymer crystallization on GPU, and the results observed perfectly agree with computations on CPU. Therefore, our single GPU codes have already provided an inexpensive alternative for macromolecular simulations on traditional CPU clusters and they can also be used as a basis to develop parallel GPU programs to further speedup the computations.Comment: 21 pages, 16 figure

CFD simulation of the influence of suspension section on the hydrodynamics of CFB riser

The gas-solid two-phase flow in a circulating fluidized bed (CFB) is affected by hydrodynamic factors (say, superficial gas velocity, solids flux, solids inventory) , material properties (say, particle diameter) and geometric factors such as the entry and exit configuration (1-5). For example, Li (5) found that the axial profile in a CFB is heavily dependent on the length of the suspension section, which refers to the part between the riser bottom and the recycle inlet of solids. The variation of the suspension section may result in exponentially decaying or S-shaped profiles. However, most of computational fluid dynamics (CFD) simulations, especially the 2D simulations, do not take into account this factor (6-8). In this work, we perform 3D, full-loop simulation of a CFB with different lengths of suspension section in the riser, as shown in Fig. 1. The simulation results reveal that the axial profiles in the riser with longer suspension section are more likely S-shaped, which is consistent with the literature (Fig. 2). This suggests a need of full-loop simulation of CFB to understand the complicated dependence of hydrodynamics on geometric factors. Please click Additional Files below to see the full abstract

3D CFD Simulation of Combustion in a 150 MWe Circulating Fluidized Bed Boiler

Eulerian granular multiphase model with meso-scale modeling of drag coefficient and mass transfer coefficient, based on the energy minimization multi-scale (EMMS) model, was presented to simulate a 150 MWe CFB boiler. The three-dimensional (3D), full-loop, time-dependent simulation results were presented in terms of the profiles of pressure, solids volume fraction and solids vertical velocity, the distributions of carbon and oxygen, as well as the temperature. The EMMS-based sub-grid modeling allows using coarse grid with proven accuracy, and hence it is suitable for simulation of such large-scale industrial reactors

Lattice Boltzmann based discrete simulation for gas-solid fluidization

Discrete particle simulation, a combined approach of computational fluid dynamics and discrete methods such as DEM (Discrete Element Method), DSMC (Direct Simulation Monte Carlo), SPH (Smoothed Particle Hydrodynamics), PIC (Particle-In-Cell), etc., is becoming a practical tool for exploring lab-scale gas-solid systems owing to the fast development of parallel computation. However, gas-solid coupling and the corresponding fluid flow solver remain immature. In this work, we propose a modified lattice Boltzmann approach to consider the effect of both the local solid volume fraction and the local relative velocity between particles and fluid, which is different from the traditional volume-averaged Navier-Stokes equations. A time-driven hard sphere algorithm is combined to simulate the motion of individual particles, in which particles interact with each other via hard-sphere collisions, the collision detection and motion of particles are performed at constant time intervals. The EMMS (energy minimization multi-scale) drag is coupled with the lattice Boltzmann based discrete particle simulation to improve the accuracy. Two typical fluidization processes, namely, a single bubble injection at incipient fluidization and particle clustering in a fast fluidized bed riser, are simulated with this approach, with the results showing a good agreement with published correlations and experimental data. The capability of the approach to capture more detailed and intrinsic characteristics of particle-fluid systems is demonstrated. The method can also be used straightforward with other solid phase solvers.Comment: 15 pages, 11 figures, 2 tables. In Chemical Engineering Science, 201