ON DIFFUSION VELOCITY AND THE MASS CONSERVATION EQUATION FOR COMPONENTS

The mass migration velocity(absolute velocitv) of component i in a multicomponent flow is equal to the convection velocity (frame velocity) plus the diffusion velocity (relative velocity). The diffusion velocity as well as the corresponding diffusion coefficient depends on how the convection velocity is adopted.In turbulent flow, the mass migration velocity of component i is( muss-weighted time average velocity). The diffusion velocity consists of turbulent diffusion velocity and molecular diffusion velocity is the simple lime average velocity of component i and a is a certain convection velocity). So, the part of turbulent diffusion velocity is independent of what convection velocity is taken.In the mdss conservation equation for component i, the expression for the diffusion term on its right-hand side will change when the convection velocity on its left-hand side changes. In turbulent flow, there could be no diffusion terms, or a turbulent diffusion term only or both the turbulent and molecular diffusion terms when or any velocity other than these two is taken as the convection velocity. The case, in which there could he molecular diffusion only without turbulent diffusion, occurs in laminar flow. The molecular diffusion term always depends on the adoption of convection velocitv.In two-phase flow, the value of the molecular diffusion term is often near or even exceeds that of the turbulent diffusion term, which is quite different from the case in gas mixture flow.

STRUCTURE CONTROL IN HYDROXYAPATITE SYNTHESIS BY HYDROTHERMAL REACTION AND ORGANIC MODULATORS

Properties of materials are determined not only by their composition, but also by their structure. It is, therefore, of great significance to develop the ability of tailoring the structure of materials according to the requirements of intended applications. In this work a novel glutamic acid-assisted hydrothermal process was developed to synthesize apatite crystals with controllable morphologies. Varied morphologies from whisk broom, fibers, mushroom, prickly spheres, to dandelion were produced through modulating both the formation of a transitory phase and its transformation into the final phase.

STUDY ON THE OVERALL PRESSURE BALANCE OF A DOWNFLOW CIRCULATING FLUIDIZED BED SYSTEM

A pressure balance model for a circulating fluidized bed unit that incorporates a downer has been proposed. The model predictions were validated with the experimental data obtained from a special cold-model circulating fluidized bed. Comparison of the operation stability between a CFB downer and a CFB riser has been carried out. Only one critical gas velocity exists in the CFB-riser for a given riser solids flux, while there can be many critical gas velocities for the operation of a CFB downer. Therefore, it is possible to achieve high solids concentration in a CFB downer if appropriate operating conditions are used.

MATHEMATICAL MODELING OF PARTICLEFLUID TWO-PHASE FLOW

Two-phase flow is modeled by considering interactions between partocles and their surrounding fluid at three scales: micro-scale of particle size, meso-scale of cluster size, and macro-scale for the overall particulate system consisting of clusters carried by the surrounding dilute phase.A further constraint has been found necessary to describe the stability of the twophase flow: the total energy always seeks a minimum for the dynamics of such particulate systems.The modeling can be used to explain numerous phenomena in particle-fluid flow, such as flow regime transitions and the heterogeneity of the two-phase system. It not only reflects the physical nature of the particle-fluid flow, but also makes possible quantitative description for both gas-solid (G/S) and liquid-solid (L/S) systems.