In this work barium sulfate precipitation is studied in a tubular reactor in a wide range of operating conditions. The effect of reactant concentrations and barium or sulfate excess on crystal-size distribution and morphology is in®estigated. Experimental results show that ion excess has a strong influence and that at high concentration aggregation takes place. Computational fluid dynamics is coupled with the finite-mode probability density function approach for taking into account both macro-and micromixing, whereas the population balance is treated by using the standard moment method. Comparison with experimental data suggests that when micromixing and agglomeration are properly taken into account, the agreement is impro®ed. Howe®er, this statement is partially affected by the lack of knowledge in barium sulfate kinetics.

This work analyzes the mechanism of spray deposition by means of computational fluid dynamics (CFD) in order to reproduce virtually the spraying of a paint gun adopted for use in the automotive industry and to predict paint drop trajectories and film builds on the target surface. The prediction of the flow of the continuous phase was obtained by solving the time averaged Navier-Stokes equations in connection with suitable closure models for turbulence (RNG and Realizable k-ε). The dispersed phase was treated by a Lagrangian approach, by tracking numerically a large number of representative particles from the gun exit to the target surface. The initial conditions for the droplets were estimated from a detailed simulation of the paint jet at the exit of the nozzle. In this way one could evaluate positions and velocities of droplets at impact and estimate the properties of the deposited layer of paint. The method was validated by comparison with experimental data obtained by phase doppler anemometry and, subsequently, the approach was applied to different geometries and operating conditions. S pray deposition processes are used for many industrial applications, mostly in the area of surface coating, but also for the manufacturing of new materials with peculiar properties. One of the reasons for their widespread use is the ability to provide a finish with very fine microstructure that results in remarkable improvement of protective and aesthetic properties.In order to obtain the appropriate characteristics of the coats, the application process of the liquid paint must be well-controlled and reproducible. As illustrated in this work, significant help can be provided by computational fluid dynamics (CFD), which can offer a detailed view of the operation and show the effect of different operating conditions by simulating the flow field generated by the spraying device. CFD can provide significant insight into the painting process and has the ability to show how changes in operating conditions, applicator type, or workpiece geometry may affect performance. A number of advances can be expected from the better understanding of the basic processes of spray painting provided by the computational methods, including: environmental issues, with improvement of paint transfer efficiency and reduction of the paint that escapes to the environment; quality issues, with more uniform coating deposition and easier identification of the reasons for maldistribution of paint; and safety issues, by assessing workers' exposure to paint as a function of work practices and local ventilation. This work will show an example of applying CFD to the simulation of an air spray painting process and to the prediction of film builds on simple surfaces. The results reveal the type of information provided by such an analysis. The approach is quite general and applicable to other conventional paint spray systems.Few authors have tried to measure and model paint sprays originated from pneumatic atomizers so far. Kwok 1 made measurements of the air flow ...

No abstract

No abstract

Steady-and unsteady-state gas permeation rates through packaging walls containing active (scavenger) materials are determined as functions of the system's physical parameters and the scavenger load. With the simplifying assumptions of constant scavenger concentration and first-order reaction kinetics, steady-state analysis shows that there is a minimum quantity of scavenger that must be added to the packaging wall if any reduction of gas permeation is to be achieved. Unsteady-state studies have established the dependence of the time needed to reach stationary behaviour on the system's physical parameters.