Nozzle geometry parameters

Controlling variables in gas atomization of melts include nozzle geometry parameters and many process parameters. An exhaustive list of these parameters is given in Tables 2.12 and 2.13, respectively, along with typical values and/or ranges of the parameters. 

The first task was to produce carriers from different recipes and in different shapes as shown schematically in Fig. 8. The raw materials diatomaceous earth, water and various binders are mixed to a paste, which is subsequently extruded through a shaped nozzle and cut off to wet pellets. The wet pellets are finally dried and heated in a furnace in an oxidising atmosphere (calcination). The nozzle geometry determines the cross section of the pellet (cf. Fig. 3) and the pellet length is controlled by adjusting the cut-off device. Important parameters in the extrusion process are the dry matter content and the viscosity of the paste. The pore volume distribution of the carriers is measured by Hg porosimetry, in which the penetration of Hg into the pores of the carrier is measured as a function of applied pressure, and the surface area is measured by the BET method, which is based on adsorption of nitrogen on the carrier surface [1]. 

The physical entrainment rate of gas varies over orders of magnitude as a function of jet stability, which acts as the surface shape generator. The controllable parameters of the system are nozzle geometry, nozzle velocity, nozzle height, jet length, and jet velocity. McCarthy et al. (Ml) have analyzed gas entrainment in such contactors in terms of the surface roughness of the jet and have proposed an entrainment ratio ... 

For the exit boundary condition studies, it will be important to study system parameters such as burned-gas velocity in the nozzle and the possibility of nonequilibrium product composition. The ability to measure burned-gas velocity near the exit of a straight tube has been demonstrated [2]. The results of exit boundary condition effects on burned-gas velocity is shown in Fig. 10.13. The same technique used to perform these measurements can be extended to investigate virtually any nozzle geometry. 

The morphology of the resulting solid material depends both on the material structure (crystalline or amorphous, composite or pure, etc.) and on the RESS parameters (temperature, pressure drop, distance of impact of the jet against the surface, dimensions of the atomization vessel, nozzle geometry, etc.)[ l It is to be noticed that the initial investigations consisted of pure substrate atomization in order to obtain very line particles (typically of 0.5-20 m diameter) with narrow diameter distribution however, the most recent publications are related to mixture processing in order to obtain microcapsules or microspheres of an active ingredient inside a carrier. 

The characteristics of the particles produced using SCF technology are influenced by the properties of the solute (drug, polymer, and other excipients), type of SCF used, and process parameters (such as flow rate of solute and solvent phase, temperature and pressure of the SCF, pre-expansion temperature, nozzle geometry, and the use of coaxial nozzles)t " ]. The influence of drug and polymer properties is discussed below. 

Lefebvre [1] has compiled droplet size correlations for variety of spray nozzles. The present chapter extends the same compilation to include more recent correlations. The correlations provided are by no means exhaustive, yet they provide commonly used correlations. These correlations are provided in Tables 24.1-24.12 at the end of this chapter. The correlations are mainly based on the (i) fluid properties (mainly density, viscosity, and surface tension), (ii) nozzle geometry, such as the exit orifice diameter, impinging angle of the air on the liquid, etc., and (iii) operational parameters such as the flow rates of the liquid or gas. While some experiments have been conducted to consider the effects of all these three types of variables, many simply choose only to deal with a handful of them, and neglect the effects of others. Obviously, the more the experimental variables, the more difficult it is to obtain an accurate correlation for the droplet size. 

Another very important parameter that often plays a role in the performance of a nozzle is its discharge coefficient. A discharge coefficient is defined as the ratio of the theoretical mass flow in a nozzle to the actual mass flow. Nozzle geometry plays a huge role in determining this. Ganippa et al. [6] derived, from theoiy, the... 

Inamura and Tomoda [28], and Inamura et al. [2] investigated the behavior of liquid sheet generated by impingement of a liquid jet onto a solid wall. They [2] combined Dombrowski s model of sheet breakup with the sheet thickness model developed based laminar boundary-layer analysis to predict the droplet size. However, they did not provide any correlation for droplet size. Fard et al. [10] studied numerically the effect of liquid properties and nozzle geometry on the droplet size distribution produced by splash plate nozzle. Again, no correlation to relate the droplet size with the studied parameters was provided. 

Since the pressure drop rate depends on the nozzle geometry (ro and L) and polymer flow properties (Ap and q) (Equation 17.11), potential in using these parameters as effective process and material variables for controlling the cell nucleation in continuous microcellular processing exists [79]. 

The gas consists of an equilibrium mixture of two conformers, in which the methyl group is trans or gauche with respect to the sulfur atom. Ab initio calculations indicate that the conformation should be %1% gauche and 18% trans. These proportions were used in the ED refinements, and the calculated differences between geometrie parameters were also incor-porated. The nozzle temperature was 355 K. 

Various correlations for mean droplet size generated by plain-jet, prefilming, and miscellaneous air-blast atomizers using air as atomization gas are listed in Tables 4.7, 4.8, 4.9, and 4.10, respectively. In these correlations, ALR is the mass flow rate ratio of air to liquid, ALR = mAlmL, Dp is the prefilmer diameter, Dh is the hydraulic mean diameter of air exit duct, vr is the kinematic viscosity ratio relative to water, a is the radial distance from cup lip, DL is the diameter of cup at lip, Up is the cup peripheral velocity, Ur is the air to liquid velocity ratio defined as U=UAIUp, Lw is the diameter of wetted periphery between air and liquid streams, Aa is the flow area of atomizing air stream, m is a power index, PA is the pressure of air, and B is a composite numerical factor. The important parameters influencing the mean droplet size include relative velocity between atomization air/gas and liquid, mass flow rate ratio of air to liquid, physical properties of liquid (viscosity, density, surface tension) and air (density), and atomizer geometry as described by nozzle diameter, prefilmer diameter, etc. 

The influence of the parameters concentration, pre- and post-expansion pressure and pre- and post-expansion temperature and geometry of the nozzle on the particle size distribution wasn t studied at all. Merely Mohamed [4] examined the influence of concentration, pre- and post-expansion pressure and temperature of the system carbon dioxide - naphthalene. No particle size distribution was measured, only the size of the smallest and biggest particles were measured. For these investigations, anthracene was used as a model substance, while the solubility of anthracene in carbon dioxide [5] is approximately more then 100 times smaler than for naphthalene. 

For the purpose of a qualitative evaluation of a complex extruder design it often may be sufficient to carry out the simulation with reduced physics, that is to pass on wall slip and to perform the simulation for example with a constant low viscosity. But the more exact the real conditions should be followed, the more activities to locate the material parameters have to be spent. A reasonable access to avoid disaccords in the interpretation of the simulations on complex geometries is to calibrate the physical model on a simple nozzle. On such a basic structure it is possible to compare the measured pressures with the calculates values and to customize the complex physics of Bingham or wall slip parameters. 

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