Numerical analysis of the outside vapor deposition process
OVD Project: The purpose of this study is to present a numerical solution of the thermophoretic deposition process which occurs in the Outside Vapor Deposition technique used in the fabrication of optical fiber preforms. In the OVD technique a mixture of gasses is combined and emitted from a nozzle. A typical mixture may include methane, oxygen, nitrogen, silicon tetrachloride, and germanium chloride. Upon exiting the nozzle the methane and oxygen ignite to form a flame jet. The large amount of heat released upon combustion induces a chemical reaction in which silicon dioxide particles doped with germanium dioxide are produced. The jet of particle laden gases flows past a cylindrical bait rod. Since the bait rod is much cooler than the flame, high temperature gradients are developed in the boundary layer over the cylinder. This temperature gradient produces thermophoretic motion of the particles in the flame jet, and the particles deposit on the bait rod. Successive layers of particles are deposited in this way to produce the preform. The thermophoretic deposition of particles involves many factors such as fluid motion and heat and mass transfer so the analysis of the process is quite difficult. Analysis has been carried out by Alam and Mehrotra (1987), and Homsy et al. (1981). In these analyses a simple uniform flow was assumed, and the target rod was assumed to have a uniform temperature. The uniform flow assumption is obviously highly simplistic. Experiments have been carried out at Ohio University (Graham et al., 1989), which indicate that the uniform temperature assumption is also not valid. In the present analysis, the flow is taken to be that of a free jet aimed at a surface with a variable temperature profile. The free jet dimensions correspond to the flame jets used in OVD systems, and the temperature profile on the target surface is based on experimental results. The heat and mass transfer problem is then solved in two phases. First, the flow equations and the energy conservation equation are solved by a finite difference technique, using upwind differentiation where appropriate. The results are compared with output of a commercial numerical software, FLUENT (Creare Inc.). The mass transport equation with the thermophoretic driving force is then incorporated into the code and deposition profiles are determined. Results from the analysis indicate that the deposition is maximum around the stagnation point, and these results are comparable to those obtained from experimentation. Dispersion Forces Project: Exact expressions for Van der Waals forces between ultra fine particles have been determined by Langbein (1974). These expressions are in the form of infinite series which sum up interactions between the direct and shielded dipoles. Calculation of these series requires only geometrical factors and the particles' dielectric permittivity. Due to the slow rate at which the series converge, Kiefer et al. (1978) have presented a set of expressions vhich are easier to apply and are reported to be in good agreement with the Langbein's expressions. Furthermore, Marlov (1980) has introduced the use of a scaling factor to account for retardation of the interaction. This factor, however, assumes that the particles are points of infinitesimal volume. The present study presents a modification of the retardation scaling factor to account for the finite size of real particles. Values of the interaction energy based on the approximations of Kiefer and including the improved retardation factor for spherical water particles will be compared with results obtained by Marlov.
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:thermophoretic deposition process numerical analysis outside vapor
Date of Publication:01/01/1990