Microscale thermofluidic modeling of LCVD fiber growth

This dissertation presents an investigation of the modeling of pyrolytic laser-induced chemical vapor deposition (LCVD) fiber growth solved with Navier-Stokes equations. A comprehensive research project is currently undertaken by FFL of Ecole Polytechnique de Montreal for producing, testing, and process control of continuous high performance fibers. It was found that some aspects of the physical process are difficult or impossible to study through experimental techniques. For example, LCVD is often used to produce three dimensional structures having characteristic radius of the order of mum. The physical phenomena associated with LCVD also occur on radius length scales of mum while axial length scale range from mum to mm depending on proximity to the reaction zone. For this reason, with the exception of the observation of bulk properties, detailed experimental observations of the fluid flow and heat transfer associated with the LCVD process are not available. Measuring spatial temperature profiles, precisely,i.e. on the order of fraction of a mum is extremely difficult.;Consequently, the project of this dissertation is to provide a virtual reactor for numerically simulating the LCVD process and to investigate the physical processes in depth and in detail. We aim at further insight into the LCVD process through numerical modeling and analysis on the effect of forced convection of precursor gas along the direction of LCVD fiber growth. It is an advanced computational model implemented into the FLUENT CFD package for studying mass, momentum and energy exchange, including species transport, as well as chemical kinetics within a forced flow LCVD environment.;Two kinds of reactors are simulated. One is the reactor for the conditions of room temperature and pressure which is currently undertaken by FFL, the other is the reactor for the conditions of low temperature and high pressure to be undertaken in the future.;Since LCVD is a thermally activated process characterized by the inter-related fluid flow, chemical reaction and mass and heat transfer phenomena, the outcomes of this numerical simulation will allow us to analyze how these factors will effect the process. They can be used to evaluate the relative importance of the different process parameters, such as laser power, pressure, the best focus position of the laser, and different flow rates of the percursor and the surrounding gases, and the mixture situation of the two species to gain insight into the LCVD process.;LCVD experiments of FFL were carried out to verify the modeling. A validating simulation was firstly be done. Since we could measure the pulling rate of the fiber by experiments which should be equal to the growth rate in steady state, we could compare the calculated deposition rate with the measure growth rate to validate our modeling.;For the simulation results, we calculated the deposition rate and the temperature and species concentration. We further investigated the insights of the phenomena of the flow, flow plus heat transfer, and flow plus heat transfer plus chemical kinetics separately.;We investigated how the growth rate depending on the laser power (or tip temperaure), precursor flow rate, and the position of the laser beam focus. Next, we compared the outcomes of two forced convection which are along and against the fiber growth direction to prove the working hypothesis which is that forced convection of the precursor gas along the direction of fiber growth contributes to enhance cooling, increase growth rate and provides better mechanical support of the growing fiber.;Then, the situation at high pressure conditions were investigated through virtual experiments. The outcomes provided a basic understanding of the chemical kinetics and the deposition mechanisms in HP-LCVD reactor.;Lastly, benefits and contributions of this dissertation are discussed. (Abstract shortened by UMI.)...