Design and analysis of a low-temperature oxygen-fuel spray system

The present work presents the first effort to develop and analyze a low-temperature oxygen-fuel (LTOF) thermal spray system. The LTOF system can spray powder materials at controllable, moderate temperatures (800--2500 K). Temperature regulation is achieved by mixing combustion products with cold nitrogen. By combining numerical and experimental approaches, the author performed a detailed study of the LTOF process which involves complex chemical and fluid dynamic phenomena. Emphasis was put on the gas and particle dynamics in the LTOF spray process. Effects of various torch designs and operating parameters on torch performance were investigated.;A comprehensive numerical model, using the Fluent CFD code, was developed to simulate the LTOF process. Both the internal flow and external flow were included in the simulation. A global reaction mechanism taking account of the dissociation of combustion products was used to model the combustion of methane in oxygen. Gas and particle dynamic characteristics of the LTOF spray were analyzed. The effects of gas flow rates, torch geometry, powder injection location, and total power on in-flight particle state were investigated as well.;Key design considerations of the LTOF system were discussed; a set of design equations were proposed and used for building the LTOF system. Based on modeling results, engineering calculations and spray tests, a LTOF spray system was built. Performance of the LTOF spray system was evaluated by monitoring in-flight particle velocity and examining coating microstructure. Experimental results have demonstrated that the LTOF system can spray particles at various temperatures without noticeable drop in particle velocity. The advent of the LTOF system makes it possible to spray powders at temperatures engineered to specific powder material and mean particle size.;A one-dimensional simplified model of gas dynamic spray was established for quick prediction of particle conditions at the nozzle exit. Using the one-dimensional model, effects of major operating parameters (gas inlet pressure and temperature), particle size, and nozzle geometry on particle acceleration and heating were investigated. The one-dimensional analysis provided a basic understanding of particle behavior in a supersonic flow and was used for selecting parameters (gas flow rates, chamber pressure, nozzle geometry etc. ) of the LTOF process.