Diagnostics of the particle field in particle-laden DC-ARC plasmas and high velocity combustion jets as applied to industrial thermal sprays

Date of Completion

January 1999

Keywords

Engineering, Mechanical

Degree

Ph.D.

Abstract

This study concerns comprehensive experimental investigations of particle fields in plasma and high velocity oxygen/fuel (HVOF) combustion flame sprays by employing state-of-the-art laser-based diagnostic instruments. Fundamental studies of particle/spray jet interaction and measurements of particle parameters (velocity, size, temperature) were conducted. A two-color pyrometer system was designed with careful selection of wavelength pairs in order to minimize the interference in the temperature measurement from strong background radiation. Simultaneous measurements of particle parameters were achieved by combining Phase Doppler Particle Analyzer (PDPA) with two-color pyrometer. For plasma sprays with transverse powder injection, particle injection visualizations indicate that particle trajectory distributions can be controlled by operating parameters such as primary and carrier gas flow rates. Particle size distributions indicate that the transverse injection of the powder results in aerodynamic classification of the powder with larger particles penetrating further across the plasma jet than smaller particles which are more readily swept by the plasma jet. Mean particle streamwise velocity distributions show that more effective acceleration of particles occurs for those particles which penetrate into the jet and fly along the jet centerline. Particle parameter distributions indicate that particle velocity, size and temperature have broad distributions for most of the measurement locations, melted and unmelted particles co-exist. Presence of a substrate has small effect on the upstream particle velocities. Significant differences in particle velocities and temperatures were observed for the nano-agglomerated powder in comparison to the conventional powder under the same plasma operating and injection conditions, which were attributed to the differences in size and particle morphology between these two powders. Experiments with HVOF spray show that particle velocity, size and temperature have relatively flat radial profiles. No apparent size gradient across the jet was observed in contrast to the transverse particle injection in plasma sprays. Particle temperature is a function of mixture stoichiometry. ^ Simultaneous measurements of particle parameters have been successfully demonstrated in both plasma and HVOF thermal sprays. The particular combination of PDPA system with superior size measurement capability and two color pyrometry is one of the first demonstrations of its successful application in the hostile thermal spray environment. ^

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