Fluid Mechanics and Heat Transfer of Liquid Precursor Droplets Injected into High-Temperature Plasmas
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JTTEE5 17:60–72 DOI: 10.1007/s11666-007-9140-6 1059-9630/$19.00 ASM International
Fluid Mechanics and Heat Transfer of Liquid Precursor Droplets Injected into High-Temperature Plasmas Saptarshi Basu, Eric H. Jordan, and Baki M. Cetegen (Submitted January 26, 2007; in revised form May 14, 2007) Thermo-physical processes in liquid ceramic precursor droplets in plasma were modeled. Models include aerodynamic droplet break-up, droplet transport, as well as heat and mass transfer within individual droplets. Droplet size, solute concentration, and plasma temperature effects are studied. Results are discussed with the perspective of selecting processing conditions and injection parameters to obtain certain types of coating microstructures. Small droplets (5 microns) tend to surface precipitate-forming shells with liquid core. They may be subjected to internal pressurization leading to shattering of shells and secondary atomization of liquid within. They arrive at the substrate as broken shells and unpyrolized material.
Keywords
influence of process parameters, plasma spray forming, spray deposition, TS coating process
1. Introduction Plasma thermal spray processes are commonly employed to generate functional coatings on hardware components. Typical thermal spray process involves injection of particles into a high-temperature plasma jet and their deposition onto a component surface following melting and re-solidification. Depending on the processing conditions, different coating microstructures can be obtained in thermal spray processes (Ref 1, 2). In a different approach, injection of a liquid spray or stream containing ceramic precursor salts has been pursued with successful results (Ref 3-8). This process, now commonly referred to as Solution Precursor Plasma Spray (SPPS) process, has gained significant interest due to the relative ease of liquid injection into the plasma as compared to powder feeding and the possibilities of different compositions and in situ composition variations. As in any thermal spray process, a detailed understanding of various thermophysical and thermochemical transformations of precursor injected into the plasma is required in order to improve the design and control of the spraying process. Some of these processes include droplet shear break-up, evaporative shrinkage, Saptarshi Basu, Eric H. Jordan, and Baki M. Cetegen, Department of Mechanical Engineering, University of Connecticut, Storrs, CT 06269-3139. Contact e-mail: cetegen@ engr.uconn.edu.
60—Volume 17(1) March 2008
and precipitation of dissolved solute resulting in different particle morphologies. The chemical, thermal, and morphological states of the plasma-processed precursor droplets ultimately determine the coating microstructure and its macroscopic properties. Parameters such as droplet size, injection velocity, location and temperature zone of plasma where droplets are injected, precursor characteristics, plasma temperature, and velocity fields can all have varying influences on the final outcome. Figure 1 shows various po
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