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of plasma, or ionized gas, is used in a variety of materials processing and metallurgical applications including plasma spray coating, arc welding, near-net-shape manufacturing, plasma vapor deposition, polymer deposition, and wire bonding in microelectronic chips.[1,2] Based on the temperature and pressure range, the plasmas used in these applications can be divided in two primary types. First are the systems that use high temperature and atmospheric or near-atmospheric pressure ionized gas, also referred to as thermal plasmas. The condition of local thermodynamic equilibrium (LTE) is reached in thermal plasma with temperatures around 10,000 K and electron densities ranging from 1021 to 1026 m
3. The high temperatures prevalent in thermal plasmas are useful for heating and melting of ceramic and metallic particles in coating and welding applications and in destruction of bio-hazardous materials. The second type of systems use the low-pressure, low-temperature plasmas. The operating pressure and gas density are very low. There is a significant difference in electron and heavy particle (neutral and ions) temperature due to the weak collision coupling between them. The temperature of ions and neutral molecules is typically close to room temperature. The abundance of ionized species in this type of plasma is used to aid in chemical reactions in vapor deposition and polymer processing. V. RAJAMANI, formerly Research Assistant with the Department of Mechanical Engineering, University of Cincinnati, is Research Engineer, Saint Gobain R&D Centre, Northborough, MA 01532. R. ANAND, formerly Research Assistant with the Department of Mechanical Engineering, University of Cincinnati, is Design Engineer, Caterpillar Corporation, Peoria, IL 61614. G.S. REDDY, Scientist, is with M.H.I. Inc., Cincinnati, OH 45215. J.A. SEKHAR, Professor, Department of Chemical and Materials Engineering, and M.A. JOG, Associate Professor of Mechanical Engineering, Department of Mechanical Engineering, are with the University of Cincinnati, OH 45221-0072. Contact e-mail: [email protected] Manuscript submitted November 28, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B

In the 1960s and 1970s, heat transfer in plasma flow received much attention in the literature, mainly in the context of electrostatic probes and aerospace applications, whereas in the last 30 years, the attention has primarily focused on plasma flow as it relates to plasma-aided manufacturing. Available reviews[3,2] provide detailed discussion of a number of factors that affect the heat transfer to a solid body from plasma. It is noted that the analysis of heat transfer from plasma to a solid surface is significantly more complicated as compared to unionized gas flow, because it not only involves the hydrodynamic and thermal boundary layers encountered in unionized gas flows, but also the electrical effects due to the presence of charged species. These electrical effects arise due to the difference in mobilities of the ion and the electron. The electrons having very high mobility travel f