Mathematical modeling of the gas and powder flow in HVOF systems
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Mathematical Modeling of the Gas and Powder Flow in HVOF Systems H.H. Tawfik and F. Zimmerman A mathematical model was developed to describe the gas dynamics and heat-transfer mechanism in the gas/particle flow of high-velocity oxyfuel (HVOF) systems. A numerical solution was carried out using a PC-based computer program. One-dimensional predictions of the temperature and velocity profiles of gas and particles along the axis of flow were obtained to conduct cost-effective parametric studies and quality optimization of thermal spray coatings produced by HVOF systems. The numerical computer model allows for the variation of the HVOF system parameters, such as air/fuel ratio and flow rates, cooling water inlet temperature and flow rate, barrel length, standoff distance, particle size, and gun geometry. Because of the negligible volume of the powder relative to the gas, the gaseous phase was modeled as continuous nonadiabatic, and friction flow with variable specific heats and changing cross-sectional areas of flow. The generalized continuity, momentum, and energy equations with the influence parameters were used to model the gaseous flow regime and predict its thermodynamic properties. Empirical formulas for the mean axial decay of both velocity and temperature in the supersonic jet plume region were generated from published measurements of these parameters using laser Doppler velocimeter and Rayleigh scattering techniques, respectively. The particle drag and heat-transfer coefficients were calculated by empirical formulas in terms of Reynolds, Nusselt, and Prandtl numbers to evaluate both the momentum and heat transferred between the combustion gases and the powder particles. The model predictions showed good agreement with the particle and gas temperature and velocity measurements that are available in the literature.
Keywords
HVOF,model, optimization, parameters, quality
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1. Introduction Thermally sprayed coatings enhance materials properties and provide surface protection against their working environments in a number of industrial applications. Recently, hexavalent chromium associated with hard chromium plating was classified as a human carcinogen and environmental pollutant (Ref 1). The aerospace industry has traditionally used hard chrome plating as a corrosion protection material. Accordingly, the National Aeronautics and Space Administration (NASA) is developing thermal spray coatings to replace the hard chrome plating that is currently being used on the space shuttle main engine. These coatings are made of tungsten carbide and cobalt, chromium oxide, and FerroTic (iron-base material with titanium carbide) (Ref 1) to provide protection against wear, corrosion, and hydrogen embrittlement of the low-pressure liquid-hydrogen-carrying ducts on the shuttle main engine. One of these sections is the low-pressure fuel turbo pump (LPFTP) discharge duct used on the shuttle main engine. The duct carries liquid hydrogen fuel at a temperature o f - 2 5 3 ~ (-423 ~ to the highpressure fuel turbo pump (Ref 2). In ad
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