Flow Characterization of High Velocity Oxy-Fuel Thermal Sprays

Abstract

High-velocity oxy–fuel (HVOF) thermal spraying system is a highly promising technique for applying durable coatings on structural materials for corrosive and high temperature environments in advanced ultrasupercritical coal-fired (AUSC) boilers, steam turbines, and gas turbines. HVOF thermal spraying is the preferred method for producing coatings with low porosity and high adhesion. The HVOF thermal spray process has demonstrated to be one of the most efficient techniques to deposit high performance coatings at moderate cost. A computational fluid dynamics (CFD) model has been developed to predict gas dynamic behavior in a HVOF thermal spray gun in which non-premixed oxygen and methane are burnt in a combustion chamber linked to a parallel-sided converging-diverging nozzle. The CFD analysis is applied to investigate axisymmetric, steady-state, turbulent, compressible, chemically reacting, subsonic and supersonic flow inside and outside the gun. The gas axial velocity, axial temperature, static pressure and Mach number distributions are presented for various locations inside and outside the gun. The calculated results show that the most sensitive parameters affecting the process are the equivalence ratio and total gas flow rate. Gas dynamic behavior along the centerline of the gun depends on both total gas flow rate and equivalence ratio. The numerical simulations show that the gas axial velocity, axial temperature, static pressure and Mach number distribution depend on both flow rate and equivalence ratio. The maximum velocity, temperature, and pressure are achieved at the highest flow rate and/or richest equivalence ratio. In addition, the results reported in this paper illustrate that the numerical simulation can be one of the most powerful and beneficial tools for the HVOF thermal spray system design, optimization and performance analysis.