Combustion Spraying

Abstract

Combustion spraying, often referred to as “Flame spraying (FS),” is at the base of the wide field of thermal spray technologies, which was developed in the early twentieth century and remains one of the most widely used spray coating processes because of their favorable economics. Its most common application is in the corrosion protection of infrastructure buildings such as bridges and steel structures. Over the years, the growth of combustion processes was driven by both scientific and technical developments and market requirements. The development of High Velocity Oxy-Fuel (HVOF) spraying by Browning in 1983 is a typical example which was pushed forward by the need to produce WC-Co cermet coatings with superior properties as a replacement for the environmentally polluting chrome-plating technologies. Detonation gun (D-gun) spraying is another example of flame spraying that generally uses an acetylene-oxygen or hydrogen-oxygen mixture contained in a tube closed at one of its ends. The shock wave generated by the reaction in the highly compressed explosive medium (about 2 MPa) heats and accelerates the particles to be sprayed, which are projected toward the substrate. Gas velocities above 2000 m/s are commonly achieved in detonation spray guns. In this chapter, each of these three techniques is described in detail reviewing the basic concepts involved, gas and particle dynamics. References are given for expanded exposure to recent developments and immerging technologies and their applications.

Nomenclature

Units are indicated in parentheses; when no units are indicated, the parameter is dimensionless.

1.1 Latin Alphabet

a _i :

local sound velocity, ${a}_i=\sqrt{\gamma \rho /{\rho}_g}$ (m/s)

A :

oxidizer volume or mass (m³ or kg)

A _i :

cross-sectional area perpendicular to the direction of the flow (m²)

A _t :

throat area (m²)

D :

detonation wave velocity (m/s)

F :

fuel volume or mass (m³ or kg)

F/A :

fuel to oxidizer molar ratio

h _i :

enthalpy (J/kg)

${\dot{m}}_{O_2}$ :

oxygen gas feed rate (kg/s)

M _a :

Mach number, Ma = vg/ai

p :

pressure (Pa)

p _t :

gas pressure at the throat (Pa)

P _F :

power dissipated in the flame (kW)

q :

chemical energy release at constant pressure (J/kg)

Q :

Gas flow rate (slm)

E _D :

specific energy of detonation (J)

R :

perfect gas constant (J/K.kg)

S _t :

stoichiometric factor ${S}_t={Q}_{O_2}/{\left({Q}_{O_2}\right)}_{St.}$

R _u :

universal gas constant (8.32 J/K.mole)

R ^, :

flame richness ratio R^, = (F/A)/(F/A)_St.

t _c :

detonation cycle duration time (s)

T _g :

gas temperature (K)

T _m :

melting temperature (K)

T _p :

particle temperature (K)

T _t :

temperature at the throat (K)

u _b :

burned gas velocity (m/s)

u _u :

unburned gas velocity (m/s)

v _f :

flame velocity (m/s)

v _g :

g velocity (m/s)

1.2 Greek Alphabet

b :

burned gases (in the immediate vicinity behind the front)

g :

gas

p :

particle

t :

throat

u :

unburned gas (before the detonation wave front)

1.3 Subscripts

Δt _i :

characteristic time intervals of a detonation (s)

ϕ :

dummy variable

ϕ’ :

time- (or Reynolds) averaged dumb variable

ϕ” :

density-averaged dumb variable

γ :

specific heats ratio γ = c_p/c_v

ρ _g :

gas mass density (kg/m³)

ρ _p :

particle mass density (kg/m³)

ρ _t :

gas mass density at the throat (kg/m³)