Combustion of Iron Particles Suspension in Laminar Premixed and Diffusion Flames

ABSTRACT

In the work, the combustion regimes of iron particles (d₁₀ = 4.7 µm) in a laminar pre-mixed flame (LPF) and in an axisymmetric laminar diffusion flame (LDF) are studied. For (LPF), we studied the effect of variations in gas suspension of the concentrations of fuel C_f = 0.4 ÷ 0.6 kg/m³ and oxygen C_O2 = 21 ÷ 40%, and for (LDF), pure oxygen blowing and variation in the iron concentration C_f = 0.4 ÷ 1.5 kg/m³. The results of particle temperature measurements in the combustion zone of LPF and LDF, the dispersed and phase composition of the combustion products (depending on the concentration of fuel and oxidizer) are presented. It is shown that, under certain conditions, iron particles in LPF switch to a “high-temperature” diffusion mode of combustion. This mode is accompanied by intensification of gas-phase reactions and leads to a two-mode particle size distribution of combustion products. In this mode of combustion of a gas suspension (at temperatures much lower than the boiling point of iron), the mass yield of a nanoscale fraction of combustion products reaches tens of percent and increases with increasing oxygen concentration in LPF. This mode of iron particles combustion was not observed in LDF. The features of the thermal structure of the diffusion dust flame are analyzed. It is shown that in LDF the burning time of metal particles in the diffusion mode is proportional to the particle radius $r_p$ ($\text{~}\sqrt{r_p}$in the kinetic mode), and should depend on the concentration of fuel and the size of the burner.

KEYWORDS:

• Iron particles combustion mode
• premixed dust flame
• diffusion dust flame
• dispersed composition combustion products

Nomenclature

d	=	Particle diameter, μm
Cf	=	Metal mass concentration, kg/ m3
ελ	=	Spectral emissivity of the flame front, W/sm3
Тк	=	Temperature of the condensed phase, K
$h_f$	=	Height of the flare, m
$q_f$	=	Thermal effect of burning from Fe to Fe3O4, J/kg
$P_{rad}$	=	Heat losses, W
$P_{chem}$	=	Heat release, W
$\alpha$	=	Fuel-oxygen equivalent ratio
${\rho }_g$	=	Air density, kg/ m3
W1	=	Volumetric flow rate of carrier (nitrogen) gase, m3/s
W2	=	Volumetric flow rate of oxidizing (pure oxygen) gase, m3/s
${\rho }_p$, ${\rho }_{ox}$	=	The density of the metal and metal oxide, kg/ m3
$D_{O2}$	=	Oxygen diffusion coefficient, m2/s
$k_f$	=	Macroscopic oxygen consumption rate in the LDF, m/s
$n_p$	=	Particle number density, m−3
$S_p$	=	Particle surface area, m2
PO	=	Pressure of atomic oxygen, Pa
E	=	Activation energy of dissociation of oxygen molecules, kJ/mol
$R$	=	Gas constant, J/kg K
Greek letters	=
λ	=	Wavelength, μm
k	=	Chemical reaction rate on particle surface, m/s
$\beta$	=	Particle mass exchange coefficient, m/s
τ	=	Burning time, s
$\sigma$	=	Stefan – Boltzmann constant, W/m2 K4
$\xi$	=	Stoichiometric coefficient

Acknowledgments

This article was written with financial support from the Ministry of Education and Science of Ukraine.