Abstract
An experimental and numerical investigation on the effect of swirl on the motion of coal particles in the near-burner region of a multi-fuel swirl-stabilised laboratory burner of total thermal input of 100 kW, has been performed. The burner was designed as a scale model of a 10 MW coal burner operating in a cement rotary kiln, produced flames of different aerodynamic characteristics and was able to burn a combination of gaseous, liquid and pulverised solid fuels. Temperature and Laser Doppler measurements of all three velocity components obtained in isothermal and reacting single and multiphase flows as a function of swirl number in the range of 0.45 to 0.9 confirmed the ability of the burner to produce close-to-industrial conditions making it a powerful tool in the field of combustion research.
In detail, the velocity measurements showed that the flow field was axisymmetric and an internal recirculation zone (IRZ) in the shape of a toroidal vortex was formed around the centreline for swirl numbers of at least 0.65. A 60% increase in the swirl number, from 0.65 to 0.9, resulted in a 30% widening of the IRZ. Solid particle measurements in the same configuration revealed that the width of the zone where coal particles recirculate is by 20% larger than that formed in the single phase case and that most of the coal particles are centrifuged away from the IRZ, particularly at high swirl numbers.
A numerical investigation on the effect of the swirl number on the fluid and particle motion has also been performed. The flow field was modeled as 2D axisymmetric and results were obtained with both the RNG and k-ε turbulence models and results were shown to be in good agreement with the measurements even at the high swirl number, when the RMG model was used. Lagrangian tracking of coal particles in the range of 1 to 150 μm was also performed by as a function of swirl number. The calculations revealed that particles of diameter larger than about 20 μm are centrifuged away from the IRZ in accordance to the measurements while particles larger than 100 μm, due to their high inertia remain on the IRZ boundary and are neither centrifuged nor entrained inside the recirculation zone, In accordance with the measurements, the calculations also showed that the effect of centrifuging is decreased when the swirl number of reduced.