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 100kW was investigated experimen-tally and also by the use of CFD. The burner was designed as a scale model of a 10MW coal burner operating in a cement rotary kiln or boiler and was able to burn a combination of gase-ous, liquid and pulverised solid fuels. Temperature and Laser Doppler measurements of all three velocity components were obtained in reacting single and multiphase flows as a function of swirl number in the range of 0.6 to 0.9 and 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 inter-nal 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, re-sulted in a 30% widening of the IRZ. Solid particle measurements 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 downstream the burner in isothermal flows has also been performed. The flow field was mod-elled as 2D axisymmetric and results were obtained with both the renormalisation group (RNG) and k-? turbulence models and compared to measurements obtained in non combusting flows. As expected, the RNG model provided a more realistic description of the flow field than the k-? turbulence model, with numerical results being in good agreement with the measure-ments even at the high swirl number. Lagrangian tracking of coal particles in the range of 1 to 150 µm has also been performed 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 ac-cordance with the measurements, the calculations also showed that the effect of centrifuging is decreased when the swirl number is reduced.
An attempt to scale-up the results, through the application of Stokes number similarity and of the standard “constant velocity scaling” concept, has also been pursued and comparisons of the present findings to the flow field downstream a 120 kW and a 12 MW industrial burner were made.