Recirculation mixing and ignition within the quarl of swirl burners for pulverised coal

An experimental and theoretical study of the flow, mixing and ignition inside the divergent quarl of high swirl double-concentric jets is presented. The study considered two jets with the ratios of primary to secondary diameter at the nozzle that were characteristic of burner geometries for pulverized bituminous and brown coals. A nineth scale isothermal model burner was constructed and extensive measurements taken of the velocity field and the boundary of the central recirculation zone, defined as the zero axial velocity line. The size and shape of the recirculation zone were determined for a range of primary and secondary swirl numbers and momentum flux rations. A secondary recirculation vortex was formed inside the divergent quarl when the primary flow was no swirled. Primary swirl (cocurrent with the secondary) removed the vortex, significantly increasing the recirculated mass flow and the size of the recirculation zone. A finite-difference mathematical model was then used to examine the stability of the secondary vortex under combustion conditions and investigate the behaviours of the recirculation zone for different nozzle geometries. Combustion was shown to have only a small influence on the secondary vortex and the recirculation zone. Mixing behaviour and turbulence characteristics were examined in detail for two flow configurations found for the brown coal burner geometry - one having a secondary vortex and one at lower swirl which did not. A heating technique was developed to simulate the constant temperature back flow within the recirculation zone during combustion. The mixing characteristics of the primary, secondary and recirculation species were determined by heating each stream and measuring the resulting temperature field. A mathematical diffusion model was then developed to calculate the turbulent eddy diffusivity distribution for the two flow configurations. A modified four-position hot wire anenometer technique was used to measure the Reynolds stress components. Calculation of the turbulent viscosity components indicated that in the two flows, turbulence was highly non-isotropic. Viscosity component μ_zz was found to be dominant and components μZ_zθ and μ_θθ were of the same order of magnitude as μ_zz at high swirl. Components μ_rθ and μ_rr were negligible in both flows. The turbulent Schmidt Number was found to vary from 0.2 to 7.0. A mathematical combustion model was also developed to predict coal heating and ignition behaviour inside the divergent quarl of an industrial brown coal swirl burner on which measurements were available. Isothermal turbulent diffusivity data was scaled to represent diffusion in the full size burner and to enable comparison of predicted gas temperature profiles and the location of ignition. Calculations showed that ignition was established inside the quarl and the residence time within the quarl was important in determining the extent of coal particle combustion. A weight loss of 53.6% occurred adjacent to the divergent quarl in the secondary vortex flow configuration (mean residence time 0.112 secs.) compared to a weight loss of 32.6% (0.064 secs.) without the vortex present. A mean turbulent eddy diffusivity was found to be adequate in predicting heating and ignition characteristics. Energy supplied by turbulent diffusion from the central recirculation zone was shown to be significant in determining gas and coal particle heating within and downstream of the divergent quarl. Combustion energy release was found to be of secondary importance. It is concluded however that ignition within the quarl is necessary to maintain the thermal stability of the recirculation zone.