On the analysis of turbulent transport processes in nonreacting multijet burner flows |
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Authors: | M V Heitor A L N Moreira |
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Institution: | (1) Dept. Mechanical Engineering, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1096 Lisbon Codex, Portugal |
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Abstract: | Measurements of time-resolved velocity characteristics have been obtained with a laser-Doppler velocimeter in the vicinity
of a model of an industrial oxy-fuel burner. The burner consists of a central axisymmetric jet surrounded by 16 circular jets,
simulating the injection of oxygen in practical burners. The experiments were carried out for isothermal flows and quantify
the effect of swirl for 0 ≤ S ≤ 0.9 on the mixing efficiency of the burner assembly.
The results show that the present flow develops faster than related coaxial free jets with similar velocity ratios between
central and peripheral air streams and, for example, for the nonswirling flow the rate of decay of the centreline velocity
increases by a factor of 2. Swirl attenuates the three-dimensional structure typical of multijet flows although the peripheral
jets limit the radial spreading of the swirling flow and give rise to increased values of mean shear strain and, therefore,
to turbulent production. The existence of zones characterized by large turbulence anisotropy indicate the need to take account
of the individual normal stresses in any proposed mathematical model to simulate the flow characteristics. Inspection of the
terms in the conservation equation for the turbulent stresses quantify the extent to which interaction of normal stresses
with normal strains influences the flow and suggests the likely combined magnitude of turbulent diffusion and dissipation. |
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