A methodology is presented in this paper how to assess and to improve burner aerodynamic stability of a premixed swirl burner by means of CFD. Steady-state RANS has been widely used for complex industrial applications for decades because of its good reproducibility of mean flow and acceptable turnaround time. With affordability of high performance cluster it becomes feasible to extend steady-state RANS to unsteady-state LES for industrial applications. It is especially beneficial for swirl burners to assess burner stability directly because swirling flow is in fact an unsteady-state phenomenon, which RANS cannot fully capture. This paper shows an industrial practice for how to improve burner design using both RANS and LES. The former helps find potential problems in the flow field, e.g. flow separation. The latter quantifies their impact on flow stability/turbulent fluctuations, e.g. helical modes generated by coupling of flow disturbances and swirling flow. Improvement measures were worked out to supress such flow fluctuations and to enhance stability of burner aerodynamics. Another critical issue for burner design, reverse flow within the burner, was also discussed because it is a potential risk. When a flame instantaneously enters the burner backwards it can be stabilized in the recirculation zone and damage hardware. The risky region was eliminated by enhancing axial momentum of the air inflow. The strong turbulent fluctuations within the burners interfere with burner stability significantly and lead to a bad flashback resistance in macroscopic view. In the second part of the paper, atmospheric tests verify the improvement of burner stability via improved flashback resistance and show an upgrading of aerodynamic behaviour via reduced burner pressure drop.
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