AbstractSwirl burners are widely used in industry. Recent environmental concerns, particularly on emissions from combustion processes, have introduced the requirement to modify these processes to reduce emissions whilst at the same time maintaining combustion efficiency. This thesis presents details of experimental and computational studies into the flowfield structures of swirl burners.
Previous investigators have concentrated on the time-average flows, but it has become apparent that these are insufficient to enable pollutant emissions to be accurately predicted. Knowledge of the time-temperature and species history is needed to obtain better predictions. Pivotal to this is a detailed determination of the time-dependent structure of the flow.
In this study, series of experiments were carried out at different inlet configurations and conditions. The flowrate and swirl number were varied as well as the injection mode, inlet length and exit geometry. The burner flow was characterised by measuring axial, tangential and radial velocities using a Laser Doppler Anemometer.
A Computational Fluid Dynamics modelling package, FLUENT was used to produce two and three-dimensional computational models to predict the flowfield structures of the burners in isothermal and combustion cases. Four turbulence models were evaluated in the prediction: the k-e Model, the Algebraic Stress Model (ASM), the Reynolds Stress Model (RSM) and the Re-normalisation Group Model (RNG). Constant velocity scaling of the 100 and 500 kW burner was examined in both experimental and computational studies.
The experimental results show that the flowfield structures in both burners are nonaxisymmetric and develop three-dimensional time-dependent coherent structures in the flow. The experimental results have been compared with the computational model predictions. The comparisons reveal very good agreement between the time average measurement and the predictive values, especially downstream of the burner exit.
This work was extended to investigate the following novel phenomena:
a). The computational prediction of the flowfield structure was extended to include different inlet boundary conditions with both the RSM and the RNG turbulence models. The model was also extended to investigate the time-dependent flows.
b). The influence of varying the inlet and exit geometries and conditions on the flow patterns and the reverse flow zone was examined in detail. A 500 kW swirl burner with scroll inlet was designed and characterized with time-dependent
flows to simulate the Precessing Vortex Core.
This investigation showed very good agreement with experimental velocity data with less constrained boundary conditions that had previously obtained. The time-dependent simulation was limited by the computer speed and processing capability but identified that such analysis is possible when computer power allows and has the potential to model the flow in greater detail yielding more accurate data on pollution emissions.
|Date of Award||Nov 1997|
|Supervisor||John Ward (Supervisor)|