Abstract
Large eddy simulations of a stratified swirling flow of a Cambridge swirl burner for both nonreacting and reacting cases are conducted using a finite rate chemistry approach represented by a partially stirred reactor model. The large eddy simulation predictions are compared with experimental measurements for velocity, temperature, and concentrations of major species. The agreement is found in overall trend of velocity prediction, but temperature and concentration of major species show slight discrepancies in the central region. Two reduced chemical mechanisms are examined in the present paper with the objective of assessing their capabilities in predicting swirling flame characteristics, and the distinct difference using two mechanisms is found in CO distribution profiles, which is considered the consequence of different kinetics of CO-CO2 equilibrium. Flow structures are qualitatively and quantitatively analyzed with numerical results. Large-scale vortex structures and precession motions are observed in both nonreacting and reacting cases. Frequency of vortex shedding is identified from the point data of instantaneous velocity in the discharging stream-induced shear layer. On this basis, the intensity and frequency of precession motion are shown to be enhanced in the presence of combustion. Large-scale wrinkling of the flame surface is resolved and characterized in the flame zone, and the effect of mixture stratification is then further discussed.
Highlights
Lean premixed combustion has been widely adopted in practical combustion systems for it can offer low NOx emission by decreasing peak reacting temperature
To assess the grid resolution, turbulent kinetic energy is collected at 30 mm downstream of the burner exit in the shear layer between the annular swirl and axial jets
For the Mech I simulation, the mean and rms of CO are too low whilst Mech II simulation gives a reasonable prediction of CO concentration the fluctuation is still too low. It indicates that the CO-CO2 equilibrium reaction in Mech II is more reasonable in this configuration. It suggests that the flexibility of chemical mechanisms is deeply dependent on practical situations
Summary
Lean premixed combustion has been widely adopted in practical combustion systems for it can offer low NOx emission by decreasing peak reacting temperature. In practical combustion systems constrained by physical space and fuel-air mixing time, inhomogeneous mixture and spatial gradient of mixture equivalence ratio often exhibit. This may lead to combustion taking place under stratified condition. Complex turbulent flow and chemistry reactions make flow field measurements in practical combustion chamber a tough challenge. To investigate the stratification and swirl effects on turbulent combustion, the Cambridge swirl burner (SwB) was designed by Sweeney et al [2,3,4] and detailed measurements of the flow fields were carried out by Zhou et al [5]. The experimental measurements are valuable for combustion model assessment and numerical simulation validation
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