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ABSTRACT
The effect of mixing tube length (partial premixing level) on the amplitude of coherent structures and acoustics, and stability of partially premixed flames (PPFs) of methane off a concentric flow conical nozzle (CFCN) swirl burner is experimentally investigated. The flowfield inside the quartz confinement is documented using particle image velocimetry (PIV), and coherent structures are captured using proper orthogonal decomposition (POD) technique. Acoustics measurements are performed using Bruel & Kjaer type 4189 microphone. Additionally, high-speed imaging and quantitative light sheet (QLS) techniques are used to qualitatively investigate the flow mixing characteristics. The results reveal that increasing partial premixing level significantly enhances flame stability. Compared to traditional dump burner/combustor, the present burner configuration promotes flame divergence with no outer recirculation zone (ORZ) and larger homogeneous and isotropic turbulent zone. More importantly, the results show that, in contrast to the traditional dump combustor, the use of a swirl in a CFCN burner makes it possible to sustain stable flame under ultra-lean conditions.
Nomenclature
| = | particle density distribution | |
| = | homogeneous particle density distribution | |
| D | = | inner diameter of the cone exit (mm) |
| d | = | inner diameter of mixing tube (mm) |
| = | dark current of the camera (electron/pixel•s) | |
| f1 | = | frequency of the first longitudinal acoustic mode (Hz) |
| = | intensity of the acquired instantaneous image in reacting flow case | |
| = | intensity of the acquired homogeneous particle density image in cold flow | |
| = | local light sheet intensity | |
| = | intensity of the background image | |
| IP | = | mean pixel intensity |
| k | = | ratio of the gas specific heats |
| L | = | combustor length (m) |
| l | = | mixing length (mm) |
| n | = | number of SPL data points |
| P0 | = | reference pressure (Pa) |
| Pvar | = | variance of pressure fluctuations (Pa2) |
| = | factor for reflections due to the use of quartz tube | |
| R | = | gas constant (kJ/kg•K) |
| r | = | radial position (mm) |
| Red | = | Reynolds number based on the inner diameter of the mixing tube |
| S | = | swirl number |
| T | = | working gas temperature (K) |
| U | = | radial velocity (m/s) |
| Urms | = | root mean square (rms) of the radial velocity fluctuations (m/s) |
| V | = | axial velocity (m/s) |
| = | factor for different viewing angles and some other effects | |
| VCL | = | centerline axial mean velocity (m/s) |
| Vj | = | mean bulk flow velocity based on the mixing tube inner diameter (m/s) |
| Vrms | = | root mean square (rms) of the axial velocity fluctuations (m/s) |
| W | = | azimuthal velocity (m/s) |
| X | = | axial position (mm) |
| z | = | radial position normal to r (mm) |
Greek symbol
| α | = | vane(s) angle of the swirl generator (°) |
| = | POD modal energy of mode i | |
| ν | = | kinematic viscosity of the mixture (m2/s) |
| = | fluid density (kg/m3) | |
| = | homogeneous fluid density (kg/m3) | |
| Σ | = | Summation |
| Φ | = | mixture/global equivalence ratio |
| ø | = | equivalence ratio of the central tube mixture |
| χ | = | mass flow rate ratio |
| ψU | = | turbulence homogeneity of the radial rms velocity |
| ψV | = | turbulence homogeneity of the axial rms velocity |
| Ω | = | normalized fluid density |
Acronym
| CCD | = | charge-coupled device |
| CFCN | = | concentric flow conical nozzle |
| CO | = | carbon monoxide |
| CRZ | = | central recirculation zone |
| FOV | = | field of view of image (mm2 or pixel2) |
| H.P.P.L. | = | high partial premixing level |
| L.P.P.L. | = | low partial premixing level |
| LSB | = | low-swirl burner |
| M.P.P.L. | = | medium partial premixing level |
| NOx | = | nitrogen oxides |
| ORZ | = | outer recirculation zone |
| PIV | = | particle image velocimetry |
| PLIF | = | planar laser-induced fluorescence |
| POD | = | proper orthogonal decomposition |
| PPFs | = | partially premixed flames |
| P.P.L. | = | partial premixing level |
| PSD | = | power spectral density (a.u.) |
| PVC | = | precessing vortex core |
| QLS | = | quantitative light sheet |
| rms | = | root mean square |
| ROI | = | region of interest in the image (mm2 or pixel2) |
| SPL | = | sound pressure level(s) as function of frequency (dB) |
| SPLtot | = | total sound pressure level (dB) |
| TiO2 | = | titanium dioxide |
| TKE | = | turbulent kinetic energy (m2/s2) |
| VB | = | vortex breakdown |
Compliance with Ethical Standards
The authors declare that they have no conflict of interest.