High-resolution imaging of dissipative structures in a turbulent jet flame with laser Rayleigh scattering

• Published in: Experiments in fluids Vol. 44; no. 2; pp. 221 - 233
• Main Authors: Frank, Jonathan H., Kaiser, Sebastian A.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01-02-2008; Springer
• Subjects: Applied sciences; Combustion of gaseous fuels; Combustion. Flame; Dissipation; Energy; Energy. Thermal use of fuels; Engineering; Engineering Fluid Dynamics; Engineering Thermodynamics; Exact sciences and technology; Fluid dynamics; Fluid flow; Fluid- and Aerodynamics; Heat and Mass Transfer; Instruments, apparatus, components and techniques common to several branches of physics and astronomy; Jet flames; Physics; Probability density functions; Rayleigh scattering; Research Article; Theoretical studies. Data and constants. Metering; Thermal instruments, apparatus and techniques; Thermometry; Turbulence; Turbulent flow; Mixture Fraction; Downstream Location; Layer Width; Thermal Dissipation; Dissipation Structure; Turbulent flame; Methane; Rayleigh scattering; High resolution; Hydrogen; Spatial analysis; Combustion; Thermal dissipation; Nitrogen; Experimental study; Spectral analysis; Temperature measurement; Imaging; Measurement by laser beam
• ISSN: 0723-4864; 1432-1114
• DOI: 10.1007/s00348-007-0396-x

High-resolution 2-D imaging of laser Rayleigh scattering is used to measure the detailed structure of the thermal dissipation field in a turbulent non-premixed CH
4
/H
2
/N
2
jet flame. Measurements are performed in the near field (
x
/
d
= 5–20) of the flame where the primary combustion reactions interact with the turbulent flow. The contributions of both the axial and radial gradients to the mean thermal dissipation are determined from the 2-D dissipation measurements. The relative contributions of the two components vary significantly with radial position. The dissipation field exhibits thin layers of high dissipation. Noise suppression by adaptive smoothing enables accurate determination of the dissipation-layer widths from single-shot measurements. Probability density functions (PDF) of the dissipation-layer widths conditioned on temperature are approximately log-normal distributions. The conditional layer width PDFs are self-similar functions with the layer widths scaling with temperature to the 0.75 power. The high signal-to-noise ratio of the Rayleigh scattering images coupled with an interlacing technique for noise suppression enable fully resolved measurements of the mean power spectral density (PSD) of the temperature gradients. These spectra are used to determine the turbulence microscales by measuring a cutoff wavelength, λ
C
, at 2% of the peak PSD. The Batchelor scale is estimated from λ
C
, and the results are compared with estimates from scaling laws in non-reacting flows. At
x
/
d
= 20, the different approaches to determining the Batchelor scale are comparable on the jet centerline. However, the estimates from non-reacting flow scaling laws are significantly less accurate in off-centerline regions and at locations closer to the nozzle exit. Throughout the near field of the jet flame, the measured ratio of a characteristic dissipation-layer width to the local Batchelor scale is larger than values previously reported for the far field of non-reacting flows.