Diagnostic considerations for optical laser-extinction measurements of soot in high-pressure transient combustion environments.

Journal Article · Tue Jun 01 04:00:00 EDT 2004 · Proposed for publication in Combustion and Flame.

OSTI ID:952820

Musculus, Mark P. B.; Pickett, Lyle M

Laser-extinction diagnostics can provide spatially and temporally resolved measurements of attenuation from combustion-generated soot within the path of the beam. When laser-extinction techniques are utilized in high-pressure combustion environments, however, a number of complications may be encountered that are not present in low-pressure environments. Several of these experimental difficulties were investigated in diesel engine environments, and solutions that facilitated acquisition of reliable laser-extinction data were demonstrated. Beam steering due to refractive index gradients within the combusting gases was observed, and a full-angle beam divergence of over 100 mrad was measured. A spatial-filtering scheme was employed to reduce the collection of forward-scattered light and background combustion luminosity while ensuring full collection of the steered beam. To further reject combustion luminosity, a narrow-bandpass laser-line filter was employed, after diffusing the transmitted light sufficiently to avoid the effects of significant spatial non-uniformities of the filter. As the windows were subjected to thermal and mechanical stresses, dynamic etaloning effects due to the photoelastic properties of synthetic fused silica were observed. Dynamic changes in the polarization of the exit beam were also observed, as stress-induced birefringence in the windows caused dynamic phase retardation of the transmitted beam. Although these photoelastic effects could not be eliminated, they were mitigated by introducing curvature to the wavefronts in the laser-extinction beam and using polarization-insensitive elements in the detection optics. Soot deposits on window surfaces were removed ablatively using a coaxial, high-energy, pulsed Nd:YAG laser beam.

• Research Organization: Sandia National Laboratories
• Sponsoring Organization: USDOE
• DOE Contract Number: AC04-94AL85000
• OSTI ID: 952820
• Report Number(s): SAND2004-2808J
• Journal Information: Proposed for publication in Combustion and Flame., Journal Name: Proposed for publication in Combustion and Flame.
• Country of Publication: United States
• Language: English

Related Subjects

33 ADVANCED PROPULSION SYSTEMS
ATTENUATION
BIREFRINGENCE
COMBUSTION
DETECTION
DIESEL ENGINES
GASES
LASERS
LUMINOSITY
OPTICS
POLARIZATION
REFRACTIVE INDEX
SILICA
SOOT
STRESSES
TRANSIENTS