Picosecond Lasers Provide Interference-Free Two-Dimensional Measurements of Atomic Oxygen in Flames

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Picosecond Lasers Provide Interference-Free Two-Dimensional Measurements of Atomic Oxygen in Flames

Quantitative measurements of atomic oxygen in flames are important for studying pollutant formation from combustion. Such measurements provide insight into the effects of turbulent flow on NO production, in which the atomic oxygen radical plays a significant role. However, accurate O-atom measurements have been hampered by the shortcomings of existing laser diagnostic methods, particularly by photolytic interference generated by the laser pulses themselves.

Using picosecond(ps) lasers to eliminate this interference, Jonathan Frank and Tom Settersten have recently demonstrated two-dimensional interference-free imaging of atomic oxygen, which is important for studying the effects of flow transients on thermal NO formation.

Figure 1. Radial profiles of O-atom LIF from photolytically produced O atoms in a lean (ø=0.70) premixed CH₄/O₂/N₂ flame at four ns-laser energies. Measurements were performed with a ns photolysis laser and a ps probe laser. For comparison, a LIF profile measured with only the ps laser is displayed on the same scale.

Building on results of a previous collaboration between CRF researchers and visiting researcher Andreas Dreizler (Technical University of Darmstadt), Frank and Settersten have also shown that the dominant source of this photolytic interference in hydrocarbon flames is the photodissociation of thermally excited CO₂, not O₂, as previously thought. Their work on interference-free two-photon laser-induced fluorescence (LIF) imaging of atomic oxygen appears in the April 20 issue of Applied Optics and will be presented at the 30th International Symposium on Combustion.

Nanosecond vs. Picosecond Lasers
Previous attempts at O-atom LIF measurements used nanosecond (ns) pulsed lasers for excitation. However, ns lasers generate significant amounts of atomic oxygen that interfere with LIF detection of combustion-generated atomic oxygen. This photolytic interference is produced by single-photon dissociation of a precursor, whereas the LIF signal generation involves two-photon absorption. The different laserintensity dependences of these processes suggest that excitation with a ps laser could be advantageous. To produce the same LIF signal, ps excitation requires signifi cantly less laser energy than does ns excitation. Because photolytic production of O atoms increases as laser energy increases, ps excitation generates less interference from single-photon photolysis.

Figure 2. Temporal evolution of atomic oxygen in an acoustically forced axisymmetric premixed CH₄/O₂/N₂ flame. Each frame in the sequence is separated by 200 µs.

The advantage of ps excitation is evident from O-atom LIF line imaging in laminar premixed Bunsen flames. Figure 1 compares the ps LIF signal due to combustion-generated O atoms (labeled ìps-only LIFî) to the signals resulting from O atoms that were photolytically produced using nonresonant ns laser pulses (labeled with the ns pulse energy). The ps pulse energy was sufficiently low to ensure that it caused negligible photodissociation. As shown in Figure 1, the interference from photolytically produced O atoms can overwhelm the signal from native O atoms in the flame

Flow flame interactions
The researchers also demonstrated the capability of two-dimensional O-atom LIF measurements of a flow-flame interaction. Measurements of transient flow-flame interactions are fundamental to understanding turbulent combustion. Repeatable flow-flame interactions provide well-controlled systems in which the effects of flow transients are studied.

In Figure 2, the image sequence, which displays the temporal evolution of atomic oxygen in an acoustically forced Bunsen flame, demonstrates the ability to perform interference-free two-dimensional O-atom LIF measurements of a flow-flame interaction. The images show localized variations in O-atom levels that may result in locally increased thermal NO production. Measurements such as these could provide insight into the effects of transient flow-flame interactions on NO production.

Article taken from the July/August 2004 CRF News Volume 26 Number 4 (PDF - 5598K)


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