Effect of soot self-absorption on color-ratio pyrometry in laminar coflow diffusion flames.

This Letter reports on the effect of self-absorption on measured temperature for color-ratio soot pyrometry with a color camera. A series of increasingly nitrogen diluted atmospheric pressure ethylene/air laminar coflow diffusion flames are studied, providing flames with different optical path lengths, soot loading, and soot optical properties. Numerical calculations are used to simulate the change in collected flame emission signal with and without light attenuation using experimentally measured maps of the soot absorption coefficient. This parameter implicitly contains information about soot volume fraction and soot optical properties. The ratio of these calculations is used to correct the raw color-channel signals, resulting in temperature maps with improved accuracy. The change in calculated temperature varies spatially within each flame, with the maximum correction quantified to be 22 K for a flame with a maximum optical depth of 0.31. This correction is as much as 42 and 75 K for simulated flames with the same optical properties, structure, and a factor of two and five increase in soot volume fraction, respectively.

Boundary condition thermometry using a thermographic-phosphor-coated thin filament.

Thermographic phosphors (TPs) exhibit a temperature sensitive emission spectrum when excited with ultraviolet radiation. In this study, 14 µm diameter SiC fibers are coated with ZnO or Dy:YAG using a ceramic binder to a total diameter of 70±9 µm. ZnO and Dy:YAG fibers were used to measure fiber temperatures in the range of 294-450 K and 450-1245 K, respectively. The coated fiber provides higher signal levels compared to TP particle seeding and is no more invasive than the commonly used thermocouple. A calibration is performed to relate fiber temperature to the ratio of luminescent signal collected within two different bands of the fiber emission spectrum. Temperature was measured along the inlet of a series of nitrogen diluted ethylene diffusion flames stabilized on the Yale coflow burner to determine suitable thermal boundary conditions for computational modeling. The boundary condition temperatures were derived from a spline fitting of data acquired from the two fiber types in order to obtain fiber temperature sensitivity from 294 to 1245 K. The peak near-burner temperature was found to be higher than ambient conditions and to increase and shift its location radially outward with increased fuel percentage.

Use of high dynamic range imaging for quantitative combustion diagnostics.

High dynamic range (HDR) imaging is applied to quantitative combustion diagnostics in coflow laminar diffusion flames as a way to improve the signal-to-noise ratio (SNR) and measurement sensitivity. The technique relies on the combination of partially saturated frames into a single unsaturated image; in this work, the effectiveness of the HDR approach is demonstrated when applied to two-color ratio pyrometry. Specifically, it is shown than an increase in SNR results in more precise temperature measurements for both soot and thin filament pyrometry. Linearity and reciprocity analysis under partially saturated conditions were performed on three selected detectors, and the camera response functions, which are required for HDR image reconstruction, were determined. The linearity/reciprocity of the detectors allowed the use of a simplified algorithm that was implemented to compute the HDR images; soot and flame temperature were calculated from those images by employing color-ratio pyrometry. The reciprocity analysis revealed that pixel cross talk can be a limiting factor in a detector's HDR capabilities. The comparison with low dynamic range results showed the advantage of the HDR approach. Due to the higher SNR, the measured temperature exhibits a smoother distribution, and the range is extended to lower temperature regions, where the pyrometry technique starts to lose sensitivity due to detector limitations.

Absorption-ablation-excitation mechanism of laser-cluster interactions in a nanoaerosol system.

The absorption-ablation-excitation mechanism in laser-cluster interactions is investigated by measuring Rayleigh scattering of aerosol clusters along with atomic emission from phase-selective laser-induced breakdown spectroscopy. For 532 nm excitation, as the laser intensity increases beyond 0.16 GW/cm^{2}, the scattering cross section of TiO_{2} clusters begins to decrease, concurrent with the onset of atomic emission of Ti, indicating a scattering-to-ablation transition and the formation of nanoplasmas. With 1064 nm laser excitation, the atomic emissions are more than one order of magnitude weaker than that at 532 nm, indicating that the thermal effect is not the main mechanism. To better clarify the process, time-resolved measurements of scattering signals are examined for different excitation laser intensities. For increasing laser intensity, the cross section of clusters decreases during a single pulse, evincing the shorter ablation delay time and larger ratios of ablation clusters. Assessment of the electron energy distribution during the ablation process is conducted by nondimensionalizing the Fokker-Planck equation, with analogous Strouhal Sl_{E}, Peclet Pe_{E}, and Damköhler Da_{E} numbers defined to characterize the laser-induced aerothermochemical environment. For conditions where Sl_{E}≫1, Pe_{E}≫1, and Da_{E}≪1, the electrons are excited to the conduction band by two-photon absorption, then relax to the bottom of the conduction band by electron energy loss to the lattice, and finally serve as the energy transfer media between laser field and lattice. The relationship between delay time and excitation intensity is well correlated by this simplified model with quasisteady assumption.

Quantitative Rayleigh thermometry for high background scattering applications with structured laser illumination planar imaging.

This work demonstrates structured laser illumination planar imaging (SLIPI) for Rayleigh thermometry with high background scattering. Two coherent laser beams were crossed to produce an interference pattern, from which the modulated Rayleigh signal was collected. The modulated signal serves as a signature that identifies information about Rayleigh scattering from the probe volume against additional contributions in the image from background scattering. This work shows that the structured nature of the illumination allows for a simplified background correction. The experimental approach is validated in a non-premixed methane/air flame, and the temperature is found to be in excellent agreement with previous experimental and computational results. Rayleigh SLIPI is then applied to a high background scattering application as part of the full-field temperature measurement of sooting non-premixed ethylene/air flames. For these flames, standard Rayleigh background corrections are impossible since scattering from soot just outside the field of view is the main source of the background. Good agreement is found between SLIPI and intensity-ratio thin-filament pyrometry-derived temperature along their adjoining interface in the flame.

Use of Rayleigh imaging and ray tracing to correct for beam-steering effects in turbulent flames.

Laser Rayleigh imaging has been applied in a number of flow and flame studies to measure concentration or temperature distributions. Rayleigh cross sections are dependent on the index of refraction of the scattering medium. The same index of refraction changes that provide contrast in Rayleigh images can also deflect the illuminating laser sheet. By applying a ray-tracing algorithm to the detected image, it is possible to correct for some of these beam-steering effects and thereby improve the accuracy of the measured field. Additionally, the quantification of the degree of beam steering through the flow provides information on the degradation of spatial resolution in the measurement. Application of the technique in a well-studied laboratory flame is presented, along with analysis of the effects of image noise and spatial resolution on the effectiveness of the algorithm.