Jerry M. Seitzman

Planar laser-fluorescence imaging of combustion gases

An overview is provided of the planar laser-induced fluorescence (PLIF) method, which currently allows simultaneous combustion measurements at more than 105 flowfield points. Important advantages of the method include its relatively high signal strength, ease of interpretation, and applicability for determining several flowfield variables (including concentration, temperature, velocity, pressure and density). Example results are shown for a turbulent non-premixed flame, a spray flame, a rod-stabilized premixed flame, and a diffusion flame from a fuel jet in cross-flow.

Instantaneous temperature field measurements using planar laser-induced fluorescence

A single-pulse, laser-induced-fluorescence diagnostic for the measurement of two-dimensional temperature fields in combustion flows is described. The method uses sheet illumination from a tunable laser to excite planar laserinduced fluorescence in a stable tracer molecule, seeded at constant mole fraction into the flow field. The temporal resolution of this technique is determined by the laser pulse length. Experimental results are presented for a rodstabilized, premixed methane-air flame, using the Q(1) (22) line of the nitric oxide A(2) Sigma(+) (v = 0) ? X(2)II((1/2))(v = 0) transition (lambda approximately 225.6 nm).

Application of quantitative two-line OH planar laser-induced fluorescence for temporally resolved planar thermometry in reacting flows

A temporally resolved approach for measurement of two-dimensional temperature fields in reacting flows is experimentally investigated. The method, based on planar laser-induced fluorescence of the hydroxyl (OH) radical, is applicable in many combustion environments, including variable density flow fields. As a means of examining the accuracy of the technique, temperature images, from 1300 to 3000 K and 0.4 to 3 atm, have been acquired in shock-heated H(2)-O(2)-Ar flows with a two-laser, two-image ratio scheme. A complete measurement system for producing accurate, effectively instantaneous temperature images is described; the system includes single-shot monitors for laser-sheet energy distributions and spectral profiles. Temperature images obtained with the OH A(2)Σ(+) ? X(2)II (1, 0) P(1)(7)-Q(2)(11) transition pair exhibit a systematic error of only 7% over the entire range of conditions, with the error most likely dominated by shot-to-shot fluctuations in the lasersspectral profiles. The largest error source in the instantaneous temperature images is photon shot noise. A group of OH transition pairs that provide good temperature sensitivity and strong signals for reduced shot-noise error over a range of flow-field conditions is also presented.

Quantitative two-photon LIF imaging of carbon monoxide in combustion gases

Two-dimensional imaging of CO concentration in combustion gases is demonstrated using two-photonexcited planar laser-induced fluorescence. A quantitative model is presented for the simultaneous twophoton excitation of several rotational transitions of the B(1)Sigma(+) ? X(1)Sigma(+) system and the subsequent visible fluorescence (B(1)Sigma(+) ? A(1)Pi). The model is verified by comparison of predicted and measured excitation spectra and of temperature-corrected relative fluorescence measurements to standard probe measurements of the center line CO distribution in a CO-air diffusion flame. In addition, CO imaging experiments in a premixed methane-air flame indicate the production of C(2) by laser photodissociation of acetylene.

Comparison of excitation techniques for quantitative fluorescence imaging of reacting flows

+ ^X2U (1,0) band, 2) KrF laser excitation of the predissociative (3,0) band, and 3) saturated pumping of the (0,0) band with a XeCl laser, are compared to find which method minimizes the overall error. The approaches are compared by calculating shot-noise limited random errors and systematic deviations between the standard scaling equations and solutions to a time-dependent five-level rate equation model of the population densities. The model is used to address saturation and depletion (bleaching) effects. Dye laser excitation has the lowest overall error for single-shot imaging in turbulent hydrocarbon-air and hydrogenair flames. XeCl pumping produces the strongest signals, with evidence of strong saturation and ground state depletion effects. KrF pumping of the weakly absorbing and predissociative (3,0) band shows potential for quantitative imaging, when frame averaging is used.

Imaging and characterization of OH structures in a turbulent nonpremixed flame

Planar laser-induced fluorescence imaging of the hydroxyl radical (OH) is used to investigate spatial structures in a number of highly turbulent (ReD≈2300 to 50,000) nonpremixed hydrogen jet flames burning in air. Hydroxyl marks the flame zone and is also expected to mark large vortical structures in the flame. At each experimental condition, more than 80 OH images are recorded within 8 seconds, permitting the compilation of statistical measurements at more than 120,000 spatial locations. Several image analysis techniques are presented. Each technique is applied to individual images within a data set, and then statistics are compiled for the complete set. Two-dimensional Fourier transform techniques are used to calculate spatial autocorrelations on each instantaneous image, from which length scale information is extracted. Two orthogonal correlation lengths are determined for each image. The correlation length along the flame exhibits a Reynolds number invariance for high ReD (>2×104). The autocorrelation technique also produces a clear, mathematically-defined flame angle. The measured flame angles indicate increased angular fluctuation of the jet at high Reynolds number. The dependence of lift-off height on jet velocity is also measured. The lift-off results agree well with previous measurements based on flame emission and schlieren photographs, with the OH measurements producing slightly lower lift-off heights.

Two-photon digital imaging of CO in combustion flows using planar laser-induced fluorescence

Two-dimensional imaging of CO distributions in combustion gases is demonstrated using planar laser-induced fluorescence. The illumination technique is based on the combination of a nonlinear absorption scheme, in which two photons at 230.1 nm excite several rotational transitions of the B (1)Sigma(+) ? X(1)Sigma(+) system, and the use of an ultraviolet multipass cell for producing the laser sheet. The subsequent visible fluorescence (B(1)Sigma(+) ? A (1)Pi) is imaged onto an intensified two-dimensional photodiode array. Experimental results are presented for carbon monoxide-air and methane-air flames.

Soot Measurements in a Simulated Engine Exhaust Using Laser-Induced Incandescence

Soot mass concentrations were measured with laser-induced incandescence (LII) in a non reacting flow. The behavior of the LII signal with respect to soot concentration, particle size, and temperature was isolated with the use of a controllable soot-generating device. This device can simulate a hot, low-soot-concentration environment similar to that of a jet engine exhaust. Reduction of interference signals and high detection sensitivity were achieved with the use of a Nd:YAG laser at its fundamental wavelength and broadband detection from 570 to ∼850 nm. The LII signals were nearly proportional to soot concentration over 4 orders of magnitude, with a soot detection limit of better than ∼1 part per trillion (∼2 μg/m 3 ). The detection setup was designed, according to a model of the LII process, to reduce dependence on local gas temperature and soot particle size. Experimental results agreed with the model predictions in terms of particle size dependence, and negligible temperature dependence (beyond gas density effects) was seen for gas temperatures from 70 to 300°C.

Double-pulse imaging using simultaneous OH/acetone plif for studying the evolution of high-speed, reacting mixing layers

A technique for recording the evolution of large-scale structures in high-speed nonpremixed combustion flows is demonstrated. A single frequency-doubled dye laser system that can produce two pulses separated by tens of microseconds is used to simultaneously excite fluorescence from acetone, seeded into the fuel stream, and from OH, which marks the combustion gases. Thus, four images, an OH/acetone pair for each of the two pulses, are recorded on four separate cameras. The accuracy of the double-pulse technique is examined in a low-speed methane-air flame, which is essentially frozen during the measurement time. The average difference, or error, between corresponding pixels in a double-pulse pair of OH images shows little spatial biasing and primarily results from signal shot-noise fluctuations. The double-pulse OH/acetone technique is demonstrated in a nominally two-dimensional, high-speed reacting mixing layer consisting of an ambient-temperature, low-speed hydrogen-containing fuel stream and a high-temperature, high-speed oxidizing stream. Two mixing layer conditions are presented: a low compressibility case, with convective Mach number M c =0.32, and a more compressible case, M c =0.70. Reasonable quality images, having minimum noise levels of 11–14%, were obtained in both cases. The peak OH levels are near 2000 ppm, and the acetone seeding is roughly 3500 ppm. The regions of the flow defined by the interface between the mixing layer and the acetone-seeded fuel stream are relatively stable during the 30–50 μs measurement times and have convective velocities slightly higher than the speed of the fuel stream. The regions of the mixing layer marked by OH move faster and exhibit greater distortion.

Instantaneous three-dimensional flow visualization by rapid acquisition of multiple planar flow images

A flow visualization system is described that provides 3-D snapshots of instantaneous flow structures by rapidly acquiring successive image planes from within the flow. For each 3-D snapshot, up to 20 planar images are collected at the rate of 10,000,000 images/s. Flow visualization at successive planes is by planar particulate scattering or planar laser-induced fluorescence using a laser sheet rapidly swept once across the flow by a 30,000-rpm polygon scanner. Variations in the laser sheet intensity are monitored by placing an on-edge image of the sheet on the periphery of every planar flow image. The camera is a high-speed image converter coupled by fiber optics to a slow-scan CCD camera. The fiber optics provide 2.5 times better light gathering efficiency than lens coupling without reducing the available spatial resolution. The overall measurement technique shows excellent potential as demonstrated with a selection of 3-D measurements.