Phillip H. Paul

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.

Fiber‐optic evanescent field absorption sensor

Waveguide evanescent field absorption has been measured as a function of cladding refractive index and bulk absorption coefficient. A simple theory for interpreting these results has been developed. An all‐fiber species concentration sensor has been constructed and its performance characterized.

Molecular velocity imaging of supersonic flows using pulsed planar laser-induced fluorescence of NO.

A technique is described for imaging components of velocity in a gaseous flow field by using pulsed planar laser-induced fluorescence. The technique is based on the fluorescence detection of Doppler-shifted absorption that results when a spectrally narrow absorption line is excited with a broadband laser. Results obtained in a Mach 7 underexpanded supersonic jet, seeded with NO, are presented. The practical extension of this technique to single-shot measurements of two velocity components is discussed.

Quantitative imaging of temperature fields in air using planar laser-induced fluorescence of O(2).

Planar laser-induced fluorescence of O(2) has been used to acquire quantitative, instantaneous two-dimensional images of temperature in heated air flows. O(2) is excited by using a broadband ArF excimer laser at 193 nm, and the resultant fluorescence signal is converted to temperature by using a theoretical calculation of the dependence of the fluorescence on temperature. This calculation has been confirmed experimentally, and validating data are presented.

Laser-fluorescence imaging of O2 in combustion flows using an ArF laser.

Planar laser-induced fluorescence images of O2 in flames have been acquired using an ArF excimer laser as the excitation source. The 193-nm radiation excites several rovibronic lines of the O2 Schumann–Runge (B3∑u− ← X3∑g−) band system. The resultant fluorescence is detected with an image-intensified 100 × 100 photodiode array camera. Images from several CH4/air flames are presented. High-speed sequences of images have been acquired using the high-repetition-rate capability of the excimer laser. These sequences display the real-time evolution of discrete flame structures. The dependence of the fluorescence signal on temperature and O2 concentration is discussed.

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.

High resolution digital flowfield imaging of jets

High resolution digital imaging, using planar laser-induced light scattering is being developed for analysis of gaseous flowfields. High resolution image data, implying both high spatial resolution and wide signal dynamic range, can be readily processed to yield two-dimensional distributions of species concentrations and, in turn, accurate two-dimensional images of concentration gradients and turbulence scales. Critical aspects of the technique are discussed; details of the design and the performance of the imaging system are presented; and results for laminar, transitional, turbulent and birfurcating nitrogen jets, using planar-laser-induced fluorescence of biacetyl, are reported. Initial results from imaging processing and the potential implications for flowfield analysis are described

Laser‐induced fluorescence imaging of laser‐ablated barium

We have applied laser‐induced fluorescence diagnostics to expanding recombining barium plasmas produced by laser ablation. For relatively modest ablation laser energy fluences (∼102–103 W cm−2), we have measured neutral barium leading edge density gradients of 1012–0.5×1013 cm−3 cm−1 and cloud expansion velocities of ∼106 cm s−1, consistent with recent results of ablation studies in copper [R. J. von Gutfeld and D. W. Dreyfus, Appl. Phys. Lett. 54, 1212 (1989)].

Scalar mixing in the supersonic shear layer

Experiments were conducted in a two-stream planar mixing layer facility at convective Mach numbers of 0.28 and 0.62. Mie scattering from condensed alcohol droplets and planar laser-induced fluorescence (PLIF) of nitric oxide were used for flow visualization in both the side and plan views. The PLIF signals were approximately proportional to mixture fraction and were used to generate statistical quantities. Visualizations using both the Mie scattering and PLIF indicate the structure is essentially two-dimensional at Mc = 0.28 and three-dimensional at Mc = 0.62. Perspective renderings of side view images show the structures are streamwise ramped at Mc = 0.28 and cross-stream ramped at Mc = 0.62. This difference appears to be associated with decreasing streamwise structure spacings at the higher Mc condition. The statistical analysis suggests that with increasing compressibility, the scalar fluctuations are smaller, and the fraction of mixed fluid is higher.

Planar laser-induced fluorescence imaging of shock-induced ignition

Planar laser-induced fluorescence (PLIF) imaging has been developed for use in studies of shock-induced ignition. PLIF measurements of OH have been made in stoichiometric mixtures of H2 and O2 diluted in 90% Ar using two shock tube endwall configurations. These OH images, which serve as indicators of ignition, were first obtained in the weak and strong ignition limits in the gas behind a shock reflected from a planar endwall. In both limits the images exhibited planar combustion waves. The absence of observable ignition kernels in the weak ignition limit presumably results from lack of optical access to the corners of the shock tube where kernel formation is expected to occur. A grooved endwall was used to introduce thermal nonuniformities and thereby promote isolated regions of ignition within the imaged region. Sequences of PLIF images defining the temporal evolution of the ignition process were acquired in the weak and strong ignition limits for this endwall configuration. These images exhibit highly nonuniform structure and serve to illustrate the utility of 2-D imaging techniques for ignition studies.