John Garman

Spatial averaging effects in CARS thermometry of a nonpremixed flame

Coherent anti-Stokes Raman scattering (CARS) allows nonintrusive in situ temperature measurements to be made in reacting flows with errors typically less than 50 K. When the temperature varies over the length of the CARS probe volume, however, the measured temperature is biased toward the cold side rather than reflecting the mean temperature or the temperature at the center of the volume. This paper details CARS measurements made on a Wolfhard-Parker slot burner to quantity these spatial averaging errors in nonpremixed flames. Results, using both analytical and experimental inputs to a CARS spectral fitting program, show that while spatial averaging errors are significant, they can be predicted accurately.

Probing Dense Sprays with Gated, Picosecond, Digital Particle Field Holography

This paper describes work that demonstrated the feasibility of producing a gated digital holography system that is capable of producing high-resolution images of three-dimensional particle and structure details deep within dense particle fields of a spray. We developed a gated picosecond digital holocamera, using optical Kerr cell gating, to demonstrate features of gated digital holography that make it an exceptional candidate for this application. The Kerr cell gate shuttered the camera after the initial burst of ballistic and snake photons had been recorded, suppressing longer path, multiple scattered illumination. By starting with a CW laser without gating and then incorporating a picosecond laser and an optical Kerr gate, we were able to assess the imaging quality of the gated holograms, and determine improvement gained by gating. We produced high quality images of 50–200 μm diameter particles, hairs and USAF resolution charts from digital holograms recorded through turbid media where more than 98% of the light was scattered from the field. The system can gate pulses as short as 3 mm in pathlength (10 ps), enabling image-improving features of the system. The experiments lead us to the conclusion that this method has an excellent capability as a diagnostics tool in dense spray combustion research.

Dependence of NO 2 degenerate four-wave-mixing signals on buffer gas pressure

Measurements of degenerate four-wave mixing (DFWM) from NO2 are presented as a function of buffer gas pressure and laser power. The signal strength first decreases and then increases with increasing buffer gas pressure. This dependence on buffer gas pressure is apparently due to the onset of thermal gratings. In the regime where thermal gratings are the dominant source of DFWM signals, the DFWM signal intensity increases as the square of the laser power until saturation.

Invited Paper: Photoignited Aluminum Nanopowder Combustion in Air

Metal combustion has recently received renewed interest as a result of the ability to manufacture and characterize nanoparticles. These nanoparticles are known to exhibit desirable traits, mainly their high specific surface area. The objective of this work was to investigate experimentally the oxidation behavior of nanosized aluminum powders in air under white light photo-ignition conditions. Powder samples with three different nominal size particles of 20, 50, and 70 nm were characterized with HRSEM and TEM. The results showed that the reactivity of photo-ignited powder decreased with increasing size whereas the energy threshold for ignition had the opposite trend. The presence of a natural oxide coating and changes in the optical properties of the powder are proposed as the cause of this behavior.

Time-gated holography to provide a glimpse into dense sprays

In all combustion devices, the flame stability and thermal efficiency are strongly influenced by the fuel and oxidizer mixing processes. In spray combustion (e.g., in gas turbines), mixing is associated with breakup and atomization of dense sprays. It is necessary to view these processes in detail to obtain knowledge vital for improving them.1 Photography, holography, and x-ray imaging, which have been used for many years in such research,2, 3 have failed to provide clear views because noise from multiple scattering obscures the signal needed to acquire a useful recording. In recent years, new methods of separating the signal from the noise have been developed, including pseudo-ballistic photon imaging,4–7 in which high-speed optical gates separate ballistic (not scattered) and only-once-scattered, near-ballistic, ‘snake’ photons from multiply scattered photons. Another problem in imaging spray particles is the wide depth of field needed to bring all of them into focus. Current ballistic imaging systems provide only the shadows of particles without resolving the third dimension.4–7 We are combining pseudo-ballistic imaging with digital holography for the first time to resolve the digitally recorded hologram of a dense spray in three dimensions. The hologram is the out-of-focus diffraction pattern of all of the spray particles, which we can reconstruct numerically at different depths throughout the viewed volume. This procedure brings every particle into focus and detects its position. Figure 1 shows the digital holocamera design. The laser output is split into three different beams: the fundamental 1.06 m beam, which is used to open the Kerr gate, a second-harmonic Figure 1. Picosecond digital holography system design. DM: Dichroic mirror. !: Fundamental beam. 2!: Second-harmonic beam. BS: Beam splitter cube. F1: Focal plane. OP: Optical path length compensation. L1: Lens, f D 400mm. L2: Lens, f D 200mm. WP: Wave plate. P1, P2: Cross polarizer. OKC: Optical Kerr cell.

Particle‐Wall Interaction in a Viscoelastic Fluid

In this work, particle‐wall interaction in viscoelastic fluids is experimentally studied. The effect of Stokes number, Weissenberg number and surface roughness on the rebound velocity of a colliding spherical particle on a wall is considered. Different steel spherical particles are released in viscoelastic solutions of different high‐molecular‐weight polymers in water with different concentration, and the coefficient of restitution is calculated for particle‐wall collision. The critical Stokes number at which no rebound occurs is studied for different Weissenberg number.