Eric James Welle

The response of a propane-air counter-flow diffusion flame subjected to a transient flow field

Abstract OH planar laser-induced fluorescence (PLIF) and particle image velocimetry have been used to study the frequency response of laminar C3H8-air counterflow diffusion flames to assess the adequacy of the steady-flamelet models. Particle image velocimetry was used to determine the flame strain rate, while OH PLIF was used both to measure temperature at the flame front, using the two-line PLIF technique, and the reaction-zone width. Both measurements demonstrate that the frequency response of flames subjected to a time-varying flow field is diffusion-limited. At the 30-Hz and 50-Hz forcing frequencies, the maximum reaction-zone temperature and width were found to respond quasi-steadily. However, at higher forcing frequencies-i.e., 100 and 200 Hz-transient behavior is evident from the phase relationship between the imposed sinusoidal strain rate and the resulting peak temperature and reaction-zone width. The measured values of the OH-field widths were well fit by an offset sine function. In all cases when the oscillation amplitude is normalized by the cycle mean strain rate and plotted against the non-dimensional flow field frequency, the results collapse onto a single line having a steep negative slope.

Simultaneous particle-imaging velocimetry and OH planar laser-induced fluorescence measurements in an unsteady counterflow propane/air diffusion flame

To study the transient response of a diffusion flame to an unsteady flowfied, quatitative measurements of velocity, using particle-imaging velocimetry, and OH measurements, using planar laser-induced fluorescence, were made simultaneously in an oscillating conterflow diffusion flame. These non-intrusive measurements were performed to spatially and tempoerally resolved flowrield and flame characteristics as a function of initial strain rate and forcing frequency. For the forcing frequencies considered in this study, the strain rate fluctuations were found to lag the velocity fluctuations, but the phase difference decresed with increasing forcing frequency. At lower forcing frequencies, the width of the OH field responded quasi-steadily, but as the forcing frequency increased, the OH field showed transient effects. The dilatation velocity, defined as the difference between the minimum velocity in the preheat zone and the maximum velocity in the reaction zone, was used as a flame temperature indicator. The dilatation velocity revealed that the phase difference between the velocity and the temperature increased with increasing forcing frequency, confirming the existence of a diffusion limited response. The resuls presented here help to illuminate the interconnecting relationships between the chemistry, fluid dynamics, and reactant transport times.

FUEL LEWIS NUMBER EFFECTS IN UNSTEADY BURKE–SCHUMANN HYDROGEN FLAMES

ABSTRACT Flame response (as determined by temperature and flame thickness) to unsteady hydrodynamics has been measured in acoustically pulsed Burke–Schumann hydrogen flames at two different oscillation frequencies and amplitudes. The effect of fuel Lewis number (Le F) on flame dynamics is isolated by investigating steady and unsteady 40% H2/60% He (Le F > 1) and 40% H2/60% Ar (Le F < 1) flames. For a given flame with Le F < 1, local temperature was found to increase with stretch imparted on the reaction zone by the unsteady flow, whereas the opposite trend was observed for the Le F > 1 flame. Unsteadiness might qualitatively alter the effect of the fuel Lewis number. Notably, for Le F < 1 flames under oscillations of sufficiently high frequency and amplitude, the temperature at the flame tip is higher than that in the shoulder regions, and is different from the temperature field of both steady and low-frequency oscillation flames. This suggests that the effect of unsteady flame stretch may overwhelm that of the flame curvature for sufficiently high unsteadiness.

Combustion structures in lifted ethanol spray flames

The development of a double flame structure in lifted ethanol spray flames is visualized using OH planar laser-induced fluorescence (PLIF). While the OH images indicate a single reaction zone exists without co-flow, the addition of low-speed co-flow facilitates the formation of a double flame structure that consists of two diverging flame fronts originating at the leading edge of the reaction zone. The outer reaction zone burns steadily in a diffusion mode, and the strained inner flame structure is characterized by both diffusion and partially premixed combustion exhibiting local extinction and reignition events.

An ultra-short pulse laser lathe for axisymmetric micromachining of explosives

Engineers have devised a novel ultra-short pulse laser lathe system for bulk micromachining of axisymmetric features in energetic material samples with three-dimensional cylindrical geometry. One hundred twenty femtosecond pulses from an 800-nm Ti:sapphire laser were utilized to machine hexanitrostilbene (HNS) rods with diameters less than 200 micrometers and greater than 5:1 aspect ratio without ignition and subsequent bulk combustion or detonation. To date, this work represents the smallest energetic material rod structures fabricated by this technology. Results indicate that surface roughness is dependent upon rotation speed and feed rate. Valuable explosive nano-particles were discovered, collected, and analyzed as a byproduct of fabrication.

Evaluation of the transient response of a counter-flow diffusion flame using two-line OH PLIF thermometry and PIV

OH planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) have been used to study the frequency response of laminar CsHg-air counterflow diffusion flames, and thereby the adequacy of the steady-flamelet model for turbulence. PIV was used to determine the flame strain rate, while OH PLIF was used both to measure temperature at the flame front, using the twoline PLIF technique, and the reaction-zone width. Both measurements demonstrate the existence of a diffusion-limited frequency response of flames subjected to a time-varying flow field. At the 30-Hz and 50-Hz forcing frequencies, the maximum reactionzone temperature and width were found to respond quasi-steadily. However, At higher forcing frequencies—that is, 100 and 200 Hz—transient behavior is evident from the phase relationship between the imposed sinusoidal strain rate and the resulting peak temperature and reaction-zone width. The measured values of the OH-field widths (FWHM) were fit well by an offset sine function. In all cases when the oscillation amplitude (from the sine-curve fit) is normalized by the cycle mean strain rate and plotted against the non-dimensional flow field frequency, it collapses onto a single line with a steep negative slope. 1 Corresponding author Published by the American Institute of Aeronautics and Astronautics, with permission

Vibrational Spectroscopy of HNS Degradation

Hexanitrostilbene (HNS) is a widely used explosive, due in part to its high thermal stability. Degradation of HNS is known to occur through UV, chemical exposure, and heat exposure, which can lead to reduced performance of the material. Common methods of testing for HNS degradation include wet chemical and surface area testing of the material itself, and performance testing of devices that use HNS. The commonly used chemical tests, such as volatility, conductivity and contaminant trapping provide information on contaminants rather than the chemical stability of the HNS itself. Additionally, these tests are destructive in nature. As an alternative to these methods, we have been exploring the use of vibrational spectroscopy as a means of monitoring HNS degradation non-destructively. In particular, infrared (IR) spectroscopy lends itself well to non-destructive analysis. Molecular variations in the material can be identified and compared to pure samples. The utility of IR spectroscopy was evaluated using pressed pellets of HNS exposed to DETA (diethylaminetriamine). Amines are known to degrade HNS, with the proposed product being a σ-adduct. We have followed these changes as a function of time using various IR sampling techniques including photoacoustic and attenuated total reflectance (ATR).