Donna Hanson-Parr

Condensed-phase species distributions about Al particles reacting in various oxidizers

Abstract Experimental results on the combustion of single, isolated aluminum particles, laser ignited in quiescent environments consisting of pure N 2 O, CO 2 , CO and in mixtures of 21% O 2 / 79% N 2 and 21%O 2 / 79% Ar are reported. Combustion measurements consisted of photographic observations and electron probe microanalysis (EPMA) of the condensed-phase product composition and radial distribution. Aluminum particles in O 2 , CO 2 , and N 2 O atmospheres were found to burn with envelope flames. Of these oxidizers, the largest flame envelope, as determined by the condensed-product distribution, occurred for Al combustion in the O 2 /Ar mixture, followed by Al combustion in the O 2 /N 2 mixture, the CO 2 atmosphere, and the N 2 O atmosphere. Combustion in the CO atmosphere appeared to occur on (near) the particle surface with only a weak envelope reaction. Consistent with previous results in the literature, Al particle disruption was not observed in O 2 /Ar environments, but was observed in O 2 /N 2 environments. Although speculated in the literature, the present work confirms the existence of aluminum nitrides (oxy-nitrides) in the fuel-rich region near the particle surface for nitrogen-containing oxidizers (i.e., O 2 /N 2 and N 2 O). Equilibrium calculations indicate that near the surface, solid-phase AlN may exist to temperatures well above the melting temperature of aluminum oxide. Thus, its presence may affect the fragmentation process. Finally, condensed-phase carbon (possibly in the form of aluminum carbide) was found throughout the surrounding gas-phase for CO combustion.

Streamwise and spanwise vortex interaction in an axisymmetric jet. A computational and experimental study

The near‐field of an azimuthally excited round jet was investigated in a combined computational/experimental study. The reaction zones in the jet were visualized using OH Planar‐Laser‐ Induced‐Fluorescence (PLIF) diagnostics. Both axisymmetric and azimuthal modes of the jet were excited to stabilize its spatial structure. Three‐dimensional flame visualization of the laboratory jet reconstructed from multiple two‐dimensional images acquired at constant phase angle, reveal a complex structure of the reaction zone. Time‐dependent numerical simulations provided insight into the underlying fluid‐dynamical processes leading to this flame structure. Simulations of reactive and non‐reactive free jets used a Monotonically Integrated Large‐Eddy‐Simulation (MILES) approach, multi‐species diffusive transport, global finite‐rate chemistry and appropriate inflow/outflow boundary conditions. The flow visualizations of the experimental and computational jets strongly resemble each other, revealing tight coupling between ...

THERMAL PROPERTIES MEASUREMENTS OF SOLID ROCKET PROPELLANT OXIDIZERS AND BINDER MATERIALS AS A FUNCTION OF TEMPERATURE

Abstract Using a fairly simple technique and small samples it was possible to obtain thermal diffusivity, specific heat capacity, and thermal conductivity, all as a function of sample temperature, for a variety of ingredients used in solid rocket propellants. The oxidizers AP, ADN, CL20, HMX, RDX, HNF, TNAZ were studied as well as the nonenergetic polymers TeflonTM, HTPB, and polyurethane, energetic binders containing GAP and BAMO and/or NMMO, and actual solid propellants XM39, N5, N12, and SB129.

On the role of large and small-scale structures in combustion control

Abstract Experiments were performed to actively control combustion between coaxial air and fuel jets. The objective was to initiate the combustion as close as possible to the flameholder surface and to maintain uniform combustion in the entire mixing region. Forcing was applied to both the air and fuel streams. at different frequencies and amplitudes. The air jet was excited at its preferred mode frequency while the fuel was forced at higher harmonics in order to trigger and amplify the initial instabilities of the coaxial jet. Nonreacting tests showed that the combined forcing was promoting an earlier transition to small-scale turbulence at the nozzles exit. Consequently. the combustion was enhanced and became uniformly distributed along the flame. contrary to the reference unforced flame where intense combustion started only at a distance from the flameholder and was limited to the region where vortical structures developed. The optimal combination of forcing parameters are presented and discussed.

Azimuthal structure of an annular diffusion flame

An annular diffusion flame was studied using planar laser induced fluorescence. The insitu concentration of one of the combustion products, hydroxyl radical, was used as an indicator of the combustion structure. The flame structure was studied both in axial and radial cross-sections. Phased average data, as well as instantaneous measurements of 18 nsec duration, revealed details on the highly three-dimensional features of the axisymmetric flame. The characteristics of these structures at different regions of the flame and their response to forcing were studied. The three-dimensional structures have a significant effect on the fine-scale mixing rate and flame stability limits.

Structure of a Controlled Ducted Flame

Abstract The structure of self-excited and controlled ducted flames was studied by imaging the CH emission and analyzing the pressure and CH intensity time variation. Self-excited combustion oscillations occur when the flame interacts with the large-scale vortices which are excited in the shear layer by the acoustic forcing at the duct resonance modes. The periodic heat release produced by the combustion inside the vortices further excite the duct acoustics. The fuel to air mixture ratio is shown to have an important effect on the interaction between the flame and vortices. The effect on the flame structure of pressure and CH control systems is described. The transition from controlled conditions lo uncontrolled and vice versa is visualized. The roll-up of coherent structures dominating the flame in the self-excited conditions is disrupted by the controller; the shear layer is forced at the exact phase necessary to cancel the perturbation due to the duct acoustic pressure. This is done more effectively wi...

RDX flame structure

Nonintrusive diagnostics were used to measure temperature and species profiles during neat RDX deflagration at 1 atm. UV-visible absorption was measured to obtain absolute concentration profiles of NO, NO 2 , CN, NH, H 2 CO, and OH. Temperature and species concentrations were obtained by spectral fitting. Planar laser-induced fluorescence (PLIF) of these same species was also measured to obtain two-dimensional (2D) profiles in the flame with excellent spatial resolution. The flame structure is characterized by a “dark zone” close to the surface and a visible flame sheet above the dark zone. For CO 2 laser-assisted deflagration, the narrow flame sheet NH profile peaks 2.3 mm above the surface at a value of 100 ppm. The CN profile is slightly wider and peaks at 2.5 mm at a value of 660 ppm. The OH profile peaks outside the CN/NH flame sheet with a mole fraction of 0.055. The dark zone species studied here were NO 2 and NO. The NO 2 peaks very close to the surface at about 0.17 mole fraction and decays rapidly to 0 at 1.5 mm. Close to the surface, the NO mole fraction is about 0.2 and falls sharply to 0.05 at 2.5 mm as NO is consumed in the CN/NH flame sheet. No formaldehyde was detected. Rotational temperature profiles were measured from OH PLIF and NO absorption spectra. The NO gas temperature near the surface is in good agreement with prior thermocouple measurements. The NO gas temperature near the surface is in good agreement with prior thermocouple measurements. The temperature rises sharply to about 1500 K at 0.3 mm and then turns over to a much more gradual slope in the dark zone. At about 2 mm, it becomes steeper again and finally levels out to 2600 K at 3.0 mm, at the top edge of the CN flame sheet.

RDX/GAP/BTTN propellant flame studies

Abstract The non-intrusive techniques of planar laser-induced fluorescence, ultraviolet-visible absorption spectroscopy and spontaneous laser Raman spectroscopy were used to map out species and temperature profiles above the surface of self-deflagrating RDX/GAP/BTTN model propellant with a pseudo pre-mixed flame. HCN, CO, and N 2 were found to be the major species near the surface, and CO, N 2 , and H 2 O in the burnt gases. A dark zone of about 1200 to 1300 K was observed in which the NO concentration was at its highest. NO 2 existed only very close to the surface. No formaldehyde was observed in the gas phase. Analysis of thermocouple measurements showed a surface temperature of 605 K. Thermal diffusivity and specific heat capacity as a function of temperature were also measured.

Compact incinerator afterburner concept based on vortex combustion

A design concept for a compact incinerator afterburner based on actively controlled vortex combustion was developed and tested at ≈ 5 kW and ≈ 50 kW. Acoustic control of fluid dynamics was used to enhance mixing and increase the DRE (destruction and removal efficiency) for a waste surrogate. A detail study of the concept of utilizing vortex combustion for incineration was undertaken in a small-scale flame using advanced laser diagnostics to elucidate and optimize the fluid dynamic mechanisms. The system was then scaled up by ≈ 10, optimized, and evaluated for performance. The open loop active control methodology is based on the concept of combustion in periodic axisymmetric vortices. Acoustic excitation was, used both to stabilize coherent vortices in the central jet air flow and to control circumferential gaseous fuel and waste injection into the shear layer at the right time during vortex formation. The gaseous-fueled actively controlled 4.5 kW incinerator was able to surpass 99.997% DRE even when the gaseous benzene waste surrogate constituted 66% of the total combustible content and the combustible to total air ratio (o) was 0.974; lowering o below this increased the DRE to beyond our detection limit. The DRE for gaseous benzene exceeded 99.999% in the 50-kW system when combustible to air ratios were kept below 0.78. Parameters found critical to maintenance of high DRE at both energy scales were the fraction of circumferentially entrained air, the entrainment geometry, the forcing levels, and the phase angle of fuel injection with respect to the vortex roll-up. The 50-kW combustor was also evaluated for stack emissions and combustion efficiency. The controller improved combustion efficiency and lowered emissions: CO dropped from 2900 ppm to as low as 2 ppm. Unburned hydrocarbons were also reduced. Under some conditions the controller also reduced NO x levels down to as low as 12 ppm.

Stabilization of a Premixed Flame by Shear Flow Excitation

Abstract The lean flarnmability limit of a premixed flame was extended by forcing the initial shear layer of the jet. It was shown that this extension depends on the forcing frequency and amplitude. The most effective control was obtained using forcing in a Strouhal number range of 0.01 to 0.06 near the most amplified frequency of the shear layer as determined by the jet exit mean velocity profile and initial boundary layer thickness. The small-scale vortices generated in the shear layer by this forcing caused reattachment of the lifted flame and held it at the jet nozzle for equivalence ratio and mean flow rate which were significantly beyond the unforced flarnmability limits. Flame intensity was augmented as well at this forcing. An adverse effect of acoustic forcing was observed when the jet was forced at the preferred mode frequency. For this condition, the generated vortices had length scales which destabilized the flame, causing intermittent flameholding and reduction in heat release. The effect of ...