Timothy P. Parr

Large-scale coherent structures as drivers of combustion instability

Abstract The role of flow coherent structures as drivers of combustion instabilities in a dump combustor was studied. Results of nonreacting tests in air and water flows as well as combustion experiments in a diffusion flame and dump combustor are discussed to provide insight into the generation process of large-scale structures in the combustor flow and their interaction with the combustion process. It is shown that the flow structures, or vortices, are formed by interaction between the flow instabilities and the chamber acoustic resonance. When these vortices dominate the reacting flow, the combustion is confined initially to the circumference of their cores and further downstream proceeds into their core, leading to periodic heat release, which may result in the driving of high amplitude pressure oscillations. These oscillations are typical to the occurrence of combustion instabilities for certain operating conditions. The basic understanding of the interaction between flow dynamics and the combustion ...

Near field dynamics of subsonic free square jets. A computational and experimental study

Results of a combined numerical and experimental investigation of the near field of low‐subsonic air square jets are presented. The study focuses on examining the role of initial conditions and other features of the jet dynamics in determining the nature and frequency of occurrence of axis switching and the related mechanisms which enhance entrainment, mixing, and turbulence production. Three different experimental square jet facilities were utilized, including orifice jets with low and high initial turbulence level, and pipe jets. Unsteady, spatially developing jets were investigated computationally using direct and monotonically‐integrated large‐eddy simulation approaches, and appropriate inflow/outflow boundary conditions. Insight on the axis‐switching process was obtained using the detailed database from the simulations to investigate how the unsteady vorticity dynamics reflects on the time‐averaged properties of the jet cross sections. The different experimental jets were chosen such that important p...

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.