Satyanarayanan R. Chakravarthy

Mechanism of Burning Rate Enhancement of Composite Solid Propellants by Ferric Oxide

This paper reports a series of experimental studies performed on sandwich propellants, wherein a matrix lamina of particulate oxidizer and polymeric binder is sandwiched between two ammonium perchlorate (AP) laminae. The catalyst (ferric oxide ) is incorporated in the matrix lamina. The variables are pressure (0.345‐ 6.9 MPa), matrix lamina thickness, catalyst concentration, matrix mixture ratio, types of oxidizer and binder, and the dispersion ability of the catalyst. The combined results indicate that, under the conditions tested, near-surface reactions associated with the particulate AP/binder contact lines on the burning surface assume signie cance in the presence of the catalyst. These reactions are further augmented by the presence of the leading-edge portion of the diffusion e ame above the interface of the matrix and AP laminae.

Understanding nanoparticle formation by a wire explosion process through experimental and modelling studies.

A wire explosion process (WEP) has been used to produce nano aluminium powder in nitrogen, argon and helium atmospheres. The impact of energy deposited into the exploding conductor on the size and shape of the particles was analysed using TEM analysis, which forms the first part of the study. It is observed that the higher the energy deposited, the smaller the particles formed. In the second part, modelling studies were carried out by solving the general dynamic equation through the nodal approach, and the particle size distributions were predicted. It is realized that, at the point of high saturation ratio and nucleation rate, the size of the critical nucleus formed is low. The particle size distribution predicted by the model correlates well with the experimental results. Time-series analysis of particle formation indicates that particles of lower dimensions form and, in the process of coagulation, larger particles are formed. It is realized that the plasma formed during the explosion plays a major role in the particle formation, and the modelling studies confirm that particle formation is not an instantaneous process but requires a certain time period to form stable sizes and shapes.

Unsteady combustion response of a ducted non-premixed flame and acoustic coupling

This paper explores some fundamental issues involved in flame–acoustic interaction in the context of non-premixed flames. The combustion model considered is a two-dimensional co-flowing non-premixed flame in a uniform flow field, as in the Burke–Schumann geometry. Both finite-rate and infinite-rate chemistry effects are examined. First, the velocity-coupled response of the flame to an externally imposed velocity fluctuation is studied at various frequencies of interest. The Damköhler number plays an important role in determining the amplitude and phase of the heat release fluctuations with respect to the velocity fluctuations. Second, the combustion model is coupled with the duct acoustics. The one-dimensional acoustic field is simulated in the time domain using the Galerkin method, taking the fluctuating heat release from the combustion zone as a compact acoustic source. The combustion oscillations are shown to cause exchange of acoustic energy between the different natural modes of the duct over several cycles of the acoustic oscillations.

Intermittent Burning of Ammonium Perchlorate-Hydrocarbon Binder Monomodal Matrixes, Sandwiches, and Propellants

The onset of the midpressure extinction of certain formulations (termed matrixes) of ammonium perchlorate (AP) of monomodal particle size distribution and hydroxyl-terminated polybutadiene binder, in the pressure range around 2-5 MPa, is examined. These matrixes, besides being tested in isolation, have been included in between AP laminas to form sandwiches and mixed with coarse AP particles to form high solids-loading (87.5%) non-aluminized propellants. The burning rates of the sandwiches show abnormal trends with pressure such as low or negative exponents in ranges corresponding to the onset of the midpressure extinction of their respective matrixes. The propellants exhibit this behavior to a lesser degree. Quenched surfaces (self-extinguished or intentionally interrupted during burning) of all the three types of samples were analyzed using a scanning electron microscope, and the burning history of the samples was captured with a high-speed digital camera. The results indicate the prevalence of intermittent burning of the matrixes as the pressure is varied across the boundary between continuous burning and self-extinction (burn/no-burn boundary) of the matrixes. The burning surfaces are marked by extreme three dimensionality coupled with a redistribution of the fine AP particles and the binder. The observations are explained based on the combined effects of the need for the AP particles and the binder to accumulate relative to each other on the burning surface depending on the difference in their pyrolysis rates and the existence of the binder in a molten state on the burning surface.

Production, Characterization, and Combustion of Nanoaluminum in Composite Solid Propellants

non- and microaluminized propellants are washed out with the addition of nano-Al, but the burning rates of nanoaluminized propellants register low pressure exponents in the elevated pressure range. The results suggest that the heat feedback from the diffusion-limited combustion of nano-Al particles near the propellant burning surface controls the propellant burning rate when sufficiently large parts of the burning surface are made up of the nanoaluminized fine ammonium-perchlorate/binder matrix.

Plateau Burning Behavior of Ammonium Perchlorate Sandwiches and Propellants at Elevated Pressures

Further results on combustion of sandwiches made of alternating layers of ammonium perchlorate (AP) and a matrix of AP particles in polymeric binder in an expanded pressure range of 0.345-13.78 MPa (50-2000 psig), over a wide range of matrix lamina thicknesses, are reported. Inclusion of a nanoparticle-size burning rate catalyst in the matrix is also considered. The sandwich burning rates indicate plateaus over the 6.89-13.78 MPa (1000-2000 psig) pressure range for select ranges of matrix lamina thickness. These are correlated with similar plateau trends in the burning rates of composite propellant formulations with bimodal particle size distribution of the oxidizer and appropriate choice of coarse AP size, wherein the fine AP/binder matrices are of identical composition to those tested in the sandwiches. The dependence of the sandwich burning rates on the matrix lamina thickness and the quenched surface features are examined to explain the plateau burning rate trends. The results indicate that the two-dimensional coupling of heat feedback from the hot, near-surface parts of the oxidizer/fuel diffusion flamelets is diminished at elevated pressures due to their greater proximity to the burning surface and the corresponding shrinking of their lateral extent.

Computer Model of Aluminum Agglomeration on Burning Surface of Composite Solid Propellant

The process of agglomeration of aluminum particles on the burning surface of an ammonium perchlorate-based composite propellant is modeled using a computer algorithm. A random pack of particulate ingredients of given size and mass specifications is cast on the computer to simulate the propellant microstructure as reported previously. The aluminum particles are tracked as they emerge at a regressing burning surface, accumulate into filigrees, and get ignited by the near-surface leading-edge oxidizer-binder diffusion flamelets attached to the exposed areas of certain ammonium perchlorate particles. An approximate heat transfer model is incorporated to estimate the ignition delays radially inward and outward from the leading-edge flamelets into the filigrees accumulated over ammonium perchlorate particles and surrounding binder-fine ammonium perchlorate matrix layers. The delay influences the number of parent aluminum particles constituting a filigree, and consequently the size of the agglomerate that the filigree rolls up into. The implementation of the algorithm is validated against experimental results available in the literature, which were specifically obtained to investigate the relationship between the decrease in the agglomerate size and attachment of leading-edge flamelets over the fine ammonium perchlorate particles in the propellant with increase in pressure.

Swirler Flow Field Characteristics in a Sudden Expansion Combustor Geometry

The present study investigates the flow field characteristics of a gas turbine swirler in a model combustion chamber, using particle image velocimetry. Detailed mean and RMS velocities, vorticity, Reynolds shear stress, and pseudoturbulent kinetic energy were obtained at various cross sections downstream of the swirler and in a plane along the inlet flow direction. The experiments were performed in a sudden expansion square geometry. A central toroidal recirculation zone and corner recirculation zone was observed and characterized. Another instability caused by swirl, called precessing vortex core, has been observed far downstream of the swirler, in the plane located at Z/D = 2.5 and 1.25 (D, diameter of the swirler) depending on the pressure drop across the swirler. High RMS velocity magnitudes are observed in several cross-sectional planes indicating high levels of turbulence generated by the swirling effect which promotes rapid mixing. The structure of the complex swirling flow field has been investigated both qualitatively and qualitatively.

Mechanism of Pipe-Tone Excitation by Flow through an Orifice in a Duct

Pipe-tone is excited by flow through a duct containing an orifice with square edges of thickness within a certain range. The mechanism of this process is investigated by time-resolved flow visualization with the orifice at the end of the duct. Vortices are primarily shed at the orifice when the acoustic pressure at that place reaches a maximum in time. Successive vortices shed during a cycle of the dominant pipe-tone mode and merge further downstream of the orifice. Under some conditions, the excitation of pipe-tone forces the jet issuing out of the orifice to fork into two trains, even for the ratio of the densities of the jet fluid to the ambient exceeding unity, unlike previously reported results. Within the range of the orifice thickness required for intense excitation of pipe-tone, the shear layer separating from the upstream edge of the orifice appears to re-attach at the downstream edge. Several mode shifts occur within the range of flow velocities tested when the orifice is placed within as well as at the end of the duct, with the amplitude maximized in the velocity range between consecutive shifts. Two modes are simultaneously present during the transition at relatively low amplitudes. Lesser number of shifts is observed when the orifice is progressively located upstream in the duct, but the simultaneous multiple-mode excitation spans a wider velocity range. The acoustic pressure amplitude of pipe-tone is found to increase linearly with the mean pressure in the duct.

Suppression of High Mach Number Rocket Jet Noise by Water Injection

DOI: 10.2514/1.43421 The present work experimentally investigates suppression of the sound level from an underexpanded jet of Mach number 2.8 by water injection. The jet is produced by a solid rocket motor being static test fired. Water is injected from a radial distance of 5.2 jet diameters, at different axial locations from the exit of the nozzle, at two different angles of injection relative to the downstream jet axis. The ratio of mass flow rates of water to the nozzle exhaust gas (referred toas themass flow rateratio)andthe injection pressure arevariedindependently. Acoustic measurements are performed at a radius of 30 jet diameters, over angles in the range of 30–130 deg, relative to the downstream jet axis.Soundlevelscontinuouslydecreaseby10dBwiththeincreaseintheangleofobservation.Withwaterinjection, higher levels of reduction in sound are observedin the upstream quadrant. Injection closer to the nozzle exit leads to better reduction, mainly due to suppression in the high-frequency range when observed from downstream, but it is almost in the entire frequency rangeas observed at the upstream locations. At intermediate mass flow rateratios, an optimum injection pressure exists for maximum noise suppression, due to the penetration of water to the potential core and its evaporation there at high injection pressures. The results affirm that the validity of many past studies obtained on water injection to suppress noise levels on simulated jets can be extended to an actual rocket situation.