Spontaneously Igniting Hybrid Fuel-Oxidizer Systems

After briefly outlining the recent developments in hybrid rockets, the work carried out by the author on self-igniting (hypergolic) solid fuel-liquid oxidiser systems has been reviewed. A major aspect relates to the solid derivatives of hydrazines, which have been conceived as fuels for hybrid rockets. Many of these N-N bonded compounds ignite readily, with very short ignition delays, on coming into contact with liquid oxidisers, like HNO3 and N2O4. The ignition characteristics have been examined as a function of the nature of the functional group in the fuel molecule, in an attempt to establish a basis for the hypergolic ignition in terms of chemical reactivity of the fuel-oxidiser combination. Important chemical reactions occurring in the pre-ignition stage have been identified by examining the quenched reaction products. Hybrid systems exhibiting synergistic hypergolicity in the presence of metal powders have been investigated. An estimation of the rocket performance parameters, experimental determination of the heats of combustion in HNO3, thermal decomposition characteristics, temperature profile by thin film thermometry and and product identification by the rapid scan FT-IR, are among the other relevant studies made on these systems. A significant recent development has been the synthesis of new N-N bonded viscous binders, capable of retaining the hypergolicity of the fuel powders embedded therein as well as providing the required mechanical strength to the grain. Several of these resins have been characterised. Metallised fuel composites of these resins having high loading of magnesium are found to have short ignition delays and high performance parameters.

Synthesis and properties of rare earth doped lamp phosphors

Red, blue and green emitting lamp phosphors such as EU(3+) doped Y2O3 (red phosphor), EU(2+) doped Ba0.64Al12O18.64, BaMgAl10O17 and BaMg2Al16O27 (blue phosphors) and Ce0.67Tb0.33MgAl11O19 and Eu2+, Mn2+ doped BaMgAl10O17 (green phosphors) have been prepared by the combustion of the corresponding metal nitrates (oxidizer) and oxalyl dihydrazide/urea/carbohydrazide (fuel) mixtures at 400 degrees-500 degrees C within 5 min. The formation of these phosphors has been confirmed by their characteristic powder X-ray diffraction patterns and fluorescence spectra. The phosphors showed characteristic emission bands at 611 nm (red emission), 430-450 nm (blue emission) and 515-540 nm (green emission). The fine-particle nature of the combustion derived phosphors has been investigated using powder density, particle size and BET surface area measurements.

Evaluation of strain-rate effects in transitional round jets using direct numerical simulation

The coherent flame model uses the strain rate to predict reaction rate per unit flame surface area and some procedure that solves for the dynamics of flame surfaces to predict species distributions. The strainrate formula for the reaction rate is obtained from the analytical solution for a flame in a laminar, plane stagnation point flow. Here, the formula's effectiveness is examined by comparisons with data from a direct numerical simulation (DNS) of a round jetlike flow that undergoes transition to turbulence. Significant differences due to general flow features can be understood qualitatively: Model predictions are good in the braids between vortex rings, which are present in the near field of round jets, as the strain rate is extensional and reaction surfaces are isolated. In several other regions, the strain rate is compressive or flame surfaces are folded close together. There, the predictions are poor as the local flow no longer resembles the model flow. Quantitative comparisons showed some discrepancies. A modified, consistent application of the strain-rate solution did not show significant changes in the prediction of mean reaction rate distributions.

Mathematical modelling of divided blast cupola

A pseudo 2-D mathematical model has been developed to simulate a cupola with one row and two rows of tuyere. The simulation results predicted higher spout temperature and combustion ratio for cupola with two rows of tuyere compared to that with one row. Further, the model has been used to study the effect of the distance of separation between the two rows of tuyere on cupola performance. The computed results shows that the spout temperature increases with tuyere level separation and attains the maximum at an optimum distance of separation between two rows of tuyere. Above the optimum, the spout temperature starts decreasing. The exit gas temperature and combustion ratio increases monotonously with the increase in tuyere level separation. These results agree well with the reported experimental observations. The mechanism behind the improved cupola performance with two rows of tuyere has been deduced from the computed temperature and composition profiles inside the cupola.

Steady state three-dimensional mathematical model for cupola

A three-dimensional mathematical model has been developed to simulate the gas flow, composition, and temperature profiles inside a cupola. Comparison of the model with the reported experimental data shows the presence of a zone with low combustion rate at the tuyere level. For a 24 in (610 mm) cupola with four rows of tuyeres, the combustion zones from each tuyere overlap each other, forming an overall combustion zone of cylindrical shape of height similar to 0.2 m. Using the model, it is found that the spout temperature initially increases with increasing blast velocity and attains a maximum. Further increase in blast velocity does not change the spout temperature. This suggests that smaller size tuyeres and higher permeability of the bed can give superior cupola performance. (C) 1997 The Institute of Materials.

Synthesis and characterization of epoxy resins based on N,N'-aliphatic dicarboxyl bis(hydrazones)

A new class of epoxy resins having N-N bonds in their structure has been synthesized by reacting N,N'-aliphatic dicarboxyl bis(hydrazones) (the aldehyde/ketone derivatives of malonic, adipic, and sebacic dihydrazides) with epichlorohydrin. The reactivity of the[GRAPHICS] protons as a function of the substituent group and the number of methylene spacer groups present in the hydrazone has been examined. The resins obtained have been characterized by elemental and epoxy equivalent analyses and IR and NMR spectra. All these resins are found to have adequate viscosity and cure easily with amine curatives at elevated temperatures. Relevant properties for their use as binders in propellant formulations, such as thermal stability, heat of combustion, density, temperature dependence of viscosity, and mechanical strength of the composites, have been evaluated. (C) 1997 John Wiley & Sons, Inc.

Universal behaviour in erosive burning of solid propellants

This paper considers the extensive data and correlations on the erosive burning of solid propellants. A relatively simple nondimensional relationship between the ratio of the actual to nonerosive burn rate (eta) and a quantity g, which is the product of g(0)-the ratio of free stream mass flux to the mass flux from the surface for nonerosive condition-and Re-0(m), where Re-0 is the Reynolds number based on the nonerosive mass flux of the propellant and port diameter, is shown to correlate most data within the accuracies of the experiments with m = -0.125. This shows the above relationship to account for the effects of pressure, aluminum, even up to a proportion of 17%, burn rate catalysts, and motor size. It is concluded that the suggested correlation between eta and g may be adopted universally for most practical propellants. (C) 1997 by The Combustion Institute.

Studies on Cu/CeO2: A new NO reduction catalyst

Fine particle and large surface area Cu/CeO2 catalysts of crystallite sizes in the range of 100-200 Angstrom synthesized by the solution combustion method have been investigated for NO reduction. Five percent Cu/CeO2 catalyst shows nearly 100% conversion of NO by NH3 below 300 degrees C, whereas pure ceria and Zr, Y, and Ca doped ceria show 85-95% NO conversion above 600 degrees C. Similarly NO reduction by CO has been observed over 5% Cu/CeO2 with nearly 100% conversion below 300 degrees C. Hydrocarbon (n-butane) oxidation by NO to CO2, N-2, and H2O has also been demonstrated over this catalyst below 350 degrees C making Cu/CeO2 a new NO reduction catalyst in the low temperature window of 150-350 degrees C. Kinetics of NO reduction over 5% Cu/CeO2 have also been investigated. The rate constants are in the range of 1.4 x 10(4) to 2.3 x 10(4) cm(3) g(-1) s(-1) between 170 and 300 degrees C. Cu/CeO2 catalysts are characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectroscopy where Cu2+ ions are shown to be dispersed on the CeO2 surface. (C) 1999 Academic Press.

Studies on strontium substituted rare earth manganites

Sintering, electrical conductivity and thermal expansion behaviour of combustion synthesised strontium substituted rare earth manganites with the general formula Ln(1-x)Sr(x)MnO(3) (Ln = Pr, Nd and Sm; x = 0, 0.16 and 0.25) have been investigated as solid oxide fuel cell cathode materials. The combustion derived rare earth manganites have surface area in the range of 13-40 m(2)/g. Strontium substitution increases the electrical conductivity values in all the rare earth manganites. With the decreasing ionic radii of rare earth ions, the conductivity value decreases. Among the rare earth manganites studied, (Pr/Nd)(0.75)Sr0.25MnO3 show high electrical conductivity ( > 100 S/cm). The thermal expansion coefficients of Pr0.75Sr0.25MnO3 and Nd0.75Sr0.25MnO3 were found to be 10.2 x 10(-6) and 10.7 x 10(-6) K-1 respectively, which is very close to that of the electrolyte (YSZ) used in solid oxide fuel cells. (C) 1999 Elsevier Science B.V. All rights reserved.

Synthesis and properties of La1-xSrxFeO3

Fine particle strontium substituted lanthanum ferrites La1-xSrxFeO3, where x = 0.0-1.0, have been synthesized by the solution combustion method using corresponding metal nitrates, oxalyl dihydrazide (ODH) or tetra formal tris azine (TFTA). Formation of La1-xSrxFeO3 was confirmed by the XRD and the fine particle nature of the ferrites investigated using SEM, particle size analysis and BET surface area measurements. La1-xSrxFeO3 (up to x = 0-0.4) exhibited low resistivity near the Neel temperatures. La1-xSrxFeO3 with x greater than or equal to 0.8 when used as bifunctional electrodes, showed oxygen evolution and reduction activity comparable with the orthoferrites prepared by the conventional solid state method. (C) 1999 Elsevier Science B.V. All rights reserved.

Synthesis and properties of willemite Zn2SiO4, and M2+: Zn2SiO4 (M = Co and Ni)

Fine particles of willemite, alpha -Zn2SiO4, were prepared by both solution combustion and sol-gel methods. Both processes yield single-phase, large-surface area (26- and 78-m(2)/g), sinteractive willemite powders. Thermal evolution of crystalline phases was studied using X-ray powder diffraction patterns. The combustion method favors low-temperature formation of willemite compared to the sol-gel method. The powders, when uniaxially pressed and sintered at 1300 degreesC, achieved 78-80% theoretical density. The microstructures of the sintered body show the presence of equiaxed 0.5- to 4-mum grains. Blue pigments of willemite doped with Co2+ and Ni2+ were also prepared by the combustion process.

Effect of Zr4+-ion substitution in CeO2 on H2O2-assisted degradation of orange G

A series of Pd ion-substituted CeO2-ZrO2 solid solutions were synthesized using the solution combustion technique. H2O2-assisted degradation of orange G was carried out in the presence of the catalysts. The activity of the catalysts was found to increase with the introduction of the second component in the solid solution, as signified by an increase in the rate constants and lowering of activation energy. The study showed the involvement of lattice oxygen and the importance of reducibility of the compound for the reaction. (C) 2011 Elsevier B.V. All rights reserved.

Assessment of conditional moment closure models of turbulent autoignition using DNS data

A systematic assessment of the submodels of conditional moment closure (CMC) formalism for the autoignition problem is carried out using direct numerical simulation (DNS) data. An initially non-premixed, n-heptane/air system, subjected to a three-dimensional, homogeneous, isotropic, and decaying turbulence, is considered. Two kinetic schemes, (1) a one-step and (2) a reduced four-step reaction mechanism, are considered for chemistry An alternative formulation is developed for closure of the mean chemical source term , based on the condition that the instantaneous fluctuation of excess temperature is small. With this model, it is shown that the CMC equations describe the autoignition process all the way up to near the equilibrium limit. The effect of second-order terms (namely, conditional variance of temperature excess sigma(2) and conditional correlations of species q(ij)) in modeling is examined. Comparison with DNS data shows that sigma(2) has little effect on the predicted conditional mean temperature evolution, if the average conditional scalar dissipation rate is properly modeled. Using DNS data, a correction factor is introduced in the modeling of nonlinear terms to include the effect of species fluctuations. Computations including such a correction factor show that the species conditional correlations q(ij) have little effect on model predictions with a one-step reaction, but those q(ij) involving intermediate species are found to be crucial when four-step reduced kinetics is considered. The "most reactive mixture fraction" is found to vary with time when a four-step kinetics is considered. First-order CMC results are found to be qualitatively wrong if the conditional mean scalar dissipation rate is not modeled properly. The autoignition delay time predicted by the CMC model compares excellently with DNS results and shows a trend similar to experimental data over a range of initial temperatures.

Autoignition in a non-premixed medium: DNS studies on the effects of three-dimensional turbulence

Direct numerical simulation (DNS) results of autoignition in anon-premixed medium under an isotropic, homogeneous, and decaying turbulence are presented. The initial mixture consists of segregated fuel parcels randomly distributed within warm air, and the entire medium is subjected to a three-dimensional turbulence. Chemical kinetics is modeled by a four-step reduced reaction mechanism for autoignition of n-heptane/air mixture. Thus, this work overcomes the principal limitations of a previous contribution of the authors on two-dimensional DNS of autoignition with a one-step reaction model. Specific attention is focused on the differences in the effects of two- and three-dimensional turbulence on autoignition characteristics. The three-dimensional results show that ignition spots are most likely to originate at locations jointly corresponding to the most reactive mixture fraction and low scalar dissipation rate. Further, these ignition spots are found to originate at locations corresponding to the core of local vortical structures, and after ignition, the burning gases move toward the vortex periphery Such a movement is explained as caused by the cyclostrophic imbalance developed when the local gas density is variable. These results lead to the conclusion that the local ignition-zone structure does not conform to the classical stretched flamelet description. Parametric studies show that the ignition delay time decreases with an increase in turbulence intensity. Hence, these three-dimensional simulation results resolve the discrepancy between trends in experimental data and predictions from DNSs of two-dimensional turbulence. This qualitative difference between DNS results from three- and two-dimensional simulations is discussed and attributed to the effect of vortex stretching that is present in the former, but not in the latter.

A Gradient-Based Comparison Measure for Visual analysis of Multifield Data

We introduce a multifield comparison measure for scalar fields that helps in studying relations between them. The comparison measure is insensitive to noise in the scalar fields and to noise in their gradients. Further, it can be computed robustly and efficiently. Results from the visual analysis of various data sets from climate science and combustion applications demonstrate the effective use of the measure.