Boron Combustion Research Articles

Kinetic model of liquid B2O3 gasification in a hydrocarbon combustion environment: I. Heterogeneous surface reactions

AbstractAs part of an ongoing program to model hydrocarbon assisted boron combustion, a kinetic model has been developed to describe gasification of the ubiquitous boron oxide coating that inhibits particulate boron ignition. This model includes homogeneous gas phase oxidation reactions, multi‐component gas phase diffusion, heterogeneous surface reactions, and oxide vaporization. The kinetic processes are treated using a generalized kinetics code, with embeded sensitivity analysis, for the combustion of a one‐dimensional (particle radius), spherical particle.This article presents the heterogeneous surface reactions used to describe oxide gasification and presents selected model results for a spherical boron oxide droplet which illustrate the dependence of the oxide gasification rates on the ambient temperature and particle diameter.

Ignition and combustion of boron in wet and dry atmospheres

A description of ignition and combustion of boron particles is developed with the aim of obtaining a theory that is as simple as possible yet consistent with available data. For the ignition stage the model involves equilibrium reactive dissolution of B in the thin B2O3(ℓ) layer to form dissolved BO, for example, surface attack of BO by O2(g) and by H2O(g) to form BO2(g), and HOBO(g), respectively, vaporization of B2O3, and later clean-surface attack of B by O2(g) to form B2O2(g). For the combustion stage the mode involves only the last of these processes. Values of relevant rate constants are identified that achieve agreement with experiments. Early laser-ignition experiments, not previously explained quantitatively, are employed for obtaining needed dry-gas rate parameters. New experiments on ignition and combustion of boron suspensions in hot combustion products of a flat-flame burner are performed to test adopted values of wet-gas rate parameters. Results provide a basis for calculation of ignition and combustion of boron in propulsions applications.

Ignition and combustion of boron particles in the flowfield of a solid fuel ramjet

Theoretical investigation on the behavior of individual boron particles in the flowfield of a solid fuel ramjet (SFRJ) combustor is presented. The study was motivated by the observed difficulties in achieving good combustion efficiencies of boron required to exploit its remarkable theoretical energetic performance. The equations describing the gas flowfield and the particle behavior are solved numerically. The solution presents the trajectory, temperature, and history of the boron particles due to the interactions with the surrounding gas, as well as the ignition envelope and combustion time. The results demonstrate the limited ranges of particle size and ejection velocity which enable ignition and sustained combustion, reveal why practical systems often exhibit poor combustion efficiencies, and predict the conditions where ignition and efficient combustion of boron are feasible. Nomenclature

Transport coefficients of boron combustion products in dry air

Transport coefficients of boron combustion products in dry air

Kinetics of boron combustion in dry air

Kinetics of boron combustion in dry air

An investigation of combustion of boron suspensions

A steady nitrogen jet transporting fine (0.04∼0.15 μ diameter) boron particles at low loading densities was injected coaxially into the hot combustion products of a flat-flame burner to study the kinetics of boron ignition and combustion. Three types of boron flame-plumes were found: a dark-yellow plume emitting, dark-brown smoke from its tip, at measured maximum flat-flame temperatures below 1800 K; a yellow plume surrounded in its lower part by green emission, between approximately 1800 K and 1900 K; and a bright-yellow plume entirely surrounded by bright-green radiation, above 1900 K. In these patterns, which were independent of the oxygen mole fraction in the product gas, over the range 0.08 to 0.80, the bright yellow was interpreted as boron ignition and the bright green as BO2 emissions from boron combustion products. Based on a theory for a one-step, Arrhenius, ignition process, a theoretical analysis of the jet flow was employed to extract overall, rate parameters from measurements of the height of the yellow plume as a function of the flat-flame temperature. The results were interpreted in terms of existing and new potential models for the ignition and combustion of boron particles.

A physical and chemical interpretation of boron particle combustion

The combustion of boron is considered from both physical and chemical points of view. The overall combustion process is separated into an ignition phase, during which the particle is covered by a layer of boron oxide, and a clean particle burning phase. During the ignition phase boron oxide production results from boron diffusion across the oxide layer, rather than the currently accepted view of oxygen diffusion. During the clean particle burning phase, boron oxidation is found to be similar to that of carbon. The chemistry of boron oxide production during this phase and the regions of kinetic and diffusion control of the burning rate are defined.

Dependence of the burning rate of transition-metal-boron systems on the component ratio

The dependence of the propagation velocity of the chemical reaction wave on the composition of the starting mixture is an important characteristic of combustion processes and is described in this article. In analyzing these dependences for group IV metal (Ti, Zr, Hf)-boron systems, the authors concluded that metastable zirconium monoboride, which was absent from the end products, may be formed in the combustion wave. Other conclusions are described. The results of comparing the calculated burning rates of mixtures of group IV metals and boron with the available experimental data are presented. A discussion of the reasons for the stage type of temperature profile in the combustion of transition metals and boron, detected in certain mixture compositions is discussed.

Direct Mixing and Combustion Efficiency Measurements in Ducted, Particle-Laden Jets

Using water-cooled probes and hot-gas valves, gas-particle samples were withdrawn from the secondary duct of an air-augmented laboratory burner. Using a boron-loaded propdlant, air/fuel ratio and secondary duct pressure were varied from 12/1 to 32/1 and from 82 to 127 psia, respectively. From sample analysis of chlorine, argon (air tracer), boron, and boron oxide, radial and axial profiles of air, gaseous fuel, participate fuel and percent of boron combustion were determined. Particles and gases mixed at significantly different rates. Measured gasphase mixing rates were comparable to model predictions, which assumed particles to be in equilibrium with gases. Boron combustion efficiency varied markedly with duct position, air/fuel ratio and secondary chamber pressure. Low boron combustion efficiency resulted principally from delayed ignition of the gaseous fuels after dilution by air reduced resulting gas-phase temperatures below the boron ignition temperature. This was especially true at low secondary duct pressures.

Boron Combustion Characteristics in Ducted Rockets

Abstract A test program was performed with the objective of experimentally investigating the boron reaction characteristics in a ducted rocket by systematically varying the primary chamber conditions. In order to vary the primary chamber conditions a test device was used which burned boron powder and gaseous propellants (hydrogen or carbon monoxide, oxygen and nitrogen) in the primary motor. The reaction characteristics in the secondary chamber were observed by color photography through quartz windows of the secondary chamber. In the tests the following parameters were varied: (1) primary chamber temperature, (2) mixture ratio of the gaseous fuel in the primary chamber from below stoichiometric to above stoichiometric at different primary chamber temperature levels in order to vary the afterburning capability of the gaseous exhaust in the secondary chamber, and (3) secondary chamber pressure. At primary motor conditions which approximated propellants with high boron and low oxidizer/fuel content the boron particle temperature at the primary nozzle exit was not sufficient to initiate self-sustained boron combustion in the air. In order to farther heat up and cause ignition of the boron particles in the secondary chamber it was necessary to burn additional gaseous fuel in the secondary chamber. It was also demonstrated that the hot (probably reacting, but not ignited) boron particles were necessary for triggering the gaseous fuel ignition in the air.

Boron Ignition and Combustion in Air-Augmented Rocket Afterburners

Abstract Results of experimental studies of the afterburning of the exhausts of highly boron-loaded, fuel-rich propellants with air and results of boron single particle combustion studies have been examined for definition of limiting processes causing incomplete combustion of boron in typical afterburners, particularly at low afterburner pressure. This examination indicates that ignition of the boron particles, inhibited by the presence of a coating of boron oxide, is likely the controlling process. Transient differential equations describing the generation and removal of oxide and the temperature history of a particle have been derived, converted to difference equation form and programmed for computer solution to permit prediction of whether or not a given particle will ignite and, if it will, the time required for ignition. Predicted ignition times for particles studied by Macek in a flat-flame burner have been compared with his experimental results—reasonably good agreement has been obtained. The progr...

Experimental investigation of boron combustion in air-augmented rockets

Studies have been made using a windowed combustion tunnel to examine the mixing and burning between subsonic air and a supersonic fuel-rich exhaust of a rocket motor burning hydrogen-oxygen-boron mixtures. The reaction processes were recorded by high-speed cinephotography. Results of experiments are presented in which the following parameters were varied: 1) boron concentration in the propellant (up to 55% by weight), 2) nonequilibrium chamber temperature of the primary rocket (750 °K to 2500 °K), and 3) air to propellant flow ratio (3 to 15). The temperature of the primary rocket was calculated by assuming boron as a heat sink. These calculations showed good agreement with experimental data at low chamber temperatures. The strong influence of the primary nonequilibrium chamber temperature on the boron combustion efficiency was demonstrated using the results of particle sampling probes and c*-efficiency measurements. By increasing the chamber temperature from 750°K to 2000°K the over-all reaction efficiency was improved from 85 to 94%.

The Thermochemistry of Boron and Some of Its Compounds. The Enthalpies of Formation of Orthoboric Acid, Trimethylamineborane, and Diammoniumdecaborane1,2

A rotating-bomb calorimetric technique was developed to solve the problem of determining accurate values of the enthalpies of combustion of boron and boron compounds. The enthalpies of combustion of crystalline boron, trimethylamineborane ((CH/sub 3/)/sub 3/NBH/sub 3/), and diammoniumdecaborane (B/sub 10/H/sub 18/N/sub 2/) were determined. The combustion products were carbon dioxide, water, fluoroboric acid in excess aqueous HF, and (in the case of the boron compounds) gaseous nitrogen. The enthalpy of solution of orthoboric acid was measured in a hydrofluoric acid solution chosen to yield the same fluoroboric acid solution obtained in the boron combustion experiments. The derived values of the enthalpies of formation, ..delta..H line integral/sup 0//sub 298.15/, from graphite, crystalline boron(..beta.., rhombohedral), and gaseous hydrogen and nitrogen are: orthoboric acid(c), -261.4/sub 7/ +- 0.2/sub 0/ kcal. mole/sup -1/; trimethylamineborane(c, III), -34.0/sub 4/ +- 0.5/sub 5/ kcal. mole/sup -1/; and diammoniumdecaborane(c), -85.8/sub 4/ +- 2.5/sub 0/ kcal. mole/sup -1/. The value of the enthalpy of formation of orthoboric acid was used with numerous other thermochemical data from the literature to derive values of the enthalpies of formation of the boron oxides, diborane, boron trichloride, and boron tribromide referred directly to crystalline boron. The value of the enthalpy of formation ofmore » trimethylamineborane was used with other literature data to derive another value of the enthalpy of formation of diborane.« less