Abdullah Ulas

Ignition and combustion of boron particles in fluorine-containing environments

Abstract The ignition and combustion of isolated boron particles in fluorine-containing environments were investigated both experimentally and theoretically. Boron particles (1-μm amorphous and 3-μm crystalline) were ignited and burned completely in the post-flame region of a multi-diffusion flat-flame burner. The high-temperature gas environment was generated by the combustion of CH4/NF3/O2/N2 gases at atmospheric pressure. From the recorded real-time images of burning boron particles in fluorine-containing environments, no clear distinction was observed to define a two-stage combustion process, which is a characteristic feature of boron oxidation without fluorine. The burning trajectories of boron particles in fluorine-containing environments showed pronounced jetting and spinning phenomena. At 1,780 K, the self-sustained ignition of boron particles required a higher oxidizer concentration in non-fluorinated environments than in fluorinated environments. From the experimental data, HF was found to increase total burning times (tb) of boron particles; whereas, F significantly reduced tb. A theoretical model was developed for simulating the combustion of an isolated boron particle in fluorine-containing environments. The oxide layer removal process was modeled using a reaction mechanism, which considered the vaporization process of the liquid B2O3/(BO)n mixture and four global surface reactions of oxide layer with O2, H2O, F, and HF. The “clean” boron combustion model included four global surface reactions of O2, H2O, F, and HF with boron. Numerical calculations showed that the oxide layer removal rate significantly increased in the presence of F and HF, as much as four times when the combustion is kinetics-controlled. In agreement with measured data, the calculated first- and second-stage combustion times decreased with increasing the ambient gas mixture temperature or the total pressure. The comparison between the current model predicted characteristic times of boron combustion and the measured data in the current study and other published experimental data in the literature is generally in good agreement.

Combustion of HTPB-Based Solid Fuels Containing Nano-sized Energetic Powder in a Hybrid Rocket Motor

An experimental investigation was conducted to determine the relative propulsive performance of various HTPB-based solid-fuel formulations containing nano-sized energetic metal particles. These particles include Alex® particles (diameter ~ 150 nm), WARP-1 aluminum particles (-70 nm), B4C (-120 nm), and a mixture of B4C and WARP-1. The nano-sized particles were cast in an HTPB solid-fuel grain and burned in the Long-Grain Center-Perforated (LGCP) hybrid rocket motor using pure oxygen as the oxidizer injected at the head-end of the motor. The LGCP hybrid rocket motor is capable of oxygen mass flow rates up to 0.36 kg/s (0.8 Ibm/s) and chamber pressures up to 12 MPa (1,750 psig). The oxidizer mass flux was varied from 140 to 850 kg/m-s at chamber pressures ranging from 2.3 to 4.6 MPa (320 to 650 psig). The addition of energetic powders showed an increase of up to 50% in mass burning rate compared to the pure HTPB fuel. Elemental compositions of quenched boron particles in oxygenated and fluorinated environments were determined by using the Energy Dispersion Spectroscopy (EDS) technique to compare the composition of quenched boron particles in oxygen-containing versus fluorine-containing environments. These results indicate no significant difference between particles recovered in oxygenated and fluorinated environments. It was found that there was no nitrogen on the particle surface, indicating no boron nitride formation.

Comparison of Different Aspect Ratio Cooling Channel Designs for a Liquid Propellant Rocket Engine

High combustion temperatures and long operation durations require the use of cooling techniques in liquid propellant rocket engines. For high-pressure and high-thrust rocket engines with long operation times, regenerative cooling is the most preferred cooling method. In regenerative cooling, a coolant flows through passages formed either by constructing the chamber liner from tubes or by milling channels in a solid liner. Traditionally, approximately square cross sectional channels have been used. However, recent studies have shown that by increasing the coolant channel height-to-width aspect ratio, the rocket combustion chamber hot-gas-side wall temperature can be reduced significantly. In this study, the regenerative cooling of a liquid propellant rocket engine has been numerically simulated. The engine has been modeled to operate on a LOX/GH2 mixture at a chamber pressure of 68 atm and LH2 (liquid-hydrogen) is considered as the coolant. A numerical investigation was performed to determine the effect of different aspect ratio cooling channels and different coolant mass flow rates on hot-gas-side wall temperature and coolant pressure drop. The variables considered in the cooling channel design were the number of cooling channels and the cooling channel cross-sectional geometry along the length of the combustion chamber.

Combustion Behavior of TAL-1308 Composite Solid Propellant for Airbag Applications.

An experimental investigation on the characterization of the combustion behavior of a composite solidpropellant ah-bag fonnulation;TAL-1308, has been conducted. The major objective of this study was to experimentally determine. several combustion characteristics of the TAL-1308 formulation, such as: 1) steady-state burning rates as a function of initial chamber pressure and propellant temperature; 2) temperature sensitivities; 3) temperature profiles in the condensed-phase; and 4) propellant activation energy. The experimental results were obtained using a high-pressure optical strand burner. Steady-state bumrates were determined for a pressure range of 20.8 to 41.5 MPa (3,000 to 6,000 psig) and initial propellant temperatures of 243 to-353 K. For the pressure and temperature ranges-tested, the temperature sensitivity was on the order of lx109 Km’. The pressure dependency on the burning rate was correlated using the Saint Robert’s law. The pressure exponent for the room temperature case (Ti = 25 “C) was 0.75. Also, the pressure exponent, n, was found to be a function of initial propellant temperature. An Arrhenius form burning rate correlation was also obtained for this propellant. The activation energy and the preexponential factor of the r for higher pressures (above 20.8 MPa), the layer-by-layer regression of the propellant strand was observed. During combustion, small spherical particles as condensed phase residues were ejected and recovered from the test run. The higher the pressure and initial propellant temperature, the smaller the spherical particles. For pressures below 41.4 MPa, the size of the particles was on the order of 900 pm. For pressures around 84..4 MPa (12,500 psig), the bead size was much smaller, on the order of 300, pm. A chemical analysis on these particles using both the ESEM and the X-ray diffraction method was conducted. The X-ray diffraction results agreed well with the ESEM results and indicated that the material of the TAL-1308 beads was mostly sodium chloride, NaCl, with a small amount of siliconcontaining compounds. Introduction first model was installed in certain 1974 General Motors automobiles3, research on airbag inflator design for Solid propellants have been used for various automobiles has been progressing since the 1960’s. The applications such as large rocket boosters and second presence of airbags in today’s automobiles provides stage motors, tactical missiles, space launch vehicles, increased protection to the vehicle operator and and gas generators.’ Over the past 30 years, gas. passengers. Recent statistical reports by the National generator technologies have been explored to meet Highway Traffic and Safety Administration (NHTSA) the stringent requirements for motor vehicle airbag reveal that there was an 11% overall reduction in fatalities applications? However, airbag technology has been for passenger car drivers and a 30% reduction in head-on rapidly increasing in the past decade. Though the collision fatalities in the last 10 years.4 ppproximately B Ph.D. Student, PSU, Student Member AIAA B Ph.D. Candidate, PSU, Student Member AIA4 # Distinguished Professor of Mechanical Engineering, PSU, Fellow AL&A’ + Research Assistant, PSU

Two dimensional numerical prediction of deflagration-to-detonation transition in porous energetic materials

Manager of Advanced Airbag Development Group, Talley Defense Systems l Principal Engineer,’ Talley Defense Systems, Associate Fellow AIAA 1 American Institute of Aeronautics and Astronautics

Ignition and combustion characteristics of RDX-based pseudopropellants

This paper describes a two-dimensional code developed for analyzing two-phase deflagration-to-detonation transition (DDT) phenomenon in granular, energetic, solid, explosive ingredients. The two-dimensional model is constructed in full two-phase, and based on a highly coupled system of partial differential equations involving basic flow conservation equations and some constitutive relations borrowed from some one-dimensional studies that appeared in open literature. The whole system is solved using an optimized high-order accurate, explicit, central-difference scheme with selective-filtering/shock capturing (SF-SC) technique, to augment central-diffencing and prevent excessive dispersion. The sources of the equations describing particle-gas interactions in terms of momentum and energy transfers make the equation system quite stiff, and hence its explicit integration difficult. To ease the difficulties, a time-split approach is used allowing higher time steps. In the paper, the physical model for the sources of the equation system is given for a typical explosive, and several numerical calculations are carried out to assess the developed code. Microscale intergranular and/or intragranular effects including pore collapse, sublimation, pyrolysis, etc. are not taken into account for ignition and growth, and a basic temperature switch is applied in calculations to control ignition in the explosive domain. Results for one-dimensional DDT phenomenon are in good agreement with experimental and computational results available in literature. A typical shaped-charge wave-shaper case study is also performed to test the two-dimensional features of the code and it is observed that results are in good agreement with those of commercial software.

Effect of Aging on Ignition Delay Times of a Composite Solid Propellant Under CO2 Laser Heating

Burning rates, ignition delay times, temperature, and OH concentration profiles in the flame zone were determined for pure RDX and pseudopropellants containing RDX with CAB (cellulose acetate butyrate) binder having 8, 11, and 14% by weight. Deduced final flame temperatures ( T f ) at 0.45 MPa from UV/visible absorption spectroscopy measurements indicated a monotonic decrease of T f with an increase of CAB percentage from 3062 K for pure RDX to 2742 K for RDX/CAB (86/14%) as departure from stoichiometry became larger. The deduced final flame temperatures are in good agreement with equilibrium calculations. The measured burning rates of the propellants at 0.45 MPa decreased with increasing CAB content. No multistage flame structure was observed for either pure RDX or RDX/CAB pseudopropellants processed by shock-precipitation procedure. Measurements indicate that the ignition delay times increase with the increase of CAB content in pseudopropellants; this increase is partly due to the endothermic surface reactions of CAB and partly caused by the increase of specific heats of pseudopropellants by adding more CAB. The simple power-law curve-fitted to the ignition data indicated that the pure RDX sample surface is almost inert during the ignition period. During the tests, it was observed that the onset of light emission occurred in the gas phase above the sample surface; therefore, the gas-phase chemistry plays an important role in the ignition processes of RDX/CAB pseudopropellants.

A numerical study on the thermal initiation of a confined explosive in 2-D geometry.

In this paper, the effect of aging on the ignitibility and reproducibility of a composite solid propellant was investigated. Ignition delay times of solid propellant samples undergone different degrees of aging were determined under various levels of incident CO2 laser energy fluxes. Micrographs obtained from an Environmental Scanning Electron Microscope (ESEM) showed that the surfaces of the aged samples exhibited fiber-like structure as compared to the heterogeneous structure of the as-machined sample. The porosity of the fiber-like structure increased with the degree of aging. Based upon the results of experiments, it was observed that aged propellant samples require much longer heating time to reach self-sustained combustion condition than the as-machined sample at lower energy fluxes (around 50 W/cm2). However, at higher energy fluxes (around 150 W/cm2), the effect of aging on the ignition delay times became smaller.

Numerical prediction of steady-state detonation properties of condensed-phase explosives

Insensitive munitions design against thermal stimuli like slow or fast cook-off has become a significant requirement for todays munitions. In order to achieve insensitive munitions characteristics, the response of the energetic material needs to be predicted against heating stimuli. In this study, a 2D numerical code was developed to simulate the slow and fast cook-off heating conditions of confined munitions and to obtain the response of the energetic materials. Computations were performed in order to predict the transient temperature distribution, the ignition time, and the location of ignition in the munitions. These predictions enable the designers to have an idea of when and at which location the energetic material ignites under certain adverse surrounding conditions. In the paper, the development of the code is explained and the numerical results are compared with available experimental and numerical data in the literature. Additionally, a parametric study was performed showing the effect of dimensional scaling of munitions and the heating rate on the ignition characteristics.

Determination of Temperature Profiles of Self-Deflagrating RDX by UV/Visible Absorption Spectroscopy and Fine-Wire Thermocouples

Within the scope of this study, a computer code named BARUT-X has been developed to calculate the detonation properties of C-H-N-O based condensed-phase explosives using the Chapman-Jouguet (C-J) theory. Determination of the detonation properties is performed in chemical equilibrium and steady-state conditions. Unlike other codes in the literature which use steepest descent optimization method, BARUT-X uses a nonlinear optimization code based on Generalized Reduced Gradient algorithm to compute the equilibrium composition of the detonation products. This optimization code provides a higher level of robustness of the solutions and global optimum determination efficiency. The Becker-Kistiakowsky-Wilsons (BKW) equation of state (EOS) is applied to the high-density gaseous detonation products at high pressures. BARUT-X uses RDX, TNT, BKWR, and BKWN set of constants in the BKW EOS. In addition, the Cowan-Ficketts EOS is applied for the compressible solid carbon in the detonation products. The calculated detonation properties for several condensed-phase explosives by BARUT-X have been compared with those computed by EXPLO5 and FORTRAN BKW codes as well as the experimental data in terms of detonation velocity and detonation pressure. Satisfactory agreement is obtained from these comparisons.