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Thermal emission from burning aluminium droplets in solid propellants has been analysed using radiative transfer theory and infrared intensity measurements from emission images of burning droplets at 1-5 atm. Below 5 atm, infrared emission is dominated by the burning droplet, not the detached flame zone around the droplet. Each droplet consists of a molten metal portion and a molten oxide (cap) portion. Emission from the metal portion of the droplet surface is spatially uniform and corresponds with opaque surface theory. Emission from the oxide cap portion of the droplet surface is spatially non-uniform and corresponds with volumetric emission from a non-opaque surface. The oxide cap was approximated as a truncated section of a sphere. The burning droplet temperature was found to be approximately 300 K below the metal boiling point. Account was also taken for non-isothermal particulate emission by the sub-micron oxide smoke cloud formed in the detached flame zone around the droplet. The analysis was able to represent infrared emitted intensity reasonably well for various oxide cap thicknesses and orientation angles. Extending the analysis to include all wavelengths suggested that smoke envelope emission becomes more important at shorter (e.g., visible) wavelengths. A simple, linear relation for the incremental increase in total (spectrally and directionally integrated) droplet emissivity owing to smoke envelope emission was obtained at 1 atm: Δ ε = 0.001D(μ m). [ABSTRACT FROM AUTHOR]

The mathematical problem of unsteady combustion of a homogeneous solid propellant with zero-order, high activation-energy, single-step decomposition and zero-activation-energy, quasi-steady gas-phase reaction is considered. Two approximate decomposition models, simple pyrolysis and leading-order asymptotic, are compared with each other and with the full, distributed reaction solution. It is shown that the leading-order asymptotic model is a reasonably good approximation of the exact solution for steady, linear-oscillatory and, with some additional limitations noted herein, nonlinear transient combustion conditions. Further, it is shown that the pyrolysis model can be made equivalent to the leading-order asymptotic model locally, i.e. for given pressure and temperature conditions, at the sensitivity parameter or linear behaviour level, but not beyond. Several misconceptions and unresolved issues associated with these models are resolved. It is shown that when equivalence is enforced, which necessarily involves a non-zero—usually negative—Jacobian parameter, ns, the previously held approximate relation between the pyrolysis and bulk decomposition activation energies, Es ≉ Ec/2, is not valid. Rather, the value of Es may exceed that of Ec. Other clarifications include the demonstration of an asymptotic inconsistency among reported leading-order model sensitivity parameters that gives rise to an error of the order of 10% in certain parameters (r, δ, ns). A blow-off behaviour is also shown to exist in the zero-activation-energy, gas-phase sub-model that can appear under nonlinear dynamic burning conditions. It is also demonstrated that previously reported nonlinear, high-frequency oscillatory behaviour associated with the leading-order asymptotic model is an artefact of the approximate asymptotic solution and not physical. [ABSTRACT FROM AUTHOR]

A current issue important to high explosive safety is deflagration-to-detonation transitions (DDTs) in accident scenarios. In order to better understand the reactive mechanisms involved in DDT and to begin to approach the fast ignition and heating rates seen in DDT, high-irradiance (≤800W/cm²) CO2 laser ignition experiments were performed on the common high explosives octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Reported data include time to ignition as a function of laser irradiance, energy, and ignition temperature. A simple dual ignition criteria model (DICM) was used to interpret the HMX results. The DICM requires two basic criteria for ignition: (1) a minimum surface temperature must be reached and (2) a minimum energy concentration must exist within the solid. The DICM sucessfully predicted the slope transition trend and the critical ignition energy for HMX to within 10% of the measured values. TATB had a single dependence on irradiance over the entire range of heating rates. [ABSTRACT FROM AUTHOR]

A two-dimensional, steady-state model of a burning composite solid propellant is developed to study the characteristics of the combustion process. The solid composite is a periodic sandwich unit comprised of two oxidizer laminates (ammonium perchlorate, AP) separated by a fuel binder layer (hydroxyl-terminated polybutadiene, HTPB). Included in the model are essential features for simulating composite propellant combustion: (1) a free surface boundary, (2) gas- and condensed-phase heat release distributions based on simplified chemical kinetics, and (3) an implicit surface regression rate (unique burning rate) determined by coupled gas-solid energy/species transport analysis. Comparisons of the model with experimental observations focus on surface geometry, flame structure, and the burning rate for variations in pressure, particle size, binder width, and propellant formulation. Experimentally observed trends for typical composite propellants are replicated. For example, the relative protrusion/recession of oxidizer and binder, recognized as an important feature of propellant surface topography, is correctly predicted. The simulation demonstrates the relation between gas-phase heat release and the heat-feedback driving the solid-phase pyrolysis. This information is critical to predicting surface geometry and regression rate. Success was also achieved in predicting the experimental burning rate pressure sensitivity without the use of arbitrary non-integer reaction order. The model provides a framework for future studies with more complex kinetic mechanisms, transient phenomena, and three-dimensional particulate propellants. [ABSTRACT FROM AUTHOR]

Ultraviolet emission imaging (305-315 nm) was used to study the combustion of sandwiches of ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene (HTPB) in nitrogen at pressures up to 32 atm, with binder layers from 50 to 450 µm in thickness. The emission imaging was combined with a novel backlighting technique to allow determination of the corresponding surface shape during combustion. The results indicated that the interface regression rate of IPDI-cured samples undergoing laser-assisted deflagration (120 W/cm 2 average flux) is nearly independent of the binder thickness for binders thicker than 100 µm. The pressure exponent of the regression rate is 0.31 up to 15 atm, then increases with pressure from 15 to 32 atm. Two primary flame regimes were identified: a regime of high Peclet and Damköhler numbers which exhibits a split base in the ultraviolet flame emission, and a regime of low Peclet and Damköhler numbers which exhibits a merged flame base. A third, "lifted" flame region, in which the strongest flame emission starts several hundred microns above the solid surface, occurs with low Damköhler numbers and high Peclet numbers. The effects of Pe and binder oxygenation (by fine-AP) on the size of the diffusion flame and the location of its leading edges relative to the AP/binder interface were observed to be in agreement with Shvab-Zeldovich theory. [ABSTRACT FROM AUTHOR]

The thermal effects of incident radiation on the burning characteristics of homogeneous solids are considered and the practical implications of these effects in solid rocket motors are assessed. The applicability of the equivalence principle (equivalence between radiation and initial temperature) is investigated experimentally and analytically. Experimental steady burning rates as a function of initial temperature and mean radiant flux are presented for a fine ammonium perchlorate (A P) composite propellant which indicate that the equivalence principle is accurate to within experimental error. The equivalence principle is also assessed analytically by considering the worst case conditions of condensed phase controlled burning. For deflagration and pyrolysis of solids controlled by condensed phase reactions it is shown that a modification of Ibiricu and Williams' high activation energy asymptotic burning rate expression allows consideration of the effect of incident radiation on steady burning rate over a wide range of propellant opacities. Numerical simulations are used to verify this modification. Thermal radiation from combustion gases in typical non-metalized solid rocket motors is examined using band model calculations. These calculations show that even pure molecular gas radiation (no particles) can reach near blackbody levels for realistic motor conditions. The effects of thermal radiation on burning rate and temperature sensitivity are also examined using the equivalence principle. It is shown that differences in burning rate and temperature sensitivity between motors and propellant strands which in the past have primarily been attributed to erosive burning, could in some instances be due to thermal gas radiation. The implications of gas radiation on combustion stability are also discussed. It is shown that radiation may have either a stabilizing or destabilizing effect, according to the initial temperature dependence of the temperature sensitivity σp(T0), although in most cases it is a stabilizing influence. [ABSTRACT FROM PUBLISHER]

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