Akira Iwama

Ignition of composite solid propellant at subatmospheric pressures

Abstract The ignition of composite solid propellants by CO2 laser irradiation at low pressures has been investigated experimentally. Three ignition modes are found at subatmospheric pressures: self-sustaining ignition, non-self-sustaining ignition, and pulsating ignition. Here, the self-sustaining and the non-self-sustaining ignition modes have been studied. Samples of an ammonium perchlorate polybutadiene propellant were ignited. The incipient flame was detected with a phototransistor, and the temperatures at the surface and in the solid phase were measured by means of Type K thermocouples. Aspects of surface regression were recorded by photography. It is found that non-self-sustaining ignition occurs in the lower pressure range and with the higher incident heat flux from the laser. At ignition, the heat loss into the solid phase in the non-self-sustaining mode is much larger than that in the self-sustaining one. It is concluded that these ignition modes are determined independently of the exposure time after the flames appearance.

SHORT COMMUNICATIOI\J

Abstract Flame spread aspect along the pool surface of gelled (O/W emulsified) II-octane with high internal phase ratio was investigated from the viewpoint of spilled-fuel fire prevention. The temperature field in the vicinity of the spreading flame front was measured by thermocouples (chromel-alurnel, 0.1 mm dia.) and Mach-Zehnder interferometry, and temperature histories on the unburned surface and in the gas phase adjacent to the surface were shown.From the measured results, the mode of heat transfer from the spreading flame to the unburned pool surface was estimated. The dependence of flame spread velocity on the initial temperature of gelled η-octane and on the oil content in gel (internal phase ratio of O/W emulsion) were also investigated.

Ignition of AP-based composite solid propellants containing nitramines exposed to CO2 laser radiation at subatmospheric pressures

Abstract Ignition characteristics of AP-based composite solid propellants that contained either HMX or RDX, were investigated using a CO 2 laser at relatively low heat flux and subatmospheric pressures, in order to simulate space environments and also simplify our understanding of the nature of ignition phenomena. The results are presented graphically as a function of surface heat flux and ambient pressure. The ignition process was found to be roughly divided into two regions of self-sustaining ignition (S.S.I.) and non-S.S.I., depending on whether the ignition reaction is self-sustained or extinguished when the external radiation is interrupted after ignition. Furthermore, the non-S.S.I. was composed of two regions of radiation-assisted combustion (R.A.C.) and radiation-assisted unstable combustion (R.A.U.). The addition of nitramines, in particular, HMX to AP-based composite propellants resulted in a marked reduction in the ignition times and made the self-sustaining ignition region shift to lower heat fluxes at each specific pressure. It was found, from the results of differential thermal analysis and thermogravimetry, that nitramine-containing propellants exhibit an exothermic decomposition at relatively low temperatures that is independent of the presence of other ingredients. This triggers the ignition reaction of the AP-based propellants and lowers temperature of the main exotherm. Consequently, it appears that the improvement of ignitability by the addition of nitramines is due to the intense low-temperature exothermic reaction of nitramines. It was found from the time-temperature histories measured with fine thermocouples on the surface during the ignition process that ignition temperatures also are lowered by the addition of nitramines. Howevver, when a propellant sample containing nitramines was exposed to higher heat flux levels, the shorter ignition times reduced self-sustaining ignitability slightly, presumably because these propellants could not form a sufficiently thick thermal layer for steady-state burning.

Ignition Characteristics of Gelled (0/W Emulsified) Hydrocarbon Fuel Pool

Abstract Ignition characteristics of the pool surface of an O/W emulsified gelled hydrocarbon fuel with high internal phase ratio were investigated from the standpoint of spilled-fuel fire prevention. Flash points of miscellaneous gelled fuels with single and multiple components, and of commercial fuels were measured and compared with those of neat hydrocarbon fuels. The Tag Closed-Cup method was used for flash point measurement. A dispersive type infrared gas analyzer (DIR)was used to measure the temperature dependence of the equilibrium vapor concentration of the emulsified fuel. Ignition aspects of an emulsified fuel pool surface were studied and compared with those for neat liquid fuel, here a small hot Nichrome wire adjacent to the pool was used to heat the surface locally. The procedure was also recorded in a 16 mm movie and inter-ferogram form. It was found that whether the neat fuel is a single substance or a multi-component mixture has an effect aside from that of emulsification on the elevation ...

Ignition of Laminated Composite Propellants Composed of Ammonium Perchlorate Single Crystal and Fuel-Binder Slab by Means of CO2 Laser

Abstract Abstract-The ignition behavior of laminated propellants composed of ammonium perchlorate (AP) single crystal oxidizer and carboxyl terminated polybutadiene (CTPB) fuel-binder slab, has been studied by means of CO2 laser under sub-atmospheric pressure. The ignition is initiated by the reaction of carbonaceous material on AP surface with AP decomposition products and ambient helium gas has appreciably preventive influence on propellant ignition, compared with Ar and N2 gases. These phenomena might be explained by a comprehensive ignition model.

Flame Spreading Along the Surface of Gelled (0/W Emulsified) Hydrocarbon Fuel Pool

Abstract Flame spreading along the pool surface of O/W (oil-in-water) emulsion type gelled hydrocarbon fuels with high internal phase ratios have been investigated from the standpoint of spilled-fuel fire prevention. Aspects of flame spreading and temperature field in the vicinity of the spreading flame front were observed and measured by 16 mm movie pictures, interferometry and thermocouples. It was found that a thin liquid layer flow ahead of and below the flame exists if the neat hydrocarbon fuel has a flash point higher than the ambient temperature and or has considerably different flash and fire points. No such liquid layer flow can be observed if the neat fuel has a flash point below the ambient temperature and consists of a single component. Whether such a liquid layer flow exists or not would have a large influence on such aspects of flame spreading as velocity, direction and steadiness. The influences of the initial temperature and the oil content of O/W emulsion on flame spreading velocity were ...

CO2 laser-induced pulsating regression behavior of GAP at sub-atmospheric pressures

Abstract Linear pyrolysis of glycidyl azide polymer (GAP) by CO 2 laser heating was studied experimentally in argon, nitrogen or oxygen at subatmospheric pressures with laser heat fluxes of 1 to 20 W/cm 2 . Surface temperature variations were measured by fine chromel-alumel thermocouples pressed onto the sample surface, and pulsating regression caused by an abrupt exothermic decomposition was observed. The behavior is very similar to the chuffing phenomenon. The momentary exothermic decomposition reaction was accompanied by the evolution of N 2 resulting mainly from breaking of the azide bond. The surface temperature in one cycle is characterized by three critical temperatures: liquefying (T l ), abrupt thermal decomposition threshold (T ad ), and maximum surface (T ms ) temperatures. Maximum surface temperatures are 700 to 800 K, which are equivalent to the surface temperature observed during self-sustained combustion controlled chiefly by exothermic reaction at the decomposition surface. As the surface heat flux increases, the period of the pulsating regression decreases, being only slightly dependent on the ambient gas. If the period becomes shorter than the characteristic time of GAP, then the pulsating regression will transition into steady regression assisted by laser irradiation. SEM photographs of the quenched surfaces reveal the presence of a liquid layer with a thickness of 7 to 8 μm, about 1/30 of the momentary regression distance, at the GAP surface just before the abrupt thermal decomposition. Spatial temperature profiles at various critical times were established by using two thermocouples embedded inside the GAP sample. Phenomenological modeling is attempted to explain the pulsating phenomena for GAP pyrolysis. From the phenomenological modeling, a subsurface layer with a relatively small temperature gradient is thought to exist at the GAP surface. In view of SEM photographs of quenched surfaces, the surface layer of the whole momentary regression distance Δl is thought to change into a liquid layer as soon as the abrupt thermal decomposition starts. One of the causes of nonself-sustained combustion of GAP in inert gases of a few atmospheres would be pulsating decomposition. At 66.7 kPa of oxygen gas the pulsating decomposition very often transitions into sustained combustion.