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

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• 作者：Saito ; Takeo ; Shimoda ; Masataka ; Tsuyuki ; Tsuyoshi ; Iwama ; Akira
• 刊名：Combustion and Flame
• 出版年：2001
• 出版时间：March, 2001
• 年：2001
• 卷：124
• 期：4
• 页码：611-623
• 全文大小：704 K

文摘

Linear pyrolysis of glycidyl azide polymer (GAP) by CO₂ laser heating was studied experimentally in argon, nitrogen or oxygen at subatmospheric pressures with laser heat fluxes of 1 to 20 W/cm². 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 N₂ 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.