Study On Characteristics Of Unstable Combustion Of Base Bleed Unit

To reduce the range dispersion of base bleed projectile, the process and characteristics of transient combustion of AP (Ammonium Perchlorate) composite base bleed charge was systematic, ally investigated in theory and experiment. The key issues of this paper are focused on the studies of the intensive unstable working process of base bleed charge in the instant when the projectile is shoting out of the gun muzzle by the using of transient pressure measurement system and high speed digital video camera. The theoretical model based on interior ballistics, heat transfer, combustion is introduced in the study.The essential characteristics of AP/Hydroxyl-Terminated Polybutadiene (HTPB) composite base bleed propellant were studied experimentally by a closed bomb. The propellant force and the covolume of the composite grains were measured, and the combustion rate equation of base bleed propellant under high pressure was fitted based on the experiment data. Furthermore, the combustion rate equation under low pressure was fitted based on the data obtained by using a low pressure capsule.The special designed semi-closed bomb and a high speed digital video camera system were used to investigate the unstable combustion of base bleed unit. The results show that, under abrupt depressurization, the combustion of composite base bleed charge consequentially experiences a successive process, which is instability, extinguishment, and reignition. A new phenomenon was found first time in this experiment, the underexpanded flow or critical flow appears in the nozzle of base bleed unit twice or more during initial 560ms under depressurization. Based on comparative tests, a new technology was proposed to control the combustion of base bleed grain and avoid burning unstably under high speed depressurization.A systematic observation and investigation on the performance and law of unstable combustion of base bleed charge were carried out, and some phenomena were observed for the first time. In the range of depressurization rate from 400MPa/s to 1.12×1O~4MPa/s, there are three kinds of transient combustion of composite base bleed charge, which are reignition, critical combustion (oscillation combustion), and irreversible extinguishment. The initial pressure and the depressurization rate are the key factors which determine the transient combustion of composite base bleed charge. If the initial pressure is higher or the depressurization rate is lower, the combustion is more stable. The base bleed charge can be ignited in bore during gun fire, the transient depressurization rate nearby the gun muzzle makes the combustion unstable. Based on the study, the mechanism to explain oscillation combustion and irreversible extinguishment was proposed, and a combustion model of base bleed charge was established, and a formula about the influence factors of combustion was conducted. Besides, the phenomenon that the over impingement of ignition jet stream to base bleed grain was found in the experiment, then part of base bleed charge is broken and blown out of base bleed chamber.To study the transient combustion characteristics and performance of different kinds of igniters under the depressurization, an experiment was performed, by using a semi-closed bomb and a high speed digital video camera. The results show that, the igniter jet stream varies according to the environment pressure and the stability of igniter combustion is affected by the depressurization. The performance of igniter is relevant to the component of ignition powder. Jet stream of igniter produces ablation holes on the circular arc burning surface of base bleed grain because of the heavy metal component in ignition powder. The direct effect of ablation intensifies progressive burning in earlier and intermediate stages of combustion, and intensifies degressive burning in late stage of combustion. In general, combustion time reduces. Consequently, the performance of base bleed unit deviates designation.Based on the analysis of ignition theory of solid energetic material, by adopting solid-phase ignition model mainly and taking working condition of composite base bleed charge in-bore into account, a simplified analytical solutions of various heat ignition models of base bleed charge were conducted with Laplace transform. Numerical calculation of heat ignition was processed also. These works are important in theory and can be applied to estimate ignition delay time and analyze the factors that affect ignition delay in various heat conditions.