Experimental Research And Numerical Analysis On The Cook-off Characteristics Of AP/HTPB Base Bleed Propellant

If the base bleed unit is exposed to the external heating environment,it will lead to unexpected ignition or explosion of energetic materials,which may cause serious damage and loss on the staff and weapon launching platform.Based on the background of the thermal safety of base bleed unit,the experimental research and numerical analysis of ammonium perchlorate(AP)/hydroxyl-terminated polybutadiene(HTPB)base bleed propellant are conducted to analyze the cook-off characteristics like the ignition time,the location of ignition,and the transient temperature distribution.Moreover,the effects of charge sizes,charge structures and heating environments on cook-off characteristics are investigated.The main research contents and results are as follow:(1)Experimental researches on the thermal decomposition and cook-off characteristics of AP/HTPB base bleed propellantThermal decomposition experiments of AP/HTPB base bleed propellant are carried by using DSC and TG-DTG at the heating rates of 20?/min and 40?/min.The results show the weight loss of the propellant is instantaneous,the dividing point between slow and instantaneous weight loss,transient weight loss phase and average weight loss rate at 40?/min are much higher than those at 20°C/min.A small endothermic peak and two large exothermic peaks are shown in DSC curves.Extracting the main characteristic of real base bleed unit,the miniature small scale base bleed simulators are designed with solid and hollow cylindrical grains of AP/HTPB propellant,respectively.The alcohol flame is adopted as heat source to envelop the experimental device in a windless environment.The experimental results show that in the cook-off experiments of solid cylindrical AP/HTPB propellant,the cook-off form is divided into fast and slow cook-off,which the cook-off times are 195s and 1281.5s,respectively.In the cook-off experiments of hollow cylindrical AP/HTPB propellant,the heating rates are simplified as 0.83K/s and 0.33K/s.The cook-off times are 369s and 840s,respectively.(2)The numerical model and verification studies on the cook-off characteristics of AP/HTPB base bleed propellantOn the basis of the different experimental conditions,the numerical simulations of base bleed simulator cook-off tests are performed.The two-step chemical reaction that accounts for the decomposition of AP/HTPB propellant is established to analyze the cook-off characteristics.For the cook-off experiments of solid cylindrical AP/HTPB propellant,the errors of the temperature of monitoring points and the cook-off time in fast cook-off are 0.67%and 1.54%,respectively,meanwhile,there are 1.45%and 0.43%in slow cook-off,respectively.For the cook-off experiments of hollow cylindrical AP/HTPB propellant,the numerical reaction time at the heating rate of 0.83K/s and 0.33K/s are 377.0s and 849.5s,respectively,which the errors between numerical and experimental tests are less than 1%.Meanwhile,the temperature-time profiles of monitoring points under the numerical tests agree with those under the experimental conditions.These results verify the rationality of the cook-off model.(3)Numerical study on the cook-off characteristics of hollow cylindrical AP/HTPB propellant in the actual base bleed unitIn order to meet the need of simplified calculation in engineering,a cook-off model for two-dimensional hollow cylindrical AP/HTPB in the actual base bleed unit is established based on the two-step chemical reaction.The results show that the critical ambient temperature of AP/HTPB base bleed unit is between 470K?475K.According to the location of the ignition and the direction of heat transfer,the cook-off is described in three forms of fast,medium and slow cook-off.When the ignition point occurs in the outer surface of the propellant and the outside temperature is higher than that of any point in the propellant,interpreting that the heat is transferred from the outside to the inside,this is called as fast cook-off.In like manner,the ignition point occurs inside annular region of the propellant in medium cook-off,meanwhile,the temperature of the ignition point is higher than that of the outside and other areas of the propellant,showing that the heat is transferred to the outside and the other areas of the propellant.The last is the slow cook-off,in this case,the ignition point generates in the inner face of the propellant,and the temperature of the ignition point is higher than that of other areas in the base bleed unit.The heat is transferred from the inner surface to the outside.The cook-off temperature decreases in slow cook-off,changes in fluctuation in medium cook-off and increases in fast cook-off with increasing the heating rate.The ignition time of AP/HTPB propellant decreases in an exponential way with the increase of the heating rate.The effect of charge size on the ignition position is small.The law of ignition time of AP/HTPB propellant to charge size is the similar at different heating rates.In slow and medium cook-off,as the charge length increases to 90mm,the cook-off time and temperature of the base bleed unit obviously changes.When the charge diameter increases from 43mm to 48mm,the cook-off time of AP/HTPB propellant decreases and the change of the cook-off temperature is small.In medium cook-off,the cook-off temperature fluctuates obviously in the range of 570K-600K with different charge sizes.In fast cook-off,the charge size has little influence on the cook-off temperature and time.The cook-off forms of the base bleed unit in the flame environments of 623?1143K belong to the medium and fast cook-off.(4)Numerical study on the cook-off characteristics of three-dimensional base bleed unit with trilobite AP/HTPB propellant in different thermal environmentsTaking into account the complex structure of the actual base bleed propellant,a cook-off model of 3D trilobite AP/HTPB propellant with slits in trim size of base bleed unit is established to study the cook-off characteristics.The numerical results show that the critical ambient temperature of the base bleed unit is between 470K-475K,which is same as that of two-dimensional hollow cylindrical AP/HTPB propellant.In slow cook-off,the ignition point of AP/HTPB propellant occurs near the interface between inner surface and air chamber.In medium cook-off,the ignition point occurs near the slit and moves outwards.In fast cook-off,the cook-off point lies in the surface of AP/HTPB propellant.Comparing to the hollow cylindrical AP/HTPB propellant,the law of ignition time and temperature of trilobite AP/HTPB propellant in the different heating rates is about the same.The cook-off time of trilobite AP/HTPB propellant is smaller than that of hollow cylindrical AP/HTPB propellant.As the charge length of trilobite AP/HTPB propellant increases from 72mm to 90mm,the cook-off time of the base bleed propellant obviously increases.Especially at the heating rate of 6K/h,the ignition position significantly changes.Meanwhile,the temperature gradient variation is large.At other heating rates,the heating environment has a less influence on the-ignition location of AP/HTPB propellant.As the charge diameter changes from 43mm to 53mm,there is a marker change in the ignition point at the heating rate of 4.6K/h-6K/h which leads to the great changes of cook-off temperature.At other heating rates,the cook-off temperature decreases with increasing the charge diameter.The law of ignition time of trilobite AP/HTPB propellant to charge size is similar in the same cook-off form.