环境气体氧含量对NEPE推进剂激光点火过程的影响

为研究环境气体氧含量对硝酸酯增塑聚醚(NEPE)推进剂激光点火过程的影响,采用CO2激光辐射点火并利用高速摄影仪记录NEPE推进剂的点火过程,讨论了环境气体氧含量对NEPE推进剂初焰位置与点火延迟时间的影响。结果表明,当环境气体氧含量小于NEPE推进剂热解产物中氧化性气体含量时,NEPE推进剂点火的气相反应发生在推进剂热解产物的分散区,初焰紧靠NEPE推进剂表面,环境气体氧含量变化不影响NEPE推进剂的点火延迟时间;当环境气体氧含量大于NEPE推进剂热解产物中氧化性气体含量时,NEPE推进剂点火的气相反应发生在推进剂热解产物与环境气体的扩散区,初焰远离NEPE推进剂表面,此时由于扩散区氧含量高于NEPE推进剂热解产物分散区氧含量,NEPE推进剂的点火延迟时间减小。     

推进剂及其组分的光学性质对点火特性的影响

通过实测的NEPE推进剂在多种热流密度下的点火延迟时间，借助于转化系数，计算获得假设忽略深度吸收的点火延迟时间，其相对误差随着热流密度的增加而显著增大，表明推进剂及其组分的光学性质对点火特性的影响是不能忽略的，且这种影响随着热流的增加而增大，因此，建立推进剂点火模型，在高热流辐射时，必须考虑深度吸收，极限吸收率和建立模型，消除其光学性质的影响提供了依据。     

Ignition of composite propellant at subatmospheric pressures by means of carbon dioxide laser

Ignition behavior of solid propellant composed of 25 wt% polybutadiene and 75% ammonium perchlorate under subatmospheric conditions has been studied using a carbon dioxide laser whose radiant flux is uniformalized. Data of ignition time in Ar, N₂ and He atmospheres, and critical radiant flux and pressures over which ignition might be achieved, were obtained. Moreover, the data were compared with theoretical values, taking account of variable heat conductivity for the ambient gases with a surface ignition hypothesis. It is concluded that these ambient gases participate in the pre- and post-ignition reactions and the ignition preventive behavior of He atmosphere is not predictable merely based on its high transport properties. The detailed observation of the ignition transient near the critical pressure revealed that the ignition in Ar atmosphere occurs in gas phase under less than 100 torr whereas in He atmosphere it occurs always at the condensed phase surface.     

Modification of Thermal Theories to Solid Propellant Ignitibility

The conventional thermal theories are inventively modified for analyzing the ignition behaviors of solid propellants. Based on the modification of the thermal theory with the boundary condition of constant heat flux, the effects of heat flux, pressure, threshold of heat flux and absorbability on the radiant ignition of solid propellant are elaborated. The innovations of theoretical analyses are consistent with most of experimental results depicted in literatures. That the increase of hot gas velocity increases the ignition time of solid propellant is verified to be attributable to the decrease of hot gas temperature, ascertaining insight of the thermal theory with the boundary condition of flowing hot gas. In addition, a tentative estimation of pressurization rate effect on ignition time of solid propellant is proposed.     

固体火箭发动机的热安全性研究

采用带源项的热传导方程,对固体火箭发动机在外界热源作用下的加热过程进行了数值模拟,分析了固体发动机内推进剂在外界热源作用下的燃烧特点,并确定了发动机产生热危险性的临界温度和起始燃烧时间。研究结果表明,在热传导方程中加入化学反应源项,可以有效地模拟发动机在外界热源作用下的加热过程;推进剂产生热危险性的临界温度为520～525K;在外界火焰作用下,发动机内的推进剂将点火燃烧,随着外界火焰温度的上升,推进剂起始燃烧的延迟时间减少。     

Composite Propellant Ignition and Extinction by CO2 Laser at Subatmospheric Pressure

Aspects of the heating, ignition and extinction of an AP/HTPB(86/14) composite propellant, induced by means of a CO₂ laser energy pulse, are investigated in the subatmospheric pressure range to determine the influence of the operating conditions on these processes. Measurements of the ignition delay time and ignition temperature for any operating condition have been done by microthermocouples able to record the temperature history of the irradiated surface and of the gas phase. Determinations of the boundaries location, defining the propellant behavior after the external energy removal, by the go-nogo techniques, have permitted to evaluate the minimum exposure time which assures to avoid the propellant extinction after the propellant ignition. Combustion transients have been studied to determine the burning propellant response to the laser energy pulse and to define the pulse features which yields the propellant extinction for different working pressure. Laser Doppler Velocimetry(LDV)technique, able to measure the velocity of the gas coming out from the burning propellant surface, was used to follow the whole combustion transient giving a precise picture of the phenomenon. Comparisons between data obtained studying the ignition and extinction by laser energy pulse phenomena show that the operating pressure has opposite effects on the propellant response to the external energy input having the same characteristics.     

NEPE推进剂拉伸断裂行为研究

通过单向拉伸力学性能实验,考察了不同测试温度和不同拉伸速率条件下NEPE推进剂力学性能的变化情况。采用扫描电镜(SEM)和原位拉伸SEM观察了推进剂拉伸断面形貌。结果表明,在低温测试条件下,NEPE推进剂最大伸长率较常温条件下显著降低,最大抗拉强度较常温和高温条件下显著升高,NEPE推进剂的破坏主要表现在黏合剂的撕裂和固体颗粒的断裂;在高温、慢拉伸速率的测试条件下,推进剂断裂时结构被破坏的程度较大,NEPE推进剂的破坏首先发生在固体颗粒堆积处,再到黏合剂网络结构。推进剂断裂的过程是推进剂拉伸取向与裂纹扩展之间的竞争过程。     

Study of solid propellant ignition by a hot gas stream

The process of fast ignition of double-base solid propellants (ignition time 0.6–40 msec) is studied in a model rocket chamber by means of fine thermocouples that are located on the surface being ignited and in the igniting gas stream under rapidly varying external conditions. The basic parameters of the process (propellant surface temperature, gas-propellant heat transfer coefficient, heat flux from the gas to the propellant, and the heat content and the heat release in the propellant) are obtained as functions of time. Three ignition regimes are found: fast, normal, and delayed. We discuss briefly the nature of the transition to steady-state combustion for the obtained regimes. The directions of future studies are noted.Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 3, pp. 20–26, May–June, 1993.     

Thermal-balanced integral model for pyrolysis and ignition of wood

The pyrolysis and ignition of wood is of great importance to understand the initial stage of combustion, helping control the occurrence and spread of unwanted building and forestry fires. The development of a thermal-balanced model is introduced for examining the analytical relationship between the ignition time and external heat flux. The critical heat flux, one of the important fire-retardant characteristics of combustible solid, is determined from a correlation study between the ignition time and external heat flux. One of the thermal-balanced integral models, considering the effect of surface heat losses, average absorptivity and moisture content, is employed to give the prediction of surface temperature rise, ignition time and ignition temperature of the Aspen. The results show that the model readily and satisfactorily predicts ignition temperature and ignition time of wood with different moisture contents.     

带延迟点火部件的发射药点火性能试验研究

为了更好地研究发射药的点火性能,在基于密闭爆发器原理的点火性能测试装置基础上增加了一个延迟点火部件,构建了一个新型点火性能模拟试验装置,根据该装置建立了简单的火药分层点火过程模型,模拟并对比了高能太根-18/1、双芳-3-18/1及NR11-18/1三种发射药的点火性能。结果表明,NR11-18/1发射药较易点火,双芳-3-18/1发射药最难点火,点火时间分别为19和45ms。增加延迟点火部件后,可将点火药的燃烧和发射药的燃烧阶段有效区分,不仅有利于对比点火性能差异较小的发射药之间的区别,还有助于分析发射药低压段的燃烧速度。随着延迟点火部件长度的增加,点火时间也增长。     

Effects of NTO Oxidizer Temperature and Pressure on Hypergolic Ignition Delay and Life Time of UDMH Organic Gel Droplet

Organic gel propellants are promising candidates for a variety of rocket motor and scramjet applications, since they are intrinsically safe and provide high performance. It is well known that organic gel fuel droplets exhibit distinct combustion characteristics compared with conventional liquid fuel droplets, and furthermore an understanding of the ignition delay and lifetime of these droplets is critical to the improvement of combustor design. In this work, investigations of the combustion of unsymmetrical dimethylhydrazine (UDMH) organic gel droplets in different nitrogen tetroxide (NTO) oxidizing atmospheres were conducted using two sets of experimental apparatus. The combustion characteristics under different conditions of temperature and pressure were compared and analyzed based on the flame shapes observed during experimentation. From these trials, an unsteady combustion model was developed and used for the numerical simulation of spray‐sized UDMH organic gel droplet combustion in an NTO atmosphere. The hypergolic ignition and burning characteristics of the organic gel droplets under conditions simulating either engine startup or steady state combustion were compared, and changes in ignition delay and droplet lifetime with ambient temperature and pressure were analyzed. The experimental and numerical results show that the UDMH organic gel droplets exhibit periodic swell‐burst behavior following the formation of an elastic film at the droplet surface. Each droplet burst results in fuel vapor ejection and flame distortion, the intensity of which declines with increasing ambient pressure. However, the swell‐burst period is extended with increasing ambient pressure, which results in potential flameout. Under conditions of low temperature and pressure similar to those at engine startup, the ignition delay and lifetime of spray‐sized gel droplets decrease with increasing temperature or pressure, although there is a sharp increase in droplet lifetime when the ambient pressure reaches a critical value associated with flameout. The ignition delay was found to be a rate‐limited phenomenon linked to the droplet heating rate. The proportion of ignition delay and droplet lifetime due to droplet heating‐up decreased with increasing temperature or decreasing pressure. Conversely, at high temperatures and pressures simulating the engine’s steady state operating conditions, the droplets were observed to flameout after several swell‐burst periods and both ignition delay and lifetime decreased monotonically with increasing temperature or pressure. The ignition delay time was determined to be rate‐limited by gas phase chemical reactions and contributed very little to the overall droplet lifetime compared with the engine startup condition.     

铝镁贫氧推进剂的点火性能

为研究镁铝富燃料固体推进剂组分对点火性能的影响,采用改进的靶线法燃速测试系统对多种含镁铝富燃料固体推进剂在常压和加压下进行了通电金属丝点火性能的对比实验。被测试推进剂的镁铝合金含量为20%～40%,或者同时含镁铝合金及硼,氧化剂含量为30%～53%。实验表明,在固定外界输入热源的情况下,推进剂的点火性能主要与氧化剂含量和粒度有关;金属的含量和种类也有一定的影响;催化剂对点火延迟时间影响很小;压强对此种点火方式几乎无影响。该点火延迟测试方法简单易行,并具有一定的可靠度,适于配方调试。     

Burning Phenomena of the Gun Propellant JA2

The combustion phenomena of the standard gun propellant JA2 are investigated in experiments and analyzed by a simplified theoretical model. Hereby energy transfer from the gas phase governs ignition and combustion of solid rocket and gun propellants. In addition to the heat conduction and convection, the radiation of the flame contributes to the heat feedback which controls the burning rate in dependence on pressure. The dependence on the initial temperature is given by physical parameters of the conversion from the solid to the gaseous state. Burning rates are measured in dependence on pressure and initial temperature confirming a simplified law for the burning rate. The evaluation yields that the pressure exponent can be directly assigned to the heat feedback and that the temperature of the conversion from the condensed to the gas phase lies at about 675 K. The experiments also comprise spectroscopic measurements at low pressures in the wavelength ranges from 300 nm to 14000 nm which are resolved spatially along the vertical flame profile. The analysis of the spectra delivers the profiles of species in the flames including di‐atomic radicals and tri‐atomic molecules of the final combustion products. In addition, gas phase temperatures are derived by application of the Single‐Line‐Group model which gives approximately 2800 K closely below the adiabatic flame temperature of 2900 K at low pressures. They are compared to temperatures assigned to soot particle emission. In summary, these data enable an estimation of the heat feedback from the flame to the burning surface.     

NEPE推进剂燃烧机理研究

应用微热电偶测温和燃烧火焰单幅照相技术测得 NEPE推进剂在稳态燃烧条件下的燃烧波温度分布及火焰结构 ,研究了该推进剂中主要组分对燃烧性能的影响 ,同时利用扫描电镜 -能谱仪观测了熄火表面形貌和元素分布规律。经过综合分析 ,提出了 NEPE推进剂的燃烧过程 ,为该类推进剂燃烧物理模型的建立奠定了基础     

Numerical study of coal particle cluster combustion under quiescent conditions

Behavior of ignition and combustion of coal particle cluster under a quiescent condition was numerically simulated by solving balance equations of mass and enthalpy with combustion kinetic models of volatiles and char. Two-flame structure, one flame penetrating into the cluster and the other moving out of the cluster, was predicted during the combustion of coal particle cluster. Effects of radiative heat transfer, group number, ambient temperature, coal particle size, and oxygen concentration on ignition and combustion of coal particle clusters were also analyzed. Simulations indicated that the gas volume fraction of coal particle cluster increases with time after devolatilization. Gas velocity passing through the cluster surface varied significantly at volatile liberation. The ignition time delay was reduced with the increase of ambient temperature. The cluster devolatilization rate and char burning rate increased while the ignition time delay decreased with the increase of ambient oxygen concentration.     

等离子体增强固体火药燃烧性能的实验研究

利用300mL电热化学密闭爆发器研究了在不同输入电能、装填密度和初始温度下等离子体点火对某混合酯高能19孔花边形固体火药颗粒燃烧性能的影响;分析了上述不同条件下等离子体对该固体火药燃烧速率影响的变化规律。结果表明,输入等离子体电能从15.4kJ增大到61.6kJ后,固体火药燃速在100MPa时提高106%,200MPa时提高30%,300MPa以上燃速无明显变化;等离子体点火对低温火药燃烧初期和中期的燃速均有显著的增强作用,对高温火药点火燃烧初始燃速的增强作用较为明显。     

Laser Ignition of Various Pyrotechnic Mixtures – an Experimental Study

Ignition of several pyrotechnic mixtures by diode‐laser was studied experimentally using a novel combustion chamber. The ignition delay times dependence on laser intensity could be fit by the expression t_ign=aI⁻ⁿ for all compositions, with I being the laser intensity at target and n=1.4–2.1. This is roughly in accordance with thermal ignition theories assuming a semi‐inert solid. Differences in ignition delay times did not depend on fuel alone or oxidizer alone. The temperature of oxidizer decomposition does not correlate with ignition delay time. Furthermore, the steady state combustion temperature, deduced from emission spectra of the composition products are not correlated with ignition delay time. It is proposed that chemical reactions, taking place in the gas‐phase or in the solid‐phase, play a significant role, but are not solely responsible for ignition delay time. The seemingly uncorrelated ignition delay results between pyrotechnics containing either the same fuel or oxidizer hamper the construction of a “unified theory” for laser ignition of pyrotechnic mixtures.     

小型固体火箭发动机尾部点火设计与实验

根据固体火箭发动机点火器设计经验,选用赛璐珞为点火器盒体材料、黑火药为点火药,并以点火压强作为发动机装药可靠点燃的判据。采用头部点火设计经验公式对端面一侧面燃烧、尾部点火的小型固体火箭发动机点火药量进行了初步估算。为获得点火器的点火压强、点火延迟时间等性能参数,设计、加工了模拟发动机尾部点火空间的试验容器,研究了电点火头、电点火管点火方案在不同条件下的试验情况。结果表明,虽然点火药量相同,但两种点火方案的点火压强、点火延迟时间、喷管堵片的打开方式却存在较大差异,基于发动机可靠性、维修性考虑,将电发火管点火方案作为优选方案,并通过发动机点火试验的成功考核。     

基于图像传感器的推进剂发烟量测试方法

提出了一种基于图像传感器的推进剂发烟量测试方法。用平行光管代替传统测试装置中的卤素钨灯,用图像传感器代替传统测试装置中的光电二极管或光敏电阻,设计了一种表面有镂空条纹的遮光罩对光源信号进行调制,采取图像数据处理方法消除环境噪声对测试结果的影响。测量了SQ2推进剂、Al-CMDB推进剂和NEPE推进剂燃烧烟雾的发烟量。结果表明,SQ2推进剂和Al-CMDB推进剂的烟雾透过率曲线在点火后50s左右基本稳定;NEPE推进剂燃烧的烟雾光透过率曲线在点火50s后稳定,但有缓慢上升趋势;3种推进剂烟雾光透过率分别为84.1%、65.9%、22.3%,与传统测试方法一致。     

An ignition criterion for combustible solids integrating surface temperature and heating rate

Surface ignition temperature has been widely used as an ignition criterion for the piloted ignition of common combustible solids. However, experimental observations have shown that the surface temperature of a solid at ignition varies with external heat flux. In addition, if the external heat flux is smaller than the critical heat flux for ignition, the solid will not ignite while the actual surface temperature may be higher than the defined surface ignition temperature. To overcome these limitations and maintain the simplicity of the surface ignition temperature criterion, a new ignition criterion integrating heating rate and surface temperature is proposed, developed, and validated. Predictions based on the new criterion compare well with experimental results on piloted ignition of a thermoplastic material (black PMMA), a thermoset composite material (E‐glass fiber reinforced polyester composite) and a cellulosic material (Red Oak) subjected to different heat flux levels. Potential factors affecting the accuracy and predictive capability of the new heating rate‐related ignition temperature criterion are discussed. The method and associated procedures to construct the heating rate‐related temperature ignition criterion can be used to obtain the same ignition criterion for other combustible solids. Copyright © 2014 John Wiley & Sons, Ltd.