摘要
为了研究NEPE推进剂的点火燃烧特性,搭建了CO2激光点火试验平台,使用高速摄影仪拍摄在不同气体环境下NEPE推进剂的燃烧过程,通过信号采集系统测量NEPE推进剂的点火延迟时间,对NEPE推进剂在0.1~3.0 MPa氮气及空气中的点火燃烧特性进行了研究。结果表明,环境压强和环境气体会影响NEPE推进剂的点火燃烧过程,环境压强越大,NEPE推进剂燃烧越激烈,且NEPE推进剂在空气中燃烧时比氮气中更加剧烈。NEPE推进剂的点火延迟时间随着环境压强的增大而减小,当环境压强从0.1 MPa增大到3.0 MPa时,氮气中的点火延迟时间由0.51 s减小到0.29 s,而空气中的点火延迟时间由0.32 s减小到0.18 s,但是当环境压强大于0.5 MPa时,环境压强对点火延迟时间的影响显著降低。同时环境压强会影响NEPE推进剂的燃烧速率,当环境压强从0.1 MPa增加到3.0 MPa时,氮气中的燃速从1.71 mm·
图文摘要
In order to comprehensively study the ignition and combustion characteristics of NEPE propellant, the combustion processes of NEPE propellant were monitored and compared for different experimental conditions through a high‑speed camera. In particular, the ignition delay time and combustion rate of NEPE propellant were measured at pressures in the range of 0.1-3.0 MPa under nitrogen and air respectively, and the effects of ambient pressure and gas environment on the ignition delay time and combustion velocity of NEPE propellant were investigated.
关键词
固体推进剂作为固体火箭发动机的能源和工质源,决定了发动机的能量特性,因此研究固体推进剂的点火燃烧特性,揭示其点火和燃烧机理、应用和开发新型推进剂具有重要意
激光点火具有输出能量高,外部干扰小,点火时间和能量可控和对环境因素不敏感等优点,被广泛应用于固体推进剂的燃烧性能研究
对NEPE推进剂激光点火的研究相对有限,目前NEPE推进剂点火延迟时间和燃烧速率是研究的重
基于此,本研究通过搭建小型封闭CO2激光点火试验平台,对NEPE推进剂在0.1~3.0 MPa氮气及空气中的点火燃烧特性进行研究。采用高速摄影仪记录其点火燃烧过程,通过光电二极管测量其点火延迟时间,研究结果有助于揭示NEPE推进剂的点火机理以及影响点火过程的因素,为NEPE推进剂的研究和应用提供了依据。
实验中使用NEPE推进剂,其基本组成部分包括黏合剂(聚乙二醇,CAB,7%)、增塑剂(1,2,4‑丁三醇三硝酸酯,BTTN,20%)、氧化剂(高氯酸铵,AP,25%)、金属添加剂(铝粉,Al,25%)、黑索今(环三亚甲基三硝铵,RDX,20%)和催化剂(3%)。在实验中,使用5 mm×5 mm×5 mm的正方体样品。为了防止试件发生侧面燃烧,影响实验现象的观察,用硅橡胶将其侧面包覆。
激光点火实验系统主要由控制系统、CO2激光器、光学系统、燃烧室、数据采集系统和燃烧产物收集系统组成,如

图1 激光点火试验系统示意图
Fig.1 Schematic diagram of the laser ignition experimental system
NEPE推进剂的点火燃烧过程可以分为4个阶段:惰性加热期、热解、初始火焰和稳定燃烧。CO2激光器发射激光辐射到推进剂试件表面后,推进剂表面开始吸收激光能量,温度逐渐升高,此阶段为惰性加热期。当推进剂温度达到发生热力学相变的熔点时,开始分解熔化,在推进剂试件表面形成由固体和液体组成的糊状区域。部分液相发生热解反应产生气体产物,同时气相产物快速从推进剂表面蒸发出来进入到周围的环境气体中,形成热解气体,如

a. 0.722 s

b. 0.803 s

c. 1.228 s
图2 NEPE推进剂的点火过程
Fig.2 The ignition process of a typical NEPE propellant
NEPE推进剂是一种物理混合的非均质推进剂,其中氧化剂AP在温度达到420 K左右时开始分解,且在高温下的燃烧非常剧
当热流密度为2.5 W·m

a. 0.1 MPa

b. 0.5 MPa

c. 1.0 MPa
图3 不同环境压强下NEPE推进剂在氮气中的燃烧过程
Fig.3 Combustion process of NEPE propellant in nitrogen at different ambient pressures

a. 0.1 MPa

b. 0.5 MPa

c. 1.0 MPa
图4 不同环境压强下NEPE推进剂在空气中的燃烧过程
Fig.4 Combustion process of NEPE propellant in air at different ambient pressures
通过在不同气体环境下拍摄到的NEPE推进剂的点火燃烧过程,可以看出NEPE推进剂在氮气和空气中的燃烧过程有很大的差异。如
如
NEPE推进剂的点火时间tig由3部分组成,tig=tpy+tmix+tchem,tpy是推进剂的热解时间,是指热解产物的量达到引燃所需最低值的热传导加热时间;tmix是推进剂热解气体的混合时间,是指热解生成的燃料蒸气穿过流体边界层向外扩散,并与周围的环境气体混合形成可燃混合物,可燃混合物和氧气浓度达到点火要求所需的时间;tchem是推进剂的气相化学反应时间,是指可燃混合物在点火源处发生燃烧所需的时
在同一激光热流密度下,NEPE推进剂点火延迟时间tig随压强变化的经验公
(1) |
式中,tig为推进剂的点火延迟时间,s;p为燃烧室压强,MPa;a,b为拟合参数。
采用最小二乘法对每个工况下5次测量结果的平均值进行拟合,拟合曲线如

a. nitrogen

b. air
图5 NEPE推进剂的点火延迟时间与环境压强的关系
Fig.5 Correlation between ignition delay time of NEPE propellant and ambient pressure
从
燃烧速率是指凝聚相燃料的质量损失速率,可以近似地用单位时间燃面退移的距离来表示,为了验证环境压强对NEPE推进剂燃烧速率的影响,在0.1、0.5、1.0、1.5、2.0、2.5、3.0 MPa环境压强下,对NEPE推进剂的燃烧速率进行测量。高速摄像法是通过记录固体推进剂的燃烧过程从而测得推进剂燃烧的一种动态测量方
从
通常使用Vielle燃烧速率公
Vielle燃速公式:
(2) |
Summerfield燃速公式:
(3) |
式中,为推进剂燃速,mm·
对NEPE推进剂在0.1~3.0 MPa下的燃速数据分别用Vielle公式和Summerfield公式进行回归分析,并得到回归曲线如

a. nitrogen

b. air
图6 NEPE推进剂的燃速拟合曲线
Fig.6 Fitting curves obtained for the burning rate of NEPE propellant
(1)NEPE推进剂在不同环境压强下点火时,由于压强影响推进剂热解气体的扩散,导致火焰的亮度和形状有很大的差别。在空气中点燃时明显比氮气中剧烈,在常压氮气中点火时不能形成明显的火焰形状,氧含量对NEPE推进剂的燃烧过程有很大的影响。
(2)NEPE推进剂的点火延迟时间随着环境压强的增加而减小,当环境压强超过0.5 MPa时,对点火延迟影响很小。当环境压强较高时,推进剂表面的气相产物可以迅速达到着火所需浓度,而当环境压强达到一定的临界值时,气相产物的浓度达到饱和值,环境压强的影响就会降低。
(3)NEPE推进剂在氮气、空气中点燃时燃速都随着环境压强的增加显著增加。当环境压强从0.1 MPa增加到3.0 MPa时,在氮气中的燃速增高165%,在空气中的燃速增加150%,而且NEPE推进剂在空气中点燃时,燃速有明显的提高,氧含量对NEPE推进剂的燃速有很大的影响。
(4)在试验压力条件下采用Vielle燃速公式和Summerfield燃速公式对NEPE推进剂在氮气、空气中的燃速进行拟合,发现Vielle燃速公式更适用于表征NEPE推进剂在0.1~3.0 MPa下的燃速特性。
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