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数值模拟高超声速再入体全目标区烧蚀流场,需要考虑除高温空气化学反应组分以外的烧蚀产物组分,把多组分化学非平衡气体动力学方程与烧蚀壁面边界条件耦合起来进行求解,因此计算量往往比不考虑烧蚀情况大很多。为了提高计算效率,对再入目标流场进行分区求解[3]:在再入体的头部和身部流场区,分别采用对粘性激波层方程和抛物化NS方程求解方法;在底部尾流区域,采用对N-S方程数值求解的LUSGS隐式离散方法。
在壁面存在热化学烧蚀情况下,烧蚀产物引射至再入体周围气体层,此时壁面速度条件可表示为:
$$ {u_\tau } = 0\quad {v_n} = {(\vec V \cdot \vec n)_w} $$ (1) 式中:
$ \vec V $ 和$ \vec n $ 表示速度和壁面法向单位矢量;$ {u_\tau } $ 和$ {v_n} $ 分别表示物面切向和法向速度。组分质量分数$ {c_{s,w}} $ 满足气-固交界面上的质量平衡关系:$$ {j_{s,w}} + {\dot m_w}{c_{s,w}} - {\dot m_g}{c_{s,g}} = {\dot m_{s,w}} $$ (2) 式中:左端第一项表示
$ s $ 组分向壁面的扩散通量;第二、三项分别表示$ s $ 组分从壁面向气体层和从材料深部向壁面的对流通量;右端项$ {\dot m_{s,w}} $ 表示壁面化学反应产生的$ s $ 组分的净质量流;其中热解气体组分质量分数$ {c_{s,g}} $ 和热解通量$ {\dot m_g} $ 与材料化学元素构成比例相关。$ s $ 组分扩散质量流$ {j_{s,w}} $ 和总的壁面烧蚀率$ {\dot m_w} $ 为:$$ {j_{s,w}} = - {(\rho {D_s}\vec \nabla {c_s} \cdot \vec n)_w} $$ (3) $$ {\dot m_w} = {\dot m_g} + \sum\limits_s {{{\dot m}_{s,w}}} = {(\rho \vec V \cdot \vec n)_w} $$ (4) 基于准定常烧蚀假设,不考虑熔化层的机械流失能量,壁面温度由气-固交界面上能量平衡关系确定:
$$ {q_w} + {q_r} + {\dot m_w}{H_w} - {\dot m_w}{h_a} = 0 $$ (5) 式中:
$ {q_w} $ 为壁面热流;$ {H_w} $ 为壁面总焓;$ {q_r} $ 为壁面辐射热流;$ {h_a} $ 为原始壁面材料的生成焓。对于碳基材料,忽略热解效应即
$ {\dot m_g} $ =0,壁面烧蚀率及组分净质量流通过材料表面氧化反应、催化反应和升华反应等热化学机制获得:$$ \begin{array}{r} \quad \quad \quad \quad\quad \quad \quad{{ C(s){\rm{+O_2 \rightarrow C O+O}}}}\quad \quad \quad \quad \quad \quad (r1)\\ {{C(s){\rm+O \rightarrow C O}}}\quad \quad \quad\quad \quad\quad \quad (r2)\\ {{C(s){\rm+O+O }\rightarrow C(s){\rm+O_2}} }\quad \quad \quad \quad\quad(r3)\\ {{C(s) \rightarrow {\rm{C}}}}\quad \quad\quad \quad \quad\quad \quad\quad(r4)\\ {{C(s) \rightarrow{\rm{ C_2}}}}\quad \quad\quad \quad \quad\quad \quad\quad(r5)\\ {{ C(s) \rightarrow{\rm{ C_3}}}}\quad \quad\quad \quad \quad\quad \quad\quad(r6) \end{array}$$ 式中:C(s)表示碳基材料固体壁面。上述反应(r1~r3)速率常数有如下形式:
$$ {k_{w,r}} = {\alpha _r}\sqrt {\frac{{R{T_w}}}{{2\pi {M_s}}}} $$ (6) 式中:
$ {\alpha _r} $ 为第$ r $ 个反应的壁面反应效率。壁面氧化和催化反应(r1~r3)产生的组分净质量流:$$ \dot{m}_{{\rm{O}}_2, w}=-\rho_{{\rm{O}}_2} k_{w, 1}+\rho_{\rm{O}} k_{w, 3} $$ (7) $$ \dot{m}_{{\rm{C O}}, w}=\frac{M_{{\rm{C O}}}}{M_{{\rm{O}}_2}} \rho_{{\rm{O}}_2} k_{w, 1}+\frac{M_{{\rm{C O}}}}{M_{\rm{O}}} \rho_{\rm{O}} k_{w, 2} $$ (8) $$ \dot{m}_{{\rm{O}}, w}=\frac{M_{\rm{O}}}{M_{{\rm{O}}_2}} \rho_{{\rm{O}}_2} k_{w, 1}-\rho_{\rm{O}} k_{w, 2}-\rho_{\rm{O}} k_{w, 3} $$ (9) 当壁面温度
$ {T_w} $ 达到3000 K时,需要考虑碳基材料表面的升华反应,升华反应(r5~r6)产生的组分C、C2和C3的质量流由Knudsen-Langmuir方程确定[5]:$$ {\dot m_{s,w}} = {a_s}({p_{v,s}} - {p_s})\sqrt {\frac{{{M_s}}}{{2\pi R{T_w}}}} $$ (10) 式中:
$ {\alpha _s} $ 为碳组分的凝结系数;$ {p_{v,s}} $ 和$ {p_s} $ 分别表示$ s $ 组分的平衡蒸气压力和壁面实际分压。平衡蒸气压力由以下关系式确定[5]:$$ {p_{v,s}} = 101 \; 300 \cdot {{\rm{e}}^{{{{P_s}} \mathord{\left/ {\vphantom {{{P_s}} {{T_w} + {Q_s}}}} \right. } {{T_w} + {Q_s}}}}} $$ (11) 在对碳基材料烧蚀流场模拟时,重点考虑高温空气组分、烧蚀气体组分以及二者相互作用产生的气体组分,采取O、N、O2、NO、NO+、N2+、N2、C、C2、C3、CO、CO2、CN和e–等14个组分及31个反应的化学模型[3]。
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忽略气体对光线的散射效应,热力学非平衡条件下高温气体热辐射传输方程可表示为[12]:
$$ \frac{{{\rm{d}}{I_\omega }}}{{{\rm{d}}\ell }} = {k_\omega }({S_\omega } - {I_\omega }) $$ (12) 式中:
$ \ell $ 为辐射传输路径;$ {I_\omega } $ 是沿该路径的光谱辐射亮度;$ {k_\omega } $ 为光谱吸收系数;$ {S_\omega } $ 是非平衡辐射的源函数。在局部热力学平衡条件下源函数还原成黑体辐射亮度:$$ {B_\omega } = \frac{{2h{c^2}{\omega ^3}}}{{{{\rm{e}}^{hc\omega /kT}} - 1}} $$ (13) 式中:
$ h $ 为普朗克常数;$ k $ 为玻耳兹曼常数;$ c $ 为光速;$ \omega $ 表示波数;$ T $ 为温度。对控制方程(13)沿气体辐射路径进行积分,可得通过长度为
$ L $ 路径后的气体光谱辐射亮度的具体形式:$$ {I_\omega } = - \int_0^L {{B_\omega }\frac{{\partial \tau }}{{\partial \ell }}} {\rm{d}}\ell $$ (14) 其中
$$ \tau (\ell ,\omega ) = \exp ( - \sum\limits_i {X(\ell ,\omega ,i)} ) $$ (15) 考虑与不考虑碰撞和多普勒加宽机制对气体组分光谱特性的影响,分别表示为:
$$ X(\ell ,\omega ,i) = f(X*,{\bar a_c},{\bar a_d}) $$ (16) $$ X(\ell ,\omega ,i) = f(X*) = \int_\ell ^L {{k_{\omega ,i}}(\ell ){\rm{d}}\ell } $$ (17) 式中:
$ \tau $ 为光谱透射率;$ {k_{\omega ,i}} $ 表示辐射组分$ i $ 的光谱吸收系数;$ {\bar a_c} $ 和$ {\bar a_d} $ 为与碰撞和多普勒加宽参数[12]。在再入目标碳-碳材料烧蚀流场红外辐射计算中,主要考虑的高温气体辐射机制有:NO基态(5.3 μm)和第一谐波带系(2.7 μm);CO的基态(4.67 μm)和第一谐波带系(2.34 μm);CO2的红外带系(15、4.3、2.7 μm等);CN的红外带系(4.76 μm);N2的第一正带系;N2的自由-自由连续辐射;O和N的自由-自由连续辐射。基于线-线辐射模型计算NO的光谱吸收系数[13],其他分子组分的吸收系数通过对燃气光谱辐射数据表中的数据插值获得。基于光学薄假设,计算任意观测方向目标流场的红外辐射特性 [12]。
Simulation of flow field infrared radiation over reentry vehicle with ablation of carbon-based thermal protection material
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摘要: 烧蚀效应是高超声速飞行器目标特性分析评估中的重要问题之一。基于高温反应气体动力学方程与辐射输运方程,建立了飞行器表面防热材料热化学烧蚀流场及其红外辐射特性的计算模型和方法。以钝锥体弹头外形及其表面防热材料碳-碳为对象,研究了材料烧蚀效应对再入目标流场红外辐射特性的影响,分析了再入体烧蚀流场及尾流在不同波段红外辐射的分布特征和变化规律。研究发现:典型状态计算结果与试验测量及文献预测结果一致,表明烧蚀流场及红外辐射模型和方法的可行性;材料热化学烧蚀现象对再入流场红外辐射特性产生严重影响,使3~8 μm波段尾流积分辐射强度增加一个量级以上,并随着尾流长度增加而增大;烧蚀流场红外辐射主要来自CO、NO和CO2等化学组分,烧蚀对1~3 μm波段流场红外辐射影响相对较弱;再入速度不变情况下,烧蚀流场在3~8 μm波段红外辐射强度随再入高度降低而增强;再入高度不变情况下,烧蚀流场在同样波段红外辐射强度随着再入速度减小而减弱。Abstract:
Objective Hypersonic vehicle travels at a very high speed in atmosphere. Due to the high flight velocity, hypersonic vehicle has to endure very high heating rates on surface, ablative material is widely used in the design of thermal protection system (TPS). During the ablation process, gaseous ablator species are injected into the flow field, these gaseous species can involve inflow air in the chemical reaction, which changes flow field species distribution and temperature distribution, thus changing the target infrared radiation characteristics of hypersonic vehicle. Infrared radiation characteristics are the foundation of aircraft detection, identification and interception. Therefore, it is necessary to study the effect of thermal protection material ablation on aircraft target infrared radiation characteristics. For this purpose, this paper focuses on strategic warhead blunt body configuration with carbon-based thermal protection material. Numerical simulation of flow field and its infrared radiation is conducted, ablation effects on infrared radiation of flow over reentry body are discussed. Methods Numerical simulation of flow field is conducted by solving three-dimensional thermal-chemical non-equilibrium Navier-Stokes equations. To simulate the surface ablation effect, surface velocity boundary condition, surface mass balance condition and surface energy balance condition are introduced into the computation process of flow field simulation. Oxidation, catalytic reaction and sublimation reaction of surface ablation material are also taken into account. To simulate the chemical reactions in the flow field, chemical reactions model of high temperature air with gaseous ablator species is used. Based on spectral band radiation model and by solving high temperature gas radiation transport equation, numerical simulation of flow field infrared radiation is conducted, the radiation mechanism of NO, CO, CO2, CN, N2, O, N is considered. Results and discussion Numerical simulation results at typical condition agree well with experiments and numerical simulation results in literature (Fig.1-3), the computation model and method are validated. The main ablation product on surface is CO, infrared radiation in the waveband of 0.8-8 μm of flow field mainly comes from the radiation of high temperature CO, NO, CO2 and CN (Fig.4). Ablation effect can increase flow field infrared radiation intensity, this phenomenon is more significant in 3-8 μm waveband than 1-3 μm waveband (Fig.5). Radiation from 3-8 μm waveband mainly comes from CO and NO, mass fraction of these species and flow field temperature increases as flight altitude decreases and flight velocity increases. Due to this, the radiation of 3-8 μm waveband increases as the flight altitude decreases and flight velocity increases (Fig.7-8). Radiation from 1-3 μm waveband in flow field around vehicle body increases as flight velocity increases, the radiation from 1-3 μm waveband in wake flow shows nonmonotonic variation due to the change of flow structure (Fig.7-8). Conclusions In this paper, the ablation effects on infrared radiation of flow over reentry body covered with carbon-based thermal protection material is studied. By solving high temperature gas dynamics equations and radiation transfer equations, numerical simulation of thermal protection material ablation flow field and its infrared radiation is conducted. The distribution and changing rules of infrared radiation in different wavebands from ablation flow are analyzed. The study shows that ablation effect has significant influence on infrared radiation of reentry flow, which makes integral radiation intensity of wake flow increase more than one order of magnitude compared with non-ablation case in 3-8 μm waveband; The infrared radiation of ablation flow mainly comes from CO, NO and CO2, and the ablation effect has less effect on the radiation in 1-3 μm waveband; The infrared radiation intensity of ablation flow increases with the decrease of reentry height at the same flight velocity, which weakens with the decreasing reentry velocity at the same flight height in 3-8 μm waveband. -
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