Numerical study of thermal nonequilibrium process in CO2 system with state-to-state model
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摘要:
以火星大气条件下飞行器非平衡流场研究为背景,采用态-态模型对定容静止CO2系统自30 km火星大气条件(181 K)瞬时升温后的热非平衡过程开展数值模拟。考虑CO2分子的对称拉伸、弯曲和非对称拉伸三个振动模态共201个振动能级,微观过程包括:振动-平动(VT)能量交换过程引起的同一模态内部的和不同模态间的振动能级跃迁,振动-振动(VV)能量交换过程引起的同一模态内部振动能级跃迁。分析不同振动模态的能级分布演化特点,并考察对应微观过程细节探究内在原因,结果表明:VT过程起支配作用,对由初始低温突然升温后保持定温的气体非平衡过程,VV过程的贡献可以忽略;CO2分子的弯曲振动模态具有最快的激发速率,平衡时也具有最高的粒子数占比,升温至
2000 K情况下,对称拉伸和非对称拉伸模态的松弛时间约为弯曲模态的2.2倍和46.1倍;更高温度下振动能级跃迁速率加大,升温至5000 K情况达到平衡的平均松弛时间比2000 K情况低了2个量级。Abstract:With the background of research on nonequilibrium flow field outside Mars probe, the thermal nonequilibrium process after temperature increase of a stationary CO2 system from Martian atmospheric condition at an altitude of 30 km (181 K) is studied with state-to-state model. The system with initial condition of Martian atmosphere is heated suddenly to a high temperature and then is kept at constant temperature and volume. Three vibrational modes (symmetric stretching, bending and asymmetric stretching modes) and totally 201 vibrational energy levels are considered. The microscopic processes include: vibration-translation (VT) energy exchange processes that cause transitions between energy levels of the same or different modes, vibration-vibration (VV) energy exchange processes that cause transitions within the same mode. The time evolution of vibrational distribution for the three modes and the specifics of the corresponding microscopic processes are analyzed. The results show that: (1) VT processes are dominant, the contribution of VV processes can be ignored for such sudden increase and then constant temperature case. (2) Among the three vibrational modes, the bending mode has the fastest excitation rate and the largest equilibrium population. For the case of temperature rise to
2000 K, the relaxation time for the symmetric and asymmetric stretching modes is respectively about 2.2 times and 46.1 times of that for the bending mode. (3) As the transition rates increase with temperature, the average relaxation time for5000 K case is lower than that for2000 K by two orders of magnitude. -
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表 1 初始时刻V1和V2模态粒子数密度(T =
2000 K)Table 1 Population of V1 and V2 modes at the initial moment (T =
2000 K)State n0vi/m–3 v0 1.20×1023 v1 = 1 2.99×1018 v2 = 1 1.19×1021 v2 = 2 5.90×1018 表 2 初始时刻V1和V2模态跃迁速率(T =
2000 K)Table 2 Transition rates of V1 and V2 modes at the initial moment (T =
2000 K)Process kf
/(m3·s–1)kb
/(m3·s–1)dnvi/dnvidtdt
/(m–3·s–1)Total dnvi/dnvidtdt
/(m–3·s–1)v1(0→1) 2.87×10–22 –7.49×10–22 4.18×1024 4.18×1024 v2(0→2) 2.85×10–21 –3.73×10–21 4.15×1025 2.12×1026 v2(1→2) 1.19×10–18 –1.93×10–18 1.71×1026 注:kf表示正向跃迁速率系数,kb表示逆向跃迁速率系数。 表 3 升温至T =
2000 K和T =5000 K两种情况下平衡时基态及三种模态粒子数占比Table 3 Fractions of equilibrium particle population in ground state and three modes under two temperature conditions
T/K nv0/n nV1/n nV2/n nV3/n 2000 23.9% 14.9% 55.9% 5.3% 5000 8.2% 17.8% 65.6% 8.4% -
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