甲烷水合物声子导热及量子修正

doi:10.11949/0438-1157.20190994

摘要/Abstract

摘要：

采用平衡分子动力学方法模拟了甲烷水合物的导热，给出了30~150 K甲烷水合物的热导率。采用量子修正对分子模拟结果进行处理，可以得到更接近实验值的结果。当模拟温度低于德拜温度时，量子效应对分子模拟结果的影响较大。通过对热流自相关函数拟合得到了声学声子和光学声子的弛豫时间。结果显示，声子弛豫时间随温度增加逐渐减小，声学声子导热在水合物的导热中比重最大。随着碳氧原子之间相互作用力的增加，碳氧原子之间振动的耦合程度增加，甲烷水合物的热导率增加。

关键词: 甲烷水合物, 分子动力学, 热导率, 声子模式分解, 量子修正

Abstract:

The equilibrium molecular dynamics method was used to simulate the thermal conductivity of methane hydrate, and the thermal conductivity of 30—150 K methane hydrate was given. The studies of thermal transport in hydrate have a lot of significance in hydrate exploitation and gas hydrate storage and transport. In this paper, an equilibrium molecular dynamics simulation for 2×2×2 type I methane hydrate periodic structure is carried out by LAMMPS and the thermal transport process is analyzed. For methane hydrate, CH₄ is modeled as OPLS-UA type while H₂O is treated by TIP4P/2005 model. The intermolecular interactions are described by the Lennard-Jones potential function and a Coulombic pairwise interaction. During the simulation, the methane hydrate is successively placed into NVT and NPT ensembles to relax for 1 ns respectively, so as to equilibrate the whole system. Then, it is transferred into NVE ensemble to run 2 ns for calculating the thermal conductivity. The thermal conductivity which is much closer to experiments results can be obtained by quantum correlation. When the simulation temperature is lower than Debye temperature, the quantum effect has a great influence on the molecular simulation results. The relaxation time of acoustic phonons and optical phonons are calculated by fitting the autocorrelation function of heat flow. The results show that the phonon relaxation time decreases with the increase of temperature and the acoustic phonon contributes most to heat conductivity. With the increase of the interaction strength between carbon and oxygen atoms, the coupling of vibration between carbon and oxygen atoms becomes stronger, and the thermal conductivity of methane hydrate increases.

Key words: methane hydrate, molecular dynamics simulation, thermal conductivity, phonon modes decomposition, quantum correction

中图分类号:

O 551

刘明, 徐哲. 甲烷水合物声子导热及量子修正[J]. 化工学报, 2020, 71(4): 1424-1431.

Ming LIU, Zhe XU. Phonon heat conduction and quantum correction of methane hydrate[J]. CIESC Journal, 2020, 71(4): 1424-1431.

图/表 12

图1 甲烷水合物模拟结构

Fig.1 Initial configuration of methane hydrate

图2 量子修正温度和分子模拟温度

Fig.2 Quantum correction temperature and simulation temperature

图3 量子修正系数

Fig.3 Quantum correction coefficient

图4 量子修正后的热导率

Fig.4 Thermal conductivity after quantum correction

图5 弛豫时间拟合

Fig.5 Relaxation time fit

表1 声子弛豫时间和光学模式峰值频率

Table 1 Phonon relaxation time and peak frequency of optical modeontribution to thermal conductivity

T/K	τ_sh,ac/ps	τ_int,ac/ps	τ_lg,ac/ps	τ_sh,opt/ps	τ_l_g,opt/ps	ω/ (rad·s^-1)
30	0.411		4.95	0.0546	0.968	172.45
50	0.2		2.33	0.0539	0.685	171.82
75	0.142		2.17	0.0521	0.570	168.68
100	0.133		1.83	0.0492	0.1836	166.79
150	0.0446	0.467	1.25	0.0479	0.1616	163.01

图6 热流自相关函数

Fig.6 HCACF at various temperatures

图7 热流自相关函数能谱

Fig.7 Power spectra of HCACF

图8 量子修正后不同水分子模型下甲烷水合物的热导率

Fig.8 Thermal conductivity of methane hydrate under different water models after quantum correction

表2 不同作用力强度下甲烷水合物的热导率

Table 2 Heat conductivity of methane hydrate with various strengths

系数	k/(W·m^-1·K^-1)	k_ww/(W·m^-1·K^-1)	k_mm/( W·m^-1·K^-1)	k_wm/(W·m^-1·K^-1)
1	0.72	0.66	0.007	0.058
2	0.74	0.67	0.010	0.062
3	0.77	0.69	0.015	0.066
4	0.79	0.70	0.023	0.072

图9 甲烷水合物的态密度图

Fig.9 DOS of methane hydrate with various strengths

图10 态密度重叠区域的能量

Fig.10 Overlapped phonon energy of DOS

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