Experimental Measurement And Molecular Simulation Of Thermal Conductivity Of Hydrate

Hydrate generally refers to gas hydrate.As a typical gas hydrate,natural gas hydrate is a cage crystal structure compound formed by water molecules and gas molecules in a certain temperature and pressure range.The natural gas hydrate reserves that have been found in nature are about 2×10¹⁶ m³,with carbon content twice that of traditional fossil energy,which can be used as strategic energy in the future.In the process of hydrate exploration,mining,storage and transportation,its stability is very sensitive to the external temperature.The formation of hydrate and the measurement of its thermophysical parameters have high requirements for laboratory conditions,and the measurement of thermal conductivity has certain technical difficulties.The research progress in the measurement of hydrate thermophysical parameters at home and abroad is relatively insufficient,and hydrate thermophysical parameters play an important role in the development and utilization of hydrate.Based on molecular dynamics simulation and Transient plane heat source method experiment,this paper studies the variation law of thermal conductivity of hydrate.The main research contents and conclusions are as follows:（1）Based on transient plane method,the thermal conductivity of ice,THF hydrate and methane hydrate was calculated experimentally,and the relationship between thermal conductivity and temperature was analyzed.The results show that the thermal conductivity of ice decreases with the increase of temperature at 253.15 K～269.15 K,and the feasibility of this experimental method is verified.When the thermal conductivity of THF hydrate is 253.15 K～267.15 K,it gradually increases with the increase of temperature,which conforms to the change law of glass thermal conductivity.Although the thermal conductivity also increases during the temperature increase at 269.15 K～277.15 K,the increasing trend does not conform to the linear change law.In this temperature range,THF hydrate is partially decomposed.At 253.15K～277.15 K,the thermal conductivity of methane hydrate measured four times ranges from 0.4875 W/（m·K）～0.5681 W/（m·K）,the average thermal conductivity obtained by compaction measurement is about 0.53 W/（m·K）,and the thermal conductivity of uncompacted methane hydrate is lower than that of compacted methane hydrate.（2）The methane hydrate model and carbon dioxide hydrate model are established by molecular dynamics simulation method.The changes of thermal conductivity of hydrate under different working conditions are calculated and analyzed according to Green-Kubo formula.The analysis shows that the thermal conductivity range of methane hydrate is between 0.6084 W/（m·K）and 0.6537 W/（m·K）at 253.15 K～277.15 K and 5 MPa.The increase of temperature makes the phonon scattering more intense,and the enhancement of inelastic scattering makes the energy transfer channels more and the thermal conductivity more.The molecular weight of carbon dioxide is larger than that of methane,the density of carbon dioxide hydrate is also larger than that of methane hydrate,and the thermal conductivity is also stronger than that of methane hydrate,that is,the larger the guest molecular weight is,the greater the thermal conductivity is.Under the same conditions,the structure of carbon dioxide hydrate is more stable than that of methane hydrate.Under certain conditions,methane hydrate can be decomposed and carbon dioxide hydrate can still be formed,so as to achieve the purpose of replacing methane in methane hydrate with carbon dioxide.（3）Based on the molecular dynamics simulation method,the theoretical framework of the simulation algorithm of thermal conductivity is deduced and improved.The results show that the improved algorithm has good energy conservation by adding coordinate integration to EHEX algorithm.The energy loss（energy drift）generated by the improved algorithm is reduced by at least 300 times compared with HEX algorithm.For TIP4P water model system,the energy loss（energy drift）generated by the improved algorithm is reduced by at least 80 times compared with HEX algorithm,and the simulation scale can be increased by two orders of magnitude.Compared with the experimental data under the same conditions,the difference between the improved simulation method and it is no more than 15%,which is closer to the theoretical experimental value of thermal conductivity than the traditional NEMD method.There are 27 figures,7 tables and 100 references.