Molecular dynamic study of methane hydrate combustion-decomposition: Fluctuation-dissipation and nonequilibrium analysis

Abstract

The combustion-decomposition of methane hydrate at micro-nano scale simulated by Reactive Force-Field molecular dynamics has characteristics of ultra-fast and nonequilibrium process. The normalized heat flux autocorrelation function of fluctuation at interface was introduced to study fluctuation-dissipation, which helped to clearly identify two distinct combustion-decomposition regimes dominated by sensible heat at burning boundary and latent heat at liquid-solid interface of decomposition. The highly synergistic rate of fluctuation period of heat flux between burning boundary and solid-liquid interface reaches up to 96.4 %. The localization characteristics and acoustic forms were observed based on the energy-spectrum of the heat flux at burning boundary and solid-liquid interface by Fourier transform. The energy-spectrum of heat flux at the two interfaces have peaks at 6 THz, and the energy-spectrum match well at lower frequency below ∼ 20 THz, justifying the highly synergistic rate. The curve of state density of methane is in good agreement with the spectral characteristics of interfacial heat flux. Finally, the mechanism of stable combustion of methane hydrate is simulated by interface adjustment and drainage, finding that continuous discharge of water from decomposed hydrate can maintain the rapid and stable combustion of methane, and the combustion rate of methane will increase by 5 times, and the number of various combustion production will increase significantly. The fluctuation period of various production meets that the later the production is generated, the shorter the fluctuation period. The fluctuation-dissipation mechanism gives deep light into the nonequilibrium process for the combustion-decomposition of methane hydrate at micro-nano scale.