Fluctuation-dissipation analysis of nonequilibrium thermal transport at hydrate dissociation interface
Nonequilibrium molecular-dynamic simulation in NVE ensemble is performed to investigate spontaneous dissociation of methane-hydrate in contact with liquid water. The nonequilibrium in the interface region is conjugated to the dissociation process of hydrate near the interface under Onsager’s hypothesis. The simulated thickness of interface is close to the acoustic phonon mean path of methane hydrate and agrees with the reference value. The normalized heat flow autocorrelation function is introduced to study the fluctuation-dissipation in terms of the thickness and moving velocity of interface and the Stefan number. This helps to identify clearly three distinct hydrate-decomposition regimes dominated by sensible heat, latent heat and intrinsically unstable lattice framework respectively. It is found that the fluctuation-dissipation theory expresses the nonequilibrium nature in the front two stages before the threshold and the dissociation rate is increased in the latter stage which is different from the thermal-driven dissociation. The Stefan number decreases rapidly with dissociation in the initial stage and then fluctuates in the intermediate stage which is analogous to the fluctuation characteristics of heat flow autocorrelation function. The Stefan number effect shows that the thermal dissipation drives the hydrate dissociation and correlates fluctuation to nonequilibrium nature. It is also found that a small Stefan number is enough to break up the residual hydrate soon after the threshold. The transient interfacial thermal resistance of the interfacial region is obtained as a typical value within the range of 10-7-10-9 m2KW-1. It justifies that the fluctuation-dissipation exists in the nonequilibrium process of hydrate dissociation no matter in terms of heat flux in this work or the diffusion of guest molecules in other works.