Origin of Self-preservation Effect for Hydrate Decomposition: Coupling of Mass and Heat Transfer Resistances
Gas hydrates could show an unexpected high stability at conditions out of thermodynamic equilibrium, which is called the self-preservation effect. The mechanism of the effect for methane hydrates is here investigated via molecular dynamics simulations, in which an NVT/E method is introduced to represent different levels of heat transfer resistance. Our simulations suggest a coupling between the mass transfer resistance and heat transfer resistance as the driving mechanism for self-preservation effect. We found that the hydrate is initially melted from the interface, and then a solid-like water layer with temperature-dependent structures is formed next to the hydrate interface that exhibits fractal feature, followed by an increase of mass transfer resistance for the diffusion of methane from hydrate region. Furthermore, our results indicate that heat transfer resistance is a more fundamental factor, since it facilitates the formation of the solid-like layer and hence inhibits the further dissociation of the hydrates. The self-preservation effect is found to be enhanced with the increase of pressure and particularly the decrease of temperature. Kinetic equations based on heat balance calculations is also developed to describe the self-preservation effect, which reproduces our simulation results well and provides an association between microscopic and macroscopic properties.
Computer Simulation of the Crystal Growth and Dissolution of Natural Gas Hydrates a
Natural gas hydrates are icelike solids in which the water molecules or “hosts” form a rigid cagelike structure enclosing voids in which light hydrocarbons or other small molecules, known as “guests,” are contained. These solids belong to a class of crystals known as clufhrutes.’ Gas hydrates are nonstoichiometric crystals. The number of water molecules per gas molecule, the hydration number, depends on the conditions in which the crystal was formed and the type of gas molecule in the hydrate. Usually, hydration numbers range from 5.75 to 19, as quoted by Sloan.2 There arc two types of well-characterized natural gas hydrate denoted as hydrate type I and hydrate type 11. The main factor deciding the type of gas hydrate formed is the diameter of the guest molecule. Propane forms the type I1 hydrate (with the larger cavity), while methane forms the type I (in which the cavity is smaller). Hydrates are important industrially since they form spontaneously in natural gas lines, even at temperatures above O’C, forming plugs that can reduce and stop the flow of gas. These plugs are difficult to remove. To avoid their formation, inhibitors like methanol are injected in the lines, increasing the cost of operation. Hence there is considerable economical incentive to try to understand how hydrates form and how to inhibit their growth. To do this, one needs to understand both the thermodynamics and the kinetics of hydrate growth and dissolution. There are extensive data on hydrate thermodynamic properties, and the phase diagram is well known. However, there are much fewer kinetic data available. Some hydrates exhibit an induction period, i.e., even when conditions are favorable for their formation, some time elapses before the hydrate forms. In contrast, if the hydrate is melted and the conditions reestablished for hydrate formation, there is essentially no induction period and the hydrate will form almost immediately.2 This hysteresis effect suggests that the water remembers its past hydrate history, presumably through the existence of subcritical sized hydrate crystal nuclei in the fluid. A hypothesis has been proposed for the existence of the induction period and the memory but this has yet to be proved.
Turbulent boundary layer wall‐pressure fluctuations downstream from an abrupt change in surface roughness
Spectra of wall‐pressure fluctuations beneath a turbulent boundary layer have been investigated immediately downstream from an abrupt change in surface roughness, from slightly rough to very rough. Over the measurable frequency band (400 Hz to 7 kHz), the spectra in the first 150 mm scale on wall variables. This indicates that the motions responsible for the pressure fluctuations measured in this region are very close to the wall. Farther downstream this scaling procedure breaks down. It is suggested that the initially thin internal layer behaves independently of the flow external to it until it has grown appreciably and only then interacts with the outer boundary layer. It was found that, for all frequencies measured, the dimensional spectral level decreases initially then rises to a maximum at 250 mm from the roughness change before decreasing once more. The position of the maximum is close to where the internal layer ceases to have a distinct edge.