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.
Gas hydrates of argon and methane synthesized at high pressures: composition, thermal expansion, and self-preservation.
For the first time, the compositions of argon and methane high-pressure gas hydrates have been directly determined. The studied samples of the gas hydrates were prepared under high-pressure conditions and quenched at 77 K. The composition of the argon hydrate (structure H, stable at 460-770 MPa) was found to be Ar.(3.27 +/- 0.17)H(2)O. This result shows a good agreement with the refinement of the argon hydrate structure using neutron powder diffraction data and helps to rationalize the evolution of hydrate structures in the Ar-H(2)O system at high pressures. The quenched argon hydrate was found to dissociate in two steps. The first step (170-190 K) corresponds to a partial dissociation of the hydrate and the self-preservation of a residual part of the hydrate with an ice cover. Presumably, significant amounts of ice Ic form at this stage. The second step (210-230 K) corresponds to the dissociation of the residual part of the hydrate. The composition of the methane hydrate (cubic structure I, stable up to 620 MPa) was found to be CH(4).5.76H(2)O. Temperature dependence of the unit cell parameters for both hydrates has been also studied. Calculated from these results, the thermal expansivities for the structure H argon hydrate are alpha(a) = 76.6 K(-1) and alpha(c) = 77.4 K(-1) (in the 100-250 K temperature range) and for the cubic structure I methane hydrate are alpha(a) = 32.2 K(-1), alpha(a) = 53.0 K(-1), and alpha(a) = 73.5 K(-1) at 100, 150, and 200 K, respectively.
Effect of elastic bandaging and prosthesis on the decrease in stump volume.
OBJECTIVES
The aim of this study was to compare the effect of elastic bandaging, pneumatic prosthesis, and temporary prosthesis on postoperative stump management.


METHODS
This study included 14 patients, between 15 and 70 years old, whose causes of amputation were tumor, diabetes, Burger's disease, osteomyelitis, and traffic accident. Fifteen stumps in 14 patients, one of whom was bilateral, were evaluated. The elastic bandaging was applied for five stumps, the pneumatic prosthesis for five stumps, and temporary prosthesis for five stumps.


RESULTS
A decrease in stump volume was observed for at least 2 or 3 weeks after the use of all three methods in patients who came to the Prosthetics and Orthotics Laboratory at Ankara University. The volume decrease induced by temporary prosthesis fitting was more significant than that achieved using elastic bandaging or pneumatic prosthetics (p<0.01). The greatest volume changes were identified distally for the elastic bandage group, proximally for the pneumatic group, and medially for the temporarily applied prosthesis group. The temporary prosthesis group took less time to permanent prosthesis (39 days) and required less prosthesis training (p<0.01). Three amputees in the elastic bandaging group had postoperative contractures.


CONCLUSION
Use of a temporary prosthesis results in ideal shaping of the stump in the shortest period of time. This approach also shortens the period of transition to permanent prosthesis, improves the quality of life of the amputee, and re-integrates the patient to normal life over a shorter period of time.