Phase-change mechanism

Kolobov, A. V. Fons, P. Frenkel, A. I. Ankudinov, A. L. Tominaga, J. Uruga, T. 2004. Understanding the phase-change mechanism of rewritable optical media. Nature Mater. 3 703-708. 

A buffer of compacted bentonite is planned to be used to prevent the movement of groundwater and the consequential escape of material from a geological repository for spent nuclear fuel. Fluid flow, phase changes, mechanical behaviour of the buffer, rock, and the waste canisters, and the heat produced by the waste constitute a coupled thermohydromechanical system. The aim of the study is to derive a general thermodynamically consistent THM model for an arbitrary mixture. The general theory is applied to the thermohydraulic modelling of a mixture of compacted bentonite, liquid water, vapour, and air. 

The same computational procedure and algorithm are used as in the single-phase model (section 3). Due to the complexity of this model with a large spatial variation in competing transport and phase-change mechanisms, the computational time required was about three times greater than in single-phase. 

Prior to proceeding with a detailed analysis of the results, it is useful to discuss some of the phase-change mechanisms. The central parameter for determining the direction of phase change is the relative humidity of the gas phase ... 

The model is shown to be able to (1) understand the many interacting, complex electrochemical, phase change mechanism, and transport phenomena that cannot be studied experimentally (2) identify limiting steps and components and (3) provide a computer-aided tool for design and optimize future fuel cell with much higher power density and lower cost. Finally, this chapter of "PEM Fuel Cell Modeling" looks at how engineers can model PEM fuel cells to get optimal results for any application. 

The electron-sea model of metallic bonding pictures metal atom cores aligned in a "sea" of delocalized electrons. The model accounts for phase changes, mechanical behavior, and electrical and thermal conductivity of metals. 

There are two mechanisms by which a phase change on the ground-state surface can take place. One, the orbital overlap mechanism, was extensively discussed by both MO [55] and VB [47] formulations, and involves the creation of a negative overlap between two adjacent atomic orbitals during the reaction (or an odd number of negative overlaps). This case was temied a phase dislocation by other workers [43,45,46]. A reaction in which this happens is... 

Heat transfer and mass transfer occur simultaneously whenever a transfer operation involves a change in phase or a chemical reaction. Of these two situations, only the first is considered herein because in reacting systems the complications of chemical reaction mechanisms and pathways are usually primary (see HeaT-EXCHANGETECHNOLOGy). Even in processes involving phase changes, design is frequendy based on the heat-transfer process alone mass transfer is presumed to add no compHcations. But in fact mass transfer effects do influence and can even limit the process rate. 

Advantages to Membrane Separation This subsertion covers the commercially important membrane applications. AU except electrodialysis are pressure driven. All except pervaporation involve no phase change. All tend to be inherently low-energy consumers in the-oiy if not in practice. They operate by a different mechanism than do other separation methods, so they have a unique profile of strengths and weaknesses. In some cases they provide unusual sharpness of separation, but in most cases they perform a separation at lower cost, provide more valuable products, and do so with fewer undesirable side effects than older separations methods. The membrane interposes a new phase between feed and product. It controls the transfer of mass between feed and product. It is a kinetic, not an equihbrium process. In a separation, a membrane will be selective because it passes some components much more rapidly than others. Many membranes are veiy selective. Membrane separations are often simpler than the alternatives. 

Phase changes also mean that pressure and or vacuum relief needs to be considered in order to maintain the mechanical integrity of the container. 

Some materials undergo transitions from one crystal structure to another as a function of temperature and pressure. Sets of Raman spectra, collected at various temperatures or pressures through the transition often provide useftil information on the mechanism of the phase change first or second order, order/disorder, soft mode, etc. 

Kawahara A, Chung PM, Kawaji M (2002) Investigation of two-phase flow pattern, void fraction and pressure drop in a micro-channel. Int J Multiphase Plow 28 1411-1435 Kawaji M (1999) Fluid mechanics aspects of two-phase flow Flow in other geometries. In Kand-likar SG, Shoji M, Dhir VK (eds) Handbook of phase change boiling and condensation. Taylor and Francis, Washington, DC, pp 205-259... 

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