Phase change, heat transfer mechanisms

In the vaporization process, the formation of a continuous vapor phase causes a transition from bubble to annular flow. This flow-pattern transition is accompanied by a gradual change in the heat-transfer mechanism. Both... 

The analysis of tubular contactors for heat transfer with phase changes in fluid-fluid systems was shown to be heavily dependent on a proper understanding of two-phase hydrodynamics. It was shown that three basic flow patterns exist within a tube, each with a different heat-transfer mechanism. The formulation of the proper mass and energy models pinpointed three key... 

Nonisothermal systems are accounted for by the introduction of temperature-control units into the generic reactor unit representation. These units consist of elements associated with the manipulation of temperature changes and constitute temperature profiles (profile-based approach) and heaters/coolers (unit-based approach). The assumption of thermal equilibrium between the contacting phases reduces the need for a single temperature per shadow reactor compartment. The profile-based system (PBS) finds the optimum profiles without considering the details of heat transfer mechanisms. Because the profiles are imposed rather than... 

The coupled heat and liquid moisture transport of nano-porous material has wide industrial applications in textile engineering and functional design of apparel products. Heat transfer mechanisms in nano-porous textiles include conduction by the solid material of fibers, conduction by intervening air, radiation, and convection. Meanwhile, liquid and moisture transfer mechanisms include vapor diffusion in the void space and moisture sorption by the fiber, evaporation, and capillary effects. Water vapor moves through textiles as a result of water vapor concentration differences. Fibers absorb water vapor due to their internal chemical compositions and structures. The flow of liquid moisture through the textiles is caused by flber-liquid molecular attraction at the surface of fiber materials, which is determined mainly by surface tension and effective capillary pore distribution and pathways. Evaporation and/or condensation take place, depending on the temperature and moisture distributions. The heat transfer process is coupled with the moisture transfer processes with phase changes such as moisture sorption and evaporation. 

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. 

Figure 7 shows the process heat loop for transporting heat from the cold side of the PCU boiler to the hot side of the HTE boiler. This loop operates as a heat pump transferring heat from a cold source to a hot sink. Heat transfer with phase change is used at both the source and sink to achieve high heat fluxes and, hence, compact heat exchangers. An important question is the mechanical power consumption of the compressor and how it compares to the overall heat transfer rate. The compressor serves to... 

As important as kinetic mechanism are the phase changes that occur in polymerization. Only a small fraction of polymerizations are carried out only in one phase thus thermodynamics, heat and mass transfer, and the kinetics of the phase change itself all play a role in determining the properties of the product polymer. Table IV indicates the principal types of kinetic mechanisms and reaction media which arise in polymerization reactors. Each of these classes of systems has its own peculiar problems so that polymerization reactor design can often be much more challenging than the design of reactors for short chain molecules. 

Carey, V. P. (1992) Liquid-Vapor Phase-Change Phenomena An Introduction to the Thermophysics of Vaporization and Condensation Processes in Heat Transfer Equipment, in Series in Chemical and Mechanical Engineering, G.F. Hewitt and C.L. Tien, Editors, Taylor Francis Series, Hemisphere Pub. Corp., Washington, D.C., p. xvii, 645. 

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