Evaporation forced convection boiling

Vohr, J. H., 1970, Evaporative Processes in Superheated Forced Convective Boiling, Rep. MTI-70TR15, p. 3, Mechanical Technology, Inc., Washington, DC. (2)... 

Forced convective boiling in channels. Here, evaporation of a liquid occurs in flow in a channel (for instance, a round tube). The vapor generated and the remaining liquid form a two-phase flow within the tube, and there are strong interactions between this two-phase flow (which can occur in a number of different forms) and the boiling process. 

General Empirical Correlations. The correlations described above were either fluid specific or related to correlations for forced convection evaporation and/or pool nucleate boiling, respectively. An alternative approach is to develop correlations based only on the data for forced convective boiling. The most widely used correlation of this form is that of Shah [265], who correlated data for convective flow boiling in both vertical and horizontal pipes in the form... 

The variety of regimes during the forced convection boiling in tubes or ducts requires different correlations in order to determine the heat transfer coefficient related to the respective boiling mechanisms. The well-established correlations have been developed for nucleate boiling controlled heat transfer - when evaporation occurs at the inner tube surface - and convective boiling heat transfer - when evaporation occurs at the liquid film interface. 

The primary mode of heat transfer at the wall is forced convection of the vapor phase. As the liquid does not wet the heating surface during film boiling, heat transfer due to drop-wall collisions is relatively small, resulting in low wall-drop heat transfer (only a few percent of the total heat input). Most of the droplet evaporation occurs because of vapor-drop heat transfer. Just after dryout, the... 

It was supposed, that for a high vapor velocity and a thin liquid film the influence of gravity is small and the correlation for up flow was used. Total boiling suppression occurs when mass quality more than 0.3 for a film thickness less than 60 pm. That value is close to the bubble departure diameter observed for flow boiling in a film. When the film thickness is smaller than the critical one, the forced convection occurs with a small heat transfer coefficient. The crisis of the heat transfer was observed for a complete liquid evaporation on a heated wall. While the mass quality less than 0.3, we have the cell or slug flow mode, so boiling is not suppressed. 

Number of researchers reports the liquid forced convection inside narrow channels is a most valuable form of heat removal from heat sinks [13-16]. However, the respective key issues are the maximum attainable heat flux by using liquid forced convection, and its value in comparison with the preeminent alternatives of boiling critical heat fluxes. We believe that maximum heat flux could be accomplished by using the inverted meniscus principle of evaporation coupled with excluding of vapor... 

Correlations for pure forced convection. At low heat fluxes and/or high mass fluxes, nucleate boiling heat transfer becomes negligible and the heat transfer is by conduction/con-vection from the wall to the interface where the evaporation is occurring. Such forced convective heat transfer represents an important limiting case. 

FIGURE 15.110 Contribution of nucleate boiling and forced convective in annular flow evaporation (from Sun et al. [272], with permission from Taylor Francis, Washington, DC. All rights reserved). 

At high enough qualities and mass fluxes, however, it would be expected that the nucleate boiling would be suppressed and the heat transfer would be by forced convection, analogous to that for the evaporation for pure fluids. Shock [282] considered heat and mass transfer in annular flow evaporation of ethanol water mixtures in a vertical tube. He obtained numerical solutions of the turbulent transport equations and carried out calculations with mass transfer resistance calculated in both phases and with mass transfer resistance omitted in one or both phases. The results for interfacial concentration as a function of distance are illustrated in Fig. 15.112. These results show that the liquid phase mass transfer resistance is likely to be small and that the main resistance is in the vapor phase. A similar conclusion was reached in recent work by Zhang et al. [283] these latter authors show that mass transfer effects would not have a large effect on forced convective evaporation, particularly if account is taken of the enhancement of the gas mass transfer coefficient as a result of interfacial waves. 

S. M. MacBain and A. E. Bergles, Heat Transfer and Pressure Drop Characteristics of Forced Convection Evaporation in Deep Spirally Fluted Tubing, in Convective Flow Boiling, J. C. Chen ed., pp. 143-148, Taylor Francis, Washington, DC, 1996. 

Convective heat exchange, natural or forced Radiant heat transfer, e.g. furnaces Evaporation, e.g. in evaporators Condensation, e.g. in shell and tube heat exchanges Heat transfer to boiling liquids, e.g. in vaporizers, boilers, re-boilers ... 

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