ESCI SJR: 0.206 SNIP: 0.446 CiteScore™: 0.5
ISSN 打印: 2152-5102
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Fluid Dynamics and Heat Transfer in Annular Two-Phase Flow
Geoffrey F. Hewitt
Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK
The processes of evaporative heat transfer in annular flow are introduced and the associated hydrodynamic phenomena described. Predictions of evaporating, vertical annular flow are presented and compared with equivalent predictions for adiabatic equilibrium and condensing flows. The importance of departures from hydrodynamic equilibrium is demonstrated and discussed. A brief discussion is then given of annular flow in horizontal channels where the dynamic difference to vertical flow is that of the mechanism for maintaining the upper part of the tube wetted by the liquid phase. It is shown that the more likely mechanism for maintaining the liquid around the tube periphery is that of wave-induced transport. The mechanisms of heat transfer in annular flow are then discussed and the conventional picture of combined nucleate boiling and forced convection introduced. An attempt at rational prediction of the heat transfer coefficient behavior is described and shown to have inadequacies which are likely to arise from phenomena not accounted for in the conventional view. An experimental investigation is described in which evaporation and condensation are compared under equilibrium conditions. This shows that, in what is conventionally regarded as forced convection, evaporative heat transfer coefficients are considerably higher than those for condensation. This suggests that bubble evaporation is occurring in addition to convective heat transfer across the liquid film, probably associated with the occurrence of the large disturbance waves on the surface which have been shown to cause bubble entrainment, thus producing centers for bubble growth in the superheated liquid film. It is concluded that the conventional view of forced convective annular flow evaporation needs modification.
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