流体力学与 及传热课件Heat Transfer to Fluids with Phase Change.pptVIP

4.5 Heat Transfer to Fluids with Phase Change;A phase change involves the addition or subtraction of considerable quantities of heat at constant or nearly constant temperature. ;The rate of phase change may be governed by the rate of heat transfer, but it is often influenced by the rate of nucleation of bubbles, drops, or crystals and by the behavior of the new phase after it is formed. ;The condensing vapor may consist of a single substance, a mixture of condensable and noncondensable substances, or a mixture of two or more condensable vapors. ; Friction losses in a condenser are normally small, so that condensation is essentially a constant-pressure process. ;Dropwise and film-type condensation; In film condensation the liquid condensate forms a film of liquid that flows over the surface of the tube under the action of gravity. ; It is the layer of liquid interposed between the vapor and the wall of the tube which provides the resistance to heat flow and therefore which fixes the value of the heat-transfer coefficient.; In dropwise condensation the condensate begins to form at microscopic nucleation sites. ;The drops grow and coalesce with their neighbors to form visible fine drops.; Because of this the heat-transfer coefficient at these areas is very high; the average coefficient for dropwise condensation may be 5 to 8 times that for film-type condensation. ; The average coefficient obtainable in pure dropwise condensation is as high as 114kW/m2.oC. ; Although attempts are sometimes made to realize practical benefits from these large coefficients by artificially inducing dropwise condensation.; Also the resistance of the layer of condensate even in film-type condensation is ordinarily small in comparison with the resistance inside the condenser tube, and increase in the overall coefficient is relatively small when dropwise condensation is achieved.;Coefficients for film-type condensation; Two forces remaining acting on the control volume ar