多相流基础 chapter -condensation.ppt

Chapter 11 —Condensation（第十一章－凝结） * 11.1 Modes of condensation Condensation can occur in two ways. First there could be filmwise condensation, as in Fig11.1(a)where the condensation forms a continuous film on the cold solid surface. The latent heat released on condensation is conducted through this film from the interface where the condensation occurs and is removed through the wall. The other mechanism is dropwise condensation, shown in Fig11.1(b). In this case the condensate forms in drops which do not form a continuous film; instead they reach a certain critical size and then run off the surface. The surface is left dry and another drop can begin to form, rather like the formation of bubbles in nucleate boiling from a nucleation site. Because in dropwise condensation the vapour is in direct contact with the solid surface, the condensation heat transfer coefficient are significantly larger than in filmwise condensation. However, dropwise condensation is difficult to promote reliably. Special surface coatings do give drop formation, but the effectiveness of such surface treatments decreases with time. The normal design practice is to assume that filmwise condensation takes place. 11.2 Filmwise condensation on a vertical surface 1. Hydrodynamics. As was done for flooding flow in Chapter 6, consider a force balance on the outer part of the film (see Fig11.2) This time, however, the gas density is taken into account. For steady flow, in which the streamlines in the liquid are vertical (11.1) And so a force balance in the vertical direction gives (11.2) Or simplifying using eqn.(11.1) (11.3) Then, putting the laminar flow value for the shear stress And so, using the boundary condition that when y=0, u=0 (11.4) (11.5) The mass flow rate M per unit width of film is then given by (11.6) And differentiating, the change in the mass flow rate can be related to the changes in the film thickness (11.7) 2. Heat transfer. As the vapor condenses, its specific enthal