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Tham khảo tài liệu 'a heat transfer textbook - third edition episode 2 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 464 Heat transfer in boiling and other phase-change configurations 9.2 Figure 9.4 Enlarged sketch of a typical metal surface. 9.2 Nucleate boiling Inception of boiling Figure 9.4 shows a highly enlarged sketch of a heater surface. Most metalfinishing operations score tiny grooves on the surface but they also typically involve some chattering or bouncing action which hammers small holes into the surface. When a surface is wetted liquid is prevented by surface tension from entering these holes so small gas or vapor pockets are formed. These little pockets are the sites at which bubble nucleation occurs. To see why vapor pockets serve as nucleation sites consider Fig. 9.5. Here we see the problem in highly idealized form. Suppose that a spherical bubble of pure saturated steam is at equilibrium with an infinite superheated liquid. To determine the size of such a bubble we impose the conditions of mechanical and thermal equilibrium. The bubble will be in mechanical equilibrium when the pressure difference between the inside and the outside of the bubble is balanced by the forces of surface tension Ơ as indicated in the cutaway sketch in Fig. 9.5. Since thermal equilibrium requires that the temperature must be the same inside and outside the bubble and since the vapor inside must be saturated at Tsup because it is in contact with its liquid the force balance takes the form psat at Tsup pambient 9.1 The p-v diagram in Fig. 9.5 shows the state points of the internal vapor and external liquid for a bubble at equilibrium. Notice that the external liquid is superheated to Tsup - Tsat K above its boiling point at the ambient pressure but the vapor inside being held at just the right elevated pressure by surface tension is just saturated. 9.2 Nucleate boiling 465 Figure 9.5 The conditions required for simultaneous mechanical and thermal equilibrium of a vapor bubble. Physical Digression 9.1 The surface tension of water in contact with its vapor is given with great accuracy by
