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Tham khảo tài liệu 'a heat transfer textbook - third edition episode 3 part 1', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 9.6 Transition boiling and system influences 489 or qmin 18 990 W m2 From Fig. 9.2 we read 20 000 W m2 which is the same within the accuracy of the graph. 9.6 Transition boiling and system influences Many system features influence the pool boiling behavior we have discussed thus far. These include forced convection subcooling gravity surface roughness and surface chemistry and the heater configuration among others. To understand one of the most serious of these the influence of surface roughness and surface chemistry we begin by thinking about transition boiling which is extremely sensitive to both. Surface condition and transition boiling Less is known about transition boiling than about any other mode of boiling. Data are limited and there is no comprehensive body of theory. The first systematic sets of accurate measurements of transition boiling were reported by Berenson 9.30 in 1960. Figure 9.14 shows two sets of his data. The upper set of curves shows the typical influence of surface chemistry on transition boiling. It makes it clear that a change in the surface chemistry has little effect on the boiling curve except in the transition boiling region and the low heat flux film boiling region. The oxidation of the surface has the effect of changing the contact angle dramatically making it far easier for the liquid to wet the surface when it touches it. Transition boiling is more susceptible than any other mode to such a change. The bottom set of curves shows the influence of surface roughness on boiling. In this case nucleate boiling is far more susceptible to roughness than any other mode of boiling except perhaps the very lowest end of the film boiling range. That is because as roughness increases the number of active nucleation sites the heat transfer rises in accordance with the Yamagata relation eqn. 9.3 . It is important to recognize that neither roughness nor surface chemistry affects film boiling because the liquid does not touch the heater. Figure 9.14
