E,8 • GENERAL RESULTS 



transfer increases sharply with the wall temperature until a maximum is 

 reached at point C. A decrease in heat transfer occurs with further in- 

 crease in wall temperature and only at very high wall temperatures does 

 this trend reverse. If electrical heating is used, the part from C to D is 

 unstable and special care has to be taken to obtain measurements in this 

 region [64\. If the heat transfer rate is increased beyond the value at C, 

 the wall temperature has to jump to a value indicated by point F. Since, 

 in many cases, this temperature is higher than the melting point of the 

 heating surface, failure of the heater may occur if the heat transfer at C 

 is exceeded. 



Point C is therefore generally 

 called the "burnout" point. This 

 name is somewhat misleading since, 

 depending on the fluid and the 

 material of the heater, physical 

 destruction does not necessarily 

 occur. The existence of a maximum 

 in the heat transfer curve, however, 

 is a significant engineering charac- 

 teristic of boiling heat transfer, since 

 an abrupt temperature increase oc- 

 curs if an attempt is made to trans- 

 fer an amount of heat greater than 

 that indicated by this point. The 

 determination of the burnout point 

 is therefore important for the design 

 of heat transfer equipment which is 

 to operate in the boiling range. An 

 exact knowledge of the shape of the 

 curve from 5 to C is often not re- 



Nucleate boiling 



Partial film boiling 



,<^ 



MM/ ' Mm y i 



(2) 



Film boiling 



(3) 



Fig. E,8b. Schematic representation of 

 typical bubble formations. 



quired because of the relatively small change in wall temperature which 

 corresponds to this range. 



Visual examination \65fil,6Ji\ of the processes at the heating surface 

 shows that, at point B, small bubbles appear at the surface (see Fig. 

 E,8b (1)). These bubbles may grow and collapse without ever leaving the 

 surface or they may leave the surface, depending on the conditions of the 

 bulk fluid. As the temperature of the surface is increased, more and more 

 bubbles appear. When it is increased beyond C, the bubbles become so 

 numerous that several of them will merge into a larger vapor mass which 

 may adhere to the surface for some time. Eventually it will detach and float 

 into the bulk fluid (see Fig. E,8b (2)). Progressively more of the larger 

 vapor masses are formed as the temperature is increased, until at point D 

 a rather stable continuous vapor film is formed, which covers the entire 

 heating surface (see Fig. E,8b (3)). The heat transfer mechanism corre- 



< 315 ) 



