E • CONVECTIVE HEAT TRANSFER AND FRICTION 



submerged [64]. Instead of a strip a wire may also be used [65], although 

 the curvature of the heating surface must then be counted as one of 

 the variables. Boihng heat transfer with forced convection may be 

 studied by pumping the test fluid through an electrically heated section 

 of metal tubing [66]. If visual observations are to be made, the fluid can, 

 for example, be made to flow in an annulus formed by an inner heating 

 tube and an outer Lucite tube [64], or an axial heating strip may be en- 

 closed in a Lucite duct [67]. Instead of electrical heating, other heating 

 methods can be used, and some experiments have been performed using a 

 condensing vapor as a heat source. Most workers in the field have pre- 

 ferred electrical heating because of its ease of control and its flexibiUty. 

 Many investigators have also found it necessary to study the vapor for- 

 mation near the heating surface in detail, making use of high speed 

 photographs for this purpose. Frame speeds up to 20,000 frames per 

 second were needed in some of these investigations [67]. 



E,8. General Results. Typical results obtained from boihng heat 

 transfer experiments are shown in a graph of heat transfer rate per unit 



Wall temperature, °F 



Fig. E,8a. Typical curve for boiling heat transfer. 



area vs. wall temperature (Fig. E,8a). The same type of curve is ob- 

 tained for forced or free convection conditions, with or without subcool- 

 ing. The first part of the curve {A to B) corresponds to the usual con- 

 vection conditions without boihng. When the wall temperature reaches a 

 certain value (somewhat above the boihng point of the hquid) the heat 



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