u. 



5. 



Assumption: 



Assumption: 



Assumption: 



Assumption: 7. 



Assumtion: 



Assumption: 3. Constant heat transfer coefficient over the face 



of the fin. Not valid, but a useful first approximation 

 in developing a fin cross section using the technique 

 of an analog computer based on resistive paper. 



Unifortz temperature in the surrounding fluid. Obviously 

 not strictly valid, but a useful approximation as in 

 Assumption 3. 



No temperature gradients occur along the length and 

 across the thickness of the fin. Invalid, but approx- 

 imately correct for very high conductivity in the 

 fins (copper, silver, etc.) and low surface coefficient. 



Temperature at the fin base is uniform, and there is 

 no contact resistance at the fin base. Valid, first 

 part approximately correct for high-conductivity 

 materials such as copper; second part valid for 

 most construction methods, including brazing, casting, 

 and soldering. 



There is no heat source within the fin itself. Valid 

 for solid fins. With internal ducting, as with heat 

 pipe channels, not valid. 



Heat transfer from the fin end and sides is negligible. 

 Valid for all but thickest and shortest (in direction 

 of water flow) fins. 



The usual industrial criterion for heat transfer surface optimization 

 is cost, and the methods outlined in the reference are based on cost 

 considerations. For the present design, the relatively low cost of the 

 heat rejection surfaces no natter how constructed appeared trivial 

 compared to the probable costs of the remainder of the RTG design. There- 

 fore, a design method based on the following criteria was adopted: 



(a) Within the physical constraints of size based en the number and 

 size of the pre-existing thermoelectric units, provide a minimum base 

 temperature at the cold junction of the thermoelectric units. 



(b) Provide as uniforn a base temperature as practicable, which 

 means a fairly heavy section of high-conductivity material. 



(c) Provide the necessary strength in longitudinal compression 

 for the intended overpressures. The thermoelectric units are fabricated 

 by an intense svagging process and can provide the necessary strength 

 in the radial direction. 



(d) Provide a maximum of protection against corrosion. 



(e) Provide- a maximum of protection against marine growth. 



(f) Fabricate by a method that will insure optimum quality control. 

 For the immediate demonstration experiment, fabricate to as nearly the 

 final design as practicable by a low-cost cethod. 



