Chapter 8- INTRODUCTION TO THERMODYNAMICS 



desirable. We will consider first a classification 

 of processes according to the type of flow and 

 then consider a classification according to the 

 type of state change. Discussion of processes as 

 "reversible" or "irreversible" isreservedfor 

 a later section. 



Type of Flow 



When classified according to type of flow of 

 the working fluid, thermodynamic processes may 

 be considered under the general headings of (1) 

 non-flow processes, and (2) steady-flow proc- 

 esses. 



A non-flow process is one in which the work- 

 ing fluid does not flow into or out of its container 

 in the course of the process. The same molecules 

 of the working fluid that were present at the be- 

 ginning of the process are therefore present at 

 the end of the process. Non-flow processes occur 

 in reciprocating steam engines, air compres- 

 sors, internal combustion engines, and other 

 kinds of machinery. Since apiston-and-cylinder 

 arrangement is typical of most non-flow proc- 

 esses, let us examine a non-flow process such 

 as might occur in the cylinder shown in figure 

 8-7. 



Suppose that we move the cylinder from 

 position 1 to position 2, thereby compressing 

 the fluid contained in the cylinder above the 

 piston. Suppose, further, that we imagine this 

 to be a completely ideal process, and one which 



is thus entirely without friction. The aspects 

 of this process that we might want to know about 

 are (1) the heat added or removed in the course 

 of the process; (2) the work done on the working 

 fluid or by the working fluid; and (3) the net 

 change in the internal energy of the working 

 substance. 



From the general energy equation, we know 

 that energy in must equal energy out. For the 

 non-flow process, the general energy equation 

 may be written as 



Wk 



Ql2 = (^2 - Uj) + 



12 



Btu 



where 



Ql2 " total heat transferred, in Btu (positive if 

 heat is added during process, negative if 

 heat is removed during process) 



Ui = total internal energy, in Btu, at state 1 



U2 = total internal energy, in Btu, at state 2 



Ug — U. = net change in internal energy from 

 state 1 to state 2 



W.C 



12 = work done between state 1 and state 2, 

 in ft-lb (positive if work is done by the 

 working substance, negative if work is 

 done on the working substance) 



'•■..■• .■•:■ ;--^ 



WORKING &,} 



SUBSTANCE ^& 



(BEFORE •^2S 



'^'- COMPRESSION) '3- 







© 



^i'ri.^ 



>i^'j WORKING ^*>i 

 >f4« SUBSTANCE liffi 

 |M (AFTER ^ 

 J,.!!*-. COMPRESSION) RS 



\'m 



© 



147.16.0 

 Figure 8-7.— Piston-and-cylinder arrangement 

 for non-flow process. 



J = the mechanical equivalent of heat, 778 ft-lb 

 per Btu 



Wk 



12 



= total work done by or on the working 

 "^ substance, in Btu (positive if work is 



done by the substance, negative if 

 work is done on the substance) 



This equation deals with total heat, total 

 work, and total internal energy. K it is more 

 convenient to make calculations in terms of 1 

 pound of the working substance, we would write 

 the equation as 



wk 



42 



(U2 - Uj ) 



12 



Btu per lb 



where the value of J remains the same and 

 where q, u, and wk have the general meanings 



175 



