Principles of naval engineering - Historic Naval Ships Association

Chapter 8- INTRODUCTION TO THERMODYNAMICSdesirable. We will consider first a classificationof processes according to the type of flow andthen consider a classification according to thetype of state change. Discussion of processes as"reversible" or "irreversible" isreservedfora later section.Type of FlowWhen classified according to type of flow ofthe working fluid, thermodynamic processes maybe considered under the general headings of (1)non-flow processes, and (2) steady-flow processes.A non-flow process is one in which the workingfluid does not flow into or out of its containerin the course of the process. The same moleculesof the working fluid that were present at the beginningof the process are therefore present atthe end of the process. Non-flow processes occurin reciprocating steam engines, air compressors,internal combustion engines, and otherkinds of machinery. Since apiston-and-cylinderarrangement is typical of most non-flow processes,let us examine a non-flow process suchas might occur in the cylinder shown in figure8-7.Suppose that we move the cylinder fromposition 1 to position 2, thereby compressingthe fluid contained in the cylinder above thepiston. Suppose, further, that we imagine thisto be a completely ideal process, and one whichis thus entirely without friction. The aspectsof this process that we might want to know aboutare (1) the heat added or removed in the courseof the process; (2) the work done on the workingfluid or by the working fluid; and (3) the netchange in the internal energy of the workingsubstance.From the general energy equation, we knowthat energy in must equal energy out. For thenon-flow process, the general energy equationmay be written aswhereQl2 = (^2 - Uj) +Wk12BtuQl2 " total heat transferred, in Btu (positive ifheat is added during process, negative ifheat is removed during process)Ui = total internal energy, in Btu, at state 1U2 = total internal energy, in Btu, at state 2Ug — U. = net change in internal energy fromstate 1 to state 2W.C 12 = work done between state 1 and state 2,in ft-lb (positive if work is done by theworking substance, negative if work isdone on the working substance)'•..• .•: ;--^WORKINGSUBSTANCE&,}^&(BEFORE •^2S'^'- COMPRESSION) '3-\'m^i'ri.^>i^'j WORKING ^*>i>f4« SUBSTANCE liffi|M (AFTER ^J,.!!*-.COMPRESSION) RSJ = the mechanical equivalent of heat, 778 ft-lbper BtuWk12= total work done by or on the working"^substance, in Btu (positive if work isdone by the substance, negative ifwork is done on the substance)©©147.16.0Figure 8-7.— Piston-and-cylinder arrangementfor non-flow process.This equation deals with total heat, totalwork, and total internal energy. K it is moreconvenient to make calculations in terms of 1pound of the working substance, we would writethe equation as42wk12(U2 - Uj )Btu per lbwhere the value of J remains the same andwhere q, u, and wk have the general meanings175