70 Pages

Unit 4 Thermodynamics

The treatment of laws and principles presented in this chapter is aligned to applications in mechanical engineering.A key to the notation used is given at the end of the chapter. Like many other subjects, thermodynamics has its own language and some key terms and definitions with which to become familiar. These are set out in the following pages. The first and second laws of thermodynamics – two laws of unchallenged importance which stand at the centre of the subject – are introduced and the analysis of work and heat transfer, the properties of working fluids, and the efficiency of cycles producing power are described. The first recorded use of the term thermodynamics was by Lord Kelvin, in a publication in 1849. The term is derived from the Greek thermos meaning heat and dynamics meaning power. This was a time when more and efficient generation of mechanical power from thermal energy released by burning coal was a pressing need for industry and the thermodynamics of power generation remains an important topic today: the plant of power stations operates on a thermodynamic

cycle and thermodynamic machines – steam turbines – drive the alternators which generate the electrical power. More generally, thermodynamics allows the performance of engines to be understood and their efficiency to be analysed. This is required to design the internal combustion engines for cars and the jet engines for aircraft. Knowledge of thermodynamics is used to design the cooling systems for computers and to develop efficient heating systems for homes. How does a refrigerator cool a cold space in a hot room? To what temperature will a brake disc rise during braking? These questions have answers provided by thermodynamics.