ABSTRACT
Objectives ◾ Physical meaning of Second Law for a cyclic process ◾ Statements of Second Law ◾ Introduction of entropy (S) a, entropy generation (σ), and entropy balance equations for a
closed and open systems ◾ Methods of evaluation of “s” and use of tables ◾ Use of Gauss divergence theorem in obtaining differential forms of entropy balance
equation ◾ Maximum entropy (Smax) and minimum energy (Umin, Hmin, Gmin, Amin) principles
3.1 Introduction Œe First Law of Œermodynamics does not limit the degree of conversion of cyclical heat input into cyclical work output, as discussed in Chapter 2. Œe Second Law establishes this limit and, for instance, it prevents heat engines from converting their entire heat input into work. Consider the example of a car. A gallon of gasoline releases 120,000 kJ of thermal energy. If the work transfer to the wheels of the car is only 40,000 kJ then the remaining 80,000 kJ must be accounted for. Assume that the heat loss from the automobile radiator accounts for 40,000 kJ while the exhaust accounts for 40,000 KJ. Œe ratio between work and heat (40,000/120,000) is called the efficiency η. For the above example η = energy sought/energy bought = 1/3. In no engines is all of the heat absorbed converted into work (i.e., η ≠ 1). A typical vehicle requires a heat input of 7400 kJ per mile with gasoline and 5800 kJ/mile with diesel. Œus a diesel cycle seems to have a higher efficiency compared to the gasoline cycle. An upper limit on the efficiency can, however, be obtained by applying the Second Law.
