ME3391 UNIT 3.pptx engineering thermodynamics

engineering thermodynamics

1. ME3391 ENGINEERING THERMODYNAMICS REGULATIONS-2021 YEAR/SEM:II/III UNIT III AVAILABILITY AND APPLICATIONS OF II LAW Prepared by. Dr.K.ARUMUGANAINAR,M.E.,Ph.D . ASSISTANT PROFESSOR, DEPARTMENT OF MECHANICAL ENGINEERING, JP COLLEGE OF ENGINEERING .

2. COURSE OBJECTIVES 1 Impart knowledge on the basics and application of zeroth and first law of thermodynamics. 2 Impart knowledge on the second law of thermodynamics in analysing the performance of thermal devices. 3 Impart knowledge on availability and applications of second law of thermodynamics 4 Teach the various properties of steam through steam tables and Mollier chart. 5 Impart knowledge on the macroscopic properties of ideal and real gases.

3. SYLLABUS UNIT III AVAILABILITY AND APPLICATIONS OF II LAW Ideal gases undergoing different processes - principle of increase in entropy. Applications of II Law. High and low-grade energy. Availability and Irreversibility for open and closed system processes - I and II law Efficiency

4. UNIT III AVAILABILITY AND APPLICATIONS OF II LAW Ideal gases undergoing different processes The basic processes with ideal gas are known as follows:  isothermal, isochoric, isobaric, isentropic, and polytropic . The isentropic process is similar to the polytropic one, however, with an exponent n equal to the adiabatic exponent . Principle of Increase In Entropy The second law of thermodynamics states that  the total entropy of a system either increases or remains constant in any spontaneous process; it never decreases . Thus the Increase in Entropy Principle states that  for any process the total change in entropy of a system together with its enclosing adiabatic surroundings is always greater than or equal to zero . This total change of entropy is denoted the Entropy Generated during the process ( S gen  [kJ/K] or s gen  [kJ/kg. K]).

5. UNIT III AVAILABILITY AND APPLICATIONS OF II LAW Applications of II Law The law states that heat always moves from a body that is warmer to a colder body. All heat engine cycles, including Otto, Diesel, etc., as well as all working fluids employed in the engines, are covered by this rule. Modern automobiles have advanced as a result of this law. Another illustration of how this idea is used is in reverse-cycle refrigerators and heat pumps. If we want to move heat from a body with a lower temperature to a body with a higher temperature, we must perform external work. The original Carnot cycle uses heat to create work, as opposed to the reversed Carnot cycle, which transfers heat from a lower temperature reservoir to a higher temperature reservoir using work.

6. UNIT III AVAILABILITY AND APPLICATIONS OF II LAW High and low-grade energy High grade is that form of energy which completely converts into some other forms like electrical energy which can be converted into thermal energy but low grade energy will not be able to convert into other forms completely like thermal energy used in thermal power plants to convert into electricity. That's why thermal power plants have lower efficiencies.

7. UNIT I BASICS, ZEROTH AND FIRST LAW Thermodynamic Properties In all thermodynamic problems energy transfer to or from the system is observed. To receive, store and deliver energy a working substance is present within the system. The characteristics which can be used to describe the condition of the system are known as properties . Thermodynamic properties are classified into two categories : intensive and extensive . Intensive properties are independent of quantity of matter or mass whereas extensive properties are dependent on mass. Consider a vessel containing air. If a membrane is assumed to be introduced into the vessel, such that it is divided into two equal parts. The properties remaining unchanged such as pressure and temperature are intensive properties. Volume of air will be reduced to half of its initial value. Hence, it is an extensive property.

8. UNIT I BASICS, ZEROTH AND FIRST LAW Processes and Cycles When a system is taken from one equilibrium state to another, the change is known as process. The series of intermediate states through which a system passes during a process is called the path of the process. If all these intermediate states are equilibrium states, the process is known as quasi equilibrium or quasi-static process. Thermodynamic State and Equilibrium When a system does not undergo any change, all the properties have fixed values. This condition is known as a thermodynamic state .

9. UNIT I BASICS, ZEROTH AND FIRST LAW The word equilibrium means balance. An equilibrium state of a thermodynamic system is a state that can not be changed without any interaction with its surroundings. The factors that cause a change without any interactions with its surroundings are: Pressure difference Temperature difference Chemical reaction There should not be any temperature difference within the system, so that the system is thermally balanced. No pressure difference exists between any two points within the system (Neglecting gravitational effects) and between the system and surroundings, so that it is mechanically balanced . No chemical reaction is taking place, so that it is chemically balanced.

10. UNIT I BASICS, ZEROTH AND FIRST LAW Hence, for a system in a state of thermodynamic equilibrium, there is no change in any macroscopic property. Displacement Work Consider a piston cylinder arrangement , If the pressure of the fluid is greater than that of the surroundings, there will be an unbalanced force on the face of the piston. Hence, the piston will move towards right.

11. UNIT I BASICS, ZEROTH AND FIRST LAW Force acting on the piston = Pressure X Area   = P X A   Work done = Force X distance   = PA X dx   = PdV where dv - change in volume. This work is known as displacement work or pdV work corresponding to the elemental displacement dx . To obtain the total work done in a process, this elemental work must be added from the initial state to the final state.  

12. UNIT I BASICS, ZEROTH AND FIRST LAW P-V DIAGRAM It is  a graph of pressure versus volume . It forms a Curve and the area under the curve is the work done (by the gas left to right, on the gas right to left). Explanation: Pressure and volume are inversely related, so as pressure increases, volume decreases, with infinite pressure and zero volume as limits .

13. UNIT I BASICS, ZEROTH AND FIRST LAW Thermal equilibrium Heat  is the flow of  energy  from a high  temperature  to a low temperature. When these temperatures balance out, heat stops flowing, then the system (or set of systems) is said to be in  thermal equilibrium . Zeroth Law

14. UNIT I BASICS, ZEROTH AND FIRST LAW Consider three bodies A, B and C. If the bodies A and B are in thermal equilibrium with C when brought into contact separately, they are also in thermal equilibrium with each other. This concept is known as zeroth law of thermodynamics. Concept of temperature and Temperature Scales Maxwell defined the temperature of a system as its Thermal state considered with reference to its ability to communicate heat to other bodies . Temperature Scales Freezing point of water known as ice point and boiling point of water known as steam point are taken as the reference states for all types of temperature scales.

15. UNIT I BASICS, ZEROTH AND FIRST LAW The various types as temperature scales in use are : Celsius scale Fahrenheit scale Kelvin scale First law “When energy is either transferred or transformed, the final total energy present in all forms must precisely equal the original total energy ”. Q=W+ U

16. UNIT I BASICS, ZEROTH AND FIRST LAW Model Problems on First Law of Thermodynamics

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22. UNIT I BASICS, ZEROTH AND FIRST LAW Problems on First law of Thermodynamics Application to closed system (Non flow Process) Prob.1 A mass of air is initially at 260 and 700kPa, and occupies 0.028m³. The air is expanded at constant pressure to 0.084m³. A polytrophic process with n=1.5 is then carried out followed by a constant temperature process which completes a cycle. All the processes are reversible. Sketch the cycle in T-S and P-V planes Find the heat received and heat rejected in the cycle. Find the efficiency of the cycle. May/June – 2016

23. UNIT I BASICS, ZEROTH AND FIRST LAW Given Data: T1 = 260°C  P1 = 700kPa = P2 V1 = 0.028 m³ V2 = 0.084 m³ Process 1-2 is constant pressure Process 2-3 is polytropic Process 3-1 is constant temperature To find: Sketch P-V and T-S diagram , Q and 

24. UNIT I BASICS, ZEROTH AND FIRST LAW Solution:

25. UNIT I BASICS, ZEROTH AND FIRST LAW Process 1-2: Constant pressure process T2 = 1599K Mass of air, m = = 0.128 kg Workdone , W 1-2 = p (V2-V1) = 39.2kJ Heat transfer, Q 1-2 = mCp (T1-T1) = 137.13kJ Process 2-3: For Polytropic process

26. UNIT I BASICS, ZEROTH AND FIRST LAW P3 = 25.93kPa From PV = mRT V3 = = 0.755 m 3 Polytropic work, W 2-3 = = 78.446 kJ Heat transfer, Q 2-3 = x W2-3 = -19.612 kJ Process 3-1 T1 = T3 = 260ºC= 533 K for constant temperature process Work transfer, W 3-1= -P3V3ln

27. UNIT I BASICS, ZEROTH AND FIRST LAW W = -64.52 kJ (work input) Heat received in this cycle, Qs = 137.13 kJ (consider only + ve heat) Heat rejected in this cycle, QR = 84.132 kJ (consider only – ve heat) Efficiency of the cycle Efficiency of the cycle , = 38.74%

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