Lecture 3: First and Second Laws of Thermodynamics and Their Applications

1. Lecture 3 First and Second Law of Thermodynamics

2. Inserting expression for flow work and regrouping terms For large processes provided all inlet/outlet conditions are steady (not changing with time) integrate both sides For rate processes dividing both sides by t and letting t0 First Law of Thermodynamics FOR OPEN SYSTEMS Recall enthalpy defn.: h=u+pv

3. Conservation of mass and energy for a steady flow process Conservation of mass Conservation of energy

4. Applications Nozzles and diffusers (e.g. jet propulsion) Turbines (e.g. power plant, turbofan/turbojet aircraft engine), compressors and pumps (power plant) Heat exchangers (e.g. boilers and condensers in power plants, evaporator and condenser in refrigeration, food and chemical processing) Mixing chambers (power plants) Throttling devices (e.g. refrigeration, steam quality measurement in power plants)

5. Applications in pictures Heat exchangers Throttling devices Source: internet

6. nozzles/diffusers Single stream

7. turbines Usually

8. compressors Usually

9. heat exchangers hot (h) cold (c) Take CV enclosing the stream that is hot at inlet Take CV enclosing the stream that is cold at inlet

10. Mixing chambers or “direct contact heat exchangers” 3 2 1 Conservation of mass Conservation of energy

11. Principles of Thermodynamics

12. Thermal efficiency, where, W = Net work transfer from the engine, and Q1 = Heat transfer to engine. Q2 = Heat transfer from cold reservoir,

13. Clausius Statement “It is impossible for a self acting machine working in a cyclic process unaided by any external agency, to convey heat from a body at a lower temperature to a body at a higher temperature”. In other words, heat of, itself, cannot flow from a colder to a hotter body

14. Kelvin-Planck Statement “It is impossible to construct an engine, which while operating in a cycle produces no other effect except to extract heat from a single reservoir and do equivalent amount of work”.

16. Why does Q flow from hot to cold? • Consider two systems, one with TA and one with TB • Allow Q > 0 to flow from TA to TB • Entropy changed by:DS = Q/TB - Q/TA • If TA > TB, then DS > 0 • System will achieve more randomness by exchanging heat until TB = TA

17. Qhot engine W Qcold Efficiencies of Engines • Consider a cycle described by:W, work done by engine • Qhot, heat that flows into engine from source at Thot • Qcold, heat exhausted from engine at lower temperature, Tcold • Efficiency is defined: Since ,

18. Carnot Engines • Idealized engine • Most efficient possible

19. Application of 2nd law isothermal expansion Carnot Engine 2T engine TA Q12 W12 adiabatic expansion adiabatic compression 1-2 4-1 2-3 W41 W23 3-4 isothermal compression W34 Q34 TB

20. Carnot Cycle

21. Efficiency of a Carnot engine apply 1st law for this cycle: then energy conversion efficiency is: for a reversible process:

22. Qhot engine W Qcold Refrigerators Given: Refrigerated region is at Tcold Heat exhausted to region with Thot Find: Efficiency Since , Note: Highest efficiency for small T differences

23. Qhot engine W Qcold Heat Pumps Given: Inside is at Thot Outside is at Tcold Find: Efficiency Since , Like Refrigerator: Highest efficiency for small DT

24. Entropy • Total Entropy always rises! (2nd Law of Thermodynamics) • Adding heat raises entropy Defines temperature in Kelvin!