King Fahd University of Petroleum & Minerals

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1 King Fahd University of Petroleum & Minerals Mechanical Engineering Thermodynamics ME 04 BY Dr. Haitham Bahaidarah My Office Office Hours: :00 0:00 am SMW 03:00 04:00 pm UT Location: Building Room # 5.4 Phone haithamb@kfupm.edu.sa ١

2 Outline Textbook Catalog Description Grading system Homework Attendance Exams What is thermodynamics Topics to be covered during the course Application Areas of Thermal-Fluid Sciences Grading System 0% 0% 0% 0% 30% Homework / Design Project Quizzes/ Class tests Exam I Exam II Final Exam (For unexcused absence, 0.5 mark will be deducted) ٢

3 Homework Homework problems are 4-6 problems every week. All homework problems assigned during a given week are due in class one week later unless stated otherwise. Late Homework will NOT be accepted Attendance. Attendance will be checked during each lecture.. Excuse should be authorized by the Deanship of Student Affairs and submitted one week later after resumption of class attendance. 3. For any unexcused absence, 0.5 marks will be deducted from the total grade. 4. Any student having more then 9 unexcused absences will receive a grade of DN for the course. ٣

4 Review Basic Review of Thermodynamics I ٤

5 Closed Systems vs. Open Systems No mass can cross its boundary But energy can. Both mass and energy can cross the boundaries of a control volume. some common steady flow devices Only one in and one out More than one inlet and exit ٥

6 T 5 Phase Change Processes on a T-v diagram 3 4 v Phase Change at higher P on a T-v diagram Repeat this experiment for higher pressures. Similar curves will be obtained but at higher sat. temperature. Note that the sat. liquid specific volume (v sat,f ) will increase while the sat. vapor specific volume (v sat,g ) will decrease ٦

7 The phase dome on a T-v diagram The saturated liquid states can be connected by a line called the saturated liquid line. The saturated vapor states can be connected by another line, called the saturated vapor line, to form a phase dome. Three main regions can be identified. Phase Change Processes on a P-v diagram Decrease P gradually but keep T constant. The pressure at which a pure substance changes phase is called the saturation pressure P sat. At P sat, Liquid and vapor phases are in equilibrium From State to 4, no weights are removed (Pconstant) and T is kept constant but heating causes liquid to vaporize. ν ٧

A-4 Saturated liquid-vapor mixture falls under the P-v (or T-v)

8 Thermodynamics Tables Table A7 Compresse Liquid Table A6 Superheated water (or vapor) Saturated Liquidvapor mixture Table A-4 Saturated liquid-vapor mixture falls under the P-v (or T-v) dome. Its properties can be obtained from Water Tables A-4 and A-5 P const. ٨

9 Saturated Liquidvapor mixture Table A-5 In Table A-5 (page 83), Pressure is listed in the left column as the independent variable. Use whichever table is convenient. Superheated Vapor Table A-6 Tconst. In the region to the right of the saturated vapor line, a substance exists as superheated vapor. ٩

Quality (x), derivation V V + V f g mv m v + m v f f g g mv ( m m ) v + m v

10 Compressed liquid Table A-7 In the region to the left of the saturated liquid line, a substance exists as compressed liquid. Quality (x), derivation V V + V f g mv m v + m v f f g g mv ( m m ) v + m v g f g g v ( x) v + xv f g v v + x( v v ) v v + xv f f g f fg υ f υ < υ < f υ g υg υ where v v v fg g f ١٠

11 Linear Interpolation A 00 B 5 30 y y m General Equations The general mass balance equation m m m m in exit system The general energy balance equation V i V e Q W + mi hi + + gz i me he + + gz e m u mu + KE + PE ١١

12 Case Q W Case Room at 5 C Q L Q H Characteristics of Heat Engines.. They receive heat from high-temperature source. They convert part of this heat to work. They reject the remaining waste heat to a low-temperature sink. They operate on (a thermodynamic) cycle. High-temperature Reservoir at T H Q H Q L HE Low-temperature Reservoir at T L W ١٢

13 Thermal efficiency Performanc e Thermal Efficiency η th W Q net,out in Desired output Required input Q Q in Q in out Q Q out in < 00 % Refrigerators High-temperature Reservoir at T H Q H Q L Q H -W Ref W Q L Low-temperature Reservoir at T L Objective ١٣

Heat Pumps Objective High-temperature Reservoir at T H Q H Q H W + Q L HP W Read to parts of pp 59 and 60 Q L Low-temperature Reservoir at T L Coefficient of Performance of a

14 Heat Pumps Objective High-temperature Reservoir at T H Q H Q H W + Q L HP W Read to parts of pp 59 and 60 Q L Low-temperature Reservoir at T L Coefficient of Performance of a Refrigerator The efficiency of a refrigerator is expressed in term of the coefficient of performance (COP R ). COP R Q Q W Q Q Desired output Required input L L net, in H L Q H Q L ١٤

15 Coefficient of Performance of a Heat Pump The efficiency of a heat pump is expressed in term of the coefficient of performance (COP HP ). COP HP Desired output Required input QH QH W Q net, in QH QL Q L H A reversible process is defined as a process that can be reversed without leaving any trace on either system or surroundings. Reversible processes Ideal processes Irreversible processes Actual processes ١٥

Heat transfer process and finite temperature difference process. For a Heat transfer process to be revisable process it has to be an Isothermal process. Q 0 T 0.

16 Heat transfer process and finite temperature difference process. For a Heat transfer process to be revisable process it has to be an Isothermal process. Q 0 T 0. For a finite temperature difference process to be revisable process it has to be an adiabatic process. T 0 Q 0 Carnot cycle can be executed in many different ways ١٦

17 Reversed Carnot Cycle How do Reversible Carnot Heat Engine compare with real engines? ηthermal ηth < η η η th > η th,rev th,rev th,rev irreversible heat engine reversible heat engine impossible heat engine ١٧

18 How do Carnot Refrigerator compare with real Refrigerator? COP of Refrigerator COP R QH Q COP R L < COP COP > COP R,rev R,rev R,rev COP of Carnot Refrigerator COP R,rev, TH T L irreversible refrigerator reversible refrigerator impossible refrigerator How do Carnot Heat Pump compare with real one? COP of real Heat Pump COP HP COP Q Q HP L H < COP COP > COP HP,rev HP,rev HP,rev COP of Carnot Heat Pump COP HP,rev T T irreversible Heat Pump reversible Heat Pump impossible Heat Pump L H ١٨

19 Derivation of Clausius Inequality δq ر T Heat Engine δq 0 T Reversible ٠ Irreversible < 0 Refrigeration ٠ < 0 The equality in the Clausius inequality holds for totally or just internally reversible cycles and the inequality for the irreversible ones. The Entropy Balance (closed system) There is some entropy generated during an irreversible process such that δq S S + T S gen Entropy change Entropy transfer with heat Entropy generation due to irreversibility This is the entropy balance for a closed system. ١٩

Summary of the increase of entropy principle > 0 S gen 0 < 0 irreversible reversible imposible process process process Let us now have an example on this concept.

We can make it adiabatic by extending the surrounding until no heat, mass, or work are crossing the boundary of the surrounding.

20 Summary of the increase of entropy principle > 0 S gen 0 < 0 irreversible reversible imposible process process process Let us now have an example on this concept. The increase of entropy principle Now suppose the system is not adiabatic. We can make it adiabatic by extending the surrounding until no heat, mass, or work are crossing the boundary of the surrounding. This way, the system and its surroundings can be viewed again as an isolated system. The entropy change of an isolated system is the sum of the entropy changes of its components (the system and its surroundings), and is never less than zero. Now, let us apply the entropy balance for an isolated system: Sgen Stotal Ssys + Ssurr 0 ٢٠

s during a process. Let us see some of them.

21 Thus Qint, rev TdS This area has no meaning for irreversible processes! It can be done only for a reversible process for which you know the relationship between T and s during a process. Let us see some of them. T-s Diagram for the Carnot Cycle Isothermal expansion Temperature Isentropic compression WQ Q in -Q out in Isentropic expansion 4 Isothermal compression Q out 3 Entropy ٢١

22 The T-ds relations: Gibbs equations or Tds relationship A- Entropy Change of Liquids and Solids Tds du + Pdv Tds dh vdp s C ln T T B- The Entropy Change of Ideal Gases B-Constant specific heats T v s Cv ln + R ln T v T P s C p ln R ln T P B-Variable specific heats s s s R ln P 0 0 P C-The Isentropic processes of Ideal Gases C-Constant specific heats k Pv constant k Tv constant TP k k constant C-Variable specific heats v v v v r r P P P P r r S in Entropy balance for Open Systems Total Total Total Change in the Entropy Entropy + Entropy total entropy In Out Generated of the system S out + S gen S system S S Q T + m s i i m s e e + S gen S cv - Heat transfer (in or out) mass (in or out) 3- Entropy generation ٢٢

23 Isentropic Efficiency of Turbines Remember, if KE and PE are ignored in the energy balance w h equation, then the work is h η Turbine η actual turbine work isentropic turbine work η Turbine Turbine wa w h h s h h a s Isentropic Efficiency of Compressors η isen,comp Isentropic compressor work Actual compressor work Remember, if KE and PE are ignored in the energy balance equation, then the work is η w h h isen,comp h h s a h h 0.75 < η isen,comp 0.85 for Well-designed compressors. ws w a ٢٣

24 Reversible steady-flow work Vs. Boundary work w rev, in vdp W b Pdv Summary ٢٤

25 m General Equations The general mass balance equation m m m m in exit system The general energy balance equation V i V e Q W + mi hi + + gz i me he + + gz e m u mu + KE + PE The general entropy balance equation Q ms ms S S S T + + ( ) i i e e gen CV ٢٥

Thermodynamics is the Science of Energy and Entropy

Definition of Thermodynamics: Thermodynamics is the Science of Energy and Entropy - Some definitions. - The zeroth law. - Properties of pure substances. - Ideal gas law. - Entropy and the second law. Some