CONCEPTS AND DEFINITIONS. Prepared by Engr. John Paul Timola

Transcription

1 CONCEPTS AND DEFINITIONS Prepared by Engr. John Paul Timola

2 ENGINEERING THERMODYNAMICS Science that involves design and analysis of devices and systems for energy conversion Deals with heat and work and those substances related to heat and work

3 Working Substance Fluid in which energy can be stored or from which energy can be removed Function is to receive, transport, or disperse heat, work, or energy Examples: Steam in a steam turbine Air in an air compressor

4 Thermodynamic System Matter or region of space which we consider or analyze

5 Surroundings Everything external to the system

6 Boundary Interface between the system and its surroundings Usually represented in a diagram by broken lines

7 Closed System Or control mass System where no mass can cross its boundary Only energy can enter and leave Example Gas inside a closed balloon Gas trapped in a cylinder by movable piston

8 Open System Or control volume Permits both mass and energy across its boundary Example Jet engine Window air conditioner

9 Identify the working substance, specify the kind of system, and sketch the system boundary. Example: Nitrogen, at a pressure of 200 kpa and a temperature of 25 C, flows at a velocity of 20 m/s through a pipe with a diameter of 35 mm. 1. Liquid water enters a pump at 25 C, 100 kpa and exits at a pressure of 5 MPa. 2. A compressor receives ambient air at 95 kpa, 20 C, with a low velocity. At the compressor discharge, air exists at 1.14 kpa, 380 C, with a velocity of 110 m/sec. 3. Steam enters a turbine at 300 C and is exhausted at 20 kpa.

10 Thermodynamic Property Any measurable characteristic of a system Quantity whose numerical value depends on the state of a system Examples: pressure, temperature, volume, specific volume

11 Extensive Property Property that depends on the amount of mass or material in a system Examples: Mass, total volume

12 Intensive Property Property that is independent of the size or the amount of mass or material in a system Examples: Specific volume, temperature, pressure density

13 Some Observable Properties Density Mass of substance per unit volume m kg lb [, m ] 3 3 V m ft where : m mass V volume

14 Specific Volume Volume per unit mass 3 3 V m ft [, ] m kg lb m where : m mass V volume

15 Specific Weight Weight of a substance per unit volume W kn N lbf = g [,, ] V m m ft where : W weight [ kn, N, lb ] g acceleration due to gravity at sea level m sec 2 f ft sec 2

16 Weight Force of gravity on a substance W mg [ kn, N, lb f ]

17 Specific Gravity Ratio of the specific weight of a substance to the specific weight of water SG Note: H2O 2 H O At 4 and kpa, H O g kg kn H O 3 cm m m 2 2

18 Example 1 Two cubic meters of air at 25 C and 1 bar has a mass of 2.34 kg a) List the values of three intensive properties and two extensive properties for this system b) If the local gravity is 9.65 m/s 2, evaluate the specific weight of the system

19 Example 2 An object has a mass of 10 kg. Calculate the following quantities: a) Weight of the object at sea level b) Weight of the object at a location where g = 9.4 m/s 2

20 Fill in the missing quantities 1. m = 3 kg V = 6 m3 υ = ρ = 2. m = 4 lb V = υ = 0.33 ft 3 /lb ρ = 3. m = V = 2 m 3 υ = 0.10 ft 3 /lb ρ =

21 Pressure Normal force per unit area Acting on the surface of a system F kg p [ kpa,, psi ] 3 A cm where : F normal force [ kn, N, kg, lb ] A area [ m, cm, ft,in ] f f

22 Pressure Measuring Devices Bourdon gauges Simple mechanical devices calibrated to read pressure directly by the movement of a needle attached to a hollow tube connected to a pressurized container

23 Pressure Measuring Devices Manometer Uses the height of a fluid column barometer p h gh gauge where : density of measuring liquid h height of column liquid

24 Absolute Pressure Actual pressure at a given position in a system

25 Gauge Pressure Difference of the absolute pressure and atmospheric pressure

26 Vacuum If a fluid exists at a pressure lower than the atmospheric pressure, its gauge pressure is negative The term vacuum is applied to the magnitude of the gauge pressure for convenience

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28 Notes for Pressure p abs = p atm + p gauge = p atm p vacuum 2 1 pascal ( Pa) 1 N / m bar 10 N / m 1 atm 760 mm Hg 101,325 Pa 14.7psi

29 Example A manometer is attached to a pressurized container. One end of the manometer is open to the atmosphere and the local atmospheric pressure is 760 mm Hg. The height of the manometer fluid is 42 cm and the fluid has a specific gravity of 1.6. Calculate the absolute pressure on the inside surface of the container.

30 Fill in the missing quantity 1. Force = 400 N Area = 14 m 2 Pressure = 2. Force = Area = 12 ft 2 Pressure = 100 lb m /ft 2 3. Force = 10 kn Area = Pressure = 60 kpa

31 Temperature Measure of the hotness or coldness of a substance 9 F C 32 5 C 5 ( F 32) 9 R F R 460 K C K 5

32 Convert F to C F to K R to C

33 State Condition of a system as indicated by its properties

34 Process Progress of a system proceeding from an initial state to a final state

35 Internally Reversible Process Or Quasi-Equilibrium An ideal process in which a system remains infinitesimally close to equilibrium condition throughout the process

36 Isothermal Process Process in which temperature remain constant

37 Isobaric Process Process in which pressure remain constant

38 Isochoric Process Also known as Isometric process Process in which volume remain constant

39 Adiabatic Process Process in which there is no heat transfer across the boundary of the system

40 Cycle Process or series of processes whose initial and final values are identical

41 Point Function Quantity whose value at any state is independent of the path or process used to reach that state Examples: pressure, temperature, specific volume, entropy, enthalpy

42 Path Function Also known as Process function Quantity whose value depends on the path followed during a particular change in state Examples: work and heat

43 Test Your Self 1. The condition of a system as indicated by its properties is its. 2. The progress of a system proceeding from an initial state to a final state is called. 3. The process in which volume remain constant is the. 4. The process or series of processes whose initial and final values are identical is named. 5. The process in which temperature remain constant is the.

44 Potential Energy Energy possessed by a body by virtue of its position PE Wz mgz [ kj, Btu ] mgz [ kg / s, Btu / hr, ft lb / s ] gz [ kj / kg, Btu / lb ] where : z elevation [ m, ft ] m mass m f [ kg, lb ] m mass flow rate [ kg / s, lb / s ] g acceleration due to gravity m s ft s m m 2 2 [ /, / ]

45 Kinetic Energy Energy possessed by an object due to its motion KE 1 mv 2 [ kj, Btu ] 2 where : v velocity [ m / s, ft / s ] m mass [ kg, lb ] m

46 Example Two identical automobiles each has a mass of 1500 kg. Both automobiles start from rest at the same location with an elevation of 1000 m. Automobile A passes a point with an elevation of 2000 m maintaining a velocity of 15 m/s while automobile B follows with a velocity of 20 m/s. Determine the change in potential and kinetic energy of both automobiles.