Thermodynamics and Energy

1. Thermodynamics and Energy Basic Concepts

2. Thermodynamics • Science of Energy • What is Energy? • Ability to cause change • Thermodynamics: • Greek words therme (heat) and dynamis (power) or “Turn heat into power” • Now includes all aspects of energy and energy transformations

3. Laws of Energy • Conservation of Energy Principle • Energy can change from one form to another but total amount remains constant • First Law of Thermodynamics • Can neither create nor destroy energy • Second Law of Thermodynamics • Energy has quality as well as quantity

4. History • Early work in 1850’s by • William Rankine • Rudolph Clausius • Lord Kelvin (William Thompson)

5. Thermodynamics • Classical thermodynamics • Macroscopic view • Looks at results of actions at overall level • Statistical thermodynamics • Microscopic view • Looks at actions at individual particle level Most engineering work at macroscopic level.

6. Units • Base units • Mass, m (or force), length, L, time, t, temperature, T • Derived units • Velocity, V, energy, E, volume, v

7. Units • Systems • SI, international • Mass based • Decimal system • English (US Customary System, USCS) • Force based (gravitational) • Unique relationships abound

8. Key Units

9. Key Units • Newton (N): force required to accelerate a mass of one kg at a rate of one meter/second2 • Pound-force (lbf): force required to accelerate a mass of 32.174 lbm (1 slug) at a rate of one foot/second2 • Weight is a force, mass isnot weight

10. Key Units • Specific weight γ: weight per unit volume or γ= ρg where ρ is density and g is the gravitational constant. • Work (energy): force times distance, newton-meter (N·m) called a joule (J) • Energy is English system is BTU: energy required to raise the temperature of 1 lbm of water at 68°F by 1°F. • 1BTU = 1.0551 kJ

11. Dimensional Homogeneity

12. Systems and Control Volumes • System: aquantity of matter or a region in space chosen for study • Surroundings: everything outside the system • Boundary: the surface that separates the system and the surroundings

13. Systems and Control Volumes

14. Systems and Control Volumes • Closed System (Control Mass): fixed amount of mass, no mass can cross boundary, energy can cross boundary • Special case: no energy crosses boundary, isolated system

15. Systems and Control Volumes

16. Systems and Control Volumes • Open Systems (Control Volumes): selected region in space, both mass and energy cross the boundary of the system

17. Systems and Control Volumes

18. Systems and Control Volumes

19. Properties of a System • Characteristics of a system are called Properties • Examples: pressure, temperature, mass, volume… • Intensive Properties: independent of mass of system • Temperature, density, pressure… • Extensive Properties: value depends on size or extent of system • Total mass, total volume, total momentum

20. Properties

21. Extensive Properties • Properties per unit mass are called Specific Properties • Examples: specific volume v = V/m specific total energy e = E/m • Convention: extensive properties, upper case, intensive properties, lower case • Exceptions: mass, pressure, temperature

22. Continuum • An assumption that allows us to work problems • Disregards atomic nature of substance • Continuum assumption allows: • Treat properties as point functions • Properties vary continually in space

23. Density and Specific Gravity • Density: mass per unit volume • ρ = m/V (kg/m3) • Specific Volume: volume per unit mass • V = V/m = 1/ρ (m3/kg) • Specific Gravity: ratio of the density of a substance to the density of a standard substance at a give temperature. • SG = ρ/ρwater (also called relative density)

24. Density and Specific Gravity

25. Specific Weight • The weight of a unit volume of a substance is called specific weight • γs = ρg (N/m3)

26. State • State is when all the properties of a system have fixed, unchanging, values • A system is said to be at a state when all the properties in the system can be measured or calculated and the system is not undergoing a change.

27. State

28. Equilibrium • Equilibrium implies a state of balance, no unbalanced driving forces in the system • Equilibrium types: • Thermal: system at same temperature • Mechanical: consistent pressure • Phase: at equilibrium level • Chemical: no chemical reactions occur

29. State Postulate • State postulate: the state of a simple compressible system is completely specified by two independent, intensive properties • Simple compressible system if no electrical, magnetic, gravitational, motion, surface tension effects • Independent if one can be varied while the other is held constant

30. State

31. Any change that a system undergoes from one equilibrium state to another is called a process The series of states through which a system passes during a process is called a path of the process Processes and Cycles

32. Processes • To describe a process completely need: • Initial and final states • Path it follows • Interactions with surroundings

33. Quasi- Processes • When a process moves so slowly that all parts of the system change at the same rate and are in equilibrium with all other parts of the system, the process is called quasi-static or a quasi-equilibrium process • A quasi-equilibrium process is an idealized process and does not occur in nature. • Serve as a standard to be compared to

34. Iso- processes • Iso- processes are processes that one property remains constant: • Isothermal: temperature • Isobaric: pressure • Isochoric, isometric: specific volume

35. Cycle • Special process where the process at the final state returns to the initial state

36. Steady-Flow Process • Steady: no change with time • Uniform: no change with location over a specific region • Opposite of steady: unsteady or transient

37. Steady-flow process is a process during which a fluid flows through a control volume steadily Steady-Flow Process

38. Under steady-flow conditions, the mass and energy contents of a control volume remain constant Steady-Flow Process

39. Temperature • Relative: freezing cold, cold, warm, hot, red-hot • Reference to know events, solidifying of water, vaporizing of water

40. Thermal equilibrium occurs when no temperature gradient exists, both objects are at same temperature Thermal Equilibrium

41. Zeroth Law • If two bodies are in thermal equilibrium with a third body, the are in thermal equilibrium with each other. • Two bodies are in thermal equilibrium if both have the same temperature, even is they are not in contact.

42. Temperature Scales • Relative or Two Point Scales • Based on temperature of ice/liquid water and liquid water/water vapor mixtures • SI system: Celsius scale based on 100 units between points (°C) • English system: Fahrenheit scale based on 180 units between points with lower point set at 32 units (°F)

43. Temperature Scales • Thermodynamic temperature scales, absolute scales • Based on absolute zero temperature • SI system: Kelvin scale, freezing point of water at 273.15 units (K) • English system: Rankine scale, freezing point of water at 459.67 units (R) • Ideal-gas temperature scale

44. Temperature Relationships • Kelvin to Celsius: T(K) = T(°C) + 273.15 • Rankine to Fahrenheit: T(R) = T(°F) + 459.67 • Between the English and SI systems: • T(R) = 1.8T(K) • T(°F) = 1.8T(°C) + 32

45. Note that: ΔT(K) = ΔT(°C) ΔT(R) = ΔT(°F) Temperature Relationships

46. Pressure • Pressure is a normal force exerted by a fluid per unit area • Units, force/unit area, N/m2, called a pascal (Pa) • 1 bar = 105 Pa = 0.1 MPa = 100 kPa • 1 atm = 101.325 kPa = 1.01325 bars

47. Pressure • Absolute pressure: relative to absolute vacuum (absolute zero pressure) • Gage pressure: relative to atmospheric pressure • Pgage = Pabs - Patm • Pvac = Patm – Pabs

48. Pressure

49. ΔP = P2 – P1 = ρgΔz = γsΔz P = Patm + ρgh or Pgage = ρgh Pressure with Depth

50. Pressure with Depth