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Energy, Energy Transfer, and General Energy Analysis

Energy, Energy Transfer, and General Energy Analysis. Chapter 2. Engineering Thermodynamics. Energy can exist in many forms. Macroscopic forms Respect to some outside reference frame Microscopic Related to the molecular structure. Macroscopic Energy. Kinetic Energy (KE) KE = ½ m V 2

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Energy, Energy Transfer, and General Energy Analysis

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  1. Energy, Energy Transfer, and General Energy Analysis Chapter 2 Engineering Thermodynamics

  2. Energy can exist in many forms • Macroscopic forms • Respect to some outside reference frame • Microscopic • Related to the molecular structure

  3. Macroscopic Energy • Kinetic Energy (KE) • KE = ½ mV2 • Potential Energy (PE) • PE = mgz

  4. Other Kinds of Macroscopic Energy • Magnetic • Electrical • Surface Tension • These are specialized, and we don’t usually need to include them

  5. Microscopic Energy • Kinetic energy of individual molecules • Potential energy of individual molecules • Binding forces • Chemical Energy • Nuclear Energy • Etc • Lump them all together into internal energy (U)

  6. Both macroscopic and microscopic forms of energy are static – they can be stored in a system

  7. A unit mass basis is often more convenient • e = E/m • u = U/m • ke = KE/m = V2/2 • pe = PE/m = gz

  8. Stationary Closed Systems • If a closed system is not moving the energy of the system is simply the internal energy of the system • Thus the change in energy of a stationary system is:

  9. Stationary Control Volumes • In a control volume matter can both flow into and out of the system • Instead of being interested in how the system changes with time (it usually doesn’t) – we are interested in how the energy of the flowing fluid changes from the time it enters the system until it exits the system

  10. In Stationary Control Volumes we are interested in: • Mass flow rates --- • Energy flow rates --- The energy flow rate is equal to:

  11. Mass Flow rate • The amount of mass flowing through a cross section per unit time • Related to the volumetric flow rate

  12. Mechanical Energy • Flowing fluids are often described as having mechanical energy • Mechanical energy is “the form of energy that can be converted to mechanical work completely and directly by an ideal mechanical device such as an ideal turbine”

  13. Mechanical energy consists of • Kinetic energy of the flowing fluid • Potential energy of the flowing fluid • Energy resulting from a pressure acting over a distance – called flow work or flow energy

  14. The rate of mechanical work flowing into a system can be found by multiplying by the mass flow rate We’ll return to this concept in Chapter 5

  15. Dynamic Energy • When energy moves from place to place we treat it differently • The only forms of energy that can cross a system boundary without matter transfer are: • Heat (Q) • Work (W)

  16. Heat • A system can not contain heat • Heat only exists as energy crossing a system boundary • What we think of as a system’s heat content is Thermal Energy • Heatis energy transferred through a temperature difference • All other forms of energy transfer are work!!

  17. Heat transfer • Heat is defined as the form of energy that is transferred between two systems by virtue of a temperature difference • A process with no heat transfer is adiabatic • Greek – not to be passed

  18. Symbols • Q • Total heat transferred • kJ or BTU • q • Heat/mass • kJ/kg or BTU/lbm • Rate of heat transfer • kJ/sec = kW

  19. Heat transferred to a system is positive

  20. Caloric Theory • Antoine Lavoisier • Heat is a fluid like substance called caloric • It can be poured from one container (system) to another • This model didn’t last very long http://scienceworld.wolfram.com/biography/Lavoisier.html

  21. The Caloric Theory was attacked by a number of scientists • The American Benjamin Thompson (Count Rumsford) showed that heat is produced continuously by friction http://scienceworld.wolfram.com/biography/Thompson.html

  22. James Joule • His careful experiments showed the mechanical equivalent of heat • He was motivated by his religious beliefs to demonstrate “the unity of forces in nature” http://scienceworld.wolfram.com/biography/Joule.html

  23. Modes of Heat Transfer • Conduction • Transfer of heat as a result of interactions between particles (atoms or molecules) • Convection • Heat transfer between a solid surface and a gas or liquid that is in motion • Radiation • Does not require an intervening medium

  24. Thermal Conductivity Conduction For the steady state condition, integrating gives

  25. Thermal Conductivities

  26. Convection Convective heat transfer coefficient

  27. Air is always in motion

  28. Convective Heat Transfer Coefficients

  29. Radiation A lot more complicated than conduction and convection

  30. Stephan Boltzman Constant All bodies at a temperature above absolute 0 emit thermal radiation e This is the maximum - Called black body radiation Real materials emit less – so we need a fudge factor, called the emissivity, e

  31. Absorbtivity Calculating the net radiative heat transfer is complicated, but For the special case where the surface is small and completely enclosed

  32. Work • Work is the energy transferred with force acting through a distance

  33. Work • W = Fd • N m = J usually we’ll use kJ • w = W/m • kJ/kg • kJ/sec • kW

  34. Work done by a system is positive

  35. We put heat in We get work out Why do work and heat have opposite sign conventions? This text is aimed at Mechanical Engineers Consider an engine

  36. We put heat in We also put work in We get petroleum products out Chemical Engineers sometimes use a different convention When you work with engineers from a variety of backgrounds, confirm that you are using the same conventions!! Consider a refinery

  37. Types of Work • Electrical Work • Mechanical Work W = FS

  38. Mechanical Work • There are many kinds of mechanical work • The most important for us will be moving boundary work • Wb Moving boundary work is covered in detail in Chapter 4

  39. Boundary work occurs because the mass of the substance contained within the system boundary causes a force, the pressure times the surface area, to act on the boundary surface and make it move.

  40. Both heat and work are dynamic forms of energy • They are recognized as they cross a boundary • Systems possess energy, but not heat or work • Both are associated with a process, not a state • Both are path functions

  41. not Path Functions • Have inexact differentials • Properties are point functions – they have exact derivatives • If we want to know how much work is done, we need to know the path

  42. D D

  43. Total Energy entering the system Total Energy leaving the system The change in total energy of the system - = Energy Balances – The First Law

  44. First Law for a Closed System How can energy get in and out of a closed system? Heat and Work

  45. 0 0 E = U + KE + PE If the system isn’t moving

  46. First Law for a Control Volume Heat, Work, And energy transfer with mass How can energy get in and out of a control volume? Chapter 4

  47. For a Control Volume Chapter 5

  48. In a cyclic process, one where you end up back where you started: You convert heat to work, or vise versa

  49. Energy Conversion Efficiencies Chapter 6

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