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## Energy, Energy Transfer, and General Energy Analysis

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**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 mV2 • Potential Energy (PE) • PE = mgz**Other Kinds of Macroscopic Energy**• Magnetic • Electrical • Surface Tension • These are specialized, and we don’t usually need to include them**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)**Both macroscopic and microscopic forms of energy are static**– they can be stored in a system**A unit mass basis is often more convenient**• e = E/m • u = U/m • ke = KE/m = V2/2 • pe = PE/m = gz**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:**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**In Stationary Control Volumes we are interested in:**• Mass flow rates --- • Energy flow rates --- The energy flow rate is equal to:**Mass Flow rate**• The amount of mass flowing through a cross section per unit time • Related to the volumetric flow rate**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”**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**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**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)**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!!**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**Symbols**• Q • Total heat transferred • kJ or BTU • q • Heat/mass • kJ/kg or BTU/lbm • Rate of heat transfer • kJ/sec = kW**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**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**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**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**Thermal Conductivity**Conduction For the steady state condition, integrating gives**Convection**Convective heat transfer coefficient**Radiation**A lot more complicated than conduction and convection**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**Absorbtivity**Calculating the net radiative heat transfer is complicated, but For the special case where the surface is small and completely enclosed**Work**• Work is the energy transferred with force acting through a distance**Work**• W = Fd • N m = J usually we’ll use kJ • w = W/m • kJ/kg • kJ/sec • kW**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**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**Types of Work**• Electrical Work • Mechanical Work W = FS**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**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.**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**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**D**D**Total Energy entering the system**Total Energy leaving the system The change in total energy of the system - = Energy Balances – The First Law**First Law for a Closed System**How can energy get in and out of a closed system? Heat and Work**0**0 E = U + KE + PE If the system isn’t moving**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**For a Control Volume**Chapter 5**In a cyclic process, one where you end up back where you**started: You convert heat to work, or vise versa**Energy Conversion Efficiencies**Chapter 6