This chapter delves into the various forms of energy, both macroscopic and microscopic, and their analysis in engineering thermodynamics. It covers kinetic and potential energy, internal energy, mechanical energy, heat transfer, work, and the modes of heat transfer. Key concepts like the mechanical equivalent of heat, thermal conductivity, and radiative heat transfer are explored in depth.
01 / 55Energy Transfer in Engineering Thermodynamics
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Energy Transfer in Engineering Thermodynamics
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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)
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
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
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
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
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
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
