Thermodynamics and Energy

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Thermodynamics and Energy

• Basic Concepts

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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

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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

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History

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

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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.

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Units

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

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Units

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

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Key Units
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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 is not weight

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Key Units

• Specific weight ? weight per unit volume or ?
  ?g where ? is density and g is the gravitational
  constant.
• Work (energy) force times distance, newton-meter
  (Nm) called a joule (J)
• Energy is English system is BTU energy required
  to raise the temperature of 1 lbm of water at
  68F by 1F.
• 1BTU 1.0551 kJ

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Dimensional Homogeneity
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Systems and Control Volumes

• System a quantity 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

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Systems and Control Volumes
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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

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Systems and Control Volumes
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Systems and Control Volumes

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

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Systems and Control Volumes
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Systems and Control Volumes
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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

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Properties
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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

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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

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Density and Specific Gravity

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

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Density and Specific Gravity
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Specific Weight

• The weight of a unit volume of a substance is
  called specific weight
• ?s ?g (N/m3)

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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.

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State
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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

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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

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State
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Processes and Cycles

• 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

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Processes

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

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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

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Iso- processes

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

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Cycle

• Special process where the process at the final
  state returns to the initial state

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Steady-Flow Process

• Steady no change with time
• Uniform no change with location over a specific
  region
• Opposite of steady unsteady or transient

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Steady-Flow Process

• Steady-flow process is a process during which a
  fluid flows through a control volume steadily

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Steady-Flow Process

• Under steady-flow conditions, the mass and energy
  contents of a control volume remain constant

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Temperature

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

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Thermal Equilibrium

• Thermal equilibrium occurs when no temperature
  gradient exists, both objects are at same
  temperature

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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.

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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)

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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

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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

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Temperature Relationships

• Note that
• ?T(K) ?T(C)
• ?T(R) ?T(F)

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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

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Pressure

• Absolute pressure relative to absolute vacuum
  (absolute zero pressure)
• Gage pressure relative to atmospheric pressure
• Pgage Pabs - Patm
• Pvac Patm Pabs

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Pressure
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Pressure with Depth

• ?P P2 P1 ?g?z ?s?z
• P Patm ?gh or Pgage ?gh

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Pressure with Depth
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Pressure
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Pressure
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Pressure
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Problem-Solving Technique

• Problem statement
• Schematic
• Assumptions approximations
• Physical laws
• Properties
• Calculations
• Reasoning, verification, discussion