Transcript of Fundamentals of Thermodynamics II Energy and Heat Transfer.
- Slide 1
- Fundamentals of Thermodynamics II Energy and Heat Transfer
- Slide 2
- References Required Introduction to Naval Engineering (Ch 2 pg.
9-14, 18, and 22-28) Optional Principles of Naval Engineering (pg.
163-173)
- Slide 3
- Objectives Comprehend stored energy and transitional energy and
give examples of each. Comprehend thermal and mechanical energy and
give examples of each. Apply the concepts of work, heat, and power.
Apply the definition of a system and correctly identify various
types of systems. Comprehend the relationship between temperature
and pressure in a working substance. Comprehend the state of a
working substance with respect to saturated conditions. Apply the
Ideal Gas Law. Comprehend the mechanisms of heat transfer and give
examples of each.
- Slide 4
- Introduction Why are we looking at thermodynamics? Naval ships
use either fossil fuel or nuclear fuel as energy for operation
Definition of energy? Types of energy/sources
- Slide 5
- Thermodynamics Definition: science concerned with the
interrelationship between thermal energy and mechanical energy
Energy conversion of greatest significance on ship is: stored
thermal mechanical Heat transfer: science that deals with methods
by which thermal energy is able to translate
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- Classifications of Energy Stored (contained in) Transitional
Mechanical Energy associated with large bodies or objects Potential
Kinetic Work Thermal Energy associated primarily with systems of
molecules Potential Kinetic Heat Potential Energy energy associated
with an objects position or elevation relative to a reference
energy level Kinetic Energy energy associated with an objects
motion.
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- Stored Energy Mechanical Energy the energy associated with
relatively large bodies Typically derived from a source outside the
object This is the ultimate goal for the systems we will study
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- Stored Energy Gravitational Potential Energy (Mechanical)
Energy stored in a system due to relative positions m = mass g =
gravitational acceleration
- Slide 9
- Potential Energy Elastic Potential Energy (Mechanical) Energy
stored in an elastic object that is deformed under tension or
compression x= change in position k = spring constant
- Slide 10
- Kinetic Energy Mechanical Kinetic Energy Energy stored in a
system by virtue of the relative velocities of the component parts
of the system
- Slide 11
- More Stored Energies Chemical Energy The energy associated with
the arrangement of atoms or molecules and the forces that bind them
together E.g. Burning fossil fuels in a gas turbine or diesel
engine.
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- More Stored Energies Nuclear Energy associated with the
arrangement of and bonds between nucleons in the nucleus of an
atom. e.g. fission, fusion, decay of radioactive materials
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- More Stored Energies Electrical Energy associated with
interactions with an electric field
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- More Stored Energies Thermal Energy associated with the force
of attraction between molecules and molecular motion Internal
Energy (U) the total stored thermal energy within a substance.
Thermodynamics is interested in CHANGES in U, rather than absolute
quantities of U.
- Slide 15
- Stored Energy Thermal Potential Energy Energy associated with
the force of attraction between molecules Similar to gravitational
potential energy Highest in Solids Lowest in Gases
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- Stored Energy Thermal Kinetic Energy Energy stored in a system
due to the motion of the molecules Thermal kinetic energy is
proportional to the temperature of a substance
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- Stored Energy Flow Work (displacement energy) The amount of
stored energy required to maintain the continuous steady flow of
working fluid Applicable to open systems Related directly to the
pressure required to move a unit volume of the substance across the
system boundary
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- Stored Energy
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- Energy in Transition WorkMechanical Energy in transition Units:
ft-lb or Joules (Newton-meter) The result of a tangible force
acting through a tangible distance (displacement) Work is
independent of the time it takes to do it Work is relative to frame
of reference Is there any work done in working out
- Slide 20
- Thermal Energy - Heat
- Slide 21
- Power
- Slide 22
- Power
- Slide 23
- Sign Convention Thermodynamics is concerned with changes in
energy rather than absolute quantities of energy. Since changes are
relative to previous states, it is necessary to define a sign
convention Heat in or Work out positive Heat out or Work in --
negative
- Slide 24
- Systems A quantity of matter contained within a prescribed
boundary (boundaries need not be physical) Systems are the most
basic unit of study in thermodynamics Three types of systems based
on how energy and mass move into and out of the system Open Closed
Isolated
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- Systems Open System Both energy and mass may cross the boundary
e.g. An open leg of piping. Both water and heat can enter and exit
the system.
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- Systems Closed System energy may cross the boundary, but mass
is constant e.g. A soda bottle with the lid on tightly. Energy may
be removed (soda cools in the fridge), but the mass of soda is
constant
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- Systems Isolated system Neither energy nor mass may cross the
boundary e.g. The universe (as we understand it) is an isolated
system. These systems are useful for analyzing closed systems in
which energy losses can be considered negligible.
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- Working Substances Any substance (or fluid) that undergoes
changes to affect a change in the sytem Examples?
- Slide 29
- Phase and State State Determined by two independent properties
of the working fluid (water) e.g. Temperature and pressure Using
steam tables and two independent properties, it is possible to
determine all other properties of the water.
- Slide 30
- Phase and State Phase Not to be confused with state. Phase
refers to the physical arrangement of the substances molecules.
Examples of phases are: solid, liquid, and gas. The working fluid
can exist in multiple states and multiple phases with in the
boundaries of the system.
- Slide 31
- Phase and State Example: A cylinder contains water at 300F and
1014 psig. What is the phase and state of the water? The
temperature and pressure of the water are two independent
properties that define the state of the water. The saturation
temperature of water at 1014 psig is ~555F. The phase of the water
is liquid ( T < T SAT for the given P)
- Slide 32
- Pressure/Temperature Relationships Ideal Gases An ideal gas is
a theoretical gas composed of randomly moving, non-interacting
point particles Under standard temperature and pressure conditions
(STP), most real gases behave like an ideal gas.
- Slide 33
- Pressure/Temperature Relationships
- Slide 34
- Pressure/Temperature Relationships Saturation Conditions
Saturation a condition in which a mixture of vapor and liquid can
exist together at a given temperature and pressure. Also known as
the boiling point Saturation Temperature the temperature, for a
given pressure, at which a fluid boils Saturation Pressure the
pressure, for a given temperature, at which a fluid boils
- Slide 35
- Terms and Definitions Subcooled liquid A liquid at a
temperature below the saturation temperature for a given pressure
Superheated vapor A vapor at a temperature above the saturation
temperature for a given pressure
- Slide 36
- Terms and Definitions Sensible Heat: The energy necessary to
heat (or cool) a fluid from a subcooled (saturated) state to
saturation (a subcooled state). Heat absorbed or rejected with a
corresponding change of temp, but no change in phase
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- Terms and Definitions Latent Heat: The energy necessary to
change the phase of a fluid. Heat absorbed or lost with a change in
phase. A change in latent heat doesnt not result in a change in
temperature of the substance. This energy is used to break the
bonds between molecules (i.e crystal structure of ice to
liquid)
- Slide 38
- Terms and Definitions Latent Heat of Vaporization: the amount
of heat necessary to change a liquid to a vapor with out a change
in temperature. Latent Heat of Fusion: That amount of heat that
must be added to a solid to melt it or must be removed from a
liquid to solidify it.
- Slide 39
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- Definitions and Terms Saturation Temperature(Pressure): The
temperature(pressure) for a corresponding pressure(temperature) at
which a liquid boils Saturated liquid/saturated vapor a
liquid(vapor) a the temperature corresponding to the boiling point,
at a given pressure Subcooled liquid A liquid at a temperature
below the saturation temperature for a given pressure Superheated
vapor A vapor at a temperature above the saturation temperature for
a given pressure
- Slide 42
- T-s Diagram Subcooled liquid Saturated liquid/vapor Superheated
vapor
- Slide 43
- Mechanisms of Heat Transfer Heat Transfer: heat flow from one
body, region or substance to another Energy moves from higher temp
-> lower temp Convention: Higher temp: heat source Lower temp:
heat sink or receiver Types: conduction, radiation, and
convection
- Slide 44
- Conduction Definition: transfer of thermal energy when source
and sink are in physical contact Energy transfer occurs layer to
layer General Conduction Equation:
- Slide 45
- Radiation Definition: electromagnetic radiation generated by
thermal motion of charged particles in all matter. All objects,
whose temp is above absolute zero (including those that do not
produce visible light), radiate thermal energy
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- Convection Not really a transfer, but a transport Definition:
transportation or movement of some portions of a fluid within a
mass of fluid Can be due to density differences caused by
temperature differences Physical contact between a portion of the
fluid mass is required, however it is not considered conduction
(convection is specific to fluids)
- Slide 47
- Convection Natural circulation: due to density only Forced
circulation: mechanical device (fan)
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- Take Aways List and describe the various types of
potential/kinetic energy (gravitational, elastic, thermal) Describe
and apply Mechanical work and its relationship to stored energy
Define the mechanisms of heat transfer and give examples of each.
Or, given an example, describe the mechanism employed (justify your
answer)
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- Take Away Define sensible heat and latent heat. Define
saturation temperature, saturation pressure, saturated
liquid/vapor, subcooled liquid, superheated vapor Indicated the
above conditions on a T-s diagram Apply the ideal gas law to
various situations Describe the mechanisms of heat transfer and
give examples.
- Slide 50
- Homework 1.7 1.14 1.21