Ch 2+ +Energy+Transfer
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Transcript of Ch 2+ +Energy+Transfer
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OBJECTIVES
Introduce the concept of the system energy and define its
various forms.
Discuss the nature of internal energy.
Define the concept of heat and the mechanisms of heattransfer.
Define the concept of work, including several forms ofmechanical work.
Introduce the energy balances, and the forms of energytransfer to or from a system including the energy associatedwith a flowing fluid.
Define energy conversion efficiencies.
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Can exist in numerous forms such as kinetic, potential,
electrical, magnetic, chemical, and nuclear, Their sum is the total energy, Eof a system.
The total energy, Eof a system can be divided into:
Macroscopic or external forms of energy:Those a systempossesses as a result of its motion and elevation in a
gravitational field with respect to some outside referenceframe, such as kinetic and potential energy, respectively.
Microscopic or internal forms of energy:Those related tothe molecular structure and the molecular activity of a system.The sum of all the microscopic forms of energy is called theinternal energy, U.
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The macroscopic energy of an object
changes with velocity and elevation.
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Total energy of a system
Kinetic energy
Kinetic energy per unit mass
Potential energy
Potential energy per unit mass
Total energy of a system per unit mass
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Mass flow rate
Energy flow rate
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The form of energy that can be converted to mechanical work
completely and directly by an ideal mechanical device such asan ideal turbine.
per unit mass of a flowing fluid
Rate of mechanical energy of a flowing fluid
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The internal energy of
a system is the sum of
all forms of the
microscopic energies.
Sensible energy:The portion of the
internal energy of a system associatedwith the kinetic energies of themolecules.
Latent energy:The internal energyassociated with the phase of a system.
Chemical energy:The internal energyassociated with the atomic bonds in amolecule.
Nuclear energy:The tremendousamount of energy associated with the
strong bonds within the nucleus of theatom itself.
Internal = Sensible + Latent + Chemical + Nuclear
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T is the
driving forcefor heat
transfer. The
larger the T,
the higher is
the rate of
heat transfer.
o Energy can cross theboundaries of a closedsystem in the form of heatand work.
o During an adiabatic
process, a systemexchanges no heat with itssurroundings.
o Heat:The form of energythat is transferred between
two systems (or a systemand its surroundings) byvirtue of a temperaturedifference.
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Radiation:The transfer of energy due to the emission ofelectromagnetic waves (or photons).
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Conduction:The transfer ofenergy from the moreenergetic particles of asubstance to the adjacent lessenergetic ones as a result ofinteraction between particles.
Convection:The transfer ofenergy between a solid surfaceand the adjacent fluid that is inmotion, and it involves thecombined effects of conductionand fluid motion.
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Energy is recognized as heat transferonly as it crosses the system boundary.
Heat transfer per unit mass
Amount of heat transfer During a timeinterval t1to t2.
When heat transfer rate is constant
Rate of Heat transfer = Heat per unit time
kWdt
QQ
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Work done per unit mass
Power is the work done per unittime (kW)
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Power is the work done per
unit time (kW) kWdt
WW
Work:The energy transfer associated with a forceacting through a distance.
A rising piston, a rotating shaft, andan electricwire crossing the system boundariesare allassociated with work interactions
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Heat transfer to a system and workdone by a system are positive;heat transfer from a system andwork done on a system arenegative.
Alternative to sign convention is touse the subscripts inand outtoindicate direction.
Use only one approach (eitherthe sign or in/out)
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Specifying the directions
of heat and work.
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Both are recognized at the boundariesof a system as they cross the
boundaries. That is, both heat andwork are boundaryphenomena.
Systems possess energy, but not heator work.
Both are associated with aprocess,not a state.
Unlike properties, heat or work has nomeaning at a state.
Both arepath functions(i.e., theirmagnitudes depend on the pathfollowed during a process as well as
the end states).
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Properties are point functions; but
heat and work are path functions.
Properties are point functions
have exact differentials (d ).
Q and W are path functions have
inexact differentials ( )
HEAT AND WORK
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There are two requirements for a workinteraction between a system and its
surroundings to exist: there must be a fo rceacting on the boundary.
the boundary must move.
Work = Force Distance
If the force is constant over the distance:
If the force is not constant:
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The work done is proportional to the force
applied (F) and the distance traveled (s).
If there is no movement,no work is done.
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Energy transmission through
rotating shafts is commonly
encountered in practice.
A force Facting through a momentarm r generates a torque T
This force acts through a distances
The power transmitted through theshaft is
Shaft work
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Elongation of a spring is directly
proportional to the force.
When the length of the spring changes bya differential amount dx under the
influence of a force F, the work done is
For linear elastic springs, the displacementx
is proportional to the force applied
k: spring constant (kN/m)
Substituting and integrating yield
x
1
and x2
:the initial and the final displacements
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1. The work transfer needed to raise a body is
equal to the change in the potential energyof the body.
2. The work transfer needed to accelerate abody is equal to the change in the kineticenergy of the body.
The energy
transferred to
a body whilebeing raised
is equal to
the change in
its potential
energy.
Non-mechanical Forms of Work
Electrical work:
Magnetic work:Electrical polarization work:
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Internal, kinetic, and potential energy changes
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Rate of heat transfer
Rate of work transfer
Rate of mass transfer 20
The energy content
of a control volumecan be changed by
mass flow as well as
heat and work
interactions.
kWdt
WW
kWdt
QQ
kWdtm
m
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A closed system involves only heat transfer and work.
For a cycle:
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Efficiencyis one of the most frequently
used terms in thermodynamics, and itindicates how well an energy conversionor transfer process is accomplished.
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Efficiency of a water heater:The
ratio of the energy delivered to thehouse by hot water to the energysupplied to the water heater.
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Heating value of the fuel:The amount of heat releasedwhen a unit amount of fuel atroom temperature iscompletely burned and thecombustion products arecooled to the roomtemperature.
Overall efficiencyof a power plant:The ratio of the netelectrical power output to the rate of fuel energy input.
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The mechanical
efficiency of a fan is the
ratio of the kinetic
energy of air at the fan
exit to the mechanical
power input.
Pump/Fan mechanical efficiency and turbine
mechanical efficiency,
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Generator efficiency
Pump-Motor overall efficiency
Turbine-Generator overall efficiency
The overall efficiency of a
turbinegenerator is the
product of the efficiency of theturbine and the efficiency of
the generator, and represents
the fraction of the mechanical
energy of the fluid converted
to electric energy.
Motor efficiency
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The conversion of energy affects theenvironment and the air we breathe in
many ways:
Pollutants emitted during thecombustion of fossil fuels areresponsible forsmog, acid rain, andglobal warming.
The environmental pollution hasreached such high levels that it becamea serious threat to vegetation, wildlife, and human health.
A 1995 report:The earth has alreadywarmed about0.5Cduring the lastcentury, and they estimate that theearths temperature will rise another2Cby the year 2100.
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A rise of the Earth temperature can cause severe changes inweather patternswith storms and heavy rains and flooding at some
parts and drought in others, major floods due to the melting of ice atthe poles, loss of wetlands and coastal areas due to rising sea levels,and other negative results.
Improved energy efficiency, energy conservation, and usingrenewable energy sourceshelp minimize global warming.
The average car produces several times its
weight in CO2every year (it is driven 20,000
km a year, consumes 2300 liters of gasoline,
and produces 2.5 kg of CO2per liter).
Renewable energies such as wind are
called green energy since they emit no
pollutants or greenhouse gases.
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Forms of energy
Macroscopic = kinetic + potential
Microscopic = Internal energy (sensible + latent + chemical +nuclear)
Energy transfer by heat
Energy transfer by work
Mechanical forms of work
Balance of energy to/from a system and the system energychange
Energy change of a system
Mechanisms of energy transfer (heat, work, mass flow)
Energy conversion efficiencies Efficiencies of mechanical and electrical devices (turbines,pumps)
Energy and environment
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