ch 2+ +energy+transfer

8/12/2019 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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