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Thermal Science
MECH 2003Y
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The Heat Engine (Revision)
Heat engine cycle
Steam boiler & steam plant
Positive displacement machine
Nozzles
Mixtures
IC engines
Hydraulic Machinery (?)
Module Outline
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Lecturer: J. Chummun
Email: [email protected]
Tel.: 403 7804
Office: Rm 2.4, Textile Bldg., FoE
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1kg of a fluid expands reversibly according to a linear lawfrom 4.2bar to 1.4 bar; the initial and final volumes are
0.004 m3 and 0.02m3. The fluid is then cooled reversibly
at constant pressure, and finally compressed reversiblyaccording to a law pv=const back to the initial conditions
of 4.2 bar and 0.004m3.
Sketch the cycle on a p-v diagram. Calculate the wor
k
done in each process and the net work of the cycle.
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Conversion of energy (heat) directly to work
can only be done by devices called HEAT
ENGINES.
INTRODUCTION:
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Heat p WorkExamples of heat energy producing other types of energy:
Air over a hot car roof is lofted, gaining kinetic energy;
That same air also gains gravitational potential energy;
All of ourwindis driven by temperature differences;
Electricity generation thrives on temperature
differences: no steam would circulate if everything was
at the same temperature
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Heat Engine
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A heat engine is a very simple
idea. It is a device thatconverts heat into another
form of energy.
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Heat engines convert heat
energy into mechanical energy.
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They receive heat from a high temperature source;They convert part of this heat to work;
They reject the remaining heat to a low temperature
sink;They involve a fluid to and from which heat is
transferred while undergoing a cycle. The fluid is called
a working fluid;
They operate on cycles.
Characteristics of heat engines
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Applying1
st law of thermodynamics to theheat engine, a closed system operating on
a cycle where the net heat supplied is
equal to the net work done,
x!x WQ
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Then,
netoutin WQQ !
Qin : heat received from source
Qout : heat rejected to sink
Wnet : net work output
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Heat Engines can be schematically represented
like this:
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SOURCE isknown as a reservoir supplying
energy as heat.
SINK is known as a reservoir absorbing energy
as heat.
Examples of Sources: furnaces, nuclear reactors,
solar energy.
Examples of Sinks : atmosphere, lakes, rivers.
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Heat Engines
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Thermal efficiency, denoted by L is a measure of
how efficiently a heat engine converts the heat
that it receives to work. Thermal efficiency is also
referred to as performance of a heat engine.Thermal efficiency, L can be defined as the ratio of
desired output to the required input, as follows
inputrequired
outputdesired!L
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Heat supplied to a system, Q, is positive.
Work input to a system, W, is positive.
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Since cyclic devices work between high
temperature reservoirs (TH) supplying heatQ
H
and low temperature reservoirs (TL) receiving
heat QL as shown below then,
H
L
H
net
Q
Q
Q
W!! 1L
L for heat engines is always less than one.
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A typical gasoline automobile engine operates at
around 25% efficiency, and a large coal-fuelled
electrical generating plant peaks at about 46%.
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The burning of a fuel-air mixture in a piston driven
heat engine produces 10kJ of heat of which 7kJ
exits the engine at the exhaust manifold.
What is the efficiency of this engine?
Problem 1
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Heat is transferred to a heat engine from a
furnace at a rate of 80 MW. If the rate of waste
heat rejection to a nearby river is 50 MW,
determine the net power output and the thermal
efficiency for this heat engine.
Problem 2
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The energy input to an engine is 3.00 times
greater than the work it performs. What is its
thermal efficiency?
What fraction of the input energy is expelled to
the cold reservoir?
Problem 3
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2nd Law of Thermodynamics -Kelvin Planck Statement
IT IS IMPOSSIBLE FOR ANY DEVICE THAT
OPERATES ON A CYCLE TO RECEIVE HEAT FROM A
SINGLE RESERVOIR AND PRODUCE A NET AMOUNT
OF WORK.
No heat engine can have a thermal efficiency of 100%and QL can never be zero.
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Refrigerator
Heat flows in the direction of decreasing
temperature, that is, from high temperature
media to low temperature ones.
The reverse process cannot occur by itself
and the transfer of heat from a low-
temperature medium to a high temperature
one requires special devices called
REFRIGERATORS.
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Refrigerator
Can be modeled as a closed system;
Operates at steady state;
Receives heat from a low temperature source;
Requires a power source;
Rejects heat to a high temperature area.
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Basic components of a refrigeration system and typical operating
conditions.
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The objective of a
refrigerator is to remove
QL from the cooled space.
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Refrigerator
RW
LQ
H
Q
For the Refrigerator operating
at steady state:
(U = Q - W
0 = QL QH W
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Coefficient of Performance for Refrigirator
InputRequired
OutputDesiredePerformanc !
Work
SpaceColdfromRemovedHeatCOPR !
R
LL
R
COP0
W
Q
W
QCOP
e
!!
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Heat Pump
Can be modeled as a closed system.
Operates at steady state
Receives heat from a low temperature source.
Requires a power source.
Rejects heat to a high temperature area.
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Heat Pump
HPW
LQ
H
Q
For the Heat Pump operating at
steady state:
(U = Q - W
0 = QL QH + W
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The objective of a heat pumpis to supply heat QH into the
warmer space.
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Coefficient of Performance - HP
InputRequired
OutputDesiredePerformanc !
Work
SpaceWarmtoSuppliedHeatCOPHP !
HP
HHHP
COP0
W
Q
W
QCOP
e
!!
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Rank in order, from largest to smallest, the workWout performed by
these four heat engines.
1. Wb > Wa > Wc > Wd2. Wb > Wa > Wb > Wc3. W
b> W
a> W
b= W
c4. Wd > Wa = Wb > Wc5. Wd > Wa > Wb > Wc
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What is the thermal efficiency of this heat engine?
1. 0.10
2. 0.25
3. 0.504. 4
5. Cant tell without knowing QC.
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What, if anything, is wrong
with this refrigerator?
1. It violates the first law of thermodynamics.
2. It violates the second law of thermodynamics.
3. It violates the third law of thermodynamics.4. Nothing is wrong.