Heat Engine 1

8/9/2019 Heat Engine 1

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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.

Heat Engine 1

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