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Chapter 1a
GAS POWER CYCLES
Principles of Power Cycles,
Carnot Cycle
MBB 2053 - Thermodynamics II
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Chapter Objectives
Evaluate the performance of gas power
cycles for which the working fluid
remains a gas throughout the entire
cycle.
Develop simplifying assumptions
applicable to gas power cycles.
Solve problems based on the Carnot,
cycle.
Review the operation of reciprocating
engines.
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Summary
Basic considerations in the
analysis of power cycles
The Carnot cycle and its value inengineering
Air-standard assumptions
An overview of reciprocatingengines
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You will recall that the product of pressure and volume is work.
Recall the term pv work or flow work
1 Pa x (1 m3
/ kg) = ?
For m kg, what do we get ?
For M kg/s, what do we get?
Consider a p-v diagram. The area in a p-v diagram represents work.
In thermodynamics, we make extensive use of the p-v diagram to seewhether work is got from the system or work is done on it.
Similarly, the product of temperature and entropy yields heat quantity.
You might recall that Q = T s (Note: Temperature T is in Kelvin) The units of entropy = ?
The units of the product of temperature x entropy = ?
In thermodynamics, we also make extensive use of the T-s diagram
to see whether heat is added to the system or rejected from it.4
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Cyclic work
Why consider thermodynamic cycles? A working medium (the simplest is air) produces work if it goes from
high pressure to low pressure (i.e. it expands) but it must go again to
high pressure if it is to continue producing work. In other words, if we
want to obtain work continuously, we need to repeat the process, i.e.
work in cycles. Imagine pedaling a bicycle!
If a volume of air is to be compressed from 1 bar to 10 bar, work
must be done on it. If this volume of air is expanded from 10 bar to 1
bar, work will be given out by it. What will be the net work obtained
from such a cycle
- In an ideal situation?
- In a real situation?
- How does irreversibility affect real life processes?
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What Thermodynamics helps us to do!
Thermodynamics, as we have seen before, is all about convertingthe abundantly available heat energy (in the form of fuels) to a more
useful form.
If after compressing air from 1 bar to 10 bar, we add heat to it, then
expand it, we can get more work than if we were to add no heat.
Some of the heat added can be converted to mechanical energy, butnot all of it. Therefore, the efficiency of conversion is < 100%.
Thermodynamic cycles help us to convert heat (or rather thermal
energy) to mechanical energy (work). This may be further changed
to electrical energy which is easy to transmit and distribute to all
parts of the country. We stated that not all of the heat added can be converted to work.
Some of it has to be rejected.
Why cant ALL of the heat be converted to work? What should be
the temperature of the sink, if we wanted to reject zero heat?
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BASIC CONSIDERATIONS IN THE ANALYSIS
OF POWER CYCLES
WHY PERFORM MODELING?
Modeling is a powerful
engineering tool that providesgreat insight and simplicity at
the expense of some loss in
accuracy.
Most power-producing devices operate on cycles.
Ideal cycle:A cycle that resembles the actual cycle closely but is made
up totally of internally reversible processes is called an ideal cycle.
Reversible cycles such as Carnot cycle have the highest thermal
efficiency of all heat engines operating between the same temperature
levels. Unlike ideal cycles, they are totally reversible, and unsuitable as
a realistic model.
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Thermal efficiency of heat engines
The analysis of many
complex processes can
be reduced to a
manageable level byutilizing some
idealizations.
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Care should be exercised in
the interpretation of the
results from ideal cycles.
On a T-s diagram, the ratio of the area enclosed by the
cyclic curve to the area under the heat-addition process
curve represents the thermal efficiency of the cycle.
Any modification that increases the ratio of these two areas
will also increase the thermal efficiency of the cycle.
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The idealizations and simplifications in the analysis of power
cycles:
1. The cycle does not involve any friction. Therefore, the working fluid
does not experience any pressure drop as it flows in pipes ordevices such as heat exchangers.
2. All expansion and compression processes take place in a quasi-
equilibrium manner.
3. The pipes connecting the various components of a system are well
insulated, and heat transferthrough them is negligible.
On both P-vand T-s diagrams, the area enclosed by
the process curve represents the net work of the cycle.
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THE VALUE OF THE CARNOT CYCLE IN ENGINEERING
A steady-flow Carnot engine.
The Carnot cycle is composed of four totally reversible processes: isothermal
heat addition, isentropic expansion, isothermal heat rejection, and isentropic
compression.
Forboth ideal and actual cycles: Thermal efficiency increases with an
increase in the average temperature at which heat is supplied to the system or
with a decrease in the average temperature at which heat is rejected from the
system.
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P-vand T-s
diagrams of a
Carnot cycle.
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The combustion process is replaced by a
heat-addition process in ideal cycles.
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AIR-STANDARD ASSUMPTIONS
Air-standard assumptions:
1. The working fluid is air, which continuously circulates in aclosed loop and always behaves as an ideal gas.
2.All the processes that make up the cycle are internally
reversible.
3. The combustion process is replaced by a heat-additionprocess from an external source.
4. The exhaust process is replaced by a heat-rejection
process that restores the working fluid to its initial state.
Air-standard cycle:A cycle for which
the air-standard assumptions are applicable.
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Nomenclature for
reciprocating engines.
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AN OVERVIEW OF RECIPROCATING ENGINES
Spark-ignition (SI) engines Compression-ignition (CI) engines
Compression ratio
Mean effective pressure