thermo ii-chapter 1a

8/6/2019 Thermo II-Chapter 1a

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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

thermo ii-chapter 1a

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