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Lecture 1 & 2

Introduction and Basic Concepts (Ch-1)

Thermodynamics

Zia Ud Din

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Objectives

Identify the unique vocabulary associated with

thermodynamics through the precise definition ofbasic concepts to form a sound foundation for thedevelopment of the principles of thermodynamics.

Review the metric SI and the English unit systems.

Explain the basic concepts of thermodynamics suchas system, state, state postulate, equilibrium,process, and cycle.

Review concepts of temperature, temperature scales,pressure, and absolute and gage pressure.

Introduce an intuitive systematic problem-solvingtechnique.

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Convert heat into power

Thermodynamics: Science of Energy

Thermodynamics is a science and, more importantly,an engineering tool used to describe processes thatinvolve changes in temperature, transformation ofenergy, and the relationships between heat and work

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Applications of Thermodynamics:

Electric or gas rangeHeating and airconditioningRefrigeratorThe humidifier

On larger scaleAutomotive enginesRocketsJet enginesConventional or Nuclear PPSolar SystemsDesinging from small cars to aeroplanes

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Applications of Thermodynamics:

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THERMODYNAMICS AND ENERGY

Thermodynamics: The science of

energy. Energy: The ability to cause changes.

The name thermodynamicsstems fromthe Greek words therme(heat) anddynamis(power).

Conservation of energy principle:During an interaction, energy can changefrom one form to another but the totalamount of energy remains constant.

Energy cannot be created or destroyed.

The first law of thermodynamics: Anexpression of the conservation of energyprinciple.

The first law asserts that energyis athermodynamic property.

Energy cannot be createdor destroyed; it can onlychange forms (the first law).

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The second law of thermodynamics:It asserts that energy has qualityaswell as quantity, and actual processesoccur in the direction of decreasingquality of energy.

Classical thermodynamics: Amacroscopic approach to the study ofthermodynamics that does not requirea knowledge of the behavior of

individual particles.

It provides a direct and easy way to thesolution of engineering problems and itis used in this text.

Statistical thermodynamics: Amicroscopic approach, based on theaverage behavior of large groups ofindividual particles.

It is used in this text only in thesupporting role.

Heat flows in the direction ofdecreasing temperature.

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SYSTEMS AND CONTROL VOLUMES System: A quantity of matter or a region

in space chosen for study.

Surroundings: The mass or regionoutside the system

Boundary: The real or imaginary surfacethat separates the system from itssurroundings.

The boundary of a system can be fixedormovable.

Systems may be considered to be closedor open.

Closed system

(Control mass):A fixed amountof mass, and nomass can crossits boundary.

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Open system (control volume): A properlyselected region in space.

It usually encloses a device that involvesmass flow such as a compressor, turbine, or

nozzle.

Both mass and energy can cross theboundary of a control volume.

Control surface: The boundaries of a controlvolume. It can be real or imaginary.

An open system (acontrol volume) with oneinlet and one exit.

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Selecting the System Boundary:

Choice of boundary defining a particular system depends upon

the convenience of analysis

(1) What is known about a possible system(2) Objective of analysis

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Selecting the System Boundary: Example

The system boundry encloses the compressor, tank and all

piping

Known; elect. InputObjective of analysis; how long compressor must operate forpressure in the tank to rise to required value

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To describe a system requires knowledgeof its properties

Property: is a macroscopic characteristic of a systempressure P, temperature T, volume V, andmass m

State: refers to the condition of a system as described byits properties

Process: When any of theproperties of a system chage, thestate changes and system is said to

undergone a process

Steady State: if a system exhibits the same value of itsproperties at two different times

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PROPERTIESOF A SYSTEM

Property: Any characteristic of asystem.

Some familiar properties arepressure P, temperature T, volumeV, and mass m.

Properties are considered to beeither intensiveor extensive.

Intensive properties: Those thatare independent of the mass of asystem, such as temperature,pressure, and density.

Extensive properties: Thosewhose values depend on the sizeor extentof the system.

Specific properties: Extensiveproperties per unit mass.

Criterion to differentiate intensiveand extensive properties.

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Continuum Matter is made up of atoms that are

widely spaced in the gas phase. Yetit is very convenient to disregard the

atomic nature of a substance andview it as a continuous,homogeneous matter with no holes,that is, a continuum.

The continuum idealization allows usto treat properties as point functions

and to assume the properties varycontinually in space with no jumpdiscontinuities.

This idealization is valid as long asthe size of the system we deal withis large relative to the space

between the molecules. This is the case in practically all

problems.

In this text we will limit ourconsideration to substances that canbe modeled as a continuum.

Despite the large gaps betweenmolecules, a substance can be treated asa continuum because of the very largenumber of molecules even in anextremely small volume.

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The State Postulate:

The number of properties required to fix the state of a system is

given by the state postulate: The state postulate requires that the two properties specified beindependent to fix the state.

The state of nitrogen is

fixed by two

independent

properties

Two properties are independent if oneproperty can be varied while the other one isheld constant.

Temperature and Specific volume are alwaysindependent properties.

Temperature and Pressure are independentproperties for only single phase systems

But Temperature and Pressure aredependent properties for multiple phase system.That is T= f(P)

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PROCESSES AND CYCLESProcess: Any change that a system undergoes from one equilibrium state to

another.

Path: The series of states through which a system passes during a process.To describe a process completely, one should specify the initial and final states,

as well as the path it follows, and the interactions with the surroundings.

Quasistatic or quasi-equilibrium process: When a process proceeds in sucha manner that the system remains infinitesimally close to an equilibrium stateat all times.

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Expansion or Compression Work inQuasiequilibrium Processes: REVERSIBILITY

Departure from thermodynamic equilibrium is infinitesimal

All states in such process are considered equilibrium states

Imagine one of the masses removed the state will depart only slightly

If masses are removed one afteranother, the gas would pass through asequence of equilibrium states withoutever being far from equilibrium

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Process:Quasi-equilibrium process is anIdealized Process: Engineers are

interested in in quasi-equilibriumprocesses for two reasons:

Easier to analyze

Quasi-equilibrium processes

serves as standards to which actualprocesses can be compared

Process diagrams plotted byemploying thermodynamic propertiesas coordinates are very useful in

visualizing the processes. Some common properties that areused as coordinates are temperatureT, pressure P, and volume V(orspecific volume v).

The P-Vdiagram of acompression process.

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The Steady-Flow Process The term steadyimplies no

change with time. Theopposite of steady isunsteady, or transient.

A large number ofengineering devices operatefor long periods of timeunder the same conditions,and they are classified as

steady-flow devices.

Steady-flow process: Aprocess during which a fluidflows through a controlvolume steadily.

Steady-flow conditions canbe closely approximated bydevices that are intended forcontinuous operation suchas turbines, pumps, boilers,condensers, and heatexchangers or power plants

or refrigeration systems.

During a steady-flow process, fluid

properties withinthe control

volume may

change withposition but not

with time.

Under steady-flow conditions, the mass

and energy contents of a control volumeremain constant.