01_ME 63_Some Concepts and Definitions

8/11/2019 01_ME 63_Some Concepts and Definitions

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ME 63

Thermodynamics

Department of

Mechanical Engineering

Some Concepts and Definitions


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Introduction

THERMODYNAMICS

the science of energyand entropy

the science that deals withheat and workand

the properties of substances that bear a

relation to heat and work

stems from the Greek word therme(heat) and

dynamis(power)


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Introduction

THERMODYNAMICS

basis is experimental observation and

formalized into basic laws which are the First,

Second, Third, and ZerothLaws of

Thermodynamics

the word thermodynamics was first used in a

publication by Lord Kelvin in 1849

The first textbook was written in 1859 by

William Rankine, at the University of Glasgow


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Applications of

Thermodynamics


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The Thermodynamic System

A systemis defined as a quantity of matter or

a region in space chosen for study. The mass

or region outside the system is called the

surroundings. The real or imaginary surfacethat separates the system from its

surroundings is called the boundary.

The extent of the system in space at any giventime is defined by the system boundary


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The envelope that represents the systemboundary which encloses the thermodynamicsystem is also known as the system control

surface The boundary can be fixed or movable


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Types of System

Closed System

Open System

Isolated System


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Closed System

Also known as control mass (amount of matter

inside control remains constant with time)

consists of a fixed amount of mass, and no mass

can cross its boundary. That is, no mass can

enter or leave a closed system, but energy, in

the form of heat or work, can cross the

boundary


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Closed System


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Open System


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Open System


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Isolated System

A system that is not influenced in any way by

the surroundings or environmentno mass and

energy flow across the system boundary


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Microscopic vs. Macroscopic

Microscopic Point of View

System behavior is described by describing the

behavior of each molecule which comprise the

system Governing equations are written for each molecule,

e.g., equations for position, velocity, etc.


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Microscopic vs. Macroscopic

Macroscopic Point of View

The gross/average effects or time-averagedinfluence of many molecules is used to describesystem behavior

Uses measurable parameters, e.g., pressure,temperature, etc.

System volume should be very large compared with

molecular dimensions (system should contain manymolecules)

System is treated as continuous, disregarding theaction of individual molecules


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Properties and State of a Substance

Phase

A quantity of matter that is homogenous

throughout; solid, liquid,gas

When more than one phase is present, each phaseis separated by phase boundaries


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State of a Substance

State

- Identified or described by certain observable,macroscopic properties


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Property of a Substance

Any quantity that depends only on the state of

the system

Independent of the path by which the state is

arrived at.

*Given a state, each property has only one definite

value

*Please remember that TWO INDEPENDENT

THERMODYNAMIC PROPERTIES determine a

state


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Some familiar properties are pressureP,

temperatureT, volumeV, and massm. The list

can be extended to include less familiar ones

such as viscosity, thermal conductivity, modulusof elasticity, thermal expansion coefficient,

electric resistivity, and even velocityand

elevation. A property of a system has significance for the

entire system only when the system is in

equilibrium.


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2 General Class of Properties

Intensive Properties

independent of mass

Examples: Pressure, Temperature

Extensive Properties

dependent of mass

Examples: Mass, Weight, Specific Volume


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Thermodynamic Equilibrium

A system is in equilibrium when it is in thermal,mechanical, and chemical equilibrium

Thermal equilibrium: uniform temperaturethroughout the system.

Mechanical equilibrium: pressure at any point inthe system does not vary with time when system isisolated

Chemical equilibrium : no tendency to changecomposition

A Thermodynamic Equilibrium a system condition thatprecludes all possible changes of state all equilibriumconditions are satisfied


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Processes and Cycles

Occurs when a change in property occurs

Any change that a system undergoes from one

equilibrium state to another is called aprocess, and

the series of states through which a system passesduring a process is called thepath of the process


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Processes

Quasi-Equilibrium Process (ideal process)

When a process proceeds in such a manner that

the system remains infinitesimally close to an

equilibrium state at all times.


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Processes

Non-Quasi-Equilibrium Process


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Cycle

a series of processes, one after the other, such

that the initial and final states are the same

initial and final system compositions are

similar.


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Units for Mass, Length, Time, and

Force

SI Units English Units

Time second (s) second (s)Length meter (m) foot (ft)

Mass kilogram (kg) pound mass (lbm)

Force Newton (N) pound force (lbf)


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SI and English Units


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Mass vs Weight

The mass of a body remains the

same regardless of its location in

the universe. Its weight, however,

changes with a change in

gravitational acceleration


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Density and Specific Volume

Densityis mass per unit volume

Specific Volume is the reciprocal of density


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Specific Gravity and Specific Weight

Specific gravity, or relative density, and is defined as the ratio

of the density of a substance to the density of some standard

substance at a specified temperature.

Substances with SG of less than 1 are lighter than water, thus

they would float on water

The weight of a unit volume is called specific weight


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Pressure

Of a liquid or gas is defined as the normal component of force perunit area

where

A = a differential area of a system

A= smallest area over which the fluid can be considered as acontinuum

Fn = component of force normal toA

Typical units,

SI: 1 Pascal (Pa) = 1 Newton / m2 (N/m2)

English: pound-force / ft2 (lbf/ft2), pound-force / in2 (lbf/in2)or pounds per square inch (psi)

Others: 1 bar = 105 Pa = 0.1 MPa

atm = 101,325 Pa = 14.696 lbf/in2


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Pressure


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Fluid pressure in relation to a movable boundary

Under equilibrium conditions,pressure P exerted by the gas on allits boundaries is the same

With no heat transfer, the pressure is

fixed by the external force Fextacting on the piston ; also, Fext =Pressure x Piston Area (from FBD ofpiston)

Heating/cooling of the gas tends toincrease/decrease pressure and

move piston to the right/left suchthat Pressure x Piston Area = Fext issatisfied.


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Pressure

Pressure is typically measured or indicated relative toeither of two references which are

Atmospheric Pressuretypically sea level pressureat standard conditions; measured by a barometer

Gauge pressure - indicates how much actual pressure isabove atmospheric pressure; measured by a pressuregauge

Vacuum pressure - indicates how much actual pressureis below atmospheric pressure; measured by a vacuumgauge

Absolute Zero Pressure zero pressure or perfectvacuum; measured by an absolute pressure gaugeor calculated from gauge/vacuum pressure


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Pressure


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Pressure Measurement

Using a Column of Fluid (Manometer)

where

= density of measuring fluid inside manometer

h= difference in level of measuring fluid inside manometer

g = gravitational constant = 9.80665 m/s2 = 32.174 ft/s2


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Pressure Measurement Example

A manometer is used to measure the pressure in atank. The fluid used has a specific gravity of 0.85, and

the manometer column height is 55 cm, as shown in

the figure. If the local atmospheric pressure is 96 kPa,

determine the absolute pressure within the tank.


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Pressure Measurement

Using Dial GaugesConsider the following

a. If Pi > Po ,

Pi = Po + Pg If Po = Patm , Pi, abs = Patm + Pg

b. If Pi < Po ,

Pi = Po - Pvac If Po = Patm , Pi, abs = Patm - Pvac

Tube

side

Dial side

Pd = pressure

reading

= Pg or Pvac

Po = pressure outside Compartment

= ambient pressure

Pi = pressureinside

compartment


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Equality of Temperature

Two bodies have equality of temperature if, when

they are in thermal equilibrium, no change in any

observable property occurs.


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The Zeroth Law of Thermodynamics

When two bodies have equality of temperature with a

third body, they in turn have equality of temperature

with each other.

"IfAis in thermal equilibrium with Band if Bis in

thermal equilibrium with C, thenAis in thermal

equilibrium with C."

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01_ME 63_Some Concepts and Definitions