Thermo Lecture Ch1

8/6/2019 Thermo Lecture Ch1

1/26

1

Thermodynamics I

Dimensions, Units, System

and Equlibrium

Lecture 1

9/21/2009

Objectives

Identify the unique vocabulary associated withthermodynamics through the precise definition

of basic concepts to form a sound foundationfor the development of the principles ofthermodynamics.

Review the metric SI and the English unitsystems.

Get familiar with the terminology ofthermodynamics


2/26

2

THERMODYNAMICS AND

ENERGY Thermodynamics: The science of

energy. Energy: The ability to cause changes.

The name thermodynamicsstemsfrom the Greek words therme(heat)and dynamis(power).

Conservation of energy principle:During an interaction, energy canchange from one form to another butthe total amount of energy remains

constant.

Energy cannot be created ordestroyed; it can only changeforms (the first law).

Applications


3/26

3

Laws of Thermodynamics The first law of

thermodynamics: Anexpression of the conservationof energy principle. The first law asserts that energyis

a thermodynamic property.

The second law ofthermodynamics: It assertsthat energy has qualityas wellas quantity, and actualprocesses occur in the directionof decreasing quality of energy.

Macroscopic and microscopic point of

view

Classical thermodynamics: A macroscopicapproach to the study of thermodynamics thatdoes not require a knowledge of the behavior of

individual particles. It provides a direct and easy way to the solution of

engineering problems and it is used in this text.

Statistical thermodynamics: A microscopicapproach, based on the average behavior oflarge groups of individual particles.

It is used in this text only in the supporting role.


4/26

4

Dimensions and Units Dimensions: Any physical quantity.

The magnitudes assigned to thedimensions are called units.

primary or fundamentaldimensions : mass m, length L, time t, and

temperature T

secondary dimensions, orderived dimensions. velocity V, energy E, and volume V

Systems of Unit

Metric SI system: A simple and logicalsystem based on a decimal relationship

between the various units. English system: It has no apparent

systematic numerical base, and variousunits in this system are related to eachother rather arbitrarily.


5/26

5

Dimensions Primary (basic) dimensions

Length, L

Time, T(t)

Mass, M

Temeprature, (T)

Secondary dimensions

Developed from primary dimensions

Area = L2

Velocity=LT-1

Density=ML-3

Ways to specify

primary dimensions

There are three basic

systems ofdimensions

MLT (MLT)

FLT (FLT

FMLT


6/26

6

Dimensional homogeneity

All theoretically derived equations are

dimensionally homogeneous, that is, the

dimensions of the left side of the equation

must be the same as those on the right hand

side, and all additive separte terms must have

the same dimensions

e.g. V=V0+at

Common systems of units

lb/ft2 (psf) or

lb/in2(psi)

Pa (N/m2)bar (106 dyne/cm2)P

R(oF)R(oF)KK

lblbN (newton)dyneF

sluglbmkggM

ssssT

ftftmcmL

British

Gravitational

System (BG)

English Engineering

System (EE)

International

System (SI)

CGS

U.S. systemMetric system


7/26

7

Unit conversion SI system should

be used in mostcases, but othersystems aresometimes

preferred in a fewfields.

Use SI system in

your work for theclass unless aparticular unitsystem is specified.

Conversion table and constants


8/26

8

Some SI and English Units

The SI unit prefixes are used in allbranches of engineering.

The definition of the force units.

Work = Force Distance1 J = 1 Nm

1 cal = 4.1868 J1 Btu = 1.0551 kJ

The relative magnitudes of the forceunits newton (N), kilogram-force

(kgf), and pound-force (lbf).

The weight of a unit

mass at sea level.

A body weighing60 kgf on earthwill weigh only 10kgf on the moon.

W weightm massg gravitationalacceleration


9/26

9

Unity Conversion Ratios

All nonprimary units (secondary units) can beformed by combinations of primary units.Force units, for example, can be expressed as

They can also be expressed more conveniently

as unity conversion ratios as

Unity conversion ratios are identically equal to 1 and

are unitless, and thus such ratios (or their inverses)can be inserted conveniently into any calculation toproperly convert units.

Dimensional homogeneity

All equations must be dimensionally homogeneous.

To be dimensionally

homogeneous, all theterms in an equationmust have the same unit.

System

Closed system

Control volume


10/26

10

System System: A quantity of matter

or a region in space chosenfor study.

Surroundings: The mass orregion outside the system

Boundary: The real orimaginary surface thatseparates the system from its

surroundings. The boundary of a system

can be fixedor movable.

Systems may be consideredto be closedor open.

Closed System

Also called Isolated SystemorControl Mass

A fixed amount of mass, and

no mass can cross itsboundary.

Always contains the samematter/mass.

No mass can cross theboundary, but energyexchange is allowed.

Composition of the massmay change, e.g. combustion.


11/26

11

Control Volume Also called Open system

A properly selected region in space.

It usually encloses a device that involves massflow such as a compressor, turbine, or nozzle.

Both mass and energy can cross the boundary ofa control volume.

Control surface is the boundaries of a controlvolume. It can be real or imaginary.

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

Property, State, Process and

Equilibrium

Property A macroscopic characteristics of a system

Assigned at a given time (state) without the knowledge of

the previous behavior (process) of the system. State

The condition of a system as described by its properties.

2 independent properties define a state.

Steady state means properties of the system do not changewith time.

Process A transformation from one state to another.

Equilibrium A state of balance.


12/26

12

Properties of a System Property: Any characteristic of a system.

A quantity if, and only if, its change in value between two statesis independent of the process.

Some familiar properties are pressure P, temperature T,volume V, and mass m.

Properties are considered to be either intensiveorextensive.

Intensive properties: Those that are independent ofthe mass of a system, such as temperature, pressure,

and density. Extensive properties: Those whose values depend on

the sizeor extentof the system.

Specific properties: Extensive properties per unitmass.

State Postulate

The number of properties required to fixthe state of a system is given by thestate postulate:

The state of a simple compressiblesystem is completely specified by

TWOindependent, intensiveproperties.

Simple compressible system: If asystem involves no electrical, magnetic,gravitational, motion, and surfacetension effects.


13/26

13

ProcessProcess: Any change that a system

undergoes from one equilibrium stateto another.

Path: The series of states through which asystem passes during a process.

To describe a process completely, oneshould specify the initial and finalstates, as well as the path it follows,and the interactions with the

surroundings.Quasistatic or quasi-equilibrium process:

When a process proceeds in such amanner that the system remainsinfinitesimally close to an equilibrium

Types of Process Process diagrams plotted by employing

thermodynamic properties as coordinatesare very useful in visualizing the processes.

Some common properties that are used ascoordinates are temperature T, pressure

P, and volume V(or specific volume v). The prefix iso- is often used to designate a

process for which a particular propertyremains constant.

Isothermal process: A process duringwhich the temperature Tremains constant.

Isobaric process: A process during whichthe pressure Premains constant.

Isochoric (or isometric) process: Aprocess during which the specific volume vremains constant.

Cycle: A process during which the initialand final states are identical.


14/26

14

Diagrams of Processes

P

v

Steady Flow Process Steadyimplies no change with time.

The opposite of steady is unsteady, ortransient.

A large number of engineering devicesoperate for long periods of time under thesame conditions, and they are classified assteady-flow devices

During a steady-flow process, fluid propertieswithin the control volume may change withposition but not with time.

Under steady-flow conditions, the mass andenergy contents of a control volume remainconstant.

Steady-flow process: A process duringwhich a fluid flows through a control volumesteadily.

Steady-flow device: turbines, pumps, boilers,condensers, and heat exchangers or powerplants or refrigeration systems.


15/26

15

Equilibrium Thermodynamics deals with equilibriumstates.

Equilibrium: A state of balance.

In an equilibrium state there are no unbalancedpotentials (or driving forces) within the system.

Thermal equilibrium: If the temperature is the samethroughout the entire system.

Mechanical equilibrium: If there is no change inpressure at any point of the system with time.

Phase equilibrium: If a system involves two phasesand when the mass of each phase reaches anequilibrium level and stays there.

Chemical equilibrium: If the chemical composition of asystem does not change with time, that is, no chemicalreactions occur.

Equilibrium

A system at two different states.A closed system reaching thermalequilibrium.


16/26

16

Important Properties

Density

Temperature

Pressure

Density

Density

Specific volume

Specific weight

Specific gravity (SG)

The ratio of the density of a substance to the density of some standardsubstance at a specified temperature (usually water at 4C).


17/26

17

Zeroth law of thermodynamics

The zeroth law of thermodynamics: If twobodies are in thermal equilibrium with a thirdbody, they are also in thermal equilibrium witheach other.

By replacing the third body with a thermometer,the zeroth law can be restated as two bodies

are in thermal equilibrium if both have the sametemperature reading even if they are not incontact.

Temperature Scale

All temperature scales are based on some easilyreproducible states such as the freezing and boilingpoints of water: the ice pointand the steam point.

Ice point: A mixture of ice and water that is inequilibrium with air saturated with vapor at 1 atmpressure (0C or 32F).

Steam point: A mixture of liquid water and watervapor (with no air) in equilibrium at 1 atm pressure(100C or 212F).


18/26

18

Temperature Scales Celsius scale: in SI unit system

Fahrenheit scale: in English unit system

Thermodynamic temperature scale: Atemperature scale that is independent of theproperties of any substance.

Kelvin scale (SI) Rankine scale (E)

A temperature scale nearly identical to the Kelvin

scale is the ideal-gas temperature scale. Thetemperatures on this scale are measured using aconstant-volume gas thermometer.

Absolute Zero: 0 K P versus Tplots of the experimental data

obtained from a constant-volume gasthermometer using four different gases atdifferent (but low) pressures.

A constant-volume gas thermometerwould read 273.15C at absolute zeropressure.

The reference temperature in the originalKelvin scale was the ice point, 273.15 K,which is the temperature at which waterfreezes (or ice melts).

The reference point was changed to amuch more precisely reproducible point,thetriple point of water (the state atwhich all three phases of water coexist inequilibrium), which is assigned the value273.16 K.


19/26

19

Conversions Between Temperature

Scales

Pressure

A normal force exerted by a fluid per unit area

Unit:


20/26

20

The standard atmosphere At sea level the U.S. Standard Atmosphere

Conditions are listed as follows:

The standard atmosphere


21/26

21

Pressure Measurements: Mercury

Manometer

Evangelista Torricelli

(1608-1647), 1644

Patm = h+Pvaporh

Pvapor=0.000023 lb/in2

(abs) at 68oF, can be

neglected

PRESSURE

The normal stress (or pressure) on thefeet of a chubby person is much greater

than on the feet of a slim person.

Somebasicpressuregages.

Pressure: A normal force exertedby a fluid per unit area

68 kg 136 kg

Afeet=300cm2

0.23 kgf/cm2 0.46 kgf/cm2

P=68/300=0.23 kgf/cm2


22/26

22

The standard atmosphere

The temperature decreases linearly with

height (up to 11 km above sea level, called

troposhere) is according to

T=(288-0.006507 z) K

=(519-0.00357z) oR

From height of 11 km to 20.1 km, theatmosphere is isothermal at a temperature of -

56.5 oC which is called stratosphere.

Measurement of pressure Absolute pressures are always

positive

Gage pressures can bepositive (negative) if the

pressure is above (below) theatmospheric pressure

A negative gage pressure isreferred to as a suction orvacuum pressure. In the text,pressures will be assumed tobe gage pressures unlessspecifically designatedabsolute 10 psi or 100 kPa : gage

pressures

10 psia or 100 kPa (abs):absolute pressure


23/26

23

Absolute, Gage and Vacuum Pressures

Absolute pressure: The actual pressure at agiven position. It is measured relative toabsolute vacuum (i.e., absolute zero pressure).

Gage pressure: The difference between theabsolute pressure and the local atmosphericpressure. Most pressure-measuring devices arecalibrated to read zero in the atmosphere, and

so they indicate gage pressure. Vacuum pressures: Pressures below

atmospheric pressure.

Absolute, Gage and Vacuum Pressures

Throughout

this text, thepressure Pwill denoteabsolutepressureunlessspecifiedotherwise.


24/26


25/26

25

ManometryU-tube manometer The pressure PA is large =>

a heavy gage fluid (mercury)can be used

The pressure PA is small =>a lighter gage fluid (water)can be used

The difference in pressurebetween two containers or

two points in a given systemP1+1h1-2h2-3h3=P5PA-PB=2h2+3h3-1h1

ManometryManometry InclinedInclined--tubetube

manometermanometer

(P1-P2)A=Alg =>P=lsin

PA+1h1-2l2sin-3h3=PB=>PA-PB=2l2sinq+3h3-1h1

For gases of A and B, then

As 0


26/26

Other Pressure Measurement Devices Bourdon tube: Consists of a hollow metal tube bent like a hook whose

end is closed and connected to a dial indicator needle.

Pressure transducers: Use various techniques to convert the pressureeffect to an electrical effect such as a change in voltage, resistance, orcapacitance.

Pressure transducers are smaller and faster, and they can be more sensitive,reliable, and precise than their mechanical counterparts.

Strain-gage pressure transducers: Work by having a diaphragm deflectbetween two chambers open to the pressure inputs.

Piezoelectric transducers: Also called solid-state pressure transducers,work on the principle that an electric potential is generated in a crystalline

substance when it is subjected to mechanical pressure.

HW Ch #1

1-2, 1-15, 1-25, 1-32, 1-34, 1-37, 1-41

Documents

Thermo Lecture Ch1