Thermo Intro

8/18/2019 Thermo Intro

1/30

1

THERMODYNAMICS AND ENERGY

• Thermodynamics: The science of

energy.• Energy: The ability to cause changes.

• The name thermodynamics stems fromthe Greek words therme (heat) anddynamis (power).

• Conservation of energy rinci!e: During an interaction energy can changefrom one form to another but the totalamount of energy remains constant.

• !nergy cannot be created or destroyed.

• The first !a" of thermodynamics: "ne#pression of the conser$ation of energyprinciple.

• The first law asserts that energy is athermodynamic property.

!nergy cannot be created

or destroyed% it can only

change forms (the first law).


2/30

&

• The second !a" of thermodynamics# 't asserts that energy has quality as wellas quantity , and actual processes occurin the direction of decreasing uality ofenergy.

• C!assica! thermodynamics: "macroscopic approach to the study ofthermodynamics that does not reuire aknowledge of the beha$ior of indi$idual

particles.

• 't pro$ides a direct and easy way to thesolution of engineering problems and itis used in this te#t.

• Statistica! thermodynamics: "microscopic approach based on thea$erage beha$ior of large groups ofindi$idual particles.

• 't is used in this te#t only in thesupporting role.

onser$ation of energy

principle for the human body.

*eat flows in the direction of

decreasing temperature.


3/30

+

A!ication Areas of Thermodynamics


4/30

,

IM$ORTANCE O% DIMENSIONS AND &NITS• "ny physical uantity can be characteri-ed by

dimensions.

• The magnitudes assigned to the dimensionsare called 'nits.

• ome basic dimensions such as mass mlength L time t and temperature T areselected as rimary or f'ndamenta!dimensions while others such as $elocity V energy E and $olume V are e#pressed interms of the primary dimensions and are calledsecondary dimensions or deriveddimensions.

• Metric SI system: " simple and logical system

based on a decimal relationship between the$arious units.

• Eng!ish system: 't has no apparentsystematic numerical base and $arious unitsin this system are related to each other rather

arbitrarily.


5/30

/

Some SI and Eng!ish &nits

The ' unit prefi#es are used in all

branches of engineering.

The definition of the force units.

0ork 2orce × Distance

1 3 1 45m

1 cal ,.1676 31 8tu 1.9//1 k3


6/30

7

The relati$e magnitudes of the force

units newton (4) kilogramforce

(kgf) and poundforce (lbf).

The weight of a unit

mass at sea le$el.

" body weighing

79 kgf on earth

will weigh only 19

kgf on the moon.

W weight

m mass

g gra$itationalacceleration


7/30

;

&nity Conversion Ratios All nonprimary units (secondary units) can be

formed by combinations of primary units.

2orce units for e#ample can be e#pressed as

They can also be e#pressed more con$eniently

as 'nity conversion ratios as


8/30


9/30

=

• Oen system (contro! vo!'me,: " properlyselected region in space.

• 't usually encloses a de$ice that in$ol$esmass flow such as a compressor turbine orno--le.

• 8oth mass and energy can cross theboundary of a control $olume.

• Contro! s'rface: The boundaries of acontrol $olume. 't can be real or imaginary.

"n open system (a

control $olume) with one

inlet and one e#it.


10/30


11/30

11

Contin''m• @atter is made up of atoms that are

widely spaced in the gas phase. Aetit is $ery con$enient to disregard the

atomic nature of a substance and$iew it as a continuoushomogeneous matter with no holesthat is a contin''m.

• The continuum ideali-ation allows usto treat properties as point functions

and to assume the properties $arycontinually in space with no Bumpdiscontinuities.

• This ideali-ation is $alid as long asthe si-e of the system we deal withis large relati$e to the space

between the molecules.• This is the case in practically all

problems.

• 'n this te#t we will limit ourconsideration to substances that canbe modeled as a continuum.

Despite the large gaps between

molecules a substance can be treated as

a continuum because of the $ery large

number of molecules e$en in an

e#tremely small $olume.


12/30

1&

DENSITY AND S$ECI%IC GRA)ITY

Density is

mass per unit$olume%

specific $olume

is $olume per

unit mass.

Secific gravity: The ratioof the density of a

substance to the density ofsome standard substance ata specified temperature(usually water at ,C).

Density

Secific "eight: The

weight of a unit $olumeof a substance.

Secific vo!'me


13/30

1+

STATE AND E.&I(I*RI&M

• Thermodynamics deals withequilibrium states.

• E/'i!i0ri'm: " state of balance.

• 'n an euilibrium state there are nounbalanced potentials (or dri$ingforces) within the system.

• Therma! e/'i!i0ri'm: 'f thetemperature is the same throughoutthe entire system.

• Mechanica! e/'i!i0ri'm# 'f there isno change in pressure at any point ofthe system with time.

• $hase e/'i!i0ri'm# 'f a systemin$ol$es two phases and when themass of each phase reaches aneuilibrium le$el and stays there.

• Chemica! e/'i!i0ri'm# 'f thechemical composition of a systemdoes not change with time that is nochemical reactions occur. " closed system reaching thermal

euilibrium.

" system at two different states.


14/30

1,

The State $ost'!ate

• The number of properties

reuired to fi# the state of asystem is gi$en by the stateost'!ate:

The state of a simplecompressible system iscompletely specified byt#o independent,intensi!e properties.

• Sim!e comressi0!esystem# 'f a system in$ol$esno electrical magneticgra$itational motion andsurface tension effects.

The state of nitrogen is

fi#ed by two independent

intensi$e properties.


15/30

1/

$ROCESSES AND CYC(ES$rocess: "ny change that a system undergoes from one euilibrium state to another.

$ath: 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 aswell as the path it follows and the interactions with the surroundings.

.'asistatic or /'asi1e/'i!i0ri'm rocess# 0hen a process proceeds in such amanner that the system remains infinitesimally close to an euilibrium state at alltimes.


16/30

17

• >rocess diagrams plotted byemploying thermodynamic propertiesas coordinates are $ery useful in$isuali-ing the processes.

• ome common properties that areused as coordinates are temperatureT pressure " and $olume V (orspecific $olume ! ).

• The prefi# iso is often used todesignate a process for which a

particularproperty remains constant.• Isotherma! rocess: " process

during which the temperature Tremains constant.

• Iso0aric rocess: " process duringwhich the pressure " remains

constant.• Isochoric +or isometric, rocess: "

process during which the specific$olume ! remains constant.

• Cyc!e: " process during which theinitial and final states are identical.

The " V diagram of a compression

process.


17/30

1;

The Steady1%!o" $rocess• The term steady implies no

change #ith time. The

opposite of steady isunsteady , or transient .

• " large number ofengineering de$ices operatefor long periods of timeunder the same conditionsand they are classified assteady$flo# de!ices.

• Steady1f!o" rocess: "process during which a fluidflows through a control$olume steadily.

•teadyflow conditions canbe closely appro#imated byde$ices that are intended forcontinuous operation suchas turbines pumps boilerscondensers and heate#changers or power plants

or refrigeration systems.

During a steady

flow process fluid

properties within

the control

$olume may

change withposition but not

with time.


18/30

16

TEM$ERAT&RE AND THE 2EROTH (A3 O%

THERMODYNAMICS• The 4eroth !a" of thermodynamics: 'f two bodies are in thermal

euilibrium with a third body they are also in thermal euilibrium witheach other.

• 8y replacing the third body with a thermometer the -eroth law can berestated as t#o bodies are in thermal equilibrium if both ha!e the sametemperature reading e!en if they are not in contact .

Two bodies reaching

thermal euilibrium

after being brought

into contact in an

isolated enclosure.


19/30

1=

Temerat're Sca!es• "ll temperature scales are based on

some easily reproducible states such asthe free-ing and boiling points of water:the ice point and the steam point.

• Ice oint: " mi#ture of ice and water thatis in euilibrium with air saturated with$apor at 1 atm pressure (9C or +&C2).

• Steam oint: " mi#ture of liuid waterand water $apor (with no air) ineuilibrium at 1 atm pressure (199C or

&1&C2).• Ce!si's sca!e: in ' unit system

• %ahrenheit sca!e: in !nglish unit system

• Thermodynamic temerat're sca!e: "temperature scale that is independent ofthe properties of any substance.

• 5e!vin sca!e (') Ran6ine sca!e (!)• " temperature scale nearly identical to

the el$in scale is the idea!1gastemerat're sca!e. The temperatureson this scale are measured using aconstant$olume gas thermometer.

" $ersus T plots

of the

e#perimental

data obtained

from a constant$olume gas

thermometer

using four

different gases

at different (but

low) pressures.

" constant$olume gas thermometer would

read &;+.1/C at absolute -ero pressure.


20/30

&9

omparison of

temperature

scales.

• The reference temperature in the original el$in scale was the ice point 7

&;+.1/ which is the temperature at which water free-es (or ice melts).

• The reference point was changed to a much more precisely reproducible

point the triple point of water (the state at which all three phases of water

coe#ist in euilibrium) which is assigned the $alue &;+.17 .

omparison of

magnitudes of

$arious

temperature

units.


21/30

&1

$RESS&RE

The normal stress (or EpressureF) on the

feet of a chubby person is much greaterthan on the feet of a slim person.

ome basic

pressure

gages.

$ress're: " normal force e#erted

by a fluid per unit area

76 kg 1+7 kg

"feet+99cm&

9.&+ kgfcm& 9.,7 kgfcm&

" 76+999.&+ kgfcm&


22/30

&&

• A0so!'te ress're: The actual pressure at a gi$en position. 't ismeasured relati$e to absolute $acuum (i.e. absolute -ero pressure).

• Gage ress're: The difference between the absolute pressure andthe local atmospheric pressure. @ost pressuremeasuring de$ices are

calibrated to read -ero in the atmosphere and so they indicate gagepressure.

• )ac''m ress'res: >ressures below atmospheric pressure.

Throughout

this te#t the

pressure P

will denote

absolute

pressure

unless

specifiedotherwise.


23/30

&+

)ariation of $ress're "ith Deth

2reebody diagram of a rectangular

fluid element in euilibrium.

The pressure of a fluid at rest

increases with depth (as a

result of added weight).

0hen the $ariation of density

with ele$ation is known


24/30

&,

'n a room filled with

a gas the $ariation

of pressure with

height is negligible.

>ressure in a liuid

at rest increases

linearly with

distance from thefree surface.

The pressure is the

same at all points on

a hori-ontal plane in

a gi$en fluid

regardless of

geometry pro$ided

that the points are

interconnected by the

same fluid.


25/30

&/

$asca!8s !a": The pressure applied to a

confined fluid increases the pressure

throughout by the same amount.

Hifting of a large weight

by a small force by the

application of >ascalIs

law.

The area ratio A& A1 is

called the ideal mechanical

ad!antage of the hydrauliclift.


26/30

&7

The Manometer

'n stackedup fluid layers the

pressure change across a fluid layer

of density ρ and height h is gh.

@easuring the

pressure drop across a

flow section or a flow

de$ice by a differential

manometer.

The basic

manometer.

't is commonly used to measure small and

moderate pressure differences. " manometer

contains one or more fluids such as mercury water

alcohol or oil.


27/30

&;

Other $ress're Meas'rement Devices

Jarious types of 8ourdon tubes used

to measure pressure.

• *o'rdon t'0e: onsists of a hollow metal tube

bent like a hook whose end is closed and

connected to a dial indicator needle.

• $ress're transd'cers: ressure transducers are smaller and faster

and they can be more sensiti$e reliable andprecise than their mechanical counterparts.

• Strain1gage ress're transd'cers# 0ork by

ha$ing a diaphragm deflect between two

chambers open to the pressure inputs.

•$ie4oe!ectric transd'cers: "lso called solidstate pressure transducers work on the principle

that an electric potential is generated in a

crystalline substance when it is subBected to

mechanical pressure.


28/30

&6

THE *AROMETER AND ATMOS$HERIC $RESS&RE

• "tmospheric pressure is measured by a de$ice called a 0arometer % thus the

atmospheric pressure is often referred to as the barometric pressure.

• " freuently used pressure unit is the standard atmosphere which is defined asthe pressure produced by a column of mercury ;79 mm in height at 9C ( ρ *g

1+/=/ kgm+) under standard gra$itational acceleration (g % =.69; ms&).

The basic barometer.

The length or thecrosssectional area

of the tube has no

effect on the height

of the fluid column of

a barometer

pro$ided that thetube diameter is

large enough to

a$oid surface tension

(capillary) effects.


29/30

&=

$RO*(EM1SO()ING TECHNI.&E

• tep 1: >roblem tatement

• tep &: chematic• tep +: "ssumptions and "ppro#imations

• tep ,: >hysical Haws

• tep /: >roperties

• tep 7: alculations

• tep ;: Keasoning Jerification and Discussion

EES +Engineering E/'ation So!ver, (>ronounced as ease):

!! is a program that sol$es systems of linear or nonlinearalgebraic or differential euations numerically. 't has a large

library of builtin thermodynamic property functions as well as

mathematical functions.


30/30

+9

S'mmary• Thermodynamics and energy

"pplication areas of thermodynamics

• 'mportance of dimensions and units ome ' and !nglish units Dimensional homogeneity

roperties of a system

• Density and specific gra$ity• tate and euilibrium

The state postulate

• >rocesses and cycles

The steadyflow process

• Temperature and the -eroth law of thermodynamics Temperature scales

• >ressure

Jariation of pressure with depth

• The manometer and the atmospheric pressure

• >roblem sol$ing techniue

Documents

Thermo Intro