Www.nptel.iitm.Ac.in Courses 101101001 Downloads Intro-Propulsion-Lect-5

8/13/2019 Www.nptel.iitm.Ac.in Courses 101101001 Downloads Intro-Propulsion-Lect-5

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In this lecture ...

Quasi-static processes

Concept of energy and its variousforms

Internal energy

Total energy

Enthalpy

Zeroth law of thermodynamics andtemperature

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay2

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When a process proceeds in such a mannerthat the system remains infinitesimallyclose to an equilibrium state at all times:

Quasi-static or Quasi-equilibrium process The process proceeds slow enough to allow

the system to the system to adjust itselfinternally so that properties in one part of

the system do not change any faster thanthose at other parts.


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W

Gasp1, v1, t1

Initial state

Final state

Stops

Weight

Piston

p1

p2

v1 v2

p

v

1

2


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Gas

p1, v1, t1

Initial state

Final state

Stops

Weights

Piston

p

v

1

2

x

x

xx x x

xx

Equilibrium states


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Engineers are interested in quasi-staticprocesses because

they are easy to analyse

work-producing devices deliver maximum workwhen they operate on quasi-static processes

Quasi-static processes serve as standardsto which actual processes can becompared


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Energy

Energy can exist in numerous forms such asthermal, mechanical, kinetic, potential,electric, magnetic, chemical, and nuclear

The sum total of the above: total energy, E

e = E/m kJ/kg

Thermodynamics provides no informationabout the absolute value of the total energy.

It deals only with the change of the totalenergy, which is what matters in engineeringproblems.


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Energy

Forms of energy:Macroscopic energy: energy that a system

possesses as a whole with respect to some

outside reference frame, eg: KE, PEMicroscopic energy: those related to the

molecular structure of a system and thedegree of the molecular activity and are

independent of outside reference framesSum of all microscopic forms of energy of a

system: Internal energy, U (kJ) or u (kJ/kg)


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Macroscopic energy


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The macroscopic energy, KE and PE, of anobject changes with velocity and elevation.


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Internal energy

Sensible energy: part of the internal energyassociated with kinetic energy of moleculesRotational KE, translational KE, vibrational KE

Latent energy: internal energy associated

with phase change of a system Chemical energy: internal energy associated

with the atomic bonds in a molecule

Nuclear energy: tremendous amount ofenergy associated with the strong bondswithin the nucleus of the atom


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12/32Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay12

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The various forms of microscopicenergies that make up sensible energy.


13/32Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

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The internal energy of a

system is the sum of allforms of the microscopicenergies.


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The macroscopickinetic energy is an organised formof energy and more useful than the disorganisedmicroscopickinetic energies of the molecules


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Kinetic and Potential energies

Kinetic energy, KE, of a system:

Potential energy, PE, of a system:


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basismassunitaon(kJ/kg)2or,(kJ)2

22V

ke

mV

KE ==

basismassunitaon(kJ/kg)or,(kJ) gzpemgzPE ==


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Total energy of a system

In the absence of magnetic, electric, andsurface tension effects, the total energy of asystem consists of the kinetic, potential, andinternal energies


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(kJ/kg)2

basismassunitaonor,

(kJ)2

2

2

gzV

upekeue

mgzmV

UPEKEUE

++=++=

++=++=


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Total energy of a system

Closed systems whose velocity and elevationof the center of gravity remain constantduring a process are frequently referred toas stationary systems.

The change in the total energy E of astationary system is identical to the change

in its internal energy U.


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Enthalpy


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The combination of internal energy u andpvis often encountered in the analysis ofcontrol volumes

Enthalpy is a combination property

Enthalpy is also often referred to as heatcontent

Process in which enthalpy is constant:isenthalpic process

kJ)(

(kJ/kg)Enthalpy,

PVUH

pvuh

+=

+=


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Enthalpy


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Control Volume

u1

p1v1

u2

p2v2

The combination u+pvis frequentlyencountered in the analysis of controlvolumes


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Zeroth law of thermodynamics

States that if two bodies are in thermalequilibrium with a third body, they are alsoin thermal equilibrium with each other.

It serves as a basis for the validity oftemperature measurement.

By replacing the third body with athermometer, the zeroth law can be restated

as two bodies are in thermal equilibrium ifboth have the same temperature readingeven if they are not in contact.


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A

B

C

Let us say TA,T

Band T

Care

the temperatures of A,B andC, respectively.

A and C are in thermalequilibrium: TA=TC

B and C are in thermal

equilibrium: TB=TC

Consequence of Zeroth law:TA=TB


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It was named the zeroth law since it shouldhave preceded (proposed in 1931) the firstand the second laws of thermodynamics thatwere defined in late 1800s.

All temperature scales based on reproduciblestates: freezing point (ice point) or boilingpoint of water (steam point).

On the Celsius scale, ice and steam pointswere assigned 0oC and 100oC respectively.


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Temperature scales

In thermodynamics, it is desirable to have atemperature scale that is independent of anyproperty of a substance.

Thermodynamic temperature scale or theKelvin scale

The lowest temperature on this scale is 0 K.


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Temperature scales

A temperature scale that turns out to benearly identical to the Kelvin scale: ideal gastemperature scale

Measured using a constant volumethermometer

Filled with hydrogen or helium

At low pressures, the temperature of a gas

is proportional to its pressure at constantvolume


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Ideal gas temperature scales

The temperature of a gas of fixed volumevaries linearly with pressure at sufficientlylow pressures.

The relationship between the temperatureand the pressure of the gas in the vessel canbe expressed as

T = a + bP

where the values of the constants a and bfor a gas thermometer are determinedexperimentally.


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Ideal gas temperature scale

Measuring the pressures of the gas in thevessel at two reproducible points (such asthe ice and the steam points) and assigningsuitable values to temperatures at those two

points. These two measurements are sufficient to

determine the constants a and b

Then the unknown temperature T of amedium corresponding to a pressure readingP can be determined from that equation by asimple calculation.


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If the ice and steam points are assigned thevalues 0C and 100C, respectively, then thegas temperature scale will be identical to theCelsius scale.

In this case the value of the constant a(which corresponds to an absolute pressureof zero) is determined to be -273.15C

regardless of the type and the amount of thegas in the vessel of the gas thermometer.


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Id l t t l


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...

.

.

.

.

.

.

.

.

. .... .

Measured data

points Gas A

Gas B

Gas C

Gas D

T(oC)0-273.15

Extrapolation

P


P vs.Tplots of the experimental data obtained from a constant-volume gas thermometer using four different gases at different(but low) pressures.


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-273.15C: is the lowest temperature thatcan be obtained by a gas thermometer, andthus we can obtain an absolute gastemperature scale by assigning a value of

zero to the constant a.

In this case, we need to specify thetemperature at only one point to define an

absolute gas temperature scale. Standard fixed point for temperature scale:

Triple point of water: 0.010C or 273.16K(for Celsius scale)


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The absolute gas temperature scale is not athermodynamic temperature scale, since itcannot be used at very low temperatures(due to condensation) and at very high

temperatures (due to dissociation andionization).

However, absolute gas temperature is

identical to the thermodynamic temperaturein the temperature range in which the gasthermometer can be used.


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Recap of this lecture


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Concept of energy and its variousforms

Internal energy Total energy

Enthalpy

Zeroth law of thermodynamics andtemperature


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In the next lecture ...

Specific heat

At constant pressure and constantvolume

Heat transferMeaning of heat transfer

Types of heat transfer

Work

Thermodynamic meaning of work

Different types of work

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Lect-5

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