Module 1 Lec 2 - THERMODYNAMICS 2nd Qtr SY1112.pdf

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11/15/201

THERMODYNAMICS

Prepared By:

Prof. Rene D. Estember

THERMODYNAMICS

• branch of physical science that treats various phenomena of energy and

the related properties of matter, especially of the law of transformation

of heat into other forms of energy and vice-versa.

Examples of everyday transformation:

• Process of converting heat into electrical work (electrical powergeneration)

• Process of converting electrical work into cooling (air conditioning)

• Process of converting work into kinetic energy (automotive

transportation)

THERMODYNAMIC S YSTEM (or simply a SYSTEM)

• refers to the quantity of matter or certain volume in space chosen for

study.

Surroundings - the mass or region outside the system.

Boundary – the real or imaginary surface that separates the system

from the surroundings. The boundary of the system can either be

fixed or movable.

Kinds of Thermodynamic System

1. Closed system (also known as control mass)

• a system in which there is no transfer of matter across the boundary. It

consists a fixed amount of mass, and no mass can cross its boundary.

That is, no mass can enter or leave a closed system.

2. Open system (also known as control volume)• a system in which there is a flow of matter through the boundary. It

usually encloses the device that involves mass flow such as

compressor, turbine, or nozzle.

Kinds of Thermodynamic System

3. Isolated System

• A system in which neither mass nor energy crosses the boundaries and it

is not influenced by the surroundings. (Δm = 0, W=0, Q=0)

PROPERTIES OF A SYSTEM

• Any characteristic of a system is called a property .

Types of Thermodynamic Properties

A. Static Properties

• refer to the physical condition of the working substance such as

temperature, pressure, density, specific volume, specific gravity, or

relative density.

B. Transport Properties

• refer to the measurement of diffusion within the working medium

resulting from molecular activity, like viscosities, thermal

conductivities, etc.

Classification of Thermodynamic Properties

A. Intensive Properties

• independent of the mass such as temperature, pressure, density, and

voltage.

B. Extensive Properties

• dependent upon the mass of the system and are total values such as total

volume and total internal energy.

The State Properties

1. Temperature

• An indication or degree of hotness and coldness and therefore a

measure of intensity of heat.

Absolute temperature – the temperature measured from absolute

zero.

Absolute zero – the temperature at which the molecules stop

moving. The absolute zero equivalent to 0oK (-273.15oC) or 0oR (-

460oF).

Conversion Formulas

The Temperature Interval (Change)• The difference between two temperature readings from the same scale,

and the change in temperature through which the body is heated.

• Note: The degree must be written after the temperature scale for it to

indicate that it is a change in temperature

ZEROTH LAW OF THERMODYNAMICS

• When any two bodies are in thermal equilibrium with the third body, they

are in thermal equilibrium with each other. (Note: the third body is

usually a thermometer)

325

9 C F o 32

9

5 F C o

460 F Ro 273 C K

o

oo C K T T oo

C F T T 5

9

OO F R

T T ooC F

T T 5

9



11/15/201

2. Density (Specific Weight)

Mass density – the mass per unit volume.

where: m = mass (kgm, g, slug, lbm)

V = volume (m3, cm3, ft3)

ρ = density (kgm/m3, g/cm3, lbm/ft3)

Weight density (Specific Weight) – the weight per unit volume.

where: Fg = force due to gravity /weight (kgf ,N, g, lbf )


γ = specific weight (kgf /m3, N/m3, g/cm3, lbf /ft

3)

3. Specific Volume

• The volume per unit mass

where: m = mass (kgm, g, lbm)


υ = specific volume (m3/kgm, cm3/g, ft3/lbm)

V

F g

1

m

V v

V

m

4. Pressure

• The force exerted per unit area.

Absolute pressure - the true pressure measured above a perfect

vacuum.

Gage Pressure

• pressure measured from the level of atmospheric pressure by most pressure

recording measurement like pressure gage and ope-ended manometer.

Atmospheric pressure

• pressure obtained from barometric reading.

where: pabs = absolute pressure

pgage = gage pressure

patm = atmospheric pressure

atm psi patm

17.14

mmHg kPa patm 760325.101

inHg cm

kg patm 92.29032.1

2

610013.1013.1

cm

dyne xbar patm

atm gageabs p p p

atmabs p p )(

atmabs p p )(

h A Ah

AV

A F p g

gag e

v gh ghh p

g

g g gage

Critical Pressure

• Minimum pressure needed to liquefy gas at its critical temperature.

5. Specific Gravity (Relative Density)

• Also known as relative density. It is the ratio of the density of a certain

gas/substance to the density of air/water at the same temperature.

CONSERVATION OF MASS

• The law of conservation of mass states that the mass is indestructible. Mass

(m1) entering the system is equal to the sum of the stored mass (Δm) and

the mass (m2) that leaves the system.

Where: A = cross sectional area of thestream

υ = average speed

ρ = density

gas

air

water air

subs gas

water air

subs gas

water air

subs gas

R

R

MW

MW GS

/

/

/

/

/

/..

222111

21

A A

mm

CONSERVATION OF ENERGY

• The law of conservation of energy states that energy is neither created nor

destroyed.

• The fist law of Thermodynamics states that one form of energy may be

converted into another.

Gravitational Potential Energy – is its energy due to its position or elevation.

Where: z = height

Fg = weight

m = mass

g = acceleration due to gravity

P = Potent ial energy, ΔP = change in potential energy

Kinetic Energy – the energy or stored capacity for performing work possessed

by a moving body, by virtue of its momentum.

Where: m = mass

υ = velocityK = kinetic energy

ΔK = change in kinetic energy

)( 1212 z zmg P P P

mgz z F P g

2

1

2

212

2

2

2

m

K K K

m

K


Internal Energy – is energy stored within the body or substance by virtue of the

activity and configuration of its molecules and of the vibration of the atoms

within the molecules.

u = specific internal energy (unit mass): Δu = u2 – u1

U = mu = total internal energy (m mass): ΔU = U2 - U1

Work (W) – is the product of the displacement of the body and the component

of the force in the direction of the displacement. Work is energy in

transition; that is, it exists only when a force is “moving through a

distance.”

• Work of a Nonflow System

Work done by the system is positive (outflow of energy).

Work done on the system is negative (inflow of energy).2

1

pdV W


Flow Work (Wf ) – of work flow energy is work done in pushing a fluid across a

boundary, usually into or out of a system.

Where: ΔWf = change in flow

work

Heat (Q) – is energy in transit (on the move) from one body or system to

another solely because of temperature difference between the bodies or systems.

Q is posit ive when heat is added to the body or system.

Q is negative when heat is rejected by the body or system.

112212 V pV pW W W

pV W pAL FLW

f f f

f

f



11/15/201


Steady Flow Energy Equation

Characteristics of steady flow system

1. There is neither accumulation nor dimunition of mass within the system.

2. There is neither accumulation nor dimunition of energy within the system.3. The state of the working substance at any point in the system remains

constant.

Energy Entering the System = Energy Leaving the System

W U W K P QU W K P f f 22221111


Enthalpy (H, h) - is a composite property applicable to all fluids . It is the

heat energy transferred to a substance at a constant pressure process. It is

defined by:

Thus, the steady flow energy equation becomes:

pV U H mh H

pvuh

W H K P Q H K P 222111

THE IDEAL GAS

• An ideal gas is ideal only in the sense that it conforms to the simple perfect

gas laws.

Boyle’s Law

• If the temperature of a given quantity of gas is held constant, the volume of

a gas varies inversely with the absolute pressure during a change of state.

2211

1

V pV p

C pV

p

C orV

pV

THE IDEAL GAS

Charles’ Law

(1) If the pressure on a particular quantity of gas is held constant, then, with

any change of state, the volume will vary directly as the absolute

temperature.

or

or

(2) If the volume of a particular quantity of gas is held constant, then, with any

change of state, the pressure will vary directly as the absolute temperature.

or

or C T

p

T p

2

2

1

1

T

p

T

p

CT p

C T

V

T V

2

2

1

1

T

V

T

V

CT V

THE IDEAL GAS

Equation of State or Characteristic Equation of a Perfect Gas

Combining Boyle’s and Charles’ Laws,

, a constant

where: p = absolute pressure

V = volume

v = specific volume

m = mass

T = absolute temperature

R = specific gas constant or gas constant

(unit mass) = u niversal gas constant

n = no. of moles

M = molecular weight

mRT

pV

mRT pV

RT pv

T Rn pV

R

M

R R

M

mn

THE IDEAL GAS

Equation of State or Characteristic Equation of a Perfect Gas

The values of Universal Gas constant:

= 8.314 kJ/moloK

= 1545 ft. lb./mol oR

= 1.986 BTU/mol oR

= 0.0821 L. atm/mol o K

Gas constant of diatomic oxygen:

= 0.2598 kJ/kg.K

= 48.28 ft.lbf /lbm.oR

Gas constant for air:

R w = 0.287 kJ/kg.K = 53.34 ft.lbf /lbm.oR

R

)()(

2

2O M

RO R

mol kg

K mol kJ O R

/32

./314.8)( 2



11/15/201

THE IDEAL GAS

Specific Heat

• The specific heat of a substance is defined as the quantity of heat required

to change the temperature of unit mass through one degree.

c

or dQ = mcdT

And for a particular mass m,

If the mean or instantaneous value of specific heat is used,

) _ _ )((

) _ (

etemperatur of changemass

unitsenergy Heat

mdT

dQc

2

1

cdT mQ

12

2

1

T T mcdT mcQ

THE IDEAL GAS

Constant Volume Specific Heat (cv)

Constant Pressure Specific Heat (cp)

12 T T mcQ

U Q

vv

v

12 T T mcQ p p

2

1

pdV U W U Q p

12

112212

12

H H Q

V pV pU U Q

V V pU Q

p

p

p

THE IDEAL GAS

Ratio of Specific Heats

Internal Energy of an Ideal Gas

• Joule’s law states that “the change of internal energy of an ideal gas is a

function of only the temperature change.”

Therefore, ΔU is given by the formula,

whether the volume remains constant or not.

1v

p

c

ck

12 T T mcU v

THE IDEAL GAS

Enthalpy of an Ideal Gas

• The change of enthalpy of an ideal gas is given by the formula,

whether the pressure remains constant or not.

Relations between cp and cv

From h = u + pv and pv = RT

dh = du + R dT

12 T T mc H p

RdT dT cdT c v p

Rcc v p

1

1

k

kRc

k

Rc

p

v

THE IDEAL GAS

Entropy (S, s)

• Entropy is that property of a substance which remains constant (if no heat

enters or leaves the substance, while it does work or alters its volume, but

which increase or diminishes should a small amount of heat enter or leave.

• The change of entropy of a substance receiving (or delivering) heat is

defined by

Where: dQ = heat transferred at the temperature T

ΔS = total change of entropy

(constant specific heat)

2

1 T

dQS

T

dQdS

1

2

2

1

2

1

lnT

T mc

T

dT mcS

T

mcdT S

THE IDEAL GAS

Temperature – Entropy Coordinates

dQ = TdS

Other Energy Relations

(Reversible steady flow,ΔP=0)

2

1

TdS Q

K W Vdp s 2

1



11/15/201

PROCESSES OF IDEAL GAS

Thermodynamic Processes

• Thermodynamic process is any change that a system undergoes from one

equilibrium state to another. It can be reversible or irreversible.

Path is the series of states through which a system passes during a

process.

a) Reversible Process (Quasi-equilibrium process)

• It is the process that can be reversed without leaving any trace on the

surroundings. That is, both the system and the surroundings are returned o

their initial states at the end of the process.

b) Irreversible Process

• It is the process that proceed spontaneously in one direction but the other.

Once having taken place, the process cannot reverse itself and always

results in an increase of molecular disorder.


Constant Volume Process (Isometric Process)

• An isometric process is a reversible constant volume process. A constant

volume process may be reversible or irreversible.

Process Formula Process Formula

p, V, T relations

n

(Wn) 0 (reversible)

Q – Δ U (irreversible)

Specific heat

c

cv

(Ws) V(p1 – p2) H2 – H1 mcp(T2 – T1)

U2 – U1 mcv(T2 – T1) S2 – S1

Q mcv(T2 – T1)

1

2

1

2

p

p

T

T

1

2lnT

T mcv

2

1

pdV

2

1

Vdp


Isobaric Process

• An isobaric process is an internally reversible process of a substance during

which the pressure remains constant.


p, V, T relations

n 0

P(v2 – V1)

Specific heat

c

cp

0 H2 – H1 mcp(T2 – T1)

U2 – U1 mcv(T2 – T1) S2 – S1

Q mcp(T2 – T1)

2

1

pdV

2

1

Vdp

1

2

1

2

V

V

T

T

1

2lnT

T mc

p


Isothermal Process

• An isothermal process is an internally reversible constant temperature

process of a substance.


p, V, T relations

n 1

Specific heat

c

H2 – H1 0

U2 – U1 0 S2 – S1

Q

2

1

pdV

2

1

Vdp

2211 V pV p

1

211 ln

V

V V p

1

211 ln

V

V V p

2

1

1

2 lnln p

pmR

V

V mR

2

1

1

211 lnln

p

pmRT

V

V V p


Isentropic Process

• An isentropic process is a reversible adiabatic process. Adiabatic simply

means no heat. A reversible adiabatic is one of constant entropy.


p, V, T

relations n k

Specific heat

c 0

H2 – H1 mcp(T2 – T1)

U2 – U1 mcv(T2 – T1) S2 – S1 0

Q 0

2

1

pdV

2

1

Vdp

k k V pV p 2211

k

k k

p

p

V

V

T

T 1

1

2

1

2

1

1

2

k

T T mR

k

V pV p

11

121122

k

T T mRk

k

V pV pk

1

)(

1

)(121122


Polytropic Process

• A polytropic process is an internally reversible process during which

and


p, V, T

relations n - to +

Specific heat

c

H2 – H1 mcp(T2 – T1)

U2 – U1 mcv(T2 – T1) S2 – S1

Q mcv(T2 – T1)

2

1

pdV

2

1

Vdp

nn

n

V pV p

C pV

2211

nn V pV p 2211

n

nn

p

p

V

V

T

T 1

1

2

1

2

1

1

2

n

T T mR

n

V pV p

11121122

n

T T mRn

n

V pV pn

1

)(

1

)( 121122

n

nk cc vn

1

1

2

T

T mcn



11/15/201

General Equation for Thermodynamic Curves

The general equation of any process is:

If

n = 0 ; Isobaric process

n = 1 ; Isothermal process

n = k ; Isentropic process

n = - to + ; Polytropic process

n = ; Isometric process

Note: pVk is steeper than pV curve.

C pV n

OTHER DEFINITION OF TERMS

1) Saturation temperature

• Saturation temperature is the temperature at which liquids start to boil or

the temperature at which vapors begin to condense.

• The saturation temperature of a given substance depends upon its pressure.

• It is directly proportional to the pressure, i.e., it increases as the pressure is

increased and decreases as the pressure is decreased.

Examples:

Water boils at 100oC at atmospheric conditions (101.325 kPa).

Water boils at 179.91oC at a pressure of 1000 kPa.

Steam condenses at 311.06oC at 10 MPa.

Steam condenses at 39oC at 0.0070 Mpa.


2. Subcooled Liquid

• A subcooled liquid is one which has a tempeature lower than the saturation

temperature corresponding to the existing pressure.

Example:

Liquid water at 60oC and 101.325 kPa is a subcooled liquid. The saturation

temperature at 101.325 kPa is 100oC. Since the actual temperature of liquid

water of 60oC is less than 100oC, therefore, it is a subcooled liquid.

3. Compressed Liquid

• A compressed liquid is one which has pressure higher than the saturation

pressure corresponding to the existing temperature.

Example

Liquid water at 110 kPa and 100oC is a compressed liquid since the actual

liquid water pressure of 110 kPa is greater than the saturation pressure of

101.325 kPa at 100oC.


4. Saturated Liquid

• A saturated liquid is a liquid at the saturations (saturation temperature or

saturation pressure) which has temperature equal to the boiling point

corresponding to the existing pressure. It is a pure liquid, i.e., it has no

vapor content.

Examples:

Liquid water at 100oC and 101.325 kPa.

Liquid water at 333.90oC and 3 Mpa.

Liquid water at 324.75oC and 12 Mpa.

5. Vapor

• Vapor is the name given to a gaseous phase that is in contact with the liquid

phase, or that is in the vicinity of a state where some of it might be

condensed.


6. Saturated Vapor

• A saturated vapor is a vapor at the saturation conditions (saturation

temperature and saturation pressure). It is 100% vapor, i.e., has no liquid or

moisture content.

Examples:

Steam (water vapor) at 100oC and 101.325 kPa.

Steam at 212.42oC and 2 Mpa.

7. Superheated Vapor

• A superheated vapor is a vapor having a temperature higher than the

saturation temperature corresponding to the existing pressure.

Examples:

Steam at 200oC and 101.325 kPa. (tsat at 101.325 kPa= 100oC)

Steam at 300oC and 5 Mpa (t sat at 5 Mpa = 263.99oC)


8. Degrees of Superheat, oSH

• The degrees of superheat is the difference between the actual temperature of

superheated vapor and the saturation temperature for the existing pressure.

• In equation form:

oSH = Actual superheated temperature – tsat at existing pressure

9. Degrees Subcooled, oSB

• The degrees subcooled of a subcooled liquid is the difference between the

saturation temperature for the given pressure and the actual subcooled

liquid temperature.

• In equation form:oSB = tsat at a given pressure – actual liquid temperature



11/15/201


10. Wet Vapor

• A wet vapor is a combination of saturated vapor and saturated liquid.

11. Quality, x

• The quality of wet vapor or wet steam is the percent by weight that is

saturated vapor.

12. Percent moisture, y

• The percent moisture of wet vapor is the percent by weight that is saturated

liquid.

13. Latent Heat of Vaporization

• The latent heat of vaporization of a pure substance is the amount of heat

added to/removed from the substance in order to convert it from saturated

liquid/saturated vapor to saturated vapor/saturated liquid with the

temperature remaining constant. It is inversely proportional to the

temperature or pressure of the substance.


14. Critical point

• The critical point represents the highest pressure and highest temperature at

which liquid and vapor can coexist in equilibrium. The state of water at

critical conditions whether it is saturated liquid or saturated vapor is

unknown. Hence, the latent heat of vaporization of water at this condition

is either zero or undefined.

15. Sensible Heat

• Heat that causes change in temperature without a change in phase.

16. Sublimation

• The term used to describe the process of changing solid to gas without

passing to the liquid state.

17. Deposition

• The reverse of sublimation. It is the process of changing gas to solid

without passing to the liquid state.


18. Latent heat of fusion

• It is the heat needed by the body to change from solid to liquid without

changing is temperature.

19. Second Law of Thermodynamics

• Heat cannot be transferred from cold body to a hot body without an input of

work. It similarly states that heat cannot be converted 100% into work.

The bottom line is that an engine must operate between a hot and a cold

reservoir. Also indicated is that energy has different levels of potential to

do work, and that energy cannot naturally move from realm of lower

potential to a realm of higher potential.

20. Third law of Thermodynamics

• The total entropy of pure substances approached zero as the absolutethermodynamic temperature approaches zero.


21. Dalton’s Law of Partial Pressure

• The pressure exerted in a vessel by a mixture of gases is equal to the sum of

the pressures that each separate gas would exert if it alone occupied the

whole volume of the vessel.

22. Avogadro’s Law

• At equal volume, at the same temperature and pressure conditions, the

gases contain the same number of molecules.

23. The Carnot Cycle

• The Carnot Cycle is the most efficient cycle conceivable. It consists of two

isothermal processes and two isentropic processes.

24. Mean effective pressure

• It is the average constant pressure that, acting through one stroke, will do

on the piston the net work of a single stroke.


25. Expansion ratio

• The ratio between the volume at the end of expansion and the volume at the

beginning of expansion.

26. Compression ratio

• The ratio between the volume at the beginnign of compression and the

volume at the end of compression.

27. Internal Combustion Engine

• It is a heat engine deriving its power from the energy liberated by the

explosion of a mixture of some hydrocarbon, in gaseous or evaporated

form, with atmospheric air.

28. Four-stroke cycle

• The four-stroke cycle is one wherein four strokes of the piston, two

revolutions, are required to complete the cycle.


29. Heat engine or thermal engine

• It is a closed system (no mass crosses its boundaries) that exchanges only

heat and work with its surrounding and that operates in cycle.

30. Elements of a thermodynamic heat engine with a fluid as the working

substance:

A working substance, matter that receives heat, rejects heat, and does

work;

A source of heat (also called a hot body, a heat resevoir, or just source),

from which the working substance receives heat;

A heat sink (also called receiver, a cold body, or ject sink), to which the

working substance can reject heat; and

An engine, wherein the woking substance may do work or have work

done on it.



11/15/201


31. Thermodynamic cycle

• It occurs when the working fluid of a system experiences a number of

processes that eventually return the fluid to its initial state.

32. Available energy

• It is that part of the heat that was converted into mechanical work.

33. Unavailable energy

• It is the remainder of the heat that had to be rejected into the receiver (sink).

34. Otto Cycle

• It is the ideal prototype of spark-ignition engine.

35. Spark-Ignition engine

• It is also referred to as gasoline engine.


36. Compression-Ignition Engine

• It is also referred to as diesel engine.

Module 1 Lec 2 - THERMODYNAMICS 2nd Qtr SY1112.pdf

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