Chapter 1 - Introduction (Thermodynamics 1)

THERMODYNAMICS 1

By:

Orley G. Fadriquel

WIND TURBINEBlade or Propeller

Turbine

Storage

Battery

Transformer

High Wind Velocity

Lower Wind VelocityGenerator

Generator

Transformer

Air Fuel

Piston Cylinder

Crank Shaft

Brake power

Indicated

Power

INTERNAL

COMBUSTION

ENGINE

Generator

Thermodynamics isthe study of energy andits transformation, thedirection of flow of heat,and the availability ofenergy to do work.

The word thermodynamics derives from the two Greek words “therme” which means “heat: and “dynamikos” which means “power”.

Approaches in the Study of Thermodynamics

1.Microscopic or statistical approach

2.Macroscopic approach

Microscopic or Statistical Approach

• Structure of matter is considered and a large number of variables are needed to describe the state of matter.

• The matter is composed of several molecules and behaviour of each individual molecule is studied.

• Each molecule is having certain position, velocity and energy at a given instant.

• The velocity and energy change very frequently due to collision of molecules.

Macroscopic approach

• In macroscopic approach the structure of matter is not considered, in fact it is simple, and only few variables are used to describe the state of matter.

• In this approach, a certainquantity of matter composed oflarge number of molecules isconsidered without the eventsoccurring at the molecular levelbeing taken into account.

• In this case, the properties of a particularmass of substance, such as itstemperature, pressure, and volume areanalyzed. Generally, in engineering, thisanalysis is used for study of heat enginesand other devices. This method gives thefundamental knowledge for the analysisof a wide variety of engineeringproblems

Applications of Thermodynamics

• Application of thermodynamicprinciples in practical design tasks,may be of a simple pressure cookeror of a complex chemical plant.The applications of thermodynamiclaws and principles are found in allfields of energy technology.

• steam and nuclear power plants

• gas turbines

• internal combustion engines

• air conditioning

• Refrigeration

• gas dynamics

• jet propulsion

• compressors

• others

DIMENSIONS AND UNITS

• Dimensions implies physical quantities. Examples are length, time, mass, force, volume and velocity. In engineering analysis, it is most important to check the dimensional homogeneity of an equation relating physical quantities. This means that the dimensions of terms on one side of the equation must equal those on the other side.

Primary dimensions impliesunits of physical quantitiesconceived of and used tomeasure other physicalquantities related by definitionand laws

Secondary dimensions implies other physical quantities measured using primary dimensions

• Unit is a definite standard or measure of a dimension.

• For example, foot, meters and angstroms are all different units with the common dimension of length.

• A unit is any specified amount of a quantity by comparison with which any other quantity of the same kind is measure.

In any dimensional system, the units of length, time, mass and forces are related through Newton’s second law of motion.

The total force acting on a body is proportional to the product of the mass and the acceleration in the direction of the force, thus,

F ma ; F = 1/k ma

where k is the proportionality constant.

UNITS OF DIFFERENT DIMENSIONAL SYSTEMS

Name of

system

Unit

of

Mass

Unit of

Lengt

h

Unit of

time

Unit

of

force

K in

F = 1/k ma

Definition of terms

SI

(mks)

kgm m Sec N 1.0

9.806

1.0 N is the force needed to accelerate a

mass of 1.0 kg at 1.0 m/s2

1.0 kgf is the force needed to accelerate a

mass of 1.0 kgm at 9.8066 m/s2

English

Eng’g

lbm Ft Sec lbf 32.17 1.0 lbf is the force needed to accelerate a

mass of 1.0 lbm at 32.174 ft/sec2

Absolute

Eng’g

Slug Ft Sec lbf 1.0 1.0 lbf is the force needed to accelerate a

mass of 1.0 slug mass at 1.0 ft/sec2

Absolute

metric

(cgs)

gm Cm Sec dyne 1.0

980.66

1.0 dyne is the force needed to accelerate a

mass of 1.0 g at 1.0 cm/sec2

1.0 gf is the force needed to accelerate a

mass 1.0 gm at 980.66 cm/s2

2

f

m

skg

mkg

2

m

sN

mkg

2

f

m

slb

ftlb

2

f slb

ftslug

2

m

sdyne

cmg

2

f

m

sg

cmg

International System Institutes(SI)

Base Unit

Specified once a set of primary dimensions is adopted

quantity unit symbol

mass kilogram kg

length meter m

time second s

(Systeme Internationale d’Unites)

Derived Unit

Also termed secondary unit, these are units derived from the base units: given new names, normally after a famous scientist.

Selected SI-derived units

Newton, N 1 N = 1 kg-m/s2

Pascal, Pa 1 Pa = 1 N/m2

Joule, J 1 J = 1 N-m

Watt, W 1 W = 1 J/s

SI unit prefixes

Factor prefix symbol Factor prefix symbol

1012 Tera T 10-2 centi c

109 Giga G 10-3 milli m

106 Mega M 10-6 micro

103 Kilo k 10-9 nano n

102 hecto h 10-12 pico p

101 deca da 10-15 femto f

10-1 deci d 10-18 atto a

English Engineering Units

Base unit

quantity unit symbol

mass pound-mass lbm

length foot ft

time second s

Derived unit

Quantity unit symbol

force pound-force lbf

pressure pound-force psi

per square inch

THERMODYNAMIC SYSTEM

System is defined as any collection of matter or any region in space bounded by a closed surface or wall.

• Surrounding

• Boundary

Piston

Cylinder

Boundary

Surrounding

System boundary

Heat

Thermodynamic System

gas

Weight

CLASSIFICATION OF THERMODYNAMIC SYSTEM

• Closed system- a system of fixed mass. In this system, energy may cross the boundary, and the total mass within the boundary is fixed. The system and it’s boundary may contract or expand in volume

PISTON MOVEMENT

gas 12

Q

Surrounding

Boundary

Energy in

Energy out

Thermodynamic System

WGas

QHeat

Open System- one in which matter crosses the boundary of the system. There may be energy transfer also, i.e, both energy and mass crosses the boundary of the system. Most engineering devices belong to this type.

PISTON MOVEMENT

gas 12

Q

Air out

Work in

Heat

boundary

Air in

Thermodynamic

System Surrounding

Energy out

Energy out

Mass in

Mass out

Note: If the inflow of mass is equal to the outflow of mass, then the mass in the system is constant and the system is known as steady flow.

Isolated System - one in which neither mass nor energy crosses the system boundary. It is of fixed mass and energy. The system is not affected by the surrounding, i.e. there is no interaction between the system and surroundings.

Thermodynamic System

Surrounding

Flow through pipe

1 2 43 5

HOMOGENEOUS AND HETEROGENEOUS SYSTEM

If the substance within the system exists in a single phase like air, steam, liquids then the system is called HOMOGENEOUS SYSTEM. In these systems, the substance exists in only one phase.

If the substance within the system exists in more than one phase, then the system is HETEROGENEOUS.

THERMODYNAMIC PROPERTIES

A property is either a directly observable or an indirectly observable characteristic of a system. Any combination of such characteristics is also a property.

THERMODYNAMIC PROPERTIES

The distinguishing characteristics of a system by which its physical condition may be described are called theproperties of the system. They are quantities that we must specify to give a macroscopic description of the system.

Two other properties -temperature and entropy- are unique to thermodynamics. Together with energy, they play a most important role in the structure of thermodynamics.

Two types of classical thermodynamic properties

• Intensive Property

These properties are independent of mass such as pressure, temperature, voltage and density.

.

• Extensive Property

These properties are dependent of mass and are total values such as total volume, total energy and entropy.

• Examples of thermodynamic properties, besides pressure, volume and Temperature, are: internal energy, enthalpy, and entropy. Other properties include: velocity, acceleration, moment of inertia, electric charge, conductivity (thermal and electrical), electromotive force, stress, viscosity, reflectivity, number of protons, and so on

Definition of properties

Mass – amount or absolute quantity of matter in a certain body.

Volume – the space occupied by a certain body

.Density 1. Mass density – the mass of substance

divided by the volume the mass occupies of simply, the mass per unit volume.

Water at standard conditions at 4oC (39.2oF)

ρwater = 1 gm/cm3 = 1000 kgm/m3 = 62.4 lbm/ft3

.2. Weight density – also known as

the specific weight, it is defined as the weight per unit volume.

Where :

g = 9.80665 m/s2 = 32.174 ft/s2

Water at standard conditions,

water = 9.81 kN/m = 62.4 lb/ft3

Specific volume - defined as the volume per unit of mass of the reciprocal of density

Weight, W – force exerted by gravity on a given mass, depends on both the mass of the substance and the gravitational field strength,

W = mg

• Specific gravity – the dimensionless parameter, it is defined as the ratio of the density (or specific weight) of a substance to some standard density(specific weight)

For liquid substances

For gaseous substances

at STP = 1.2 kg/m3 at 1 atm, 21.1 oC

air

gas

air

gas.G.S

OH

LIQUID

OH

LIQUID

22

.G.S

.Pressure – defined as the normal

force exerted by a system on a unit area of its boundary.

Manometer – the instrument used in measuring pressure.

Standard reference atmospheric pressure

1 atm = 14.7 psia

= 760 mm Hg

= 29.92 in Hg at 32oF

= 760 torrs

= 101.325 kpa

= 34 ft H20

= 1.033 kg/cm2

Types of pressurea. Gage pressure, Pg – pressure of a fluid

and the atmospheric pressure, measured using manometer or bourdon gage

Note:

-Vacuum pressure is negative pressure;

-measured using fluid pressure < atmospheric pressure

b. Atmospheric pressure, Patm

- measured using a barometer, refer to standard atmospheric cited above

c. Absolute pressure, Pabs – sum of the gage pressure and atmospheric pressure.

Relationships among the types of pressure

For Pabs > Patm

Pabs = Patm + Pg

For Pabs < Patm

Pabs = Patm – Pv

Also, Pg = h = ρgh

Temperature – it is an

intensive property, originates with our sense perceptions, rooted in the notion of hotness or coldness of a body

Types of Temperature

a. Arbitrary, t – man made calibrated

a.1 Celsius scale, oC (used to be Centigrade scale). Named after Anders Celsius, a Swedish

– - steam point - equilibrium temperature of pure liquid water in contact with its vapor at one atmosphere; 100oC

– - ice point – equilibrium temperature of ice and air-saturated liquid pressure at a pressure of one atmosphere. 0oC

a.2 Fahrenheit scale ,oF

Named after Gabriel Fahrenheit, a German who devised the first mercury-in-glass thermometer; earlier thermometer fluids used were alcohol and linseed oil

- steam point : 212oF

- ice point : 32oF

b. Absolute, T – measured from absolute zero, all molecular motion cease. 0 Kelvin or 0 Rankine

b.1 Kelvin Scale, K

Named after William Thomson, aka Lord Kelvin who related absolute scale to the Celsius scale. The ice point is assigned with a value of 273.15 K and the steam point is assigned with the value 373.15 K. The triple point of water is 273.16 K.

b.2 Rankine Scale, R

Named after William MacquornRankine, a Scotish. The ice point is assigned with a value of 491.67 R and the steam point is assigned with the value 671.67 R The triple point of water is 491.69 R.

THERMODYNAMIC STATE OF A SYSTEM

Thermodynamic

property B

Thermodynamic

property A

1

2

ZEROTH LAW

This law states that when two bodies, isolated from other environment are in thermal equilibrium with a third body, the two are in thermal equilibrium with each other.

ZEROTH LAW

If two closed system with different temperatures are brought together in thermal contact with a third system, the heat will flow from the system with high temperature to the system with low temperature until the bodies reach thermal equilibrium with each other.

Coffee in a cup

Tcoffee = Tcup=Tsurroundings

THERMODYNAMIC PROCESS

If any one or more properties of a system change, the system is said to have undergone a process; there has been a change of state.

Thermodynamic processes that are commonly experienced in

engineering practice:

1. Constant pressure/ Isobaric process2. Constant volume/ Isochoric process3. Constant temperature/ Isothermal

process4. Reversible adiabatic/ Isentropic process5. Polytropic process6. Throttling process/ Iso-enthalpic process

REVERSIBLE AND IRREVERSIBLE

PROCESS

Reversible Process

• A reversible process for a system is an ideal process which once having taken place can be reversed in such a way that the initial state and all energies transformed during the process can be completely regained in both systems and surroundings.

This process does not leave any net change in the system or in the surroundings. A reversible process is always, quasi-static.

Irreversible Process

• If the initial state and energies transformed cannot be restored without net change in the system after the process has taken place, it is called irreversible.

Quasi-Static Process

•Quasi means “almost”. Infinite slowness is the characteristic feature of this process. It is also a reversible process.

THERMODYNAMIC CYCLE

•When a certain mass of fluid in a particular state passes through a series of processes and returns to its state, it undergoes a cycle.

p

v

v1

p2

p1

v2

A

B

1

2

C

Cycles. By our convention of signs, cycles that

trace a clockwise path, as A-1-B-C-A or A-1-B-2-

A are delivering work; cycles tracing a

counterclockwise path, as A-C-B-1-A or A-2-B-

1-A, are receiving work.

LAW OF CONSERVATION OF MASS

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

The verbal form of the law is

systemtheinstored

massofchange

leaving

mass

entering

mass

m mvelocity

area

density

Assume that each point of any cross section where the fluid flows, the properties are the same and use average velocity normal to the section and assumed to be the same at each point. Thus, if the density is the same in all points of the cross section of area A, then mass rate of flow is

m = ρelA

A steady flow system is an open system in which there is no change of stored mass; having an equation called the continuity equation of steady flow;

m1 = m2 = m

m = ρ11A1 = ρ22A2

GENERAL METHODOLOGY FOR PROBLEM SOLVING IN

ENGINEERING THERMODYNAMICS

As suggested by HUANG

1.Read the problem carefully

2.Since a sketch almost always aids in visualization, draw a simple diagram of all the components of the system involved. This could be a pump, a heat exchanger, gas inside a tank, or an entire power plant.

3. Select the system whose behavior we want to study by properly and clearly locating the boundary of the system. Do we have an isolated system, a closed system, or an open system.

4. Make use of the appropriate thermodynamic diagrams to locate the state points, and possibly the path of the process. These diagrams are extremely helpful as visual aids in our analysis.

5. Show all interactions (work, heat, and mass) across the boundary of the selected system.

6. Extract from the statement of the problem the unique features of the process and list them. Is the process isothermal, constant pressure, constant volume, adiabatic, isentropic, or constant enthalpy?

7. List all the assumptions that one might need to solve the problem. Are we neglecting a change of kinetic energy and change of potential energy?

8. Apply first law equation appropriate to the system we have selected.

9.Apply the principle of mass conservation appropriate to the system that we have selected.

10.Apply the second law equation appropriate to the system we have selected

11.Apply the appropriate property relations. That is, bring in data from tables, charts, or appropriate property equations.

12.Try to work with general equations as long as possible before substituting in numbers

13.Watch out for units. For example, when we use h = u +pv, h, u, and pv must all have the same units.

14.Make sure that absolute temperature, in degrees Rankine or Kelvin, is used in calculation.