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Mechanical Theory I

MET3405

1. Thermodynamics

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Lecture 1

1.1 Fundamental Concepts of Thermodynamics

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1.1.1 Introduction

Heat

Dynamis

Force

Therme

Thermodynamics

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Thermodynamics

capacity of hot bodies to produce work

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Thermodynamics

It is a branch of engineering science that deals with the relationship between

�energy associated to heat

�and other forms of energy

•••• mechanical

•••• electrical

•••• chemical

•••• ….

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Resources in nature

Fossil Fuels

Radioactive Substances

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People’s needs for energy

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People’s needs for energy

Air conditioning

Heating

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Provision of energy is one

of the main tasks of ME

The branch of science that

�explains how much energy people may extract from various sources

�and predicts how efficiently people may use the extracted energy in a particular situation is called:

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1.1.2 Basic Concepts

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Fluids

Fluids are substances that:

• flow, even under the action of small forces;

• and take the shape of their container.

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GasesLiquids

Incompressible

FluidsVolume = const

Compressible

FluidsVolume ≠≠≠≠ const

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Thermodynamic System

System is the subject of analysis.

It may include:

•••• single simple body,

•••• or very complex assembly of many component and parts.

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Schematic presentation

of a System

System

Surroundings

System Boundary

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Closed System

Closed Reservoir

m = constThe substance in the reservoir cannot leave.New substance cannot enter in the reservoir.

m : mass of the substance

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Cylinder - Piston Assembly

Cylinder

Piston

Boundary

System: Working

Fluid

m = const

V ≠≠≠≠ const

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V2 > V1 Expansion

V1

State 1

V2

State 2

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V1 > V2 Compression

V2

State 2

V1

State 1

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Cylinder - Piston Assembly

This device has very important role in engineering practice and is commonly used in many gas power cycles such as internal combustion engines.

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Closed System ≡ Control Mass

In some textbooks, the closed system is also called Control Massbecause in a closed system the mass of the working substance is under consideration.

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Open System

Pipe

Control Volume

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Open System

Examples of open system include pipes, nozzles and diffusers, boilers, heat exchangers, valves, turbines, pumps and compressors.

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Thermodynamic Property

Properties are quantities that can be measured,

so their values can describe the condition of the system

without knowing how the system came to that condition.

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Examples of Properties

• Mass of the working fluid employed in the process

• Volume occupied by the working fluid

• Temperature of the working fluid

• Pressure of the working fluid

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State

State is a term for a condition of the system

as it is described by its properties.

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Example for a State

• Closed System

Our lecture theatre

• Working Fluid in the lecture theatre:

Air

• Temperature of Air: 20oC

• Pressure of Air: 1 bar

• The state of the system is given with: 20oC and 1 bar.

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Process

Process is a transformation of the system

from one state

to another state.

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Example for a Process

• Closed System: Our Lecture Theatre

• Initial State: 20oC, 1 bar

• Air-conditioning system switched off

• Process: Heating of the air (due to transfer of heat from outside and from students)

• Final State: 30oC, 1 bar

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1.1.3 The SI System of Units in Thermodynamics

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International System of Units: Basic Units

Mass Length Time

Symbol m (Note)

L t

Unit kilogramme metre second

Symbol for unit

kg m s

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Note (Very Important!)

�Quantity mass with unit kg is characteristic for Closed Systems.

�For Open System, corresponding quantity is

called Mass Flow Rate

m&the symbol is

and the unit is kg/s

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Additional Basic Unit

for Heat Interactions:

Temperature

No negative values for temperature

Kelvin Scale

Only positive values for temperature

Absolute Zero Temperature

Temperature (T) expressed in Kelvin degrees: K

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Additional Basic Unit

for Heat Interactions:

TemperatureCelsius Scale

Negative values for temperature

Positive values for temperature

Zero Temperature

Temperature (t) expressed in Celsius degrees: oC

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Relation between Kelvin and Celsius Scales

0 K = - 273.15 oC

0 oC = 273.15 K

T (K) = t (oC) + 273.15

t (oC) = T (K) - 273.15

∆∆∆∆T (K) = ∆∆∆∆t (oC)∆∆∆∆T = T2 - T1; ∆∆∆∆t = t2 - t1

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SI System of Units:

Derived Units

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Examples

• Area

aA=a2

a

b

A=ab d

A=d2ππππ/4

Unit: m 2

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Examples

Volume

ab

c V = abc

Unit: m 3

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Examples: Volume

Liter (l)

• 1 liter = 1 dm3

• 1 dm = 10 cm

• 1 m = 10 dm

• 1 m3 = 1000 dm3 = 1000 l

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Force

F = maF: force acting on the body;

m: mass of the body, kg;

a: acceleration, m/s2;

Unit for force: Newton (N)

2s

m X kg F forcefor Unit =

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Weight

w = mgw: weight of the body, N;

m: mass of the body, kg;

g: acceleration due to gravity, m/s2;

g = 9.81 m/s2

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Density

ρρρρ = m/V

ρρρρ: density, kg/m3;

m: mass of the body, kg;

V: volume occupied by the mass, m3;

m = ρρρρV

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Specific Volume

v = 1/ρρρρ = V/m

v: specific volume, m3/kg;

V: volume, m3;

m: mass of the body, kg;

m = V/v

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Relative Density

s = ρρρρsubstance/ρρρρwaters: relative density, no unit;

Example:

ρρρρmercury = 13595 kg/m3;

ρρρρwater = 1000 kg/m3;

⇒⇒⇒⇒ s = 13595/1000 = 13.595

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Pressure

p = F/A

p: pressure

F: normal component of the force acting over the body, N;

A: area over which the force acts, m2;

Unit for pressure = N/m2 = Pascal = Pa

1 bar = 105 Pa

1 atmosphere pressure = 101325 Pa

= 1.01325 bar

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Reference: Basic Thermodynamics

by K. Iynkaran and D. J. Tandy

�Suggested Additional ReadingChapter 1: Introduction to Thermodynamics

�Suggested Examples1.1 - 1.5

�Suggested Tutorial Problems1.1 - 1.8