Chapter 1... Page 1 of 12 Chapter 1 Introductory Concepts and Definition 1.1 ... The fluid...

Engineering Thermodynamics__________________________________________________________ _ AAiT

_____________________________________________________________________________________Compiled by Yidnekachew M. of 12

Chapter 1

Introductory Concepts and Definition

1.1 Introduction

Thermodynamics may be defined as follows :

Thermodynamics is an axiomatic science which deals with the relations among

heat, work and properties of system which are in equilibrium. It describes state

and changes in state of physical systems.

Thermodynamics is the science of the regularities governing processes of energy

conversion.

Thermodynamics is the science that deals with the interaction between energy

and material systems.

Thermodynamics can be defined as the science of energy. Although everybody has a feeling of

what energy is, it is difficult to give a precise definition for it. Energy can be viewed as the ability

to cause changes.

The name thermodynamics stems from the Greek words therme (heat) and dynamis (power), which

is most descriptive of the early efforts to convert heat into power. Today the same name is broadly

interpreted to include all aspects of energy and energy transformations, including power

generation, refrigeration, and relationships among the properties of matter.

Some Energy Conversion application 1

Steam Power Plant Nuclear Power Plant Internal Combustion Engines Gas Turbines Refrigeration Systems

1 Read the additional hand out.



Area of Application

Selected Areas of Application of Engineering Thermodynamics

Aircraft and rocket propulsion

Alternative energy systems

o Fuel cells

o Geothermal systems

o Magnetohydrodynamic (MHD) converters

o Ocean thermal, wave, and tidal power generation

o Solar-activated heating, cooling, and power generation

o Thermoelectric and thermionic devices

o Wind turbines

Automobile engines

Bioengineering applications

Biomedical applications

Combustion systems

Compressors, pumps

Cooling of electronic equipment

Cryogenic systems, gas separation, and liquefaction

Fossil and nuclear-fueled power stations

Heating, ventilating, and air-conditioning systems

o Absorption refrigeration and heat pumps

o Vapor-compression refrigeration and heat pumps

Steam and gas turbines

o Power production

o Propulsion



1.2 Closed, Open, and Isolated System

A thermodynamic system, or simply system, is defined as a quantity of matter or a region in space

chosen for study (A system is a finite quantity of matter or a prescribed region of space).

The region outside the system is called the surroundings. The real or imaginary surface that

separates the system from its surroundings is called the boundary. The boundary of a system may

be fixed or movable.

Surroundings are physical space outside the system boundary.

Figure 1.1 The system. It is not difficult to visualize a real boundary but an example of imaginary boundary would be one

drawn around a system consisting of the fresh mixture about to enter the cylinder of an I.C. engine

together with the remnants of the last cylinder charge after the exhaust process.

Figure 1.2 The real and imaginary boundaries



Systems may be considered to be closed or open, depending on whether a fixed mass or a fixed

volume in space is chosen for study.

Closed system

A closed system consists of a fixed amount of mass and no mass may cross the system boundary.

The closed system boundary may move. Examples of closed systems are sealed tanks and piston

cylinder devices note the volume does not have to be fixed). However, energy in the form of heat

and work may cross the boundaries of a closed system.

Figure 1.3 Closed system. Figure 1.4 Closed system with movable boundary.

Open system

An open system, or control volume, has mass as well as energy crossing the boundary, called a

control surface. Examples of open systems are pumps, compressors, turbines, valves, and heat

exchangers.

Figure 1.5 Open system



Isolated system

An isolated system is a general system of fixed mass where no heat or work may cross the

boundaries. An isolated system is a closed system with no energy crossing the boundaries and is

normally a collection of a main system and its surroundings that are exchanging mass and energy

among themselves and no other system.

Figure 1.6 Isolated system

Since some of the thermodynamic relations that are applicable to closed and open systems are

different, it is extremely important that we recognize the type of system we have before we start

analyzing it.

1.3 State, Equilibrium, Process and Properties

State

Consider a system that is not undergoing any change. The properties can be measured or calculated

throughout the entire system. This gives us a set of properties that completely describe the

condition or state of the system. At a given state all of the properties are known; changing one

property changes the state.

Equilibrium

A system is said to be in thermodynamic equilibrium if it maintains thermal (uniform temperature),

mechanical (uniform pressure), phase (the mass of two phases, e.g., ice and liquid water, in

equilibrium) and chemical equilibrium.



Figure 1.7 Thermal Equilibrium

Process

Any change from one state to another is called a process. During a quasi-equilibrium or quasi-

static process the system remains practically in equilibrium at all times. We study quasi-

equilibrium processes because they are easy to analyze (equations of state apply) and work-

producing devices deliver the most work when they operate on the quasi-equilibrium process.

Figure 1.8 A process between states 1 and 2 and the process path.

In most of the processes that we will study, one thermodynamic property is held constant. Some

of these processes are

Process Property held constant

Isobaric Pressure

Isothermal Temperature

Isochoric Volume

Isentropic Entropy



Process diagrams plotted by employing thermodynamic properties as coordinates are very useful

in visualizing the processes. Some common properties that are used as coordinates are temperature

T, pressure P, and volume V (or specific volume v).

Figure 1.9 The P-V diagram of a compression process.

Cycle

A process (or a series of connected processes) with identical end states is called a cycle. Below is

a cycle composed of two processes, A and B. Along process A, the pressure and volume change

from state 1 to state 2. Then to complete the cycle, the pressure and volume change from state 2

back to the initial state 1 along process B. Keep in mind that all other thermodynamic properties

must also change so that the pressure is a function of volume as described by these two processes.

Figure 1.10 Cyclic Process



Property

Any characteristic of a system is called a property. Some familiar properties are pressure P,

temperature T, volume V, and mass m. The list can be extended to include less familiar ones such

as viscosity, thermal conductivity, modulus of elasticity, thermal expansion coefficient, electric

resistivity, and even velocity and elevation.

Properties are considered to be either intensive or extensive. Intensive properties are those that

are independent of the mass of a system, such as temperature, pressure, and density. Extensive

properties are those whose values depend on the size—or extent—of the system. Total mass,

total volume and total momentum are some examples of extensive properties. An easy way to

determine whether a property is intensive or extensive is to divide the system into two equal

parts with an imaginary partition, as shown in the figure below.

Figure 1.11 Intensive and Extensive Properties

Steady-Flow Process

Consider a fluid flowing through an open system or control volume such as a water heater. The

flow is often defined by the terms steady and uniform. The term steady implies that there are no

changes with time. The term uniform implies no change with location over a specified region.

Engineering flow devices that operate for long periods of time under the same conditions are

classified as steady-flow devices. The processes for these devices is called the steady-flow process.

The fluid properties can change from point to point with in the control volume, but at any fixed

point the properties remain the same during the entire process.

State Postulate

As noted earlier, the state of a system is described by its properties. But by experience not all

properties must be known before the state is specified. Once a sufficient number of properties are



known, the state is specified and all other properties are known. The number of properties required

to fix the state of a simple, homogeneous system is given by the state postulate:

The thermodynamic state of a simple compressible system is completely specified by

two independent, intensive properties.

1.4 Dimension and Units

Any physical quantity can be characterized by dimensions. The magnitudes assigned to the

dimensions are called units. Some basic dimensions such as mass m, length L, time t, and

temperature T are selected as primary or fundamental dimensions, while others such as velocity

V, energy E, and volume V are expressed in terms of the primary dimensions and are called

secondary dimensions, or derived dimensions.

Quantity Dimension Units

Mass M Kg

Length L m

Time T s

1.5 Specific volume, Pressure and Temperature

Specific Volume

It is the volume occupied by a unit mass of a substance (the reciprocal of density), and it is

designated by the letter .

3 /Volume V

v m kgmass m

(1.1)

Sometimes the density of a substance is given relative to the density of a well-known substance.

Then it is called specific gravity, or relative density, and is defined as the ratio of the density of

a substance to the density of some standard substance at a specified temperature (usually water at

4°C, for which rH2O =1000 kg/m3). That is

Specific gravity: 2

( )sH O

SG

(1.2)



Pressure

Pressure is defined as a normal force exerted by a fluid per unit area. We speak of pressure only

when we deal with a gas or a liquid. Since pressure is defined as force per unit area, it has the unit

of newtons per square meter (N/m2), which is called a pascal (Pa).

2

Force F NP Pascal Pa

Area A m (1.3)

The pressure unit pascal is too small for pressures encountered in practice. Therefore, its multiples

kilopascal (1 kPa= 103 Pa) and megapascal (1 MPa= 106 Pa) are commonly used. Three other

pressure units commonly used in practice, especially in Europe, are bar, standard atmosphere, and

kilogram-force per square centimeter:

51bar=10 0.1 100Pa MPa kPa

1 atm = 101,325 Pa = 101.325 kPa = 1.01325 bars

The actual pressure at a given position is called the absolute pressure, and it is measured relative

to absolute vacuum (i.e., absolute zero pressure). Most pressure-measuring devices, however, are

calibrated to read zero in the atmosphere and so they indicate the difference between the absolute

pressure and the local atmospheric pressure. This difference is called the gage pressure. Pressures

below atmospheric pressure are called vacuum pressures and are measured by vacuum gages that

indicate the difference between the atmospheric pressure and the absolute pressure. Absolute,

gage, and vacuum pressures are all positive quantities and are related to each other by

gage abs atmP P P (1.4)

vac atm absP P P (1.5)



Figure 1.12 Absolute, gage, and vacuum pressures.

A device called manometer, it is commonly used to measure small and moderate pressure

differences. A manometer mainly consists of a glass or plastic U-tube containing one or more

fluids such as mercury, water, alcohol, or oil. To keep the size of the manometer to a manageable

level, heavy fluids such as mercury are used if large pressure differences are anticipated.

Consider the manometer shown in Fig. 1-13 that is used to measure the pressure in the tank. Since

the gravitational effects of gases are negligible, the pressure anywhere in the tank and at position

1 has the same value. Furthermore, since pressure in a fluid does not vary in the horizontal direction

within a fluid, the pressure at point 2 is the same as the pressure at point 1, P2 = P1. The differential

fluid column of height h is in static equilibrium, and it is open to the atmosphere. Then the pressure

at point 2 is determined directly from Eq. 1.6 to be

2 atmP P gh (1.6)

2 ,atm

W W mg Vg AhgP P hg P

A A A A A

2 atmP P hg

2 atmP P P gh

This pressure difference is determined from the manometer fluid displaced height as

P gh



Temperature

Although we are familiar with temperature as a measure of “hotness” or “coldness,” it is not easy

to give an exact definition of it. However, temperature is considered as a thermodynamic property

that is the measure of the energy content of a mass. When heat energy is transferred to a body, the

body's energy content increases and so does its temperature. In fact it is the difference in

temperature that causes energy, called heat transfer, to flow from a hot body to a cold body. Two

bodies are in thermal equilibrium when they have reached the same temperature. If two bodies are

in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. This

simple fact is known as the zeroth law of thermodynamics.

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