CH-1 - Introductory Concepts and Definitions PDF

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ENRE 230 ENGINEERING THERMODYNAMICS Chapter one Introductory Concepts and Definitions

Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad

Introductory Concepts and Definitions Thermodynamics Thermodynamics is the study of energy and its transformations, and its relationship to the properties of matter. Although, it is difficult to give a precise definition for energy, it can be viewed as the ability to cause changes. Thermodynamics studies the behavior of how objects and systems behave as energy is transferred between them, what happen to objects as energy is added or subtracted.

Areas of Application of Thermodynamics: All natural processes are governed by the principles of thermodynamics. However, the following engineering devices are typically designed based on the principles of thermodynamics: Automotive engines, Turbines, Compressors, Pumps, Fossil and Nuclear Power Plants, Propulsion systems for the Aircrafts, Refrigeration, Air-conditioning and Heating Devices. The principles of thermodynamics are summarized in the form of four laws known as zeroth, first, second, and the third laws of thermodynamics.

The Zeroth Law

The First Law

Fourth Laws of Thermodynamics

The Third Law

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The Second Law

Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad 

The Zeroth Law deals with thermal equilibrium and provides a means for measuring temperatures.



The First Law deals with the conservation of energy and introduces the concept of internal energy.



The Second Law of thermodynamics provides with the guidelines on the conversion of internal energy of matter into work. It also introduces the concept of entropy.



The Third Law of thermodynamics defines the absolute zero of entropy. The entropy of a pure crystalline substance at absolute zero temperature is zero.

Thermodynamic system Thermodynamic system is defined as a quantity of matter or a region in space chosen for study. The region outside the system is called the surroundings. The real or imaginary surface that separates the system from its surrounding is called the boundary. SURROUNDINGS

SYSTEM

BOUNDARY

The boundary of a system can be fixed or movable, real or imaginary. The boundary is the contact surface shared by both the system and the surroundings, and through which energy and mass may enter or leave the system. A system together with its surroundings is said to constitute a universe. Imaginary boundary

Real boundary

Moving boundary

System

System

a nozzle Fixed boundary

A control volume with

A control volume with

real and imaginary boundaries

Moving and fixed boundaries

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Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad

Types of Systems Systems may be considered to be closed or open: Closed system (also known as a control mass) is one in which the system mass cannot cross the boundary, but energy (in the form of heat or work), can.

mass

NO

Closed system m = constant

energy

YES

As a special case of closed system, even energy is not allowed to cross the boundary, that system is called an isolated system.

mass

NO

energy

NO

Isolated system m = constant

Open system or a control volume, as it is often called, is one in which mass can cross the system boundary as well as energy. mass

YES

Open System (CV) energy

YES

Macroscopic and Microscopic Approaches It is well-known that a substance consists of a large number of particles called molecules. The properties of the substance naturally depend on the behavior of these particles. For example, the pressure of a gas in a container is the result of momentum transfer between the molecules

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Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad

and the walls of the container. However, one does not need to know the behavior of the gas particles to determine the pressure in the container. It would be sufficient to attach a pressure gage to the container. This macroscopic approach to the study of thermodynamics that based on empirical laws to describe matter treated as a continuum, and does not require a knowledge of the behavior of individual particles, is called classical thermodynamics. It provides a direct and easy way to the solution of engineering problems. Another approach which concerns directly with the structure of matter and characterize by statistical means the average behavior of the molecules making up a system of interest, is called Microscopic approach , sometimes called statistical thermodynamics. In the macroscopic approach, we are always concerned with volumes that are very large compared to molecular dimensions, and therefore a system contains many molecules, and this is called continuum. The concept of continuum loses validity when the mean free path of molecules approaches the order of typical system dimensions.

Units: SI- units are used exclusively Fundamental units: Mass

kilograms

kg

Length

meter

m

Time

seconds

s

Temperature

Celsius / Kelvin

o

Force (F)

Newton

N

Pressure (P)

Pascal

Pa

Energy (E)

Joule

J

C /K

Derived units:

Newton’s Law: Force = mass × acceleration F = m⋅a

P =

F/A

[N] = [kg] [m/s2]

[Pa] = [kg⋅m/s2] [m2]

→ 1 N = 1 kg⋅m/s2

→ 1 Pa = 1 kg/m⋅s2

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E

=

F

⋅ x

[J] = [kg⋅m/s2] [m] → 1 J = 1 kg⋅m2/s2

Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad

Properties of a System Properties are characteristics of a system to which a numerical value can be assigned at given time without knowledge of the history of the system. x The property of a system should have a definite value when the system is in a particular state. Examples of properties: temperature, density, pressure, energy Examples of non-properties: mass flow, heat, work

Not all properties are independent. Some are defined in terms of other ones. For example, density is defined as mass per unit volume: ρ

m V

(kg/m3)

Sometimes the density of a substance is given relative to the density of a better known substance. Then it is called specific gravity, or relative density (sg), 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 U H 2o = 1000 kg/m3), That is: sg

U UH O 2

x A more frequently used property in thermodynamics is the specific volume. It is the reciprocal of density and is defined as the volume per unit mass: Q

V m

1 U

Properties are considered to be either intensive or extensive.

x Extensive property is one whose value depends on the size or extent of the system, (i.e. the property is divided when the system is divided). Mass m, volume V, and total energy E are some examples of extensive properties.

x Intensive property is one whose value is independent of the size or extent of the system, (i.e. the property doesn’t change when the system is divided), such as temperature, pressure, and density. An easy way to determine whether a property is intensive or extensive is to divide the system into two equal parts with a partition; Each part will have the same value of intensive properties as the original system, but half the value of the extensive properties.

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Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad

x Specific property is an intensive property which has been obtained by dividing the extensive property by the mass of the system. Some examples of specific properties are: o specific volume (ν = V/m), o specific total energy (e = E/m), and o specific internal energy (u = U/m).

State and Equilibrium State is the condition of a system as described by the values of its properties (P,T,….). The number of properties required to fix the state of a system is given by the state postulate. The State Postulate states that if two independent intensive property values of simple compressible system are defined, then all the other intensive property values (and thus the state of the system) are also defined. Two properties are independent if one property can be varied while the other one is held constant. A system is called a simple compressible system in the absence of electrical, magnetic, gravitational, motion, and surface tension effects. These effects are due to external force fields and are negligible for most engineering problems.

Thermodynamics deals with equilibrium states. When the properties of the system show no tendency to change, a state of thermodynamic equilibrium exists. x

When the property of a system is defined, it is understood that the system is in equilibrium.

x

If a system is in thermal equilibrium, the temperature will be same throughout the system.

x

If a system is in mechanical equilibrium, there is no tendency for the pressure to change.

x

In a single phase system, if the concentration is uniform and there is no tendency for mass

transfer or diffusion, the system is said to be in chemical equilibrium. x

A system which is simultaneously in thermal, mechanical, and chemical equilibrium is said

to be in thermodynamic equilibrium. 1-6

Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad

Processes and Cycles Process: It is any transformation of a system from one equilibrium state to another. The series of states through which a system passes during a process is called the path of the process. P

Final state

2 Process path Initial state

1

V2

V1

V

System

(2)

(1)

P- V diagram of a compression process of a gas

Cycle: It is a sequence of processes which return the system to its initial state. P

P 2

3

2 4

1

1

V

V (b) A four-process cycle

(a) A two-process cycle

Pressure Pressure (P): is defined as a normal force exerted by a fluid per unit area.

P  F A (Pa) Units: 1 Pa = 1 N/m2 1 standard atmosphere = 101325 Pa 1 bar = 105 Pa = 100 kPa = 0.1 MPa

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Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad

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: Pgage = Pabs - Patm > 0 (for pressures above Patm ) Pvac = Patm - Pabs < 0 (for pressures below Patm )

Pgage

Patm Pabs

Pvac Patm

Patm

Pabs

Pabs = 0

The pressure at a point in a fluid has the same magnitude in all directions. The variation of pressure with elevation is given by

dP  g dz

1

P = Patm

Δz

where the positive z direction is taken to be upward. When the density of the fluid is constant, the pressure difference across a fluid layer of thickness Δz is ΔP = P2 - P1 = ρgΔz

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2 P = Patm + ρgh

Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad

Example The piston of the piston-cylinder devise containing a gas has a mass 60 kg and the gravitational acceleration is 9.81(m.s-2). The cross-sectional area of the piston is 0.04 m2. Determine the pressure inside the cylinder when the local atmospheric pressure is 0.98 bar.?

Solution: m = 60 kg

F  0  Ap P  Ap Patm  W p  P  Patm   0.98 

mg Ap

Gas

60  9.81  105  1.117bar 0.04

The basic method of measuring pressure is by means of a Manometer, as shown below:

The atmospheric pressure is measured by means of a Mercury Barometer as follows:

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Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad

Temperature The concept of temperature is fundamental to thermodynamics. We know that a body at high temperature will transfer energy to one at lower temperature. Consider two bodies with different temperatures in contact with each other. Net energy transfer will be from the hotter body to the colder body. At some point, the net energy transfer will be zero, and the bodies are said to be in thermal equilibrium. Bodies in thermal equilibrium are defined to have the same temperature. Temperature ( T),in units of degrees Celsius, oC, is a measure of “hotness” relative to the freezing and boiling point of water. A thermometer is based on the thermal expansion of mercury. 100oC

0 oC

Ice Bath

Boiling Water

Microscopic point of view: Temperature is a measure of the internal molecular motion, e.g., average molecule kinetic energy, At a temperature of –273.15oC molecular motion ceases. Temperature in units of degrees kelvin, K, is measured relative to this absolute zero temperature, so 0 K = -273oC

in general,

T in K = T in oC + 273.13

Zeroth Law of Thermodynamics If a system A is in thermal equilibrium with another system B and also with a third system C, then all of the systems are in thermal equilibrium with each other. This is called the zeroth law of thermodynamics. This is how a thermometer works. If a thermometer is placed in a substance for temperature measurement, the thermometer's glass comes into thermal equilibrium with the substance. The glass then comes into thermal equilibrium with the liquid

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Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad

(mercury, alcohol, etc . . .) inside the thermometer. Because the substance is in thermal equilibrium with the glass and the glass is in thermal equilibrium with the inner liquid, the substance and liquid must be in thermal equilibrium by the zeroth law. And because they are thermally equivalent, they must have the same temperature. TA = TC

0 oC

A

B

C

TA = TB

Zeroth law of thermodynamics: TA = TB = TC

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Ice bath

Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad

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Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad

c) Flow Energy When fluid is pumped along a pipe, energy is used to do the pumping. This energy is carried along in the fluid and may be recovered (as for example with an air tool or a hydraulic motor). Consider a piston pushing fluid into a cylinder.

The fluid pressure is P N/m2. The force needed on the piston is F= PA The piston moves a distance x metres. The work done is W = Fx = PAx Since Ax =V and is the volume pumped into the cylinder the work done is W = PV Since energy has been used doing this work, it must now be stored in the fluid and carried along with it as FLOW ENERGY. F.E = PV

(J)

d) Internal Energy The molecules of a matter are in random motion, as a result they possess kinetic energy. Usually this is regarded simply as the energy due to the temperature . INTERNAL ENERGY (U) is a measure of kinetic energy of the molecules and atoms that make up the matter. The internal energy of matter is measured by its temperature. Hot water has more internal energy than the same amount of cold water.

e) Enthalpy When a fluid has pressure and temperature, it must possess both flow and internal energy. It is often convenient to add them together and the result is ENTHALPY (H). H = U + F.E = U + PV

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(J)

Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad

Tutorial # (1) Problem 1 The level of the water in an enclosed water tank is (40 m) above ground level. The pressure in the air space above the water is (120 kPa), and the density of the water is (1000 kg/m3). What is the water pressure at a ground level?

-------------------------------------------------------------------------------------------------Problem 2 A gas is contained in a vertical frictionless piston-cylinder device. The piston has a mass of (4 kg) and cross sectional area of (35 cm2). A compressed spring above the piston exerts a force of (60 N). If the atmospheric pressure is (95 kPa), Determine the pressure inside the cylinder.

-------------------------------------------------------------------------------------------------Problem 3 Two piston/cylinder arrangements, A and B, have their gas chambers connected by pipe, see following figure. Cross-sectional areas are AA = 75 cm2 and AB = 25 cm2, with the piston mass in A being mA = 25 kg. Assume outside pressure is 100 kPa and standard gravitation. Find the mass mB so that none of the pistons have to rest on the bottom.

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Energy Engineering Department (Fundamentals of Thermodynamics/ 2nd Year) Course Tutor: Jasim M. Mahdi / University of Baghdad

H.W # (1) Q1:

Four cubic meters of water at 25 ºC and 1 bar have a mass of 3990 kg. (a) List the values of two extensive and three intensive properties of the system. (b) If the local gravity g for the system is 9.7 m/s2, evaluate the specific weight.

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Q2:

You are entering into a steam room filled with 5 kg of water vapor and 50 kg of air. Suppose that the volume of the room is 50 m3, calculate specific volume of water vapor and air, respectively. Ans.: 10 m3/kg, 1 m3/kg ------------------------------------------------...