ES312 Energy Transfer Fundamentals

Unit A: Fundamental Concepts

ROAD MAP . . .

A-1: Introduction to Thermodynamics

A-2: Engineering Properties

ES312 Energy Transfer Fundamentals

Unit A-1: List of Subjects

What is Thermodynamics?

First and Second Law of Thermodynamics

Definition of Terminology in Thermodynamics

Thermodynamic Process and Cycle

Fundamental Concept of Continuum Mechanics

Basic Engineering Unit System

OBJECTIVES OF PART I (THERMODYNAMICS):

Understand the basic principles and theories of thermodynamics

Understand the first and second laws of thermodynamics

Prepare for the fundamentals of heat transfer analysis in computer simulations in part II/III

OBJECTIVES OF PART II/III (ANSYS HEAT TRANSFER ANALYSIS):

Learn fundamentals of heat transfer analysis in commercial software (ANSYS)

ANSYS SEMESTER COURSE PROJECT

A simple heat transfer analysis will be performed, using ANSYS

Project proposal will be due at the beginning of part III

Final project presentation will be on the day of the scheduled final exam

Final project report will be due by the end of the day of the final presentation

UNIT A-1PAGE 1 of 9

What’s Thermodynamics?

The term “thermodynamics” stems from the Greek words “theme” (heat) and “dynamis” (motion)

Thermodynamics is both a branch of physics and engineering science

Science v.s. Engineering . . .

Scientists are interested in gaining a fundamental understanding of the physical behavior

Engineers are interested in studying systems and how they interact with their surroundings

CONSERVATION OF ENERGY PRINCIPLE

First Law of Thermodynamics: energy cannot be created or destroyed: energy is conserved, it can

only change forms

Second Law of Thermodynamics: energy has “quality,” means that the actual thermodynamic

processes occur in the direction of decreasing quality of energy

LAWS OF THERMODYNAMICS: POKER-PLAYER’S ANALOGY (Bob Riggins, Rice University)

“The universe is the House, the great Casino. The great dealer, who controls the deck, always need to

take His percentage; so that in the long run the player is broke and his chip (energy) is dissipated into the

void (and unrecoverable).”

You can’t win (you can’t even break-even) and you can’t get out of the game

OTHER LAWS OF THERMODYNAMICS

“Zeroth Law” of Thermodynamics: if two systems are in thermal equilibrium respectively with a

third system, they must be in thermal equilibrium with each other (this law helps define the motion

of temperature)

“Third Law” of Thermodynamics: the entropy of a system approaches a constant value as the

temperature approaches absolute zero: the entropy of a system at absolute zero is typically close to

zero

UNIT A-1PAGE 2 of 9

First and Second Law of Thermodynamics

Conservation of energy principle

1st Law of Thermodynamics

2nd Law of Thermodynamics

Energy cannot be created or

destroyed: it can only change forms

(the 1st Law of Thermodynamics)

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SYSTEM OF THERMODYNAMICS

System: an object of focus or attention, enclosed by surroundings (boundaries)

Control Mass (CM): typically a “closed” system, defined by a fixed amount of mass in space (a

system without “convection” or “flow”)

Control Volume (CV): typically an “open” system, defined by a fixed volume in space (a system

with “convection” or “flow”)

ENGINEERING PROPERTIES

Property: characteristics that can be measured or quantified

Extensive properties: properties that depends on the size of the system

“Energy” is an extensive property

Intensive properties: properties that are independent to the size of the system

“Energy per unit mass (energy density)” is an intensive property

Properties are somewhat inter-related and a set of few properties can specify others by these

relations

STATE OF THE SYSTEM

The “state” can often be specified by providing the values of a subset of the properties

“State of the system” can be defined by a set of particular properties of a system

UNIT A-1PAGE 3 of 9

Definition of Terminologyin Thermodynamics

System, surroundings,

and boundary

The design of many engineering

systems, such as this solar hot

water system, involves

thermodynamics

A system at 2 different states

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EQUILIBRIUM

“Equilibrium” implies a state of balance: a system in equilibrium experiences no changes when it is

isolated from its surroundings

A system is in “thermal equilibrium” if the temperature is the same throughout the system

A system is in “mechanical equilibrium” if there is no change in pressure at any point of the system

with time

A system is in “phase equilibrium” if a multi-phase system’s mass of each phase does not change

with time

A system is in “chemical equilibrium” if its chemical composition does not change with time

PROCESS

“Process” is any change that a system undergoes from one equilibrium state to another: the series of

states through which a system passes during a process is called process “path”

“Quasi-static” or “quasi-equilibrium” process: a sufficiently slow process that allows the system to

adjust itself internally so that properties in one part of the system do not change any faster than those

at other parts

UNIT A-1PAGE 4 of 9

Thermodynamic Process and Cycle

The P-V diagram of a

compression process

Quasi-equilibrium and

non-quasi-equilibrium

compression processes

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CONTINUUM ASSUMPTION

Substances are made up of atoms that are, in reality, widely spaced in gas phase; however, it is

convenient to disregard the “atomic nature” of a substance and view it as “continuous” and

“homogeneous” matter with no imperfections (continuum)

The engineering mechanics, based on this continuum assumption is called continuum mechanics:

Statics, Fluid Mechanics, Solid Mechanics, and Thermodynamics . . . are all continuum mechanics

CONTROL MASS (CM) ANALYSIS

Often referred as “closed” system

Collection of a matter of fixed amount (mass) that we focus our attention

CONTROLVOLUME (CV) ANALYSIS

Often referred as “open” system

A fixed region in space (volume) that allows flow in and out of the region

UNIT A-1PAGE 5 of 9

Fundamental Concept of Continuum Mechanics

Despite the large gaps

between molecules, a

substance can be treated

as a continuum because

of the very large number

of molecules even if

extremely small volume

Mass cannot cross the

boundaries of a closed

system, but energy can

(control mass)

A closed system with a

moving boundary

Volume in the space is fixed, and mass and energy can move across boundaries (control volume)

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SI (INTERNATIONAL STANDARD) UNITS

Basic units for mass, length, and time:

kilogram (kg), meter (m), and second (s)

Force (weight) unit: Newton (N), where: 1 N = (1 kg)(1 m/s2)

Temperature unit: Celsius (C) / Kelvin (K), where: K = C + 273

US CUSTOMARY (ENGLISH) UNITS

Basic units for mass, length, and time:

slug, foot (ft), and second (s)

Force (weight) unit: pound (lb), where: 1 lb = (1 slug)(1 ft/s2)

Temperature unit: Fahrenheit (F) / Rankine (R), where: R = F + 460

NON-STANDARD UNITS

“Pound mass” (lbm): weight of one “pound mass” on the earth’s surface (gravity is 32.2 ft/s2) is

equal to one “pound force” (lb)

“Kilogram force” (kgf): weight of one “kilogram” on the earth’s surface (gravity is 9.8 m/s2) is

equal to one “kilogram force” (kgf)

UNIT CONVERSION

Non-standard units cannot be mixed up against standard units (important)

Convert non-standard units into standard units:

1 slug = 32.2 lbm and 9.8 N = 1 kgf

UNIT A-1PAGE 6 of 9

Basic Engineering Unit System

Definition of ForceWeight of a unit mass

(at sea-level)

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Solution

The volume of an oil tank is given. The mass of oil is to be determined.

Assumptions

Oil is an incompressible substance and thus its density is constant.

Analysis

Given the density and volume of oil: 3850 kg/m and V = 2 m3

The mass is density times volume, therefore: m V

Therefore, 3 3850 kg/m 2 mm 1,700 kg

UNIT A-1PAGE 7 of 9

Oil tank

A tank is filled with oil (density is 850 kg/m3). If the volume of the tank is 2m3, determine the amount of mass (in “kg”) in the tank.

EXERCISE A-1-1(Do-It-Yourself)

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Solution

A mass of 1 lbm is subjected to standard earth gravity. Its weight in lb (lbf) is to be determined.

Assumptions

Standard sea-level condition ( 232.2 ft/sg ).

Analysis

Applying Newton’s second law, the weight (force) can be calculated.

21 slug1 lbm 32.2 ft/s

32.2 lbmW mg

1 lb (this is “pound force” or “lbf”)

UNIT A-1PAGE 8 of 9

A mass of 1lbm weighs 1 lbf

on earth, under standard

gravity (at sea-level)

Applying appropriate unit conversions, show that 1 lbmweighs 1 lbf on earth, under the gravity of 32.2 ft/s2.

EXERCISE A-1-2(Do-It-Yourself)

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Assumptions

The density of air is constant throughout the room.

Properties

The density of air is given: 31.16 kg/m

Analysis

The mass of air in the room is: m V

Therefore, 3 31.16 kg/m 6 6 8 mm 334.1 kg

Weight of air in the room is:

2334.1 kg 9.8 m/sW mg 3,274 N

In “kilograms”:

1 kgf3,274 N

9.8 N

334.1 kgf

UNIT A-1PAGE 9 of 9

Determine the mass and the weight of air (both in “kilograms”) contained in a room (dimension: 6 m 6 m 8 m). Assume that the density of air is 1.16 kg/m3.

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