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Introduction and Basic Concepts

MAE 320- Chapter 1

The content and the pictures are from the text book: Çengel, Y. A. and Boles, M. A., “Thermodynamics: An Engineering Approach,” McGraw-Hill, New York, 6th Ed., 2008

Identify the unique vocabulary associated with thermodynamics through the precise definition of basic concepts.

Review the metric SI and the English unit systems.

Explain the basic concepts of thermodynamics such as system,

Objectives

state, state postulate, equilibrium, process, and cycle.

Review concepts of temperature, temperature scales

Review concepts of pressure, absolutepresuure and gage pressure.

Thermodynamics and Energy

• The name thermodynamics stems from the Greek words therme (heat) and dynamis (power).

• Thermodynamics: The science of energy.

• Energy: The ability to cause changes.

What is the difference between thermodynamics and kinetics?

Thermodynamics and Energy

• Storage of energy- internal energy, (T)- potential energy, (gravity, h)- kinetic energy (motion, V)- chemical energy (reaction)

What topics are covered in this course?

• Second law of thermodynamics- quantity- quality- direction

• Transfer of energy- from one form to another form- from one state to another state

• First law of thermodynamics- Energy cannot be created- Energy cannot be destroyed- Energy can be transferred

• Entropy

• Substances- H2O (ice, water, vapor)- ideal gas

Thermodynamics and Energy

Conservation of energy principle: During an interaction, energy can change from one form to another but the total amount of energy remains constant. gy

Energy cannot be created or destroyed.

Application Areas of Thermodynamics

Heat Pump

Electric heater

How to save energy bills?

Which one do you choose between a heat pump and an electric heater?

How does a heat pump work?

What is the efficiency of the heat pump?

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Application Areas of Thermodynamics

Double-layer glass windowWhat is the efficiency of the

hot water tank?

What is the optimal temperature for the water tank?

What is minimum tank capacity needed for your house?

Application Areas of ThermodynamicsHybrid carConventional car

brakingbraking

Pad-drum friction

Heat dissipation battery

Mechanical energy converted to heat

electric motor

Electric generator

accelerating

Mechanical energy converted to electric energy, converted to mechanical energy

Application Areas of ThermodynamicsHow Hybrid car workshttp://www.youtube.com/watch?v=2HrQeGzIZoI&feature=related

Hybrid SUV-3D animationhttp://tw.youtube.com/watch?v=Fk7xbcnzxks

• The second law of thermodynamics:It asserts that energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy.

• Classical thermodynamics: A macroscopic approach to the study of thermodynamics that does not require a knowledge of the behavior of Conservation of energy

Concepts of Thermodynamics

a o edge o e be a o oindividual particles.

• It provides a direct and easy way to the solution of engineering problems and it is used in this text.

• Statistical thermodynamics: A microscopic approach, based on the average behavior of large groups of individual particles.

• It is used in this text only in the supporting role.

gyprinciple for the human body.

Heat flows in the direction of decreasing temperature.

Dimensions 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, • velocity V, energy E, and volume V are expressed in terms of the primary

dimensions and are called secondary dimensions, or derived dimensions.

Review SI units

SI Primary units (base units)

Quantity Name of Unit SymbolLength (l) meter mMass (m) kilogram kgTime (t) second sElectrical current (I) ampere AThermodynamic temperature (T) Kelvin K

Amount of substance mole mol

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Dimensions and Units• English system: It has no apparent systematic numerical

base, and various units in this system are related to each other rather arbitrarily.

SI unit system will be adopted in our course:SI system has a systematic numerical base (clear physical meaning)• SI system has a systematic numerical base (clear physical meaning).

• SI system is a decimal system (x10).• Better for communication. All the industrial country use the SI system.

Only USA is employing a dual-unit system. However, all the car manufactures use the SI system. You need a (wrench) kit for metric bolts instead English sockets

• Easy to remember

Density and Specific Gravity

specific volume is volume per unit mass:

Density:

Specific volume:

Density is mass per unit volume:

(m3/kg)Specific volume: (m /kg)

Density and Specific GravitySpecific gravity: The ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4°C).

Specific weight: The weight of a funit volume of a substance.

ggVm

Vmg

S ργ =⋅==

In SI, one newton (1 N) is the force required to cause a mass of one kilogram (1 Kg) to accelerate at a rate of one meter per second squared (1 m/s2)

Dimension and Units

In the English system, the force unit, 1 lbf, is the force required to cause a mass of 32.174 lbm to accelerate at a rate of one foot per second squared (1 ft/s2)

In daily life, people usually use weight to express mass.In science, weight is a force:

W = mg

Where m is the mass, g is the local gravitational acceleration, g=9.807 m/s2 or g =32.174 ft/s2 at sea level and 45o latitude.

On the earth, at sea level, a mass of 1 kg weighs 9.8 N

Dimension and Units

W = mg = 1 kg x 9.8 m/s2 = 9.8 kg.m/s2 = 9.8 N

On the moon, a mass of 1 kg weighs 1.63 NW = mg = 1 kg x 1.63 m/s2 = 1.63 kg.m/s2 = 1.63 N

Mass does not change with location, but weight does

Unit Conversion Ratios

All non-primary units (secondary units) can be formed by combinations of primary units. Force units, for example, can be expressed as

All equations must be dimensionally homogeneous.

To be dimensionally homogeneous, all the terms in an equation must have the same unit system.

They can also be expressed more conveniently as unity conversion ratiosas

Unity conversion ratios are identically equal to 1 and are unitless, and thus such ratios (or their inverses) can be inserted conveniently into any calculation to properly convert units.

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How much is the weight of a mass of 1 lbm?

Apply Newton’s second law:

Dimension and Units

In SI system:1 lbm= 0.454 kg and g =9.8 m/s2

W = mg = 0.454 kg x 9.8 m/s2 = 4.45 kg.m/s2 = 4.45 N

A mass of 1 lbm weighs 4.45 N

Work (W) = Force × Distance

1 J = 1 N·m

1 cal = 4.1868 J

1 Btu = 1 0551 kJ

Work, a form of energy:

Dimension and Units

1 Btu 1.0551 kJ

specific energy (e), e=E/m (J/kg)

power = W/t Unit: J/s or watt, w

Review SI units – Derived Units

Quantity Description SymbolExpression in

other SI units

Expression in SI base units

Area (A) square meter m2 m2

Volume (V) cubic meter m3 m3

velocity (or speed) (V) meters per second m/s m s-1

dAcceleration (a) meter per second squared m/s2 m s-2

density (ρ), ρ=m/V kilogram per cubic meter kg/m3 kg m-3

specific volume (v), v=V/m

cubic meter per kilogram m3/kg m3 kg-1

specific gravity (SG), SG=ρ/ρH2O

Dimensionless

force (F), F=ma Newton N kg•m/s² m kg s-2

Quantity Description Symbol

Expression in other SI units Expression in

terms of SI base units

Weight (W), W=mg Newton N kg•m/s² m kg s-2

pressure (P), P=F/A pascal Pa N/m2 m-1 kg s-2

work (energy heat) (W), Joule J N⋅m m2 kg s-2

Review SI units – Derived Units

( gy ) ( )W=F*d Joule J m2 kg s 2

specific energy (e), e=E/m joule per kilogram J/kg m2 s-2

energy density joule per cubic meter J/m3 m-1 kg s-2

power Watt, W J/s m2 kg s-3

Continuum• Matter is made up of atoms that

are widely spaced in the gas phase. disregard the atomic nature of a substance and view it as a continuous, homogeneous matter with no holes, that is, a continuum.

• The continuum idealization allows us to treat properties as point functions and to assume thefunctions and to assume the properties vary continually in space with no jump discontinuities.

• This idealization is valid as long as the size of the system is large relative to the space between the molecules.

• In this course we will limit our consideration to continuum.

Despite the large gaps between molecules, a substance can be treated as a continuum because of the very large number of molecules even in an extremely small volume.

Systems and Control Volumes• System: A quantity of matter or a region in space chosen for study.• Surroundings: The mass or region outside the system• Boundary: The real or imaginary surface that separates the system

from its surroundings.• The boundary of a system can be fixed or movable.• Systems may be considered to be closed or open.

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Systems and Control Volumes• Closed system (Control mass): A fixed amount of mass, and no

mass can cross its boundary.• Isolated system: in such a close system, even energy is not allowed

to cross the boundary

Coke Can

• Demonstrate Concept of system, boundary and surroundings

• Demonstrate closed system

• Demonstrate open system

• Open system (control volume): A properly selected region in space.

• It usually encloses a device that involves mass flow such as a compressor, turbine, or nozzle.

• Both mass and energy can cross the boundary of a control volume.

• Control surface: The boundaries of a control volume. It can be real or imaginary.

Systems and Control Volumes

An open system (a control volume) with one inlet and one exit.

In an open system, the volume can vary with time!

Properties of System• Property: Any characteristic of a system. • Some familiar properties are pressure P,

temperature T, volume V, and mass m. • Properties are considered to be either

intensive or extensive. • Intensive properties: Those that are

independent of the mass of a system, such as temperature, pressure, and densitydensity.

• Extensive properties: Those whose values depend on the size or extent of the system.

• Specific properties: Extensive properties per unit mass.

Criterion to differentiate intensive and extensive properties.

Thermodynamics deals with equilibrium states. • Equilibrium: A state of balance. there are no unbalanced

potentials (or driving forces) within the system. • Thermal equilibrium: If the temperature is the same

throughout the entire system.

State and Equilibrium

A closed system reaching thermal equilibrium.

The Zeroth Law of Thermodynamics

• The zeroth law of thermodynamics: If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.

• By replacing the third body with a thermometer, the zeroth law can be restated as two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact.

Two bodies reaching thermal equilibrium after being brought

into contact in an isolated enclosure.

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Thermodynamics deals with equilibrium states.

• Mechanical equilibrium: If there is no change in pressure at any point of the system with time.

• Chemical equilibrium: If the chemical composition of a system does not change with time, that is, no h i l ti

State and Equilibrium

a phase is a region of space (a h d i )

• Phase equilibrium: If a system involves two phases and when the mass of each phase reaches an equilibrium level and stays there.

chemical reactions occur. thermodynamic system), throughout which all physical properties of a material are essentially uniform.

State and Equilibrium

State: a certain condition that can be completely described a set of properties

A closed system reaching thermal equilibrium.

A system at two different states.

The State Postulate

• The number of properties required to fix the state of a system is given by the state postulate:

The state of a simple compressible system is completely specified by twocompletely specified by two independent, intensive properties.

• Simple compressible system: If a system involves no electrical, magnetic, gravitational, motion, and surface tension effects.

The state of nitrogen is fixed by two independent, intensive properties.

Processes and CyclesProcess: Any change that a system undergoes from one equilibrium state to

another.Path: The series of states through which a system passes during a process.To describe a process completely, one should specify the initial and final

states, as well as the path it follows, and the interactions with the surroundings.

• Process diagrams plotted with thermodynamic properties as coordinatesthermodynamic properties as coordinates are use to visualize the processes.

• Some common properties as coordinates are temperature T, pressure P, and volume V (or specific volume v).

• The prefix iso- is often used to designate a process for which a particular property remains constant.

• Cycle: A process during which the initial and final states are identical.

• Isothermal process: A process during which the temperature T remains constant.

• Isobaric process: A process during which the pressure P remains constant.

Processes and Cycles

e a s co sta t

• Isochoric (or isometric) process: A process during which the specific volume v remains constant.

The P-V diagram of a compression process.

Quasi-static or quasi-equilibrium process:When a process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times.

Processes and Cycles

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The Steady-Flow Process• The term steady implies no change with

time. The opposite of steady is unsteady, or transient.

During a steady-flow process, fluid properties within the control volume may change with position but not with time.

Under steady-flow conditions, the mass and energy contents of a control volume remain

• Steady-flow process: A process during which a fluid flows through a control volume steadily.

• Steady-flow conditions can be closely approximated by devices that are intended for continuous operation such as turbines, pumps, boilers, condensers, and heat exchangers or power plants or refrigeration systems.

energy contents of a control volume remain constant.

Temperature Scales• Ice point: A mixture of ice and water that is in equilibrium with air

saturated with vapor at 1 atm pressure (0°C or 32°F). • Steam point: A mixture of liquid water and water vapor (with no air) in

equilibrium at 1 atm pressure (100°C or 212°F).• Celsius scale: in SI unit system, ice point=0°C, steam point=100°C

• Fahrenheit scale: in English unit system, ice point= 32°F, steam point 212°F.

T (oF) =1.8 T(oC) +32• Rankine scale:

T (R) = T(oF) +459.67

• Thermodynamic temperature scale: A temperature scale that is independent of the properties of any substance.

• Thermodynamic temperature scale is the Kelvin scale (SI); T (K) = T (°C) + 273.15 (K)

Temperature ScalesThermodynamic temperature scale

Atomic-resolution STM image of reconstructed Si(111)-(7×7), Frame size: 31nm

Temperature arises from the random submicroscopic vibrations of the particle constituents of matter. These motions comprise the kinetic energy in a substance

Thermodynamic temperature at 0 K, absolute zero, is the temperature at which the particle constituents of matter are as close as possible to complete rest

0 K = - 273.15 oCSTM (scanning tunneling microscope)

Temperature Scales

Tools for temperature measurement:

• Alcohol Thermometer

• Thermocouple

The International temperature Scale of 1990 (ITS-90):

Ice point: 0 oC (273.15 K)

Steam point: 99.975 oC

• Infrared sensor

PressurePressure: A compressive force per unit area

68 kg 136 kg

The normal stress (or “pressure”) on the feet of a chubby person is much greater than on the feet of a slim person.

Some basic pressure gages.

Afeet=300cm2

0.23 kgf/cm2 0.46 kgf/cm2

P=68/300=0.23 kgf/cm2

• Absolute pressure: The actual pressure at a given position. It is measured relative to absolute vacuum (i.e., absolute zero pressure).

• Gage pressure: The difference between the absolute pressure and the local atmospheric pressure. Most pressure-measuring devices are calibrated to read zero in the atmosphere, and so they indicate gage pressure.

• Vacuum pressure: Pressures below atmospheric pressure.

Gage Pressure and Vacuum Pressure

Throughout this text, the pressure Pwill denote absolute pressureunless specified otherwise.

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A pressure gauge connected to a tank displays 75 kPa in Morgantown (the atmosphere pressure is 100 kPa in Morgantown WV). After the tank was moved to Denver, CO, the pressure gauge shows a pressure of 92 kPa. What is the atmosphere pressure in Denver?

Pgauge = Pabs -Patm

Pgauge, morgantown = Pabs –Patm, morgantown

Gauge Pressure and Vacuum Pressure

75 kPa

100 kPa in Morgantown

Pabs

92 kPa = 175 kPa – Patm, denver

75 KPa = Pabs – 100 kPa

Pgauge, denver = Pabs –Patm, denver

Pabs = 175 kPa

Patm, denver = 83 kPa

Morgantown

92 kPa

??? kPa in Denver

Pabs

Pascal’s law: The pressure applied to a confined fluid increases the pressure throughout by the same amount.

The area ratio A2/A1 is called the ideal mechanical advantage of the hydraulic lift.

Pascal’s Law

Lifting of a large weight by a small force by the application of Pascal’s law.

Variation of Pressure with Depth

ΔxΔy

Δz

0:0 12 =−ΔΔ−ΔΔ=∑ mgyxPyxPFz

012 =ΔΔΔΔ−ΔΔ−ΔΔ zgxyxyxPyxP ρ

012 =Δ−− zgPP ρ

Free-body diagram of a rectangular fluid element in equilibrium.

If take Point 1 at the free surface of the liquid Where P1=Patm.

(1). Pressure in a liquid at rest is independent of the shape or cross section of the container.

(2). It changes with the vertical distance by remains in other directions.

In a room filled with a gas, the variation of pressure with height is negligible.ghPP ρ=− 12

Pascal’s Law

The pressure is the same at all points on a horizontal plane in a given fluid regardless of geometry, provided that the points are interconnected by the same fluid.

The ManometerIt is commonly used to measure small and moderate pressure differences. A manometer contains one or more fluids such as mercury, water, alcohol, or oil.

P2 = Patm + ρgh

Pgas = P1 = P2

Patm

The basic manometer

P2

Pgas Pgas = P1 = P2 = Patm + ρgh

P1The gage pressure measured by manometer:

Pgage = ρgh

Pascal’s Law

In stacked-up fluid layers, the pressure change across a fluid layer of density ρ and height h is ρgh.

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The ManometerMeasuring the pressure drop across a flow section or a flow device by a differential manometer.

PA= PB

P1 + ρ1g(a+h)= P2 + ρ1ga + ρ2gh

PBPA

Alternatively

The Manometer

Measure pressure with multi-fluid manometer

A BC

D F

Other Pressure Measurement Devices• Bourdon tube: Consists of a hollow metal tube

bent like a hook whose end is closed and connected to a dial indicator needle.

• Pressure transducers: Use various techniques to convert the pressure effect to an electrical effect such as a change in voltage, resistance, or capacitance.

Pressure transducers are smaller and faster, and they can be more sensitive, reliable, and precise th th i h i l t t

Various types of Bourdon tubes used to measure pressure.

than their mechanical counterparts.

• Strain-gage pressure transducers: Work by having a diaphragm deflect between two chambers open to the pressure inputs.

• Piezoelectric transducers: Also called solid-state pressure transducers, work on the principle that an electric potential is generated in a crystalline substance when it is subjected to mechanical pressure.

The Barometer and Atmospheric Pressure• Atmospheric pressure is measured by a device called a barometer; thus,

the atmospheric pressure is often referred to as the barometric pressure.

• A frequently used pressure unit is the standard atmosphere, which is defined as the pressure produced by a column of mercury 760 mm in height at 0°C (ρHg = 13,595 kg/m3) under standard gravitational acceleration (g = 9.807 m/s2).

The length or the

The basic barometer.

The length or the cross-sectional area

of the tube has no effect on the height of

the fluid column of a barometer, provided

that the tube diameter is large enough to

avoid surface tension (capillary) effects.

All equations must be dimensionally homogeneous.

To be dimensionally homogeneous, all the terms in an equation must have the same unit system.

Dimension and Units

On a day when the barometer reads 760 Torr, a tire pressure gage reads 204 kPa. What is the absolute pressure in Pascals in the tire?

kPaPPP gaugeatmabs 764204760 =+=+= ?

PaPatorr

PatorrPPP gaugeatmabs 30530810002043.133)760( =×+×=+=

1 mmHg = 1 torr

The barometer (with a diameter 3 mm in diameter) reads 10 inch H2O. The density of water is 1 g/cm3. What is this pressure in kiloPascals?

The Barometer and Atmospheric Pressure

Attention: the pressure reading is independent on the diameter of the barometer.

)10)(/8.9)(/1( 23 inchsmcmgghP == ρ (unit?)

3

kPaPa

msmm

kgghP

489.22489

)0254.010)(/8.9]()101(

101[ 232

3

=

=

×××

== −

−

ρ

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Summary• Importance of dimensions and units

Some SI and English units, Dimensional homogeneity, Unity conversion ratios

• Systems and control volumes• Properties of a system• Density and specific gravity• State and equilibrium

The state postulateThe state postulate• Processes and cycles

The steady-flow process• Temperature and the zeroth law of thermodynamics

Temperature scales• Pressure

Variation of pressure with depth• The manometer and the atmospheric pressure