Ch01 thermo

transcript

Summer 2013

Lec Mubashir Gulzar

LECTURE 1

Instructor:–Lec Mubashir Gulzar

– Phone: 03346862433– Email: mubashirgulzar@ceme.nust.edu.pk

Course Overview and PolicyRequired Textbook M. J. Moran and H. N. Shapiro, Fundamentals of Engineering Thermodynamics, 5th editionReference Book Cengel and Boles Thermodynamics: An Engineering ApproachPrerequisites(Calculus) & (Mechanics)

Course Overview and Policy (1)Topics Covered

–Basic concepts of Thermodynamics. –Work, Heat and Energy. –Conservation of Energy (First Law),

Internal Energy. Evaluating propersties–Second Law of Thermodynamics. –Applying second law to engineering

systems. –Entropy and its application. –Vapor power systems. –Refrigeration and heat pumps systems.

Course Overview and Policy (2)

Grades Weight FactorsQuiz - 10%Midterm Exam - 30%Final Exam – 50%Class Participation/Assignments – 10%

Chapter 1

Introductory Concepts and Definitions

Learning Outcomes

►Demonstrate understanding of several fundamental concepts used throughout this book . . . Including closed system, control volume, boundary and surroundings, property, state, process, the distinction between extensive and intensive properties, and equilibrium.

Learning Outcomes, cont.

►Apply SI and English Engineering units, including units for specific volume, pressure, and temperature.

►Work with the Kelvin, Rankine, Celsius, and Fahrenheit temperature scales.

►Apply the problem-solving methodology used in this book.

What is Thermodynamics?• Greek: therme (θέρμη) = heat and dynamis

(δύναμις) = power (The science of energy)• N. L. Sadi Carnot (1796 – 1832) Father of

Thermodynamics• Lord Kelvin used for the first time the

word thermodynamics in 1849• Macroscopic Approach = Classical

Thermodynamics (State and Process)• Microscopical Approach = Statistical

thermodynamics• Application areas … are many.

Defining Systems►System: whatever we want to study; (Quantity of mass or region in space)►Surroundings: everything external to the

system. ►Boundary: separates system from its

surroundings. Can be fixed or movable.

System

Boundary

Surroundings

Closed System (Control mass)►A system that always

contains the same matter (same mass).

►No transfer of mass across its boundary can occur.

►Isolated system: special type of closed system that does not interact in any way with its surroundings (no energy crosses its boundary)

Control Volume (Open System)►A given region of space through which mass

flows. ►Mass may cross the boundary of a CV►Boundary of CV is called Control Surface (real or

imaginary)

Automobile Engine

Selecting System BoundaryExample: compressor and tank

►Boundary 1: How long will the compressor operate? ►Boundary 2: What is the electric power input?

Boundary 1

Boundary 2

Macroscopic and Microscopic Views►Systems can be described from the macroscopic and

microscopic points of view. ►The microscopic approach aims to characterize by statistical

means the average behavior of the particles making up a system and use this information to describe the overall behavior of the system.

►The macroscopic approach describes system behavior in terms of the gross effects of the particles making up the system – specifically, effects that can be measured by instruments such a pressure gages and thermometers.

►Engineering thermodynamics predominately uses the macroscopic approach.

LECTURE 2

Property

►Mass►Volume►Energy►Pressure►Temperature

►A macroscopic characteristic of a system to which a numerical value can be assigned at a given time without knowledge of the previous behavior of the system.

►For the system shown, examples include:

State►The condition of a system as described by its

properties.►Example: The state of the system shown is

described by p, V, T,….►The state often can be specified by providing the

values of a subset of its properties. All other properties can be determined in terms of these few.

State: p, V, T, …

Process►A transformation from one state to another.►When any of the properties of a system

changes, the state changes, and the system is said to have undergone a process.

►Example: Since V2 > V1, at least one property value changed, and the gas has undergone a process from State 1 to State 2.

State 1: p1, V1, T1, … State 2: p2, V2, T2, …

Gas Gas

►Depends on the size or extent of a system.►Examples: mass, volume, energy.►Its value for an overall system is the sum of its

values for the parts into which the system is divided.

►Its value may vary with time but not position.

Extensive Property

Intensive Property

►Independent of the size or extent of a system.►Examples: pressure, temperature.►Its value is not additive as for extensive

properties.►May vary from place to place within the system

at any moment – function of both position and time.

Equilibrium►When a system is isolated, it does not interact with

its surroundings; however, its state can change as a consequence of spontaneous events occurring internally as its intensive properties such as temperature and pressure tend toward uniform values. When all such changes cease, the system is at an equilibrium state.

►Equilibrium states and processes from one equilibrium state to another equilibrium state play important roles in thermodynamic analysis.

Units, System of Units►A unit is any specified amount of a quantity by

comparison with which any other quantity of the same kind is measured (e.g., meter, kilometers, feet, and miles are all units of length).

►Two systems of units:►Metric SI System (Système International

d’Unités)• Simple, based on decimal relationship

►English Engineering System of units.• no apparent systematic numerical base

►Any physical quantity can be characterized by dimensions ant their magnitude is called unit.

►Primary or • Fundamental • Dimensions

►Secondary • or derived • dimensions: • Expressed in terms of the primary dimensions)• Eg: Velocity v, energy E, and volume V

Dimensions and Units

SI Unit Prefixes

Other prefixes

1024 - yotta, Y1021 - zetta, Z1018 - exa, E1015 - peta, P

10-15 - femto, a10-18 – atto, a10-21 - zepto, z10-24 - yocto, y

SI: 1 N = (1 kg)(1 m/s2) = 1 kg∙m/s2

Units: Example

Mass, length, and time are base units and force has a unit derived from them using,

F = ma – Newton Second Law of Motion

English: 1 lbf = (1 lb)(32.1740 ft/s2) = 32.1740 lb∙ft/s2

Derived Units: ExamplesWork = force x distance [J]

SI: 1J = 1 N * mEnglish system: Btu (British thermal unit) = energy required to raise the temperature of 1 lbm of H2O at 68°F by 1°F.Calorie(cal): 1g H2O: 14,5°C by 1°C.1 cal = 4.1868 J1Btu = 1055.1J = 1.0551 KJ

Power [W] = time rate of energy [J/s]1 kW = 1.34 Horsepower KW and KWh! | 1KWh = 3600 kJ

Dimensional homogeneity►All equations must be dimensionally

homogeneous.►Excellent tool to spot errors

IMPORTANT!To be dimensionally homogeneous,

all the terms in an equation must have the same unit.

Do not mix apples and oranges!

Source of errors: lb, lbf, lbm

►lbm (lb) – pound mass = 0.4536 Kg (MASS)

►lbf – pound force (FORCE)

UM: lbf (lbm·ft/s2)

Force and mass are fundamentally different!

Weight and Mass!► Weight is not mass,

it is force: F = m·g g = 9.807 m/s2

= 32.174 ft/s2

(gravitational acc.)► Weight of 200 lb or 200 Kg is wrong!(Scales should show Newtons or lbf)► If you jump on scale the weight changes!► In space w/ no g scale will show “0” NO MASS?

Weight and Mass: Solution?

► The Problem: We measure mass indirectly (we measure the gravity force it exerts)

► The Solution: The correct way is to measure the mass of an object is to compare it to a known mass. − Not used in real live much. − Only used for calibration or measuring

precious metals.

Continuum Hypothesis►From a macroscopic perspective, description of

matter is simplified by considering it to be distributed continuously throughout a region.

►For Gas: Valid when size of system >> space between molecules

►More Scientific: Continuum model valid if characteristic length of system >> mean free path of molecules►When substances are treated as continua, it is

possible to speak of their intensive thermodynamic properties “at a point.”

►Three important intensive properties are: specific volume, pressure and temperature. 1-31

Density (r) and Specific Volume (v)►At any instant the density (r ) at a point is

defined as

where V ' is the smallest volume for which a definite value of the ratio exists.V ‘ - contains enough particles for statistical average

- small enough so it can be considered a “point”

►The density is an intensive property so it can vary from point to point. So the mass in a volume V can be calculate by integration

►So, the density is mass per unit volume r = m / V kg/m3 (SI) or lb/ft3(English)

►Specific volume is volume per unit mass and is the reciprocal of density: v = 1/r .

►Specific volume is an intensive property that may vary from point to point.

►SI units: (m3/kg), English units: (ft3/lb).

Density (r) and Specific Volume (v)

Specific volume is usually preferred for thermodynamic analysis when working with gases that typically have small density values.

Pressure (p)►Consider a small area A passing through a point

in a fluid at rest.►The fluid on one side of the area exerts a

compressive force that is normal to the area, Fnormal. An equal but oppositely directed force is exerted on the area by the fluid on the other side.

►The pressure (p) at the specified point is defined as the limit

normal

'AAlim

where A' is the area at the “point”: the smallest area for which a definite value of the ratio exists.

►SI unit of pressure is the pascal: 1 Pascal = 1 N/m2

Multiples of the Pascal are frequently used:►1 kPa = 103 N/m2 ; 1 MPa = 106 N/m2

►1 bar = 105 N/m2

►1 atm (standard atmosphere) = 101,325 N/m2

►Other: mmHG, torr(1 mmHG)►English units for pressure are:

►pounds force per square foot, lbf/ft2 ►pounds force per square inch, lbf/in.2 (PSI)►1 Atm = 14.696 PSI

Pressure Units (Many!)

►Absolute pressure: Pressure with respect to the zero pressure of a complete vacuum.

►Absolute pressure MUST be used in thermodynamic relations.

►Pressure-measuring devices often indicate the difference between the absolute pressure of a system and the absolute pressure of the atmosphere outside the measuring device.

Absolute Pressure

►When system pressure is greater than atmospheric pressure, the term gage pressure is used.

p(gage) = p(absolute) – patm(absolute)

►When atmospheric pressure is greater than system pressure, the term vacuum pressure is used.

p(vacuum) = patm(absolute) – p(absolute)

Gage and Vacuum Pressure

Variation of Pressure with Depth

Pressure of a fluid at restincreases with depth (as a result of added weight).

Pressure in a liquid at rest increaseslinearly with distance from the free surface.

Variation of Pressure with Depth

Free-body diagram of a rectangularfluid element in equilibrium.

Buoyancy: Archimedes’ Principle

The upward pushing force on horizontal area of a submerge block is:F=P2·A − P1·A= A(patm+ρgL2) − A(patm+ ρ gL1)= ρgA(L2-L1) F= ρgV

Archimedes’ Principle Buoyant force equal to weight of displaced fluid

The pressure applied to a confined fluid increases the pressure throughout by the same amount

Force applied by a fluid is proportional to the surface area.

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

Pascal’s Law

The Manometer

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

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

• Atmospheric pressure is measured by a device called a barometer thus it is also called barometric pressure.

• Standard atmosphere = pressure produced by 760 mm column of mercury at 0°C under standard g = 9.81 m/s2).

Basic barometer

Length or the cross-sectional area of the tube has no effect on reading,

Large Diameter (no capillarity effects)

The Barometer (Torricelli)

1 mmHg = 1 Torr

= 133.3Pa

Other Pressure Measurement Devices

Bourdon tubes

Bourdon tube: Closed hollow metal tube bent like a hook with end connected to a dial indicator needle.

Pressure transducers: Use change in voltage, resistance, or capacitance to read pressure effect (smaller and faster, more sensitive, reliable, and precise)

Strain-gage pressure transducers: Have 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.

Bourdon Tube Gage: Details

Example of Pressure Gages

LECTURE 3

Temperature (T)►If two blocks (one warmer than the other) are brought into contact and isolated from their surroundings, they would interact thermally with changes in observable properties.

►When all changes in observable properties cease, the two blocks are in thermal equilibrium.

►Temperature is a physical property that determines whether the two objects are in thermal equilibrium. 1-49

• 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 in isolated enclosure.

Temperature (T)

►Any object with at least one measurable property that changes as its temperature changes can be used as a thermometer.

►Such a property is called a thermometric property.

►The substance that exhibits changes in the thermometric property is known as a thermometric substance.

Thermometers

►Example: Liquid-in-glass thermometer►Consists of glass capillary tube connected to a bulb filled

with liquid and sealed at the other end. Space above liquid is occupied by vapor of liquid or an inert gas.

►As temperature increases, liquid expands in volume and rises in the capillary. The length (L) of the liquid in the capillary depends on the temperature.

►The liquid is the thermometric substance.►L is the thermometric property.

►Other types of thermometers:►Thermocouples►Thermistors►RTD - Resistance Temp. Detector►Optical – Thermoreflectance, IR►Radiation thermometers and optical pyrometers

Thermometers

Temperature Scales

All temperature scales are based on some easily reproducible states such as the freezing and boiling points of water: the ice point and the steam point. 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).The reference point was changed to more precisely reproducible point, the triple point of water(the state at which all three phases of water coexist in equilibrium), which is assigned the value 273.16 K

►Kelvin scale: An absolute thermodynamic temperature scale whose unit of temperature is the kelvin (K); the SI base unit for temperature.

►Rankine scale: An absolute thermodynamic temperature scale with absolute zero that coincides with the absolute zero of the Kelvin scale; the English base unit for temperature.

Temperature Scales(1)

T(oR) = 1.8T(K)

►Celsius scale (oC):

T(oC) = T(K) – 273.15

►Fahrenheit scale (oF):

T(oF) = T(oR) – 459.67

Temperature Scales(2)

►Celsius scale (oC):

T(oC) = T(K) – 273.15

►Fahrenheit scale (oF):

T(oF) = T(oR) – 459.67 1-55

►Rankine Scale(oR):

T(oR) = 1.8T(K)

Problem-Solving Methodology

Use step-by-step process: reduce complicated problem to solution of series of simple problems

Design Constrains

Analysis: Engineering Model

Problem-Solving Methodology

►Problem Statement / Known: Read the problem, think about it, and identify what is known.

►Find: State what is to be determined.►Schematic: Draw a sketch of system and label

with all relevant information/data.►Engineering Model: List all simplifying

assumptions/ idealizations and approximations made. Physical Laws. Properties.

►Calculations / Analysis: Reduce appropriate governing equations and relationships to forms that will produce the desired results.

Example: Solving Methodology

►Known►Find ►Schematic: ►Engineering Model ►Calculations / Analysis

Ch01 thermo

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