termofluidos i diapositivas 1ra unidad

7/22/2019 Termofluidos I Diapositivas 1ra Unidad

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CLASSINTRODUCTION AND

BASIC CONCEPTS

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Objectives

Identify the unique vocabulary associated with

thermofluids through the precise definition of

basic concepts to form a sound foundation for

the development of the principles of

thermofluids.

Explain the basic concepts of thermofluids. Review concepts of temperature, temperature

scales, pressure, and absolute and gage

pressure.


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FIGURE 15

Some application areas of

thermodynamics.


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FIGURE 15


thermodynamics.


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FIGURE 15


thermodynamics.


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FIGURE 15


thermodynamics.


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FIGURE 15

Some application areas of Fluid Mechanics


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Aerodynamics

Bioengineering and biological systems

Combustion

Energy generation

Geology

Hydraulics and Hydrology

Hydrodynamics

Meteorology

Ocean and Coastal Engineering

Water Resources

numerous other examples

Fluid Mechanics is beautiful


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Some application areas of Fluid Mechanics,

Aerodynamics

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Bioengineering


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Energy generation


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C i ht Th M G Hill C i I P i i i d f d ti di l


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Geology


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C i ht Th M G Hill C i I P i i i d f d ti di l


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River Hydraulics


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Hydraulic Structures


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Hydrodynamics


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Meteorology

Copyright The McGraw Hill Companies, Inc. Permission required for reproduction or display.

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Fluid Mechanics is beautiful

Copyright The McGraw Hill Companies, Inc. Permission required for reproduction or display.

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Dimensions and Units

Any physical quantity can be characterized by dimensions.

The magnitudes assigned to dimensions are called units.

Primary dimensions include: mass m, lengthL, time t, andtemperature T.

Secondary dimensions can be expressed in terms of primarydimensions and include: velocity V, energyE, and volume V.

Unit systems include English system and the metric SI(International System). We'll use both.

Dimensional homogeneity is a valuable tool in checking for

errors. Make sure every term in an equation has the same units. Unity conversion ratios are helpful in converting units. Use

them.



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py g p q p p y

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Dimensions and Units


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Accuracy, Precision, and Significant

DigitsEngineers must be aware of three principals that govern the proper use of

numbers.

1. Accuracy error : Value of one reading minus the true value. Closeness ofthe average reading to the true value. Generally associated with repeatable,fixed errors.

2. Precision error : Value of one reading minus the average of readings. Is ameasure of the fineness of resolution and repeatability of the instrument.Generally associated with random errors.

3. Significant digits : Digits that are relevant and meaningful. When

performing calculations, the final result is only as precise as the leastprecise parameter in the problem. When the number of significant digits isunknown, the accepted standard is 3. Use 3 in all homework and exams.



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Accuracy, Precision, and Significant

Digits

SYSTEMS AND CONTROL VOLUMES


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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 itssurroundings.

The boundary of a system can be fixed ormovable.

Systems may be considered to be closedor open.

Closed system

(Control mass):A fixed amountof mass, and nomass can crossits boundary.


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Open system (control volume):A properlyselected region in space.

It usually encloses a device that involvesmass flow such as a compressor, turbine, ornozzle.

Both mass and energy can cross theboundary of a control volume.

Control surface: The boundaries of a controlvolume. It can be real or imaginary.

An open system (acontrol volume) with one

inlet and one exit.


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Criterion to differentiate intensive

and extensive properties.

PROPERTIES

OF A SYSTEM Property:Any characteristic of a

system.

Some familiar properties are

pressure P, temperature T, volumeV, and mass m.

Properties are considered to beeither intensive or extensive.

Intensive properties: Those thatare independent of the mass of asystem, such as temperature,pressure, and density.

Extensive properties: Thosewhose values depend on the sizeor extentof the system.

Specific properties: Extensiveproperties per unit mass.

Continuum


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Continuum Matter is made up of atoms that are

widely spaced in the gas phase. Yetit is very convenient to disregard theatomic nature of a substance andview it as a continuous,homogeneous matter with no holes,that is, a continuum.

The continuum idealization allows usto treat properties as point functionsand to assume the properties varycontinually in space with no jump

discontinuities. This idealization is valid as long as

the size of the system we deal withis large relative to the spacebetween the molecules.

This is the case in practically allproblems.

In this text we will limit ourconsideration to substances that can

be modeled as a continuum.

Despite the large gaps between

molecules, a substance can be treated as

a continuum because of the very largenumber of molecules even in an

extremely small volume.

Wh t i fl id?


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What is a fluid?

A fluid is a substance in the gaseous or liquid form

Distinction between solid and fluid?

Solid: can resist an applied shear by deforming. Stress isproportional to strain

Fluid: deforms continuously under applied shear. Stress is

proportional to strain rate

F

A =

F V

h =

FluidSolid

Wh t i fl id?


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What is a fluid?

Stress is defined as the

force per unit area.

Normal component:

normal stress

In a fluid at rest, thenormal stress is called

pressure

Tangential component:shear stress


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What is a fluid?

solid liquid gas

STATE AND EQUILIBRIUM


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STATE AND EQUILIBRIUM

Thermodynamics deals with

equilibrium states.

Equilibrium:A state of balance.

In an equilibrium state there are nounbalanced potentials (or driving

forces) within the system. Thermal equilibr ium: If the

temperature is the same throughoutthe entire system.

Mechanical equilibrium: If there isno change in pressure at any pointof the system with time.

Phase equilibrium: If a systeminvolves two phases and when themass of each phase reaches anequilibrium level and stays there.

Chemical equilibrium: If thechemical composition of a system

does not change with time, that is,no chemical reactions occur. A closed system reaching thermalequilibrium.

A system at two different states.

The State Postulate


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The State Postulate

The number of propertiesrequired to fix the state of asystem is given by the statepostulate:

The state of a simplecompressible system iscompletely specified by

two independent,intensive properties.

Simple compressible

system: If a system involvesno electrical, magnetic,gravitational, motion, and

surface tension effects.

The state of nitrogen is

fixed by two independent,

intensive properties.

PROCESSES AND CYCLES


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Process:Any change that a system undergoes from one equilibrium state toanother.

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.

Quasistatic or quasi-equilibr ium process: When a process proceeds in sucha manner that the system remains infinitesimally close to an equilibrium stateat all times.

PROCESSES AND CYCLES

Process diagrams plotted by


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Process diagrams plotted byemploying thermodynamic propertiesas coordinates are very useful in

visualizing the processes. Some common properties that are

used as coordinates are temperatureT, pressure P, and volume V (orspecific volume v).

The prefix iso- is often used todesignate a process for which aparticularproperty remains constant.

Isothermal process:A process

during which the temperature Tremains constant.

Isobaric process:A process duringwhich the pressure P remainsconstant.

Isochoric (or isometric) process:Aprocess during which the specificvolume v remains constant.

Cycle:A process during which the

initial and final states are identical.

The P-V diagram of a compression

process.

The Steady-Flow Process


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The Steady Flow Process The term steady implies no

change with time. Theopposite of steady isunsteady, or transient.

A large number ofengineering devices operatefor long periods of timeunder the same conditions,and they are classified assteady-flow devices.

Steady-flow process:Aprocess during which a fluidflows through a controlvolume steadily.

Steady-flow conditions can

be closely approximated bydevices that are intended forcontinuous operation suchas turbines, pumps, boilers,condensers, and heatexchangers or power plantsor refrigeration systems.

During a steady-

flow process, fluid

properties withinthe control

volume may

change with

position but not

with time.

Under steady-flow conditions, the massand energy contents of a control volume

remain constant.


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CLASEPROPERTIES OF FLUIDS


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Density

The density of a fluid, denoted by , is its mass per unitvolume. Density is highly variable in gases and increases

nearly proportionally to the pressure level. Density in liquids

is nearly constant; the density of water (about 1000 kg/m3)increases only 1 percent if the pressure is increased by a

factor of 220. Thus most liquid flows are treated analytically

as nearly incompressible.

( )

3

3

volumen,

masa,

mV

kgm

mkg

V

m

=

=

=Specific volume


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Density of Ideal Gases

Equation of State: equation for the relationshipbetween pressure, temperature, and density.

The simplest and best-known equation of state is theideal-gas equation.

P v = R T or P = R T

Ideal-gas equation holds for most gases.

However, dense gases such as water vapor andrefrigerant vapor should not be treated as ideal gases.


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Specific weight

The specific weight of a fluid, denoted by , is its weight perunit volume. Just as a mass a weight W = mg, density and

specific weight are simply related by gravity

( )

2

3

3

/gravedad,ladenaceleraci/densidad,

smgmkg

mNg

==

=

( )( )

3323

3323

4,629790807,9998

0752,08,11807,9205,1

ftlbfmNsmmkg

ftlbfmNsmmkg

water

air

===

===


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Specific gravity

The specific gravity, denoted by SG, is the ratio of a fluiddensity to a standar reference fluid, water (for liquids), and air

(for gases)

3

3

998SG

205,1SG

mkg

mkg

liquid

water

liquid

liquid

gas

air

gasgas

==

==

For example, the specific gravity of mercury (Hg) is

SGHg = 13580/998 13,6. These dimensionless ratios

easier to remember than the actual numerical values

of density of a variety of fluids


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Specific gravity

Vapor Pressure and Cavitation


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Vapor Pressure and Cavitation

Vapor Pressure Pv is defined asthe pressure exerted by its vaporin phase equilibrium with itsliquid at a given temperature

If P drops below Pv, liquid islocally vaporized, creatingcavities of vapor.

Vapor cavities collapse whenlocal P rises above Pv.

Collapse of cavities is a violent

process which can damagemachinery.

Cavitation is noisy, and cancause structural vibrations.

E d S ifi H


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Energy and Specific Heats

Total energyEis comprised of numerous forms: thermal,mechanical, kinetic, potential, electrical, magnetic, chemical,

and nuclear. Units of energy arejoule (J) orBritish thermal unit(BTU).

Microscopic energy

Internal energy u is for a non-flowing fluid and is due to molecularactivity.

Enthalpy h=u+Pv is for a flowing fluid and includes flow energy (Pv).

Macroscopic energy

Kinetic energy ke=V2/2

Potential energype=gz

In the absence of electrical, magnetic, chemical, and nuclear

energy, the total energy is eflowing=h+V2/2+gz.

Compressibility and expansion


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Compressibility and expansion

How does fluid volume change with P and T?

Fluids expand as T or P

Fluids contract as T or P

Need fluid properties that relate volume changes to changes in P and T.

Coefficient of compressibility

Coefficient of volume expansion

Combined effects of P and Tcan be written as

T T

P Pv

v

= =

1 1

P P

v

v T T

= =

P T

v vdv dT dP

T P

= +


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Coefficient of Compressibility

( )psiPa,TT

P

v

Pvk

=

=

in terms of finite changes as

( )psiPa,

P

vv

Pk


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Coefficient of Compressibility

The density of seawater at a free surface where the pressure is 98

kPa is approximately 1030 kg/m3. Taking the bulk modulus of

elasticity of seawater to be 2.34 X 10

9

N/m

2

and expressingvariation of pressure with depth z as dP =gdz determine the

density and pressure at a depth of 2500 m. Disregard the effect

of temperature


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Coefficient of Volume Expansion

( )K

111

PP T

P

T

v

v

=

=

in terms of finite changes as

( )

( )K

11

K1

gasidealT

TT

vv

=


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The combined effects of pressure and temperature changes on the

volume change of a fluid can be determined by taking the specific

volume to be a function of T and P

( )vdPdTdPP

vdT

T

vdv

TP

=

+

=

Then the change in volume (or density) due to changes in pressure

and temperature can be expressed approximately as

PTv

v

=


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Surface Tension


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Liquid droplets behave like smallspherical balloons filled with liquid,and the surface of the liquid acts likea stretched elastic membrane under

tension.

The pulling force that causes this is

due to the attractive forces betweenmolecules

called surface tensions.

Attractive force on surface moleculeis not symmetric.

Repulsive forces from interiormolecules causes the liquid tominimize its surface area and attaina spherical shape.

Capillary Effect


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Capillary effect is the rise orfall of a liquid in a small-diameter tube.

The curved free surface in thetube is call the meniscus.

Water meniscus curves upbecause water is a wettingfluid.

Mercury meniscus curvesdown because mercury is a

nonwetting fluid. Force balance can describe

magnitude of capillary rise.

Viscosity


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Viscosity is a propertythat represents theinternal resistance of afluid to motion.

The force a flowing

fluid exerts on a body inthe flow direction iscalled the drag force,

and the magnitude ofthis force depends, inpart, on viscosity.

The behavior of a fluid inViscosity


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The behavior of a fluid in

laminar flow between two

parallel plates when theupper plate moves with a

constant velocity.

Fluids for which the rate of deformation is

proportional to the shear stress are called

Newtonian fluids.

Shear stress

Shear force

dynamic viscositykg/m s or N s/m2 or Pa s

1 poise = 0.1 Pa s

Viscosity


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Viscosity


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Viscosity


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Viscosity


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Viscosity


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This equation can be used tocalculate the viscosity of a fluid

by measuring torque at a

specified angular velocity.

Therefore, two concentric

cylinders can be used as a

viscometer, a device that

measures viscosity.

L length of the cylinder

number of revolutions per unit time

TEMPERATURE AND THE ZEROTH LAW OF

THERMODYNAMICS


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THERMODYNAMICS

The zeroth law of thermodynamics: If two bodies are in thermalequilibrium with a third body, they are also in thermal equilibrium witheach 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 thesame temperature reading even if they are not in contact.

Two bodies reaching

thermal equilibrium

after being brought

into contact in an

isolated enclosure.

All temperature scales are based onil d ibl t t h

Temperature ScalesP versus T plots

of the


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some easily reproducible states such asthe freezing and boiling points of water:the ice point and the steam point.

Ice point:A mixture of ice and waterthat is in equilibrium with air saturatedwith vapor at 1 atm pressure (0C or32F).

Steam point:A mixture of liquid waterand water vapor (with no air) inequilibrium at 1 atm pressure (100C or212F).

Celsius scale: in SI unit system

Fahrenheit scale: in English unitsystem

Thermodynamic temperature scale:Atemperature scale that is independent ofthe properties of any substance.

Kelvin scale (SI) Rankine scale (E)

A temperature scale nearly identical tothe Kelvin scale is the ideal-gastemperature scale. The temperatures

on this scale are measured using aconstant-volume gas thermometer.

of the

experimental

data obtainedfrom a constant-

volume gas

thermometer

using four

different gases

at different (but

low) pressures.

A constant-volume gas thermometer wouldread 273.15C at absolute zero pressure.

Comparison of

temperature


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scales.

The reference temperature in the original Kelvin scale was the ice point,

273.15 K, which is the temperature at which water freezes (or ice melts).

The reference point was changed to a much more precisely reproducible

point, the triple point of water (the state at which all three phases of watercoexist in equilibrium), which is assigned the value 273.16 K.

Comparison ofmagnitudes of

various

temperature

units.

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