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
Some application areas of
thermodynamics.
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FIGURE 15
Some application areas of
thermodynamics.
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FIGURE 15
Some application areas of
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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Some application areas of Fluid Mechanics,
Bioengineering
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Some application areas of Fluid Mechanics,
Energy generation
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Some application areas of Fluid Mechanics,
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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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Some application areas of Fluid Mechanics,
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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Some application areas of Fluid Mechanics,
River Hydraulics
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Some application areas of Fluid Mechanics,
Hydraulic Structures
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Some application areas of Fluid Mechanics,
Hydrodynamics
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Some application areas of Fluid Mechanics,
Meteorology
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Some application areas of Fluid Mechanics,
Fluid Mechanics is beautiful
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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
1-2
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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Coefficient of Volume Expansion
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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Coefficient of Volume Expansion
1-15
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.