Transcript Fluid dynamics and Heat transfer

Lecture 2
Noor Shazliana Aizee Bt Abidin
FORMS OF ENERGY
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THERMAL ENERGY – HEAT
CHEMICAL ENERGY – IN FUELS OR BATTERIES
KINETIC ENERGY – IN MOVING SUBSTANCES
ELECTICAL ENERGY
GRAVITATIONAL ENERGY – POTENTIAL ENERGY
OTHERS
EQUATIONS, UNITS
 FORCE (N) = MASS (kg) X ACCERATION (m s-2)
 ENERGY(J) = FORCE (N) X DISTANCE (m)
 POWER (W) = RATE AT WHICH ENERGY IS
CONVERTED FROM ONE FORM TO ANOTHER OR
TRANSFERRED FROM ONE PLACE TO ANOTHER.
MORE DEFINITIONS
 ONE WATT (power) = ONE JOULE PER SECOND
 1kWh (energy) = 1000 w X 3600 s/h = 3.6x106 J
 ENERGY IS OFTEN MEASURED IN QUANTITIES
OF FUEL SUCH AS tonnes OIL OR COAL (1 tonne
= 2,205 pounds)
 NATIONAL ENERGY STATISTICS OFTEN USE
THE UNIT OF “million tonnes of oil equivalent”
1Mtoe = 41.9PJ = 41.9x1015J
FORMS OF ENERGY
 FUNDAMENTALLY 4 TYPES
 kinetic, gravitational, electrical, nuclear
 KINETIC ENERGY = ½ X mass X speed2
 THERMAL ENERGY IS FORM OF KINETIC ENERGY –
i.e. movement of molecules
 0 degrees Kelvin corresponds to zero molecular motion
 REMEMBER: temp(K) = temp(oC) + 273
GRAVITATIONAL ENERGY
 POTENTIAL ENERGY
 PE = FORCE X DISTANCE = WEIGHT X HEIGHT = m
xgxh
 IMPORTANT FOR SOME ENERGY STORAGE
TECHNOLOGIES
 HYDROPOWER – PUMPED STORAGE
ELECTRICAL ENERGY
 CHEMICAL ENERGY (BATTERY) IS ELECTRICAL
ENERGY
 CHEMICAL ENERGY FROM BURNING A FUEL IS
CONVERTED TO THERMAL (kinetic energy)
 Power (watts) = voltage (volts) x current (amps)
 ELECTROMAGNETIC ENERGY – RADIATION (XRAYS, UV, IR, VISIBLE, MICROWAVES, RADIO
WAVES)
 frequency x wavelength = velocity of light
 fxl=c
THERMODYNAMICS
 CONSERVATION OF ENERGY = FIRST LAW OF
THERMODYNAMICS
 SECOND LAW OF THERMODYNAMICS = THERE IS
A LIMIT TO THE EFFICIENCY OF ANY HEAT
ENGINE (SOME OF THE ENERGY MUST BE
REJECTED AS LOWER TEMPERATURE HEAT)
WORLD CONSUMPTION OF ENERGY
 IN 2002
 451 EJ (exajoules) = 451 x 1018 J = 10,800Mtoe
 WORLD POPULATION = 6.2 billion people
 AVERAGE ANNUAL CONSUMPTION per PERSON =
350 GJ per year
Fluid dynamics
 Fluid dynamics has a wide range of applications,
including calculating forces and moments on aircraft,
determining the mass flow rate of petroleum through
pipelines, predicting weather patterns, understanding
nebulae in interstellar space and reportedly modeling
fission weapon detonation.
 The solution to a fluid dynamics problem typically
involves calculating various properties of the fluid,
such as velocity, pressure, density, and temperature, as
functions of space and time.
Equations of fluid dynamics
 The foundational axioms of fluid dynamics are the
conservation laws, specifically, conservation of mass,
conservation of linear momentum (also known as
Newton's Second Law of Motion), and conservation of
energy (also known as First Law of Thermodynamics).
These are based on classical mechanics and are
modified in quantum mechanics and general relativity.
They are expressed using the Reynolds Transport
Theorem.
Conservation of Momentum
 In the absence of an external force, the momentum of
a system remains unchanged.
total momentum before = total momentum after
(m1v1 + m2v2)before = (m1v1 + m2v2)after
 As an example lets look at the cannon below. Let's
assume that the cannon has a mass of 500 kg (mc) and
the cannonball has a mass of 10 kg (mb). If the cannon
launches the cannonball at a velocity of 200 m/s, what
is the velocity of the cannon?
We can attack this problem using the conservation of
momentum formula in equation above. Before the
cannon is fired we know that its velocity, vc, is zero and
the velocity of the cannonball, vb, is zero.
(mcvc + mbvb) = (500 kg x 0 m/s + 10 kg x 0 m/s) = 0
 Now if the total momentum is zero before the cannon
firing, the conservation of momentum tells us that it must
be zero after the cannon fires.
(500 kg x vc + 10 kg x 200 m/s) = 0
(500 kg x vc + 2000 kg.m/s) = 0
500 kg vc = -2000 kg.m/s
 200 kg.m / s
vc 
500 kg
vc = -4 m/s
 We have used the conservation of momentum to calculate
that the cannon recoils with an initial velocity of -4 m/s,
that is, 4 m/s in the opposite direction of the cannonball.
Viscosity
 Viscous problems are those in which fluid friction has significant
effects on the fluid motion.
 Viscosity is a measure of the resistance of a fluid which is being
deformed by either shear stress or extensional stress. In everyday
terms (and for fluids only), viscosity is "thickness." Thus, water is
"thin," having a lower viscosity, while honey is "thick," having a
higher viscosity.
 Dynamic viscosity is measured with various types of rheometer.
Close temperature control of the fluid is essential to accurate
measurements, particularly in materials like lubricants, whose
viscosity can double with a change of only 5 °C. For some fluids,
it is a constant over a wide range of shear rates. These are
Newtonian fluids.
 The fluids without a constant viscosity are called non-Newtonian
fluids. Their viscosity cannot be described by a single number.
Non-Newtonian fluids exhibit a variety of different correlations
between shear stress and shear rate.
Turbulence
 In fluid dynamics, turbulence or turbulent flow is a
fluid regime characterized by chaotic, stochastic
property changes. This includes low momentum
diffusion, high momentum convection, and rapid
variation of pressure and velocity in space and time.
 Flow that is not turbulent is called laminar flow. While
there is no theorem relating Reynolds number to
turbulence, flows with high Reynolds numbers usually
become turbulent, while those with low Reynolds
numbers usually remain laminar.
Heat transfer
Conduction
Convection
 Convection occurs when heat travels along with a
moving fluid. In mass transfer, convection ( convective
mass transfer ) refers to a situation whereby molecular
diffusion occurs simultaneously with bulk flow.
Radiative heat transfer
 The term radiation covers a vast array of phenomena
that involve energy transport in the form of waves.
 In this section, we deal only with a particular kind of
radiation, called thermal radiation .
 Thermal radiation refers to electromagnetic radiation
in the wavelength range of 10 7 to 10 4 m and
encompasses mainly the range of infrared radiation.
 It is so called because its practically sole effect is
thermal, i.e. cooling of the emitting body and heating
of the receiving body.
 Above the absolute temperature of zero °K, all
substances emit electromagnetic radiation.
 The intensity and the ‘ color ’ (wavelength
distribution) of the radiation strongly depend on the
temperature of the source.
 In contrast with conduction and convection, heat
transfer by radiation does not require the presence of a
material medium.
Thank you.