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Fluid dynamics and Heat transfer Lecture 2 Noor Shazliana Aizee Bt Abidin
FORMS OF ENERGY • 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 x g x h • 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 (X-RAYS, UV, IR, VISIBLE, MICROWAVES, RADIO WAVES) • frequency x wavelength = velocity of light • f x l = 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 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.