non-continuum energy transfer: gas dynamics
Post on 22-Feb-2016
67 Views
Preview:
DESCRIPTION
TRANSCRIPT
AME 60614 Int. Heat Trans.
D. B. Go Slide 1
Non-Continuum Energy Transfer: Gas Dynamics
AME 60614 Int. Heat Trans.
D. B. Go Slide 2
Phonons – What We’ve Learned• Phonons are quantized lattice vibrations
– store and transport thermal energy– primary energy carriers in insulators and semi-conductors (computers!)
• Phonons are characterized by their– energy– wavelength (wave vector)– polarization (direction)– branch (optical/acoustic) acoustic phonons are the primary thermal
energy carriers
• Phonons have a statistical occupation (Bose-Einstein), quantized (discrete) energy, and only limited numbers at each energy level – we can derive the specific heat!
• We can treat phonons as particles and therefore determine the thermal conductivity based on kinetic theory
AME 60614 Int. Heat Trans.
D. B. Go Slide 3
Electrons – What We’ve Learned• Electrons are particles with quantized energy states
– store and transport thermal and electrical energy– primary energy carriers in metals– usually approximate their behavior using the Free Electron Model
• energy• wavelength (wave vector)
• Electrons have a statistical occupation (Fermi-Dirac), quantized (discrete) energy, and only limited numbers at each energy level (density of states)
– we can derive the specific heat!
• We can treat electrons as particles and therefore determine the thermal conductivity based on kinetic theory
– Wiedemann Franz relates thermal conductivity to electrical conductivity
• In real materials, the free electron model is limited because it does not account for interactions with the lattice– energy band is not continuous– the filling of energy bands and band gaps determine whether a material is a
conductor, insulator, or semi-conductor
AME 60614 Int. Heat Trans.
D. B. Go Slide 4
• We will consider a gas as a collection of individual particles– monatomic gasses are simplest and can be analyzed from first
principles fairly readily (He, Ar, Ne)– diatomic gasses are a little more difficult (H2, O2, N2) must account for
interactions between both atoms in the molecule– polyatomic gasses are even more difficult
Gases – Individual Particles
gas … gas
AME 60614 Int. Heat Trans.
D. B. Go Slide 5
Gases – How to Understand One• Understanding a gas – brute force
– suppose we wanted to understand a system of N gas particles in a volume V (~1025 gas molecules in 1 mm3 at STP) position & velocity
• Understanding a gas – statistically– statistical mechanics helps us understand microscopic properties and
relate them to macroscopic properties– statistical mechanics obtains the equilibrium distribution of the
particles
• Understanding a gas – kinetically– kinetic theory considers the transport of individual particles (collisions!)
under non-equilibrium conditions in order to relate microscopic properties to macroscopic transport properties thermal conductivity!
just not possible
AME 60614 Int. Heat Trans.
D. B. Go Slide 6
Gases – Statistical MechanicsIf we have a gas of N atoms, each with their own kinetic energy ε, we can organize them into “energy levels” each with Ni atoms
gas … gas
total atoms in the system: internal energy of the system:
• We call each energy level εi with Ni atoms a macrostate
• Each macrostate consists of individual energy states called microstates• these microstates are based on quantized energy related to the quantum
mechanics Schrödinger’s equation• Schrödinger’s equation results in discrete/quantized energy levels
(macrostates) which can themselves have different quantum microstates (degeneracy, gi) can liken it to density of states
AME 60614 Int. Heat Trans.
D. B. Go Slide 7
Gases – Statistical Mechanics• There can be any number of microstates in a given macrostate
called that levels degeneracy gi• this number of microstates the is thermodynamic probability, Ω, of a
macrostate• We describe thermodynamic equilibrium as the most probable
macrostate
• Three fairly important assumptions/postulates(1)The time-average for a thermodynamic variable is equivalent to the
average over all possible microstates(2) All microstates are equally probable(3)We assume independent particles
• Maxwell-Boltzmann statistics gives us the thermodynamic probability, Ω, or number of microstates per macrostate
AME 60614 Int. Heat Trans.
D. B. Go Slide 8
Gases – Statistics and DistributionsThe thermodynamic probability can be determined from basic statistics but is dependant on the type of particle. Recall that we called phonons bosons and electrons fermions. Gas atoms we consider boltzons
boltzons: distinguishable particles
bosons: indistinguishable particles
fermions: indistinguishable particles and limited occupancy (Pauli exclusion)
Maxwell-Boltzmann statistics
Bose-Einstein statistics
Fermi-Dirac statisticsFermi-Diracdistribution
Bose-Einsteindistribution
Maxwell-Boltzmanndistribution
AME 60614 Int. Heat Trans.
D. B. Go Slide 9
Gases – What is Entropy?
Thought Experiment: consider a chamber of gas expanding into a vacuum
A B A B
• This process is irreversible and therefore entropy increases (additive)
• The thermodynamic probability also increases because the final state is more probable than the initial state (multiplicative)
How is the entropy related to the thermodynamic probability (i.e., microstates)? Only one mathematical function converts a multiplicative operation to an additive operation
Boltzmann relation!
AME 60614 Int. Heat Trans.
D. B. Go Slide 10
Gases – The Partition Function
The partition function Z is an useful statistical definition quantity that will be used to describe macroscopic thermodynamic properties from a microscopic representation
The probability of atoms in energy level i is simply the ratio of particles in i to the total number of particles in all energy levels
leads directly to Maxwell-Boltzmann distribution
AME 60614 Int. Heat Trans.
D. B. Go Slide 11
Gases – 1St Law from Partition Function
Heat and Work
adding heat to a system affects occupancy at each energy level
a system doing/receiving work does changes the energy levels
First Law of Thermodynamics – Conservation of Energy!
AME 60614 Int. Heat Trans.
D. B. Go Slide 12
Gases – Equilibrium PropertiesEnergy and entropy in terms of the partition function Z
Classical definitions & Maxwell Relations then lead to the statistical definition of other properties
chemical potential Gibbs free energy
Helmholtz free energy pressure
AME 60614 Int. Heat Trans.
D. B. Go Slide 13
Gases – Equilibrium Properties
enthaply
but classically … ideal gas law
the Boltzmann constant is directly related to the Universal Gas Constant
AME 60614 Int. Heat Trans.
D. B. Go Slide 14
Recalling that the specific heat is the derivative of the internal energy with respect to temperature, we can rewrite intensive properties (per unit mass) statistically
internal energy entropy
Gibbs free energy
Helmholtz free energy
enthaply
specific heat
Gases – Equilibrium Properties
AME 60614 Int. Heat Trans.
D. B. Go Slide 15
Gases – Monatomic Gases• In diatomic/polyatomic gasses the atoms in a molecule can vibrate between each
other and rotate about each other which all contributes to the internal energy of the “particle”
• monatomic gasses are simpler because the internal energy of the particle is their kinetic energy and electronic energy (energy states of electrons)
• an evaluation of the quantum mechanics and additional mathematics can be used to derive translational and electronic partition functions
consider the translational energy only
we can plug this in to our previous equations
internal energy entropy
specific heat
AME 60614 Int. Heat Trans.
D. B. Go Slide 16
Gases – Monatomic Gases
Where did P (pressure) come from in the entropy relation?
pressure
plugging in the translational partition function ….
the derivative of the ln(CV) is 1/V
ideal gas law
AME 60614 Int. Heat Trans.
D. B. Go Slide 17
Gases – Monatomic GasesThe electronic energy is more difficult because you have to understand the energy levels of electrons in atoms not too bad for monatomic gases(We can look up these levels for some choice atoms)
Defining derivatives as
internal energy entropy
specific heat
AME 60614 Int. Heat Trans.
D. B. Go Slide 18
Gases – Monatomic HeliumConsider monatomic hydrogen at 1000 K …I can look up electronic degeneracies and energies to give the following table
level g
1 0 0 0 0 0
2 3 229.9849711 2.282E+100 5.2484E+102 1.207E+105
3 0 239.2234393 0 0 0
4 8 243.2654669 3.564E+106 8.67E+108 2.1091E+111
5 3 246.2119245 2.5445E+107 6.2648E+109 1.5425E+112
6 3 263.622928 9.2705E+114 2.4439E+117 6.4427E+119
AME 60614 Int. Heat Trans.
D. B. Go Slide 19
Gases – Monatomic Heliumfrom Incropera and Dewitt
AME 60614 Int. Heat Trans.
D. B. Go Slide 20
Gases – A Little Kinetic TheoryWe’ve already discussed kinetic theory in relation to thermal conductivity individual particles carrying their energy from hot to cold
G. Chen
The same approach can be used to derive the flux of any property for individual particles individual particles carrying their energy from hot to cold
general flux of scalar property Φ
AME 60614 Int. Heat Trans.
D. B. Go Slide 21
Gases – Viscosity and Mass DiffusionConsider viscosity from general kinetic theory (flux of momentum) Newton’s Law
Consider mass diffusion from general kinetic theory (flux of mass) Fick’s Law
Note that all these properties are related and depend on the average speed of the gas molecules and the mean free path between collisions
AME 60614 Int. Heat Trans.
D. B. Go Slide 22
Gases – Average SpeedThe average speed can be derived from the Maxwell-Boltzmann distribution
We can derive it based on assuming only translational energy, gi = 1 (good for monatomic gasses – recall that translation dominates electronic)
This is a ratio is proportional to a probability density function by definition the integral of a probability density function over all possible states must be 1
probability that a gas molecule has a given momentum p
AME 60614 Int. Heat Trans.
D. B. Go Slide 23
Gases – Average SpeedFrom the Maxwell-Boltzmann momentum distribution, the energy, velocity, and speed distributions easily follow
AME 60614 Int. Heat Trans.
D. B. Go Slide 24
Gases – Mean Free PathThe mean free path is the average distance traveled by a gas molecule between collisions we can simply gas collisions using a hard-sphere, binary collision approach (billiard balls)
rincident
rtarget
incident particle
rincident
collision with target particle
d12cross section defined as:
General mean free path Monatomic gas
AME 60614 Int. Heat Trans.
D. B. Go Slide 25
Gases – Transport PropertiesBased on this very simple approach, we can determine the transport properties for a monatomic gas to be
M is molecular weight
Recall, that
more rigorous collision dynamics model
AME 60614 Int. Heat Trans.
D. B. Go Slide 26
Gases – Monatomic Heliumfrom Incropera and Dewitt
only 2% difference!
AME 60614 Int. Heat Trans.
D. B. Go Slide 27
Gases – What We’ve Learned• Gases can be treated as individual particles
– store and transport thermal energy– primary energy carriers fluids convection!
• Gases have a statistical (Maxwell-Boltzmann) occupation, quantized (discrete) energy, and only limited numbers at each energy level – we can derive the specific heat, and many other gas properties using an
equilibrium approach
• We can use non-equilibrium kinetic theory to determine the thermal conductivity, viscosity, and diffusivity of gases
• The tables in the back of the book come from somewhere!
top related