Thermal Science
Rub your hands together for 15 seconds.
Are your hands warm?
Thermal energy
What is thermodynamics?
• The study of thermodynamics is concerned with ways energy
is stored within a body and how energy transformations,
which involve heat and work, may take place.
• Approaches to studying thermodynamics
– Macroscopic (Classical thermodynamics)
• study large number of particles (molecules) that make
up the substance in question
• does not require knowledge of the behavior of
individual molecules
– Microscopic (Statistical thermodynamics)
• concerned within behavior of individual particles
(molecules)
• study average behavior of large groups of individual
particles
Applications of Thermodynamics
Thermodynamic Systems
Thermodynamic System
– quantity of matter or a region of
space chosen for study
Boundary
– real or imaginary layer that
separates the system from its
surroundings
Surroundings
– physical space outside the
system boundary
Types of Systems
– Closed
– Open
Closed Systems (fixed masses)
Energy, not mass, crosses closed-system boundaries
Closed System with Moving Boundary
Open Systems (Control Volumes)
Mass and Energy Cross Control Volume Boundaries
Isolated System
•
Closed system where no heat or work (energy) may cross the system
boundary
– typically a collection of a main system (or several systems) and its
surroundings is considered an isolated system
Properties
•
•
Any characteristic of a system in
equilibrium is called a property.
Types of properties
– Extensive properties - vary directly
with the size of the system
Examples: total volume, total mass,
total energy
– Intensive properties - are
independent of the size of the
system
•
Examples: temperature, pressure.
Extensive properties per unit mass are
intensive properties.
specific volume, v = Volume/Mass = V/m
density, r = Mass/Volume = m/V
Gas properties Interactive tool
(https://phet.colorado.edu/)
State & State Postulate
• State Postulate
•
•
•
•
•
The state of a simple compressible system is completely specified by two
independent, intensive properties.
A system is called a simple compressible system in the absence of electrical,
magnetic, gravitational, motion, and surface tension effects.
These effects are due to external force fields and are negligible for most
engineering problems.
Otherwise, an additional property needs to be specified for each effect that is
significant.
Two properties are independent if one property can be varied while the other one
is held constant.
• State of a system
– system that is not undergoing any change
– all properties of system are known & are not changing
– if one property changes then the state of the system changes
Equilibrium
• Thermodynamic equilibrium
– “equilibrium” - state of balance
– A system is in equilibrium if it maintains thermal (uniform
temperature), mechanical (uniform pressure), phase (mass
of two phases), and chemical equilibrium
– Thermal equilibrium: If the temperature is the same
throughout the entire system.
– Mechanical equilibrium: If there is no change in pressure at
any point of the system with time.
– Phase equilibrium: If a system involves two phases and
when the mass of each phase reaches an equilibrium level
and stays there.
– Chemical equilibrium: If the chemical composition of a
system does not change with time, that is, no chemical
reactions occur.
Processes & Paths
• Process
– when a system changes from one equilibrium state to
another one
– some special processes:
• isobaric process
- constant pressure process
• isothermal process
- constant temperature process
• isochoric process
- constant volume process
• isentropic process
- constant entropy
• isenthalpic process
- constant enthalpy
• Path
– series of states which a system passes through during a
process
Terminology
Compression Process
Quasi-Equilibrium Processes
• System remains practically in
equilibrium at all times
• Easier to analyze (equations of
state can apply)
• Sufficiently slow process
• Properties in one part of the
system do not change any faster
than those at other parts
Thermodynamic Cycle
•
Cycles
– A process (or a series of connected processes) with identical initial
and final states
Heat and Temperature
➢ Though we all have a feel for temperature (‘like when we are feeling hot’); in the context of
TD temperature is technical term with ‘deep meaning’.
➢ Temperature is a measure of the ‘intensity of heat’. ‘Heat flows’ (energy is transferred as heat) from a
body at higher temperature to one at lower temperature. (Like pressure is a measure of the
intensity of ‘force applied by matter’→ matter (for now a fluid) flows from region of higher pressure
to lower pressure).
➢ That implies (to reiterate the obvious!) if I connect two bodies− (A)-one weighing 100kg at 10C and
the other (B) weighing 1 kg at 500C, then the ‘heat will flow’ from the hotter body to the
colder body (i.e. the weight or volume of the body does not matter).
Temperature #1
Temperature #2
Heat
TEMPERATURE AND THE ZEROTH
LAW OF THERMODYNAMICS
➢ Temperature comes in two important ‘technical’ contexts in
TD:
1. It is a measure of the average kinetic energy (or velocity)
of the constituent entities (say molecules)
2. It is the parameter which determines the distribution of
species (say molecules) across various energy states
available.
➢ The zeroth law of thermodynamics : If two bodies are in
thermal equilibrium with a third body, they are also in
thermal equilibrium with each 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 the same temperature reading even if they are not
in contact.
THE ZEROTH LAW OF THERMODYNAMICS
Two bodies reaching thermal equilibrium after being
brought into contact in an isolated enclosure
Temperature Scales
➢ Any body with at least one measurable property that changes as its
temperature changes can be used as a thermometer.
➢ Such a property is called a Thermometric Property.
➢ The particular substance that exhibits changes in the thermometric
property is known as a Thermometric Substance.
➢ Temperature scales enable us to use a common basis for temperature
measurements.
➢ All temperature scales are based on some easily reproducible states
such as the freezing and boiling points of water: the ice point and the
steam point.
➢ Ice point: A mixture of ice and water that is in equilibrium with air
saturated with vapor at 1 atm pressure (0°C or 32°F).
➢ Steam point: A mixture of liquid water and water vapor (with no air) in
equilibrium at 1 atm pressure (100°C or 212°F).
Temperature Scales
Scale
Freezing
Boiling point
point of water
of water
Celsius
0°C
100°C
Fahrenheit
32°F
212°F
Kelvin
273K
373K
• Matter is made up of molecules in motion (kinetic energy)
• Absolute Zero: occurs when all kinetic energy is removed from an
object, 0 K = -273° C
Liquid-in-Glass thermometer
➢ Consists of a glass capillary tube
connected to a bulb filled with a liquid
such as alcohol and sealed at the other end.
➢ Space above the liquid is occupied by the
vapor of the liquid or an inert gas.
➢ As temperature increases, the liquid
expands in volume and rises in the
capillary.
➢ Length L of the liquid in the capillary
depends on the temperature.
➢ Liquid is the thermometric substance and
L is the thermometric property.
Ideal-Gas Temperature Scale
➢ The temperatures on this scale are measured
using a Constant-Volume Gas Thermometer
➢ Thermometer is based on the principle that at
low pressures, the temperature of a gas is
proportional to its pressure at constant volume.
➢ Temperature of a gas of fixed volume varies
linearly with pressure at sufficiently low
pressures.
➢ ΔT = (V / R) ΔP
➢ Constant-volume gas thermometer is basically a
rigid vessel filled with a gas, usually hydrogen or
helium, at low pressure.
Problem
Applications of Thermal Energy
http://www.nrel.gov
Stress in a fluid
Stress: Force per unit area.
Normal stress: The normal
component of a force acting
on a surface per unit area.
Shear stress: The tangential
component of a force acting
on a surface per unit area.
Pressure: The normal stress
in a fluid at rest.
Zero shear stress: A fluid at
rest is at a state of zero shear
stress.
When the walls are removed
or a liquid container is tilted, a
shear develops as the liquid
moves to re-establish a
horizontal free surface.
The normal stress and shear stress at
the surface of a fluid element. For
fluids at rest, the shear stress is zero
and pressure is the only normal
stress.
Pascal’s Law
➢
➢
➢
➢
➢
➢
In absence of shear stress only 3 components of normal
stresses are present.
As static fluid has isotropic nature, so these three
components have same magnitude for all direction.
𝜎𝑥𝑥 = 𝜎𝑦𝑦 = 𝜎𝑧𝑧 =-p
‘p’ is thermodynamic pressure (-ve sign shows
compressive nature).
Pressure at a point in a fluid at rest, or in motion, is
independent of direction as long as there are no
shearing stresses present- Pascal’s Law
The pressure applied to a confined fluid increases the
pressure throughout by the same amount.
Pressure variation in a static fluid
Pressure variation in a static fluid
➢ For a fluid at rest, pressure gradient must be
balanced by the gravity force
➢ Recall: ∇p is perpendicular everywhere to surface of
constant pressure p.
➢ In our customary coordinate z is “upward” and the
gravity vector is:
➢ where g = 9.807 m/s2. The pressure gradient vector
becomes:
Pressure variation in a static fluid
➢ For incompressible
fluid:
➢ 𝑝1 − 𝑝2 = ρgh = 𝛾 h
➢ H = 𝑧2 - 𝑧1 =
𝑝1 −𝑝2
𝛾
=
∆𝑝
𝛾
➢ The quantity, p\γ is
a length called the
pressure head of
the fluid.
Hydrostatic pressure distribution
➢ Pressure in a continuously distributed uniform static fluid
varies only with vertical distance and is independent of the
shape of the container.
➢ The pressure is the same at all points on a given
horizontal plane in a fluid.
Pressure-Terminology
➢ Absolute pressure: The actual pressure at a given position. It is
measured relative to absolute vacuum (i.e., absolute zero pressure).
➢ Gage pressure: The difference between the absolute pressure and
the local atmospheric pressure. Most pressure-measuring devices
are calibrated to read zero in the atmosphere, and so they indicate
gage pressure.
➢ Vacuum pressures: Pressures below atmospheric pressure.
The mercury barometer
➢ Atmospheric pressure is measured by a
device called a barometer; thus, the
atmospheric pressure is often referred to as
the barometric pressure.
➢ A frequently used pressure unit is the
standard atmosphere, which is defined as the
pressure produced by a column of mercury
760 mm in height at 0°C (rHg = 13,595 kg/m3)
under standard gravitational acceleration
(g = 9.807 m/s2)
➢ Mercury has an extremely small vapor
pressure at room temperature (almost
vacuum), thus p1 = 0. One can write:
The Manometers and Manometry
➢ Science of pressure measurement –
Manometry.
➢ The device similar to barometer and used
to measure pressure is called Manometer.
➢ Manometers
measure
a
pressure
difference by balancing the weight of a
fluid column between the two pressures of
interest.
➢ Three common types of manometers are
Piezometer tube, U-tube Manometer
and Inclined tube manometer.
Piezometer tube
➢ An elevation change of h1 in
a fluid at rest corresponds to
Dp/rg.
➢ A device based on this is
called a Piezometer tube.
➢ pA = p1 - patm = rgh1
➢ pAgage = p1gage = rgh1
➢ pA ∝ h1
➢ Disadvantages:
•
•
•
Can’t measure Gas pressures
Can’t measure pressure in
vacuum
Difficult to measure large
pressure.
U-tube Manometer
➢ A manometer consists of a Utube containing one or more
fluids such as mercury, water,
alcohol, or oil.
➢ Heavy fluids such as mercury
are used if large pressure
differences are anticipated.
➢ Overcomes the disadvantages
of Piezometer tube.
➢ It is commonly used to
measure small and moderate
pressure differences.
Manometric calculation
1.
2.
3.
4.
5.
6.
Begin writing pressures from any end
of manometer.
Keep on adding pressure increments
on going down through the fluid in
same column. Use equation:
pdown = pup +𝜸 ∆𝒉
Jump across to other column when
two points in same fluid are at equal
elevations (Pascal’s Law).
Keep on repeating steps 2 and 3 until
you reach other end.
Solve the system of equations of
obtain unknown pressures.
Inclined tube manometer
➢ Its difficult to detect small pressure changes in U-tube manometer.
➢ To detect small pressure changes, inclined tube manometer is used.
➢ One leg is inclined at angle θ (variable) with horizontal.
➢ Difference in level as seen in inclined tube is 𝑙2 .
➢ Pressure difference is not due to 𝑙2 but due to its vertical projection i.e.
𝒍𝟐 sinθ.
➢ 𝑝𝐴 - 𝑝𝐵 =𝛾2 𝑙2 sinθ+ 𝛾3 ℎ3 - 𝛾1 ℎ1
➢ If A and B contain gas (gas pressures are usually neglected), so
➢ 𝑝𝐴 - 𝑝𝐵 =𝛾2 𝑙2 sinθ
𝑝 −𝑝
➢ or 𝑙2 = 𝐴 𝐵
𝛾2 sinθ
Other Pressure Measurement Devices
Bourdon tube: Consists of a hollow metal
tube bent like a hook whose end is closed
and connected to a dial indicator needle.
Pressure transducers: Use various
techniques to convert the pressure effect
to an electrical effect such as a change in
voltage, resistance, or capacitance.
Pressure transducers are smaller and
faster, and they can be more sensitive,
reliable, and precise than their mechanical
counterparts.
Strain-gage pressure transducers:
Work by having a diaphragm deflect
between two chambers open to the
pressure inputs.
Piezoelectric transducers: Also called
solid-state pressure transducers, work on
the principle that an electric potential is
generated in a crystalline substance when
it is subjected to mechanical pressure.
References
1. Thermodynamics An Engineering Approach, Third Edition,
Yunus A. Çengel & Michael A. Boles, McGraw-Hill Publication.
2. Basic and Applied Thermodynamics by P. K. Nag, Tata McGraw
Hill (2002).
3. Fluid Mechanics: Fundamentals and Applications, 3rd Edition,
Yunus A. Cengel, John M. Cimbala, McGraw-Hill, 2014.
4. Fluid Mechanics, Frank M. White, McGraw Hill, 4th Ed.
5. Fundamentals of Fluid Mechanics by Bruce R. Munson, Donald
F. Young, Theodore H. Okiishi, Wade W. Huebsch, Wiley, 6th
Ed.