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Transcript
ME 267: Fundamentals of Mechanical Engineering
Department of Mechanical Engineering, BUET
Principles of Thermodynamics
System
The system is whatever we want to study. It may be as simple as a free body
or as complex as an entire chemical refinery. We may want to study a quantity
of matter contained within a closed, rigid-walled tank, or we may want to
consider something such as a pipeline through which natural gas flows. The
composition of the matter inside the system may be fixed or may be changing
through chemical or nuclear reactions. The shape or volume of the system
being analyzed is not necessarily constant, as when a gas in a cylinder is
compressed by a piston or a balloon is inflated.
Boundary
The system is distinguished from its surroundings by a specified boundary,
which may be at rest or in motion.
Closed System
A closed system is defined when a particular quantity of
matter is under study. A closed system always contains
the same matter. There can be no transfer of mass
across its boundary. A special type of closed system
that does not interact in any way with its surroundings
is called an isolated system. The figure shows a gas in a
piston–cylinder assembly. When the valves are closed,
we can consider the gas to be a closed system.
Fig.: piston cylinder assembly
Control Volume
In most cases it is simpler to think
instead in terms of a given region of
space through which mass flows. With
this approach, a region within a
prescribed boundary is studied. The
region is called a control volume.
Mass may cross the boundary of a
control volume.
Fig.: control volume
Property
A property is a macroscopic characteristic of a system such as mass, volume,
energy, pressure, and temperature to which a numerical value can be assigned
at a given time without knowledge of the previous behavior of the system. In
the physical sciences, an intensive property is a physical property of a system
that does not depend on the system size or the amount of material in the
system. By contrast, an extensive property of a system does depend on the
system size or the amount of material in the system. For example, density is
an intensive quantity while mass and volume are extensive quantities.
State
The word state refers to the condition of a system as described by its
properties. Since there are normally relations among the properties of a
system, the state often can be specified by providing the values of a subset of
the properties. All other properties can be determined in terms of these few.
Process
When any of the properties of a system change, the state changes and the
system is said to have undergone a process. A process is a transformation
from one state to another. However, if a system exhibits the same values of its
properties at two different times, it is in the same state at these times. A
system is said to be at steady state if none of its properties changes with time.
© Nusair Mohammed Ibn Hasan
ME 267: Fundamentals of Mechanical Engineering
Department of Mechanical Engineering, BUET
Cycle
A thermodynamic cycle is a sequence of processes that begins and ends at the
same state. At the conclusion of a cycle all properties have the same values
they had at the beginning
First Law of Thermodynamics
When any closed system is taken through a cycle, the net work delivered to the surrounding
is proportional to the net heat taken from the surroundings and the converse is also true. These
statements may be expressed in mathematical form by –
ΣdQ ∝ ΣdW
dQ = dU + dW
Internal Energy
Internal energy of a system may be defined as the amount of energy summing
all the energies of all the atoms, ions and molecules of that system. The value
of internal energy depends on temperature and pressure. Measurement of
total internal energy is not possible. But change in internal energy is equal to
amount energy transferred by heat and work together. A change in the
internal energy between two states is independent of the path between them.
Internal energy of an isolated system is constant.
Second Law of Thermodynamics
It is impossible to construct a system which will operate in a cycle, extract heat from a
reservoir, and do an equivalent amount of work on the surroundings. In other words, it is impossible
to construct a system which will operate in a cycle and transfer heat from a cooler to a hotter body
without work being done on the system by the surrounding.
Enthalpy
Enthalpy is a thermodynamic property of a system that can be defined as the
summation of internal energy and the product of pressure and volume of that
system. A change in enthalpy under constant pressure condition is equal to the
change in internal energy of the system and the work done by the system on
its surroundings.
∆H = ∆U + ∆(PV)
Entropy
In thermodynamics, entropy is a measure of how much of the energy of a
system is potentially available to do work and how much of it is potentially
manifest as heat. In classical thermodynamics, the entropy is defined only for
a system in thermodynamic equilibrium. Statistical mechanics explains entropy
as the amount of uncertainty which remains about a system, after its
observable macroscopic properties have been taken into account. For a given
set of macroscopic variables, like temperature and volume, the entropy
measures the degree to which the probability of the system is spread out over
different possible quantum states.
∆S =
© Nusair Mohammed Ibn Hasan
ME 267: Fundamentals of Mechanical Engineering
Department of Mechanical Engineering, BUET
Steady Flow Energy Equation
Q
W
m
h
V
z
=
=
=
=
=
=
rate of heat transfer between the control volume and its surroundings
rate of work transfer between the control volume and its surroundings
mass flow through the control volume
specific enthalpy of the working fluid
velocity of the working fluid
reference height of the system
Some Steady Flow Engineering Devices
Device
Steady Flow Energy Equation
Heat Exchanger (Boiler & Condenser)
Q = m (h2 – h1)
Turbine & Compressor
W = m (h1 – h2)
Throttling
h2 = h1
© Nusair Mohammed Ibn Hasan
ME 267: Fundamentals of Mechanical Engineering
Department of Mechanical Engineering, BUET
Properties of Fluids
Pressure
Pressure (often termed as absolute pressure) is the force per unit area applied
in a direction perpendicular to the surface of an object. Gauge pressure is the
pressure relative to the local atmospheric or ambient pressure.
Absolute Pressure = Atmospheric Pressure + Gauge Pressure
Because pressure is commonly measured by its ability to displace a column of
liquid in a manometer, pressures are often expressed as a depth of a particular
fluid (e.g., inches of water). The most common choices are mercury (Hg) and
water. The standard atmosphere (atm) is an established constant. It is
approximately equal to typical air pressure at earth mean sea level and is
defined as follows:
1 atm = 101.325 kPa = 14.7 psi = 10.3 m of H2O = 760 mm of Hg
Temperature
In physics, temperature is a physical property of a system that underlies the
common notions of hot and cold; something that feels hotter generally has the
higher temperature. Temperature is one of the principal parameters of
thermodynamics. If no net heat flow occurs between two objects, the objects
have the same temperature; otherwise heat flows from the hotter object to the
colder object. This is the content of the zeroth law of thermodynamics. The
basic unit of temperature in the International System of Units (SI) is the kelvin.
The kelvin and Celsius scales are defined by two points, absolute zero and the
triple point water. Absolute zero is defined as being precisely 0 K and −273.15
°C. Absolute zero is where all kinetic motion in the particles comprising matter
ceases and they are at complete rest. At absolute zero, matter contains no
thermal energy.
Phases of a Pure Substance
Fig.: arrangement of atoms in different phases
Fig.: illustration of constant pressure change from liquid to vapor for water
© Nusair Mohammed Ibn Hasan
ME 267: Fundamentals of Mechanical Engineering
Department of Mechanical Engineering, BUET
Fig.: T – v diagram for the heating process of water at constant pressure
Fig.: T – v diagram of constant pressure phase change process at different pressures
A state at which a phase change begins or ends is called a saturation state. The liquid states
along the line segment 1–2 are sometimes referred to as subcooled liquid states because the
temperature at these states is less than the saturation temperature at the given pressure. These
states are also referred to as compressed liquid states because the pressure at each state is higher
than the saturation pressure corresponding to the temperature at the state. When a mixture of liquid
and vapor exists in equilibrium, the liquid phase is a saturated liquid and the vapor phase is a
saturated vapor. If the system is heated further until the last bit of liquid has vaporized, it is brought
to point 4, the saturated vapor state. The intervening two phase liquid–vapor mixtures can be
distinguished from one another by the quality, an intensive property. For a two phase liquid–vapor
mixture, the ratio of the mass of vapor present to the total mass of the mixture is its quality, x. In
symbols,
© Nusair Mohammed Ibn Hasan
ME 267: Fundamentals of Mechanical Engineering
Department of Mechanical Engineering, BUET
The value of the quality ranges from zero to unity: at saturated liquid states, x = 0, and at saturated
vapor states, x = 1. When the system is at the saturated vapor state, further heating at fixed
pressure results in increases in both temperature and specific volume. A state such as this is often
referred to as a superheated vapor state because the system would be at a temperature greater than
the saturation temperature corresponding to the given pressure.
Triple Point
In thermodynamics, the triple point of a substance is the
temperature and pressure at which three phases (for
example, gas, liquid, and solid) of that substance coexist in
thermodynamic equilibrium. For example, the triple point of
mercury occurs at a temperature of −38.8344 °C and a
pressure of 0.2 mPa. The single combination of pressure
and temperature at which water, ice, and water vapor can
coexist in a stable equilibrium occurs at exactly 273.16 k
and a partial vapour pressure of 0.611 kPa.
Critical Point
In thermodynamics, a critical point, also called a critical state, specifies the
conditions (temperature, pressure and sometimes composition) at which a
phase boundary ceases to exist. There are multiple types of critical points such
as vapor-liquid critical points and liquid-liquid critical points. The vapor-liquid
critical point denotes the conditions above which distinct liquid and gas phases
do not exist. In water, the critical point occurs at around 647 k and 22.064
MPa. As the critical temperature is approached, the properties of the gas and
liquid phases approach one another, resulting in only one phase at the critical
point: a homogeneous supercritical fluid. The heat of vaporization is zero at
and beyond this critical point, so there is no distinction between the two
phases. Above the critical temperature a liquid cannot be formed by an
increase in pressure, but with enough pressure a solid may be formed. The
critical pressure is the vapor pressure at the critical temperature.
Fig.: T – v diagram of a pure substance
© Nusair Mohammed Ibn Hasan
ME 267: Fundamentals of Mechanical Engineering
Department of Mechanical Engineering, BUET
Fig.: P – v diagram of a pure substance
** See Attached Table for Properties of Water
© Nusair Mohammed Ibn Hasan