Thermo-Fluids
ERE 221
Lecture 2: Introduction - Energy
Dr. Ahmed Elwardany
Energy Resources Engineering (ERE) Department
Email: ahmed.Elwardany@ejust.edu.eg
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Introduction: history
Types of thermodynamics
Units and Dimensions
Systems
Property, State, Process and Cycle
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Temperature and Zeroth Law of T.D
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How could we define it?
A measure of hotness or coldness.
It determines whether they will be in thermal equilibrium
Freezing cold, cold, warm, hot, and red-hot (Sensation)
A metal chair will feel much colder than a wooden one even
when both are at the same temperature !
Two bodies at different T
 Heat is transferred from the body at higher T to
the one at lower T until both bodies attain the
same temperature.
 The heat transfer stops, and the two bodies are
said to have reached thermal equilibrium.
Two bodies at different T
The zeroth law of thermodynamics states that if two bodies are in thermal
equilibrium with a third body, they are also in thermal equilibrium with each other.
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Temperature Scales
 All temperature scales are based on some easily reproducible
states such as the freezing and boiling points of water, which
are also called the ice point and the steam point, respectively.
 Temperature scales are defined by the
numerical value assigned to a standard fixed
point.
 By international agreement the standard fixed
point is the easily reproducible triple point of
water: the state of equilibrium between
steam, ice, and liquid water.
 The triple point of pure water is at 0.01°C
(273.16 K, 32.01°F)
 The ice point is (273.15 K)
 The steam point is (373.15 K)
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Temperature Scales
The Rankine scale is
The temperature scales in the two unit systems
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Temperature Rise
During a heating process, the temperature of a system rises
by 10°C. Express this rise in temperature in K, °F, and R.
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Pressure
Pressure is defined as a normal force exerted by a fluid per
unit area.
P=F/A (Pa = N/m2)
 Absolute pressure: The actual pressure at a given position,
and it is measured relative to absolute vacuum (i.e.,
absolute zero pressure).
 Most pressure-measuring devices, however, are calibrated
to read zero in the atmosphere.
 Gage pressure is the difference between the absolute
pressure and the local atmospheric pressure and measured
by pressure gages.
 Pgage can be positive or negative, but pressures
below atmospheric pressure are sometimes called
vacuum pressures
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Pressure
Atmospheric pressure: represents the weight of the
atmosphere around us. (standard pressure=101.3 kPa)
Dr Ahmed Elwardany, ERE, Spring 2019
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Pressure: example
A vacuum gage connected to a chamber reads 5.8 psi at a
location where the atmospheric pressure is 14.5 psi.
Determine the absolute pressure in the chamber.
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Pressure: with Depth
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Pressure: with Depth
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Atmospheric Pressure
The Barometer
The pressure at point B is equal to the atmospheric
pressure, and the pressure at point C can be taken to
be zero since there is only mercury vapor above point
C and the pressure is very low relative to Patm and can
be neglected to an excellent approximation.
 The standard atmospheric pressure is 760 mmHg
(29.92 in Hg) at 0°C.
 The unit mmHg is also called the torr in honor of
Torricelli.
 Therefore, 1 atm = 760 torr and 1 torr = 133.3 Pa.
 The length or the cross-sectional area of
the tube has no effect on the height of the
fluid column of a barometer
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Energy
Energy (E)
Energy is defined as the capacity of a system to perform work or
produce heat.
 Energy exists in numerous forms such as thermal, mechanical, electric,
chemical, and nuclear.
 Even mass can be considered a form of energy.
 Energy can be transferred to or from a closed system (a fixed mass) in
two distinct forms: heat and work.
 An energy transfer to or from a closed system is heat if it is caused by a
temperature difference.
 Otherwise it is work, and it is caused by a force acting through a
distance.
 For control volumes, energy can also be transferred by mass flow.
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Forms of Energy
 The total energy of a system on a unit mass basis is denoted by e and
is expressed as
 Thermodynamics provides no information about the absolute value of
the total energy.
 It deals only with the change of the total energy, which is what
matters in engineering problems.
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Forms of Energy
Energy
Work
Energy
Heat
Energy
Internal
Energy
Flow
Energy
Potential
energy
Kinetic
energy
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Forms of Energy
 The various forms of total energy could be considered a system
in two groups: macroscopic and microscopic.
 The macroscopic forms of energy are those a system possesses
as a whole with respect to some outside reference frame, such
as kinetic and potential energies.
 The microscopic forms of energy are those related to the
molecular structure of a system and the degree of the
molecular activity, and they are independent of outside
reference frames.
 The sum of all the microscopic forms of energy is called the
internal energy of a system and is denoted by U.
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Kinetic Energy (KE)
 KE is the energy that a system possesses as a result of
its motion relative to some reference frame
 The kinetic energy is expressed as
or, on a unit mass basis,
V: Velocity (m/s)
m: Mass (kg)
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Potential Energy (PE)
 PE is the energy that a system possesses as a result of
its elevation in a gravitational field
 The potential energy is expressed as
or, on a unit mass basis,
g: Gravitational acceleration (9.81 m/s2)
z: Elevation of the center of gravity of a system relative to reference level.
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Internal Energy (U)
 Internal energy is the sum of all the microscopic forms of energy
of a system.
 It is related to the molecular structure and
the degree of molecular activity, and can be
viewed as the sum of the kinetic and
potential energies of the molecules.
Translational energy: molecules of a gas move through
space with some velocity, and thus possess some kinetic
energy.
Rotational and vibrational energy: for polyatomic
molecules. Electron spin and in an atom rotate about the
nucleus, and thus possess rotational kinetic energy.
Electrons at outer orbits have larger kinetic energies.
Electrons also spin about their axes, and the energy
associated with this motion is the spin energy.
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Energy
Total Energy
 Stationary system: A closed system its velocity and elevation of the
center of gravity remain constant during a process.
 Control volumes typically involve fluid flow for long periods of time,
and it is convenient to express the energy flow associated with a fluid
stream in the rate form.
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Flow Energy (FE)
 It arises from the pressure (P) and the volume (V) of a fluid.
FE =
where:
P = pressure (kN/m2)
V = total volume (m3)
v = specific volume (m3/kg)
m = mass (kg).
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Enthalpy (H)
 Enthalpy is a property of a substance, like pressure, temperature,
and volume, but it cannot be measured directly.
 Normally, the enthalpy of a substance is given with respect to
some reference value.
h = u + P v = internal energy + flow energy
Flow energy means that enthalpy appear when dealing with open
systems
For ideal gases:
h = cp T
(kJ/kg)
u = cv T (kJ/kg)
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Mechanical Energy
The form of energy that can be converted to mechanical work
completely and directly by an ideal mechanical device such as an
ideal turbine.
 Kinetic and potential energies are the familiar forms of mechanical
energy.
 Thermal energy is not mechanical energy.
 The mechanical energy of a flowing fluid
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Example
A site evaluated for a wind farm is observed to have steady
winds at a speed of 8.5 m/s.
Determine the wind energy
(a) per unit mass,
(b) for a mass of 10 kg, and
(c) for a flow rate of 1154 kg/s for air.
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Thank You!
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