Inter - Bayamon
Lecture
Thermodynamics I
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Thermodynamics I
MECN 4201
Professor: Dr. Omar E. Meza Castillo
omeza@bayamon.inter.edu
http://facultad.bayamon.inter.edu/omeza
Department of Mechanical Engineering
Inter American University of Puerto Rico
Bayamon Campus
Inter -del
Bayamon
Turabo
Universidad
Thermodynamics
Systems IDesign
Thermal
Course Objective
 To Develop the conservation of mass principle.
 To apply the conservation of mass principle to
various
systems
including
steadyand
unsteady-flow control volumes.
 To apply the first law of thermodynamics to
control volumes.
 To identify the energy carried by a fluid stream
crossing a control volume
 To solve energy balance problems for common
steady-flow
devices
such
as
nozzles,
compressors, turbines, throttling valves, mixer,
heaters and heat exchangers.
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Thermodynamics I
Energy Analysis
Control Volumes
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Thermodynamics I
Topics
 Conservation of mass principle applied to
steady- and unsteady-flow control volumes.
 Conservation of energy principle applied to
control volumes.
 Energy carried by a fluid stream crossing a
control surface.
 Energy balances for common steady-flow
devices such as nozzles, compressors,
turbines, throttles, mixers, heaters, and
heat exchangers.
 Energy
balances
for
unsteady-flow
processes.
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Thermodynamics I
Conservation of Mass
 Conservation of mass: Mass, like energy, is
a conserved property, and it cannot be created
or destroyed during a process.
 Closed systems: The mass of the system
remain constant during a process.
 Control volumes: Mass can cross the
boundaries, and so we must keep track of the
amount of mass entering and leaving the
control volume
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Mass and Volume Flow Rates
Definition of
average velocity
Volume flow rate
Thermodynamics I
Mass flow rate
The average velocity Vavg is defined as the
average speed through a cross section.
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Conservation of Mass Principle
The conservation of mass principle for a control volume: The net
mass transfer for a control volume during a time interval t is equal to
the net change in the total mass within the control volume during the
interval.
Thermodynamics I
General conservation of mass
General conservation of mass in rate form
Conservation of mass principle
for an ordinary bathtub.
or
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Mass Balance for Steady-Flow Processes
During a steady-flow process, the total amount of mass contained
within a control volume does not change with time (mCV = constant).
Then the conservation of mass principle requires that the total amount
of mass entering a control volume equal the total amount of mass
leaving it.
For steady-flow processes, we are
interested in the amount of mass
flowing per unit time, that is, the mass
flow rate.
Thermodynamics I
Multiple inlets
and exits
Devices
such
as
nozzles,
diffusers,
turbines, compressors, and pumps involve
a single stream (only one inlet and one
outlet).
Single
stream
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Thermodynamics I
Flow Work
Flow work, or flow energy: The work (or energy) required to
push the mass into or out of the control volume. This work is
necessary for maintaining a continuous flow through a control
volume.
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Thermodynamics I
Total Energy of a Stream
The flow energy is
automatically taken
care of by enthalpy.
This is the main
reason for defining
the
property
enthalpy.
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Energy Transport by Mass
Thermodynamics I
When the kinetic and potential energies of a fluid stream are negligible
When the properties of the mass at each inlet or exit change with time
as well as over the cross section
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Thermodynamics I
Steady-Flow Systems
Under steady-flow conditions, the
mass and energy contents of a
control volume remain constant.
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Under steady-flow conditions,
the fluid properties at an inlet
or exit remain constant.
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Balances for a Steady-Flow Process
Mass
balance
Single stream
Thermodynamics I
Energy
balance
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Energy balance relations with sign conventions
General, unit time basis
Single entrance, single exit,
unit time basis
Unit mass basis
Thermodynamics I
KE and PE negligible
Some energy unit
equivalents
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Thermodynamics I
Steady-Flow Engineering Devices
Many engineering devices operate essentially under the
same conditions for long periods of time. Therefore, these
devices can be conveniently analyzed as steady-flow
devices.
1.
2.
3.
4.
5.
6.
Nozzles and Diffusers
Turbines and Compressors
Throttles
Mixing Chambers
Heat Exchangers
Pipes and Ducts
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Nozzles and Diffusers
A nozzle is a device that
increases the velocity of a fluid
at the expense of pressure.
Thermodynamics I
A diffuser is a device that
increases the pressure of a fluid
by slowing it down.
Nozzles and diffusers are
shaped so that they cause
large changes in fluid
velocities.
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Turbines and Compressors
Turbine: drives the electric generator in a
steam, gas, or hydroelectric power plant.
As the fluid passes through the turbine,
work is done against the blades, which are
attached to the shaft. As a result, the
shaft rotates, and the turbine produces
work.
Thermodynamics I
Compressors, pumps and fans, are
devices used to increase the pressure of a
fluid. Work is supplied to these devices
from an external source through a
rotating shaft.
Energy balance for the
compressor in this figure:
A fan increases the pressure of a gas
slightly.
A compressor is capable of compressing
the gas to very high pressures.
Pumps work very much like compressors
except that they handle liquids instead of
gases.
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Throttles
Throttling valve: flow-restricting device that
cause a significant pressure drop in the fluid.
The pressure drop in the fluid is accompanied
by a large drop in temperature, and for that
reason throttling devices are commonly used in
refrigeration and air-conditioning applications.
Thermodynamics I
Energy
balance
Isenthalpic
During a throttling process, the
enthalpy of a fluid remains
constant.
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Mixing Chamber
In engineering applications, the section where the mixing process
takes place is commonly referred to as a mixing chamber.
60C
Thermodynamics I
Energy balance for the
adiabatic mixing chamber
in the figure is:
140
kPa
10C
43C
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Thermodynamics I
Heat Exchangers
Heat exchangers are
devices in which two
moving fluid streams
exchange
energy
without mixing.
Mass and energy balances for the
adiabatic heat exchanger :
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Pipe and Duct Flow
Pipe or duct flow may involve more
than one form of work at the same
time.
Thermodynamics I
Energy balance for the pipe flow
shown on the right is
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Thermodynamics I
Unsteady-Flow Processes
Many processes of interest,
Charging of a
however, involve changes
rigid tank from a
within the control volume
supply line is an
with time. Such processes
unsteady-flow
are called unsteady-flow, or
process since it
transient-flow, processes.
involves changes
within the control
volume.
Uniform-flow process: The
fluid flow at any inlet or exit
is uniform and steady, and
thus the fluid properties do
not change with time or
The shape and
position over the cross
size of a control
section of an inlet or exit.
volume may
change during
an unsteadyflow process.
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Mass balance
Thermodynamics I
Energy balance
A uniform-flow system may
involve electrical, shaft, and
boundary work all at once.
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Thermodynamics I
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Homework5  Web Page
Thermodynamics I
Due Date:
Omar E. Meza Castillo Ph.D.
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