Transcript ENGR 2213 Thermodynamics F. C. Lai School of Aerospace and Mechanical Engineering

ENGR 2213 Thermodynamics
F. C. Lai
School of Aerospace and Mechanical
Engineering
University of Oklahoma
First Law of Thermodynamics
Energy can be neither created or destroyed; it can
only change forms.
Conservation of Energy Principle
Closed Systems
Change in
Net energy
Net energy
the total energy = transferred in - transferred out
of the system
by heat
by work
E  Q  W
First Law of Thermodynamics
E  Q  W
E: total energy
includes kinetic energy, potential energy and
other forms of energy
All other forms of energy are lumped together as
the internal energy U.
Internal energy U is an extensive property.
Specific internal energy u = U/m
is an intensive property
Internal Energy
● Sensible Energy
It is associated with the kinetic energies of the
molecules
- Translational kinetic energy
- Rotational kinetic energy
- Vibrational kinetic energy
Internal Energy
● Latent Energy
It is associated with the phase change of a system.
It is the energy used to overcome the intermolecular
forces.
● Chemical (Bond) Energy
It is associated with the atomic bonds of a molecule
● Nuclear Energy
It is associated with the bonds within the nucleus of
an atom.
Internal Energy
For saturated mixtures
u  (1  x)uf  xug
 uf  x(ug  uf )
For compressed liquid
u uf
based on the given temperature
Example 1
A piston-cylinder assembly contains 8 kg of
superheated water vapor at 500 kPa and 300 ºC.
Steam is now cooled at constant pressure until 70%
of it, by mass, condenses. Find the work associated
with this process.
p
2
1
W
v
Example 2
An insulated rigid tank contains saturated water
vapor at 100 ºC. The water is rapidly stirred until the
pressure becomes 200 kPa. Find the water
temperature at the end of the process and work
associated with this process.
p
2
1
v
Example 3
Steam in a radiator (V = 20 L) is at P1 = 300 kPa and
T1 = 250 ºC. If the valves are closed now, determine
the amount of heat that will transfer to the room when
the pressure drops to 100 kPa.
p
Q
1
2
v
Example 4
Water contained in a piston-cylinder assembly
undergoes two processes in series. Initially, water is
at 1 MPa and 400 ºC.
1 → 2 water is cooled as it is compressed at
constant pressure to saturated vapor at p2 =
1 MPa.
2 → 3 water is cooled at constant volume to 150 ºC.
Find heat and work involved in these processes.
Example 4 (continued)
p
2
3
1
W12
v
Example 5
A water mixture with an initial quality of 25% is
contained in a piston-cylinder assembly. The mass
of the piston is 40 kg, and its diameter is 10 cm. The
atmospheric pressure is 100 kPa. As the water is
heated, the pressure inside the cylinder remains
constant until the piston hits the stops. Heat transfer
to the water continues until the pressure becomes
300 kPa. Neglect the friction between the piston and
cylinder wall. Determine the total amount of heat
transfer.
Example 5 (continued)
p
3
4 cm
1
2
W12
1 cm
v
Example 6
Two rigid tanks are connected by a valve. Tank A
contains 0.2 m3 of water at 400 kPa and 80% quality.
Tank B contains 0.5 m3 of water at 200 kPa and
250 ºC. The valve is now opened, and the two tanks
eventually come to the same state. Determine the
pressure and the amount of heat transfer when the
system reaches thermal equilibrium with the
surroundings at 25 ºC.
Example 6 (continued)
Tank B
Tank A
pA1 = 400 kPa
xA1 = 0.8
pB1 = 200 kPa
TB1 = 250 ºC
Q