Sections 2.12.7

1
Sections 2.1-2.7

• Engr 260 Thermodynamics

2
What happens here?
3
Forms of Energy

• Total Energy Sum of all the Energies in a
  system
• Thermal
• Mechanical
• Kinetic
• Potential
• Electric
• Magnetic
• Chemical
• Nuclear

4
Energy per unit mass
5
Forms of Energy

• Internal Energy, U Sum of all of the
  microscopic forms of energy.

6
Mass Flow rate
7
Energy of Closed/Open Systems

• Only two forms of energy associated with closed
  system
• Heat Transfer driven by temperature difference
• Work everything else
• Open system adds
• Energy transfer by mass flow

8
Flow work

• For open systems, obviously work must be done to
  move the fluid into and out of the control
  volume.
• It is a form of boundary work.

L
Flow
piston of fluid having m, P, V. A is
crossectional area of pipe.
9
Forms of Energy

• Mechanical Energy forms of energy that can be
  converted to mechanical work completely by an
  ideal mechanical device
• Kinetic
• Potential
• Flow Work (pressure acting on fluid over a
  distance)

10
Example Wind Energy

• 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
  11.43 kg/sec for air.

11
Energy Transfer by Heat

• Heat Form of energy that is transferred between
  systems by the virtue of temperature difference

12
Adiabatic No heat transfer

• Can happen two ways
• System is well insulated or
• System and the surrounding are at the same
  temperature

13
Heat Equations
kJ or BTU
kJ/sec kW
For variable heat flow
For constant heat flow
14
Heat Transfer Mechanisms

• Conduction transfer of heat between more
  energetic particles of a substance and less
  energetic ones as a result of the interaction of
  the particles (ex two solid surfaces)
• Convection transfer between a solid surface and
  an adjacent fluid in motion
• Radiation transfer of heat due to the emission
  of electro magnetic waves.

15
Energy Transfer by Work

• Work Energy that crosses a system boundary that
  is not heat is work
• Also work is the energy transfer associated with
  a force acting over a distance.

16
Heat and work are directional
17
Sign Conventions

• W gt 0 Work done by the system (Wout)
• W lt 0 Work done on the system (Win)
• Q gt 0 Heat Transfer to the system (Qin)
• Q lt 0 Heat transfer from the system (Qout)

18
Heat and work are directional

• Qin gt 0
• Qout lt 0
• Win lt 0
• Wout gt 0

19
Example Candle

• A candle is burning in a well-insulated room.
  Taking the room (the air and the candle) as the
  system, determine (a) if there is any heat
  transfer during this burning process and (b) if
  there is any change in the internal energy in the
  system.

20
Example Baked Potato

• A potato initially at room temperature (25oC) is
  being baked in an oven that is maintained at
  200oC. Is there any heat transfer during this
  process?

21
Example Oven

• A well-insulated electric oven is being heated
  through its heating element. If the entire oven,
  including the heating element, is taken to be the
  system, determine whether this is a heat or work
  interaction.
• Answer the question again if the system is taken
  as only the air in the oven without the heating
  element.

22
Heat and Work are Path Dependent

• Path functions have inexact differentials
  designated by the symbol d.
• A differential amount of heat or work is
  represented by dW or dQ, instead of dW or dQ.
• Properties are point functions. They have exact
  differentials.

23
The magnitude of path functions depend on the
path followed.
24
Notes on Heat and Work

• Both are boundary phenomena.
• Systems possess energy, not heat or work.
• Both are associated with a process and not a
  state. Heat has no meaning at a state.
• Both are path functions.

25
Types of Work

• Electrical Work
• Mechanical Work
• Shaft Work
• Work on Elastic Solid Bars
• Spring Work
• Work to stretch a liquid film
• Work to raise or accelerate a body
• Note Please review these in your text!

26
First Law of ThermodynamicsConservation of Energy

• Energy can neither be created nor destroyed
  during a process, it can only change forms.
• The increase in the internal energy of a
  thermodynamic system is equal to the amount of
  heat energy added to the system minus the work
  done by the system on the surroundings.

27
Mechanisms of Energy Transfer

• Heat Transfer
• Work Transfer
• Mass Flow

28
Conservation of EnergyFirst Law of Thermodynamics
Viewed in this way, E on the left hand side is a
property of the system. However, the right side
has Q and W which depend on the path, in general.

29
Conservation of EnergyFirst Law of Thermodynamics

• Consider a closed, adiabatic system.

0
0
adiabatic
closed
30
Conservation of EnergyFirst Law of Thermodynamics

• So for an adiabatic closed system the work is
  equal to a path independent quantity, which means
  adiabatic work is independent of path.
• This is another form of the first law
• For all adiabatic processes between two
  specified states of a closed system, the work is
  the same, regardless of the nature of the closed
  system and the details of the process.

31
Energy Balance

• Look at energy entering and leaving system
• 2) Look at change in energy between two states

32
Example Fan

• A fan that consumes 20W of electric power when
  operating is claimed to discharge air from a
  ventilated room at a rate of 1.0 kg/sec at a
  discharge velocity of 8 m/s. Is this reasonable??

33
NOTE THE CONVERSION TO GET FROM m2/s2 to kJ/kg
REMEMBER IT! YOU WILL NEED IT.
34
Example Another Fan

• A room is initially at the outdoor temperature of
  25oC. Now a large fan that consumes 200W of
  electricity when running is turned on. The heat
  transfer rate between the room and the outdoor
  air is given as
• Where U 6 W/m2/oC is the overall heat transfer
  coefficient, A 30 m2 is the exposed surface
  area of the room, and T1 and T0 are the indoor
  and outdoor air temperatures, respectively.
  Determine the indoor air temperature when steady
  operating conditions are established.

35
Energy Efficiency
36
Hot Water Heater Purchase??

• High-Efficiency Electric 95
• High-Efficiency Gas 65
• Energy Costs
• 0.095 / kWH (electricity)
• 1.4 / Therm (gas)
• Assume heating water takes 300 kWH each month