Chapter 2 Energy and Energy Transfer wrc

PDF of wrc lecture

1. ENERGY AND ENERGY TRANSFER
Shacheendra Kishor Labh
Assistant Professor
Department of Mechanical and Automobile Engineering
IOE, Paschimanchal Campus
1
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

2. Energy
2
•Traditionally, energy is defined as the capacity of doing work.
•It can also be defined as the capacity to exert a force through a distance.
•Energy can be classified into two general types:
i.as stored energy in materials and
ii.as energy in transition from one system to another system or the
surroundings.
•The stored energy is the energy possessed by a system within its boundaries. The
potential energy, kinetic energy and internal energy are the examples of
stored energy.
•The energy in transition is the energy possessed by a system which is capable of
crossing its boundaries. The heat, work and electrical energy are the examples of
transit energy.
•The stored energy is a thermodynamic property whereas the transit energy is not
a thermodynamic property
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

3. Energy
3
Potential Energy
•It is the energy possessed by a body or a system for doing work, by virtue of its
position above the ground level.
Potential Energy (PE) = mgz
Where,
m = mass of the system
z = height of the system from ground
g = acceleration due to gravity
Kinetic Energy
•It is the energy possessed by a body or a system, for doing work, by virtue of its
mass and velocity of motion.
Kinetic Energy (KE) =
1
2
????????????
2
where, ‘m’ is the mass and ‘v ‘ is the velocity of the
system
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

4. Energy
4
Internal Energy
•It is the energy possessed by a body or a system due to its molecular arrangement
and motion of the molecules. It is usually represented by U.
•In thermodynamics, we are concerned with the change in Internal Energy (dU)
dU = mc
vdT
Where,
m = mass of the system
c
v = specific heat at constant volume
dT = change in temperature of system
Total Energy
•It is the summation of Kinetic, Potential and Internal Energy.
E = PE + KE + U = mgz +
1
2
????????????
2
+ U
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

5. Work Transfer: Definition of work
5
•Work is defined as the product of a force and the distance moved in the direction of
the force.
•In mathematical terms, the amount of work done ‘δW’ by a force in moving through
a differential distance ‘ds’ directed outward from the system boundary is
δW = F. ds
•The work done over a finite path between points 1 and 2 is
W
12 = ׬
1
2
??????.????????????
•In terms of thermodynamics, work can be defined as,
Work is done by a system if the sole effect on the surroundings (everything
external to the system) could be the raising of a weight. The raising of a weight is in
effect a force acting through a distance.
•Work is designated by the symbol ‘W’ and its SI unit is Joule (J).
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

6. Work Transfer: Definition of work
6
•The sign convention generally adopted for thermodynamic work is that work done by
the system on the surroundings is positive, while work done on the system by the
surroundings is negative.
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

7. Moving boundary works
7
•There are a variety of ways in which work can be done on or by a system. These
include work done by a rotating shaft, electrical work, and work done by the
movement of the system boundary.
•Consider a system of a cylinder and piston that contains gas. Removing one of the small
weights from the piston will cause the piston to move upward a distance dL.
•The total force on the piston is PA, where P is the pressure of the gas and A is the area
of the piston. Therefore, the work δW is
δW = ׬�??????.??????�
A.dL = Incremental volume = dV
δW = ׬�.????????????
Work done in process 1-2,
W
12 = ׬
�
�
??????.????????????
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

8. Moving boundary works
8
•The assumption of a quasi-equilibrium process is essential here because each point
on line 1–2 represents a definite state, and these states correspond to the actual state
of the system only if the deviation from equilibrium is infinitesimal.
•It is possible to go from state 1 to state 2 along many different quasi-equilibrium paths,
such as A, B, or C.
•Since the area under each curve represents the work for each process, the amount of
work done during each process depends on the path followed in going from one state
to another.
•For this reason, work is called a path function or, in
mathematical parlance, δW is an inexact differential.
•Thermodynamic properties are point functions. The
differentials of point functions are exact differentials.
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

9. Expression for work
9
•Work transfer during different processes can be derived as:
Constant Volume (Isochoric) Process
Constant Pressure (Isobaric) Process
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

10. Expression for work transfer
10
Constant temperature (Isothermal) Process
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

11. Polytropic Process
Thermodynamic process which follows the relation PV
n
= constant is called a
polytropic process and the index ‘n’ is called polytropic index.
Pressure at any intermediate state during polytropic process is
Hence,
Expression for work transfer
11
1
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

12. Other system involving work
12
•There are other types of systems in which work is done at a moving boundary. Three
such systems are: a stretched wire, a surface film, and electrical work.
•Consider as a system a stretched wire that is under a given tension T. When the length
of the wire changes by the amount dL, the work done by the system is
δW = - TdL
This equation can be integrated to have
W
12 = ׬
�
�
−??????.????????????
•Electrical energy flowing across the boundary of a system is work. Examples of such a
system include a charged condenser, an electrolytic cell, and fuel cell. Consider a quasi-
equilibrium process during which the potential difference be E and the amount of
electrical charge that flows into the system be dZ. The work is given by the relation
δW = - EdZ
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

13. Other system involving work
13
•Considering a system that consists of a liquid film with a surface tension ‘S’. When the
area of the film is changed, for example, by sliding the movable wire along the frame,
work is done on or by the film.
• When the area changes by an amount dA, the work done by the system is
δW = - SdA
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

14. Other system involving work
14
•In each of the quasi-equilibrium processes, work is expressed by the integral of the
product of an intensive property and the change of an extensive property
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

15. Energy transfer
15
•There are two ways in which a closed system can interact with the surrounding:
i.Heat transfer
ii.Work transfer
Heat Transfer
•Heat is energy transferred, without transfer of mass, across the boundary of a
system because of a temperature difference between system and surroundings.
•Heat is transferred across a boundary from a system at higher temperature to a
system at lower temperature by the mechanisms of conduction, convection and
radiation.
•The symbol for heat is Q . By convention heat flowing into a system is considered
positive while heat flowing out of a system is considered negative.
•Heat is a transient quantity.
•It is not a property of a system.
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

16. Examples
16
1.A gas undergoes compression from an initial state of V
1= 0.5 m
3
, P
1= 300 kPa to a final
state of V
2= 0.125 m
3
, P
2= 1200 kPa. If the pressure varies linearly with volume during the
process, determine the work transfer. [281.25 KJ]
2.In a non-flow process, a gas expands from a volume of 0.2 m
3
to a volume of 0.5 m
3

according to the law
P=
5
??????
+2.5
Where P is the pressure in bar and v is the volume in m
3
. Determine
i.The pressure at the end of expansion [12.5 bar]
ii.The work done by the gas in the expansion in kJ
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

17. Examples
17ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh
4.Nitrogen (5 kg) is contained in a cylinder device as shown below. The initial
pressure and temperature is 800 kPa and 127
o
C respectively. There is a heat
transfer to the system until the temperature reaches to 527
o
C. It takes a pressure
of 1500 kPa to lift the piston. Sketch the process on P-V and T-V diagrams and
determine the total work in the process. [Take R = 297 J/kg.K]

18. 6.A piston cylinder device restrained by a linear spring (k = 200 kN/m) shown in figure contains a
gas initially at a pressure of 400 kPa and a volume of 0.5 m
3
. Initially spring touches the piston
but exerts no force on it. Heat is transferred to the system until its volume doubles. If the cross-
sectional area of the piston is 0.4 m
2
, determine
i.The final pressure inside the cylinder
ii.The total work done by the gas
iii.The fraction of total work done consumed for the compression of spring
Examples
18
Gas
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

19. 7.A 0.5-m-long steel rod with a 1-cm diameter is stretched in a tensile test. What is the
work required to obtain a relative strain of 0.1%? The modulus of elasticity of steel is 2 ×
10
8
kPa.
[7.85 kJ]
8.A soap bubble has a surface tension of S = 3 × 10
−4
N/cm as it sits flat on a rigid ring of
diameter 5 cm. You now blow on the film to create a half sphere surface of diameter 5
cm. How much work was done?
[0.23 mJ]
Examples
19ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

20. Heat Transfer Modes
20
•Heat transfer is the transport of energy due to a temperature difference between
different amounts of matter.
•There are three modes of heat transfer:
i.Conduction
ii.Convection
iii.Radiation
Conduction
•Energy transfer by conduction can take place in solids, liquids, and gases.
•Conduction can be thought of as the transfer of energy from the more energetic
particles of a substance to adjacent particles that are less energetic due to
interactions between particles.
•The time rate of energy transfer by conduction is quantified macroscopically by
Fourier’s law.
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

21. Heat Transfer Modes
21
•By Fourier’s law, the rate of heat transfer across any plane normal to the x direction, ሶ�
?????? is
proportional to the wall area, A, and the temperature gradient in the x direction,
�??????
�??????
ሶ�
??????=−�??????
�??????
�??????

ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh
where the proportionality constant K is a property called the thermal
conductivity. The minus sign is a consequence of energy transfer in
the direction of decreasing temperature.
•For a plane wall of thickness L and cross sectional area A as shown in figure.
ሶ�
??????=−�??????
??????
2
−??????
1
??????

•Substances with large values of thermal conductivity such as copper are good conductors,
and those with small conductivities (cork and polystyrene foam) are good insulators.

22. Heat Transfer Modes
22
Convection
•Energy transfer between a solid surface at a temperature T
b and an adjacent gas or
liquid at another temperature T
f referred to as convection.
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh
•In the figure alongside, energy is transferred in the
direction indicated by the arrow due to the
combined effects of conduction within the air and
the bulk motion of the air.
•The rate of energy transfer from the surface to the
air can be quantified by Newton’s Law of cooling
ሶ�
??????=ℎ??????????????????=ℎ?????? (???????????? −????????????)
where A is the surface area and the proportionality factor h is called the
convective heat transfer coefficient.

23. Heat Transfer Modes
23
•The heat transfer coefficient is not a thermodynamic property.
•It is an empirical parameter that incorporates into the heat transfer relationship the
nature of the flow pattern near the surface, the fluid properties, and the
geometry.
•When fans or pumps cause the fluid to move, the value of the heat transfer coefficient
is generally greater than when relatively slow buoyancy induced motions occur.
•The two general categories of convection are called forced and free (or natural)
convection.
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

24. Heat Transfer Modes
24
Radiation
•Thermal radiation is emitted by matter as a result of changes in the electronic
configurations of the atoms or molecules within it.
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh
•The energy is transported by electromagnetic waves (or
photons).
•Unlike conduction, thermal radiation requires no
intervening medium to propagate and can even take place
in a vacuum.
•Solid surfaces, gases, and liquids all emit, absorb, and transmit thermal radiation to
varying degrees.
•The rate at which energy is emitted, ሶ�
� , from a surface of area A is quantified by
Stefan–Boltzmann law
ሶ�
�=εσAT
b
4

25. Heat Transfer Modes
25
•Thermal radiation is associated with the fourth power of the absolute temperature of
the surface, T
b.
•The emissivity (ε) is a property of the surface that indicates how effectively the
surface radiates (0 ≤ ε ≤ 1.0), and σ is the Stefan–Boltzmann constant whose value is
5.67 x 10
-8
W/m
2
.K
4
•The net rate of energy transfer by thermal radiation between two surfaces involves
relationships among the properties of the surfaces, their orientations with respect to
each other, the extent to which the intervening medium scatters, emits, and absorbs
thermal radiation, and other factors.
•For radiation exchange between a surface at temperature T
b and a much larger
surrounding surface at T
s

ሶ�
�=εσA[T
b
4
- T
s
4
]
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

26. 11.A 2-m
2
window has a surface temperature of 15
o
C, and the outside wind is blowing air at 2
o
C
across it with a convection heat transfer coefficient of h = 125 W/m
2
.K. What is the total heat
transfer loss?
[3250 W]
12.A radiant heating lamp has a surface temperature of 1000 K with ε = 0.8. How large a surface area is
needed to provide 250 W of radiation heat transfer?
[55.1 cm
2
]
Examples
26
ME 104 Engineering Thermodynamics I BME I/II Shacheendra K Labh

27. 27
Thank you for your time and attention!