Faculty:  Faculty of Civil Engineering

 

Department: Department of Civil Engineering

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UNIVERSITY
OF ENGINEERING AND TECHNOLOGY, LAHORE

 

Subject:

 Basic Mechanical Technologies

 

Submitted to: Sir
Rahmat Bashir

Submitted From:  Taha
Usama

                               [email protected]

                              
0333-6776767   

 

 

 

 

 

THERMODYNAMICS

“Thermo”
means “related to heat “and “dynamics” means “related to motion”.
Thermodynamics is actually concerned with heat and temperature and there
relation to energy and work. The behavior of these quantities are controlled by
the four laws of thermodynamics.1

Heat
and temperature are dissimilar properties. The difference between Heat and
Temperature is given below 2

 

Only K.ETRANS
is involved in temperature measurement.

System

The
system is the part of the universe being studied.

Surroundings

Surroundings
are the rest of the universe that interacts with the system.

Open
System

An open
system is a system that spontaneously exchanges energy and matter with its
surroundings. Volume of a system remains uniform but amount of matter and
energy may increase or decrease.

Closed
System

A closed
system is a system that exchanges only energy with its surroundings, not the
matter. The mass remains same but volume and energy may vary within the
boundaries.

Isolated
System

An
isolated system does not exchange energy or matter with its surroundings.
Everything remains Constant. 3

Properties of a System

These
are the inputs and outputs of a system. Properties can be intensive or
extensive.

 

 

Specific
Properties

Specific
properties are written with small letter. The specific properties are derived
from intensive and extensive properties. For example, the density of steel
(intensive property) can be derived from measurements of the mass of a steel
bar (an extensive property) divided by the volume of the bar.4

Work

Work
done by a system is the energy transferred by the system to its surroundings.
Work is not a State function rather it is a Path function. Work done is the shifting of energy from one point
to another.5

When
heat enters the system, the work is done by the gas present inside the cylinder
and the piston moves up and there is a change in volume, and is given as

State
Function

A state
function is a function whose value does not depend on the path taken to reach
that particular value. State functions do not depend on the path taken.
E.g.  Enthalpy, Volume6

Path
Function

Path
functions are the quantities whose values vary with the change of a system from
the initial state to the final state. The two most common path functions are
heat and work. 7 7 7 7 7 7 7 7 7 7

Boyle’s
Law:

For
fixed amount of a gas, the pressure is inversely proportional to volume, at
constant temperature.

 

   As the volume decreases, the pressure of the
gas increases and vice versa. 8

 

 

 

Derivation

                         Since PV=C, Therefore

                                                                     

           

 

 

 For
Polytropic Process

PVn
 = C

P=C
V-n

    W=  C/1-n

Since C = PVn

=P2 V2n
.V21-n  – P1 V1n.V11-n    / 1-n

Work
Done = P2V2 – P2V2   / 1-n

Work done by a Polytropic Process in case of temperature.

=
m R (T2-T1 ) / 1-n

 

1st
LAW OF THERMODYNAMICS:

It
says that the change in the internal energy ?U of a closed system is equal to
the amount of heat Q given to the system, subtracting the amount of work W done
by the system on its surroundings.

.

Where
?E is the sum of all the kinds of energies.9        

·        
Supplying heat at constant pressure,
more work is done

·        
Supplying Heat constant Volume ,
increases internal energy

INTERNAL
ENERGY:

The
sum of all the kinds of energies in a system is referred as the Internal
energy.

Internal
energy may be represented at constant volume or at constant pressure.

But,

                                                       

But
in both the cases, internal energy remains the same. So internal energy is the
intrinsic property.10

 

 

SPECIFIC
HEAT CAPACITY:

The
amount of heat required to raise one degree temperature by one kg of a
substance at constant volume and constant pressure.

It
may be described at the constant Volume, Cv   or at the constant pressure Cp.

    
But in case of liquids, as ?V ? 0, so

                                                                                          
Cp
=   Cv

     Soil gets
warm 5 times quickly than water. It means that water absorbs 5 times more heat
to get warm or to rise its temperature through one degree.   

·        
In case of fix piston cylinder, all the energy
we will supply to the system, it will increase the internal energy. 11

 

SPECIFIC
INTERNAL ENERGY:

So,

Lim?u?0   ?µ =   Cv   Lim ?T?0   ?T

 =   Cv 

U2
– U1   =   Cv 

 

This
is the mathematical form of internal energy in case of ideal volume.10

 

ENTHALPY:

The total heat content of a system
at particular pressure is called the enthalpy of the

System.12

                                                               
H  =  U 
+  PV

                                                             
?h  =  ?u + 
P?V

At the constant pressure,

                                                                   

Qp   = ?U + W

 

Qp   =  ?h

   

Cp ?T 
=  ?h

Cp  

  = 
h2   –  h1

SPECIFIC
HEAT:

The heats in which physical state of
the system remains the same are called the specific heats. Once it reach 100C
then this heat is absorbed by the water so temperature remains the same also
the pressure is same.11

Specific
heat capacity at constant Volume Cv:

                                                                                     
The specific heat capacity of a substance is the heat required to
increase the temperature of 1 kg substance by 1k at constant volume is called
specific heat capacity at constant volume.

Specific
heat capacity at constant pressure Cp:

                                                                                 
The specific heat capacity of a substance is the heat required to
increase the temperature of 1 kg substance by 1k at constant pressure is called
specific heat capacity at constant pressure.

Latent
Heat: 

The
heat with which physical state of the system is changed. Physical changes
always occur at contant pressure.13

 1st Law of Thermodynamics:

Dividing both sides by ?t

 =

dE
/ dT = Q. – W.

This is for the closed system.

Derivation
of Enthalpy:

                                   H = U + PV

At
constant volume:

At
constant volume heat supplied to the system will increase the internal energy (DU).

Qv µ m

DU
= CvDT

Cv  = DU/DT

Cv = du/dt

du = Cv.dt

 

At
constant pressure:

                                      If the
energy is provided to the system at constant pressure then the system change
its enthalpy.

Dh = Cp DT

Lim Dh = Lim Cp DT

Dh®0                  Dh®
0

First law of thermodynamics for open
system

 

 

 

For example;
if system is in steady state i.e. (there is no change in quantity with respect
to time) so energy of this control volume will not change

 

 

Wout=m0 (h1-h2)

It is work
performed by control volume.

WP=mo (h2-h1)

First law is
also known as quantitative law.

Refrigeration cycle

It is also known as vapour
compression cycle. There are four main parts of refrigerator.

 Compressor

     
It is used to raise the pressure of fluid which are in gaseous state
.The refrigerant coming towards the compressor should be completely in the
vapour state. Only compressor utilizes electricity.14

 

 

Condenser   

Condenser is a device which is used
to condense a substance from its gaseous to its liquid state. When refrigerant
comes from compressor to condenser than both temperature and pressure increases.15

 

 

 

 

 

 

 

 

 

 

 

 

Expansion valve

It is a
component that controls the amount of refrigerant released into the evaporator
by controlling super heat. When refrigerant come towards the expansion valve
then energy utilizes and both temperature and   
pressure decreases. 16

  Evaporator

 It is a device that absorb heat from
surroundings so liquid is converted into vapours so temperature of refrigerator
decreases.17

Otto Cycle

The Otto Cycle is the ideal air
universal cycle for the petrol engine, the gas engine, and the high speed oil
engine.18

 

 

Process 1 to 2 is isentropic
compression.

Process 2 to 3 is reverse constant
volume heating.

Process 3 to 4 is isentropic
expansion.

Process 4 to
1 is reverse constant volume cooling.

                
i.           
Air-fuel mixture gets into the
intake stroke.

              
ii.           
 In second step Compression occurs.

Isentropic process:

In thermodynamics, a process involving change without any
increase or decrease of entropy.19. Some examples of
theoretically isentropic thermodynamic devices are pumps, gas
compressors, turbines, nozzles, and diffusers.

Removal of heat from walls of engine is equal to CV.
Efficiency of the system is given as:

According
to general gas equation,

 

 

 

 

 

Combustion stroke

                                                                

   

                            

V2  and V3 are equal during this phase.
So,

Power stroke

Since,

So,

This
is T4 in terms of T3.

 

For T4   in terms of T2 put   

      
in above equation. Hence

Where compression ratio is

 Where V1 – V2 =
swept volume.

This
means

 (efficiency) only
depends on C.R. If C.R value increases then

 decreases as a result efficiency
increases.

Knocking:  

Knocking takes place in diesel engine but It
can also occur in petrol engine. Knocking is due to self-ignition. The value
must range from 9 to 13. It should be greater than 9 but less than 13. When
combustion of the air/fuel mixture in the cylinder does not start off
correctly in response to ignition by the sparkplug, but one or more
pockets of air/fuel mixture explode outside the envelope of the normal
combustion front.20

Heat Transfers

This differentiates from thermodynamics in a way that it deals with
heat transfers with respect to time i.e. rate of heat is considered. Heat can
be transferred from one place to another by three methods.21

1.   
Conduction

The
mode of transfer of heat by vibrating atoms and free electrons in solids from
hot to cold parts of a body is called conduction of heat. Wood is an insulator
there are no free electrons.

 

2.      Convection

Transfer of heat by actual movement of molecules from hot
place to a cold place is known as convection. Mostly due to the bulk movement
of the molecules.

 

3.     
Radiation

 

The mode of transfer of heat from one place to another in the
form of waves called electromagnetic waves. All matter continously emits
electromagnetic radiation unless its temperature falls absolute zero.

 

Fourier Law :

           Fouriers Law
states that the rate of flow of heat through a single uniform solid is directly
propotional to Area A of the section at right angles to the direction of heat
flow,  and to the change of temperature
with respect to the length of the path.22

                                                             

          

 

From these equations,

 

 

Where K is the material property, is the thermal
conductivity constant.

 

        

               (K=1)

 

In this expression, ?T
is the driving force and ?x÷?A acts like thermal resistance ( RT ) to
the the heat flowing in the body. Just like the ohm’s law,    where V is driving force and R is the
resistance to the current flowing in the circuit.  

Similarly, if 3
bodies are there in series, there thermal resistances will be added to
calculate total thermal resistance ( RT ), just like an electrical
circuit when resistances are connected in series.

 

 

If
the heat is passing through the same material then,

Hence,

 

Lets consider

     and

A1m1
= A2m2

As,

And

This means temperature movement
for lesser area will be greater as temperature gradient is more.

Newton’s Law of Cooling

Newton
Law of cooling states that the heat transfer from a solid surface of area A, at
a temperature tw , to a fluid temperature is given by

Change
in temperature with respect to time is directly proportional to difference
between its own temperature and the cambium temperature.

Where,

 is an ambient temperature
(temperature of the surrounding )

                                                                                

 

     Where h
is convective constant. This heat of transfer is also termed as Newton’s
law of cooling.23

 

AIR CONDITIONING

There are 2 temperatures involved in air conditioning, Wet Bulb Temperature and Dry Bulb Temperature.

 

Dry Bulb Temperature

       It is the temperature measured by
thermometer when it is freely exposed to air.

 

 

Wet bulb temperature

         It is the temperature measured by  thermometer when tip of  bulb is wet.

 

              Relative humidity

          At specific temperature the amount
of vapours that sustained in given sample of air divided by total sample of air
at that temperature .    If relative
humidity is less then wet bulb temperature is less and vise versa.24

 If relative humidity is 100% then

Dry bulb
temp= wet bulb temp

WBD =
DBT – WBT

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1.            Wikipedia. Thermodynamics.  cited 1
December ,2017; Available from: https://en.wikipedia.org/wiki/Thermodynamics.

2.            Difference between Heat and Temperature.  21 November 2017; Available from: http://keydifferences.com/difference-between-heat-and-temperature.html.

3.            A System and its Surroundings.  21 November , 2017; Available from:
https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Thermodynamics/Fundamentals_of_Thermodynamics/A_System_and_Its_Surroundings.

4.            Wikipedia. Specific Properties.  cited 14 November , 2017; Available from:
https://en.wikipedia.org/wiki/Specific_properties.

5.            Wikipedia. Work (thermodynamics).  14 November , 2017; Available from:
https://en.wikipedia.org/wiki/Work_(thermodynamics)#Formal_definition.

6.            State Functions. Chemistry LibreTexts;
Available from: https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Thermodynamics/State_Functions.

7.            Wikipedia. Path Functions. Chemistry LibreTexts;
Available from: https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Thermodynamics/Path_Functions.

8.            Wikipedia. Boyle’s Law.  cited 2017 1 December 2017 12:03 UTC;
Available from:
https://en.wikipedia.org/w/index.php?title=Boyle%27s_law=812258447.

9.            First law of thermodynamics.  1 December 2017 13:23 UTC; Available from:
https://en.wikipedia.org/w/index.php?title=First_law_of_thermodynamics=809731205.

10.          Wikipedia. Internal Energy. 2017  4 December 2017 15:47 UTC; Available from:
https://en.wikipedia.org/w/index.php?title=Internal_energy=812131728.

11.          Wikipedia. Heat capacity. 2017  4 December 2017 15:54 UTC; Available from:
https://en.wikipedia.org/w/index.php?title=Heat_capacity=813559876.

12.          Wikipedia. Enthalpy. 2017  4 December 2017 15:59 UTC; Available from:
https://en.wikipedia.org/w/index.php?title=Enthalpy=805941542.

13.          Wikipedia. Latent heat. 2017  4 December 2017 16:09 UTC; Available from:
https://en.wikipedia.org/w/index.php?title=Latent_heat=806944640.

14.          Wikipedia. Compressor. 2017  4 December 2017 16:16 UTC; Available from:
https://en.wikipedia.org/w/index.php?title=Compressor=803380477.

15.          Wikipedia. Condenser (heat transfer). 2017 4
December 2017 17:08 UTC; Available from: https://en.wikipedia.org/w/index.php?title=Condenser_(heat_transfer)=796021494.

16.          Wikipedia. Thermal expansion valve. 2017  4 December 2017 17:14 UTC; Available from:
https://en.wikipedia.org/w/index.php?title=Thermal_expansion_valve=808575843.

17.          Wikipedia. Evaporator. 2017 4 December 2017 17:21
UTC; Available from:
https://en.wikipedia.org/w/index.php?title=Evaporator=803030624.

18.          McConkey,
T.D.E.A., Applied Thermodynamics. 5th
ed.: Pearson Education Ltd.

19.          Engineering.com.
Isentropic process.  cited 2018; Available from: http://www.engineering.com/[email protected]/qactid/2/qaqid/2334.aspx.

20.          Wikipedia. Engine knocking. 22 January 2018 19:45
UTC; Available from: https://en.wikipedia.org/w/index.php?title=Engine_knocking=817048490.

21.          Wikipedia. Heat transfer. 2018  19 January 2018 20:07 UTC; Available from:
https://en.wikipedia.org/w/index.php?title=Heat_transfer=821330521.

22.          T.W., D. FOURIERS LAW.  cited 2018; Available from: http://www.thermopedia.com/content/781/.

23.          Wikipedia. Newton’s law of cooling. 2018  cited 2018; Available from:
https://en.wikipedia.org/w/index.php?title=Newton%27s_law_of_cooling&oldid=821785729.

24.          Wikipedia. Relative Humidity. 2018  cited 2018; Available from:
https://en.wikipedia.org/w/index.php?title=Relative_humidity&oldid=820765474.