Download Thermal Engineering - Nilachal Polytechnic

Thermal Engineering
(MET - 303)
3RD MECHANICAL
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Chapter – 1
Concepts and Terminology
Group –A
1.
Define thermodynamic
Ans. It is the branch of science which deals with the study of energy transfer and its effect on the
physical properties of the substance.
2.
What is quasistatic process ?
Ans. If every state passed through by the system will be in equilibrium state, then the process is called
quasistatic process. It is also called as reversible process.
3.
Define system.
Ans. Thermodynamic system is defined as quantity of matter or any region in space upon which
attention is concentrated to study its properties.
4.
Define closed system & open system.
Ans. Closed System : In this type of system mass transfer across the system is not possible but energy
may transfer across the system.
Open System : If energy and mass both can transfer across the boundary of the system then the
system is called as open system.
5.
What is isolated system?
Ans. If neither energy nor mass can transfer across the boundary of the system then that system is
called as isolated system. It is of fixed mass and energy.
6.
Differentiate between path and process.
[2014wn]
Ans. Path : The succession of states passed through during a change of state is called the path of
change of state.
Process : When the path is completely specified the change of state is called a process.
6.
Distinguish between intensive properties & extensive properties.
Ans. Intensive Property : Intensive properties are independent of mass
Ex. : Pressure, temperature, density etc.
Extensive Property : Extensive properties are depends on mass.
Ex.: volume, heat energy, work energy etc.
8.
What is thermodynamic cyclic process?
[pg-28, R.S. Khurmi]
Ans. A thermo dynamic cycle is defined as a series of the state changes such that final state is
identical with the initial state.
9.
What is thermodynamic equilibrium ?
Ans. A system is said to be in a state of thermodynamic equilibrium when no change in macroscopic
property is registered if the system is isolated from the surrounding.
10. Define continuum.
Ans. The concept of continuum assumes a continuous distribution of mass with the matter or system
with no empty space.
11. Differentiate between reversible and irreversible process.
Ans. Reversible Process : A process which can work in the reverse direction without effecting the
system and surrounding is called as reversible process.
Irreversible Process : The process which can not work in the reverse direction and it works in the
reverse direction then it has some effects on system and surrounding is called irreversible
process.
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12.
Ans.
13.
Ans.
Define state.
State can be defined as the condition of a system at a particular instant of time.
State Zeroth Law of Thermodynamics.
When a body ‘A’ is in thermal equilibrium with a body ‘B’and also separately with a body ‘C’
then B & C will be in thermal equilibrium with each other.
14. Define path.
Ans. The succession of states passed through during a change of state is called the path of change of
state.
Chapter – 2
Energy and Work Transfer
Group –A
1.
Define heat, work and state their units.
[2006(w), 2007(w),2011(w)]
Ans. Heat is one form of energy which gets transmitted across the boundary of the system due to the
temperature difference between system & surrounding.
Units- joule or KJ.
Work - When there occurs a physical displacement of system boundary due to the action of an
unbalance force. Across the system boundary then work is done by the system or on the system.
Units- joule or KJ.
2.
What are the different forms of work transfer ?
Ans. i)
Electrical work
ii) Shaft work
iii) Paddle wheel work
iv) flow work.
3.
Explain specific heat and give its unit.
[2011(W)]
Ans. It is defined as the amount of heat required to raise the temperature of unit mass of any substance
through 1º.
Unit – KJ/KgK.
4.
Name the modes of heat transfer.
[2014wn]
Ans. The different modes of heat transfer are : Conduction, Convection, Radiation.
5.
Define thermal conductivity.
Ans. It is the property of material to conduct heat. It may depend upon temperature.
6.
What is the high grade and low grade energy ?
Ans. High Grade energy : High grade energy that can be completely transferred into mechanical work
Ex: Mechanical Energy, Tidal energy, Electrical energy etc.
Low grade Energy : Low grade energy is the energy that can not be completely converted in to
mechanical work.
Ex. – Thermal energy, Nuclear energy etc.
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Chapter – 3
First Law of thermodynamics
Group –A
1.
State 1st law of thermodynamics based upon system and its change of state points.
[2014wn]
Ans. When ever heat is absorbed by system it goes to increase its internal energy plus to do some
work.
2.
Give expression for 1st law of thermodynamics undergoing a cyclic process.
Ans.
 dQ   dW
dQ = Amount of heat supplied
dW = work done by the system.
3.
Define enthalpy
Ans. Enthalpy is a state of property of a system. It is defined by
H=U+PV
U=Internal Energy, PV = flow energy
4.
Write the general energy equation for a steady flow system.


1
2


1
2
Ans.  H1  m1v12  mgz 1  Q  H 2  m 2 v 22  mgz 2  w 
Where Q= heat supplied
W = work done
H1 = Enthalpy at inlet
H2 = Enthalpy at outlet
5.
Define PMMI.
Ans. It is a hypothetical machine that will continuously churn out work but without absorbing heat
from its surrounding.
6.
Define internal energy.
Ans. It is a energy stored in a system due to molecular arrangement.
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Chapter – 4
Second Law of Thermodynamics
Group –A
1)
nd
State Kelvin-planck statement of 2 Law of Thermodynamics.
[2014S]
Ans. No heat engine while operating in a cycle, can produce net work while exchanging heat from a
single heat reservoir.
2)
[2014wn]
Write clausius inequality equation for reversible cycle.
Ans. Transfer of heat iron a low temperature body to a high temperature one is not feasible
spontaneously, but this is possible only at the expense of the external work input to the system.
3)
What is thermal energy reservoir .
Ans. A thermal reservoir is a heat source or heat sink that remains at constant temperature regardless
of energy interaction.
4)
Define COP.
Ans. The co-efficient of performance is the desired effect upon the external work supplied for
obtaining the desired effect.
COP 
5)
Desired Effect
Work input
Define PMM – II
Ans. It is the hypothetical machine that will continuously pump out heat from low temperature and
deliver the same to a high temperature reservoir without taking up any input work energy from its
surrounding .
6)
Define entropy.
Ans. The sum of all
Q
terms of all the reversible process completing a cycle is zero.
T
term
Q
represents thermo dynamic property of a closed system and is called entropy.
T
7)
Write clausius inequality equation for reversible cycle.
Q
T
 0 . This
Ans. Transfer of heat from a low temp. body to a high temperature one is not feasible spontaneously,
but this possible only at the expense of the external work input to the system.
8)
[2014wn]
Define refrigerator ?
Ans. A refrigerator is similar to heat pump. It operates as a reversed heat engine. Its duty is to extract
heat as much as possible from the cold body & deliver the same to high temperature body.
9)
Define Heat pump.
Ans. A heat pump is a reversed heat engine. It receives heat from a low temperature reservoir and
rejects it to a high temperature reservoir.
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10) Define Heat Engine
Ans. A heat engine is a device that converts heat into work. It does so by operating between the two
temperature limits.
Chapter – 5
Working Substances
Group –A
1
Define ‘pure substance’.
[2014wn]
Ans. A pure substance is a substance that maintains its chemical homogeneity and constant chemical
composition i.e. it does not undergo a chemical change during heat transfer process.
2
Explain the properties of steam?
Ans. The different properties of steam are Enthalpy, entropy, internal energy, specific volume,
specific enthalpy.
3
What is the difference between wet steam and dry saturated steam?
Ans. Wet Steam : Wet steam is generated from heating of water which turns to steam. The water
content is high.
Dry saturated steam : During the further heating of wet steam the temperature is raised and super
heated steam is generated & water content is much lower.
4
What is latent heat of evaporation?
[A.R. Basu-P-229, Q-1B]
Ans. It is the amount of heat required to vaporize unit mass of a liquid.
5
Define total heat of wet steam.
Ans. Total heat of wet steam (h) = hf+xhfg
Where hf = Specific enthalpy of the fluid (Water)
hg = Specific enthalpy of the gas (steam)
hfg=hg-hf(change in specific enthalpy due to the liquid to vapour transition)
6
What is the dryness fraction of dry saturated steam?
Ans. It is a parameter that defines the quality of wet steam. It is the percentage of dry steam present in
a sample of wet steam.
7
What is critical temp?
[A.R. basu, P-229, Q-10]
Ans. The temperature at which the phase change occurs without absorbing the latent heat is known as
critical temperature.
8
What is critical point?
Ans. It refers to a particular set of pressure and temperature condition at which the density of vapour
equals the density of liquid of a substance. That is, it refers to the point of the highest pressure
and temperature condition at which a liquid coexists with its vapour for the last time.
9
What is the use of steam table?
Ans. Steam is a pure substance having constant properties at different pressures and temperatures.
These properties are estimated and presented in a tabulated form as steam table.
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10
What does mollier diagram represent?
Ans. The molier diagram represents the enthalpy vs Entropy graph or h-s graph.
11
What is the expression for specific entropy of a superheated steam?
Ans. Ssup=Sg+Cp ln(Tsup / Tg)
Where Sg = specific entropy of the steam.
Cp = Molar specific heat at constant pressure.
Chapter – 6
Ideal Gases and Real Gases
Group –A
1
Differentiate between gas and vapour.
Ans. Vapour : i) The word vapour in its nature state is a solid or liquid at room temperature.
Ex-steam.
ii) A vapour is a substance above its boiling point temp.
Gas : i) A gas in its natural state at room temperature would still gas.
Ex- Nitrogen.
ii) A gas is a substance above its critical temperature but below its critical pressure.
2
What is an ideal gas (Perfect gas)? Under what conditions does a real gas behave as a perfect
gas?
[Pg. 48, R.S Khurmi]
Ans. A perfect gas may be defined as a state of a substance whose evaporation from its liquid state is
complete & also obeys all the gas laws under all conditions of temp. & Pressure.
The real gases which originally difficult to liquefy such as oxygen, hydrogen & air within certain
temperature & pressure limit may be regarded as perfect gas.
3
State Boyle’s law
Ans. For a given mass of a gas at a constant temperature (T). its volume is inversely proportional to
pressure(p), i.e. V
1
P
4
State Charle’s Law.
[Pg. 48, R.S. Khurmi]
Ans. For a given mass of a gas at a constant pressure, volume of a gas is proportional to its absolute
temperature
VT
Where T is in Kelvin.
5
What is characteristic gas constant?
Ans. If the volume in general gas equation is taken as that of 1kg of gas (Vs) then the constant ‘C’ is
represented by another constant ‘R’.
Then the general gas equation PVs = RT
‘R’ is known as characteristic gas constant.
6
What is the difference between universal gas constant and characteristics gas constant?
Ans. The universal gas constant (Ru) of a gas is the product of the gas constant and the molecular
mass of the gas.
Ru=MR
M = Molecular mass of the gas.
R = Gas constant.
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7
Explain polytropic process.
Ans. The polytropic process is also known as the general law for the expansion and compression of
gases and is given by PVn = constant
n = polytropic index
8
What is throttling process?
Ans. The throttling process is an irreversible steady flow expansion process in which a perfect gas is
expanded through an orifice of minute dimensions such as a narrow throat or a slightly opened
valve.
9.
State Avogadro’s hypothesis.
[2014wn]
Ans. Equal volume of all gases at the same temperature & pressure, contain equal no. of molecules.
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