Download ME6301- ENGINEERING THERMODYNAMICS UNIT – I BASIC

ME6301- ENGINEERING THERMODYNAMICS
UNIT – I
BASIC CONCEPT AND FIRST LAW
PART-A
1. What is meant by thermodynamics system?
(A/M 2006)
Thermodynamics system is defined as any space or matter or group of matter where
energy transfer or energy conversion takes place.
2. What is meant by thermodynamics property?
(A/M 2006)
Thermodynamics property is characteristics of a substance which is used to identify
the state of the system and can be measured when the system remains in an equilibrium
state.
3. What are the conditions for a system to be in "thermodynamics equilibrium"?
(N/D 2007)
When a system is in thermodynamics equilibrium it should satisfy the following three
conditions:
a. Mechanical Equilibrium - Pressure remains constant
b. Thermal Equilibrium
- Temperature remain constant
c. Chemical Equilibrium
- There is no chemical reaction
4. State Zeroth Law and the First Law of Thermodynamics.
(A/M 2008)
Zeroth law of thermodynamics states when two systems are separately in thermal
equilibrium with a third system then they themselves are in thermal equilibrium with each
other.
The first law of thermodynamics states that when a system undergoes a cyclic
process, net heat transfer is equal to work transfer.
ɸQ = ɸw
5. What is meant by "Perpetual motion machine of first kind?
(A/M 2008)
PMM of first kind delivers work continuously without any input. It violates the first
law of thermodynamics. It is impossible to construct an engine working with this
principle.
6. Prove that there is no change in internal energy for an isolated system.
(N/D 2009)
For any isolated system there is no heat, work and mass transfer
Q=W=0
According to the first law of thermodynamics
Q = W + Δu
Δu = 0
7. Prove that the difference in specific heat capacities equal to Cp – Cv = R (N/D 2009)
Heat supplies Q = mCp(T2-T1)
Work done w
= P (V2-V1)
= mR (T2-T1)
Change in internal energy Δu = mCv (T2-T1)
Q = W + Δu
mCp(T2-T1) = mR(T2 - T1) + mCv(T2 - T1)
Cp = R + C v
Cp – Cv = R
8. What is a steady flow process?
(A/M 2010)
Steady flow means that the rates of flow of mass and energy across the control surface are
constant.
9. Define – Properties of a System
(N/D 2010)
Every system has certain mass or energy transfer characteristics’ which it is physical
condition may be described. Eg: volume, Temperature etc., Such characteristics are
called properties of the system.
10. Define – Concept Of Continuum
(N/D 2011)
A continuous homogenous medium is called as continuum. Continuum is based on
macroscopic approach, and ‘the matter is treated as continuous instead of disregarding
the behaviors’ of individual molecules.
11. What is meant by reversible process?
(N/D 2011)
A reversible process is one, which is performed in such a way that at the conclusion
of process, both system and surroundings may be restored to their initial state, without
producing any changes in rest of the universe.
12. Define – Cycle
(N/D 2012)
It is defined as a series of state changes such that the final state is identical with
the initial state.
13. Define – Path Function
(A/M 2012)
The work done by a process does not depend upon the end of the process. It depends
on the path the system follows from one state to another state. Hence work is called a path
function.
14. Define – Point Function
(A/M 2013)
Thermodynamics properties are point functions. The change in a thermodynamics
property of a system is a change of state independent of the path and depends only on the
initial and final states of the system.
15. List out the practical applications of steady flow energy equation.
a. Turbine
b. Nozzle
c. Condenser
d. Compressor
(A/M 2013)
UNIT-2
SECOND LAW OF THERMODYNAMICS
PART-A
1. State the Kelvin- Planck statement of the second law of thermodynamics. (N/D 2006)
Kelvin-Planck states that it is impossible to construct a heat engine working on cyclic
process, whose only purpose is to convert all the heat energy given to it in an equal
amount of work.
2. Write a short note on the term “availability”.
(A/M 2006)
The availability (A) of a given system is defined as the maximum useful work that is
obtainable in a process in which the system comes to equilibrium with its surroundings.
Availability is thus a composite property depending on the state of both the system and
surroundings.
3. What is the difference between Ideal gas and Real gas?
(N/D 2007)
Ideal Gas
Real Gas
a) Ideal gas obeys all gas laws under Real gas obeys gas laws only at law
all conditions of pressure and pressure and high temperature
temperature
b) Force of attraction among the Forces of attractions are not negligible
molecules negligible as compared
to the total volume occupied by the
gas
c) Obeys ideal gas equation
Obeys Vander Waals equation
4. List out any four reasons for irreversibility in a process.
a.
b.
c.
d.
e.
(A/M 2007)
Lack of equilibrium.
Heat transfer through a finite temperature difference.
Force expansion,
Dissipative effects.
Lack of pressure equilibrium within the interior of the system.
5. State Carnot Theorem.
(A/M 2007)
No heat engine operating in a cyclic process between two fixed temperatures can be
more efficient than a reversible engine operating between the same temperature limits.
6. What are the Corollaries of Carnot theorems?
(A/M 2008)
a. All the reversible engines operating between the two given thermal reservoirs with
fixed temperature have the same efficiency.
b.The efficiency of any reversible heat engine operating between two reservoirs is
independent of the nature of the working fluid and depends only on the temperature of
the reservoirs.
7. Define – PMM of Second Kind
(N/D 2008)
Perpetual motion machine of second kind draws heat continuously from single
reservoir and converts it into equivalent amount of work.
Thus, it gives 100%
efficiency.
8. What is the difference between a heat pump and refrigerator?
(A/M 2009)
Heat pump is a device which operating in a cyclic process maintains the temperature
of a hot body at a temperature higher that the temperature of the surrounding.
A refrigerator is a device which operating in a cycle process, maintains the
temperature of a cold body at a temperature lower than the temperature of the
surrounding.
9. Why cannot the Carnot cycle be realized in practice?
(N/D 2009)
a. In a Carnot cycle, all the four processes are reversible but in actual practice there is no
process is reversible.
b. It is not possible to avoid friction between moving parts completely.
10. What is meant by “Clausius Inequality”?
(A/M 2010)
It is impossible for a self acting machine working in a cyclic process unaided by any
external agency to convert heat from a body at a lower temperature to a body at a higher
temperature.
11. Define – Entropy
(N/D 2010)
It is an important thermodynamic property of the substance. It is the measure of
molecular disorder. It is denoted by S. The measurement of change in entropy for
reversible process is obtained by the quantity of heat received or rejected to absolute
temperature.
12. Define – Available Energy and Unavailable Energy
(A/M 2011)
Available energy is the maximum thermal useful work under ideal condition. The
remaining part, which cannot be converted into work, is known as unavailable energy.
13. What are the important characteristics of entropy?
(A/M 2012)
a. If the heat is supplied to the system then the entropy will increase.
b. If the heat is rejected to the system then the entropy will decrease.
c. The entropy is constant for all adiabatic frictionless process.
14. Define – Heat Engine
(N/D 2013)
Heat engine is a machine, which is used to convert the heat energy into mechanical
work in a cyclic process.
15. What are the assumptions made on heat engine?
a. The source and sink are maintained at constant temperature.
b. The source and sink has infinite heat capacity.
(N/D 2013)
UNIT -3
PROPERTIES OF PURE SUBSTANCE AND STEAM POWER CYCLE
PART – A
1. Define – Latent Heat Of Vaporization
(A/M 2006)
The amount of heat added during heating of water from boiling point to dry saturated
state is called latent heat of vaporization or enthalpy of vaporization or latent heat of
steam.
2. Define – ‘Boiling Point’ and ‘Melting Point’
(N/D 2006)
Boiling point:
It is the temperature at which the liquid starts to change its state from liquid to vapour.
Melting point:
It is the temperature at which the solid starts to change its state from solid to liquid.
3. What is meant by super heated steam? And indicate its use.
(N/D 2007)
If the dry steam is further heated, then the process is called super heating and steam
obtained is known as super heated steam.
4. Define – Dryness Fraction of Steam
(A/M 2007)
It is defined as the ratio of the mass of the total steam actually present to the mass of
the total steam.
Dryness fraction =
5. Write short notes on the terms: Degree of super heat and Degree of sub cooling.
(N/D 2008)
Degree of Super heat:
It is the difference between superheated temperature and saturated temperature at the
same pressure.
Degree of sub cooling:
It is the amount by which the water is cooled beyond the saturated temperature at the
same pressure.
6. Define – Triple Point and Critical Point for Pure Substance
(A/M 2008)
Triple point:
Triple point is the state at which all the three phases i.e. solid, liquid and vapour exist
in equilibrium.
Critical point:
It represents the highest pressure and temperature at which the liquid and vapour
phases coexist in equilibrium. At the critical point the liquid and vapour phases are
indistinguishable i.e. Liquid is directly converted into vapour.
7. One kg of steam at 10 bar has an enthalpy of 2500 kJ/kg. Find its quality.(A/M 2008)
h=2500 kJ/kg
h=hf+ X x hfg
At 10 bar from steam tables
Hf = 762.6 kJ/kg; hfg = 2013.6 kJ/kg
2500 = 762.6 + X x 2013.6
X=
8.
Draw a skeleton P-V diagram for water and show an isothermal passing room
compressed liquid state to superheated vapour state through vaporization process.
(N/D 2009)
9. Write the different process of Ranking cycle on T-S diagram.
Process:
1-2: isentropic expansion
2-3: Constant pressure and temperature heat rejection
3-4: water is pumped to boiler pressure
4-5: constant pressure heat addition in boiler of to saturation temperature
5-1: Constant pressure and temperature in boiler
(A/M 2009)
10. List out the advantages of reheat cycle.
(N/D 2010)
a. Marginal increase in thermal efficiency.
b. Increase in work done per kg of steam which results in reduced size of boiler and
auxiliaries for the same output.
c. Prevention of the turbine from erosion.
11. List out the improvements made to increase the ideal efficiency of Rankine cycle.
(N/D 2011)
a. Lowering the condenser pressure.
b. Superheated steam is supplied to the turbine.
c. Increasing the boiler pressure to a certain limit.
d. Implementing reheat and regeneration in the cycle.
12. What are the advantages of reheat cycle?
(A/M 2011)
a. It increases the turbine work.
b. It increases the heat supply.
c. It increases the efficiency of the plant.
d. It reduces the wear on the blade because of low moisture content in LP state of the
turbine.
13. What is the purpose of reheating?
(N/D 2012)
The purpose of reheating is to increase the dryness fraction of the steam passing out
of the later stages of the turbine.
13. List out the various vapour power cycle.
(A/M 2012)
a. Carnot cycle
b. Rankine cycle
c. Reheat Rankine cycle
d. Regenerative cycle.
14. Define – Pure Substance
(N/D 2013)
A pure substance is defined as one that is homogeneous and invariable in chemical
composition throughout its mass.
15. Why is reheat cycle not used for low boiler pressure?
(N/D 2013)
At the low reheat pressure the heat cycle efficiency may be less than the Rankine cycle
efficiency, since the average temperature during heating will then be low.
UNIT – 4
IDEAL AND REAL GASES, THERMODYNAMIC RELATIONS
PART-A
1. State Boyle’s Law.
(N/D 2006)
Boyle’s law states that the volume of a given mass of a gas varies inversely as its
absolute pressure, when the temperature remains constant.
2. State Charle’s law.
(A/M 2006)
Charle’s law states that, “The volume of a given mass of a gas, varies directly as its
absolute temperature, when the pressure remains constant’’.
3. State Joule’s law.
(N/D 2007)
Joule’s law states that, “The internal energy of a given quantity of a gas depends only
on the temperature”.
4. State Regnault’s law.
(A/M 2007)
Regnault’s law states that constant pressure and constant volume of a gas always
remain constant.
5. State Avagadro’s law.
(N/D 2008)
Avogadro’s law states, “Equal volume of different perfect gases at the same
temperature and pressure, contain equal number of molecules”
6. State Daltons Law of partial pressure.
(A/M 2008)
Dalton’s law of partial pressure states, “The total pressure of a mixture of gases is
equal to the sum of the partial pressures exerted by individual gases if each one of them
occupied separately in the total volume of the mixture at mixture temperature”.
P = P1 + P2 + P3 + . . . . Pk
7. State the assumptions made in kinetic theory of gases.
(N/D 2009)
a. There are no intermolecular force between particles
b.The volume of the molecules is negligible in comparison with the gas.
8. State Helmholtz and Gibbs function.
(A/M 2009)
Helmholtz function is property of a system and is given by subtracting the product of
absolute temperature (T) and entropy (S) from the internal energy
i.e. Helmholtz function = u-Ts
Gibbs function is property of a system and is given by
G=u – Ts + PV = H - Ts (H = U + PV)
Where,
H – Enthalpy
T- Temperature
S –Entropy
9. State the third law of thermodynamics.
(A/M 2010)
It is states that the entropy of any pure substance in thermodynamics equilibrium
approaches zero as the absolute temperature approaches zero.
10. What is Joule – Thomson coefficient? Why is it zero for an ideal gas?
(N/D 2010)
Joule – Thomson Coefficient is defined as the Change in temperature with change in
pressure, keeping the enthalpy remains constant. It is denoted by
We know that the equation of State as
PV=RT
Differentiating the above equation of state with respect to T by keeping pressure, P is
constant.
µ=0
It implies that the Joule –Thomson coefficient is Zero for ideal gas.
11. What is compressibility factor?
(N/D 2011)
The gas equation for an ideal gas is given by (PV/RT) = 1, for real gas (PV/RT) is
not equal to 1 (PV/RT) = Z, for real gas is called the compressibility factor.
12. What is equation of state?
(A/M 2011)
The relation between the independent properties such as pressure, specific volume and
temperature of a pure substance is known as the equation of state.
13. What is partial pressure?
(N/D 2012)
The partial pressure of each constituent is that pressure which the gas would exert if it
occupies, and that volume is occupied by the mixtures at the same temperature.
14. State the assumptions made in kinetic theory of gases.
a. There are no intermolecular forces between the particles.
b. The volume of the molecules is negligible in comparison with the gas.
(N/D 2012)
15. What is meant by reduced properties?
(N/D 2013)
The ratios of pressure, temperature and specific volume of a real gas to the
corresponding critical values are called the reduced properties
UNIT 5
PSYCHROMETRY
PART A
1. What is the difference between air conditioning and refrigeration?
(A/M 2006)
Refrigeration is the process of providing and maintaining the temperature in space
below atmospheric temperature.
Air conditioning is the process of supplying sufficient volume of clean air containing
a specific amount of water vapour and maintaining the predetermine atmospheric
condition within a selected enclosure.
2. What is meant by adiabatic mixing?
(N/D 2006)
The process of mixing two or more stream of air without any heat transfer to the
surrounding is known as adiabatic mixing. It is happened in air conditioning system.
3. Define – Dry Bulb and Wet Bulb Temperature
(A/M 2007)
The temperature which is measured by an ordinary thermometer is known as dry bulb
temperature It is generally denoted by td.
It is the temperature of air measured by a thermometer when its bulb is covered with
wet cloth and exposed to a current rapidly moving air. It is denoted by tw.
4. Define – Relative Humidity and Specific Humidity
(A/M 2008)
Relative humidity is the ratio of the mass of water vapour (mv) in a certain volume of
moist air at a given temperature to the mass of water vapour (mvs) in the same volume of
saturated air at the some temperature.
i.e. RH (or)  = mv/mvs
Specific humidity (w) is the ratio of mass of water vapour (mv) to the mass of dry air
in the given volume of mixture is w =mv/ma.
5. Differentiate absolute humidity from relative humidity.
(N/D 2008)
Absolute humidity is defined as the ratio of the mass of water vapour (m v) in a certain
volume of moist air at a given temperature to the mass of water vapour (mv)s at atm
conditions.
Relative humidity is the ratio of the mass of water vapour (mv) in a certain volume of
moist air at a given temperature to the mass of water vapour (mv)s in the same volume of
saturated air at the same temperature.
RH (or)  = mv/mvs
6. Define – DPT and Degree of Saturation
(A/M 2009)
DPT (Dew point temperature) is the temperature to which moist cut is to be cooled
before it starts condensing.
Degree of saturation is the ratio of specific humidity of most air to the specific
humidity of saturated air temperature.
7.
Define – Apparatus Dew Point (ADP) of Cooling Coil
(A/M 2010)
For dehumidification, the cooling coil is to be kept at a mean temperature which is
below the dew point temperature (DPT) of the entering. This temperature of the coil is
called ADP temperature.
8. Write the psychometric processes:
(N/D 2010)
1) Sensible heating process
2) Sensible cooling process
3) Humidification process
4) Dehumidification process
5) Heating and humidification process
6) Cooling and dehumidification process
7) Adiabatic mixing airstreams process
8) Evaporative cooling process.
9. Define – Bypass Factor (BPF) of a Coil
(N/D 2011)
The ratio of the amount of air which does not contact the cooling coil (amount of
bypassing air) to the amount of supply air is called BPF.
BPF =
10. Define – Humidification Process
(A/M 2011)
Humidification is defined as the process of adding moisture at constant dry bulb
temperature so W2 > W1: but td1 = td2, so the humidity ratio increases from ø1 to ø2.
11. What is psychrometer?
(A/M 2012)
Psychrometer is an instrument which measures both dry bulb temperature and wet
bulb
temperature.
12. What is psychrometric chart?
(N/D 2012)
It is the graphical plot with specific humidity and partial pressure of water vapour in y
axis and dry bulb temperature along x axis. The specific volume of mixture, wet bulb
temperature, relative humidity and enthalpy are the properties that appear in the
psychrometric chart.
14. Define – Throttling Process
(N/D 2013)
When a fluid expands through a minute orifice or slightly opened valve, the process is
called throttling. During this process, pressure and velocity are reduced.
15. Define – Sensible Heat and Latent Heat
(N/D 2013)
Sensible heat is the heat that changes the temperature of the substance when added to
it or when abstracted from it.
Latent heat is the heat that does not affect the temperature but the change of state
occurred by adding the heat or by abstracting the heat.