ENGINEERING THERMO DYNAMICS (ME 2202) QUESTION BANK WITH ANSWERS ENGINEERING THERMO DYNAMICS (ME 2202) QUESTION BANK WITH ANSWERS
QUESTION BANK
ANNA UNIVERSITY QUESTION BANK FOR MECHANICAL ENGINEERING
QUESTION BANK
1. ENGINEERING THERMO DYNAMICS (ME 2202)
1. What do you understand by pure substance?
A pure substance is defined as one that is homogeneous and invariable in
chemical composition throughout its mass.
2. Define thermodynamic system.
A thermodynamic system is defined as a quantity of matter or a region in space,
on which the analysis of the problem is concentrated.
3. Name the different types of system.
Closed system (only energy transfer and no mass transfer)
Open system (Both energy and mass transfer)
Isolated system (No mass and energy transfer)
4. Define thermodynamic equilibrium.
If a system is in Mechanical, Thermal and Chemical Equilibrium then the system
is in thermodynamically equilibrium. (or)
If the system is isolated from its surrounding there will be no change in the
macroscopic property, then the system is said to exist in a state of thermodynamic equilibrium.
5. What do you mean by quasi-static process?
Equilibrium state
Quasi-static process
Infinite slowness is the characteristic feature of a quasi-static process. A quasistatic process is that a succession of equilibrium states. A quasi-static process is also called as reversible process.
6. Define Path function.
The work done by a process does not depend upon the end of the process. It
depends on the path of the system follows from state 1 to state 2. Hence work is
called a path function.
7. Define point function.
Thermodynamic properties are point functions. The change in a thermodynamic
property of a system is a change of state is independent of the path and depends
only on the initial and final states of the system.
8. Name and explain the two types of properties.
The two types of properties are intensive property and extensive property.
Intensive Property: It is independent of the mass of the system. Example:
pressure, temperature, specific volume, specific energy, density.
Extensive Property: It is dependent on the mass of the system. Example:
Volume, energy. If the mass is increased the values of the extensive properties
also increase.
9. Explain homogeneous and heterogeneous system.
The system consist of single phase is called homogeneous system and the system consist of more than one phase is called heterogeneous system.
10. What is a steady flow process?
Steady flow means that the rates of flow of mass and energy across the control
surface are constant.
11. Prove that for an isolated system, there is no change in internal energy.
In isolated system there is no interaction between the system and the
surroundings. There is no mass transfer and energy transfer. According to first
law of thermodynamics as dQ = dU + dW; dU = dQ – dW; dQ = 0, dW = 0,
There fore dU = 0 by integrating the above equation U = constant, therefore the
internal energy is constant for isolated system.
12. Indicate the practical application of steady flow energy equation.
1. Turbine, 2. Nozzle, 3. Condenser, 4. Compressor.
13. Define system.
It is defined as the quantity of the matter or a region in space upon which we
focus attention to study its property.
14. Define cycle.
It is defined as a series of state changes such that the final state is identical with
the initial state.
15. Show that work is a path function and not a property.
16. Explain Mechanical equilibrium.
If the forces are balanced between the system and surroundings are called
Mechanical equilibrium
17. Explain Chemical equilibrium.
If there is no chemical reaction or transfer of matter form one part of the system to another is called Chemical equilibrium
18. Explain Thermal equilibrium.
If the temperature difference between the system and surroundings is zero then it
is in Thermal equilibrium.
19. Define Zeroth law of Thermodynamics.
When two systems are separately in thermal equilibrium with a third system then
they themselves are in thermal equilibrium with each other.
20. What are the limitations of first law of thermodynamics?
a. According to first law of thermodynamics heat and work are mutually convertible during any cycle of a closed system.
b. But this law does notspecify the possible conditions under which the heat is converted intowork.
c. According to the first law of thermodynamics we can’t prove that it is impossible to transfer heat from lower temperature to higher temperature.
d. It does not give any information regarding change of state or whether theprocess is possible or not.
e. The I law does not specify the direction of heat and work.
21. What is perpetual motion machine of first kind?
It is defined as a machine, which produces work energy without consuming an
equivalent of energy from other source. It is impossible to obtain in actual
practice, because no machine can produce energy of its own without consuming
any other form of energy.
22. Define Clausius statement.
It is impossible for a self-acting machine working in a cyclic process, to transfer
heat from a body at lower temperature to a body at a higher temperature without
the aid of an external agency.
23. What is Perpetual motion machine of the second kind?
A heat engine, which converts whole of the heat energy into mechanical work is
known as Perpetual motion machine of the second kind.
24. Define Kelvin Planck Statement.
It is impossible to construct a heat engine to produce network in a complete cycle
if it exchanges heat from a single reservoir at single fixed temperature.
25. Define Heat pump.
A heat pump is a device, which is working in a cycle and transfers heat from
lower temperature to higher temperature.
26. Define Heat engine.
Heat engine is a machine, which is used to convert the heat energy into mechanical work in a cyclic process.
27. 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.
28. State Carnot theorem.
It states that no heat engine operating in a cycle between two constant temperature heat reservoir can be more efficient than a reversible engine operating betweenthe same reservoir.
29. What is meant by reversible process?
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.
30. What is meant by irreversible process?
The mixing of two substances and combustion also leads to irreversibility. All
spontaneous process is irreversible.
31. Explain entropy?
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.
32. What is absolute entropy?
The entropy measured for all perfect crystalline solids at absolute zero
temperature is known as absolute entropy.
33. Define availability.
The maximum useful work obtained during a process in which the final condition
of the system is the same as that of the surrounding is called availability of the
system.
34. Define available energy and unavailable energy.
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.
35. Explain the term source and sink.
Source is a thermal reservoir, which supplies heat to the system and sink is a
thermal reservoir, which takes the heat from the system.
36. What do you understand by the entropy principle?
The entropy of an isolated system can never decrease. It always increases and
remains constant only when the process is reversible. This is known as principle
of increase in entropy or entropy principle.
37. What are the important characteristics of entropy?
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.
d. The entropy increases if temperature of heat is lowered without work being done as in throttling process.
e. If the entropy is maximum, then there is a minimum availability for conversion in to work.
f. If the entropy is minimum then there is a maximum availability forconversion into work.
38. What is reversed carnot heat engine? What are the limitations of carnot cycle?
No friction is considered for moving parts of the engine.
There should not be any heat loss.
39. Why Rankine cycle is modified?
The work obtained at the end of the expansion is very less. The work is too
inadequate to overcome the friction. Therefore the adiabatic expansion is
terminated at the point before the end of the expansion in the turbine and pressure decreases suddenly, while the volume remains constant.
40. Name the various vapour power cycle.
a. Carnot cycle and
b. Rankine cycle.
41. Define efficiency ratio.
The ratio of actual cycle efficiency to that of the ideal cycle efficiency is termed as efficiency ratio.
42. Define overall efficiency.
It is the ratio of the mechanical work to the energy supplied in the fuel. It is also
defined as the product of combustion efficiency and the cycle efficiency.
43. Define specific steam consumption of an ideal Rankine cycle.
It is defined as the mass flow of steam required per unit power output.
44. Name the different components in steam power plant working on Rankine cycle. Boiler, Turbine, Cooling Tower or Condenser and Pump.
45. What are the effects of condenser pressure on the Rankine Cycle?
By lowering the condenser pressure, we can increase the cycle efficiency. The main disadvantage is lowering the back pressure in rease the wetness of steam. Isentropic compression of a very wet vapour is very difficult.
46. Mention the improvements made to increase the ideal efficiency of Rankine cycle.
1. Lowering the condenser pressure.
2. Superheated steam is supplied to the turbine.
3. Increasing the boiler pressure to certain limit.
4. Implementing reheat and regeneration in the cycle.
47. Why reheat cycle is not used for low boiler pressure?
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.
48. What are the disadvantages of reheating?
Reheating increases the condenser capacity due to increased dryness fraction,
increases the cost of the plant due to the reheats and its very long connections.
49. What are the advantages of reheat cycle?
1. It increases the turbine work.
2. It increases the heat supply.
3. It increases the efficiency of the plant.
4. It reduces the wear on the blade because of low moisture content in LP state of the turbine.
50. Define latent heat of evaporation or Enthalpy of evaporation.
The amount of heat added during heating of water up to dry steam from boiling point is known as Latent heat of evaporation or enthalpy of evaporation.
51. Explain the term super heated steam and super heating.
The dry steam is further heated its temperature raises, this process is called as superheating and the steam obtained is known as superheated steam.
52. Explain heat of super heat or super heat enthalpy.
The heat added to dry steam at 100oC to convert it into super heated steam at the temperature Tsup is called as heat of superheat or super heat enthalpy.
53. Explain the term critical point, critical temperature and critical pressure.
In the T-S diagram the region left of the waterline, the water exists as liquid. In
right of the dry steam line, the water exists as a super heated steam. In between
water and dry steam line the water exists as a wet steam. At a particular point, the water is directly converted into dry steam without formation of wet steam. The point is called critical point. The critical temperature is the temperature above which a substance cannot exist as a liquid, the critical temperature of water is 374.15oC. The corresponding pressure is called critical pressure.
54. Define dryness fraction (or) What is the quality of steam?
It is defined as the ratio of mass of the dry steam to the mass of the total steam.
55. Define enthalpy of steam.
It is the sum of heat added to water from freezing point to saturation temperature
and the heat absorbed during evaporation.
56. How do you determine the state of steam?
If V>vg then super heated steam, V= vg then dry steam and V< vg then wet steam.
57. Define triple point.
The triple point is merely the point of intersection of sublimation and vapourisation curves.
58. Define heat of vapourisation.
The amount of heat required to convert the liquid water completely into vapour
under this condition is called the heat of vapourisation.
59. Explain the terms, Degree of super heat, degree of sub-cooling.
The difference between the temperature of the superheated vapour and the
saturation temperature at the same pressure. The temperature between the saturation temperature and the temperature in the sub cooled region of liquid.
60. What is the purpose of reheating?
The purpose of reheating is to increase the dryness fraction of the steam passing
out of the later stages of the turbine.
61. Define Ideal gas.
It is defined as a gas having no forces of intermolecular attraction. These gases
will follow the gas laws at all ranges of pressures and temperatures.
62. Define Real gas.
It is defined, as a gas having the forces of attraction between molecules tends to
be very small at reduced pressures and elevated temperatures.
63. What is equation of state?
The relation between the independent properties such as pressure, specific volume and temperature for a pure substance is known as the equation of state.
64. State Boyle’s law.
It states that volume of a given mass of a perfect gas varies inversely as the
absolute pressure when temperature is constant.
65. State Charle’s law.
It states that if any gas is heated at constant pressure, its volume changes directly as its absolute temperature.
66. Explain the construction and give the use of generalized compressibility chart.
The general compressibility chart is plotted with Z versus Pr for various values of
Tr. This is constructed by plotting the known data of one of mole gases and can
be used for any gas. This chart gives best results for the regions well removed
from the critical state for all gases.
67. What do you mean by reduced properties?
The ratios of pressure, temperature and specific volume of a real gas to the
corresponding critical values are called the reduced properties.
68. Explain law of corresponding states.
If any two gases have equal values of reduced pressure and reduced temperature, then they have same values of reduced volume.
69. Explain Dalton’s law of partial pressure.
The pressure of a mixture of gases is equal to the sum of the partial pressures of
the constituents. The partial pressure of each constituent is that pressure which
the gas would expect if it occupied alone that volume occupied by the mixtures at
the same temperatures. m = mA+mB+mC+……. = _mi
mi = mass of the constituent.
P=PA+PB+PC+……. = _Pi, Pi – the partial pressure of a constituent.
70. State Avogardo’s Law.
The number of moles of any gas is proportional to the volume of gas at a given pressure and temperature.
71. What is Joule-Thomson coefficient?
The temperature behaviors of a fluid during a throttling (h=constant) process is described by the Joule-Thomson coefficient defined as
µ =[dT/dP]n
72. What is compressibility factor?
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.
73. What is partial pressure?
The partial pressure of each constituent is that pressure which the gas would exert if it occupied alone that volume occupied by the mixtures at the same
temperature.
74. Define Dalton’s law of partial pressure.
The total pressure exerted in a closed vessel containing a number of gases is equl to the sum of the pressures of each gas and the volume of each gas equal to the volume of the vessel.
75. How does the Vander Waal’s equation differ from the ideal gas equation of state?
The ideal gas equation pV=mRT has two important assumptions,
1. There is little or no attraction between the molecules of the gas.
2. That the volume occupied by the molecules themselves is negligibly small compared to the volume of the gas. This equation holds good for low pressure and high temperature ranges as the intermolecular attraction and the volume of the molecules are not of much significance.
As the pressure increases, the inter molecular forces of attraction and repulsion
increases and the volume of the molecules are not negligible. The real gas deviate considerably from the ideal gas equation
[p+(a/V2)](V-b) = RT
76. What is humidification and dehumidification?
The addition of water vapour into air is humidification and the removal of water
vapour from air is dehumidification.
77. Differentiate absolute humidity and relative humidity.
Absolute humidity is the mass of water vapour present in one kg of dry air.
Relative humidity is the ratio of the actual mass of water vapour present in one kg of dry air at the given temperature to the maximum mass of water vapour it can with hold at the same temperature. Absolute humidity is expressed in terms of kg/kg of dry air. Relative humidity is expressed in terms of percentage.
78. What is effective temperature?
The effective temperature is a measure of feeling warmth or cold to the human
body in response to the air temperature, moisture content and air motion. If the
air at different DBT and RH condition carries the same amount of heat as the heat carried by the air at temperature T and 100% RH, then the temperature T is
known as effective temperature.
79. Represent the following psychrometric process using skeleton psychrometric
chart? a) Cooling and dehumidification, b) Evaporative cooling.
80. Define Relative humidity.
It is defined as the ratio of partial pressure of water vapour (pw) in a mixture to the
saturation pressure (ps) of pure water at the same temperature of mixture.
81. Define specific humidity.
It is defined as the ratio of the mass of water vapour (ms) in a given volume to the
mass of dry air in a given volume (ma).
82. Define degree of saturation.
It is the ratio of the actual specific humidity and the saturated specific humidity at
the same temperature of the mixture.
83. What is dew point temperature?
The temperature at which the vapour starts condensing is called dew point
temperature. It is also equal to the saturation temperature at the partial pressure of water vapour in the mixture. The dew point temperature is an indication of
specific humidity.
84. What is meant by dry bulb temperature (DBT)?
The temperature recorded by the thermometer with a dry bulb. The dry bulb
thermometer cannot affected by the moisture present in the air. It is the measure
of sensible heat of the air.
85. What is meant by wet bulb temperature (WBT)?
It is the temperature recorded by a thermometer whose bulb is covered with
cotton wick (wet) saturated with water. The wet bulb temperature may be the
measure of enthalpy of air. WBT is the lowest temperature recorded by
moistened bulb.
86. Define dew point depression.
It is the difference between dry bulb temperature and dew point temperature of air vapour mixture.
87. What is meant by adiabatic saturation temperature (or) thermodynamic wet bulb temperature?
It is the temperature at which the outlet air can be brought into saturation state by
passing through the water in the long insulated duct (adiabatic) by the evaporation of water due to latent heat of vapourisation.
88. What is psychrometer?
Psychrometer is an instrument which measures both dry bulb temperature and wet bulb temperature.
89. What is psychrometric chart?
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 appeared
in the psychrometric chart.
90. Define sensible heat and latent heat.
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 change of state occurred by adding the heat or by abstracting the heat.
91. What are the important psychrometric process?
1.Sensible heating and sensible cooling, 2. Cooling and dehumidification, 3.Heating and humidification, 4. Mixing of air streams, 5. Chemical
dehumidification, 6. Adiabatic evaporative cooling.
92. What is meant by adiabatic mixing?
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.
93. What are the assumptions made in Van Der waal’s equation of state?
There is no inter molecular forces between particles. The volume of molecules is negligible in comparison with the gas.
94. Define coefficient of volume expansion.
The coefficient of volume expansion is defined as the change in volume with the change in temperature per unit volume keeping the pressure constant. It is denoted by b.
95. State Helmholtz function.
Helmholtz function is the property of a system and is given by subtracting the product of absolute temperature (T) and entropy (S) from the internal energy (U).
Helmholtz function = U – TS
96. What are thermodynamic properties?
Thermodynamic properties are pressure (p), temperature (T), volume (V), internal energy (U), enthalpy(H), entropy (S), Helmholtz function (a) and Gibbs function
97. Define throttling process.
When a fluid expands through a minute orifice or slightly opened valve, the
process is called as throttling process. During this process, pressure and velocity
are reduced.
98. Define Molecular mass.
Molecular mass is defined as the ratio between total mass of the mixture to the total number of moles available in the mixture.
99. Define isothermal compressibility.
Isothermal compressibility is defined as the change in volume with change in pressure per unit volume keeping the temperature constant.
100. Define psychrometry.
The science which deals with the study of behaviour of moist air (mixture of dry
air and water vapour) is known as psychrometry.
1. A system receives 42 kJ of heat while expanding with volume change of 0.123 m3 against an atmosphere of
12 N/cm2. A mass of 80 kg in the surroundings is also lifted through a distance of 6 metres.
(i) Find the change in energy of the system.
(ii) The system is returned to its initial volume by an adiabatic process which requires 100 kJ of work. Find the change in energy of system.
(iii) Determine the total change in energy of the system.
[Ans. (i) 22.54 kJ, (ii) 100 kJ, (iii) 122.54 kJ]
2. A tank contains 2.26 m3 of air at a pressure of 24.12 bar. If air is cooled until its pressure and temperature becomes 13.78 bar and 21.1°C respectively. Determine the decrease of internal energy.
[– 5857.36 kJ]
3. Determine work done by fluid in the thermodynamic cycle comprising of following processes :
(a) Unit mass of fluid at 20 atm and 0.04 m3 is expanded by the law PV1.5 = constant, till volume gets doubled.
(b) Fluid is cooled isobarically to its original volume.
(c) Heat is added to fluid till its pressure reaches to its original pressure, isochorically.
[18.8 kJ]
4. Air at 8 bar, 100°C flows in a duct of 15 cm diameter at rate of 150 kg/min. It is then throttled by a valve upto 4 bar pressure. Determine the velocity of air after throttling and also show that enthalpy remains constant before and after throttling.
[37.8 m/s]
5. Determine the power required by a compressor designed to compress atmospheric air (at 1 bar, 20°C)
to 10 bar pressure. Air enters compressor through inlet area of 90cm2 with velocity of 50 m/s and leaves with velocity of 120 m/s from exit area of 5 cm2. Consider heat losses to environment to be 10% of power input to compressor. [50.4 kW]
6. A frictionless piston is free to move in a closed cylinder. Initially there is 0.035 m3 of oxygen at 4.5 bar, 60°C on one side of the piston and 0.07 m3 of methane at 4.5 bar and – 12°C on the other side. The cylinder walls and piston may be regarded as perfect thermal insulators but the oxygen may be heated electrically. Heating takes place so that the volume of oxygen doubles. Find :
(i) Final state condition ; (ii) Work done by the piston ;
(ii) Heat transferred to oxygen.
Treat both gases as perfect and take :
For oxygen cp = 0.88 kJ/kg K, R = 0.24 kJ/kg K
For methane cp = 1.92 kJ/kg K, R = 0.496 kJ/kg K.
7. The specific heat at constant pressure of one kg fluid undergoing a non-flow constant pressure process is
given by
where T is in °C.
The pressure during the process is maintained at 2 bar and volume changes from 1 m3 to 1.8 m3
and temperature changes from 50°C to 450°C.
Determine :
(i) Heat added (ii) Work done
(iii) Change in internal energy (iv) Change in enthalpy.
8. The resistance of the winding in a certain motor is found to be 75 ohms at room (25oC). When operating at full load under steady state conditions, the motor is switched off and the resistance of the winding s is immediately measured again, and found to be 90 ohms. The windings are made of copper whose resistance at temperature toC id given by Rt = Ro [1+0.00393t] where R0 is the resistance at 00C. Find the temperature of the coil during full load
9. A certain working fluid undergoes a process in such a way that pressure and volume are related as , Where p is in kPa and V is in m3 . During the process the volume changes from 0.15 m3 to 0.1 m3. Determine the work done in the process.
10. A gas occupies 0.3m3 at 2 bar. It undergoes a cycle consisting of the following processes a)1-2 constant pressure process with work interaction of 15kJ b)2-3 constant temperature process and E3=E2 c) 3-1 constant volume and change in internal energy E1-E3 is-40kJ. Determine network and net heat transfer for the cycle.
11. A piston-cylinder assembly contains 1kg or nitrogen at 100 kPa. The initial volume is 0.5 m3. Heat is transferred to the substance in an amount necessary to cause a slow expansion at constant temperature. This process is terminated when the final volume is twice the initial volume. Estimate the amount of heat transferred.[Molecular wt. 28 & Cv=0.773 kJ/kgK]
12. 0.115m3 of gas has a pressure of 138 kN /m2. It is compressed to 680 kN /m2according to the law PV1.4 =C, Determine the volume of the gas, work done and heat transferred.
13. The gas is compressed from initial state of 0.35 m3 and 105 kPa to a final state of 0.14 m3 and to the same pressure .Determine the change in internal energy of the gas which transfers 38 kJ of heat.
14. A piston and cylinder device contains 1 kg of air, Initially, v = 0.8 m3/kg and
T = 298 K. The air is compressed in a slow frictionless process to a specific volume of 0.2 m3/kg and a temperature of 580 K according to the equation pV1.3 = 0.75 ( p in bar, v in m3/kg). If Cv of air is 0.78 kJ/kg determine :
work and
heat transfer (both in kJ)
15. A piston-cylinder assembly contains 1kg or nitrogen at 100 kPa. The initial volume is 0.5 m3. Heat is transferred to the substance in an amount necessary to cause a slow expansion at constant temperature. This process is terminated when the final volume is twice the initial volume. Estimate the amount of heat transferred.[Molecular wt. 28 & Cv=0.773 kJ/kgK](16)
16. A closed system undergoes a cycle consisting of three process 1-2, 2-3 and 3-1. Given that Q12 = 30 kJ, Q23 = 10 kJ, 1w2 = 5 kJ, 3w2 = 5 kJ and DE31 = 15 kJ, determine Q31, w23, DE12 and DE23.
17. The following cycle involves 3 kg of air : Polytropic compression from 1 to 2 where P1 = 150 kPa, T1 = 360 K, P2 = 750 kPa and n = 1.1 ; constant-pressure cooling from 2 to 3; and constant - temperature heating from 3 to 1. Draw the pV diagram and find temperatures, pressures and volumes at each state and determine the net work and heat.
18. A reversible engine is operating between 1000 K and 300 K. Find the efficiency of the engine. Also find the power developed, in kW if the amount of heat received from high temperature reservoir is 500 kJ per min.
19. A refrigerator operating on reversible cycle is pumping heat from -5oC to the atmosphere at 40oC. If it has to pump 24000 kJ/hr, find the power required to run the refrigerator.
20. Calculate the power developed and diameter of the inlet pipe, if air enters at 5 kg/s, 50m/s with an enthalpy of 0.9 MJ/kg and leaves at 150 m/s with an enthalpy of 0.4 MJ/kg. The heat loss to the surroundings is 0.025MJ/kg. Assume 100kPa and 300 K at the inlet
21. In an isentropic flow through nozzle, air flow at the rate of 600 kg/hr. At inlet to
the nozzle, pressure is 2MPa and temperature is 127oC. The exit pressure is 0.5
MPa. Initial air velocity is 300 m/s determine exit velocity of air, inlet and exit
area of nozzle.
22. Two heat engines operating on carnot cycle are arranged in series. The engine A
receives heat at 1000 K and rejects heat at a constant temperature T2. The engine
B receives heat rejected by A, and in turn rejects heat to a reservoir at 300 K.
Calculate the T2 when a) the efficiency of the two engines are equal b) the work
output of the two engines are equal.
23. A carnot engine receives 90kJ from a reservoir at 900 K. It rejects heat to he
environment at 300 K. One-fifth of its work output is used to drive a carnot
refrigerator. The refrigerator rejects 60 kJ to the environment at 300 K. Find work
output and efficiency of the engine, the temperature of the sink for the refrigerator in
degree celsius.
24. Three reversible engines of Carnot type are operating in series between the limiting temperatures of 1100 K and 300 K. Determine the intermediate temperatures if the work output from engines is in proportion of 3 : 2 : 1.
25. In a compressor the air enters at 27°C and 1 atm and leaves at 227°C and 1 MPa. Determine the work done per unit mass of air assuming velocities at entry and exit to be negligible. Also determine the additional work required, if velocities are 10 m/s and 50 m/s at inlet and exit respectively.
26. In a gas turbine installation air is heated inside heat exchanger upto 750°C from ambient temperature of 27°C. Hot air then enters into gas turbine with the velocity of 50 m/s and leaves at 600°C. Air leaving turbine enters a nozzle at 60 m/s velocity and leaves nozzle at temperature of 500°C. For unit mass flow rate of air determine the following assuming adiabatic expansion in turbine and nozzle,
(i) heat transfer to air in heat exchanger
(ii) power output from turbine
(iii) velocity at exit of nozzle.
Take cp for air as 1.005 kJ/kg°K.
27. A refrigerator operates on reversed Carnot cycle. Determine the power required to drive refrigerator between temperatures of 42ºC and 4ºC if heat at the rate of 2 kJ/s is extracted from the low temperature region.
28. A piston and cylinder machine containing a fluid system has a stirring device as shown in Fig. 1.1 The piston is frictionless, and it is held down against the fluid due to atmospheric pressure of 101.3 kPa. The stirring device is turned 9500 revolutions with an average torque against the fluid of 1.25 Nm. Meanwhile the piston of 0.65 m diameter moves out 0.6 m. Find the net work transfer for the system.
Fig. 1.1
29. At the inlet to a certain nozzle the enthalpy of fluid passing is 2800 kJ/kg, and the velocity is 50 m/s. At the discharge end the enthalpy is 2600 kJ/kg. The nozzle is horizontal and there is negligible heat loss from it.
a. Find the velocity at exit of the nozzle.
b. If the inlet area is 900 cm2 and the specific volume at inlet is 0.187 m3/kg, find the mass flow rate.
c. If the specific volume at the nozzle exit is 0.498 m3/kg, find the exit area of nozzle. [8]
30. A fluid system, contained in a piston and cylinder machine, passes through a complete cycle of four processes. The sum of all heat transferred during a cycle is – 340 kJ. The system completes 200 cycles per min. Complete the following table showing the method for each item, and compute the net rate of work output in kW. [8]
Process Q (kJ/min) W (kJ/min) ΔE (kJ/min)
1—2 0 4340 —
2—3 42000 0 —
3—4 –4200 — – 73200
4—1 — — ___
31. During flight, the air speed of a turbojet engine is 250 m/s. Ambient air temperature is – 14°C. Gas temperature at outlet of nozzle is 610°C. Corresponding enthalpy values for air and gas are respectively 250 and 900 kJ/kg. Fuel air ratio is 0.0180. Chemical energy of fuel is 45 MJ/kg. Owing to incomplete combustion 6% of chemical energy is not released in the reaction. Heat loss from the engine is 21 kJ/kg of air. Calculate the velocity of the exhaust jet. [8]
32. The connections of a reversible engine to three sources at 500 K, 400 K and 300 K are shown in Fig. 1.3. It draws 1500 kJ/min of energy from the source at 500 K and develops 200 kJ/min of work.
a. Determine the heat interactions with the other two sources of heat.
b. Evaluate the entropy change due to each heat interaction with the engine.
c. Total entropy change during the cycle. [8]
33. 3 kg of water at 80°C is mixed with 4 kg of water at 15°C in an isolated system. Calculate the change of entropy due to mixing process.
34. Two tanks A and B contain 1 kg of air at 1 bar, 50ºC and 3 bar, 50ºC when atmosphere is at 1 bar,15ºC. Identify the tank in which stored energy is more. Also find the availability of air in each tank.
35. A cold storage plant of 40 tonnes of refrigeration capacity runs with its performance just
1/4 th of itsCarnot COP. Inside temperature is –15ºC and atmospheric temperature is 35ºC. Determine the powerrequired to run the plant. [Take : One ton of refrigeration as 3.52 kW]
36. A vessel of volume 0.2 m3 has 2 kg of water at 200 o C find p, h and x
37. In a power station, the saturated steam is generated at 200ºC by transferring the heat from hot gases in a steam boiler. Find the increase in total entropy of the combined system of gas and water and increase in unavailable energy due to irreversible heat transfer. The gases are cooled from 1000°C to 500°C and all the heat from gases goes to water. Assume water enters the boiler at saturated condition and leaves as saturated steam
38. One kg of air is compressed polytropically from 1 bar pressure and temperature of 300 K to a pressure of 6.8 bar and temperature of 370 K. Determine the irreversibility if the sink temperature is 293 K. Assume R = 0.287 kJ/kg K, cp = 1.004 kJ/kg K and cv = 0.716 kJ/kg K.
39. water is heated from 40oC in a piston cylinder arrangement until it becomes s aturated
vapour at a constant pressure of 50 bar. Find the heat to added per kg.
40. A steam power plant works between 40 bar and 0.05 bar. If the steam supplied is dry saturated and the cycle of operation is Rankine, find : (i) Cycle efficiency (ii) Specific steam consumption.
41. In a regenerative cycle the inlet conditions are 40 bar and 400°C. Steam is bled at 10 bar in Regenerative heating. The exit pressure is 0.8 bar. Neglecting pump work determine the efficiency of the cycle.
42. A steam power plant operates on ideal Rankine cycle using reheater and regenerative feed water heaters. It has one open feed heater. Steam is supplied at 150 bar and 600°C.The condenser pressure is 0.1 bar. Some steam is extracted from the turbine at 40 bar for closed feed water heater and remaining steam is reduced at 40 bar to 600°C. Extracted steam is completely condensed in this closed feed water heater and is pumped to 150 bar before mixing with the feed water heater. Steam for the open feed water heater is bled from L.P. turbine at 5 bar. Determine :
(i) Fraction of steam extracted from the turbines at each bled heater, and
(ii) Thermal efficiency of the system.
Draw the line diagram of the components and represent the cycle on T-s diagram.
43. The atmospheric conditions are 30ºC and specific humidity of 0.0215 kg/kg of air. Determine :
(i) Partial pressure of air
(ii) Relative humidity
(iii) Dew point temperature.
Atmospheric pressure = 756 mm Hg.
44. 1 kg of air at 24ºC and a relative humidity of 70% is to be mixed adiabatically in a steady state, steady flow device with 1 kg of air at 16ºC and a relative humidity of 10%. Assuming that the mixing is to be carried out at a constant pressure of 1.0 atm, determine the temperature and relative humidity of the stream leaving the device.
45. In a laboratory test, a psychrometer recorded 36ºC DBT and 30ºC WBT. Calculate :
(i) Vapour pressure
(ii) Relative humidity
(iii) Specific humidity
(iv) Degree of saturation
(v) Dew point temperature
(vi) Enthalpy of the mixture.
46. An air-water vapour mixture enters an adiabatic saturator at 30ºC and leaves at 20ºC, which is the adiabatic saturation temperature. The pressure remains constant at 1 bar. Determine the relative humidity and the humidity ratio of the inlet mixture.
47. The following observation were made during a testing of moist air
Dry bulb temperature Tdb = 29oC
Dew point temperature Tdb = 15oC
Total pressure = 1 bar
Determine the relative humidity and degree of saturation
48 In a combined heating and humidification process, moist air enters heating coil with the dry bulb temperature 20oC and 30% R.M. After the process the drug bulb temperature and the RH of Air were found to be 40oC and 55% RH. The air passes through the heating at the rate of 350 kg/min
49 In an air conditioning plant, fresh air is used to mix up with returned air from conditioned space. The fresh air drawn from atmosphere has the drug bulb temperature 32oC and we bulb temperature 25oC. The fresh air is drawn at the rate of 100m3/min. The returned air from conditioned space has the drug bulb temperature 23oC and relative humidity 50%. The volume flow rate of it 540 m3/min. Determine the (i) dry bulb and web bulb temperature (ii) specific humidity of mixture
50 Find the value of co-efficient of volume expansion β and isothermal compressibility K for a Van der Waals’ gas obeying
QUESTION BANK
ANNA UNIVERSITY QUESTION BANK FOR MECHANICAL ENGINEERING
QUESTION BANK
1. ENGINEERING THERMO DYNAMICS (ME 2202)
1. What do you understand by pure substance?
A pure substance is defined as one that is homogeneous and invariable in
chemical composition throughout its mass.
2. Define thermodynamic system.
A thermodynamic system is defined as a quantity of matter or a region in space,
on which the analysis of the problem is concentrated.
3. Name the different types of system.
Closed system (only energy transfer and no mass transfer)
Open system (Both energy and mass transfer)
Isolated system (No mass and energy transfer)
4. Define thermodynamic equilibrium.
If a system is in Mechanical, Thermal and Chemical Equilibrium then the system
is in thermodynamically equilibrium. (or)
If the system is isolated from its surrounding there will be no change in the
macroscopic property, then the system is said to exist in a state of thermodynamic equilibrium.
5. What do you mean by quasi-static process?
Equilibrium state
Quasi-static process
Infinite slowness is the characteristic feature of a quasi-static process. A quasistatic process is that a succession of equilibrium states. A quasi-static process is also called as reversible process.
6. Define Path function.
The work done by a process does not depend upon the end of the process. It
depends on the path of the system follows from state 1 to state 2. Hence work is
called a path function.
7. Define point function.
Thermodynamic properties are point functions. The change in a thermodynamic
property of a system is a change of state is independent of the path and depends
only on the initial and final states of the system.
8. Name and explain the two types of properties.
The two types of properties are intensive property and extensive property.
Intensive Property: It is independent of the mass of the system. Example:
pressure, temperature, specific volume, specific energy, density.
Extensive Property: It is dependent on the mass of the system. Example:
Volume, energy. If the mass is increased the values of the extensive properties
also increase.
9. Explain homogeneous and heterogeneous system.
The system consist of single phase is called homogeneous system and the system consist of more than one phase is called heterogeneous system.
10. What is a steady flow process?
Steady flow means that the rates of flow of mass and energy across the control
surface are constant.
11. Prove that for an isolated system, there is no change in internal energy.
In isolated system there is no interaction between the system and the
surroundings. There is no mass transfer and energy transfer. According to first
law of thermodynamics as dQ = dU + dW; dU = dQ – dW; dQ = 0, dW = 0,
There fore dU = 0 by integrating the above equation U = constant, therefore the
internal energy is constant for isolated system.
12. Indicate the practical application of steady flow energy equation.
1. Turbine, 2. Nozzle, 3. Condenser, 4. Compressor.
13. Define system.
It is defined as the quantity of the matter or a region in space upon which we
focus attention to study its property.
14. Define cycle.
It is defined as a series of state changes such that the final state is identical with
the initial state.
15. Show that work is a path function and not a property.
16. Explain Mechanical equilibrium.
If the forces are balanced between the system and surroundings are called
Mechanical equilibrium
17. Explain Chemical equilibrium.
If there is no chemical reaction or transfer of matter form one part of the system to another is called Chemical equilibrium
18. Explain Thermal equilibrium.
If the temperature difference between the system and surroundings is zero then it
is in Thermal equilibrium.
19. Define Zeroth law of Thermodynamics.
When two systems are separately in thermal equilibrium with a third system then
they themselves are in thermal equilibrium with each other.
20. What are the limitations of first law of thermodynamics?
a. According to first law of thermodynamics heat and work are mutually convertible during any cycle of a closed system.
b. But this law does notspecify the possible conditions under which the heat is converted intowork.
c. According to the first law of thermodynamics we can’t prove that it is impossible to transfer heat from lower temperature to higher temperature.
d. It does not give any information regarding change of state or whether theprocess is possible or not.
e. The I law does not specify the direction of heat and work.
21. What is perpetual motion machine of first kind?
It is defined as a machine, which produces work energy without consuming an
equivalent of energy from other source. It is impossible to obtain in actual
practice, because no machine can produce energy of its own without consuming
any other form of energy.
22. Define Clausius statement.
It is impossible for a self-acting machine working in a cyclic process, to transfer
heat from a body at lower temperature to a body at a higher temperature without
the aid of an external agency.
23. What is Perpetual motion machine of the second kind?
A heat engine, which converts whole of the heat energy into mechanical work is
known as Perpetual motion machine of the second kind.
24. Define Kelvin Planck Statement.
It is impossible to construct a heat engine to produce network in a complete cycle
if it exchanges heat from a single reservoir at single fixed temperature.
25. Define Heat pump.
A heat pump is a device, which is working in a cycle and transfers heat from
lower temperature to higher temperature.
26. Define Heat engine.
Heat engine is a machine, which is used to convert the heat energy into mechanical work in a cyclic process.
27. 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.
28. State Carnot theorem.
It states that no heat engine operating in a cycle between two constant temperature heat reservoir can be more efficient than a reversible engine operating betweenthe same reservoir.
29. What is meant by reversible process?
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.
30. What is meant by irreversible process?
The mixing of two substances and combustion also leads to irreversibility. All
spontaneous process is irreversible.
31. Explain entropy?
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.
32. What is absolute entropy?
The entropy measured for all perfect crystalline solids at absolute zero
temperature is known as absolute entropy.
33. Define availability.
The maximum useful work obtained during a process in which the final condition
of the system is the same as that of the surrounding is called availability of the
system.
34. Define available energy and unavailable energy.
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.
35. Explain the term source and sink.
Source is a thermal reservoir, which supplies heat to the system and sink is a
thermal reservoir, which takes the heat from the system.
36. What do you understand by the entropy principle?
The entropy of an isolated system can never decrease. It always increases and
remains constant only when the process is reversible. This is known as principle
of increase in entropy or entropy principle.
37. What are the important characteristics of entropy?
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.
d. The entropy increases if temperature of heat is lowered without work being done as in throttling process.
e. If the entropy is maximum, then there is a minimum availability for conversion in to work.
f. If the entropy is minimum then there is a maximum availability forconversion into work.
38. What is reversed carnot heat engine? What are the limitations of carnot cycle?
No friction is considered for moving parts of the engine.
There should not be any heat loss.
39. Why Rankine cycle is modified?
The work obtained at the end of the expansion is very less. The work is too
inadequate to overcome the friction. Therefore the adiabatic expansion is
terminated at the point before the end of the expansion in the turbine and pressure decreases suddenly, while the volume remains constant.
40. Name the various vapour power cycle.
a. Carnot cycle and
b. Rankine cycle.
41. Define efficiency ratio.
The ratio of actual cycle efficiency to that of the ideal cycle efficiency is termed as efficiency ratio.
42. Define overall efficiency.
It is the ratio of the mechanical work to the energy supplied in the fuel. It is also
defined as the product of combustion efficiency and the cycle efficiency.
43. Define specific steam consumption of an ideal Rankine cycle.
It is defined as the mass flow of steam required per unit power output.
44. Name the different components in steam power plant working on Rankine cycle. Boiler, Turbine, Cooling Tower or Condenser and Pump.
45. What are the effects of condenser pressure on the Rankine Cycle?
By lowering the condenser pressure, we can increase the cycle efficiency. The main disadvantage is lowering the back pressure in rease the wetness of steam. Isentropic compression of a very wet vapour is very difficult.
46. Mention the improvements made to increase the ideal efficiency of Rankine cycle.
1. Lowering the condenser pressure.
2. Superheated steam is supplied to the turbine.
3. Increasing the boiler pressure to certain limit.
4. Implementing reheat and regeneration in the cycle.
47. Why reheat cycle is not used for low boiler pressure?
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.
48. What are the disadvantages of reheating?
Reheating increases the condenser capacity due to increased dryness fraction,
increases the cost of the plant due to the reheats and its very long connections.
49. What are the advantages of reheat cycle?
1. It increases the turbine work.
2. It increases the heat supply.
3. It increases the efficiency of the plant.
4. It reduces the wear on the blade because of low moisture content in LP state of the turbine.
50. Define latent heat of evaporation or Enthalpy of evaporation.
The amount of heat added during heating of water up to dry steam from boiling point is known as Latent heat of evaporation or enthalpy of evaporation.
51. Explain the term super heated steam and super heating.
The dry steam is further heated its temperature raises, this process is called as superheating and the steam obtained is known as superheated steam.
52. Explain heat of super heat or super heat enthalpy.
The heat added to dry steam at 100oC to convert it into super heated steam at the temperature Tsup is called as heat of superheat or super heat enthalpy.
53. Explain the term critical point, critical temperature and critical pressure.
In the T-S diagram the region left of the waterline, the water exists as liquid. In
right of the dry steam line, the water exists as a super heated steam. In between
water and dry steam line the water exists as a wet steam. At a particular point, the water is directly converted into dry steam without formation of wet steam. The point is called critical point. The critical temperature is the temperature above which a substance cannot exist as a liquid, the critical temperature of water is 374.15oC. The corresponding pressure is called critical pressure.
54. Define dryness fraction (or) What is the quality of steam?
It is defined as the ratio of mass of the dry steam to the mass of the total steam.
55. Define enthalpy of steam.
It is the sum of heat added to water from freezing point to saturation temperature
and the heat absorbed during evaporation.
56. How do you determine the state of steam?
If V>vg then super heated steam, V= vg then dry steam and V< vg then wet steam.
57. Define triple point.
The triple point is merely the point of intersection of sublimation and vapourisation curves.
58. Define heat of vapourisation.
The amount of heat required to convert the liquid water completely into vapour
under this condition is called the heat of vapourisation.
59. Explain the terms, Degree of super heat, degree of sub-cooling.
The difference between the temperature of the superheated vapour and the
saturation temperature at the same pressure. The temperature between the saturation temperature and the temperature in the sub cooled region of liquid.
60. What is the purpose of reheating?
The purpose of reheating is to increase the dryness fraction of the steam passing
out of the later stages of the turbine.
61. Define Ideal gas.
It is defined as a gas having no forces of intermolecular attraction. These gases
will follow the gas laws at all ranges of pressures and temperatures.
62. Define Real gas.
It is defined, as a gas having the forces of attraction between molecules tends to
be very small at reduced pressures and elevated temperatures.
63. What is equation of state?
The relation between the independent properties such as pressure, specific volume and temperature for a pure substance is known as the equation of state.
64. State Boyle’s law.
It states that volume of a given mass of a perfect gas varies inversely as the
absolute pressure when temperature is constant.
65. State Charle’s law.
It states that if any gas is heated at constant pressure, its volume changes directly as its absolute temperature.
66. Explain the construction and give the use of generalized compressibility chart.
The general compressibility chart is plotted with Z versus Pr for various values of
Tr. This is constructed by plotting the known data of one of mole gases and can
be used for any gas. This chart gives best results for the regions well removed
from the critical state for all gases.
67. What do you mean by reduced properties?
The ratios of pressure, temperature and specific volume of a real gas to the
corresponding critical values are called the reduced properties.
68. Explain law of corresponding states.
If any two gases have equal values of reduced pressure and reduced temperature, then they have same values of reduced volume.
69. Explain Dalton’s law of partial pressure.
The pressure of a mixture of gases is equal to the sum of the partial pressures of
the constituents. The partial pressure of each constituent is that pressure which
the gas would expect if it occupied alone that volume occupied by the mixtures at
the same temperatures. m = mA+mB+mC+……. = _mi
mi = mass of the constituent.
P=PA+PB+PC+……. = _Pi, Pi – the partial pressure of a constituent.
70. State Avogardo’s Law.
The number of moles of any gas is proportional to the volume of gas at a given pressure and temperature.
71. What is Joule-Thomson coefficient?
The temperature behaviors of a fluid during a throttling (h=constant) process is described by the Joule-Thomson coefficient defined as
µ =[dT/dP]n
72. What is compressibility factor?
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.
73. What is partial pressure?
The partial pressure of each constituent is that pressure which the gas would exert if it occupied alone that volume occupied by the mixtures at the same
temperature.
74. Define Dalton’s law of partial pressure.
The total pressure exerted in a closed vessel containing a number of gases is equl to the sum of the pressures of each gas and the volume of each gas equal to the volume of the vessel.
75. How does the Vander Waal’s equation differ from the ideal gas equation of state?
The ideal gas equation pV=mRT has two important assumptions,
1. There is little or no attraction between the molecules of the gas.
2. That the volume occupied by the molecules themselves is negligibly small compared to the volume of the gas. This equation holds good for low pressure and high temperature ranges as the intermolecular attraction and the volume of the molecules are not of much significance.
As the pressure increases, the inter molecular forces of attraction and repulsion
increases and the volume of the molecules are not negligible. The real gas deviate considerably from the ideal gas equation
[p+(a/V2)](V-b) = RT
76. What is humidification and dehumidification?
The addition of water vapour into air is humidification and the removal of water
vapour from air is dehumidification.
77. Differentiate absolute humidity and relative humidity.
Absolute humidity is the mass of water vapour present in one kg of dry air.
Relative humidity is the ratio of the actual mass of water vapour present in one kg of dry air at the given temperature to the maximum mass of water vapour it can with hold at the same temperature. Absolute humidity is expressed in terms of kg/kg of dry air. Relative humidity is expressed in terms of percentage.
78. What is effective temperature?
The effective temperature is a measure of feeling warmth or cold to the human
body in response to the air temperature, moisture content and air motion. If the
air at different DBT and RH condition carries the same amount of heat as the heat carried by the air at temperature T and 100% RH, then the temperature T is
known as effective temperature.
79. Represent the following psychrometric process using skeleton psychrometric
chart? a) Cooling and dehumidification, b) Evaporative cooling.
80. Define Relative humidity.
It is defined as the ratio of partial pressure of water vapour (pw) in a mixture to the
saturation pressure (ps) of pure water at the same temperature of mixture.
81. Define specific humidity.
It is defined as the ratio of the mass of water vapour (ms) in a given volume to the
mass of dry air in a given volume (ma).
82. Define degree of saturation.
It is the ratio of the actual specific humidity and the saturated specific humidity at
the same temperature of the mixture.
83. What is dew point temperature?
The temperature at which the vapour starts condensing is called dew point
temperature. It is also equal to the saturation temperature at the partial pressure of water vapour in the mixture. The dew point temperature is an indication of
specific humidity.
84. What is meant by dry bulb temperature (DBT)?
The temperature recorded by the thermometer with a dry bulb. The dry bulb
thermometer cannot affected by the moisture present in the air. It is the measure
of sensible heat of the air.
85. What is meant by wet bulb temperature (WBT)?
It is the temperature recorded by a thermometer whose bulb is covered with
cotton wick (wet) saturated with water. The wet bulb temperature may be the
measure of enthalpy of air. WBT is the lowest temperature recorded by
moistened bulb.
86. Define dew point depression.
It is the difference between dry bulb temperature and dew point temperature of air vapour mixture.
87. What is meant by adiabatic saturation temperature (or) thermodynamic wet bulb temperature?
It is the temperature at which the outlet air can be brought into saturation state by
passing through the water in the long insulated duct (adiabatic) by the evaporation of water due to latent heat of vapourisation.
88. What is psychrometer?
Psychrometer is an instrument which measures both dry bulb temperature and wet bulb temperature.
89. What is psychrometric chart?
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 appeared
in the psychrometric chart.
90. Define sensible heat and latent heat.
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 change of state occurred by adding the heat or by abstracting the heat.
91. What are the important psychrometric process?
1.Sensible heating and sensible cooling, 2. Cooling and dehumidification, 3.Heating and humidification, 4. Mixing of air streams, 5. Chemical
dehumidification, 6. Adiabatic evaporative cooling.
92. What is meant by adiabatic mixing?
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.
93. What are the assumptions made in Van Der waal’s equation of state?
There is no inter molecular forces between particles. The volume of molecules is negligible in comparison with the gas.
94. Define coefficient of volume expansion.
The coefficient of volume expansion is defined as the change in volume with the change in temperature per unit volume keeping the pressure constant. It is denoted by b.
95. State Helmholtz function.
Helmholtz function is the property of a system and is given by subtracting the product of absolute temperature (T) and entropy (S) from the internal energy (U).
Helmholtz function = U – TS
96. What are thermodynamic properties?
Thermodynamic properties are pressure (p), temperature (T), volume (V), internal energy (U), enthalpy(H), entropy (S), Helmholtz function (a) and Gibbs function
97. Define throttling process.
When a fluid expands through a minute orifice or slightly opened valve, the
process is called as throttling process. During this process, pressure and velocity
are reduced.
98. Define Molecular mass.
Molecular mass is defined as the ratio between total mass of the mixture to the total number of moles available in the mixture.
99. Define isothermal compressibility.
Isothermal compressibility is defined as the change in volume with change in pressure per unit volume keeping the temperature constant.
100. Define psychrometry.
The science which deals with the study of behaviour of moist air (mixture of dry
air and water vapour) is known as psychrometry.
1. A system receives 42 kJ of heat while expanding with volume change of 0.123 m3 against an atmosphere of
12 N/cm2. A mass of 80 kg in the surroundings is also lifted through a distance of 6 metres.
(i) Find the change in energy of the system.
(ii) The system is returned to its initial volume by an adiabatic process which requires 100 kJ of work. Find the change in energy of system.
(iii) Determine the total change in energy of the system.
[Ans. (i) 22.54 kJ, (ii) 100 kJ, (iii) 122.54 kJ]
2. A tank contains 2.26 m3 of air at a pressure of 24.12 bar. If air is cooled until its pressure and temperature becomes 13.78 bar and 21.1°C respectively. Determine the decrease of internal energy.
[– 5857.36 kJ]
3. Determine work done by fluid in the thermodynamic cycle comprising of following processes :
(a) Unit mass of fluid at 20 atm and 0.04 m3 is expanded by the law PV1.5 = constant, till volume gets doubled.
(b) Fluid is cooled isobarically to its original volume.
(c) Heat is added to fluid till its pressure reaches to its original pressure, isochorically.
[18.8 kJ]
4. Air at 8 bar, 100°C flows in a duct of 15 cm diameter at rate of 150 kg/min. It is then throttled by a valve upto 4 bar pressure. Determine the velocity of air after throttling and also show that enthalpy remains constant before and after throttling.
[37.8 m/s]
5. Determine the power required by a compressor designed to compress atmospheric air (at 1 bar, 20°C)
to 10 bar pressure. Air enters compressor through inlet area of 90cm2 with velocity of 50 m/s and leaves with velocity of 120 m/s from exit area of 5 cm2. Consider heat losses to environment to be 10% of power input to compressor. [50.4 kW]
6. A frictionless piston is free to move in a closed cylinder. Initially there is 0.035 m3 of oxygen at 4.5 bar, 60°C on one side of the piston and 0.07 m3 of methane at 4.5 bar and – 12°C on the other side. The cylinder walls and piston may be regarded as perfect thermal insulators but the oxygen may be heated electrically. Heating takes place so that the volume of oxygen doubles. Find :
(i) Final state condition ; (ii) Work done by the piston ;
(ii) Heat transferred to oxygen.
Treat both gases as perfect and take :
For oxygen cp = 0.88 kJ/kg K, R = 0.24 kJ/kg K
For methane cp = 1.92 kJ/kg K, R = 0.496 kJ/kg K.
7. The specific heat at constant pressure of one kg fluid undergoing a non-flow constant pressure process is
given by
where T is in °C.
The pressure during the process is maintained at 2 bar and volume changes from 1 m3 to 1.8 m3
and temperature changes from 50°C to 450°C.
Determine :
(i) Heat added (ii) Work done
(iii) Change in internal energy (iv) Change in enthalpy.
8. The resistance of the winding in a certain motor is found to be 75 ohms at room (25oC). When operating at full load under steady state conditions, the motor is switched off and the resistance of the winding s is immediately measured again, and found to be 90 ohms. The windings are made of copper whose resistance at temperature toC id given by Rt = Ro [1+0.00393t] where R0 is the resistance at 00C. Find the temperature of the coil during full load
9. A certain working fluid undergoes a process in such a way that pressure and volume are related as , Where p is in kPa and V is in m3 . During the process the volume changes from 0.15 m3 to 0.1 m3. Determine the work done in the process.
10. A gas occupies 0.3m3 at 2 bar. It undergoes a cycle consisting of the following processes a)1-2 constant pressure process with work interaction of 15kJ b)2-3 constant temperature process and E3=E2 c) 3-1 constant volume and change in internal energy E1-E3 is-40kJ. Determine network and net heat transfer for the cycle.
11. A piston-cylinder assembly contains 1kg or nitrogen at 100 kPa. The initial volume is 0.5 m3. Heat is transferred to the substance in an amount necessary to cause a slow expansion at constant temperature. This process is terminated when the final volume is twice the initial volume. Estimate the amount of heat transferred.[Molecular wt. 28 & Cv=0.773 kJ/kgK]
12. 0.115m3 of gas has a pressure of 138 kN /m2. It is compressed to 680 kN /m2according to the law PV1.4 =C, Determine the volume of the gas, work done and heat transferred.
13. The gas is compressed from initial state of 0.35 m3 and 105 kPa to a final state of 0.14 m3 and to the same pressure .Determine the change in internal energy of the gas which transfers 38 kJ of heat.
14. A piston and cylinder device contains 1 kg of air, Initially, v = 0.8 m3/kg and
T = 298 K. The air is compressed in a slow frictionless process to a specific volume of 0.2 m3/kg and a temperature of 580 K according to the equation pV1.3 = 0.75 ( p in bar, v in m3/kg). If Cv of air is 0.78 kJ/kg determine :
work and
heat transfer (both in kJ)
15. A piston-cylinder assembly contains 1kg or nitrogen at 100 kPa. The initial volume is 0.5 m3. Heat is transferred to the substance in an amount necessary to cause a slow expansion at constant temperature. This process is terminated when the final volume is twice the initial volume. Estimate the amount of heat transferred.[Molecular wt. 28 & Cv=0.773 kJ/kgK](16)
16. A closed system undergoes a cycle consisting of three process 1-2, 2-3 and 3-1. Given that Q12 = 30 kJ, Q23 = 10 kJ, 1w2 = 5 kJ, 3w2 = 5 kJ and DE31 = 15 kJ, determine Q31, w23, DE12 and DE23.
17. The following cycle involves 3 kg of air : Polytropic compression from 1 to 2 where P1 = 150 kPa, T1 = 360 K, P2 = 750 kPa and n = 1.1 ; constant-pressure cooling from 2 to 3; and constant - temperature heating from 3 to 1. Draw the pV diagram and find temperatures, pressures and volumes at each state and determine the net work and heat.
18. A reversible engine is operating between 1000 K and 300 K. Find the efficiency of the engine. Also find the power developed, in kW if the amount of heat received from high temperature reservoir is 500 kJ per min.
19. A refrigerator operating on reversible cycle is pumping heat from -5oC to the atmosphere at 40oC. If it has to pump 24000 kJ/hr, find the power required to run the refrigerator.
20. Calculate the power developed and diameter of the inlet pipe, if air enters at 5 kg/s, 50m/s with an enthalpy of 0.9 MJ/kg and leaves at 150 m/s with an enthalpy of 0.4 MJ/kg. The heat loss to the surroundings is 0.025MJ/kg. Assume 100kPa and 300 K at the inlet
21. In an isentropic flow through nozzle, air flow at the rate of 600 kg/hr. At inlet to
the nozzle, pressure is 2MPa and temperature is 127oC. The exit pressure is 0.5
MPa. Initial air velocity is 300 m/s determine exit velocity of air, inlet and exit
area of nozzle.
22. Two heat engines operating on carnot cycle are arranged in series. The engine A
receives heat at 1000 K and rejects heat at a constant temperature T2. The engine
B receives heat rejected by A, and in turn rejects heat to a reservoir at 300 K.
Calculate the T2 when a) the efficiency of the two engines are equal b) the work
output of the two engines are equal.
23. A carnot engine receives 90kJ from a reservoir at 900 K. It rejects heat to he
environment at 300 K. One-fifth of its work output is used to drive a carnot
refrigerator. The refrigerator rejects 60 kJ to the environment at 300 K. Find work
output and efficiency of the engine, the temperature of the sink for the refrigerator in
degree celsius.
24. Three reversible engines of Carnot type are operating in series between the limiting temperatures of 1100 K and 300 K. Determine the intermediate temperatures if the work output from engines is in proportion of 3 : 2 : 1.
25. In a compressor the air enters at 27°C and 1 atm and leaves at 227°C and 1 MPa. Determine the work done per unit mass of air assuming velocities at entry and exit to be negligible. Also determine the additional work required, if velocities are 10 m/s and 50 m/s at inlet and exit respectively.
26. In a gas turbine installation air is heated inside heat exchanger upto 750°C from ambient temperature of 27°C. Hot air then enters into gas turbine with the velocity of 50 m/s and leaves at 600°C. Air leaving turbine enters a nozzle at 60 m/s velocity and leaves nozzle at temperature of 500°C. For unit mass flow rate of air determine the following assuming adiabatic expansion in turbine and nozzle,
(i) heat transfer to air in heat exchanger
(ii) power output from turbine
(iii) velocity at exit of nozzle.
Take cp for air as 1.005 kJ/kg°K.
27. A refrigerator operates on reversed Carnot cycle. Determine the power required to drive refrigerator between temperatures of 42ºC and 4ºC if heat at the rate of 2 kJ/s is extracted from the low temperature region.
28. A piston and cylinder machine containing a fluid system has a stirring device as shown in Fig. 1.1 The piston is frictionless, and it is held down against the fluid due to atmospheric pressure of 101.3 kPa. The stirring device is turned 9500 revolutions with an average torque against the fluid of 1.25 Nm. Meanwhile the piston of 0.65 m diameter moves out 0.6 m. Find the net work transfer for the system.
Fig. 1.1
29. At the inlet to a certain nozzle the enthalpy of fluid passing is 2800 kJ/kg, and the velocity is 50 m/s. At the discharge end the enthalpy is 2600 kJ/kg. The nozzle is horizontal and there is negligible heat loss from it.
a. Find the velocity at exit of the nozzle.
b. If the inlet area is 900 cm2 and the specific volume at inlet is 0.187 m3/kg, find the mass flow rate.
c. If the specific volume at the nozzle exit is 0.498 m3/kg, find the exit area of nozzle. [8]
30. A fluid system, contained in a piston and cylinder machine, passes through a complete cycle of four processes. The sum of all heat transferred during a cycle is – 340 kJ. The system completes 200 cycles per min. Complete the following table showing the method for each item, and compute the net rate of work output in kW. [8]
Process Q (kJ/min) W (kJ/min) ΔE (kJ/min)
1—2 0 4340 —
2—3 42000 0 —
3—4 –4200 — – 73200
4—1 — — ___
31. During flight, the air speed of a turbojet engine is 250 m/s. Ambient air temperature is – 14°C. Gas temperature at outlet of nozzle is 610°C. Corresponding enthalpy values for air and gas are respectively 250 and 900 kJ/kg. Fuel air ratio is 0.0180. Chemical energy of fuel is 45 MJ/kg. Owing to incomplete combustion 6% of chemical energy is not released in the reaction. Heat loss from the engine is 21 kJ/kg of air. Calculate the velocity of the exhaust jet. [8]
32. The connections of a reversible engine to three sources at 500 K, 400 K and 300 K are shown in Fig. 1.3. It draws 1500 kJ/min of energy from the source at 500 K and develops 200 kJ/min of work.
a. Determine the heat interactions with the other two sources of heat.
b. Evaluate the entropy change due to each heat interaction with the engine.
c. Total entropy change during the cycle. [8]
33. 3 kg of water at 80°C is mixed with 4 kg of water at 15°C in an isolated system. Calculate the change of entropy due to mixing process.
34. Two tanks A and B contain 1 kg of air at 1 bar, 50ºC and 3 bar, 50ºC when atmosphere is at 1 bar,15ºC. Identify the tank in which stored energy is more. Also find the availability of air in each tank.
35. A cold storage plant of 40 tonnes of refrigeration capacity runs with its performance just
1/4 th of itsCarnot COP. Inside temperature is –15ºC and atmospheric temperature is 35ºC. Determine the powerrequired to run the plant. [Take : One ton of refrigeration as 3.52 kW]
36. A vessel of volume 0.2 m3 has 2 kg of water at 200 o C find p, h and x
37. In a power station, the saturated steam is generated at 200ºC by transferring the heat from hot gases in a steam boiler. Find the increase in total entropy of the combined system of gas and water and increase in unavailable energy due to irreversible heat transfer. The gases are cooled from 1000°C to 500°C and all the heat from gases goes to water. Assume water enters the boiler at saturated condition and leaves as saturated steam
38. One kg of air is compressed polytropically from 1 bar pressure and temperature of 300 K to a pressure of 6.8 bar and temperature of 370 K. Determine the irreversibility if the sink temperature is 293 K. Assume R = 0.287 kJ/kg K, cp = 1.004 kJ/kg K and cv = 0.716 kJ/kg K.
39. water is heated from 40oC in a piston cylinder arrangement until it becomes s aturated
vapour at a constant pressure of 50 bar. Find the heat to added per kg.
40. A steam power plant works between 40 bar and 0.05 bar. If the steam supplied is dry saturated and the cycle of operation is Rankine, find : (i) Cycle efficiency (ii) Specific steam consumption.
41. In a regenerative cycle the inlet conditions are 40 bar and 400°C. Steam is bled at 10 bar in Regenerative heating. The exit pressure is 0.8 bar. Neglecting pump work determine the efficiency of the cycle.
42. A steam power plant operates on ideal Rankine cycle using reheater and regenerative feed water heaters. It has one open feed heater. Steam is supplied at 150 bar and 600°C.The condenser pressure is 0.1 bar. Some steam is extracted from the turbine at 40 bar for closed feed water heater and remaining steam is reduced at 40 bar to 600°C. Extracted steam is completely condensed in this closed feed water heater and is pumped to 150 bar before mixing with the feed water heater. Steam for the open feed water heater is bled from L.P. turbine at 5 bar. Determine :
(i) Fraction of steam extracted from the turbines at each bled heater, and
(ii) Thermal efficiency of the system.
Draw the line diagram of the components and represent the cycle on T-s diagram.
43. The atmospheric conditions are 30ºC and specific humidity of 0.0215 kg/kg of air. Determine :
(i) Partial pressure of air
(ii) Relative humidity
(iii) Dew point temperature.
Atmospheric pressure = 756 mm Hg.
44. 1 kg of air at 24ºC and a relative humidity of 70% is to be mixed adiabatically in a steady state, steady flow device with 1 kg of air at 16ºC and a relative humidity of 10%. Assuming that the mixing is to be carried out at a constant pressure of 1.0 atm, determine the temperature and relative humidity of the stream leaving the device.
45. In a laboratory test, a psychrometer recorded 36ºC DBT and 30ºC WBT. Calculate :
(i) Vapour pressure
(ii) Relative humidity
(iii) Specific humidity
(iv) Degree of saturation
(v) Dew point temperature
(vi) Enthalpy of the mixture.
46. An air-water vapour mixture enters an adiabatic saturator at 30ºC and leaves at 20ºC, which is the adiabatic saturation temperature. The pressure remains constant at 1 bar. Determine the relative humidity and the humidity ratio of the inlet mixture.
47. The following observation were made during a testing of moist air
Dry bulb temperature Tdb = 29oC
Dew point temperature Tdb = 15oC
Total pressure = 1 bar
Determine the relative humidity and degree of saturation
48 In a combined heating and humidification process, moist air enters heating coil with the dry bulb temperature 20oC and 30% R.M. After the process the drug bulb temperature and the RH of Air were found to be 40oC and 55% RH. The air passes through the heating at the rate of 350 kg/min
49 In an air conditioning plant, fresh air is used to mix up with returned air from conditioned space. The fresh air drawn from atmosphere has the drug bulb temperature 32oC and we bulb temperature 25oC. The fresh air is drawn at the rate of 100m3/min. The returned air from conditioned space has the drug bulb temperature 23oC and relative humidity 50%. The volume flow rate of it 540 m3/min. Determine the (i) dry bulb and web bulb temperature (ii) specific humidity of mixture
50 Find the value of co-efficient of volume expansion β and isothermal compressibility K for a Van der Waals’ gas obeying
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