Me 6301 Engineering Thermodynamics Short Questions and Answers

ME 6301 ENGINEERING THERMODYNAMICS [FOR THIRD SEMESTER B.E MECHANICAL ENGINEERING STUDENTS]

COMPILED BY

BIBIN.C ASSISTANT PROFESSOR DEPARTMENT OF MECHANICAL ENGINEERING RMK COLLEGE OF ENGINEERING AND TECHNOLOGY GUMMIDIPOONDI TALUK TIRUVALLUR DIST

ME 6301 - ENGINEERING THERMODYNAMICS UNIT I - BASIC CONCEPT AND FIRST LAW 1. What is meant by Thermodynamics? Thermodynamics is the science of energy transfer and its effects on physical properties of substances. 2. Define the term thermal engineering. Thermal engineering is the science that deals with the energy transfer to practical applications such as energy transfer power generation, refrigeration, gas compression and its effect on the properties of working substance. 3. What is meant by Macroscopic approach in thermodynamics? Macroscopic thermodynamics deals with the effects of the action of many molecules concerned. 4. What is Microscopic (or) Statistical thermodynamics? Microscopic approach in thermodynamics deals with the study of the behaviour of the system by summing up the behaviour of each molecule 5. The two approaches by which the behaviour of matter can be studied in thermodynamics are ................. and ........................ [Ans: Macroscopic, Microscopic] 6. Give few applications of thermodynamic laws and principles: The laws and principles are applied in the steam and nuclear power plants, IC engines, Gas turbines, refrigeration etc. 7. What is thermodynamic medium? A matter of growing material chosen for the conversion of one form of energy into another is called thermodynamic medium. COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS 8. What is meant by thermodynamic system? Thermodynamic system is defined as the any space or matter or group of matter where the energy transfer or energy conversions are studied. OR A thermodynamic system is defined as a quantity of matter or a region in space, on which the analysis of the problem is concentrated. 9. How do you classify thermodynamic system? It may be classified into three types. i. Closed system (only energy transfer and no mass transfer) ii. Open system (Both energy and mass transfer) iii. Isolated system (No mass and energy transfer) 10. What is meant by closed system? Give an example. When a system has only heat and work transfer, but there is no mass transfer, it is called as closed system. Example:

Piston and cylinder arrangement. Compression of a gas in a piston -. cylinder arrangement.

11. What is meant by open system? Give an example. When a system has both mass and energy transfer it is called as open system. Example:

Air Compressor.

12. What is meant by isolated system? Give an example. Isolated system is not affected by surroundings. There is no heat; work and mass transfer take place. In this system total energy remains constant. Example:

Entire Universe

COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS 13. Differentiate closed and open system. Closed System

Open System

1. There is no mass transfer. Only heat 1. Mass transfer will take place, in and work will transfer.

addition to the heat and work transfer.

2. System boundary is fixed one

2. System boundary may or may not change.

3.

Example:

Piston

&

cylinder 3. Example: Air compressor, boiler

arrangement, Thermal power plant 14. 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. 15. What is boundary? System and surroundings are separated by an imaginary line is called boundary. 16. What is meant by surroundings? Any other matter outside the system boundary is called as surroundings. 17. What is universe ? A system and· its surroundings together is called an universe. 18. What are the various types of open system? The types of open system are: i. ii.

Steady fiow system Unsteady flow system.


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ME 6301 - ENGINEERING THERMODYNAMICS 19. In an isolated system, the total energy of the system............................ Ans: remains constant. 20. What is meant by control volume? Control volume is an imaginary region within which interactions are studied. 21. What is known as control surface? The boundary of the control volume is called as control surface. 22. Define change of state of a system Any process in which one or more of the properties of the system changes is called change of state. 23. What is meant by thermodynamic property? Thermodynamic property is any characteristic 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. 24. How do you classify the property? Thermodynamic property can be classified into two types. i.

Intensive or Intrinsic property

ii.

Extensive and Extrinsic property

25. What is meant by intensive or intrinsic property? Give an example. The properties which are independent on the mass of the system is called intensive properties. Example: Pressure, Temperature, Specific Volume etc.


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ME 6301 - ENGINEERING THERMODYNAMICS 26. What is meant by extensive or extrinsic property? Give an example. The properties which are dependent on the mass of the system are called extensive properties. Example: Total energy, Total volume, weight etc. 27. Differentiate Intensive and Extensive properties Intensive Properties

Extensive Properties

1. Independent on the mass of the Dependent on the mass of the system. system 2. If we consider part of the system these If we consider part of the system it will properties remain same. Example:

pressure,

have a lesser value. Temperature Example: Total energy, Total volume,

specific volume etc.,

weight etc.,

3. Extensive property/mass is known as intensive property 28. What do you understand by equilibrium of a system? When a system remains in equilibrium state, it should not undergo any charges to its own accord. 29. 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.


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ME 6301 - ENGINEERING THERMODYNAMICS 30. What are the conditions for thermodynamic equilibrium? The system in thermodynamic equilibrium must essentially be in i.

Mechanical Equilibrium

ii.

Thermal equilibrium

iii.

Chemical equilibrium

31. What is meant by thermodynamic equilibrium? When a system is in thermodynamic equilibrium, it should satisfy the following three conditions. i.

Mechanical Equilibrium: - Pressure remains constant

ii.

Thermal equilibrium: - Temperature remains constant

iii.

Chemical equilibrium: There is no chemical reaction.

32. Explain Mechanical equilibrium. If the forces are balanced between the system and surroundings are called Mechanical equilibrium 33. 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 34. Explain Thermal equilibrium. If the temperature difference between the system and surroundings is zero then it is in Thermal equilibrium. 35. Define nuclear equilibrium A system is said to be in nuclear equilibrium when nuclear reactions such as fusion, fission reaction does not cause any change.


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ME 6301 - ENGINEERING THERMODYNAMICS 36. Define magnetic equilibrium A system is said to be in magnetic equilibrium when the magnetisation and demagnetisation effects do not change its properties 37. when there is uniform pressure at all points of the system, then it is said to be in ......... Ans: Mechanical equilibrium 38. When a system wiII be in a state of thermodynamic equilibrium? A system is said to exist in a state of thermodynamic equilibrium, when no change in any microscopic property is registered, if the system is isolated from the surroundings. 39. Define continuum The Atomie structure of a substance is disregarded in classical thermodynamics and the substance is viewed to be a continuous, homogenous matter without microscopic holes. This concept is called as continuum. 40. Define the term process It is defined as the change of state undergone by a gas due to energy flow. 41. Define the term thermodynamic Cycle When a system undergoes a series of processes and return to its initial condition, it is known as thermodynamic cycle. 42. What is meant by open cycle? In an open cycle, the same working substance will be exhausted to the surroundings after expansion. 43. What is meant by closed cycle? In a closed cycle, the same working substance will recalculates again and again.


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ME 6301 - ENGINEERING THERMODYNAMICS 44. What is meant by reversible process? A process is said to be reversible, it should trace the same path in the reverse direction when the process is reversed. It is possible only when the system passes through a continuous series of equilibrium state. 45. What is meant by irreversible process? If a system does not pass through continuous equilibrium state, then the process is said to be irreversible. 46. What is Quasi – Static process? The process is said to be quasi – static, it should proceed infinitesimally slow and follows continuous series of equilibrium states. A quasi- static process is that a succession of equilibrium states. A quasi-static process is also called as reversible process.

47. State True or False: Quasi static process is also called as Reversible process. [Ans: True] 48. What is meant by Point function? The quantity which is independent on the process or path followed by the system is known as point functions. Example:

Pressure, volume, temperature, etc.,


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ME 6301 - ENGINEERING THERMODYNAMICS 49. What is meant by Path function? The quantities which are dependent on the process or path followed by the system is known as path functions. Example:

Heat transfer, work transfer.

50. Define absolute pressure The total algebric sum of the atmospherie pressure and gauge pressure is called absolute pressure. 51. What is gauge pressure? The pressure measured above the atmospherie pressure gauge pressure. 52. Define atmospheric pressure Pressure exerted by air on a column of 760 mm of mercury is called atmospherie pressure. 53. What is meant by state of a system? State is an unique condition of the system and at equilibrium. It can be identified by its properties, such as pressure, volume. 54. Define path of change of state The succession equilibrium states passed through, is called the path of change of state. 55. Define energy Energy of system is the ability or capacity to do work. 56. Define stored energy Energy possessed by a system within its boundaries is called stored energy. Example:

Potential energy.


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ME 6301 - ENGINEERING THERMODYNAMICS 57. Define power The rate of energy transfer is called power. 58. Define transit energy Energy possessed by a system which is capable of crossing its boundaries is.caIled transit energy. Example:

Heat energy, Electrical energy'.

59. Define internal energy Energy possessed by a substance due to its molecular arrangement and. motion of its molecules is called internal energy. 60. What is meant by thermodynamic work? A thermodynamic work is said to be do ne by a system if its sole effect outside its boundary is equivalent to raising a weight against the force of gravity. 61. What is meant by thermodynamic work? It is the work done by the system when the energy transferred across the boundary of the system. It is mainly due to intensive property difference between the system and surroundings. 62.What is the work done in a closed system in terms pressure and volume? Workdone = Pdv. 63. Work done by a system is............. Ans: Positive. 64. Work done on a system is ............... Ans: Negative 65. Heat supplied to the system is...................... Ans: Positive.


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ME 6301 - ENGINEERING THERMODYNAMICS 66. Heat rejected by the system is.................... Ans: Negative 67. State True or False: Heat and work are not properties of the system. Ans: True 68. State True or False: During all adiabatic process, the change in internal energy is equal to work done. Ans: True 69. State True or False: During an isothermal process, the net heat interchange is el to the work done by the system. Ans: True 70. Work done is a ------------------- function. Ans: Path 71. Define specific heat The heat required by a'unit mass of a substanee to raise its temperature by one degree is called the specific heat of subs.tance Unit:

kJ I kg I K.

72. Define Latent heat Latent heat is the amount of energy required to convert liquid completely into vapour per unit mass of a substance at a given pressure. 73. Explain Zeroth Law of thermodynamics? Zeroth law of thermodynamics states that when two systems are separately in thermal equilibrium with a third system, then they themselves is in thermal equilibrium with each other. 74.

Zeroth law is the basic of ................. measurement. Ans: Temperature COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS 75. State the law of conservation of energy Energy can neither be created nor destroyed, but it can be transferred from one form to another. 76. State the First law of thermodynamics First law of thermodynamics states that when system undergoes a cyclic process the net heat transfer is equal to work transfer. 77. 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. But this law does not specify the possible conditions under which the heat is converted into work. b. According to the first law of thermodynamics it is impossible to transfer heat from lower temperature to higher temperature. c. It does not give any information regarding change of state or whether the process is possible or not. d. The law does not specify the direction of heat and work. 78. Define the term enthalpy? The Combination of internal energy and flow energy is known as enthalpy of the system. It may also be defined as the total heat of the substance. Mathematically, Enthalpy (H) = U + p v ……..KJ Where, U – internal energy p – Pressure v – Volume In terms of Cp & T → H = m Cp (T2-T1) ……… KJ


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ME 6301 - ENGINEERING THERMODYNAMICS 79. Give the general gas energy equations. dH = dE + dW. 80. Define the term internal energy Internal energy of a gas is the energy stored in a gas due to its molecular interactions. It is also defined as the energy possessed by a gas at a given temperature. 81. Define Heat. Heat is the energy crossing the boundary due to the temperature difference between the system and surroundings. 82. Define Specific heat capacity at constant pressure. It is defined as the amount of heat energy required to raise or lower the temperature of unit mass of the substance through one degree when the pressure kept constant. It is denoted by CP. 83. Define Specific heat capacity at constant volume. It is defined as the amount of heat energy required to raise or lower the temperature of unit mass of the substance through one degree when volume kept constant. It is denoted by CV. 84. 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. 85. Define entropy of a pure substance. Entropy is an important thermodynamic property, which increases with addition of heat and decreases with its removal. Entropy is a function of temperature only. It is an unavailability of energy during energy transfer.


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ME 6301 - ENGINEERING THERMODYNAMICS 86. Define perpetual motion machine (PMM) of first kind PMM of first kind delivers work continuously without any input. It violates first law of thermodynamics, it is impossible to construct an engine working with this principle. OR 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. 87. 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. OR 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. 88. Work transfer is equal to heat transfer in case of ________ process. Isothermal process 89. Define an isentropic process. Isentropic process is also called as reversible adiabatic process. It is a process which follows the law of pVy = C is known as isentropic process. During this process entropy remains constant and no heat enters or leaves the gas. 90. Explain the throttling process. When a gas or vapour expands and flows through an aperture of small size, the process is called as throttling process. 91. Define free expansion process. When a gas expands suddenly into a vacuum through a large orifice is known as free expansion process. COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS 92. Write down the characteristic gas equation. Characteristic gas equation is pV = mRT Where, p = pressure V = Volume R = Characteristic gas constant T = Temperature. 93. What is meant by steady flow process? During the process the rate of flow of mass and energy across the boundary remains constant, is known as steady flow process. OR Steady flow means that the rates of flow of mass and energy across the control surface are constant. 94. What is the difference between steady flow and non – flow process? During the steady flow process the rate of flow of mass and energy across the boundary remains constant. In case of non – flow across the system and boundary. 95. Indicate the practical application of steady flow energy equation. i.

Turbine

ii.

Nozzle

iii.

Condenser

iv.

Compressor

v.

Pump

96. Work done in a free expansion process is _________ Ans: Zero


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ME 6301 - ENGINEERING THERMODYNAMICS 97. 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. 98. What are the important characteristics of entropy? i.

If the heat is supplied to the system then the entropy will increase.

ii.

If the heat is rejected to the system then the entropy will decrease.

iii.

The entropy is constant for all adiabatic frictionless process.

iv.

The entropy increases if temperature of heat is lowered without work being done as in throttling process.

v.

If the entropy is maximum, then there is a minimum availability for conversion in to work.

vi.

If the entropy is minimum then there is a maximum availability for conversion into work.

99. 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. 100.

Which property is constant during throttling? Enthalpy


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ME 6301 - ENGINEERING THERMODYNAMICS 101.

If in the equation PVn = C, the value of n =∞ then the process is called _______ Constant Volume process

102.

The polytropic index (n) is given by ________ n = ln (P2/P1)/ ln (V1/V2)

103. 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, Therefore dU = 0 by integrating the above equation U = constant, therefore the internal energy is constant for isolated system.


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ME 6301 - ENGINEERING THERMODYNAMICS UNIT II - SECOND LAW AND AVAILABILITY ANALYSIS 1. State the Kelvin – Plank statement of second law of thermodynamics Kelvin – Plank 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 into an equal amount of work. OR 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. 2. State Clausius statement of second law of thermodynamics. It states that heat can flow from hot body to cold without any external aid but heat cannot flow from cold body to hot body without any external aid. OR 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. 3. State Carnot theorem. It states that no heat engine operating in a cycle between two constant temperature heat reservoirs can be more efficient than a reversible engine operating between the same reservoirs. 4. What is absolute entropy(Third law of Thermodynamics)? The entropy measured for all perfect crystalline solids at absolute zero temperature is known as absolute entropy.

5. What are the Corollaries of Carnot theorem? COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS i.

In all reversible engine operating between the two given thermal reservoirs with fixed temperature, have the same efficiency.

ii.

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.

6. Define Heat pump. A heat pump is a device, which is working in a cycle and transfers heat from lower temperature to higher temperature. 7. Define Heat engine. Heat engine is a machine, which is used to convert the heat energy into mechanical work in a cyclic process. OR A heat engine is a device which is used to convert the thermal energy into mechanical energy. 8. What are the assumptions made on heat engine? i.

The source and sink are maintained at constant temperature.

ii.

The source and sink has infinite heat capacity.

9. What is the difference between a heat pump and a refrigerator? Heat pump is a device which operating in cyclic process, maintains the temperature of a hot body at a temperature higher than the temperature of surroundings. A refrigerator is a device which operating in a cyclic process, maintains the temperature of a cold body at a temperature lower than the temperature of the surroundings. 10. Define the term COP? COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS Co-efficient of performance is defined as the ratio of heat extracted or rejected to work input.

COP =

Heat extracted or Rejected Work input

11. Write the expression for COP of a heat pump and a refrigerator?

=

T2 T2  T1

COP of Refrigerator (COP) REF =

T1 T2  T1

COP of heat pump (COP) HP

12. What is the relation between COPHP and COP ref?

(COP) HP = (COP) REF 1 13. Why Carnot cycle cannot be realized in practical? i.

In a Carnot cycle all the four processes are reversible but in actual practice there is no process is reversible.

ii.

There are two processes to be carried out during compression and expansion. For isothermal process the piston moves very slowly and for adiabatic process the piston moves as fast as possible. This speed variation during the same stroke of the piston is not possible.

iii.

It is not possible to avoid friction moving parts completely.

14. Name two alternative methods by which the efficiency of a Carnot cycle can be increased. i.

Efficiency can be increased as the higher temperature T2 increases.

ii.

Efficiency can be increased as the lower temperature T1 decreases.

15. Why a heat engine cannot have 100% efficiency? COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS For all the heat engines there will be a heat loss between system and surroundings. Therefore we can’t convert all the heat input into useful work. 16. When will be the Carnot cycle efficiency is maximum? Carnot cycle efficiency is maximum when the initial temperature is 0°K. 17. What are the processes involved in Carnot cycle. Carnot cycle consist of i.

Reversible isothermal compression

ii.

Isentropic compression

iii.

Reversible isothermal expansion

iv.

Isentropic expansion

18. Write the expression for efficiency of the Carnot cycle.

Carnot =

T2  T1 T2

19. What are the limitations of Carnot cycle? i.

No friction is considered for moving parts of the engine.

ii.

There should not be any heat loss.

20. 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. 21. 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. 22. Explain the term source and sink. COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS Source is a thermal reservoir, which supplies heat to the system and sink is a thermal reservoir, which takes the heat from the system. 23. 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. 24. Power requirement of a refrigerator is _________ Ans: Inversely proportional to cop 25. In SI Units, one ton of refrigeration is equal to __________ Ans: 210KJ/min 26. The capacity of a domestic refrigerator is in the range of __________ Ans: 0.1 to 0.3 tonnes. 27. The vapour compression refrigerator employs the __________cycle Ans: Reversed Carnot 28. In vapour compression cycle the condition of refrigerant is dry saturated vapour ________ Ans: Before entering the compressor 29. Define the unit for refrigeration Unit of refrigeration is expressed in terms of tonne of refrigeration (TR). A tonne of refrigeration is defined as the quantity of heat required to be removed form one tonne of water at 0oC to convert into ice at 0oC in 24 hours.

30. What is the unit of refrigeration? COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS The capacity of refrigeration is expressed in tonnes of refrigeration (TOR). 1 tonnes of refrigeration = 210 kJ/min (or) = 3.5 kJ/sec (kW) A tonne of refrigeration is defined as the quantity of heat to be removed in order to form one tonne of ice at 0oC in 24 hours. 31. Define refrigeration effect. The amount of heat extracted in a given time is known as refrigeration effect. 32. What is the refrigeration effect of the refrigerant? Refrigeration effect is the total heat removed from the evaporator by the refrigerant. It is called as Tonne of Refrigeration of kW. 33. Define COP of refrigeration. The COP of a refrigeration system is the ratio of net refrigeration effect to the work required to produce the effect.


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ME 6301 - ENGINEERING THERMODYNAMICS UNIT III - PROPERTIES OF PURE SUBSTANCE AND STEAM POWER CYCLE PROPERTIES OF PURE SUBSTANCES 1. Define pure substance A pure substanee is a homogenous and invariable chemical composition through out the mass. 2. What is Triple point? The point at which Solid, liquid and vapour phases are equilibrium is called as triple point. OR The triple point is merely the point of intersection of sublimation and vapourisation curves. 3. 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. 4. 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. 5. 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. 6. Explain heat of super heat or super heat enthalpy. The heat added to dry steam at 1000 C to convert it into super heated steam at the temperature Tsup is called as heat of superheat or super heat enthalpy. COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS 7. 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. 8. 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 374.150 C. The corresponding pressure is called critical pressure. 9. 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. If S>Sg then super heated steam, S= Sg then dry steam and S< Sg then wet steam. 10. 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. 11. 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.


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ME 6301 - ENGINEERING THERMODYNAMICS STEAM POWER CYCLE 12. 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. 13. What are the assumptions made on the analysis of ideal Rankine cycle? i.

Each component of the working fluid is internally reversible.

ii.

All processes of the working fluid are internally reversible.

iii.

The pump and turbine operate adiabatically.

iv.

Potential and kinetic energy affects are neglected.

v.

Condensate leaves the condenser as saturated liquid.

14. What are the various methods used to improve the efficiency of Rankine cycle? i.

Increase the boiler pressure (or) Temperature

ii.

Decrease the condenser pressure

iii.

Increase the temperature of steam at superheated condition

iv.

Reheating the steam

v.

Adopting regeneration of steam

15. What are the advantages of reheat cycle? i.

It increases turbine work

ii.

It increases the efficiency of the plant

iii.

It reduces wear of turbine blades by reducing moisture content in steam.

16. Define specific steam consumption of an ideal Rankine cycle. It is defined as the mass of steam required per unit power output. Specific steam consumption = (Steam flow kg/h) / (Power) COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS 17. What is the effect of reheating the steam on the specific output and the cycle efficiency? The specific output and the cycle efficiency are increased due to reheating the steam in vapour power cycles. 18. What is meant by vapour power cycle? Give some examples Thermodynamic cycles which uses vapour as the working fluid is called vapour power cycle. Example:

Carnot cycle, Rankine cycle.

19. Define efficiency ratio. The ratio of actual cycle efficiency to that of the ideal cycle efficiency is termed as efficiency ratio. 20. 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. 21. Define specific steam consumption of an ideal Rankine cycle. It is defined as the mass flow of steam required per unit power output. 22. Name the different components in steam power plant working on Rankine cycle. Boiler, Turbine, Cooling Tower or Condenser and Pump. 23. What is meant by work ratio? What is the importance of work ratio in vapour cycles? Work ratio is defined as the ration of network transfer to the positive work transfer. Work ratio affects the actual cycle efficiency. Comparing two cycles with the same ideal efficiency, the cycle having lower work ratio would have smaller actual efficiency. Higher work ratio, smaller the plant size.


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ME 6301 - ENGINEERING THERMODYNAMICS 24. Compare Carnot and Rankine cycles Carnot cycle

Rankine cycle

1. Lower work ratio

1. Higher work ratio

2. Specific steam consumption is more

2. Specific steam consumption is less

3. Size of the power plant for a given 3. Size of the power plant for a given output is big

output is small

4. Higher thermal efficiency

4. Lower thermal efficiency

25. What is the function of feed water heater? The main function of feed water heater is to increase the temperature of feed water to the saturation temperature corresponding to the boiler pressure before it enters into the boiler. 26. Why reheat cycle is not used for low boiler pressure? At low boiler pressure the reheat cycle efficiency may be less than the Rankine cycle efficiency. This may be due to the lower temperature during heating. 27. What are the advantages of vapour power cycles over gas power cycle? i.

The isothermal heat transfer is possible in condenser and evaporator

ii.

The work ratio is high compared to the gas power cycle.

28. Why carnot cycle cannot be realised in practice for vapour power cycles? The main difficulty to attain the cycle in practice is that isothermal condensation is stopped before it reaches to saturated liquid condition. Therefore the compressor has to deal with a non-homogeneous mixture of water and steam. Because of the large specific volume of liquid vapour mixture before compression, the compressor size and work input have to be large. The higher power requirement reduces the plant efficiency as well as work ratio. COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS 29. What are the effects of condenser pressure in Rankine cycle? By lowering the condenser pressure, we can increase the cycle efficiency. The main disadvantage is lowering the backpressure increases the wetness of steam. Isentropic compression of a very wet vapour is very difficult. 30. What are the disadvantages of bleeding? Cost of the plant increased and the work done per kg is reduced which results in higher boiler capacity for given output. 31. What are the advantages of bleeding? i.

It increases the thermodynamic efficiency as the heat of the bled steam is not lost in the condenser but is utilized in feed water heating

ii.

By bleeding, the volume flow at the low-pressure end is considerably reduced, this reduces the design difficulties of blades, and also condenser size is reduced.

32. Mention the improvements made to increase the ideal efficiency of Rankine cycle. i.

Lowering the condenser pressure.

ii.

Superheated steam is supplied to the turbine.

iii.

Increasing the boiler pressure to certain limit.

iv.

Implementing reheat and regeneration in the cycle.

33. 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 increase the wetness of steam. Isentropic compression of a very wet vapour is very difficult. 34. 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.


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ME 6301 - ENGINEERING THERMODYNAMICS 35. What are the disadvantages of reheating? i.

The cost of the plant increases

ii.

It increases the condenser capacity due to increased dryness fraction.

36. What are the advantages of reheat cycle? i.

It increases the turbine work.

ii.

It increases the heat supply.

iii.

It increases the efficiency of the plant.

iv.

It reduces the wear on the blade because of low moisture content in LP

v.

State of the turbine.

37. 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.


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ME 6301 - ENGINEERING THERMODYNAMICS UNIT IV - IDEAL AND REAL GASES AND THERMODYNAMIC RELATIONS 1. 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. 2. 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. 3. 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. 4. State Charle's law. It states that if any gas is heated at constant pressure, its volume changes directly as its absolute temperature. 5. 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. 6. 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. 7. 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. COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS 8. 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. 9. Define Dalton's law of partial pressure. The total pressure exerted in a closed vessel containing a number of gases is equal to the sum of the pressures of each gas and the volume of each gas equal to the volume of the vessel. 10. 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 = m A+m B+m C+. = mi mi = mass of the constituent. P=P A+P B+P C+. = P i P i - the partial pressure of a constituent. 11. 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. 12. Explain the construction and give the use of generalized compressibility chart. The general compressibility chart is plotted with Z versus P r 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.


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ME 6301 - ENGINEERING THERMODYNAMICS 13. State Avogardo's Law. The number of moles of any gas is proportional to the volume of gas at a given pressure and temperature. 14. 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 =[T/P]n 15. 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, i.

There is little or no attraction between the molecules of the gas.

ii.

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 deviates considerably from the ideal gas equation [p+(a/V 2)](V-b) = RT 16. What are the assumptions made in Vanderwaal's equation of state? i.

There is no intermolecular force between particles.

ii.

The volume of molecules is negligible in comparison with the gas.

17. 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.


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ME 6301 - ENGINEERING THERMODYNAMICS 18. 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 19. What are thermodynamic properties? Thermodynamic properties are pressure (p), temperature (T), volume (V), internal energy (U), enthalpy(H), entropy (S), Helmholtz function () and Gibbs function (g). 20. 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. 21. Define isothermal compressibility. Isothermal compressibility is defined as the change in volume with change in pressure per unit volume keeping the temperature constant.


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ME 6301 - ENGINEERING THERMODYNAMICS UNIT V - GAS MIXTURES AND PSYCHROMETRY 1. 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. 2. Represent the following Psychrometric process using skeleton Psychrometric chart? i.

Cooling and dehumidification

ii.

Evaporative cooling.

3. Define Relative humidity. It is defined as the ratio of partial pressure of water vapour (p w) in a mixture to the saturation pressure (p s) of pure water at the same temperature of mixture. 4. Define specific humidity. It is defined as the ratio of the mass of water vapour (m s) in a given volume to the mass of dry air in a given volume (m a). COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS 5. 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. 6. 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. 7. 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. 8. Define dew point depression. It is the difference between dry bulb temperature and dew point temperature of air vapour mixture. 9. 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. 10. 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.


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ME 6301 - ENGINEERING THERMODYNAMICS 11. 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. 12. What is psychrometer? Psychrometer is an instrument which measures both dry bulb temperature and wet bulb temperature. 13. 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. 14. 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. 15. What are the important psychrometric process? i.

Sensible heating and sensible cooling,

ii.

Cooling and dehumidification,

iii.

Heating and humidification,

iv.

Mixing of air streams,

v.

Chemical dehumidification,

vi.

Adiabatic evaporative cooling.

16. What is humidification and dehumidification? The addition of water vapour into air is humidification and the removal of water vapour from air is dehumidification. COMPILED BY BIBIN, AP/MECH, RMKCET

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ME 6301 - ENGINEERING THERMODYNAMICS 17. Define dew point depression. It is the difference between dry bulb temperature and dew point temperature of air vapour mixture. 18. Define RSHF. Room sensible heat factor is defined as the ratio of room sensible heat load to the room total heat load. 19. How does humidity affect human comfort? Human beings want to feel comfortable. They want to live in an environment that is neither hot not cold, neither very humid nor very dry. The desires of human body could not be adopted with high or low humidity. They feels comfortable only when they can freely dissipate their waste heat to the environment. 20. What do you mean by the "Infiltration" in heat load calculations? The amount of heat load added due the air entering into the A/c system through small opening in the doors and windows, cracks in the walls etc., are termed as infiltration. 22. 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. 23. 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.


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ME 6301 - ENGINEERING THERMODYNAMICS 24. 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.


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ME 6301 - ENGINEERING THERMODYNAMICS ANNA UNIVERSITY: CHENNAI SYLLABUS ME 6301

ENGINEERING THERMODYNAMICS

LTPC3104

OBJECTIVES:

To familiarize the students to understand the fundamentals of thermodynamics and to perform thermal analysis on their behavior and performance. (Use of Standard and approved Steam Table, Mollier Chart, Compressibility Chart and Psychrometric Chart permitted)

UNIT I BASIC CONCEPTS AND FIRST LAW

9

Basic concepts - concept of continuum, comparison of microscopic and macroscopic approach. Path and point functions. Intensive and extensive, total and specific quantities. System and their types. Thermodynamic Equilibrium State, path and process. Quasi-static, reversible and irreversible processes. Heat and work transfer, definition and comparison, sign convention. Displacement work and other modes of work .P-V diagram. Zeroth law of thermodynamics – concept of temperature and thermal equilibrium– relationship between temperature scales –new temperature scales. First law of thermodynamics –application to closed and open systems – steady and unsteady flow processes.

UNIT II SECOND LAW AND AVAILABILITY ANALYSIS

9

Heat Reservoir, source and sink. Heat Engine, Refrigerator, Heat pump. Statements of second law and its corollaries. Carnot cycle Reversed Carnot cycle, Performance. Clausius inequality. Concept of entropy, T-s diagram, Tds Equations, entropy change for - pure substance, ideal gases - different processes, principle of increase in entropy. Applications of II Law. High and low grade energy. Available and non-available energy of a source and finite body. Energy and


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ME 6301 - ENGINEERING THERMODYNAMICS irreversibility. Expressions for the energy of a closed system and open systems. Energy balance and entropy generation. Irreversibility. I and II law Efficiency.

UNIT III PROPERTIES OF PURE SUBSTANCE AND STEAM POWER CYCLE 9

Formation of steam and its thermodynamic properties, p-v, p-T, T-v, T-s, h-s diagrams. p-v-T surface. Use of Steam Table and Mollier Chart. Determination of dryness fraction. Application of I and II law for pure substances. Ideal and actual Rankine cycles, Cycle Improvement Methods - Reheat and Regenerative cycles, Economiser, preheater, Binary and Combined cycles.

UNIT IV IDEAL AND REAL GASES, THERMODYNAMIC RELATIONS

9

Properties of Ideal gas- Ideal and real gas comparison- Equations of state for ideal and real gases- Reduced properties-.Compressibility factor-.Principle of Corresponding states. Generalised Compressibility Chart and its use-. Maxwell relations, Tds Equations, Difference and ratio of heatcapacities, Energy equation, Joule-Thomson Coefficient, Clausius Clapeyron equation, Phase Change Processes. Simple Calculations.

UNIT V GAS MIXTURES AND PSYCHROMETRY

9

Mole and Mass fraction, Dalton’s and Amagat’s Law. Properties of gas mixture – Molar mass, gas constant, density, change in internal energy, enthalpy, entropy and Gibbs function. Psychrometric properties, Psychrometric charts. Property calculations of air vapour mixtures by using chart and expressions. Psychrometric process – adiabatic saturation, sensible heating and cooling, humidification, dehumidification, evaporative cooling and adiabatic mixing. Simple Applications TOTAL : 45 PERIODS


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ME 6301 - ENGINEERING THERMODYNAMICS OUTCOMES:

Upon completion of this course, the students can able to apply the Thermodynamic Principles to Mechanical Engineering Application. Apply mathematical fundamentals to study the properties of steam, gas and gas mixtures.

TEXT BOOKS : 1. Nag.P.K., “Engineering Thermodynamics”, 4thEdition, Tata McGraw-Hill, New Delhi, 2008. 2. Natarajan E., "Engineering Thermodynamics: Fundamentals and Applications", Anuragam Publications, 2012.

REFERENCES :

1. Cengel. Y and M.Boles, "Thermodynamics - An Engineering Approach", 7th Edition, Tata McGraw Hill, 2010. 2. Holman.J.P., "Thermodynamics", 3rd Edition, McGraw-Hill, 1995. 3. Rathakrishnan. E., "Fundamentals of Engineering Thermodynamics", 2nd Edition, PrenticeHall of India Pvt. Ltd, 2006 4. Chattopadhyay, P, "Engineering Thermodynamics", Oxford University Press, 2010. 5. Arora C.P, “Thermodynamics”, Tata McGraw-Hill, New Delhi, 2003. 6. Van Wylen and Sonntag, “Classical Thermodynamics”, Wiley Eastern, 1987 7. Venkatesh. A, “Basic Engineering Thermodynamics”, Universities Press (India) Limited, 2007. 8. Kau-Fui Vincent Wong, "Thermodynamics for Engineers", CRC Press, 2010 Indian Reprint. 9. Prasanna Kumar: Thermodynamics "Engineering Thermodynamics" Pearson Education, 2013


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Me 6301 Engineering Thermodynamics Short Questions and Answers

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