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DEPARTMENT OF MECHANICAL ENGINEERING
ME 6301 - ENGINEERING THERMODYNAMICS
1. Define the term thermal engineering. Ans: 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. 2. What is meant by thermodynamic system? How do you classify it? (MU – Oct’99, Apr’2000: BRU – Apr’96; MSU – Apr’96) Ans: Thermodynamic system is defined as the any space or matter or group of matter where the energy transfer or energy conversions are studied. It may be classified into three types. (a) Open system (b) Closed system (c) Isolated system 3. What is meant by closed system? Give an example.(BNU – Nov’95) Ans: 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. 4. Define a open system, Give an example. Ans: When a system has both mass and energy transfer it is called as open system. Example: Air Compressor. 5. Differentiate closed and open system. (MU – Apr’98, MKU – Nov’95) Closed System Open System
- There is no mass transfer. Only heat and work will transfer.
- Mass transfer will take place, in addition to the heat and work transfer.
- System boundary is fixed one 2. System boundary may or may not change.
- Ex: Piston & cylinder arrangement, Thermal power plant
- Air compressor, boiler 6. Define an isolated system Ans: 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
7. Define: Specific heat capacity at constant pressure. (MU – Oct’99) Ans: It isdefined 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. 8. Define: Specific heat capacity at constant volume. Ans: 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. 9. What is meant by surroundings? Ans: Any other matter out side the system boundary is called as surroundings. 10. What is boundary? Ans: System and surroundings are separated by an imaginary line is called boundary. 11. What is meant by thermodynamic property? (MU – Apr’2001; BRU – Nov’96; BNU – Nov’94) Ans: 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. 12. How do you classify the property? Ans: Thermodynamic property can be classified into two types. 1. Intensive or Intrinsic and 2. Extensive and Extrinsic property. 13. Define Intensive and Extensive properties. (MU – Oct’96,98; MKU – Apr’96) Ans: The properties which are independent on the mass of the system is called intensive properties. e.g., Pressure, Temperature, Specific Volume etc., The properties which are dependent on the mass of the system is called extensive properties. e.g., Total energy, Total volume, weight etc. **14. Differentiate Intensive and Extensive properties (MU – Apr’99, Apr’2001; MSU
- Nov’96)** Intensive Properties Extensive Properties
- Independent on the mass of the system Dependent on the mass of the system.
- If we consider part of the system these properties remain same. e.g. pressure, Temperature specific volume etc., If we consider part of the system it will have a lesser value. e.g., Total energy, Total volume, weight etc.,
- Extensive property/mass is known as intensive property
Example: Pressure, volume, temperature, etc., The quantities which are dependent on the process or path followed by the system is known as path functions. Example: Heat transfer, work transfer.
24. What is Quasi – Static process? (MU – Oct’98, Apr’2000 & 2001; BNU – Nov’95) Ans: The process is said to be quasi – static, it should proceed infinitesimally slow and follows continuous series of equilibrium states. Therefore, the quasi static, it should proceed infinitesimally slow and follows continuous series of equilibrium states. Therefore, the quasi static process may be an reversible process. 25. Explain Zeroth Law of thermodynamics? (MU – Nov’94, Apr’2001; BRU – Apr’96) Ans: Zeroth law of thermodynamics states that when two systems are separately in thermal equilibrium with a third system, then they themselves are in thermal equilibrium with each other. 26. Define the term enthalpy? (MU – Oct’99) Ans: 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 + pv KJ) Where, U – internal energy p – pressure v – volume In terms of Cp & T → H = mCp (T 2 - T 1 )KJ 27. Define the term internal energy (MKU – Apr’96) Ans: 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. 28. What is meant by thermodynamic work? Ans: 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. 29. Define Heat. Ans: Heat is the energy crossing the boundary due to the temperature difference between the system and surroundings. 30. Give the general gas energy equations. (MU – Apr’95 & 98) Ans: dH = dE + dW. 31. State the law of conservation of energy (BRU – Nov’95)
Ans: Energy can neither be created nor destroyed, but it can be transferred from one form to another.
32. Define entropy of a pure substance. (MU – Oct’2000; MKU – Nov’96; BRU – Nov’95) Ans: 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. 33. Define an isentropic process. (MU – Oct’99) Ans: 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. 34. Explain the throttling process. Ans: When a gas or vapour expands and flows through an aperture of small size, the process is called as throttling process. 35. Work done in a free expansion process is (MU – Apr’97) Ans: Zero 36. Define free expansion process. Ans: When a gas expands suddenly into a vacuum through a large orifice is known as free expansion process. 37. Which property is constant during throttling? (MU – Oct’98, Oct’2000) Ans: Enthalpy 38. If in the equation PVn^ = C, the value of n = then the process is called Ans: Constant Volume process 39. The polytropic index (n) is given by (MU – Apr’95 & 96) Ans: n = log (P2/P1)/ log (V1/V2) 40. Work transfer is equal to heat transfer in case of process.MU – Nov’94) Ans: Isothermal process. 41. Write down the characteristic gas equation. Ans: Characteristic gas equation is pV = mRT Where, p = pressure V = Volume R = Characteristic gas constant T = Temperature. 42. What is meant by steady flow process? (BNU – Nov’96)
51. Define the term COP? Ans: Co-efficient of performance is defined as the ratio of heat extracted or rejected to work input. COP = Heat extracted or rejected Work input 52. Write the expression for COP of a heat pump and a refrigerator? Ans: COP of heat pump Heat Supplied T 2 COP (^) HP = = Work input T 2 - T 1 COP of Refrigerator Heat extrated T 1 COP (^) HP = --------------- = -------- Work input T 2 - T 1 53. What is the relation between COPHP and COP (^) ref? Ans: COPHP = COPref + 54. Why Carnot cycle cannot be realized in practical? Ans: (i) In a Carnot cycle all the four process 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. 55. Name two alternative methods by which the efficiency of a Carnot cycle can be increased. Ans: (i) Efficiency can be increased as the higher temperature T 2 increases. (ii) Efficiency can be increased as the lower temperature T 1 decreases. 56. Why a heat engine cannot have 100% efficiency? Ans: 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. 57. When will be the Carnot cycle efficiency is maximum? Ans: Carnot cycle efficiency is maximum when the initial temperature is 0 °K.
58. What are the processes involved in Carnot cycle. Ans: Carnot cycle consist of i) Reversible isothermal compression ii) isentropic compression iii) reversible isothermal expansion iv) isentropic expansion 59. Write the expression for efficiency of the carnot cycle. T2 – T Ans: n = --------- T 60. Define: Thermodynamic cycles. Ans: Thermodynamic cycle is defined as the series of processes performed on the system, so that the system attains to its original state. 61. Define the term compression ratio. Ans: Compression ratio is the ratio between total cylinder volume to clearance volume. It is denoted by the letter ‘r’ 62. What is the range of compression ratio for SI and diesel engine? Ans: For petrol of SI engine 6 to 8 For diesel engine 12 to 18. 63. Which cycle is more efficient for the same compression ratio and heat input, Otto cycle or Diesel cycle? Ans: Otto cycle is more efficient than diesel cycle 64. Write the expression for efficiency of the otto cycle? Ans: 1 Efficiency n =1 - --------- (r)r-^1 65. The efficiency of the diesel cycle approaches the otto cycle efficiency when the cut off ratio is Ans: reduced 66. Which device is used to control the Air – fuel ratio in the petrol engine? Ans: Carburettor 67. Which device is used to control the Air fuel ratio in the diesel engine? Ans: Injection nozzle
Ans: Constant volume cycle.
77. What is meant by air standard efficiency of the cycle? Ans: It is defined as the ratio of work done by the cycle to the heat supplied to the cycle. Work done Efficiency n = -------------- Heat supplied 78. Define: Mean effective pressure of an I.C. engine. Ans: Mean effective pressure is defined as the constant pressure acting on the piston during the working stroke. It is also defined as the ratio of work done to the stroke volume or piston displacement volume. 79. What will be the effect of compression ratio on efficiency of the diesel cycle? Ans: Efficiency increases with the increase in compression ratio and vice – versa. 80. What will be the effect of cut off ratio on efficiency of the diesel cycle. Ans: Efficiency decreases with the increase of cut off ratio and vice – versa. 81. The thermal efficiency of a two stroke cycle engine is than the four stroke cycle engine. Ans: Lesser. 82. Define: Specific fuel consumption. Ans: SFC is defined as the amount of fuel consumed per brake power hour of work. 83. What is meant by calorific value of a fuel. Ans: Calorific value of a fuel is defined as the amount of heat liberated by the compete combustion of unit quantity of a fuel. 84. Give the expression for efficiency of the Dual cycle. Ans: where, 1 Kpr - 1 Efficiency n = 1 - ------- ----------------- (r)r- 1 (K-1) + yK(p-1) r – Compression ratio k – pressure or Expassion ratio p – cut off ratio and y – adiabatic index 85. The efficiency of the Dual cycle is than the diesel cycle and than the otto cycle for the same compression ratio.
Ans: greater, less.
86. What are the factors influencing of the Dual cycle? Ans: 1. Compression ratio 2.cut off ratio 3. pressure ratio and 4. heat supplied at constant volume and constant pressure. 87. The Brayton cycle is mainly used in Ans: Gas turbine power plant. 88. Give the expression for efficiency of the Brayton cycle. Ans: 1 Efficiency n = 1 ------------- where Rp – pressure ratio. (Rp)y-^1 89. The two stroke cycle engine gives the number of power strokes as compared to the four stroke cycle engine, at the same engine speed. Ans : double. 90. In petrol engine, the charge is ignited with the help of Ans : Spark plug 91. The diesel engine draws the mixture of diesel and air during suction stroke (True / False) Ans : False. 92. What is the fuel injector? Ans : Fuel injector is used in diesel engine to inject and atomize the diesel at the end of the compression stroke. 93. What is meant by SI engine? Why it is called so? Ans : SI engine means spark ignition engine. In SI engine air fuel mixture is ignited by spark plug hence it is called spark ignition engine. It is also called as petrol engine. 94. Give four major difference between two stroke and four stroke IC engine. No Two stroke cycle engine Four Stroke cycle engine 1 One cycle is completed in two stroke of the piston or one revolution of the crank shaft. One cycle is completed in four stroke of the piston or two revolution of the crank shaft. 2 For the same speed, twice the number of power strokes are produced than 4 stroke engine. For the same speed, half of the number of power strokes are produced than 2 stroke engine. 3 Turning moment is more uniform and hence lighter flywheel is used. Turning moment is not uniform and hence bigger flywheel is used.
Ans: In open cycle gas turbine, the exhaust gas form turbine is exhausted to the atmosphere and fresh air is taken in compressor for every cycle.
103. Gas turbine is working on ---- cycle Ans: Brayton or Jules cycle. 104. How can we increase the efficiency of the gas turbine? Ans: By providing inter cooler, re-heater along with heat exchanges. 105. Differentiate open and closed cycle gas turbines. Open cycle gas turbine Closed cycle gas turbine
- Working substance is exhausted to the atmosphere after one cycle.
- The same working substance is recirculated again and again.
- Pre-cooler is not required 2. Pre-cooler is required to cool the exhaust gas to the original temperature.
- High quality fuels are used 3. Low quality fuels are used
- For the same power developed size and weight of the plant is small
- Size and weight are bigger. 106. What is the function of intercooler in gas turbines? Where it is placed? Ans: The intercooler is placed between L.P. and H.P. compressors. It is used to cool the gas coming form L.P. compressor to its original temperature. 107. Why re-heater is necessary in gas turbine? What are its effects? Ans: The expansion process is very often performed in two sperate turbine stages. The re-heater is placed between the H.P. and L.P. turbines to increase the enthalpy of the exhaust gas coming from H.P. turbine. Effects:
- Turbine output is increased for the same compression ratio
- Thermal efficiency is less. 108. What is the function of regenerator in gas turbine? Ans: The main function of heat regenerator is to exchange the heat from exhaust gas to the compressed air for preheating before combustion chamber. It increases fuel economy and increase thermal efficiency. 109. What is meant by single acting compressor? In single acting compressor, the suction, compression and delivery of air take place on one side of the piston. 110. What is meant by double acting compressor? In double acting reciprocating compressor, the suction compressin and delivery of air take place on both side of the piston. 111. What is meant by single stage compressor?
In single stage compressor, the compression of air from the initial pressure to the final pressure is carried out in one cylinder only.
112. Define clearance ratio Clearance ratio is defined as the ratio of clearance volume to swept volume (or) stroke volume. Vc Vc – clearance volume C = -------- Vs – swept volume Vs 113. What is compression ratio? Compression ratio is defined as the ratio between total volume and clearance volume. Total volume Compression ratio = ------------------- Clearance Volume 114. What are the factors that effect the volumetric efficiency of a reciprocating compressor?
- Clearance volume 2) Compression ratio. 115. Compressor Capacity is a) Volume of air delivered b) Volume of air sucked c) Both a and b d) Nine of the above Ans: (a) 116. Compressor capacity is highest, when the intake air temperature is Ans: Lowest 117. Compressor capacity is expressed in Ans: m^3 /min 118. As the compression ratio increases, the volumetric efficiency of air compressor Ans: Decreases. 119. A 50 m^3 /min compressor can a) Compress 50m^3 /min of free air b) Compress 50m^3 /min of standard air c) Deliver 50m^3 /min of standard air d) Deliver 50m^3 /min of free air.
Ans: Before entering the compressor.
130. Give the 4 important parameters that are to be measured and controlled of an air conditioning system. 1. Temperature of air 2. Humidity of air 3. Purity of air 4. Motion of air 131. Name the cycles on which an Air refrigeration system works. Ans: 1. Reversed carnot cycle 2. Bell – coleman cycle 132. Name four important properties of a good refrigerant Ans: 1. Low boiling point 2. High critical temperature & pressure 3. Low sp.heat of liquid 4. Non – flammable and non explosive. 133. Name some of the equipments used in air conditioning system Ans: 1. Filter 2. Cooling coil 3. Heating coil 4. Compressor 5. Condeser 6. Evaporator 134. Name any four commonly used refrigerants Ans; 1. Ammonia (NH 3 ) 2. Carbon di oxide (CO 2 ) 3. Sulphur di oxide (SO 2 ) 4. Freon – 12. 135. What are the factors to be considered in air conditioning a room? Ans: 1. Temperature of air 2. Humidity of air 3. Purity of air 4. Motion of air. 136. The door of a running refrigerator inside a room was left open. What will happen? Ans: The room will be gradually warmed up. 137. Fourier’s Law is based on assumption that Ans: Heat transfer in steady state 138. A perfect black body is one which
Ans: Absorb heat radiation of all wave lengths falling on it.
139. The valve of the wave length for maximum emissive power is given by Ans: Wein’s Law 140. Thermal diffusivity of a substance is given by Ans: k pCp 141. The unit for Stefan – Boltzman constant is Ans: Watt/m^2 /k^4 142. Two plates spaced 150mm apart are maintained at 1000 °C and 70°c. The heat transfer will take place mainly by Ans: Radiation. 143. Heat conducted through unit area and unit thick face per unit time when temperature difference between opposite faces is unity is called Ans: Thermal conductivity 144. The amount of radiation mainly depends on Ans: Nature of body, temperature of body and type of surface of body 145. What is meant by radiation shape factor? The space factor is defined as the fraction of the radiative energy that is diffused from one surface element and strikes the other surface directly with no intervening reflections. It is represented by Fiji. Other names for the radiation shape factor are view factor, angle factor, and configuration factor. 146. Give the 3 basic expressions governing the 3 modes of heat transfer.
- Fourier Law of conduction dT Q = - KA ---------- dx where A – Area in m^2 dT
- ----- Temperature gradient in K/m dx K – Therma conductivity W/mK
- Newton’s Law of cooling Q = hA(Ts - T∞)
