Application of Thermodynamics Multiple Choice Questions
Q1. Which one of the following is correct?
✅ Correct: In the wet vapor region, temperature and pressure are dependent, so an isothermal line is also a constant pressure line.
Q2. With increase in pressure, the latent heat of the steam
✅ Correct: Latent heat of steam decreases with increase in pressure.
Q3. When wet steam flows through a throttle valve and remains wet at exit
✅ Correct: In throttling, enthalpy stays constant, so both temperature and quality decrease if steam remains wet.
Q4. Constant pressure lines in the superheated region of the Mollier diagram will have
✅ Correct: In the superheated region, constant pressure lines show a positive slope on the Mollier diagram.
Q5. Which one of the following statements is correct when saturation pressure of water vapor increases?
✅ Correct: As saturation pressure rises, the enthalpy of evaporation decreases.
Q6. For a pure substance, what are the numbers of the thermodynamic degree of freedom for saturated vapor and superheated vapor, respectively?
✅ Correct: Saturated vapor has 1 d.o.f. while superheated vapor has 2 d.o.f.
Q7. Which one of the following is correct? At critical point the enthalpy of vaporization is
✅ Correct: At the critical point, liquid and vapor phases merge, so enthalpy of vaporization is zero.
Q8. Water (\(c_p = 4 \,\text{kJ/kgK}\)) is fed to a boiler at 30°C, the enthalpy of vaporization at atmospheric pressure in the boiler is 2400 kJ/kg; the steam coming from the boiler is 0.9% dry. What is the net heat supplied in the boiler?
Q9. In steam power cycle, reheat factor is usually in the range
✅ Correct: Reheat factor in steam cycles is typically 1.02–1.05.
Q10. In a steam power plant, feed water heater is a heat exchanger to preheat feed water by
✅ Correct: Feed water is preheated using extracted steam from turbine (regeneration).
Q11. A condenser of a refrigeration system rejects heat at a rate of 120 kW, while its compressor consumes a power of 30 kW. The coefficient of performance of the system would be
✅ Correct: COP = QL/W = (120−30)/30 = 90/30 = 3.
Q12. In a Rankine cycle, with the maximum steam temperature being fixed from metallurgical considerations, as the boiler pressure increases
Q15. Which one of the following is correct? In ideal regenerative cycle, the temperature of steam entering the turbine is same as that of
✅ Correct: In ideal regeneration, feedwater is heated to turbine exhaust steam temperature.
Q16. In a convergent-divergent nozzle, normal shock can generally occur
✅ Correct: Normal shock usually forms in the divergent section or near throat.
Q17. In flow through steam nozzles, the actual discharge will be greater than the theoretical value when
✅ Correct: Supersaturation delays condensation, giving higher actual discharge.
Q18. In flow through a convergent nozzle, the ratio of back pressure to the inlet pressure is given by the ratio \(\frac{p_B}{p_1} = \left(\frac{2}{\gamma+1}\right)^{\frac{2\gamma}{\gamma-1}}\). If the back pressure is lower than \(p_B\) given by the above equation, then
✅ Correct: If back pressure is too low, expansion continues outside nozzle with shock outside.
Q19. For adiabatic expansion with friction through a nozzle, the following remains constant:
✅ Correct: In adiabatic flow with friction, stagnation enthalpy remains constant.
Q20. The effect of friction on flow of steam through nozzle is to
✅ Correct: Friction lowers mass flow rate and reduces wetness at nozzle exit.
Q21. If the velocity of propagation of small disturbance in air at 27°C is 330 m/s, then at a temperature of 54°C, its speed would be
✅ Correct: Speed of sound ∝ √T, so at 54°C it becomes 330 × √(327/300).
Q22. For one-dimensional isentropic flow in a diverging passage, if the initial static pressure is p₁, and the initial Mach number is M₁, then for the downstream flow
✅ Correct: In subsonic diverging flow, Mach decreases while pressure also decreases.
Q23. The stagnation temperature of an isentropic flow of air (γ = 1.4) is 400 K. If the temperature is 200 K at a section, then the Mach number of the flow will be
✅ Correct: Using T₀/T = 1 + (γ−1)/2·M² ⇒ M = √5 = 2.236.
Q24. Isentropic flow is
✅ Correct: Isentropic flow is a reversible adiabatic process.
Q25. An aeroplane is cruising at a speed of 800 kmph at an altitude, where the air temperature is 0°C. The flight Mach number at this speed is nearly
✅ Correct: At 0°C, speed of sound ≈ 331 m/s, so Mach ≈ 222/331 ≈ 0.67.
Q26. The isentropic heat drop in the nozzle of an impulse turbine with nozzle efficiency 0.9, blade velocity ratio 0.5 and mean blade velocity 150 m/s in kJ/kg is
✅ Correct: From velocity ratio and blade speed, isentropic heat drop ≈ 50 kJ/kg.
Q27. The critical pressure ratio for maximum discharge through a nozzle is
✅ Correct: Critical pressure ratio is \( (2/(n+1))^{n/(n-1)} \).
Q28. Under ideal conditions, the velocity of steam at the outlet of a nozzle for a heat drop of 400 kJ/kg will be, approximately,
✅ Correct: Velocity = √(2×1000×400) ≈ 900 m/s.
Q29. In a steam nozzle, inlet pressure of superheated steam is 10 bar. The exit pressure is decreased from 3 bar to 1 bar. The discharge rate will
✅ Correct: Once choked, further lowering exit pressure does not change mass flow rate.
Q30. In a steam nozzle, to increase the velocity of steam above sonic velocity by expanding steam below critical pressure
✅ Correct: Supersonic velocity requires a divergent section after throat.
Q31. Steam pressure values at the inlet and exit of a nozzle are 16 bar and 5.2 bar, respectively, and discharge is 0.28 m³/s. Critical pressure ratio is 0.5475. If the exit pressure is reduced to 3.2 bar, then what will be the flow rate in m³/s?
✅ Correct: Once choked, lowering exit pressure further does not change flow rate.
Q32. In a steam condenser, the partial pressure of steam and air are 0.06 bar and 0.007 bar, respectively. The condenser pressure is
✅ Correct: Total condenser pressure = steam + air partial pressures = 0.067 bar.
Q33. The main aim of compounding in steam turbines is
✅ Correct: Compounding reduces turbine speed to practical limits.
Q34. In a de Laval nozzle expanding superheated steam from 10 to 0.1 bar, the pressure at the minimum cross-section will be
✅ Correct: At throat, pressure = critical ratio × inlet pressure = 0.5475×10 = 5.46 bar.
Q35. For a Parson’s reaction turbine, if α₁ and α₂ are fixed blade angles at inlet and exit, respectively, and β₁ and β₂ are the moving blade angles at entrance and exit, respectively, then
✅ Correct: In Parson’s turbine, fixed and moving blade angles are symmetrical so α₁ = β₂ and α₂ = β₁.
Q36. Match List I with List II and select the correct answer using the codes given below
List-I (Turbine) List-II (Classification)
A. Parson’s → ?
B. De Laval → ?
C. Rateau → ?
D. Curtis → ?
✅ Correct: Parson’s–reaction, De Laval–simple impulse, Rateau–pressure compounded, Curtis–velocity compounded.
Q37. In Parson’s reaction turbine stage, blade velocity is 320 m/s at the mean radius and the rotor blade exit angle is 30°. For minimum kinetic energy of the steam leaving the stage, the steam velocity at the exit of the stator will be
✅ Correct: For minimum exit KE, V = 2Vb cosβ = 640/√3 m/s.
Q38. A reaction turbine stage has angles α, β, γ as nozzle angle, inlet blade angle, and outlet blade angle, respectively. The expression for maximum efficiency of the turbine is given by
✅ Correct: Max efficiency of reaction turbine = 2cos²β / (1+cos²β).
Q39. The degree of reaction of a turbine is defined as the ratio of
✅ Correct: Degree of reaction = energy from pressure drop / total energy transfer.
Q40. Given that Vb = Blade speed, V = Velocity of steam entering the blade, α = Nozzle angle. The efficiency of an impulse turbine is maximum when
✅ Correct: Impulse turbine efficiency is max when blade speed = 0.5 V cosα.
Q41. The Rateau turbine belongs to the category of
✅ Correct: Rateau turbine is a pressure‑compounded impulse turbine.
Q42. A Curtis stage, Rateau stage and a 50% reaction stage in a steam turbine are examples of
✅ Correct: Curtis is velocity‑compounded, Rateau is pressure‑compounded, and Parson’s is 50% reaction.
Q43. A steam plant has the boiler efficiency of 92%, turbine efficiency (mechanical) of 94%, generator efficiency of 95% and cycle efficiency of 44%. If 6% of the generated power is used to run the auxiliaries, the overall plant efficiency is
✅ Correct: Multiplying efficiencies and subtracting auxiliaries gives ≈34% overall.
Q44. For a single stage impulse turbine with a rotor diameter of 2 m and a speed of 3000 rpm when the nozzle angle is 20°, the optimum velocity of steam in m/s is?
Q45. If in a steam turbine stage, heat drop in moving blade ring is 40 kJ/kg and heat in the fixed blade ring is 60 kJ/kg, then the degree of reaction is
Q46. If in an impulse turbine designed for free vortex flow, tangential velocity of steam at the root radius of 250 mm is 430 m/s and the blade height is 100 mm, then the tangential velocity of steam at the tip will be
Q47. The maximum blade efficiency of single stage impulse turbine in terms of nozzle angle α under ideal conditions is proportional to
✅ Correct: For equiangular, frictionless blades, maximum efficiency ∝ cos²α.
Q48. In a reaction turbine, the enthalpy drop in the fixed blade ring is 50 kJ per kg and the enthalpy drop in the moving blade ring is 25 kJ per kg. The degree of reaction of the turbine is
Q49. Symmetrical blading is used in a turbine when its degree of reaction is
✅ Correct: Symmetrical blading occurs at 50% reaction.
Q51. The impulse turbine rotor efficiency will have a maximum value cos²α where α is the nozzle exit flow angle, if the
✅ Correct: With equiangular, frictionless blades, efficiency ∝ cos²α.
Q52. Energy conversion takes place only in one row of rotor of nozzle blades and later the steam glides over the rotor and guide rows in the case of
✅ Correct: De Laval turbine is a single‑stage impulse turbine.
Q53. In a 50% reaction turbine stage, the tangential component of absolute velocity at rotor inlet is 537 m/s and blade velocity is 454 m/s. The power output in kW of steam will be
✅ Correct: Calculation gives ~282 kW output.
Q54. Employing superheated steam in turbines leads to
Q55. In which one of the following steam turbines, steam is taken from various points along the turbine, solely for feedwater heating?
✅ Correct: Regenerative turbines extract steam for feedwater heating.
Q56. The degree of reaction of an impulse turbine:
✅ Correct: Impulse turbines have degree of reaction = 0.
Q57. A 4-row velocity compounded steam turbine develops a total power of 6400 kW. What is the power developed by the last row?
✅ Correct: In velocity compounding, each row develops progressively less power. For 4 rows and 6400 kW total, the last row develops 400 kW.
Q58. Blade erosion in steam turbines takes place
✅ Correct: Moisture droplets in wet steam strike the blades, causing erosion. Superheating reduces this problem.
Q59. In a simple impulse turbine the nozzle angle at the entrance is 30°. For maximum diagram efficiency, what is the blade-speed ratio?
✅ Correct: For impulse turbines, optimum blade speed ratio = cosα / 2. With α = 30°, ratio = 0.866/2 = 0.433.
Q60. If the enthalpy drop in the moving blades and fixed blades of a steam turbine is 10 kJ/kg and 15 kJ/kg, respectively, then what is the degree of reaction?
✅ Correct: Degree of reaction = enthalpy drop in moving blades ÷ total = 10 ÷ (10+15) = 0.4 = 40%.
Q61. In steam and other vapor cycles, the process of removing non-condensables is called
✅ Correct: Deaeration removes dissolved gases and non‑condensables to prevent corrosion and improve efficiency.
Q62. Given that ηs = Stage efficiency, ηn = Nozzle efficiency, ηb = Blade efficiency. Which one of the following is correct?
✅ Correct: Stage efficiency is the product of nozzle and blade efficiencies, so ηs = ηb × ηn.
Q63. In a spark ignition engine working on the ideal Otto cycle, the compression ratio is 5.5. The work output per cycle is 23.625 × 10⁵ × vc J. The indicated mean effective pressure is
✅ Correct: IMEP = Work per cycle ÷ displacement volume. Substituting values gives 5.25 bar.
Q64. The correct sequence of the decreasing order of brake thermal efficiency of the three given basic types of IC engines is
Correct: 4-stroke CI > 4-stroke SI > 2-stroke SI.
Diesel (CI) engines have higher compression ratios and thus higher thermal efficiency.
4-stroke SI engines are less efficient than CI but better than 2-stroke SI, which wastes more fuel and has higher losses.
Q65. An IC engine has a bore and stroke of 2 units each. The area to calculate heat loss can be taken as
Correct: 6π.
Heat loss area = piston crown area + cylinder liner area.
With bore = 2, stroke = 2: piston area = π(1²) = π, liner area = circumference × stroke = 2π×1×2 = 4π.
Adding head area gives total ≈ 6π.
Q66. With increasing temperature of intake air, IC engine efficiency
Correct: Remains the same.
Theoretical efficiency of Otto/Diesel cycles depends on compression ratio, not intake temperature.
Practically, higher intake temperature reduces charge density, but cycle efficiency formula itself is unaffected.
Q67. For the same maximum pressure and heat input
Correct: Petrol engine exhaust temperature is higher.
Petrol (SI) engines operate closer to constant volume heat addition, leading to higher exhaust temperatures compared to Diesel engines under same max pressure and heat input.
Q68. Which cycle consists of two reversible isotherms and two reversible isobars?
Correct: Ericsson cycle.
It has two isothermal processes (heat addition and rejection) and two isobaric processes.
Carnot has isothermal + adiabatic, Stirling has isothermal + isochoric, Brayton has isentropic + isobaric.
Q69. In the case of a Diesel cycle, increasing the cut-off ratio will increase
Correct: Mean effective pressure.
Higher cut‑off ratio means more fuel is injected, increasing work output per cycle.
Efficiency actually decreases with higher cut‑off ratio, but mean effective pressure rises.
Q70. For the same compression ratio and the same heat input, the correct sequence of the increasing order of the thermal efficiencies of the given cycles is
Correct: Diesel < Dual < Otto.
For the same compression ratio and heat input:
- Otto cycle (constant volume heat addition) has the highest efficiency.
- Diesel cycle (constant pressure heat addition) has the lowest.
- Dual cycle lies in between.
Q71. In an air standard Otto cycle, r is the volume compression ratio and γ is an adiabatic index (cp/cv), the air standard efficiency is given by
Correct: η = 1 − 1/r^(γ−1).
This comes from the isentropic compression and expansion relations in the Otto cycle.
Efficiency depends only on compression ratio r and γ, not on the working fluid or temperature.
Q72. The bore and stroke of the cylinder of a 6-cylinder engine working on an Otto-cycle are 17 cm and 30 cm, respectively, total clearance volume is 9225 cm³, then what is the compression ratio?
Correct: 5.4.
- Swept volume per cylinder = (π/4) × (0.17²) × 0.30 ≈ 681 cm³.
- For 6 cylinders, swept volume = 681 × 6 ≈ 4086 cm³.
- Compression ratio r = (Swept + Clearance) / Clearance = (4086 + 9225)/9225 ≈ 1.44 → but per cylinder clearance = 9225/6 = 1537.5 cm³.
- So r = (681 + 1537.5)/1537.5 ≈ 5.4.
Q73. The air–fuel ratio for idling speed of an automobile petrol engine is closer to
Correct: 10:1.
At idling, engines require a richer mixture (more fuel, less air) to maintain smooth combustion at low speeds.
Stoichiometric ratio is ~15:1, but idling needs richer ~10:1.
Q74. In combustion process, the effect of dissociation is to
Correct: Reduce the flame temperature.
Dissociation absorbs part of the heat energy to break molecules into simpler components, lowering the effective flame temperature.
This is why actual flame temperatures are lower than theoretical adiabatic values.
Q75. As compared to air standard cycles, in actual working, the effect of variation in specific heats is to
✅ Correct: In actual engines, specific heats vary with temperature.
- As temperature rises, cp and cv increase, which lowers the effective temperature rise for the same heat input.
- This results in lower maximum pressure and lower maximum temperature compared to the ideal air‑standard cycle assumption (constant specific heats).
Q76. The essential function of carburettor in a spark engine is to:
✅ Correct: The carburettor’s main role is to meter the right amount of fuel into the air stream depending on load and speed.
Mixing and vaporization also occur, but the essential function is precise fuel metering.
Q77. Match List I with List II related to operation SI engines and select the correct option.
List-I (Operating mode)
A. Idling
B. Cold starting
C. Cruising
D. Maximum power
✅ Correct:
- Idling needs a slightly rich mixture → A-2 (≈9:1).
- Cold starting requires very rich mixture → B-5 (≈3:1).
- Cruising runs lean for economy → C-3 (≈16:1).
- Maximum power requires rich mixture → D-1 (≈12.5:1).
Q78. In some carburettors, meter rod and economizer device is used for
✅ Correct: The meter rod and economizer provide extra fuel during acceleration to avoid hesitation or flat spots.
They momentarily enrich the mixture when throttle is opened quickly.
Q81. By higher octane number of SI fuel, it means that the fuel has
✅ Correct: A higher octane number means the fuel has greater resistance to knocking.
This corresponds to a longer ignition delay before auto‑ignition, allowing higher compression ratios in SI engines.
It does not imply higher heating value or flash point.
Q83. In the operation of four-stroke diesel engines, the term ‘squish’ refers to the
✅ Correct: Squish is the rapid entry of air into the combustion chamber from the piston–cylinder clearance, improving turbulence and mixing.
Q84. Which one of the following quantities is assumed constant for an internal combustion engine while estimating its friction power by extrapolation through Willan’s line?
✅ Correct: Willan’s line assumes indicated thermal efficiency remains constant while extrapolating to find friction power.
Q85. In a Morse test for a 2-cylinder, 2-stroke, spark ignition engine, the brake power was 9 kW whereas the brake powers of individual cylinders with spark cut-off were 4.25 kW and 3.75 kW, respectively. The mechanical efficiency of the engine is
Q86. An engine produces 10 kW brake power while working with a brake thermal efficiency of 30%. If the calorific value of the fuel used is 40,000 kJ/kg, then what is the fuel consumption?
✅ Correct: Brake power = 10 kW = 10 kJ/s.
Brake thermal efficiency ηb = BP / (ṁf × CV).
So, ṁf = BP / (ηb × CV) = 10 / (0.30 × 40,000) = 0.000833 kg/s.
Converting to kg/h → 0.000833 × 3600 ≈ 3.0 kg/h.
Q87. A 40 kW engine has a mechanical efficiency of 80%. If the frictional power is assumed to be constant with load, what is the approximate value of the mechanical efficiency at 50% of the rated load?
✅ Correct: At full load, BP = 40 kW, ηm = 0.8 → IP = 50 kW, so FP = 10 kW.
At 50% load, BP = 20 kW, IP = BP + FP = 30 kW.
ηm = BP/IP = 20/30 = ≈65%.
Q88. The main object of Morse test is to find out
✅ Correct: The Morse test is used for multi‑cylinder SI engines.
By cutting off spark in each cylinder and measuring drop in brake power, the frictional power can be estimated.
Q89. The amount of CO₂ produced by 1 kg of carbon on complete combustion in kg is
✅ Correct: Reaction: C + O₂ → CO₂.
M(C) = 12, M(CO₂) = 44.
So, 12 kg C → 44 kg CO₂.
1 kg C → 44/12 = 11/3 kg CO₂.
Q90. The presence of nitrogen in the products of combustion ensures that:
✅ Correct: Nitrogen is inert in combustion and enters only with air.
Its presence in exhaust gases confirms that air was used as the oxidizer.
Q91. The combustion analysis carried out by the Orsat Apparatus renders which one of the following?
✅ Correct: The Orsat apparatus measures CO₂, O₂, CO in flue gases.
It gives the percentage composition by volume on a dry basis.
Q92. Air cooling is used for freon compressors, whereas water jacketing is adopted for cooling ammonia compressors. This is because
✅ Correct: Ammonia has a higher latent heat and produces more heat during compression.
Hence, more effective cooling (water jacketing) is required, while Freon can be cooled by air.
Q93. Global warming is caused by
✅ Correct: Carbon dioxide is the primary greenhouse gas responsible for global warming.
It traps infrared radiation, leading to a rise in Earth’s average temperature.
Q94. The pipes and fitting in an ammonia refrigeration system should be made of
✅ Correct: Ammonia reacts with copper and its alloys, causing corrosion and leakage.
Therefore, ammonia refrigeration systems use cast steel or wrought iron pipes and fittings for safety and durability.
Q95. The color of the flame of halide torch, in case of leakage of Freon refrigerant, will change to
✅ Correct: In a halide torch test, Freon refrigerant decomposes in the flame and produces compounds of copper.
This causes the flame to turn a bright green color, indicating leakage.
Q96. The significant advantage of using ammonia as a refrigerant is its
✅ Correct: Ammonia has a high latent heat of vaporization, which means it can absorb large amounts of heat per unit mass.
This makes it highly efficient as a refrigerant, requiring less mass flow for the same cooling effect.
Q97. A one ton capacity water cooler cools water steadily from 35°C to 20°C. The specific heat of water is 4.18 kJ/kgK. The water flow rate will be, nearly
Q98. When a refrigerator system is started from ambient conditions, the evaporator temperature decreases from ambient temperature to design value. This period is known as a pull-down period. The power requirement of compressor during pull down
✅ Correct: During pull‑down, the evaporator temperature is initially high, so suction pressure is high and the compressor requires more power.
As the evaporator cools down to design temperature, suction pressure drops and the compressor power requirement decreases.
Hence, the power first increases and then decreases.
Q99. A refrigeration system operating on reversed Brayton cycle is used for maintaining 250 K. If the temperature at the end of constant pressure cooling is 300 K and rise in the temperature of air in the refrigerator is 50 K, then the network of compression will be (assume cp = 1 kJ/kg°C)
✅ Correct: In reversed Brayton cycle, compression work = cp × (T₂ − T₁).
Here, T₁ = 250 K, T₂ = 275 K (since rise = 50 K/2 across two stages).
So, W = 1 × (275 − 250) = 25 kJ/kg.
Therefore, the network of compression is 25 kJ/kg.
Q100. In a vapor compression refrigeration plant, the refrigerant leaves the evaporator at 195 kJ/kg and the condenser at 65 kJ/kg. For every kg of refrigerant, the plant can supply per second a cooling load of
✅ Correct: Cooling effect per kg = hevap out − hcond out = 195 − 65 = 130 kJ/kg.
If 1 kg of refrigerant flows per second, cooling load = 130 kJ/s = 130 kW.