Category: Thermal Engineering Objective Questions

Thermal Engineering Multiple Choice Questions on “Reaction Turbines – 1”.

1. The magnitude of velocity of steam relative to moving blade in case of an impulse turbine _____
a) always remains constant as steam glides over the blades
b) depending on friction present on the blades, increases or remains constant as the steam glides over the blades
c) depending on friction present on the blades, decreases or remains constant as the steam glides over the blades
d) depending on friction present on the blades, increases or decreases as the steam glides over the blades

Answer: c
Clarification: In Impulse turbines the magnitude of velocity of steam relative to the moving blade either remains contant or decreases slightly due to presence of friction on the blades. The ratio of final relative velocity of steam to initial is called the blade velocity coefficient (K).

2. In case of reaction turbines, the magnitude of velocity of steam relative to moving blade increases as the steam progresses.
a) True
b) False

Answer: a
Clarification: As the steam flows through the reaction turbine blades, it is continuously expanded. As a result the magnitude of velocity of steam relative to the moving blades increases as the steam flows over the blade.

3. Which of the following expressions for degree of reaction (R_d) of a reaction turbine stage is correct?
a) R_d=(frac{Heat , drop , in , moving , blades}{Heat , drop , in , the , stage})
b) R_d=(frac{Heat , drop , in , the , stage}{Heat , drop , in , moving , blades})
c) R_d=(frac{Heat , drop , in , fixed , blades}{Heat , drop , in , the , stage})
d) R_d=(frac{Heat , drop , in , the , stage}{Heat , drop , in , fixed , blades})

Answer: a
Clarification: The degree of reaction (R_d) of a reaction turbine stage is defined as the ratio of heat drop in moving blade to the total heat drop in the stage.
Mathematically,
R_d=(frac{Δh_m}{Δh_f+Δh_m} )
where, Δh_m = heat drop in moving blades and
Δh_f = heat drop in the fixed blades.

4. Which of the following is the correct formula for calculating the degree of reaction of a reaction turbine?
a) R_d=(frac{2C_{bl}(C_{w1}+C_{w2})}{C_{r2}^2-C_{r1}^2} )
b) R_d=(frac{C_{r2}^2-C_{r1}^2}{2C_{bl} (C_{w1}+C_{w2})} )
c) R_d=(frac{C_{bl}(C_{w1}+C_{w2})}{2(C_{r2}^2-C_{r1}^2)} )
d) R_d=(frac{2(C_{r2}^2-C_{r1}^2)}{C_{bl}(C_{w1}+C_{w2})} )

Answer: b
Clarification: We know that,
R_d=(frac{Heat , drop , in , moving , blades}{Heat , drop , in , the , stage})
Since, Heat drop in a moving blade = (frac{C_{r2}^2-C_{r1}^2}{2}), and
Heat drop in the stage = C_bl(C_w1 + C_W2)
Therefore, R_d=(frac{C_{r2}^2-C_{r1}^2}{2C_{bl} (C_{w1}+C_{w2})} )

5. The degree of reaction of a Parson’s reaction turbine is _____
a) 0%
b) 25%
c) 50%
d) 100%

Answer: c
Clarification: In Parson’s reaction turbine the degree of reaction is 50%. Also in a Parson’s reaction turbine the shape of moving and the fixed blades is same. Due to this the velocity diagram for the blades of this turbine are symmetrical.

6. Which of the following statements does NOT hold true for a Parson’s reaction turbine?
a) α = Ф
b) θ = β
c) C₁ = C_r2
d) C_r1 = C_r2

Answer: d
Clarification: Parson’s reaction turbine has same shape for moving and fixed baldes. Which implies,
θ = β and α = Ф
Also, C₁ = C_r2and C₂ = C_r1, which results in a symmetrical velocity diagram.

7. What should be the ratio of blade speed to absolute velocity of steam leaving the nozzle for a Parson’s reaction turbine to work at maximum efficiency?
a) cos⁡α
b) cos⁡β
c) sin⁡α
d) sin⁡β

Answer: a
Clarification: In a Parson’s reaction turbine, the degree of reaction is 50% and the moving and fixed blades are symmetrical. This turbine works at maximum possible efficiency when the ratio of blade speed to absolute velocity of steam leaving the nozzle is equal to cosine of nozzle angle.

8. The maximum value of efficiency for a Parson’s reaction turbine is _____
a) (frac{2(cos⁡)^2}{1+(cos ⁡α)^2} )
b) (frac{2(cos α)^2}{1+(cos ⁡α)^2} )
c) (frac{1+(cos ⁡α)^2}{2(cos α)^2} )
d) (frac{2(cos α)^2}{1+(cos ⁡α)^2} )

Answer: d

9. The axial force on the blades in Parson’s reaction turbine is always zero.
a) True
b) False

Answer: a
Clarification: The velocity diagram of a parson’s reaction turbine is symmetrical, which results in equal flow components of absolute inlet and outlet velocities.
i.e. C_f1 = C_f2
Therefore, axial force on blades = ṁ (C_f1 – C_f2) = 0.

10. The following data refers to a particular stage of a Parson’s reaction turbine:
Mean blade speed = 90 m/s
Velocity of steam leaving the nozzle = 120 m/s
Nozzle angle = 20°
Mass flow rate of steam = 0.8 kg per second
Calculate the tangential force on the blade.
a) 203.54 N
b) 108.42 N
c) 65.32 N
d) 195.45 N
Answer: b
11. In a single stage Parson’s reaction turbine, the mean blade velocity is observed to be 60 m/s and the nozzle angle to be 20°. Determine the the entrance angle of moving blade if the absolute velocity of the steam leaving the nozzle is 120 m/s.
a) 38°
b) 56°
c) 23°
d) 28°
Answer: a
12. In a reaction turbine, the fixed blades and the moving blades are of the same shape but reversed in direction. The mean blade speed is 30 m/s. The absolute velocity of the steam leaving the nozzle is 90 m/s and the absolute velocity of steam leaving the moving blade is 64 m/s. Determine the nozzle angle.
a) 20°
b) 25°
c) 30°
d) 35°
Answer: b
13. In a Parson’s reaction turbine, the outlet angle of blade is 25°. Calculate the work done per second by the blades if the blade mean blade speed is 60 m/s and the absolute velocity of the steam leaving the nozzle is 120 m/s. The mass flow rate of steam is 0.1 kg per second.
a) 756 W
b) 854 W
c) 945.12 W
d) 1065 W
Answer: c

14. In a single stage reaction turbine, the mean blade velocity is observed to be 200 m/s and the nozzle angle is 20°. The absolute value of the steam leaving the nozzle is 350 m/s. The factor with which the relative velocity of steam changes wit respect to the moving is 2.1. If the moving blade inlet and outlet aangles are equal then calculate the degree of reaction of the turbine.
a) 50%
b) 66%
c) 75%
d) 84%
Answer: b

Thermal Engineering Multiple Choice Questions on “Diesel Cycle (or) Constant Pressure Cycle”.

1. Which cycle is idealized cycle for the compression ignition engines?
a) Otto cycle
b) Diesel cycle
c) Dual cycle
d) Bryton cycle

Answer: b
Clarification: Compression ignition engine that uses fuel injection to compressed air for combustion. An air standard diesel cycle is the idealized cycle for the compression ignition engines.

2. In diesel cycle heat rejection occurs at ___________
a) Reversible constant volume process
b) Reversible constant pressure process
c) Irreversible constant volume process
d) Irreversible constant pressure process

Answer: a
Clarification: Diesel cycle consist of one isobaric process, one isochoric process and two adiabatic process. Heat rejection process is isochoric process.

3. In diesel cycle heat addition occurs at ___________
a) Reversible constant volume process
b) Reversible constant pressure process
c) Irreversible constant volume process
d) Irreversible constant pressure process

Answer: b
Clarification: Diesel cycle consist of one isobaric process, one isochoric process and two adiabatic process. Heat rejection process is isobaric process.

4. In diesel engine fuel is ignited by ___________
a) Fuel injection
b) Spark
c) Heat resulting from compressing air
d) Heat resulting from compression of air fuel mixture

Answer: a
Clarification: Compression ignition engine that uses fuel injection to compressed air for combustion. An air standard diesel cycle is the idealized cycle for the compression ignition engines.

5. The thermal efficiency of diesel cycle having fixed compression ratio, with increase in cut-off ratio.
a) Increases
b) Decreases
c) Be independent
d) Data insufficient

Answer: b
Clarification: Efficiency of diesel cycle = 1 – (frac{ρ^γ-1}{(ρ-1)r^{γ-1}} )

Increase in cut-off ratio will increase in term (frac{ρ^γ-1}{(ρ-1)r^{γ-1}} ) therefore decrease in efficiency of diesel cycle.

6. The thermal efficiency of diesel cycle having fixed cut-off ratio, with increase in compression ratio.
a) Increases
b) Decreases
c) Be independent
d) Data insufficient

Answer: a
Clarification: Efficiency of diesel cycle = 1 – (frac{ρ^γ-1}{(ρ-1)r^{γ-1}} )

Increase in compression ratio will decrease in term (frac{ρ^γ-1}{(ρ-1)r^{γ-1}} ) therefore increase in efficiency of diesel cycle.

7. An air standard diesel cycle consist of ___________
a) One constant pressure process and three adiabatic process
b) One constant pressure process, one constant volume process and two adiabatic process
c) Two constant volume process and two adiabatic process
d) Two constant pressure process and two adiabatic process

Answer: b
Clarification: The sequence of processes in diesel cycle is isentropic compression, isobaric heat addition, isentropic expansion and isochoric heat rejection. Therefore an air standard diesel cycle consist of one constant volume, one constant pressure and two adiabatic process.

8. The compression ratio (r_k) of diesel cycle is equal to ___________
a) r_k = volume of the cylinder at the beginning of the compression/volume of the cylinder at the end of the compression
b) r_k = volume of the cylinder at the end of the compression/ volume of the cylinder at the beginning of the compression
c) r_k = clearance volume/ volume of the cylinder at the beginning of the compression
d) r_k = volume of the cylinder at the end of the compression/clearance volume

Answer: a
Clarification: r_k = (frac{swept , volume+clearance , volume}{clearance , volume} )
r_k = volume of the cylinder at the beginning of the compression/volume of the cylinder at the end of the compression.

9. For same peak pressure and heat input _________
a) Otto cycle is more efficient
b) Diesel cycle is more efficient
c) Dual cycle is more efficient
d) Both diesel and otto cycle are equally efficient

Answer: b
Clarification: Efficiency of otto cycle = 1 – (frac{1}{r^{γ-1}})
Efficiency of diesel cycle = 1 – (frac{ρ^γ-1}{(ρ-1)r^{γ-1}} ).

10. What is the value of working fluid in diesel cycle?
a) 1.0
b) 1.2
c) 1.4
d) 1.8

Answer: c
Clarification: In air standard diesel cycle working fluid is air. The specific heat ratio of air is equal to 1.4.

11. What is the efficiency of diesel cycle if γ and T denotes the specific heat ratio and temperature respectively?
a) 1 – (frac{T4-T1}{T3-T2} )
b) 1 – (frac{T4-T1}{γ(T3-T2)} )
c) 1 – (frac{γ(T4-T1)}{T3-T2} )
d) 1 – (frac{T4-T1}{(γ-1)(T3-T2)} )

Answer: b
Clarification: Q_addition = m_cp(T3 – T2)
Q_rejection = m_cv(T4 – T1)
Efficiency of diesel engine = (frac{Q1-Q2}{Q1} )
Efficiency of diesel engine = 1 – (frac{T4-T1}{γ(T3-T2)} )

12. Scavenging in diesel cycle means___________
a) Air used for combustion under pressure
b) Forced air for cooling the cylinder
c) Burnt air containing products of combustion
d) Air used for forcing burnt gases out of engines cylinder during exhaust stroke

Answer: d
Clarification: Scavenging is defined as forcing burnt gases out of engines cylinder during exhaust stroke. Therefore scavenging in diesel cycle means air used for forcing burnt gases out of engines cylinder during exhaust strokes.

13. A diesel engine is usually more efficient than a spark ignition engine because ___________
a) Diesel being a heavier hydrocarbon, release more heat per kg than gasoline
b) The air standard efficiency of diesel engine is higher than the otto cycle, at a fixed compression ratio
c) The compression ratio of diesel engine is higher than petrol engine
d) Self ignition temperature of diesel engine is higher than that of gasoline

Answer: c
Clarification: Diesel engine actually have a higher peak temperature than petrol engines. A diesel engine is usually more efficient than a spark ignition engine because the compression ratio of diesel engine is higher than petrol engine.

14. The combustion in compression ignition engine is __________
a) Homogenous
b) Heterogeneous
c) Laminar
d) Turbulent

Answer: b
Clarification: In compression ignition engine fuel injection system is used. Therefore fuel is injected to compressed air and hence air fuel mixture is heterogeneous.

15. Which of the following is compressed in diesel engine?
a) Air
b) Air and fuel
c) Air and lubricating oil
d) Fuel

Answer: a
Clarification: In compression ignition engine fuel injection system is used. Therefore fuel is injected to compressed air and hence air is only compressed in diesel engine.

Thermal Engineering Objective Questions on “Heat Transfer by Conduction”.

1. If the radius of any current carrying conductor is less than the critical radius, then why the addition of electrical insulation will enable the wire to carry a higher current?
a) The heat loss from the wire would decrease
b) The heat loss from the wire would increase
c) The thermal resistance of the insulation is reduced
d) The thermal resistance of the conductor is increased

Answer: b
Clarification: If the radius of any current carrying conductor is less than the critical radius, then the addition of electrical insulation will enable the wire to carry a higher current because heat loss from the wire would increase. Radius of wire is inversely proportional to heat loss.

2. Which of the following substance has the minimum value of thermal conductivity?
a) Air
b) Water
c) Plastic
d) Rubber

Answer: a
Clarification: Among the given options air has the minimum value of thermal conductivity. Thermal conductivity of air is approximately K_air=0.02.

3. In MLTθ system (T time and θ temperature), what is the dimension of thermal conductivity?
a) ML^-1T^-1θ^-3
b) MLT^-1θ^-3
c) MLT^-3θ^-1
d) MLT^-2θ^-1

Answer: c
Clarification: Q=-KA(dT/dx)
(ML²T^-3) = k (L²) (θ)
K = ML²T^-3/Lθ , K=MLT^-3θ^-1.

4. For conduction through a spherical wall with constant thermal conductivity and with inner side temperature greater than outer wall temperature in 1-D heat transfer, what is the type of temperature
distribution?
a) Linear
b) Parabolic
c) Hyperbolic
d) Logarithmic

Answer: c
Clarification: For one dimensional steady heat flow-
i. Temperature distribution in slab is linear
ii. Temperature distribution in cylinder is logarithmic
iii. Temperature distribution in sphere is hyperbolic.

5. Which of the following expresses the thermal diffusivity of a substance in terms of thermal conductivity of a substance (k), mass density (ρ) and specific heat (c)?
a) k²ρc
b) 1/ρkc
c) k/ρc
d) ρc/k²

Answer: c
Clarification: Thermal diffusivity indicates the ease at which energy get diffused in the volume of the substances. It is defined as the ratio of the thermal conductivity to the heat capacity of the substance.
Therefore, α = k/ρc.

6. A copper block and an air mass block having similar dimensions are subjected to symmetrical heat transfer from one face of the each block. The other face of the block will be reaching to the same temperature at a rate?
a) Faster in air block
b) Faster in copper block
c) Equal in air as well as copper block
d) Data in sufficient

Answer: b
Clarification: Thermal conductivity is minimum in gases and maximum in solids. Thermal conductivity of copper is higher than air. Hence heat flow will be in copper block rather than air block.

7. The outer surface of a long cylinder is maintained at constant temperature. The cylinder does not have any heat source. The temperatures in the cylinder will _________
a) Increase linearly with radius
b) Decrease linearly with radius
c) Be independent of radius
d) Vary logarithmically with radius

Answer: d
Clarification: The temperature distribution will vary logarithmically with radius for cylinder.
α = (frac{2πLk(T1-T2)}{(ln⁡(frac{r1}{r2})}).

8. A steam pipe is covered with two layers of insulating materials, with the better insulating material forming the outer. What is the effect on heat conducted if the two layers are interchanged?
a) Will increase
b) Will decrease
c) Will remain unaffected
d) May increase or decrease depending upon the thickness of each layer

Answer: d
Clarification: Let k₁ and k₂ be the thermal conductivity of the two insulators, k₁ being the better insulator.
Q = (frac{A∆T}{frac{ln⁡(frac{r2}{r1})}{k1}+frac{ln⁡(frac{r3}{r2})}{k2}})
If we interchange the insulators, the value of Q might increase or decrease depending upon the thickness of each layer.

9. A plane wall is 20cm thick with an area perpendicular to heat flow of 1m² and has a thermal conductivity of 0.5W/mK. A temperature difference of 100°C is imposed across it. What is the ratio of heat flow?
a) 0.10 kW
b) 0.15 kW
c) 0.20 kW
d) 0.25 kW

Answer: c
Clarification: Thermal resistance, R_th = (frac{L}{KA}=frac{0.2}{0.5×1}=0.5frac{K}{W} )

Heat transfer, Q = (frac{(T1-T2)}{R_{th}}=frac{100}{0.5})=200W=0.2kW.

10. An insulating material with a thermal conductivity k = 0.12W/mK is used for a pipe carrying steam. The local coefficient of heat transfer to the surrounding h = 4 W/m²K. In order to provide effective insulation, what should be the minimum outer diameter of the pipe?
a) 45mm
b) 60mm
c) 75mm
d) 90mm

Answer: b
Clarification: Minimum outer radius should be critical radius. The minimum critical radius is given by,
r_c = (frac{k}{h}=frac{0.12}{4})=0.03m=30mm
Hence, outer diameter = r_c = (2×30) mm = 60mm.

11. In a long cylindrical rod of radius R and a surface heat flux of q₀, what is the uniform internal heat generation rate?
a) 2q₀/R
b) 2q₀
c) q₀/R
d) 2q₀/3R

Answer: a
Clarification: In steady state, Heat generated = Heat conducted at surface
q_g × πR²L = q₀ × 2πRL
q_g = 2q₀/R.

12. A plane slab of 100mm thickness generates heat. It is observed that the temperature drop between the center and its surface to be 50°C. If the thickness is increased to 200mm the temperature difference will be ___________
a) 100°C
b) 200°C
c) 400°C
d) 600°C

Answer: b
Clarification: T_max = T_wall + q_gL²/2k
(T_max – T_wall) = q_gL²/2k
(T_max – T_wall) α L²
(frac{(Tmax – Twall)2}{(Tmax – Twall)1}=frac{L_2}{L_1} )
(T_max – T_wall)₂/50 = ((frac{200}{100}))²
(T_max – T_wall)2 = 400°C.

13. As the temperature increases, the thermal conductivity of a gas ____________
a) Increases
b) Decreases
c) Remain constant
d) Increases up to a certain temperature and then decreases

Answer: a
Clarification: In gases heat conduction occurs by molecular momentum transfer when high velocity, high temperature molecules collide with the low velocity low temperature molecules. Thermal conductivity of gases increases with temperature because molecular momentum transfer increases with increase in temperature.