For pipe flow, the friction factor f in the Moody diagram depends on both Reynolds number and relative roughness (ε/D). For a rough pipe with high Re, f approaches an asymptotic value independent of Re. This region is called:

The correct answer is B: Fully turbulent region (complete turbulence). At very high Reynolds numbers in rough pipes, friction factor depends only on relative roughness, not Re. This region is called the 'fully turbulent' or 'zone of complete turbulence' region in the Moody diagram.

In a Diesel cycle, which process represents constant pressure heat addition?

The correct answer is B: Isobaric process. The Diesel cycle has constant pressure heat addition (isobaric process) between the initial compression and subsequent expansion, distinguishing it from the Otto cycle

A hollow circular shaft has outer diameter 50 mm and inner diameter 30 mm. Calculate the polar moment of inertia.

The correct answer is C: 257,484 mm⁴. J = π/32 × (D⁴ - d⁴) = π/32 × (50⁴ - 30⁴) = π/32 × (6,250,000 - 810,000) ≈ 257,484 mm⁴

In centrifugal pump design, the impeller exit diameter D₂ and speed N (rpm) determine the peripheral velocity U₂ = πD₂N/60. For a pump with D₂ = 300 mm running at 1500 rpm, the peripheral velocity is:

The correct answer is A: 23.56 m/s. U₂ = π × 0.3 × 1500/60 = 23.56 m/s. This velocity is critical in pump design as it affects the Euler's head and pump efficiency in Indian water supply and irrigation projects.

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Mechanical Engineering
Thermodynamics

Thermodynamics, hydraulics, machine design

45 Q 3 Topics Take Mock Test

Difficulty: All Easy Medium Hard 21–30 of 45

Topics in Mechanical Engineering

All Thermodynamics 100 Fluid Mechanics 79 Machine Design 80

Q.21 Medium Thermodynamics

Steam at 80 kPa with enthalpy 2660 kJ/kg and entropy 7.31 kJ/kg·K. What is the thermodynamic state of this steam?

A Saturated liquid

B Saturated vapor

C Wet steam (two-phase mixture)

D Superheated vapor

Correct Answer: D. Superheated vapor

EXPLANATION

At 80 kPa, saturated vapor properties are h_g ≈ 2660.3 kJ/kg and s_g ≈ 7.31 kJ/kg·K. Given values match saturated vapor line, indicating superheated or at saturation point; properties at saturation boundary indicate superheated vapor.

Test

Q.22 Medium Thermodynamics

A heat pump operating between 280 K and 350 K has a coefficient of performance (COP) of 5. How much work input is required to transfer 1000 J of heat to the hot reservoir?

A 200 J

B 250 J

C 300 J

D 175 J

Correct Answer: A. 200 J

EXPLANATION

COP = Q_h / W = 5. Therefore, W = Q_h / COP = 1000 / 5 = 200 J

Test

Q.23 Medium Thermodynamics

For an ideal gas in a constant pressure (isobaric) process from state 1 to state 2, the change in specific entropy is:

A Cp ln(T2/T1)

B Cv ln(V2/V1)

C R ln(P2/P1)

D Cp ln(P2/P1)

Correct Answer: A. Cp ln(T2/T1)

EXPLANATION

For isobaric process: dS = dQ/T = nCp dT/T. Integrating: ΔS = nCp ln(T2/T1) or specific entropy Δs = Cp ln(T2/T1)

Test

Q.24 Medium Thermodynamics

In a turbocharger application, compressed air enters a turbine. If the isentropic efficiency of the turbine is 0.85, what does this indicate?

A 85% of input heat is converted to work

B Actual work output is 85% of isentropic work output

C 85% of the system is reversible

D The process is 85% adiabatic

Correct Answer: B. Actual work output is 85% of isentropic work output

EXPLANATION

Isentropic efficiency = Actual work output / Isentropic work output = 0.85. It compares real process with ideal isentropic process.

Test

Q.25 Medium Thermodynamics

During throttling of a real gas through an expansion valve, which thermodynamic property remains constant?

A Temperature

B Entropy

C Enthalpy

D Internal energy

Correct Answer: C. Enthalpy

EXPLANATION

Throttling is an isenthalpic process (constant enthalpy). Temperature may change for real gases due to Joule-Thomson effect.

Test

Q.26 Medium Thermodynamics

During an adiabatic expansion of an ideal gas, the temperature of the gas decreases. This is because:

A Heat is removed from the gas

B The gas does work on its surroundings, reducing internal energy

C The surroundings do work on the gas

D The gas absorbs heat from surroundings

Correct Answer: B. The gas does work on its surroundings, reducing internal energy

EXPLANATION

In adiabatic process, Q = 0. From first law: dU = -W. During expansion, W > 0, so dU < 0, meaning internal energy and temperature decrease.

Test

Q.27 Medium Thermodynamics

Which of the following statements is correct regarding the first law of thermodynamics?

A Energy can be created in an isolated system

B The internal energy of an isolated system can increase indefinitely

C Heat and work are forms of energy transfer, not properties of a system

D Internal energy is path-dependent

Correct Answer: C. Heat and work are forms of energy transfer, not properties of a system

EXPLANATION

The first law states that dU = Q - W, where heat (Q) and work (W) are energy transfer mechanisms, not state properties. Internal energy (U) is a state property.

Test

Q.28 Medium Thermodynamics

A reversible heat engine operates between two thermal reservoirs. If the temperature of the hot reservoir is 500 K and the cold reservoir is 300 K, what is the maximum possible efficiency of this engine?

A 40%

B 60%

C 50%

D 75%

Correct Answer: A. 40%

EXPLANATION

Maximum efficiency occurs in a Carnot engine: η = 1 - (T_cold/T_hot) = 1 - (300/500) = 1 - 0.6 = 0.4 or 40%

Test

Q.29 Medium Thermodynamics

A heat engine receives 2000 J from a hot reservoir and rejects 1200 J to a cold reservoir. What is the thermal efficiency and work output?

A η = 40%, W = 800 J

B η = 60%, W = 1200 J

C η = 40%, W = 1200 J

D η = 50%, W = 1000 J

Correct Answer: A. η = 40%, W = 800 J

EXPLANATION

Efficiency η = W/Q_in = (Q_in - Q_out)/Q_in = (2000-1200)/2000 = 800/2000 = 0.4 = 40%. Work output W = 800 J

Test

Q.30 Medium Thermodynamics

At what condition is the specific heat capacity at constant pressure equal to infinity?

A At critical point

B During phase change at constant temperature and pressure

C At absolute zero

D At very high pressures

Correct Answer: B. During phase change at constant temperature and pressure

EXPLANATION

During phase transitions (like vaporization at constant T and P), infinite heat can be absorbed without temperature change, making C_p → ∞.

Test

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