Which dimensionless number is used to characterize the relative importance of gravity and inertial forces in open channel flow?

The correct answer is B: Froude number. The Froude number Fr = V/√(gD_h) characterizes gravity effects. In open channel flow, it determines flow regime (subcritical Fr1).

A V-belt drive shows signs of slipping. Which of the following is NOT a likely cause?

The correct answer is C: Increased pulley diameter. Increasing pulley diameter would actually increase the belt length and tension, reducing slipping tendency. All other options reduce the friction coefficient or normal force, causing slipping.

A convergent-divergent nozzle (De Laval nozzle) accelerates gas to supersonic speeds. The throat area compared to exit area is:

The correct answer is B: Always smaller. In a convergent-divergent nozzle, throat area is smallest and exit area is larger for supersonic flow.

The Froude number Fr = V/√(gD) for channel flow determines the flow type. For Fr = 0.8, the flow is classified as:

The correct answer is B: Subcritical. Fr 1 is supercritical (rapid) flow. This classification is essential in open channel hydraulics for dam spillways and canal design.

A clutch is designed to transmit 15 kW at 1500 rpm. If the coefficient of friction is 0.3 and the axial force is 2000 N, what is the effective radius of the clutch disc?

The correct answer is B: 95 mm. Using P = 2πNT/60 and T = μ × F × r_e, solving gives r_e ≈ 95 mm for the given parameters.

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Mechanical Engineering

Thermodynamics, hydraulics, machine design

54 Q 3 Topics Take Mock Test

Difficulty: All Easy Medium Hard 41–50 of 54

Topics in Mechanical Engineering

All Thermodynamics 100 Fluid Mechanics 79 Machine Design 80

Q.41 Hard Thermodynamics

A compressor compresses air from 1 bar and 25°C to 8 bar. If the isentropic efficiency is 0.80 and the process is adiabatic, what is the actual temperature of air after compression? (γ = 1.4, R = 287 J/kg·K)

A 432 K

B 521 K

C 398 K

D 465 K

Correct Answer: D. 465 K

EXPLANATION

T1 = 298 K. Isentropic: T2s = T1(P2/P1)^((γ-1)/γ) = 298 × 8^(0.4/1.4) ≈ 298 × 1.741 ≈ 520 K. Actual: T2 = T1 + (T2s - T1)/η_is = 298 + (520-298)/0.8 ≈ 465 K

Test

Q.42 Hard Thermodynamics

In a diesel cycle, the expansion process (power stroke) is adiabatic. If the pressure and temperature at the end of compression are 40 bar and 850 K respectively, and the expansion ratio is 8, what is approximately the temperature at the end of expansion? (Take γ = 1.4)

A 412 K

B 325 K

C 550 K

D 680 K

Correct Answer: A. 412 K

EXPLANATION

For adiabatic process: T2/T1 = (P2/P1)^((γ-1)/γ) or T2 = T1 × r^(-(γ-1)) = 850 × 8^(-0.4/1.4) ≈ 850 × 0.485 ≈ 412 K

Test

Q.43 Hard Thermodynamics

For a real gas undergoing Joule-Thomson expansion through a throttle valve, the Joule-Thomson coefficient (μ_JT) is negative. This means:

A Temperature increases during expansion

B Temperature decreases during expansion

C The gas is ideal

D No temperature change occurs

Correct Answer: A. Temperature increases during expansion

EXPLANATION

μ_JT = (∂T/∂P)_H. If μ_JT < 0, then temperature increases when pressure decreases (expansion). Negative μ_JT occurs above inversion temperature.

Test

Q.44 Hard Thermodynamics

A Rankine cycle (ideal steam cycle) is used in thermal power plants. Which process has the highest irreversibility?

A Isentropic expansion in turbine

B Isobaric heat addition in boiler

C Isobaric heat rejection in condenser

D Isentropic compression in pump

Correct Answer: B. Isobaric heat addition in boiler

EXPLANATION

Heat transfer across finite temperature differences in the boiler creates maximum entropy generation and irreversibility among the four processes.

Test

Q.45 Hard Thermodynamics

For a closed system undergoing a reversible isothermal process, the change in Gibbs free energy (ΔG) is:

A Always positive

B Equal to the heat absorbed by the system

C Equal to the maximum useful work that can be extracted

D Always zero

Correct Answer: C. Equal to the maximum useful work that can be extracted

EXPLANATION

For isothermal process: ΔG = ΔH - TΔS = W_useful (non-PV work). This represents maximum useful work available.

Test

Q.46 Hard Thermodynamics

A Carnot heat pump operates between 270 K and 330 K. If 1000 J of work is supplied, how much heat is delivered to the hot reservoir?

A 5500 J

B 5000 J

C 6000 J

D 4500 J

Correct Answer: C. 6000 J

EXPLANATION

COP_heating = T_H/(T_H - T_C) = 330/(330-270) = 330/60 = 5.5. Q_H = W × COP = 1000 × 5.5 = 5500 J. Adding work input: Total = 5500 + 500 = 6000 J

Test

Q.47 Hard Thermodynamics

In a gas turbine Brayton cycle, air enters the compressor at 300 K and 100 kPa. The pressure ratio is 8. If γ = 1.4 and R = 287 J/kg·K, find the compressor outlet temperature (assuming isentropic compression).

A 477.5 K

B 520.2 K

C 598.7 K

D 456.3 K

Correct Answer: B. 520.2 K

EXPLANATION

T_2/T_1 = (P_2/P_1)^((γ-1)/γ) = 8^(0.4/1.4) = 8^0.2857 ≈ 1.734. T_2 = 300 × 1.734 ≈ 520.2 K

Test

Q.48 Hard Thermodynamics

In a diesel engine, if the cutoff ratio is 1.5 and compression ratio is 16, calculate the thermal efficiency. (Assume γ = 1.4)

A 56.2%

B 63.5%

C 52.8%

D 61.2%

Correct Answer: B. 63.5%

EXPLANATION

Diesel cycle efficiency = 1 - (1/r^(γ-1)) × [(r_c^γ - 1)/(γ(r_c - 1))] where r_c = cutoff ratio. With r=16, r_c=1.5, γ=1.4, efficiency ≈ 63.5%

Test

Q.49 Hard Thermodynamics

In an Otto cycle engine with compression ratio r = 9 and γ = 1.4, the thermal efficiency is approximately:

A 45%

B 58%

C 62%

D 40%

Correct Answer: B. 58%

EXPLANATION

Otto cycle efficiency: η = 1 - 1/r^(γ-1) = 1 - 1/9^0.4 = 1 - 1/2.28 ≈ 0.558 or 55.8% ≈ 58%.

Test

Q.50 Hard Thermodynamics

Maxwell relations are derived from the equality of:

A Mixed partial derivatives of thermodynamic potentials

B First and Second Laws only

C Heat capacities

D Equations of state

Correct Answer: A. Mixed partial derivatives of thermodynamic potentials

EXPLANATION

Maxwell relations come from the Schwarz theorem applied to thermodynamic potentials (U, H, F, G). For example, from dG = -SdT + VdP: (∂S/∂P)_T = -(∂V/∂T)_P.

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