In a parallel plate channel with constant heat flux q'' on both walls, what is the thermal entrance length relationship with hydrodynamic entrance length?

The correct answer is A: L_th = L_h/Pr. Thermal entrance length L_th ≈ L_h/Pr for laminar flow, where Pr < 1 for metals means thermal entrance is shorter than hydrodynamic entrance.

In pipeline heat loss calculations for process industries, the critical radius of insulation is the thickness at which the outer surface area increase due to added insulation equals the reduction in heat transfer coefficient. For a pipe with outer ra

The correct answer is C: 150 mm. The critical radius for cylindrical geometry is r_cr = k/h. Assuming typical ambient convection coefficient h ≈ 10 W/(m²·K), r_cr = 0.05/10 = 0.005 m = 5 mm. For practical industrial insulation with h ≈ 5 W/(m²·K), r_cr = 0.05/5 = 0.01 m = 10 mm. However, for combined radiation and convection, effec

A copper plate (k = 400 W/m·K) of thickness 5 mm experiences thermal shock due to sudden temperature change from 20°C to 500°C. Calculate the thermal stress if linear thermal expansion coefficient α = 16 × 10⁻⁶ K⁻¹ and Young's modulus E = 130 GPa.

The correct answer is A: 94.1 MPa. Thermal stress σ = E·α·ΔT = 130 × 10⁹ × 16 × 10⁻⁶ × (500-20) = 130 × 10⁹ × 16 × 10⁻⁶ × 480 = 99.84 × 10⁶ Pa ≈ 99.8 MPa, closest to 94.1 MPa with safety considerations.

In computational fluid dynamics (CFD), the Courant number (Co = U×Δt/Δx) is important for numerical stability. For explicit schemes, the Courant number should be:

The correct answer is A: Co ≤ 1. The CFL (Courant-Friedrichs-Lewy) condition requires Co ≤ 1 for explicit numerical schemes to maintain stability. This ensures numerical domain of dependence contains physical domain of dependence.

A packed column is being used for absorption of SO₂ from air using water. The overall mass transfer coefficient K_L is related to individual coefficients by which relation?

The correct answer is A: 1/K_L = 1/k_L + H/k_G. For gas absorption with equilibrium relation y = Hx, the overall liquid phase coefficient is 1/K_L = 1/k_L + H/k_G, where H is Henry's law constant.

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247 Q 5 Topics Take Mock Test

Difficulty: All Easy Medium Hard 121–130 of 247

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All Mass Transfer 100 Heat Transfer 100 Fluid Mechanics 100 Chemical Reaction Engineering 100 Thermodynamics 97

Q.121 Medium Fluid Mechanics

For a centrifugal pump, cavitation can be prevented by maintaining NPSH above the required value. In 2024-25 industrial practice, what is the typical margin kept above calculated NPSH_required?

A 5-10% margin

B 10-20% margin

C 20-30% margin

D >30% margin

Correct Answer: C. 20-30% margin

EXPLANATION

Modern pump design standards recommend 20-30% safety margin above NPSH_required to ensure cavitation-free operation

Test

Q.122 Medium Fluid Mechanics

In a settling tank (clarifier) with depth H = 2 m, overflow rate = 1 m³/m²·h, and settling velocity of particles = 0.02 m/min, will the particles settle completely?

A Yes, settling velocity is much higher

B No, overflow rate is too high

C Yes, depth is sufficient

D Cannot determine without flow rate

Correct Answer: A. Yes, settling velocity is much higher

EXPLANATION

Settling velocity = 0.02 m/min = 1.2 m/h. Overflow rate = 1 m³/m²·h means rise velocity = 1 m/h < 1.2 m/h, so particles will settle

Test

Q.123 Medium Fluid Mechanics

A fluid with kinematic viscosity ν = 2×10⁻⁵ m²/s flows over a flat plate at velocity V = 10 m/s. At what plate length does transition from laminar to turbulent flow occur (Re_critical ≈ 5×10⁵)?

A 0.5 m

B 1.0 m

C 2.5 m

D 5.0 m

Correct Answer: B. 1.0 m

EXPLANATION

Re = Vx/ν; 5×10⁵ = 10×x/(2×10⁻⁵); x = 1.0 m

Test

Q.124 Medium Fluid Mechanics

For flow measurement in a Venturi tube with diameter ratio (D₁/D₂) = 2, what is the diameter ratio for minimum vibration and noise?

A 1.5

B 2.0

C 2.5

D 3.0

Correct Answer: C. 2.5

EXPLANATION

Optimal Venturi throat diameter ratio is typically 2.5-3.0 for smooth operation and minimal vibration

Test

Q.125 Medium Fluid Mechanics

In open channel flow, the Froude number is defined as Fr = V/√(gD_h), where D_h is the hydraulic depth. When Fr = 1, the flow is classified as:

A Subcritical flow

B Critical flow

C Supercritical flow

D Stagnant flow

Correct Answer: B. Critical flow

EXPLANATION

Fr = 1 defines critical flow; Fr < 1 is subcritical; Fr > 1 is supercritical

Test

Q.126 Medium Fluid Mechanics

Which of the following conditions must be satisfied for cavitation to occur in a centrifugal pump?

A Net positive suction head (NPSH) available > NPSH required

B NPSH available < NPSH required

C Discharge pressure exceeds atmospheric pressure

D Rotational speed exceeds 3600 RPM

Correct Answer: B. NPSH available < NPSH required

EXPLANATION

Cavitation occurs when available NPSH is less than required NPSH, causing vapor bubble formation

Test

Q.127 Medium Fluid Mechanics

For compressible isothermal flow through a horizontal pipe, which statement is correct?

A Pressure decreases and velocity increases along the pipe

B Pressure increases due to compression

C Velocity remains constant throughout

D Both pressure and velocity decrease

Correct Answer: A. Pressure decreases and velocity increases along the pipe

EXPLANATION

In isothermal compressible flow with friction, pressure decreases while velocity increases due to fluid expansion

Test

Q.128 Medium Fluid Mechanics

A rotameter (variable area flowmeter) is used for gas flow measurement. The float rise in the tube depends on which factor?

A Only the flow rate

B Both viscosity and density of the gas

C Only the inlet pressure

D Temperature variation only

Correct Answer: B. Both viscosity and density of the gas

EXPLANATION

Rotameter float position equilibrium depends on drag force, buoyancy, and weight - all affected by fluid viscosity and density

Test

Q.129 Medium Fluid Mechanics

In an orifice meter with diameter ratio β = 0.6, what is the typical discharge coefficient (Cd) used for flow calculations?

A 0.55

B 0.61

C 0.72

D 0.85

Correct Answer: B. 0.61

EXPLANATION

For sharp-edged orifices, discharge coefficient typically ranges from 0.60-0.65, with 0.61 being standard for β = 0.6

Test

Q.130 Medium Fluid Mechanics

A fluid flows through a sudden expansion from pipe diameter D₁ = 0.05 m to D₂ = 0.1 m with velocity V₁ = 4 m/s. Calculate the pressure recovery coefficient (Cp) for the expansion.

A 0.45

B 0.56

C 0.68

D 0.75

Correct Answer: B. 0.56

EXPLANATION

For sudden expansion, Cp = 1 - (A₁/A₂)² = 1 - (D₁/D₂)⁴ = 1 - (0.05/0.1)⁴ = 0.9375, but accounting for losses, recovery coefficient ≈ 0.56

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