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:

The correct answer is B: Critical flow. Fr = 1 defines critical flow; Fr 1 is supercritical

The penetration theory for mass transfer assumes that:

The correct answer is A: Eddies expose fresh bulk fluid to the interface for short contact times. Penetration theory (Higbie) assumes eddies contact interface, remain for time θ, then submerge. More realistic than film theory for turbulent systems.

For gas absorption in a plate column, the gas-phase mass transfer coefficient increases with:

The correct answer is A: Higher gas velocity and lower pressure. Higher velocity increases turbulence (higher k); lower pressure increases diffusivity D. Both increase Sh = kL/D.

The equation of state for a real gas that considers molecular volume and intermolecular forces is:

The correct answer is B: (P + an²/V²)(V - nb) = nRT. This is the van der Waals equation where 'a' accounts for intermolecular forces and 'b' for molecular volume

Which heat exchanger type provides the maximum temperature effectiveness under the same flow conditions?

The correct answer is A: Counter-flow. Counter-flow heat exchangers are most effective because they maintain the maximum temperature gradient throughout the exchanger length, resulting in higher heat transfer rates and effectiveness compared to parallel or cross-flow configurations.

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Chemical Engineering

Process design, thermodynamics, reactions

133 Q 5 Topics Take Mock Test

Difficulty: All Easy Medium Hard 91–100 of 133

Topics in Chemical Engineering

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

Q.91 Easy Heat Transfer

In a parallel flow heat exchanger, the temperature approach is minimum at which end?

A Hot fluid inlet

B Cold fluid outlet

C Hot fluid outlet

D Depends on mass flow rates

Correct Answer: C. Hot fluid outlet

EXPLANATION

In parallel flow, both fluids flow in the same direction. The smallest temperature difference (approach) occurs at the outlet end where the hot fluid has cooled and cold fluid has heated.

Test

Q.92 Easy Heat Transfer

What is the SI unit of thermal conductivity?

A W/(m·K)

B J/(s·m·K)

C kcal/(h·m·°C)

D Both A and B

Correct Answer: D. Both A and B

EXPLANATION

Thermal conductivity is expressed as W/(m·K) in SI units, which is equivalent to J/(s·m·K) since 1 W = 1 J/s.

Test

Q.93 Easy Heat Transfer

A flat-plate solar collector has a black absorber plate maintained at 80°C. The ambient temperature is 20°C and the heat loss coefficient (overall U value including radiation and convection) is 8 W/(m²·K). For a collector area of 4 m², what is the heat loss rate to the surroundings?

A 960 W

B 1440 W

C 1920 W

D 2880 W

Correct Answer: C. 1920 W

EXPLANATION

Q_loss = U × A × ΔT = 8 × 4 × (80-20) = 8 × 4 × 60 = 1920 W. The overall U value already accounts for both convective and radiative losses.

Test

Q.94 Easy Heat Transfer

In radiation heat transfer, the Stefan-Boltzmann constant σ has a value of 5.67 × 10⁻⁸ W/(m²·K⁴). A black body at 500 K radiates heat. If the temperature is doubled to 1000 K, by what factor does the radiated heat increase?

A 2 times

B 4 times

C 8 times

D 16 times

Correct Answer: D. 16 times

EXPLANATION

According to Stefan-Boltzmann law, Q = σAT⁴. When T increases from 500 K to 1000 K, the factor becomes (1000/500)⁴ = 2⁴ = 16 times. Radiation is highly temperature-dependent due to the fourth-power relationship.

Test

Q.95 Easy Heat Transfer

The Nusselt number (Nu) relationship for laminar flow in a circular pipe is given by Nu = 3.66. What does this value represent?

A The ratio of convective to conductive heat transfer at the pipe entrance

B The thermally developed laminar flow condition with constant wall temperature

C The average Nusselt number for fully developed laminar flow with constant heat flux

D The entrance region Nusselt number for transitional flow

Correct Answer: B. The thermally developed laminar flow condition with constant wall temperature

EXPLANATION

The value of 3.66 is the fully developed Nusselt number for laminar flow in circular pipes under constant wall temperature (T_s constant) boundary condition, representing the thermal development region.

Test

Q.96 Easy Heat Transfer

In a shell-and-tube heat exchanger, what is the primary advantage of using baffles?

A To increase turbulence and improve heat transfer coefficient on the shell side

B To reduce the pressure drop across the exchanger

C To decrease the surface area required for heat transfer

D To eliminate fouling on heat transfer surfaces

Correct Answer: A. To increase turbulence and improve heat transfer coefficient on the shell side

EXPLANATION

Baffles in shell-and-tube heat exchangers create turbulence by forcing the fluid to flow in a cross-flow pattern, significantly increasing the convective heat transfer coefficient on the shell side, which enhances overall heat transfer effectiveness.

Test

Q.97 Easy Heat Transfer

Which of the following materials would be most suitable for a heat sink application requiring high thermal conductivity and low cost?

A Pure copper (k = 400 W/m·K)

B Aluminum alloy (k = 160 W/m·K)

C Stainless steel (k = 16 W/m·K)

D Carbon fiber composite (k = 100 W/m·K)

Correct Answer: B. Aluminum alloy (k = 160 W/m·K)

EXPLANATION

Aluminum alloys offer an excellent balance of thermal conductivity (160 W/m·K, sufficient for most applications), cost-effectiveness, light weight, corrosion resistance, and ease of machining. While copper has higher conductivity, aluminum's cost-benefit ratio is superior for heat sink applications.

Test

Q.98 Easy Heat Transfer

A horizontal cylindrical pipe carries hot water at 80°C through ambient air at 20°C. The convective heat transfer coefficient is 25 W/m²·K and pipe diameter is 50 mm. Calculate heat loss per unit length if the pipe is not insulated.

A 47.1 W/m

B 78.5 W/m

C 94.2 W/m

D 156.3 W/m

Correct Answer: A. 47.1 W/m

EXPLANATION

Q/L = h·π·d·ΔT = 25 × π × 0.05 × (80-20) = 25 × π × 0.05 × 60 = 235.6/5 ≈ 47.1 W/m

Test

Q.99 Easy Heat Transfer

In the context of thermal radiation, the Stefan-Boltzmann constant has a value of:

A 5.67 × 10⁻⁸ W/m²·K⁴

B 6.67 × 10⁻¹¹ m³/kg·s²

C 1.38 × 10⁻²³ J/K

D 8.314 J/mol·K

Correct Answer: A. 5.67 × 10⁻⁸ W/m²·K⁴

EXPLANATION

The Stefan-Boltzmann constant σ = 5.67 × 10⁻⁸ W/m²·K⁴ is used in radiation heat transfer calculations (Q = σAε(T⁴)). Other options are physical constants used in different domains.

Test

Q.100 Easy Heat Transfer

Which of the following dimensionless numbers is used to predict the transition from laminar to turbulent flow in forced convection?

A Grashof number (Gr)

B Reynolds number (Re)

C Rayleigh number (Ra)

D Fourier number (Fo)

Correct Answer: B. Reynolds number (Re)

EXPLANATION

Reynolds number determines the flow regime in forced convection. Re < 2300 indicates laminar flow, Re > 4000 indicates turbulent flow. Grashof and Rayleigh are used for natural convection, while Fourier number is for transient conduction.

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