For a non-isothermal diffusion process with coupled heat and mass transfer, the Lewis number Le is defined as:

The correct answer is A: Ratio of thermal diffusivity to mass diffusivity. Lewis number Le = α/D_AB, where α is thermal diffusivity. It indicates the relative importance of heat and mass transfer in coupled processes. Le ≈ Sc/Pr for most gases.

For simultaneous diffusion and reaction (fast reaction regime) in a porous catalyst, the effectiveness factor η approaches:

The correct answer is B: 1/Φ where Φ is the Thiele modulus. In the fast reaction regime where diffusion limits the overall rate, η ≈ 1/Φ for spherical pellets, indicating severe internal diffusion limitations.

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?

The correct answer is A: Yes, settling velocity is much higher. 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

In membrane separation, the permeate flux in reverse osmosis is given by:

The correct answer is B: J = A(ΔP - Δπ). The reverse osmosis permeate flux follows: J = A(ΔP - Δπ), where A is membrane permeability, ΔP is applied pressure, and Δπ is osmotic pressure difference.

The compressibility factor Z for an ideal gas is:

The correct answer is B: Always equal to 1. For ideal gas, Z = PV/nRT = 1 always. For real gases, Z ≠ 1 due to intermolecular forces and molecular volume.

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247 Q 5 Topics Take Mock Test

Difficulty: All Easy Medium Hard 151–160 of 247

Topics in Chemical Engineering

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

Q.151 Medium Fluid Mechanics

Water flows through a venturimeter with inlet diameter 0.1 m and throat diameter 0.05 m. The pressure difference is 5 kPa. Assuming ideal flow, calculate the velocity at the inlet (ρ = 1000 kg/m³).

A 1.44 m/s

B 2.88 m/s

C 4.32 m/s

D 5.76 m/s

Correct Answer: B. 2.88 m/s

EXPLANATION

Using continuity and Bernoulli: V₁ = √[2ΔP/(ρ(A₁²/A₂² - 1))]. With area ratio 4, V₁ = √[2×5000/(1000×15)] = 2.88 m/s.

Test

Q.152 Medium Fluid Mechanics

For turbulent flow in rough pipes at high Reynolds numbers, which friction factor equation is most applicable?

A Laminar equation: f = 64/Re

B Colebrook-White equation

C Blasius equation: f = 0.316/Re⁰·²⁵

D Swamee-Jain equation

Correct Answer: B. Colebrook-White equation

EXPLANATION

Colebrook-White equation is implicit but accurate for all turbulent regimes including rough pipes. For very rough pipes at high Re, relative roughness dominates.

Test

Q.153 Medium Heat Transfer

In a multipass heat exchanger design, increasing the number of shell passes from 1 to 2 will predominantly affect which parameter?

A Increases the heat transfer coefficient h

B Increases the correction factor F closer to 1

C Decreases required heat exchanger area

D All of the above

Correct Answer: B. Increases the correction factor F closer to 1

EXPLANATION

Adding shell passes (1-2 or 2-4 configuration) brings the temperature distribution closer to counterflow arrangement, increasing the correction factor F (reducing mismatch with LMTD). This increases effective heat transfer driving force without changing h significantly.

Test

Q.154 Medium Heat Transfer

The Prandtl number (Pr = Cp·μ/k) represents the ratio of which two transport properties?

A Momentum diffusivity to thermal diffusivity

B Thermal conductivity to viscosity

C Kinematic viscosity to thermal diffusivity

D Heat capacity to thermal conductivity

Correct Answer: A. Momentum diffusivity to thermal diffusivity

EXPLANATION

Prandtl number = ν/α where ν = μ/ρ (momentum diffusivity) and α = k/(ρCp) (thermal diffusivity). Pr << 1 means heat diffuses faster than momentum; Pr >> 1 means momentum diffuses faster.

Test

Q.155 Medium Heat Transfer

In pool boiling, the critical heat flux (CHF) occurs at which point on the boiling curve?

A Onset of nucleate boiling (ONB)

B Peak of nucleate boiling curve before transition to film boiling

C At transition from nucleate to film boiling

D During saturated liquid heating

Correct Answer: B. Peak of nucleate boiling curve before transition to film boiling

EXPLANATION

Critical heat flux (CHF) is the maximum heat flux in nucleate boiling. Beyond this point, further heat input causes transition to film boiling with lower heat transfer coefficient, leading to surface temperature rise (burnout).

Test

Q.156 Medium Heat Transfer

The overall heat transfer coefficient U in a composite system (series resistances) is determined by which method?

A U = 1/(R_total) where R_total = R_conv,1 + R_cond + R_conv,2

B U = h₁ + h₂ + (k/L)

C U is constant regardless of resistance arrangement

D U must be measured experimentally

Correct Answer: A. U = 1/(R_total) where R_total = R_conv,1 + R_cond + R_conv,2

EXPLANATION

Overall heat transfer coefficient is the reciprocal of total thermal resistance: 1/U = 1/(h₁A) + L/(kA) + 1/(h₂A). This accounts for series arrangement of convection and conduction resistances.

Test

Q.157 Medium Heat Transfer

For radiation heat transfer between two surfaces, increasing the absolute temperature of the hot surface by 10% will increase radiative heat transfer by approximately what percentage?

A 10%

B 21%

C 40%

D 46%

Correct Answer: D. 46%

EXPLANATION

Radiative heat transfer follows Stefan-Boltzmann law: Q ∝ T⁴. If T increases by 10%, new flux = (1.1T)⁴ = 1.464T⁴ ≈ 46.4% increase.

Test

Q.158 Medium Heat Transfer

In convective heat transfer, which dimensionless number represents the ratio of buoyant to viscous forces?

A Rayleigh number (Ra)

B Grashof number (Gr)

C Richardson number (Ri)

D Both A and B

Correct Answer: D. Both A and B

EXPLANATION

Grashof number (Gr = gβΔT L³/ν²) directly represents buoyancy to viscous force ratio. Rayleigh number (Ra = Gr·Pr) combines Grashof and Prandtl numbers for natural convection.

Test

Q.159 Medium Heat Transfer

A shell-and-tube heat exchanger with one shell pass and two tube passes (1-2 STHE) has a correction factor F. What is the typical range of F values?

A 0.5 to 0.9

B 0.9 to 1.0

C 1.0 to 1.2

D 0.2 to 0.8

Correct Answer: A. 0.5 to 0.9

EXPLANATION

The correction factor F (applied to LMTD) for 1-2 STHE typically ranges from 0.5 to 0.9 depending on temperature ratios and heat capacity ratios. F is always ≤ 1.

Test

Q.160 Medium Heat Transfer

The Peclet number (Pe = Re·Pr) is used to determine the relative importance of convection to conduction. For Pe

A Convection is dominant

B Conduction is dominant

C Both are equally important

D Neither is significant

Correct Answer: B. Conduction is dominant

EXPLANATION

The Peclet number Pe = Re·Pr = (velocity × characteristic length × ρ·cp)/k represents the ratio of convection to conduction. When Pe << 1, conduction dominates over convection because advective transport is very slow compared to thermal diffusion.

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