A gas A diffuses through a stagnant film of gas B. If the diffusivity D_AB = 0.1 cm²/s, film thickness δ = 0.01 cm, and concentration difference = 0.05 mol/cm³, the diffusive flux is:

The correct answer is B: 5 × 10⁻³ mol/(cm²·s). Using Fick's law for stagnant diffusion: J = D_AB·ΔC/δ = 0.1 × 0.05 / 0.01 = 0.5 × 10⁻² = 5 × 10⁻³ mol/(cm²·s).

Which dimensionless number is used to characterize the ratio of inertial forces to viscous forces in fluid flow?

The correct answer is B: Reynolds number. Reynolds number (Re = ρVD/μ) represents the ratio of inertial to viscous forces and determines the flow regime (laminar, transitional, or turbulent).

The Damköhler number (Da) in chemical reactor design represents:

The correct answer is B: Ratio of residence time to reaction time. Da = kτ = (reaction rate constant × residence time), dimensionless number indicating reaction extent

For a process where ΔG < 0 at all temperatures, the process must be:

The correct answer is B: Exothermic with ΔS > 0. From ΔG = ΔH - TΔS, for ΔG 0 (entropy increases). This is a spontaneous process at all temperatures.

The virial equation of state truncated after second term is: PV = nRT(1 + B(T)P/RT). What does B(T) represent?

The correct answer is D: All of the above. B(T) is the second virial coefficient that accounts for molecular interactions. It corrects ideal gas behavior and is temperature-dependent, directly representing non-ideality.

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Chemical Engineering
Heat Transfer

Process design, thermodynamics, reactions

26 Q 5 Topics Take Mock Test

Difficulty: All Easy Medium Hard 21–26 of 26

Topics in Chemical Engineering

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

Q.21 Hard Heat Transfer

In a counter-flow double pipe heat exchanger, the outlet temperature of hot fluid becomes lower than the outlet temperature of cold fluid. This is:

A Thermodynamically impossible

B Possible only with infinite heat transfer area

C Always possible in counter-flow arrangement

D Possible only if cold fluid enters at higher temperature

Correct Answer: C. Always possible in counter-flow arrangement

EXPLANATION

In counter-flow, the temperature profiles allow hot outlet to be lower than cold outlet with sufficient heat transfer area, limited by second law of thermodynamics.

Test

Q.22 Hard Heat Transfer

For a microfin tube (enhanced surface), the heat transfer improvement comes primarily from:

A Increased surface area

B Increased turbulence and boundary layer disruption

C Combination of increased area and disrupted boundary layer

D Reduced friction factor only

Correct Answer: C. Combination of increased area and disrupted boundary layer

EXPLANATION

Microfins provide dual benefits: (1) increased surface area A, and (2) enhanced convection coefficient h by disrupting boundary layer development, thus improving overall heat transfer coefficient.

Test

Q.23 Hard Heat Transfer

Which statement best describes radiation heat transfer in participating media?

A Identical to vacuum conditions

B Enhanced by absorption and emission within the medium

C Always negligible for gases

D Independent of medium properties

Correct Answer: B. Enhanced by absorption and emission within the medium

EXPLANATION

In participating media (like CO₂, H₂O vapor), radiation is absorbed and re-emitted by gas molecules, affecting net radiative heat transfer significantly.

Test

Q.24 Hard Heat Transfer

The Graetz number in laminar heat transfer represents:

A Ratio of entrance effects to fully developed conditions

B Ratio of buoyancy to viscous forces

C Ratio of heat capacity to thermal conductivity

D Ratio of conduction to convection

Correct Answer: A. Ratio of entrance effects to fully developed conditions

EXPLANATION

Graetz number Gz = (D_h/L) × Re × Pr indicates relative importance of entrance effects. Gz > 100 means entrance region dominates.

Test

Q.25 Hard Heat Transfer

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

A L_th = L_h/Pr

B L_th = L_h × Pr

C L_th = L_h

D L_th ≈ 0.05 × Re × D_h × Pr

Correct Answer: A. L_th = L_h/Pr

EXPLANATION

Thermal entrance length L_th ≈ L_h/Pr for laminar flow, where Pr < 1 for metals means thermal entrance is shorter than hydrodynamic entrance.

Test

Q.26 Hard Heat Transfer

For a long thin fin with adiabatic tip condition, the fin efficiency is given by:

A η_fin = tanh(mL)/(mL)

B η_fin = 1 - (tanh(mL))/(mL)

C η_fin = sinh(mL)/(mL·cosh(mL))

D η_fin = 1/(mL)

Correct Answer: A. η_fin = tanh(mL)/(mL)

EXPLANATION

For adiabatic tip condition: η_fin = tanh(mL)/(mL), where m = √(hP/kA_c). This represents the efficiency of heat transfer through the fin.

Test

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