An engineering application of throttling includes:

The correct answer is D: All of the above. Throttling is isenthalpic (ΔH = 0) and occurs in expansion valves, regulators, and orifices. Used in refrigeration, HVAC systems. Entropy increases (irreversible) while enthalpy remains constant.

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

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

In a regenerative heat exchanger (rotary wheel type), the effectiveness depends on the capacity rate ratio and heat capacity of the wheel material. If the wheel rotates slowly (high residence time), what effect does this have on effectiveness?

The correct answer is A: Increases effectiveness due to longer heat transfer contact time. In rotary regenerators, slower rotation provides longer residence time for each sector in contact with hot and cold fluids, increasing heat transfer duration and thereby increasing the overall effectiveness of heat recovery.

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Chemical Engineering
Heat Transfer

Process design, thermodynamics, reactions

49 Q 5 Topics Take Mock Test

Difficulty: All Easy Medium Hard 31–40 of 49

Topics in Chemical Engineering

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

Q.31 Medium Heat Transfer

In radiation view factor calculations, the reciprocity relation F₁₂·A₁ = F₂₁·A₂ ensures:

A Energy conservation in radiation exchange

B Symmetry of radiating surfaces

C Both (a) and (b)

D Gray body approximation validity

Correct Answer: A. Energy conservation in radiation exchange

EXPLANATION

Reciprocity relation ensures energy conservation: heat leaving surface 1 striking surface 2 equals heat leaving 2 striking 1 on reciprocal basis.

Test

Q.32 Medium Heat Transfer

The convective heat transfer coefficient 'h' for natural convection from a vertical surface at constant temperature increases with height due to:

A Increase in boundary layer thickness

B Increase in buoyancy-driven velocity

C Both (a) and (b) effects competing

D Decrease in local Grashof number

Correct Answer: C. Both (a) and (b) effects competing

EXPLANATION

While boundary layer grows (reducing h), buoyancy increases velocity and local Gr increases (increasing h). Combined effect shows h varies as x^(-1/4).

Test

Q.33 Medium Heat Transfer

The mean free path (λ) in gas kinetic theory at standard conditions is approximately 60 nm. This implies that at atmospheric pressure, heat conduction in gases is primarily through:

A Molecular diffusion

B Radiation

C Convection

D Bulk fluid motion

Correct Answer: A. Molecular diffusion

EXPLANATION

Small mean free path (60 nm << device dimension) ensures continuous medium behavior and heat transfer via molecular diffusion.

Test

Q.34 Medium Heat Transfer

The Peclet number (Pe = Re·Pr) in convective heat transfer indicates that:

A Pe >> 1 means convection dominates over conduction

B Pe

C Both (a) and (b) are correct

D Pe is independent of flow velocity

Correct Answer: C. Both (a) and (b) are correct

EXPLANATION

Pe represents relative importance of convection to diffusion: Pe >> 1 indicates convection dominance, Pe << 1 indicates diffusion dominance.

Test

Q.35 Medium Heat Transfer

In a recuperative heat exchanger, the effectiveness (ε) is defined as the ratio of actual heat transfer to:

A Maximum possible heat transfer (C_min·ΔT_max)

B Mean heat transfer of inlet conditions

C Logarithmic mean temperature difference

D Outlet temperature difference

Correct Answer: A. Maximum possible heat transfer (C_min·ΔT_max)

EXPLANATION

Effectiveness ε = Q_actual/Q_max = Q/(C_min(T_h,in - T_c,in)) for counterflow and parallel flow exchangers.

Test

Q.36 Medium Heat Transfer

In laminar flow through a circular tube with constant wall temperature, the Nusselt number is constant at Nu ≈ 3.66. This means:

A Heat transfer coefficient varies linearly with tube length

B Heat transfer coefficient is constant along the tube

C The entrance effects are negligible

D Both (b) and (c) are correct

Correct Answer: C. The entrance effects are negligible

EXPLANATION

Constant Nu in fully developed laminar flow indicates entrance effects are negligible and thermal profile is established.

Test

Q.37 Medium Heat Transfer

In forced convection heat transfer, the Colburn factor (j_H) is related to which dimensionless numbers?

A j_H = St·Pr^(2/3) = (Nu)/(Re·Pr^(1/3))

B j_H = Nu/Re

C j_H = Bi·Fo

D j_H = Gr/Ra

Correct Answer: A. j_H = St·Pr^(2/3) = (Nu)/(Re·Pr^(1/3))

EXPLANATION

The Colburn analogy relates Stanton number to Nusselt, Reynolds, and Prandtl numbers: j_H = St·Pr^(2/3) = Nu/(Re·Pr^(1/3)).

Test

Q.38 Medium Heat Transfer

For a circular fin of diameter d and length L attached to a surface at temperature T₀, the fin effectiveness approaches zero when:

A mL → 0 (where m = √(hP/kA_c))

B mL → ∞

C h/k ratio is very high

D L/d ratio is minimized

Correct Answer: B. mL → ∞

EXPLANATION

When mL → ∞, the temperature along the fin drops rapidly and efficiency decreases, approaching zero as the fin becomes ineffective.

Test

Q.39 Medium Heat Transfer

In steady-state heat conduction through a composite wall with three layers in series, if the thermal conductivities are k₁ > k₂ > k₃, which layer will have the maximum temperature drop?

A Layer 1

B Layer 2

C Layer 3

D Temperature drop is equal in all layers

Correct Answer: C. Layer 3

EXPLANATION

Temperature drop across a layer is inversely proportional to thermal conductivity (ΔT ∝ 1/k). Since k₃ is smallest, layer 3 experiences maximum temperature drop.

Test

Q.40 Medium Heat Transfer

In unsteady-state conduction with Bi

A Lumped capacitance method - uniform internal temperature

B Temperature varies spatially and temporally

C Steady-state conditions apply

D Conduction resistance is infinite

Correct Answer: A. Lumped capacitance method - uniform internal temperature

EXPLANATION

When Bi < 0.1, convective resistance is much larger than conductive resistance, allowing assumption of uniform temperature throughout the object.

Test

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