Which of the following is the SI unit of thermal conductivity?
Answer: A
Thermal conductivity is expressed as W/(m·K) in SI units, equivalent to J/(s·m·K). This represents heat flow rate per unit area per unit temperature gradient.
Q.102Easy
In heat conduction through a composite wall, the overall heat transfer rate is determined by which principle?
Answer: A
The thermal resistance network (Ohm's law analogy) is used where total thermal resistance equals the sum of individual resistances in series for composite walls.
Q.103Medium
What is the Biot number (Bi) used to determine in transient heat conduction?
Answer: A
Biot number = hL_c/k determines whether lumped capacitance method is applicable. Bi << 0.1 indicates uniform internal temperature.
Q.104Medium
For fully developed laminar flow in a circular pipe with constant wall heat flux, which Nusselt number (Nu) is approximately correct?
Answer: B
For constant heat flux boundary condition (H1 type) in circular pipes with fully developed laminar flow, Nu ≈ 4.36. For constant wall temperature (H2), Nu ≈ 3.66.
Q.105Easy
Which dimensionless number represents the ratio of buoyancy forces to viscous forces in natural convection?
Answer: A
Grashof number (Gr = gβΔT L³/ν²) represents the ratio of buoyancy to viscous forces. Ra = Gr × Pr combines both effects.
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Q.106Medium
In a counter-current heat exchanger with equal heat capacities, what is the maximum possible effectiveness?
Answer: A
For counter-current arrangement, maximum effectiveness can approach 1.0 (100%) with infinite NTU. For co-current, it's limited to (1-exp(-NTU(1+C_r)))/(1+C_r).
Q.107Medium
The Number of Transfer Units (NTU) is defined as which of the following?
Answer: A
NTU = UA/(ṁC_p)_min is the ratio of overall heat transfer capacity to minimum heat capacity rate, used in ε-NTU method.
Q.108Hard
For a long thin fin with adiabatic tip condition, the fin efficiency is given by:
Answer: A
For adiabatic tip condition: η_fin = tanh(mL)/(mL), where m = √(hP/kA_c). This represents the efficiency of heat transfer through the fin.
Q.109Easy
Which of the following best describes the behavior of thermal conductivity with temperature for most metals?
Answer: B
For most pure metals, thermal conductivity decreases with increasing temperature due to increased lattice vibrations causing phonon scattering.
Q.110Hard
In a parallel plate channel with constant heat flux q'' on both walls, what is the thermal entrance length relationship with hydrodynamic entrance length?
Answer: A
Thermal entrance length L_th ≈ L_h/Pr for laminar flow, where Pr < 1 for metals means thermal entrance is shorter than hydrodynamic entrance.
Q.111Medium
The log mean temperature difference (LMTD) correction factor F is required primarily for which configuration?
Answer: B
F-correction factor is used for non-counter-current arrangements (cross-flow, 1-2 shell-tube, etc.) where LMTD doesn't directly apply without correction.
Q.112Medium
For turbulent flow in smooth pipes (Pr > 0.6), which correlation is most commonly used?
Answer: A
Dittus-Boelert correlation is the most widely used for turbulent flow with Pr > 0.6. Valid for Re > 10,000, smooth pipes, and fully developed flow.
Q.113Easy
The thermal diffusivity (α) has dimensions of which of the following?
Answer: A
Thermal diffusivity α = k/(ρ·C_p) has dimensions L²/t, representing how fast thermal disturbances propagate through a material.
Q.114Medium
In a shell-and-tube heat exchanger (1 shell pass, 2 tube passes), the effectiveness is lower than counter-current primarily because:
Answer: A
In 1-2 arrangement, portion of shell-side flows co-currently with tube-side, reducing overall effectiveness compared to true counter-current.
Q.115Medium
For a flat plate solar collector with selective coating, which of the following is most desirable?
Answer: B
Selective coatings should absorb solar radiation (high α_sol) but minimize thermal radiation losses (low ε_IR), maximizing collector efficiency.
Q.116Hard
The Graetz number in laminar heat transfer represents:
Answer: A
Graetz number Gz = (D_h/L) × Re × Pr indicates relative importance of entrance effects. Gz > 100 means entrance region dominates.
Q.117Medium
For natural convection from a vertical isothermal plate in air, which correlation applies best?
Answer: A
For natural convection: Nu = C(Ra)^n where Ra = Gr×Pr. For laminar boundary layer (Ra < 10^9), n ≈ 0.25. For turbulent (Ra > 10^9), n ≈ 0.33.
Q.118Medium
In unsteady-state conduction with Bi << 0.1, what assumption is valid?
Answer: A
When Bi < 0.1, convective resistance is much larger than conductive resistance, allowing assumption of uniform temperature throughout the object.
Q.119Hard
Which statement best describes radiation heat transfer in participating media?
Answer: B
In participating media (like CO₂, H₂O vapor), radiation is absorbed and re-emitted by gas molecules, affecting net radiative heat transfer significantly.
Q.120Hard
For a microfin tube (enhanced surface), the heat transfer improvement comes primarily from:
Answer: C
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.
