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Question 1. Which of the following best defines the difference between temperature and heat? A) Temperature measures the total kinetic energy of a system, while heat is the rate of energy transfer. B) Temperature is a scalar property indicating the average kinetic energy of particles, whereas heat is energy transferred due to a temperature difference. C) Heat is a state function, while temperature is not. D) Temperature can be created, but heat cannot. Answer: B Explanation: Temperature reflects the average kinetic energy of particles in a material, while heat is the energy that moves from a higher‑temperature region to a lower‑temperature region. Question 2. The First Law of Thermodynamics for a closed system can be expressed as: A) ΔU = Q – W B) ΔU = Q + W C) ΔU = Q·W D) ΔU = Q/W Answer: A Explanation: For a closed system, the change in internal energy (ΔU) equals heat added to the system (Q) minus the work done by the system (W). Question 3. Fourier’s Law for one‑dimensional steady‑state conduction through a plane wall is: A) q = h (Ts – T∞) B) q = – k (dT/dx) C) q = εσ(T⁴s – T⁴∞) D) q = ρ c v ΔT Answer: B
Explanation: Fourier’s Law states that heat flux q is proportional to the negative temperature gradient, with thermal conductivity k as the proportionality constant. Question 4. Which mode of heat transfer does NOT require a material medium? A) Conduction B) Convection C) Radiation D) All require a medium Answer: C Explanation: Thermal radiation can propagate through a vacuum because it is electromagnetic energy. Question 5. In Newton’s Law of Cooling, the convective heat transfer coefficient (h) is: A) Independent of fluid properties. B) Dependent on flow regime, fluid properties, and geometry. C) Always equal to 10 W/m²·K. D) Only a function of surface temperature. Answer: B Explanation: h varies with fluid velocity, viscosity, thermal conductivity, and the shape of the surface. Question 6. The Stefan‑Boltzmann Law for a black surface is expressed as: A) q = h (Ts – T∞) B) q = εσ(T⁴s – T⁴∞) C) q = σ(T⁴s) D) q = k (dT/dx) Answer: C
Answer: B Explanation: U = 1/(R₁ + R₂ + … + Rₙ), where each R includes conduction and any surface resistances. Question 10. The fin efficiency (η_f) is defined as: A) Ratio of actual heat transferred by the fin to the heat that would be transferred if the entire fin were at base temperature. B) Ratio of fin surface area to base area. C) Ratio of convective coefficient to conductive coefficient. D) Ratio of fin length to thickness. Answer: A Explanation: Fin efficiency compares real fin performance to an ideal fin with uniform base temperature. Question 11. Which condition justifies the use of the lumped capacitance method for transient conduction? A) Bi < 0. B) Bi > 10 C) Fo > 1 D) Re > 4000 Answer: A Explanation: When the Biot number is less than 0.1, internal temperature gradients are negligible, allowing lumped analysis. Question 12. The Fourier number (Fo) in transient heat conduction is defined as: A) Fo = α t/L² B) Fo = h L/k C) Fo = Re·Pr
D) Fo = (k ΔT)/q Answer: A Explanation: Fo is a dimensionless time parameter, where α is thermal diffusivity, t is time, and L is a characteristic length. Question 13. In Heisler charts, the abscissa typically represents: A) Radial position normalized by radius. B) Fourier number (Fo). C) Nusselt number (Nu). D) Grashof number (Gr). Answer: B Explanation: Heisler charts plot temperature ratios versus the Fourier number for various geometries. Question 14. The thermal boundary layer thickness (δ_t) is generally: A) Larger than the velocity boundary layer for fluids with Pr > 1. B) Smaller than the velocity boundary layer for fluids with Pr > 1. C) Equal to the velocity boundary layer for all fluids. D) Independent of the Prandtl number. Answer: B Explanation: For high Prandtl numbers (liquid metals), thermal diffusion is slower, making δ_t thinner than the velocity boundary layer. Question 15. For laminar flow over a flat plate, the local Nusselt number (Nu_x) is given by: A) Nu_x = 0.332 Re_x^0.5 Pr^0. B) Nu_x = 0.0296 Re_x^0.8 Pr^0.
B) The Grashof number plays a role similar to Reynolds number in forced convection. C) It requires a fan to induce flow. D) It is independent of temperature difference. Answer: B Explanation: Gr = g β ΔT L³/ν² characterizes buoyancy‑driven flow, analogous to Re for forced convection. Question 19. The Rayleigh number (Ra) is defined as: A) Ra = Re·Pr B) Ra = Gr·Pr C) Ra = Nu·Re D) Ra = Pr/Gr Answer: B Explanation: Ra = Gr Pr combines buoyancy and thermal diffusion effects, governing the onset of natural convection. Question 20. In pool boiling, the nucleate boiling regime is characterized by: A) A steep rise in heat flux with a small temperature rise. B) Film formation that insulates the surface. C) No bubble formation. D) Constant heat flux independent of surface temperature. Answer: A Explanation: Nucleate boiling efficiently transfers heat via bubble formation, causing a rapid increase in heat flux for modest temperature differences.
Question 21. Drop‑wise condensation generally yields a higher heat‑transfer coefficient than film‑wise condensation because: A) Liquid films act as thermal resistances. B) Drops have larger surface area. C) Vapor pressure is lower. D) It requires higher surface temperatures. Answer: A Explanation: In drop‑wise condensation, discrete droplets shed, minimizing thermal resistance compared with a continuous liquid film. Question 22. According to Planck’s law, the spectral emissive power of a blackbody depends on: A) Only temperature. B) Wavelength and temperature. C) Emissivity and surface roughness. D) Pressure and volume. Answer: B Explanation: Planck’s law gives emissive power as a function of wavelength (or frequency) and absolute temperature. Question 23. Wien’s Displacement Law states that the wavelength at which emission is maximum (λ_max) is: A) Directly proportional to temperature. B) Inversely proportional to temperature. C) Independent of temperature. D) Proportional to the square of temperature. Answer: B
Explanation: The radiosity‑irradiation method accounts for surface emissivities and view factors in gray, diffuse systems. Question 27. A radiation shield placed between two surfaces primarily works by: A) Increasing convection. B) Reflecting infrared radiation. C) Absorbing all incident radiation. D) Enhancing conduction. Answer: B Explanation: High‑reflectivity shields reflect thermal radiation, reducing net radiative heat transfer. Question 28. In a parallel‑flow heat exchanger, the temperature difference between the fluids is: A) Constant along the length. B) Largest at the outlet. C) Smallest at the inlet. D) Varies but never exceeds the inlet temperature difference. Answer: D Explanation: In parallel flow, the temperature difference changes gradually but remains less than or equal to the inlet temperature difference. Question 29. The Log Mean Temperature Difference (LMTD) for a counter‑flow heat exchanger is calculated as: A) (ΔT₁ – ΔT₂)/ln(ΔT₁/ΔT₂) B) (ΔT₁ + ΔT₂)/ C) √(ΔT₁·ΔT₂) D) (ΔT₁ – ΔT₂)·ln(ΔT₁/ΔT₂)
Answer: A Explanation: LMTD = (ΔT₁ – ΔT₂)/ln(ΔT₁/ΔT₂) provides an average driving temperature difference for heat‑exchanger analysis. Question 30. The effectiveness‑NTU (ε‑NTU) method is preferred over LMTD when: A) Inlet and outlet temperatures are unknown but flow rates and U‑value are known. B) The heat exchanger is very small. C) The fluids have identical heat capacities. D) The exchanger is adiabatic. Answer: A Explanation: ε‑NTU uses the Number of Transfer Units (NTU) and effectiveness to predict performance without requiring outlet temperatures. Question 31. The fouling factor (R_f) in heat‑exchanger design accounts for: A) Increased thermal conductivity of the fluid. B) Thermal resistance due to deposits on heat‑transfer surfaces. C) Changes in fluid density. D) Pressure drop only. Answer: B Explanation: Fouling adds an extra thermal resistance, reducing heat‑transfer efficiency. Question 32. Critical insulation thickness for a cylindrical pipe occurs when: A) The heat loss rate begins to increase with additional insulation. B) The pipe reaches its melting point. C) The outer surface temperature equals ambient temperature.
C) 0.
D) 1.
Answer: A Explanation: Bi = h L_c/k = 25 · 0.005/200 = 0.000625 ≈ 0.025 (using L_c = thickness/2 ≈ 0.0025 m gives similar small value <0.1). Question 36. For turbulent flow inside a smooth pipe, the Dittus‑Boelter correlation for heating is: A) Nu = 0.023 Re^0.8 Pr^0. B) Nu = 0.027 Re^0.8 Pr^0. C) Nu = 4. D) Nu = 0.332 Re^0.5 Pr^0. Answer: A Explanation: The Dittus‑Boelter equation for heating (Pr > 0.7) uses exponent 0.4 on Pr. Question 37. In a shell‑and‑tube heat exchanger, the “tube‑side” refers to: A) The fluid flowing inside the tubes. B) The fluid flowing outside the tubes. C) The fluid in the shell only. D) The fluid that does not exchange heat. Answer: A Explanation: Tube‑side fluid is inside the tubes; shell‑side fluid surrounds the tubes. Question 38. When two surfaces have the same temperature but different emissivities, the net radiative exchange is: A) Zero.
B) Positive from the higher‑ε surface to the lower‑ε surface. C) Negative from the lower‑ε surface to the higher‑ε surface. D) Dependent on view factor only. Answer: A Explanation: Radiative exchange depends on temperature difference; equal temperatures give zero net exchange regardless of emissivity. Question 39. The Prandtl number (Pr) for air at room temperature is approximately: A) 0. B) 1. C) 2. D) 5. Answer: A Explanation: For air at ~20 °C, Pr ≈ 0.71, indicating momentum diffusivity dominates over thermal diffusivity. Question 40. In a fin array with closely spaced pin fins, the overall fin efficiency tends to: A) Increase with decreasing fin spacing. B) Decrease with decreasing fin spacing. C) Remain unchanged. D) Become zero. Answer: B Explanation: Crowding reduces individual fin effectiveness due to limited space for heat to spread, lowering overall efficiency. Question 41. Which of the following statements about the Grashof number (Gr) is correct?
Question 44. The term “thermal resistance network” is analogous to which electrical concept? A) Series and parallel resistors. B) Capacitors in series. C) Inductors in parallel. D) Voltage sources. Answer: A Explanation: Thermal resistances can be added in series or parallel just like electrical resistors. Question 45. In a transient conduction problem where the surface temperature is suddenly raised and held constant, the interior temperature response is best described by: A) An exponential decay function. B) A sinusoidal function. C) A linear increase with time. D) A step function. Answer: A Explanation: The solution involves exponential terms (e.g., Fourier series) representing the decay of temperature gradients over time. Question 46. For a sphere of radius R conducting heat radially, the steady‑state temperature distribution is: A) Linear with radius. B) Logarithmic with radius. C) Inverse proportional to radius. D) Quadratic with radius. Answer: C Explanation: For spherical symmetry, q = – 4πkR² (dT/dr), integrating gives T ∝ 1/r.
Question 47. In a plate fin with adiabatic tip, the temperature distribution is best approximated by assuming: A) Zero heat flux at the tip. B) Constant temperature at the tip. C) Infinite conductivity at the tip. D) Convective heat loss equal to base. Answer: A Explanation: An adiabatic tip means no heat leaves the tip, so the derivative dT/dx = 0 there. Question 48. The term “thermal diffusivity” (α) is defined as: A) k/(ρ cₚ) B) ρ cₚ/k C) k·ρ·cₚ D) k·cₚ/ρ Answer: A Explanation: α measures the rate at which temperature changes diffuse through a material. Question 49. In the context of heat exchangers, the term “NTU” stands for: A) Number of Transfer Units. B) Normalized Temperature Uniformity. C) Net Thermal Utilization. D) None of the above. Answer: A Explanation: NTU quantifies the size of the heat exchanger relative to the heat capacity rate.
Explanation: Low Pr implies high thermal diffusivity, so heat spreads quickly, making the thermal boundary layer thick. Question 53. In a parallel‑flow heat exchanger, the temperature difference between the hot and cold fluids at the outlet is: A) Larger than at the inlet. B) Smaller than at the inlet. C) Equal to the inlet difference. D) Zero. Answer: B Explanation: Both fluids move in the same direction, so the temperature gap narrows downstream. Question 54. The “critical radius” for insulation around a cylinder is given by: A) r_crit = k/h B) r_crit = h/k C) r_crit = √(k/h) D) r_crit = k·h Answer: A Explanation: r_crit = k/h is the radius at which additional insulation no longer reduces heat loss. Question 55. Which dimensionless group governs the onset of turbulence in internal pipe flow? A) Reynolds number (Re) B) Prandtl number (Pr) C) Grashof number (Gr) D) Nusselt number (Nu)
Answer: A Explanation: Transition to turbulence in pipes typically occurs when Re exceeds ~2300. Question 56. In a radiation enclosure with all surfaces black (ε = 1), the net radiative heat exchange between any two surfaces is: A) Zero. B) Determined solely by view factors and temperatures. C) Independent of temperature. D) Maximized when surfaces are parallel. Answer: B Explanation: With ε = 1, surface properties drop out; exchange depends on geometry (view factors) and temperature differences. Question 57. The term “effective thermal conductivity” of a composite wall refers to: A) A weighted average based on layer thicknesses. B. The conductivity of the thickest layer only. C. The arithmetic mean of all conductivities. D. The harmonic mean of conductivities. Answer: A Explanation: Effective conductivity accounts for series resistances; it is a thickness‑weighted average. Question 58. In a fin, the parameter η_f (fin efficiency) approaches 1 when: A) The fin is very short or has very high conductivity. B) The fin is very long and thin. C) The convective coefficient is zero.
