Heat transfer by convection MCQs With Answer

Heat transfer by convection MCQs With Answer

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Heat transfer by convection MCQs With Answer is an essential study area for B. Pharm students involved in unit operations like drying, tablet coating, sterilization, and fluidized bed processing. This concise, keyword-rich introduction covers convection fundamentals—natural and forced convection, convective heat transfer coefficient, Nusselt, Reynolds and Prandtl numbers, boundary layers, and practical correlations—linked to pharmaceutical equipment design and process optimization. Understanding convective heat transfer helps in predicting drying rates, heat exchanger performance, and maintaining product quality during thermal processing. Clear MCQs with answers will reinforce concepts and application in pharma contexts. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary mechanism of heat transfer in convection?

  • Heat transfer by electromagnetic waves
  • Heat transfer by particle collisions within a body
  • Heat transfer due to bulk fluid motion carrying thermal energy
  • Heat transfer through stationary solids only

Correct Answer: Heat transfer due to bulk fluid motion carrying thermal energy

Q2. Which dimensionless number represents the ratio of inertial to viscous forces and influences convective heat transfer?

  • Prandtl number
  • Nusselt number
  • Reynolds number
  • Biot number

Correct Answer: Reynolds number

Q3. The Nusselt number (Nu) is defined as which of the following?

  • Ratio of conductive to convective heat transfer
  • Ratio of convective to conductive heat transfer across a boundary layer
  • Ratio of inertial to gravitational forces
  • Ratio of thermal to momentum diffusivity

Correct Answer: Ratio of convective to conductive heat transfer across a boundary layer

Q4. For flow over a flat plate, transition from laminar to turbulent flow commonly occurs around which Reynolds number based on distance from the leading edge?

  • Re_x ≈ 5 x 10^2
  • Re_x ≈ 5 x 10^3
  • Re_x ≈ 5 x 10^5
  • Re_x ≈ 5 x 10^7

Correct Answer: Re_x ≈ 5 x 10^5

Q5. The Prandtl number (Pr) expresses the ratio of which two diffusivities?

  • Thermal diffusivity to concentration diffusivity
  • Momentum diffusivity to thermal diffusivity
  • Thermal diffusivity to momentum diffusivity
  • Mass diffusivity to thermal diffusivity

Correct Answer: Thermal diffusivity to momentum diffusivity

Q6. In pharmaceutical spray drying, which type of convection predominates inside the dryer chamber?

  • Purely natural convection
  • Forced convection due to airflow and atomization
  • Radiative heat transfer only
  • Conduction through static powder bed only

Correct Answer: Forced convection due to airflow and atomization

Q7. Convective heat transfer coefficient (h) has which SI units?

  • W
  • W/m
  • W/m^2·K
  • W·s/m^2

Correct Answer: W/m^2·K

Q8. Which correlation is commonly used to estimate average Nusselt number for turbulent pipe flow?

  • Newton’s law of cooling only
  • Dittus-Boelter equation
  • Fourier’s conduction correlation
  • Stokes-Einstein equation

Correct Answer: Dittus-Boelter equation

Q9. The Dittus-Boelter equation for turbulent flow gives Nu = 0.023 Re^0.8 Pr^n. For heating of the fluid, what is n typically?

  • 0.2
  • 0.4
  • 1.0
  • −0.3

Correct Answer: 0.4

Q10. Natural convection is driven primarily by which physical effect?

  • External pumping of fluid
  • Temperature-induced density variations generating buoyancy forces
  • Electromagnetic forces
  • Mechanical stirring only

Correct Answer: Temperature-induced density variations generating buoyancy forces

Q11. In a coating pan for tablets, improving air velocity around tablets mainly increases which parameter?

  • Thermal conductivity of tablets
  • Convective heat transfer coefficient between air and tablet surfaces
  • Specific heat of coating solution
  • Viscosity of coating solution

Correct Answer: Convective heat transfer coefficient between air and tablet surfaces

Q12. The Biot number (Bi) compares which resistances relevant to convective heating of solids?

  • Radiative to convective resistance
  • Internal conductive resistance to external convective resistance
  • Viscous resistance to inertial forces
  • Mass transfer resistance to heat transfer resistance

Correct Answer: Internal conductive resistance to external convective resistance

Q13. A small Biot number (Bi << 1) implies which simplification for transient heating of a particle?

  • Significant internal temperature gradients must be considered
  • Particle can be treated as lumped capacitance (uniform temperature)
  • Radiation dominates over convection
  • Conduction through the particle is negligible

Correct Answer: Particle can be treated as lumped capacitance (uniform temperature)

Q14. Which boundary layer thickness is most relevant for convective heat transfer at a solid-fluid interface?

  • Momentum boundary layer and thermal boundary layer both influence heat transfer
  • Only the molecular viscous sublayer matters
  • Only thermal radiation penetration depth
  • None—only bulk flow matters

Correct Answer: Momentum boundary layer and thermal boundary layer both influence heat transfer

Q15. In forced convection over a sphere (e.g., bead in a dryer), empirical correlations often use which form of Reynolds number?

  • Reynolds number based on particle diameter
  • Reynolds number based on thermal diffusivity
  • Reynolds number based on mean free path
  • Reynolds number based on film thickness

Correct Answer: Reynolds number based on particle diameter

Q16. Which effect reduces convective heat transfer coefficient in a duct or pipe?

  • Increasing turbulence intensity
  • Decreasing fluid velocity
  • Roughening the internal surface
  • Adding fins to increase surface area

Correct Answer: Decreasing fluid velocity

Q17. For gas-phase convective heat transfer, which physical property change with temperature often affects heat transfer correlations?

  • Magnetic susceptibility
  • Electrical conductivity
  • Viscosity and thermal conductivity
  • Optical refractive index

Correct Answer: Viscosity and thermal conductivity

Q18. In a fluidized bed dryer, effective convective heat transfer to particles is enhanced primarily by:

  • Reducing gas velocity below minimum fluidization velocity
  • Creating intimate contact and relative motion between gas and particles
  • Eliminating agitation
  • Using only natural convection

Correct Answer: Creating intimate contact and relative motion between gas and particles

Q19. Which equation represents local convective heat flux at a surface under Newton’s law of cooling?

  • q = k dT/dx at the surface
  • q” = h (T_surface − T_fluid)
  • q” = ε σ (T_surface^4 − T_sky^4)
  • q = m Cp ΔT

Correct Answer: q” = h (T_surface − T_fluid)

Q20. When designing heat exchangers for pasteurization, which convective factor is critical to ensure product safety?

  • Low Prandtl number of product
  • High convective heat transfer coefficient to achieve required thermal lethality
  • Minimizing Reynolds number to reduce shear
  • Using only conductive heat transfer

Correct Answer: High convective heat transfer coefficient to achieve required thermal lethality

Q21. Which correlation would you use for estimating Nu for laminar flow over a flat plate for thermal entrance region?

  • Dittus-Boelter correlation
  • Churchill-Bernstein correlation
  • Local solution from boundary layer energy equation (e.g., similarity solution)
  • Poiseuille’s law

Correct Answer: Local solution from boundary layer energy equation (e.g., similarity solution)

Q22. In convective mass and heat transfer analogy, which dimensionless number often replaces Prandtl for mass transfer?

  • Schmidt number
  • Péclet number
  • Grashof number
  • Reynolds number

Correct Answer: Schmidt number

Q23. Forced convection heat transfer coefficient typically increases with which of the following?

  • Decreasing fluid velocity
  • Increasing surface smoothness beyond a point
  • Increasing velocity leading to higher Reynolds number
  • Increasing thickness of a stagnant boundary layer

Correct Answer: Increasing velocity leading to higher Reynolds number

Q24. For a sphere in creeping (Stokes) flow (very low Re), convective heat transfer is controlled mainly by:

  • Turbulent eddies increasing Nu dramatically
  • Diffusive transport, giving low Nusselt numbers near 2
  • Radiation dominating over convection
  • Boiling heat transfer mechanisms

Correct Answer: Diffusive transport, giving low Nusselt numbers near 2

Q25. Which parameter is needed to calculate Reynolds number for flow in a circular pipe?

  • Fluid thermal conductivity
  • Characteristic length equal to pipe diameter, fluid density, velocity, and viscosity
  • Solid specific heat capacity
  • Surface emissivity

Correct Answer: Characteristic length equal to pipe diameter, fluid density, velocity, and viscosity

Q26. In calculating convective heat transfer inside a drying oven, which assumption is often acceptable for thin films or small particles?

  • Neglecting external convection entirely
  • Lumped capacitance model if Biot number is small
  • Assuming infinite thermal resistance inside the particle
  • Assuming zero heat capacity for the solid

Correct Answer: Lumped capacitance model if Biot number is small

Q27. The Grashof number (Gr) is important in natural convection because it represents the ratio of:

  • Buoyancy to viscous forces
  • Inertial to viscous forces
  • Convective to conductive heat transfer
  • Thermal to mass diffusivity

Correct Answer: Buoyancy to viscous forces

Q28. Which effect would reduce convective heat transfer from a heated tablet surface in coating processes?

  • Increasing air temperature while maintaining velocity
  • Reducing relative velocity between air and tablet
  • Using perforated pans to enhance flow
  • Maintaining turbulent flow around tablets

Correct Answer: Reducing relative velocity between air and tablet

Q29. Which measurement technique is commonly used to experimentally determine convective heat transfer coefficient in lab studies?

  • Calorimetry only without temperature measurement
  • Measuring surface and fluid temperatures and heat flux and applying q” = h (Ts − T∞)
  • Using gas chromatography
  • Measuring sound speed in the fluid

Correct Answer: Measuring surface and fluid temperatures and heat flux and applying q” = h (Ts − T∞)

Q30. In convective drying of granules, increasing bed porosity typically affects convective heat transfer by:

  • Eliminating airflow through the bed
  • Enhancing penetration of hot air, improving convective transfer
  • Reducing surface area available to air
  • Converting convection to conduction only

Correct Answer: Enhancing penetration of hot air, improving convective transfer

Q31. The Churchill and Bernstein correlation is used for which flow situation?

  • Forced convection over cylinders including wide Re range
  • Laminar flow inside microchannels only
  • Radiative heat transfer between surfaces
  • Pure conduction in solids

Correct Answer: Forced convection over cylinders including wide Re range

Q32. In convective heat transfer, what role does surface roughness play in turbulent flow?

  • It always reduces heat transfer coefficient
  • It can enhance turbulence and increase heat transfer coefficient
  • It eliminates boundary layer formation
  • It converts convection to radiation

Correct Answer: It can enhance turbulence and increase heat transfer coefficient

Q33. Which scenario is an example of mixed convection?

  • Only buoyancy-driven flow in a tall still column
  • High-speed forced air over a heated plate where buoyancy is negligible
  • Flow in a heated duct where both forced flow and buoyancy are comparable
  • Heat conduction through a thick wall with no fluid motion

Correct Answer: Flow in a heated duct where both forced flow and buoyancy are comparable

Q34. In design of a pharmaceutical heat exchanger, fouling reduces convective performance by:

  • Increasing thermal conductivity between fluid and wall
  • Adding thermal resistance and lowering effective heat transfer coefficient
  • Decreasing flow path length only
  • Eliminating viscosity effects

Correct Answer: Adding thermal resistance and lowering effective heat transfer coefficient

Q35. Which dimensionless group combines Grashof and Reynolds numbers to assess relative importance of natural vs forced convection?

  • Péclet number
  • Richardson number
  • Biot number
  • Schmidt number

Correct Answer: Richardson number

Q36. For external forced convection over a cylinder in crossflow, which parameter strongly affects drag and heat transfer?

  • Electrical conductivity of the cylinder
  • Flow Reynolds number and surface roughness
  • Magnetic field strength
  • Time of day

Correct Answer: Flow Reynolds number and surface roughness

Q37. In pharmaceutical processing, why is convective drying often preferred over conductive drying for particulate products?

  • Convection provides uniform heating without airflow
  • Convective drying allows faster moisture removal through direct contact with heated air and better mass transfer
  • Conduction is always faster than convection
  • Convective drying eliminates the need for temperature control

Correct Answer: Convective drying allows faster moisture removal through direct contact with heated air and better mass transfer

Q38. The local Nusselt number for flow along a plate generally decreases with increasing distance in laminar flow because:

  • The thermal boundary layer thickens, reducing local heat transfer
  • The fluid suddenly becomes compressible
  • The plate temperature rises indefinitely
  • Conduction becomes dominant over the whole domain

Correct Answer: The thermal boundary layer thickens, reducing local heat transfer

Q39. In forced convection to a fluid flowing inside a tube, entrance region heat transfer is higher because:

  • The bulk fluid temperature never changes
  • Boundary layers are developing, producing higher local gradients
  • The fluid viscosity becomes infinite
  • There is no shear stress at the wall

Correct Answer: Boundary layers are developing, producing higher local gradients

Q40. Which of the following increases the convective heat transfer coefficient for gas in a duct?

  • Decreasing turbulence intensity
  • Increasing hydraulic diameter without changing velocity
  • Increasing mean flow velocity
  • Reducing fluid temperature uniformly

Correct Answer: Increasing mean flow velocity

Q41. In an industrial spray dryer, droplet drying time depends strongly on convective heat transfer and on:

  • Droplet color only
  • Mass transfer of moisture from droplet to air (evaporation kinetics)
  • Electrical charge of droplets
  • Presence of magnetic particles

Correct Answer: Mass transfer of moisture from droplet to air (evaporation kinetics)

Q42. What effect does increasing air humidity have on convective drying rate at constant temperature and velocity?

  • Increases drying rate by increasing convective coefficient
  • Decreases drying rate due to reduced driving force for mass transfer
  • Has no effect on drying rate
  • Converts convection into conduction

Correct Answer: Decreases drying rate due to reduced driving force for mass transfer

Q43. Which method is commonly used to enhance convective heat transfer in heat exchangers without increasing flow velocity excessively?

  • Using smooth plain tubes only
  • Adding fins, turbulators, or extended surfaces
  • Decreasing surface area
  • Lowering fluid thermal conductivity

Correct Answer: Adding fins, turbulators, or extended surfaces

Q44. For laminar internal flow in a circular pipe with constant wall temperature, the fully developed Nusselt number is approximately:

  • 3.66
  • 0.023 Re^0.8 Pr^0.4
  • 100
  • 0.5

Correct Answer: 3.66

Q45. What is the main practical challenge in measuring h (convective coefficient) in pharmaceutical equipment?

  • Lack of mathematical definitions for h
  • Complex geometry, unsteady conditions and coupled mass transfer make accurate determination difficult
  • h is independent of process conditions
  • There is only one universal value of h for all cases

Correct Answer: Complex geometry, unsteady conditions and coupled mass transfer make accurate determination difficult

Q46. Which of the following increases natural convection heat transfer from a vertical hot wall?

  • Decreasing temperature difference between wall and ambient
  • Increasing wall height and temperature difference to enhance buoyancy-driven flow
  • Sealing the enclosure to prevent flow
  • Making the wall perfectly insulated

Correct Answer: Increasing wall height and temperature difference to enhance buoyancy-driven flow

Q47. In convective heat transfer, the matching of which two resistances is important for optimal design of heating of a particle?

  • Electrical and magnetic resistances
  • External convective resistance and internal conductive resistance
  • Radiative and chemical resistances
  • Membrane and osmotic resistances

Correct Answer: External convective resistance and internal conductive resistance

Q48. When using empirical correlations like Dittus-Boelter, validity is limited to certain ranges of Re and Pr. Why is this caution important?

  • Correlations are exact physical laws valid always
  • Outside those ranges the correlation may give large errors because flow and thermal behavior differ
  • Because Re and Pr are irrelevant in heat transfer
  • Because Dittus-Boelter accounts for chemical reactions automatically

Correct Answer: Outside those ranges the correlation may give large errors because flow and thermal behavior differ

Q49. Which of the following statements about convective heat transfer in pharmaceutical spray coating is true?

  • Convective cooling of droplets is negligible during coating
  • Convective heat transfer affects solvent evaporation rate and film formation
  • Only radiation controls coating drying
  • Mass transfer is independent of convection

Correct Answer: Convective heat transfer affects solvent evaporation rate and film formation

Q50. To increase convective heat transfer in a drying tunnel without changing air temperature, you can:

  • Decrease airflow to reduce turbulence
  • Increase air velocity and promote turbulence for higher h
  • Polish surfaces to reduce turbulence
  • Eliminate ventilation to force natural convection only

Correct Answer: Increase air velocity and promote turbulence for higher h

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