Heat Transfer Practice Questions (Mechanical Engineering) Set 1

The characteristic length, in the non-dimensional Biot-number, is the ratio of


Options are :

  • Perimeter to surface area of solid
  • None of these
  • Volume of solid to its surface area
  • Surface area to perimeter of solid

Answer : Volume of solid to its surface area

Heat Transfer Practice Questions (Mechanical Engineering) Set 1

Which among the following have the same units?


Options are :

  • Boltzmann constant and Plank’s constant
  • Boltzmann constant and Stefan’s constant
  • Plank’s constant and angular momentum
  • Plank’s constant and Stefan’s constant

Answer : Plank’s constant and angular momentum

Which among the following statements is correct?


Options are :

  • Only some substances emit radiation
  • Black colour bodies are called as black body
  • All substances emit radiation, the quality and quantity depend on absolute temperature and the properties of the material
  • A substance will emit radiation at a particular wavelength only

Answer : All substances emit radiation, the quality and quantity depend on absolute temperature and the properties of the material

In convective heat transfer, the Nusselt number


Options are :

  • Represents the ratio of viscous to inertia force
  • Is the ratio of conduction to convection resistance
  • Is the ratio of molecular momentum diffusivity to thermal diffusivity
  • Signifies the velocity gradient at the surface

Answer : Is the ratio of conduction to convection resistance

Heat Transfer Practice Questions (Mechanical Engineering) Set 1

Selective surfaces have same value of emissivity throughout the entire range of wavelength


Options are :

  • Rapid heat transfer by forced convection to natural convection
  • Rapid diffusion of momentum by viscous action compared to diffusion of energy
  • Relative heat transfer by conduction to convection
  • All of the above

Answer : Rapid diffusion of momentum by viscous action compared to diffusion of energy

Which among the following is an incorrect statement?


Options are :

  • Thermal conductivity is always higher in the purest form of metal
  • Heat treatment causes considerable variation in thermal conductivity
  • Thermal conductivity of a damp material is considerably higher than the thermal conductivity of the dry material and water taken individually
  • Thermal conductivity decreases with increase in the density of the substance

Answer : Thermal conductivity decreases with increase in the density of the substance

In case of natural convection, the Nusselt number is a function of


Options are :

  • Weber number and Mach number
  • Reynolds number and Prandtl number
  • Reynolds number
  • Grashoff number and Prandtl number

Answer : Grashoff number and Prandtl number

In shell of a tubular heat exchanger, why are the expansion bellows provided?


Options are :

  • To impart structural strength to exchanger
  • To reduce pressure drop
  • To facilitate increase in length of boiler shell
  • To account for uneven expansion of shell and tube bundles

Answer : To account for uneven expansion of shell and tube bundles

Which of the following statements is incorrect?


Options are :

  • Selective surfaces have same value of emissivity throughout the entire range of wavelength
  • At thermal equilibrium, the emissivity and absorptivity are same
  • Glasses are transparent to thermal radiations at short wavelengths
  • The emissivity of a smooth surface is lower compared to a rough surface of same material

Answer : Selective surfaces have same value of emissivity throughout the entire range of wavelength

Which of the following terms does not pertain to radiation heat transfer?


Options are :

  • Configuration factor
  • Spectral distribution
  • Reynolds analogy
  • Solid angle

Answer : Reynolds analogy

Which among the following statement defines film co-efficient?


Options are :

  • None of these
  • (specific heat x viscosity)/ thermal conductivity
  • Thermal conductivity / equivalent thickness of film
  • Inside diameter of tube / equivalent thickness of film

Answer : Thermal conductivity / equivalent thickness of film

The radial heat transfer rate through hollow cylinder increases as the ratio of outer radius to inner radius


Options are :

  • Decreases
  • Constant
  • Increases
  • None of these

Answer : Decreases

The quantity of heat radiation is dependent on


Options are :

  • None of these
  • Area of the body only
  • Shape of the body only
  • Temperature of the body only

Answer : Temperature of the body only

Most unsteady heat flow occurs


Options are :

  • During annealing of castings
  • Through lagged pipes carrying steam
  • Though the walls of a furnace
  • Through the wall of refrigerator

Answer : During annealing of castings

Which statement is incorrect with regard to thermal diffusivity?


Options are :

  • It is an important characteristic for unsteady heat conduction
  • It represents a physical property of the material
  • None of these
  • It is a dimensionless quantity

Answer : It is a dimensionless quantity

Transient conduction mean


Options are :

  • Conduction when the temperature at a point varies with time
  • Heat transfer with a very small temperature difference
  • Very little heat transfer
  • Heat transfer for a short time

Answer : Conduction when the temperature at a point varies with time

For a finned surface, it is considered appropriate that area of cross-section be


Options are :

  • None of these
  • Maintained constant along the length
  • Increased along the length
  • Reduced along the length

Answer : Reduced along the length

Thermal diffusivity of a substance is


Options are :

  • Inversely proportional to specific heat
  • Directly proportional to the thermal conductivity
  • Inversely proportional to density of substance
  • All of these

Answer : All of these

When the thickness of insulation on a pipe exceeds the critical value the heat flow rate


Options are :

  • Increases
  • None of these
  • Remains constant
  • Decreases

Answer : Increases

The amount of heat flow through a body by conduction is


Options are :

  • Directly proportional to the temperature
  • All of these
  • Inversely proportional to the thickness of the body
  • Dependent upon the material of the body

Answer : All of these

Which among the following is the reason for providing floating heads in heat exchanger?


Options are :

  • To regulate the flow
  • To avoid the deformation of tubes due to thermal expansion
  • To increase pressure drop
  • To decrease pressure drop

Answer : To avoid the deformation of tubes due to thermal expansion

Which of the following number is generally associated with natural convection heat transfer?


Options are :

  • Prandtl
  • Grashoff
  • Weaker
  • Nusselt

Answer : Grashoff

Two insulating materials are used to insulate a steam pipe. The best result would be obtained when


Options are :

  • Better insulation is put over pipe and inferior one over it
  • Both maybe put in any order
  • Inferior insulation is put over pipe and better one over it
  • None of these

Answer : Better insulation is put over pipe and inferior one over it

In heat exchangers, the degree of approach is defined as the difference between temperatures of


Options are :

  • Hot medium inlet and outlet
  • Hot medium outlet and cold water inlet
  • Hot medium outlet and cold water outlet
  • Cold water inlet and outlet

Answer : Hot medium outlet and cold water outlet

In regenerative type heat exchanger, heat transfer takes place by


Options are :

  • flow of hot and cold fluids alternatively over a surface
  • direct mixing of hot and cold fluids
  • generation of heat again and again
  • indirect transfer

Answer : flow of hot and cold fluids alternatively over a surface

Fin effectiveness increases due to high value of


Options are :

  • Sectional area
  • Both (a) and (b)
  • Circumference
  • Thermal conductivity

Answer : Both (a) and (b)

…………….number is relevant in transient heat condition


Options are :

  • Prandtl
  • Reynolds
  • Grashoff
  • Fourier

Answer : Fourier

For a cylindrical rod with uniformly distributed heat sources, what will be the thermal gradient (dt/dr) at r = 0.5R with respect to that at the surface?


Options are :

  • One-half
  • Twice
  • Four times
  • One-forth

Answer : One-half

What happens to the emissivity when metallic surfaces are oxidised?


Options are :

  • Decreases
  • Increases
  • Unpredictable
  • Remains unaltered

Answer : Remains unaltered

How can the temperature drop in a plane wall with uniformly distributed heat generation be decreased?


Options are :

  • By reducing heat generation rate
  • By reducing thermal conductivity of wall material
  • By reducing wall thickness
  • By reducing convection co-efficient at the surface

Answer : By reducing wall thickness

Comment / Suggestion Section
Point our Mistakes and Post Your Suggestions