   ## 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

##### 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

##### 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

##### 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

##### 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

##### 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

##### 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

##### 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

Options are :

• Prandtl
• Grashoff
• Weaker
• Nusselt

##### 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)

Options are :

• Prandtl
• Reynolds
• Grashoff
• Fourier

Options are :

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

##### What happens to the emissivity when metallic surfaces are oxidised?

Options are :

• Decreases
• Increases
• Unpredictable
• 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