Heat Transfer - Mechanical Engineering test

Heat Transfer - Mechanical Engineering test

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

a) Constant.
b) Increases.
c) Decreases.
d) None of these.

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2) The quantity of heat radiation is dependent on

a) Temperature of the body only.
b) Shape of the body only.
c) Area of the body only.
d) None of these.

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3) The amount of heat flow through a body by conduction is

a) Inversely proportional to the thickness of the body.
b) Directly proportional to the temperature.
c) Dependent upon the material of the body.
d) All of these.

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4) Thermal diffusivity of a substance is

a) Directly proportional to the thermal conductivity.
b) Inversely proportional to density of substance.
c) Inversely proportional to specific heat.
d) All of these.

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5) Most unsteady heat flow occurs

a) During annealing of castings.
b) Through the wall of refrigerator.
c) Through lagged pipes carrying steam.
d) Though the walls of a furnace.

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6) Which statement is incorrect with regard to thermal diffusivity?

a) It represents a physical property of the material.
b) It is a dimensionless quantity.
c) It is an important characteristic for unsteady heat conduction.
d) None of these.

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7) Which among the following statement defines film co-efficient?

a) Thermal conductivity / equivalent thickness of film.
b) Inside diameter of tube / equivalent thickness of film.
c) (specific heat x viscosity)/ thermal conductivity.
d) None of these.

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

a) Remains constant.
b) Decreases.
c) Increases.
d) None of these.

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

a) Reduced along the length.
b) Increased along the length.
c) Maintained constant along the length.
d) None of these.

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10) Transient conduction mean

a) Conduction when the temperature at a point varies with time.
b) Heat transfer for a short time.
c) Heat transfer with a very small temperature difference.
d) Very little heat transfer.

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

a) By reducing heat generation rate.
b) By reducing convection co-efficient at the surface.
c) By reducing wall thickness.
d) By reducing thermal conductivity of wall material.

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

a) Grashoff.
b) Prandtl.
c) Fourier.
d) Reynolds.

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

a) Prandtl.
b) Weaker.
c) Nusselt.
d) Grashoff.

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

a) Cold water inlet and outlet.
b) Hot medium inlet and outlet.
c) Hot medium outlet and cold water inlet.
d) Hot medium outlet and cold water outlet.

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

a) Both maybe put in any order.
b) Better insulation is put over pipe and inferior one over it.
c) Inferior insulation is put over pipe and better one over it.
d) None of these.

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16) What happens to the emissivity when metallic surfaces are oxidised?

a) Increases
b) Decreases
c) Remains unaltered
d) Unpredictable

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

a) To increase pressure drop
b) To decrease pressure drop
c) To regulate the flow
d) To avoid the deformation of tubes due to thermal expansion

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18) Fin effectiveness increases due to high value of

a) Thermal conductivity
b) Circumference
c) Both (a) and (b)
d) Sectional area

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19) In regenerative type heat exchanger, heat transfer takes place by

a) indirect transfer
b) flow of hot and cold fluids alternatively over a surface
c) direct mixing of hot and cold fluids
d) generation of heat again and again

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20) 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?

a) One-forth
b) One-half
c) Twice
d) Four times

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21) Which among the following have the same units?

a) Plank’s constant and Stefan’s constant
b) Plank’s constant and angular momentum
c) Boltzmann constant and Plank’s constant
d) Boltzmann constant and Stefan’s constant

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22) Which among the following statements is correct?

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

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23) Which among the following is an incorrect statement?

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

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

a) Surface area to perimeter of solid.
b) Volume of solid to its surface area.
c) Perimeter to surface area of solid.
d) None of these.

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25) In convective heat transfer, the Nusselt number

a) Signifies the velocity gradient at the surface.
b) Is the ratio of molecular momentum diffusivity to thermal diffusivity.
c) Represents the ratio of viscous to inertia force.
d) Is the ratio of conduction to convection resistance.

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

a) To impart structural strength to exchanger
b) To reduce pressure drop
c) To account for uneven expansion of shell and tube bundles
d) To facilitate increase in length of boiler shell

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

a) Solid angle
b) Reynolds analogy
c) Configuration factor
d) Spectral distribution

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28) Which of the following statements is incorrect?

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

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29) What does a high value of Prandtl number indicate?

a) Rapid heat transfer by forced convection to natural convection
b) Rapid diffusion of momentum by viscous action compared to diffusion of energy
c) Relative heat transfer by conduction to convection
d) All of the above

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

a) Reynolds number
b) Reynolds number and Prandtl number
c) Weber number and Mach number
d) Grashoff number and Prandtl number

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