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Mechanical Engineering :: Heat and Mass Transfer

  1. In convection heat transfer from hot flue gases to water tube, even though flow may be turbulent, a laminar flow region (boundary layer of film) exists close to the tube. The heat transfer through this film takes place by

  2. A.

     Convection

    B.

     Radiation

    C.

     Conduction

    D.

     Both convection and conduction

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  3. Wien’s law states that the wave length corresponding to ________ is proportional to the absolute temperature.

  4. A.

     Minimum energy

    B.

     Maximum energy

    C.

     Both (A) and (B)

    D.

     None of these

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  5. The amount of radiation mainly depends on

  6. A.

     Nature of body

    B.

     Temperature of body

    C.

     Type of surface of body

    D.

     All of the above

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  7. The heat transfer by conduction through a thick cylinder (Q) is given by (where T₁ = Higher temperature, T₂ = Lower temperature, r₁ = Inside radius, r₂ = Outside radius, l = Length of cylinder, and k = Thermal conductivity)

  8. A.

     Q = [2πlk (T₁ - T₂)]/2.3 log (r₂/r₁)

    B.

     Q = 2.3 log (r₂/r₁)/[2πlk (T₁ - T₂)]

    C.

     Q = [2π (T₁ - T₂)]/2.3 lk log (r₂/r₁)

    D.

     Q = = 2πlk/2.3 (T₁ - T₂) log (r₂/r₁)

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  9. Thermal diffusivity of a substance is given by (where h = Thermal diffusivity, ρ = Density of substance, S = Specific heat, and k = Thermal conductivity)

  10. A.

     h = k/ ρS

    B.

     h = ρS/k

    C.

     h = S/ρk

    D.

     h = kρ/S

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  11. Which of the following is expected to have highest thermal conductivity?

  12. A.

     Steam

    B.

     Solid ice

    C.

     Melting ice

    D.

     Water

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  13. Which of the following has maximum value of thermal conductivity?

  14. A.

     Aluminium

    B.

     Steel

    C.

     Brass

    D.

     Copper

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  15. Free convection flow depends on

  16. A.

     Density

    B.

     Coefficient of viscosity

    C.

     Gravitational force

    D.

     All of these

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  17. The critical thickness of insulation for a sphere is

  18. A.

     k/h₀

    B.

     2k/h₀

    C.

     h₀/k

    D.

     h₀/2k

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  19. Unit of thermal conductivity in M.K.S. units is

  20. A.

     K cal/kg m² °C

    B.

     K cal m/hr m² °C

    C.

     K cal/hr m² °C

    D.

     K calm/hr °C

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