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Mechanical Engineering :: Heat and Mass Transfer

  1. Thermal conductivity of air with rise in temperature

  2. A.

     Increases

    B.

     Decreases

    C.

     Remain constant

    D.

     May increase or decrease depending on temperature


  3. Thermal conductivity of water in general with rise in temperature

  4. A.

     Increases

    B.

     Decreases

    C.

     Remain constant

    D.

     May increase or decrease depending on temperature


  5. Heat is closely related with

  6. A.

     Liquids

    B.

     Energy

    C.

     Temperature

    D.

     Entropy


  7. The thermal diffusivities for gases are generally

  8. A.

     More than those for liquids

    B.

     Less than those for liquids

    C.

     More than those for solids

    D.

     Dependent on the viscosity


  9. Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be

  10. A.

     One

    B.

     Two

    C.

     Three

    D.

     Four


  11. Reynolds number (RN) is given by (where h = Film coefficient, l = Linear dimension, V = Velocity of fluid, k = Thermal conductivity, t = Temperature, ρ = Density of fluid, cp = Specific heat at constant pressure, and μ = Coefficient of absolute viscosity)

  12. A.

     RN = hl/k

    B.

     RN = μ cp/k

    C.

     RN = ρ V l /μ

    D.

     RN = V²/t.cp


  13. An ordinary passenger aircraft requires a cooling system of capacity.

  14. A.

     2 TR

    B.

     4 TR

    C.

     8 TR

    D.

     10 TR


  15. A cube at high temperature is immersed in a constant temperature bath. It loses heat from its top, bottom and side surfaces with heat transfer coefficients of h₁, h₂ and h₃ respectively. The average heat transfer coefficient for the cube is

  16. A.

     h₁ + h₂ + h₃

    B.

     (h₁.h₂.h₃)1/3

    C.

     1/h₁ + 1/h₂ + 1/h₃

    D.

     None of these


  17. Moisture would find its way into insulation by vapour pressure unless it is prevented by

  18. A.

     High thickness of insulation

    B.

     High vapour pressure

    C.

     Less thermal conductivity insulator

    D.

     A vapour seal


  19. The ratio of surface convection resistance to the internal conduction resistance is known as

  20. A.

     Grashoff number

    B.

     Biot number

    C.

     Stanton number

    D.

     Prandtl number