Fluid Mechanics and Hydraulic Machinery Miscellaneous


Fluid Mechanics and Hydraulic Machinery Miscellaneous

Fluid Mechanics and Hydraulic Machinery

  1. For a Pelton wheel with a given water jet velocity, the maximum output power from the Pelton wheel is obtained when the ratio of the bucket speed to the water jet speed is ______ (correct to two decimal places).









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    In pelton wheel:-

    η=
    2(vu -u2)
    v2

    where,
    u = bucket speed
    v = water jet speed
    So, For maximum efficiency:-
    dx
    = 0
    du

    2(v – 2u) = 0 v = 2u
    u =
    v
    2

    Correct Option: A

    In pelton wheel:-

    η=
    2(vu -u2)
    v2

    where,
    u = bucket speed
    v = water jet speed
    So, For maximum efficiency:-
    dx
    = 0
    du

    2(v – 2u) = 0 v = 2u
    u =
    v
    2


  1. Which one of the following statements is TRUE?









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    Kaplan turbine is an axial-flow reaction turbine

    Correct Option: D

    Kaplan turbine is an axial-flow reaction turbine



  1. The blade and fluid velocities for an axial turbine are as shown in the figure.

    The magnitude of absolute velocity at entry is 300 m/s at an angle of 65° to the axial direction, while the magnitude of the absolute velocity at exit is 150 m/s. The exit velocity vector has a component in the downward direction. Given that the axial (horizontal) velocity is the same at entry and exit, the specific work (in kJ/kg) is___________ .









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    Given: V1 = 300 m/sec
    u =150 m/sec
    Vf1 = Vf2
    (α = 25°)
    V2 = 150 m/sec

    Specific work = [Vw1 + Vw2]. u
    Vw1 = V1 cos 25°
    Vw2 = 300 cos 25°
    = 271.89 m/s
    Vf1 = Vf2 = V1 sin 25
    Vf2 = 300 sin 25
    = 126.78 m/s
    Vw2 = √Vf12 - Vf22
    = √1502 - 126.782 = √6426.83
    Vw2 = 80.16 m/s
    Specific work = [Vw1 + Vw2]. u
    = [271.89 + 80.76] × 150
    = 52807.5 J/kg
    = 52.81 kJ/kg

    Correct Option: C

    Given: V1 = 300 m/sec
    u =150 m/sec
    Vf1 = Vf2
    (α = 25°)
    V2 = 150 m/sec

    Specific work = [Vw1 + Vw2]. u
    Vw1 = V1 cos 25°
    Vw2 = 300 cos 25°
    = 271.89 m/s
    Vf1 = Vf2 = V1 sin 25
    Vf2 = 300 sin 25
    = 126.78 m/s
    Vw2 = √Vf12 - Vf22
    = √1502 - 126.782 = √6426.83
    Vw2 = 80.16 m/s
    Specific work = [Vw1 + Vw2]. u
    = [271.89 + 80.76] × 150
    = 52807.5 J/kg
    = 52.81 kJ/kg


  1. Consider two hydraulic turbines having identical specific speed and effective head at the inlet. If the speed ratio (N1 /N2) of the two turbines is 2, then the respective power ratio (P1 /P2) is ____.









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    Ns
    N√P
    H5/4

    Given Ns1 = Ns2, H1 H2
    N1
    = 2,
    P1
    = ?
    N2P2

    N1P1 = N2P2
    P1
    =
    N1
    2
    P2N2

    =
    1
    2= 0.25
    2

    Correct Option: C

    Ns
    N√P
    H5/4

    Given Ns1 = Ns2, H1 H2
    N1
    = 2,
    P1
    = ?
    N2P2

    N1P1 = N2P2
    P1
    =
    N1
    2
    P2N2

    =
    1
    2= 0.25
    2



  1. Which of the following statements are TRUE, when the cavitation parameter σ = 0?
    i. the local pressure is reduced to vapor pressure
    ii. cavitation starts
    iii. boiling of liquid starts
    iv. cavitation stops









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    σ = 0 implies (i), (ii) and (iii)

    Correct Option: D

    σ = 0 implies (i), (ii) and (iii)