Why does the cross sectional area of the Spiral casing gradually decrease along the circumference of the Francis turbine from the entrance to the tip?

Mechanical Engineering : Hydraulic Machines - Quiz(MCQ)

To prevent leakage from the turbine

To reduce material costs in order to make the turbine more economical

To ensure constant velocity of water during runner entry

To prevent loss of efficiency of the turbine due to impulsive forces caused by extra area

Correct Answer : Option (C) : To ensure constant velocity of water during runner entry

Explanation : The primary purpose of the gradual decrease in area is so that the runner sees constant velocity of water at each point of entry. Absence of this may lead to inefficiency. The spiral casing is used to prevent leakage from the turbine but the gradual decrease in area is not for that reason.

Recently Updated in Hydraulic Machines Questions

It is given that the input water power of the Kaplan turbine is 1.10 times the runner power. What would be the hydraulic efficiency of the turbine (in %)?

70.71

80.81

90.91

100.00

Correct Answer : 90.91

Explanation : The definition of Hydraulic efficiency in a Kaplan turbine states that it is the ratio between the Runner power to the shaft power. If input water power is 1.10 times the runner power, the runner power is 1/1.10 = 0.90909 Times the water power. Hence, hydraulic efficiency = 90.91%.

Which of the following efficiencies for Kaplan Turbine is described as the ratio between the power produced by runner to the power supplied by water at the inlet?

Overall efficiency

Volumetric efficiency

Mechanical efficiency

Hydraulic efficiency

Correct Answer : Hydraulic efficiency

Explanation : The definition of Hydraulic efficiency in a Kaplan turbine states that it is the ratio between the Runner power to the shaft power. Hence, the correct option is Hydraulic efficiency.

Which of the following turbines will have the lowest number of blades in it?

Kaplan turbine

Steam turbine

Pelton turbine

Francis turbine

Correct Answer : Kaplan turbine

Explanation : As the head for Kaplan turbine is low, the discharge of water through the turbine is high and hence, blade resistance should be low. That’s why Kaplan turbine will have the lowest number of blades.

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