Hydraulic Machines Multiple Choice Questions on “Francis Turbine Velocity Diagrams”.
1. What is the water flow direction in the runner in a Francis turbine?
a) Axial and then tangential
b) Tangential and then axial
c) Radial and then axial
d) Axial and then radial
Answer: c
Clarification: Francis turbine is radial flow reaction turbine. Though the water enters the turbine tangentially, it enters the runner radially inward and flows outward along the axis of the runner.
2. Which of the following is true in case of flow of water before it enters the runner of a Francis Turbine?
a) Available head is entirely converted to velocity head
b) Available head is entire converted to pressure head
c) Available head is neither converted to pressure head nor velocity head
d) Available head is partly converted to pressure head and partly to velocity head
Answer: d
Clarification: Since Francis Turbine is a reaction turbine, part of the available head is converted to velocity head. It is not entirely converted to velocity head. The rest of the available head is converted into pressure head.
3. 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?
a) To ensure constant velocity of water during runner entry
b) To prevent loss of efficiency of the turbine due to impulsive forces caused by extra area
c) To prevent leakage from the turbine
d) To reduce material costs in order to make the turbine more economical
Answer: a
Clarification: 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.
4. Which of the following profiles are used for guide vanes to ensure smooth flow without separation?
a) Rectangular
b) Bent Rectangular
c) Elliptical
d) Aerofoil
Answer: d
Clarification: Smooth flow and flow without separation (eddiless flow) can be ensured when the cross sectional profile of the guide vanes are aerofoil in nature. Aerofoil shape is used in airplane wings to ensure smooth flow too. Rectangular profiles are not effective in guiding the water into the runner. Elliptical profiles will cause more drag, finally ending up with turbine inefficiency.
5. In which of the following type of runners the velocity of whirl at inlet is greater than the blade velocity?
a) Such a case is practically impossible
b) Slow Runner
c) Medium Runner
d) Fast Runner
Answer: b
Clarification: Considering the velocity diagram of Francis turbine at the inlet for a slow runner, we notice that the whirl velocity exceeds the blade velocity along the same direction. They are equal in case of a medium runner.
6. Which of the following runner types will have the highest vane angle at inlet (β1 value)?
a) Slow Runner
b) Medium Runner
c) Fast Runner
d) Vane angle is defined only for Kaplan Turbines and not Francis turbines