Analyzing Hydraulic Turbine Statements

The question asks us to identify the statement that is factually incorrect concerning hydraulic turbines. We need to examine each description of the turbine types and their operational characteristics.

Statement 1: The relative velocity over the moving blades increases in impulse turbines

This statement is incorrect. In an impulse turbine, like the Pelton wheel, water is directed by a nozzle as a high-speed jet onto the runner buckets. The fundamental principle is the conversion of the jet's kinetic energy into rotational mechanical energy. As the water jet impacts the bucket and flows along its curved surface, it transfers momentum to the bucket. This interaction causes a change in the water's velocity relative to the moving blade. Ideally, for maximum energy transfer, the relative velocity of the water leaving the bucket should be minimal. In practical terms, the relative velocity ($V_{rel}$) typically decreases as the fluid performs work on the blade, transferring its kinetic energy. The statement claims an increase, which contradicts the basic energy conversion process in impulse turbines.

Statement 2: A pelton wheel is a tangential flow impulse turbine

This statement is correct. The Pelton wheel is designed such that the water jet strikes the buckets tangentially to the path of rotation. It operates based on the impulse principle, where the pressure of the water remains constant (approximately atmospheric) as it passes through the runner, and the force is generated due to the change in momentum of the jet. It is well-suited for high-head, low-flow applications.

Statement 3: A francis turbine is a mix of radial and axial flow reaction turbine

This statement is correct. A Francis turbine is classified as a mixed-flow turbine. Water is typically guided radially inwards towards the runner and then flows axially outwards through the runner as it exits. As a reaction turbine, it operates with a pressure difference across the runner blades, converting both pressure and kinetic energy into mechanical work.

Statement 4: A kaplan turbine is a axial flow reaction turbine

This statement is correct. The Kaplan turbine is characterized by water flowing parallel to the axis of rotation as it passes through the runner blades. Like the Francis turbine, it is a reaction turbine, meaning pressure changes occur across the runner. Kaplan turbines are particularly effective for low-head, high-flow conditions and often feature adjustable blades to optimize performance across varying flow rates.

Identifying the Incorrect Statement

By evaluating each statement, we find that the description of relative velocity increasing in impulse turbines is inaccurate. The process of energy extraction involves a decrease in the fluid's relative velocity as it interacts with the turbine blades.