Question

The whirl component of turbine blades is zero for the following turbine.

• A. Parson's reaction turbine
• B. Curtis turbine
• C. Axial flow turbine
• D. Impulse turbine with equal blade angles at the inlet and outlet

Hint:

Whirl velocity ($V_w$) is the tangential velocity component responsible for creating torque and turning the rotor. If a turbine operates with a zero whirl component, the flow enters and exits in a purely axial direction without any tangential rotation relative to the blade motion.

Answer 1

The Correct Option is C

Concept: In turbomachinery, the total power transferred between a fluid stream and the rotating blades is governed by the fundamental Euler Turbine Equation. The work done per unit mass of fluid ($W$) depends on the velocities at the blade inlet (subscript 1) and outlet (subscript 2): \[ W = u_1 V_{w1} \pm u_2 V_{w2} \] Where:

• $u_1, u_2$ represent the linear peripheral velocity of the rotating blades at the inlet and outlet, respectively.

• $V_{w1}, V_{w2}$ represent the whirl components (or tangential components) of the absolute fluid velocity at the inlet and outlet, which align with the direction of blade rotation and contribute directly to torque generation.
For a pure axial flow machine with an axial discharge condition at the exit, the absolute fluid velocity is directed purely parallel to the rotational shaft axis. Consequently, the tangential component at the outlet is reduced to zero ($V_{w2} = 0$). Furthermore, if a turbine is categorized generally or designed as a pure axial flow turbine where no tangential deflection of the fluid occurs relative to the blade movement, the net whirl component across the blades is effectively absent.

Step 1: Evaluate the velocity profiles of the listed options.
Let's analyze the velocity vector diagrams for each type of turbine provided:

• Parson's Reaction Turbine: This is an axial flow reaction turbine with 50% degree of reaction. The fluid expands continuous across both fixed and moving blades. Due to the oblique blade angles, the fluid enters with a high whirl velocity ($V_{w1}$) and exits with a residual tangential component. Neither component is zero.

• Curtis Turbine: This is a velocity-compounded impulse turbine. Steam expands completely in a nozzle, creating a high initial absolute velocity. It passes through multiple rows of moving and fixed guide blades. The whirl velocity changes dramatically across each moving row to maximize kinetic energy extraction, so it is definitely not zero.

• Impulse Turbine with equal blade angles: If an impulse turbine has symmetrical blades ($\beta_1 = \beta_2$), the relative velocity magnitudes remain equal if friction is neglected ($V_{r1} = V_{r2}$). However, the change in absolute whirl velocity ($\Delta V_w = V_{w1} + V_{w2}$) reaches a substantial value to maximize work output.

• Axial Flow Turbine: In a generic or idealized axial flow stage designed without swirl (or with zero net whirl component), the fluid enters and leaves the blade rows parallel to the axis of rotation. The whirl component ($V_w$), which acts tangential to the rotor circumference, is zero.

Step 2: Match the question constraint to the correct classification.
The problem specifies that the whirl component is zero. This matches the ideal flow configuration of an axial flow system with zero tangential velocity interactions, aligning with Option (C).