A reaction turbine is a type of steam turbine that operates based on the principle of both pressure and velocity compounding. Unlike impulse turbines, which only utilize the kinetic energy of the steam jet to drive the turbine blades, reaction turbines also harness the pressure energy of the steam as it expands through the turbine stages.
Reaction Turbine Working Principle
A reaction turbine, also known as an impulse-reaction turbine, operates by utilizing both the kinetic energy and pressure energy of steam as it passes through the turbine stages. Unlike impulse turbines where only the kinetic energy of steam is utilized, reaction turbines extract energy from both the pressure and velocity changes of the steam.
In a reaction turbine, the rotor blades are designed to resemble convergent nozzles. As steam is directed onto the rotor by the fixed vanes of the stator, it forms a jet that encompasses the entire circumference of the rotor. The steam then changes direction and accelerates relative to the speed of the rotor blades. This acceleration through the rotor blades generates a reaction force, driving the rotation of the turbine rotor.
Throughout the turbine stages, steam expands continuously over both the fixed and moving blades, leading to a gradual decrease in pressure and temperature. This expansion results in a pressure drop and heat drop across both the fixed and moving blades, reflecting the work performed in driving the rotor.
An example of a reaction turbine is Parsons turbine, where steam expands over the moving blades, imparting a reaction force to them. The turbine operates through multiple stages, each comprising sets of fixed and moving blades, to efficiently extract energy from the steam.
Velocity Diagram for Reaction Turbine
Degree of Reaction
The degree of reaction in a turbine is a measure of how the total heat drop is distributed between the moving and fixed blades within a turbine stage. It is expressed as the ratio of the isentropic heat drop occurring in the moving blades to the total isentropic heat drop across the entire stage of a reaction turbine.
A commonly used design in turbines, such as the Parsons turbine, features a half degree of reaction or 50% reaction. This means that half of the total heat drop occurs in the moving blades, while the other half occurs in the fixed blades. In the Parsons turbine, both the rotor and stator blades are symmetrical.
This distribution of heat drop results in symmetrical velocity triangle s within the turbine stage, facilitating efficient energy transfer and turbine operation.
Comparing Efficiencies of Impulse and Reaction turbines
How reaction turbine differs from Impulse turbine?
Reaction turbines operate differently from impulse turbines in several important aspects:
- Energy Conversion Process: In impulse turbines, the potential energy of the water is entirely converted to kinetic energy by the nozzles before entering the runner. The pressure within the runner remains constant at atmospheric level. However, in reaction turbines, only part of the potential energy is converted to kinetic energy by the stationary guide blades. The remaining potential energy is gradually converted to kinetic energy as the water passes through the runner. This leads to a varying pressure inside the runner along the flow path.
- Blade Engagement: In impulse turbines, only a few buckets are engaged by the water jet at any given time. Conversely, in reaction turbines, all blades or vanes are constantly engaged by water throughout the operation.
Other differences include the suitability of reaction turbines for low and medium heads (up to 300 meters), while impulse turbines are preferred for higher heads. Additionally, in reaction turbines, the relative velocity at the outlet is higher due to the drop in pressure in the vane passages, whereas in impulse turbines, the relative velocity may decrease due to surface friction or remain constant.
Furthermore, the flow area between two blades in a reaction turbine changes gradually to accommodate the change in static pressure, while in an impulse turbine, this area remains constant. Finally, reaction turbines do not have a fixed speed ratio for optimum efficiency like impulse turbines, allowing them to be operated at higher speeds.