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Concept
Degree of reaction
Summary
In turbomachinery, degree of reaction or reaction ratio (R) is defined as the ratio of the static pressure rise in the rotating blades of a compressor (or drop in turbine blades) to the static pressure rise in the compressor stage (or drop in a turbine stage). Alternatively it is the ratio of static enthalpy change in the rotor to the static enthalpy change in the stage.
Degree of reaction (R) is an important factor in designing the blades of a turbine, compressors, pumps and other turbo-machinery.
Various definitions exist in terms of enthalpies, pressures or flow geometry of the device.
In case of turbines, both impulse and reaction machines, Degree of reaction (R) is defined as the ratio of energy transfer by the change in static head to the total energy transfer in the rotor i.e.
For a gas turbine or compressor it is defined as the ratio of isentropic heat drop in the moving blades (i.e. the rotor) to the sum of the isentropic heat drops in the fixed blades (i.e. the stator) and the moving blades i.e.
In pumps, degree of reaction deals in static and dynamic head. Degree of reaction is defined as the fraction of energy transfer by change in static head to the total energy transfer in the rotor i.e.
Most turbo machines are efficient to a certain degree and can be approximated to undergo isentropic process in the stage.
Hence from ,
it is easy to see that for isentropic process ∆H ≃ ∆P. Hence it can be implied
The same can be expressed mathematically as:
Where 1 to 3ss in Figure 1 represents the isentropic process beginning from stator inlet at 1 to rotor outlet at 3. And 2 to 3s is the isentropic process from rotor inlet at 2 to rotor outlet at 3. The velocity triangle (Figure 2.) for the flow process within the stage represents the change in fluid velocity as it flows first in the stator or the fixed blades and then through the rotor or the moving blades. Due to the change in velocities there is a corresponding pressure change.
Another useful definition used commonly uses stage velocities as:
is the enthalpy drop in the rotor and
is the total enthalpy drop.
Official source
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