Degree of reaction

In turbomachinery, degree of reaction or reaction ratio (denoted R) is defined as the ratio of the change in static pressure in the rotating blades of a compressor or turbine, to the static pressure change in the compressor or turbine stage. Alternatively it is the ratio of static enthalpy change in the rotor to the static enthalpy change in the stage.

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 is defined as the ratio of energy transfer by the change in static head to the total energy transfer in the rotor:^[1] $${\displaystyle R={\frac {\text{Isentropic enthalpy change in rotor}}{\text{Isentropic enthalpy change in stage}}}}$$

For a gas turbine or compressor it is defined as the ratio of isentropic heat drop in the moving blades (the rotor) to the sum of the isentropic heat drops in both the fixed blades (the stator) and the moving blades: $${\displaystyle R={\frac {\text{Isentropic heat drop in rotor}}{\text{Isentropic heat drop in stage}}}}$$

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: $${\displaystyle R={\frac {\text{Static pressure rise in rotor}}{\text{Total pressure rise in stage}}}}$$