Degree of Reaction

A certain amount of distributionof pressure (a rise in static pressure) takes place as the air passes through the rotor as well as the stator; the rise in pressure through the stage is in general, attributed to both the blade rows. The term degree of reaction is a measure of the extent to which the rotor itself contributes to the increase in the static head of fluid. It is defined as the ratio of the static enthalpy rise in the rotor to that in the whole stage. Variation of over the relevant temperature range will be negligibly small and hence this ratio of enthalpy rise will be equal to the corresponding temperature rise.

It is useful to obtain a formula for the degree of reaction in terms of the various velocities and air angles associated with the stage. This will be done for the most common case in which it is assumed that the air leaves the stage with the same velocity (absolute) with which it enters ( ).

This leads to . If and are the static temperature rises in the rotor and the stator respectively,

then from Eqs (9.4),(9.5),(9.6),

	(10.1)

Since all the work input to the stage is transferred to air by means of the rotor, the steady flow energy equation yields,

With the help of Eq. (10.1), it becomes

But and , and hence

	(10.2)