Handbook of Turbomachinery Second Edition Revised - Ventech!

In an impulse design, most of the pressure drop is across the stationary
blade or nozzle. In a ‘‘pure’’ impulse design, all the pressure drop would
occur across the nozzle; this would be termed 0% ‘‘reaction.’’ In practice,
there must be some pressure drop across the rotating blades in order to
generate flow. For example, the control stage of the HP turbine is typically
an ‘‘impulse’’ stage by design but still has on average about 5% reaction at
full load. Note that a stage will pass flow even if there is negative reaction;
what is required is that the ratio of the total to static pressure drop across
the rotating row be a positive value (see Fig. 24).
In a reaction design, work is performed by a jet exiting from stationary
blades similar to the impulse design; however, an additional reaction from
the jet created by the moving blades also occurs. For a symmetric reaction
design, equal pressure drop occurs across the stationary and rotating blades
and thus reaction is equal to 50%. Figures 24 and 25 show the percent
reaction versus stage number for fossil and nuclear units, respectively, with
both impulse and reaction turbine types plotted on each figure. Note that
the tip and root reaction percentages are different to counteract the effect of
Figure 24 Percentage stage reactions for fossil steam turbines.
Copyright © 2003 Marcel Dekker, Inc.