Impulse stage

Because of the smaller blade angle the reaction stage is more efficient than the impulse stage, but it requires more stages for the same... 

Most axial flow turbines eonsist of more than one stage, the front stages are usually impulse (zero reaetion) and the later stages have about 50% reaetion. The impulse stages produee about twiee the output of a eompar-able 50% reaetion stage, while the effieieney of an impulse stage is less than that of a 50% reaetion stage. 

One or more impulse stages followed by several stages of impulse reaction blading (Rateau-Parsons type). 

Figurg 15.2 Diagram of impulse stage showing velocities.

This fraction of the kinetic eneigy possessed by the mid-stage gas stream will be converted into additional enthalpy. There is no mechanism for reconverting this enthalpy into kinetic energy in an impulse blade, so there will be no recovery of any energy lost at the blade entry in an impulse stage. A reasonable estimate may be 1. The loss correction factor to be applied to the initial value of blade efficiency is simply (l-X ). The final calculation of blade efficiency for an operating impulse blade is thus... 

Calculate the mid-stage pressures, p j. In the case of an impulse stage these will be the same as the inlet pressure to the next stage. The case of a reaction stage is more complex and is covered in Section 15.4 above. 

Since we know the gas speed at the exit of the nozzle, we may calculate the ratio of gas speed to nozzle outlet speed, Rp. If the stage is an impulse stage we calculate the blade efflciency at off-design conditions using equations (15.50) and (15.66). The specific work of the stage will then be given by ... 

We may at this point calculate the outlet velocity, C2, from equation (15.55) for an impulse stage or from equation (15.110) for a 50% reaction stage. 

But an accurate calculation of the performance of the nozzle requires a knowledge of both the stagnation and the local values of pressure. For example, the equation for mass flow for an impulse stage (e.g. equation (14.63)) will depend on the ratio pi.i/por.z. where the absence of a pressure drop over the blade implies p j = p2j = po.i+il unfortunately we will not have po.i+i available, only the value p oj+i, which will have to stand for bothpo.i+i and Por./+i- In fact, the assumption of zero interstage velocities means that we cannot distinguish between the stagnation and local values of any of the thermodynamic variables, and must be content with the approximations ... 

The specific work, w, for an impulse stage may be written in terms of the blade efficiency, ijb, using equations (15.31) and (15.32) ... 

Substituting into equation (16.15) gives the stage efficiency for all impulse stages after the first as ... 

If the stage is the first stage and is an impulse stage, calculate overall stage efficiency from equation (16.17) ... 

Peak efficiency is obtained in an impulse stage with more work per stage than in a reaction stage for a given stage diameter. It is normal, therefore, for an impulse turbine section to requhe either fewer stages on the same diameter or the same number of stages on a smaller diameter. 

FIGURE 6.79 Typical impulse stages, wheel and diaphragm construction. 

Single-row impulse stages have a maximiun efficiency of about 86 percent at a velocity ratio of 0.45. Figure T-69a shows a combination of impulse buckets with an expanding nozzle, and Fig. T-69d shows multistage impulse blading with nonexpanding nozzles. 

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