Turbines impulse

An impulse-type turbine experiences its entire enthalphy drop in the nozzle, thus naving a very high velocity entering the rotor. The velocity entering the rotor is about twice the velocity of the wheel. The reaction type turbine divides the enthalphy drop in the nozzle and in the rotor. Thus, for example, a 50 percent reaction turbine has a velocity leaving the nozzle equal to the wheel speed and produces about V2 the work of a similar size impulse turbine at about 2-3 percentage points higher efficiency than the impulse turbine (0 percent reaction turbine). The effect on the efficiency and ratio of the wheel speed to inlet velocity is shown in Fig. 29-27 for an impiilse turbine and 50 percent reaction turbine. 

Impulse Turbine The impulse turbine is the simplest type of turbine. It consists of a group of nozzles followed by a row of blades. The gas is expanded in the nozzle, converting the high thermal energy into kinetic energy. This conversion can be represented by the following relationship ... 

As mentioned earlier, turboexpander are generally of radial reaetion turbine design beeause this geometry is often most effieient. In an ordinary impulse turbine the high veloeity stream from the nozzles makes a U-turn in the rotor blades, and this U-turn eonsumes 8%-10% of the energy. 

The two types of turbines—axial-flow and radial-inflow turbines—can be divided further into impulse or reaction type units. Impulse turbines take their entire enthalpy drop through the nozzles, while the reaction turbine takes a partial drop through both the nozzles and the impeller blades. 

The axial-flow turbine, like its eounterpart the axial-flow eompressor, has flow, whieh enters and leaves in the axial direetion. There are two types of axial turbines (1) impulse type, and (2) reaetion type. The impulse turbine has its entire enthalpy drop in the nozzle therefore it has a very high veloeity entering the rotor. The reaetion turbine divides the enthalpy drop in the nozzle and the rotor. Figure 1-37 is a sehematie of an axial-flow turbine, also depleting the distribution of the pressure, temperature and the absolute veloeity. 

The two conditions that vary the most in a turbine are the inlet pressure and temperature. Two diagrams are needed to show their characteristics. Figure 3-12 is a performance map that shows the effect of turbine inlet temperature and pressure, while power is dependent on the efficiency of the unit, the flow rate, and the available energy (turbine inlet temperature). The effect of efficiency with speed is shown in Figure 3-13. Figure 3-13 also shows the difference between an impulse and a 50% reaction turbine. An impulse turbine is a zero-reaction turbine. 

From the previous relationship, it is obvious that for a zero-reaetion turbine (impulse turbine) the relative exit veloeity is equal to the relative inlet veloeity. Most turbines have a degree of reaetion between 0 and 1 negative reaetion turbines have mueh lower effieieneies and are not usually used. 

The value of the work faetor for an impulse turbine (zero reaetion) with a maximum utilization faetor is two. In a 50% reaetion turbine with a maximum utilization faetor the work faetor is one. 

Figure 9-6 shows a diagram of a single-stage impulse turbine. The statie pressure deereases in the nozzle with a eorresponding inerease in the absolute veloeity. The absolute veloeity is then redueed in the rotor, but the statie pressure and the relative veloeity remain eonstant. To get the maximum energy transfer, the blades must rotate at about one-half the veloeity of the gas jet veloeity. Two or more rows of moving blades are sometimes used in eonjunetion with one nozzle to obtain wheels with low blade tip speeds and stresses. In-between the moving rows of blades are guide vanes that redireet the gas from one row of moving blades to another as shown in Figure 9-7. This type of turbine is sometimes ealled a Curtis turbine.

Another impulse turbine is the pressure eompound or Ratteau turbine. In this turbine the work is broken down into various stages. Eaeh stage eonsists of a nozzle and blade row where the kinetie energy of the jet is absorbed into the turbine rotor as useful work. The air that leaves the moving blades enters the next set of nozzles where the enthalpy deereases further, and the veloeity is inereased and then absorbed in an assoeiated row of moving blades. 

Figure 9-6. Schematic of an Impulse turbine showing the variation of the thermodynamic and fluid mechanic properties.

Figure 9-8. Pressure and velocity distributions in a Ratteau-type impulse turbine.

The power developed by the flow in an impulse turbine is given by the Euler equation... 

The relative veloeity fV remains unehanged in a pure impulse turbine, exeept for frietional and turbulenee effeet. This loss varies from about 20% for very high-veloeity turbines (3000ft/see) to about 8% for low-veloeity turbines (500ft/see). Sinee the blade speed ratio is equal to (eosa)/2 for maximum utilization, the energy transferred in an impulse turbine ean be written... 

In most designs, the reaetion of the turbine varies from hub to shroud. The impulse turbine is a reaetion turbine with a reaetion of zero (R = 0). The utilization factor for a fixed nozzle angle will increase as the reaction approaches 100%. For = 1, the utilization factor does not reach unity but reaches some maximum finite value. The 100% reaction turbine is not practical because of the high rotor speed necessary for a good utilization factor. For reaction less than zero, the rotor has a diffusing action. Diffusing action in the rotor is undesirable, since it leads to flow losses. 

Figure 14-17A. Basic reaction and impulse turbine principles. (Used by permission Rowley, L N., B. G. A. Skrotzki and W. A. Vopat. Power, Dec. 1945. McGraw-Hill, Inc. All rights reserved.)...

Impulse turbine design employs a stationary, circular diaphragm onto which a large number of fixed-position, tear-shaped nozzle blades (vanes) are mounted. High-velocity steam moves across the vanes and produces steam jets that are directed into waterwheel-type buckets, mounted onto discs around the turbine rotor. The pressure of the steam in the buckets forces the shaft to rotate. The kinetic energy of the jets is translated into mechanical work as the shaft turns. 

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