Nozzle guide vane rows

We concentrate here on open loop cooling in which compressor air mixes with the mainstream after cooling the blade row, the system most widely used in gas turbine plants (but note that a brief reference to closed loop. steam cooling in combined cycles is made later, in Chapter 7). For a gas turbine blade row, such as the stationary entry nozzle guide vane row where most of the cooling is required, the approach first described here (called the simple approach) involves the following ... 

A more sophisticated approach would not only take account of Eqs. (4.45) and (4.46) to give the two stagnation temperatures at exit from control surfaces A and B, but it would also not assume the total pressures of coolant and mainstream to be the same. Eor the first nozzle guide vane row these can be derived by accounting for losses as follows ... 

The choice of these values is arbitrary. In practice, the cooling fraction will depend not only on the combustion temperature but also on the compressor delivery temperature (i.e. the pressure ratio), the allowable metal temperature and other factors, as described in Chapter 5. But with ip assumed for the first nozzle guide vane row, together with the extra total pressure loss involved (k = 0.07 in Eq. (4.48)), the rotor inlet temperature may be determined. These assumptions were used as input to the code developed by Young [11] for cycle calculations, which considers the real gas properties. 

The results of calculations for the cooling air flow fractions in the first (nozzle guide vane) row of the turbine, ba.sed on the assumptions outlined in Section 5.2 for film cooled blading, are illustrated in Fig. 5.1. The entry gas temperature Tgi was taken as the combustion temperature Tcoi = Ty and the cooling air temperature as the compressor delivery temperature T2. The cooling air required is. shown here as a fraction of the exhaust gas flow, i.e. as ip/( 1 + ip), plotted against compressor pressure ratio and combustion temperature for an allowable blade metal temperature, Tpi = 800°C. Also shown are... 

Finally, it may be noted that there is little or no point in adding steam directly to the turbine alone—say into the first nozzle guide vane row—because its enthalpy even at best would only be equal to the enthalpy of the steam leaving the turbine (/is6 h A). 

In order to make a preliminary assessment of the importance of turbine cooling in cycle analysis, the real gas calculations of a simple open uncooled cycle, carried out in Chapter 3 for various pressure ratios and combustion temperatures, are now repeated with single step turbine cooling, i.e. including cooling of the first turbine row, the stationary nozzle guide vanes. 

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.

The turbine normally consists of several stages, each stage combined with a row of stationary guide vanes or nozzles followed by a row of moving blades or buckets. The guide vanes are moimted to the tiu-bine casing and the buckets are fitted to turbine discs, mostly by means of fir-tree roots. See Fig. T-40. 

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