Single step cooling

The cooling fraction obviously increases with combustion temperature, but the compressor pressure ratio (and hence the cooling air temperature Tj) is also critically important. It is seen that the arbitrary assumptions made for i/ in Chapter 4 (linearly increasing with the combustion temperature cot would be approximately valid for a cycle with a pressure ratio just below 30. 

The results of a set of computer calculations for a CBT plant with single-step cooling (i.e. of the first stage nozzle guide vanes) are illustrated in Fig. 5.2, in the form of (arbitrary) overall thermal efficiency (tjq) against pressure ratio (r) with the combustion temperature T. oi as a parameter, and in Fig. 5.3 as tjq against with r as a parameter. 

At very high combustion temperatures, it is not sufficient that the first blade row alone needs to be cooled. In practice, up to half a dozen rows may be cooled in an industrial gas turbine, if the combustion temperature is high and the allowable blade metal temperature is low. The cooling fractions for each of the cooled rows must be estimated and u.sed in the cycle calculations, which now become complex. 

Illustrations of such calculations, for an open cycle [CBTjic, plant, were given by Horlock et al. 12, in which it was assumed that three blade rows were film cooled, the two 

The cycle calculations for this multi-cooling then proceeded in a similar fashion to those for the single-.step cooling calculations of Section 5.4 (full details are given in Ref. [2]). 

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Cycle [CHTJj ci with single step cooling... 

Fig. 4.1. Temperature-entropy diagram for single-step cooling—reversible eycle (CHTIr (after Ref. [5]).

Efficiency as a function of combustion temperature or rotor inlet temperature (for single-step cooling)... 

Fig. 4.10. Calculation of efficiency of. -iimple CBT plant—single-step cooled [CBT n ] as a function of iseniropic temperature ratio with maximum temperature (7 ) as a parameter.

Fig. 4.11. Calculation of efficiency of simple CBT plants—single-.step cooled ICBTlica uncooled [CBT ]ii—a.s a function of specific work with pressure ratio (r) and maximum temperature as parameters and with r)p< = t), = 0.9. 7hi = 1073 K (after Ref. 5 ).

Fig. 4.12. Calculation of efficiency of ICBT] plant uncoolcd CBT uj a.s a function of combustion lemperalurc (7eoi) single-step cooled (CBT)k i as a function of rotor inlet temperature (TVi,). Pres,sure ratio r = 30. t)c — 0.8.

These conclusions are broadly confirmed by real gas calculations for single step cooling with arbitrary assumptions for cooling flow fractions. 

Fig. 5.2 shows that for the single-step cooled CBT plant at a given combustion temperature, the overall efficiency of the cooled gas turbine efficiency increases with pressure ratio initially but, compared with an uncooled cycle, reaches a maximum at a lower optimum pressure ratio. Fig. 5.3 shows that for a given pressure ratio the efficiency generally increases with the combustion temperature even though the required cooling fraction increases. 

Fig. 5.2. Overall efficiency of fCBT]ici plant with single-step cooling of NGVs, as a function of pressure ratio with combustion temperature as a parameter.

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