Compressors, and Turbines

Full-Form Shaping. The third appHcation of ECM, hill-form shaping, uti1i2es a constant gap across the entire workpiece, and a constant feed rate in order to produce the type of shape used for the production of compressor and turbine blades. In this procedure, current densities as high as 100 A/cm ate used, and the current density remains high across the entire face of the workpiece. 

Improved materials, coatings, and cooling techniques permit newer machines to operate at higher turbine inlet temperatures, yielding both increased output and efficiency. Further efficiency gains result from improved aerodynamics in the hot gas path, compressor, and turbine sections. Use is also made of variable inlet guide vanes (IGV). 

To obtain a more accurate relationship between the overall thermal efficiency and the inlet turbine temperatures, overall pressure ratios, and output work, consider the following relationships. For maximum overall thermal cycle efficiency, the following equation gives the optimum pressure ratio for fixed inlet temperatures and efficiencies to the compressor and turbine ... 

The simple cycle is the most common type of cycle being used in gas turbines in the field today. The actual open simple cycle as shown in Figure 2-9 indicates the inefficiency of the compressor and turbine and the loss in pressure through the burner. Assuming the compressor efficiency is rjc and the turbine efficiency is t], then the actual compressor work and the actual turbine work is given by ... 

This chapter examines the overaii performance characteristics of compressors and turbines. This materiai is presented here to famiiiarize the reader with the behavior of these machines, ciassified under the broad term tur-bomaciiinery. Pumps and compressors are used to produce pressure turbines produce power. These machines have some common characteristics. The main eiement is a rotor with biades or vanes, and the path of the fluid in the rotor may be axiai, radiai, or a combination of both. 

Figure 3-16 shows the effeet of efficiency as a function of the load for both the compressor and turbine. Part-load turbine efficiencies are affected more than compressor efficiencies. The discrepancy results from the compressor operating at a relatively constant inlet temperature, pressure, and pressure ratio, while the turbine inlet temperature is greatly varied (Figure 3-17). 

Figure 3-16. Compressor and turbine efficiency as a function of ioad.

Brown, L.E., Axial Flow Compressor and Turbine Loss Coefficients A Comparison of Several Parameters, Journal of Engineering for Power, ASME Transactions 94A, pp. 193-201, 1972. 

The most simple combustor is a straight-walled duct connecting the compressor and turbine as seen in Figure 10-1. Actually, this arrangement is impractical because of the excessive pressure loss resulting from combustion at high velocities. The fundamental pressure loss from combustion is proportional to the air velocity squared. Since compressor discharge... 

Childs, D.W., and Vance, J.M., Annular Gas Seals and Rotordynamics of Compressors and Turbines Proceedings of the 26th Turbomachinery Symposium, Texas A M University, p. 201, 1997. 

Compressor and turbine seetions ean be analyzed effeetively by eombining vibration speetra with ehanges in performanee data. Major problem areas in eaeh of these eomponents ean be identified with proper monitoring and analysis. 

Since aerothermal performance of compressors and turbines is very sensitive to inlet temperature and pressure variations, it is essential to normalize the aerothermal performance parameters such as flow, speed, horsepower, etc., to standard-day conditions. When these corrections to standard conditions are not applied, a performance degradation may appear to occur when in fact it was a performance change resulting merely from ambient pressure and temperature changes. Some of the equations for obtaining correction to standard-day conditions are given in Table 19-3. 

Fig. 2.9 illustrates this approach of tracing exergy through a plant. The various terms in Eq. (2.49) are shown for an irreversible open gas turbine plant based on the JB cycle. The compressor pressure ratio is 12 1, the ratio of maximum to inlet temperature is 5 1 (T,nax = 1450 K with To = 290 K), the compressor and turbine polytropic efficiencies are... 

Calculation of the specific work and the arbitrary overall efficiency may now be made parallel to the method used for the a/s cycle. The maximum and minimum temperatures are specified, together with compressor and turbine efficiencies. A compressor pressure ratio (r) is selected, and with the pressure loss coefficients specified, the corresponding turbine pressure ratio is obtained. With the compressor exit temperature T2 known and Tt, specified, the temperature change in combustion is also known, and the fuel-air ratio / may then be obtained. Approximate mean values of specific heats are then obtained from Fig. 3.12. Either they may be employed directly, or n and n may be obtained and used. 

Fig. 3.13 shows the overall efficiency for the [CBTJic, plant plotted against the i.sentropic temperature ratio for various maximum temperatures Tj (and 6= Ty/Ti, with T, = 27°C (3(X) K)). The following assumptions are also made polytropic efficiency, rjp = 0.9 for compressor and turbine pressure loss fraction in combustion 0.03 fuel (methane) and air supplied at 1 bar, 27°C (3(X) K). 

From the study of uncooled cycles in Chapter 3, we next move to consider irreversible cycles with compressor and turbine isentropic efficiencies, tjc and r/p, respectively. 

For the (CICBT)iXr, (CBCBT)iXr and (CICBTBT)iXr cycles, with equal pressure ratios across the split compressors and turbines, it may be shown that the corresponding expressions for efficiency are... 

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