Tham khảo tài liệu 'turbo machinery dynamics part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | INDUSTRIAL GAS AND STEAM TURBINES 63 In order to improve thermal efficiency a turbine s hot path components are exposed to gases at extraordinarily high temperatures sometimes beyond the capability of the material. Sophisticated cooling methods are required in the combustor and the turbine to avoid degradation of the material s strength and durability characteristics. New breeds of superalloys are constantly under development to deliver the required characteristics and still can be formed to the required component dimensions during manufacturing. The differential thermal growth between mating rotating and static parts must be predicted with a high level of precision. Besides normal operating conditions dynamic loads arise from rotating unbalance fluid flow forces and misalignment. Manufacturing techniques have also seen vast changes in the past 20 years to allow machining and fabrication of highly contoured airfoils to close tolerances. SIMPLE-CYCLE GAS TURBINE A gas turbine operates on the Brayton cycle. The Brayton principle consists of two isobaric and two isentropic processes as shown in Fig. . The former take place in the gas turbine s combustor and in the steam generator s gas side and the latter represents the compression of air and expansion of gases in the turbine. Gas turbine cycle efficiency depends on the compression ratio and turbine firing temperature increasing with both parameters according to the relation shown in Eq. . It is assumed in the derivation that the pressure ratios in the compressor and the turbine remain the same the fuel flow rate is considerably smaller than that for air the specific heat of the gases stays constant and that all components operate without incurring any losses where h is the ideal cycle efficiency r represents the compression ratio and gis the ratio of specific heats at constant pressure and constant volume. Turbine and compressor efficiencies FIGURE Standard Brayton cycle for gas turbine. 64 .
