tailieunhanh - GasTurbine Engineering HandbookSecond Edition phần 4

gần với vận tốc âm thanh lớn hơn, một sóng xung kích diễn ra trong các phần cảm ứng. Một làn sóng xung kích sản xuất giảm sốc và cuộn cảm các chất cảm ứng. Hình 6-12 cho thấy tác động của prewhirl đầu vào máy nén hiệu quả. Có ba loại prewhirl: 1. Free-xoáy prewhirl. Loại này được đại diện bởi r1 Â 1 V | Centrifugal Compressors 231 close to the sonic velocity or greater than it a shock wave takes place in the inducer section. A shock wave produces shock loss and chokes the inducer. Figure 6-12 shows the effect of inlet prewhirl on compressor efficiency. There are three kinds of prewhirl 1. Free-vortex prewhirl. This type is represented by n Ve constant with respect to the inducer inlet radius. This prewhirl distribution is shown in Figure 6-13. Vq is at a minimum at the inducer inlet shroud radius. Therefore it is not effective in decreasing the relative Mach number in this manner. 2. Forced-vortex prewhirl. This type is shown as Vỹi Ịvx constant. This prewhirl distribution is also shown in Figure 6-14. Vq is at a maximum at the inducer inlet shroud radius contributing to a decrease in the inlet relative Mach number. Figure 6-13. Prewhirl distribution patterns. Figure 6-14. Euler work distribution at an impeller exit. 232 Gas Turbine Engineering Handbook 3. Control-vortex prewhirl. This type is represented by Ve AR B r where A and B are constants. This equation shows the first type with A 0 B Ạ 0 and the second type with B 0 A Ạ 0. Euler work distributions at an impeller exit with respect to the impeller width are shown in Figure 6-14. From Figure 6-14 the prewhirl distribution should be made not only from the relative Mach number at the inducer inlet shroud radius but also from Euler work distribution at the impeller exit. Uniform impeller exit flow conditions considering the impeller losses are important factors in obtaining good compressor performance. Impeller An impeller in a centrifugal compressor imparts energy to a fluid. The impeller consists of two basic components 1 an inducer like an axial-flow rotor and 2 the radial blades where energy is imparted by centrifugal force. Flow enters the impeller in the axial direction and leaves in the radial direction. The velocity variations from hub to shroud resulting from these changes in flow directions complicate