Rotating stall prevention system for compressor

Abstract: A compressor has inlet guide vanes 6, rotor blades 9 and stator blades 7, and comprises further baffle vanes 5 installed upstream of said inlet guide vanes 6 and capable of varying their attached angles, actuators 11 to vary the attached angles of said baffle vanes 5, sensors 12 installed in the upstream side of said baffle vanes 5 to detect a rotating stall condition inside the flow passage of the compressor, and control means 13 to receive signals from said sensors 12 and to output control signals to said actuators 11 to operate said baffle vanes 5 so as to prevent the rotating stall condition.

Description: The invention relates to an axial compressor, comprising the featurs of the first part of claim 1. Such an axial compressor is disclosed in the GB-A-2 191 606. In a high pressure ratio axial compressor having a plurality of stator blade rows and a plurality of rotor blade rows arranged between the stator blade rows, there are some cases where a partial stalling region, called as a cell, is caused by flow separation from the blades and rotationally propagates at a speed of approximately a half rotating speed of the compressor, that is so-called rotating stall. The rotating stall phenomena is caused by the circumferential inlet distortion of the fluid flowing into the blade rows. Therefore, the rotating stall phenomena has been understood to include circumferential distortion of the fluid before flowing into the blade rows. A system preventing such a rotating stall is described in a paper ASME paper 91-GT-88 issued in July 1991. The system described in this paper has a plurality of hot wire anemometers arranged in the peripheral direction of a casing to detect a rotating stall (circumferential inlet distortion of flow), and the setting angles of a plurality of inlet guide vanes are controlled with mutual phase differences based on the detected signals so as to eliminate the rotating stall to the peripheral direction. The setting angles of the inlet guide canes are changed with DC motors operated by the command from a control circuit. The axial compressor disclosed in the GB-A-2 191 606 is provided with a control system for controlling the rotor blade flutter, the rotating stall, the surge, force vibrations and acoustic resonances. For conceiving such unsteady motion phenomenon a sensor array is provided in the compressor and an electronic controller having a selector/filter selects those signal components from the output of the sensor array. On the basis of said selected signal components an actuator array adjust the angles of the inlet guide vanes. The disadvantages in conventional rotating stall prevention systems may be eliminated by the above mentioned prior technology since unsteady state flow field is directly controlled in the prior technology. However, varying the setting angles of a plurality of the inlet guide vanes with mutual phase differences may deteriorate the essential function of the inlet guide vanes. As a result, the inlet flow directions under a steady state flow condition into the blades in the rear stages differ depending on the peripheral positions, which leads to serious effects on the performances under a normal operation of the compressor, such as efficiency drop. An object of the invention is to solve problems existing in the prior technology described above and to provide a rotating stall prevention system for compressor which is high in preventing efficiency against the rotating stall and low in affecting deterioration in the fluid performance of compressor. First and second solutions of the above object, according to the present invention, are defined in claim 1 and 8 resp. Rotating stall is a phenomena where a partial stalling region, called as a cell, is caused by flow separation from the blades and rotationally propagates at a speed of approximately a half rotating speed of the compressor. The flow velocity in the peripheral direction in a partially stalling region is larger than that in a non-stalling region, or the axial flow velocity in a partially stalling region is smaller than that in a non-stalling region, and the blade angle of attack against flow in a partially stalling region is larger than that in a non-stalling region. Further, the pressure in a partially stalling region is higher than that in a non-stalling region. That is, when a rotating stall takes place, the flow velocity and the pressure become uneven distributions over the peripheral direction. The most dominant factor causing initiation of the rotating stall is the inlet distortion (uneven distributions) in flow velocity, pressure and temperature at an inlet of the compressor due to the asymmetry in its shape and so on. The distortion gradually increases from the inlet of the compressor to the inlet of the blade rows to cause the rotating stall inside the blade rows. This rotating stall can be predicted or detected based on the signals from flow sensors. The detected signals are input into control means. The control means carries out calculation to obtain the angles of the baffle vanes or the jet flow rates which make the flow in the passage of the compressor uniform in order to prevent the occurrence of rotating stall and controls the actuators or the control valves. Brief Description of the Drawings FIG.1 is a longitudinal sectional view of a compressor having an embodiment of a rotating stall prevention system in accordance with the present invention. FIG.2 is a front view showing an arrangement of baffle vanes in the compressor shown in FIG.1. FIG.3 is a front view showing an arrangement of hot wire anemometer in the compressor shown in FIG.1. FIG.4 is an enlarged view being seen from the plane of the line IV-IV in FIG.3. FIG.5 is a block diagram showing a detailed structure of control means in the compressor shown in FIG.1. FIG.6 is a peripherally unfolded graph showing velocity vectors under a condition of occurrence of a rotating stall to be prevented by a rotating stall prevention system in accordance with the present invention. FIG.7 is a characteristic graph showing velocity vector versus time under a condition of occurrence of a rotating stall to be prevented by a rotating stall prevention system in accordance with the present invention. FIG.8 is a characteristic graph on baffle vane angles versus time on a rotating stall prevention system in accordance with the present invention. FIG.9 is a longitudinal sectional view of a compressor having another embodiment of a rotating stall prevention system in accordance with the present invention. FIG.10 is a longitudinal sectional view of a compressor having a further embodiment of a rotating stall prevention system in accordance with the present invention. Detailed Description of the Preferred Embodiments FIG.1 shows a compressor having an embodiment of a system according to the present invention. In FIG.1, a compressor 1 has a compressor flow passage 4 formed between a casing 2 and a rotor 3 installed therein. In the casing 2, there provided from the upstream side baffle vanes 5, inlet guide vanes 6, stator blades 7 and exit guide vanes 8. The rotor 3 has rotor blades 9 at the positions between the inlet guide vanes 6 and the stator blades 7, and between the stator blades 7 and the stator blades 7. The setting angles of the inlet guide vanes 6 described above are changed depending on the operating condition (rotating speed of rotor 3) of the compressor with an angle varying mechanism 10 so that the flow rate matches to the rotating speed. The baffle vanes 5 installed the upstream of the inlet guide vanes 6 are, as shown in FIG.2, pivotably attached peripherally onto the casing 2 with circumferentially equal intervals. In this embodiment, four baffle vanes 5 are provided. The baffle vanes 5 are individually driven by actuators 11 such as motors to change their setting angles. Hot wire anemometers 12 as flow sensors for detecting the rotating stall or the circumferential distortion of flow are provided in the downstream of the baffle vanes 5 or the upstream of the inlet guide vanes 6 with circumferentially equal intervals. The hot wire anemometer 12 has, as shown in FIG.3 and FIG.4, two hot wires perpendicular to each other, one is a first hot wire 12a which detects the magnitude of the flow velocity in the axial direction, and the other is a second hot wire 12b which detects the magnitude of the flow velocity in the peripheral direction. Referring to FIG.5, control means 13 for varying the angles of the baffle vanes 5, illustrated in FIG.1, comprises a flow angle processor 14 which receives the signals from the first hot wire 12a and the second hot wire 12b in the hot wire anemometer 12 to obtain the flow angle of fluid velocity , a memory for standard flow angle 15 which stores standard flow angle data, a comparator 16 which compares the standard flow angle values from the memory for standard flow angle 15 with the detected flow angle values from the flow angle processor 14 to obtain the difference between them, a phase difference circuit 17 which produces phase difference for the difference from the comparator 16 to compensate the positional delay and the fluid inertial delay due to the setting position interval between the baffle vanes 5 and the hot wire anemometers 12, a reversing circuit 18 which changes sign of the difference from the phase difference circuit 17, a memory for standard angle of baffle vanes 19 which stores the standard angle data for the baffle vanes 5, an adder 20 which adds the standard angle for baffle vanes from the memory for standard angle of baffle vanes 19 to the difference from the reversing circuit 18, and a subtracter 21. The signals for controlling the baffle vanes angles from the adder 20 are led to the actuator 11 through a subtracter 21. The subtracter 11 receives the angle signals as negative feedback from a position detector 22 installed in the actuator 11. The memory for standard flow angle 15 described above is set to store the standard flow angle value obtained in advance, however, it is also possible to store an average value of a plurality of the flow angles obtained from a plurality of the anemometers 12 as the standard angle value. Next, the operation of the embodiment of a rotating stall prevention system according to the present invention described above will be explained. In a case, for example, where a stalling region takes place at a position of the peripheral angle of 180° as shown in FIG.6, in the interval between the upstream of the blade rows and the inlet of the blade rows the flow angle , angle between the flow velocity vector and the axial flow direction, increases at the region corresponding to the stalling region. By controlling so as to lessen the angle of the baffle vane corresponding to the peripheral position of the region where the angle  is large, the direction of fluid flow vector is forced to turn to decrease the stalling region in the blade rows. By performing this manner with following the peripheral travelling of the stalling region, the rotating stall in the blade rows can be prevented. Therein, the hot wire anemometer 12 corresponding to the peripheral angle described above detects the flow velocity in the axial direction and the flow velocity in peripheral direction. The flow angle processor 14 receives the detected signal from the hot wire anemometer 12 to obtain the flow angle of fluid velocity . The flow angle of fluid velocity  changes, for example, sinusoidally as the time passed as shown in FIG. 7. In the comparator 16, the flow angle of fluid velocity  is compared with the standard flow angle values stored in the memory for standard flow angle 15 to obtain the difference between them. The difference is input to the phase difference circuit 17 to produce an advance phase difference to compensate the positional delay and the fluid inertial delay due to the setting position interval between the baffle vanes 5 and the hot wire anemometers 12 as shown in FIG.8. In the reversing circuit 18, the difference given the phase difference is changed its sign. In the adder 20, the difference reversed its signal is added to the standard angle of the baffle vanes 5 from the memory for standard angle of baffle vane 19. Thus the angles for controlling the baffle vane angles are obtained. The angles for controlling the baffle vane angles are led to the actuator 11 through a subtracter 21. The actuator 11 controls, as described above, so as to lessen the angle of the baffle vane corresponding to the peripheral position of the region where the angle  is large. As the result, the direction of fluid flow vector is forced to turn to decrease the stalling region in the blade rows. The control for the baffle vanes 5 is performed with a certain period of cycle so as to follow the peripheral travelling of the stalling region, since the stalling region travels in such a manner. On the other hand, for the region not corresponding to the stalling region where the fluid flow angle  is small, the control is preformed in the same manner as described above such that the angle of the baffle vane 5 approaches to the standard angle for the baffle vane to stabilize fluid flow. The controlled angles of the baffle vanes 5 are detected by the position detector 22 and are fed back to the subtracter 21 to control so as to keep the controlled angles agreeing with the setting values. According to the present invention, since the unsteady state flow field under a rotating stalling condition of compressor is actively controlled by using the baffle vanes 5, the rotating stall can certainly be prevented. And since there is no need unsteadily to change the angles of the inlet guide vanes 6 for preventing the rotating stall, the performance of the compressor is hardly affected. Although the hot wire anemometers 12 are used as flow sensors in the embodiment described above, pressure sensors or temperature sensors may be used instead of the hot wire anemometers. In this case, since the pressure and the temperature in the stalling region rise, the control may be performed such that the angles of baffle vanes 5 in the peripheral position corresponding to the high pressure or high temperature region are lessened. Further, although the flow sensors 12 and the baffle vanes 5 are provided four in number respectively in the embodiment described above, the more accurate control is capable the more number thereof provided. However, at least three sensors are sufficient. FIG.9 shows a compressor having another embodiment of a system according to the present invention. In this figure, the numerals refers to same parts in FIG.1. In this embodiment, hot wire anemometers 12 are installed in the upstream side of the baffle vanes 5. Such structure is also capable of obtaining the same effect as the embodiment described above. FIG.10 shows a compressor having a further embodiment of a system according to the present invention. In this figure, the numerals refers to same parts in FIG.1. In this embodiment, a compressor comprises nozzles 23 to supply jet flow on a casing 2 in an upstream of inlet guide vanes 6, a compressed fluid supply 25 being connected to the nozzles 23 through valves 24, pressure signals from pressure sensors 26 provided in an upstream side of the inlet guide vanes 6 are input into control means 27, the control means 27 regulates said valves 24. This control means 27 may be formed by changing the flow angle in the control means 13 in FIG.1 to pressure. According to this embodiment, when a rotating stall takes place, the pressure in the region corresponding to the stalling region between the upstream of the blade rows and the inlet of the blade rows is high and the pressure in the non-stalling region is low. By increasing the jet flow from the nozzle 23 on the position corresponding to the low pressure region, the unevenness of pressure distribution in the peripheral direction can be eliminated to decrease the stalling region inside the blade rows. By performing this manner with following the peripheral travelling of the stalling region, the rotating stall in the blade rows can be prevented. According to the present invention, since the unsteady state flow field under a rotating stalling condition of compressor is actively controlled, a high prevention effect against the rotating stall can be attained. And by providing nozzles 23 for jet flow in an upstream of the inlet guide vanes 6, the performance of the compressor is hardly affected. Furthermore, there is an advantage that the structure is simpler than that for the embodiment using the baffle vanes 5. In the embodiment described above, an air compressor may be used as the pressurized fluid supply for the jet flow, or instead of using an air compressor the fluid from the compressor itself may be utilized. And temperature sensors may be used instead of the pressure sensors 26. Furthermore, a plurality of nozzles 23 to supply jet flow may be provided in a downstream of the inlet guide vanes 6. According to the present invention, since the rotating stall can be prevented without deteriorating the performance of compressor, the efficiency of the compressor increases and the reliability of components connected downstream thereof can be improved.

Claims: Axial compressor comprising rotor blades (9) mounted on a rotational driven rotor (3), stator blades (7) mounted in a casing (2), inlet guide vanes (6) mounted in the casing (2) upstream of the compressing blade rows (7; 9), flow sensors (12) for detecting flow conditions in the flow passage (4) defined between the rotor (3) and the casing (2) and an angle varying mechanism (10) for adjusting the angles of said inlet guide vanes (6) depending on the operating conditions, characterized by baffle vanes (5) provided in the flow passage (4) upstream of said guide vanes (6), control means (13) receiving the signals of the flow sensors (12) and actuators (11) for adjusting the angles of said baffle vanes (5) on the basis of the output signals of the control means (13) for preventing the occurrence of rotating stall phenomena. Compressor according to claim 1, characterized in that said flow sensors (12) are located between said inlet guide vanes (6) and said baffle vanes (5). Compressor according to claim 1, characterized in that said sensors (12) are provided upstream of the baffle vanes (5). Compressor according to any of claim 1 to 3, characterized in that said flow sensors (12) produce signals corresponding to the flow velocity or pressure of fluid in the flow passage (4). Compressor according to claims 1 to 3, characterized in that said flow sensors (12) produce signals corresponding to an axial flow velocity and a circumferential flow velocity of fluid in the flow passage (4). Compressor according to claim 5, characterized in that said flow sensor (12) is a hot wire anemometer having two hot wires (12a, 12b) perpendicular to each other, one hot wire (12a) for detecting the magnitude of the axial flow velocity and the other hot wire (12b) for detecting the magnitude of the circumferential flow velocity. Compressor according to anyone of claims 1 to 6, characterized in that four buffle vanes (5) are provided with circumferentially equal intervals and individually driven by the actuators (11). Axial compressor comprising rotor blades (9) mounted on a rotational driven rotor (3), stator blades (7) mounted in a casing (2), inlet guide vanes (6) mounted in the casing (2) upstream of the compressing blade rows (7; 9), flow sensors (26) for detecting flow conditions in the flow passage (4) defined between the rotor (3) and the casing (2) and a control system including control means (27) receiving the output signals of said sensors (26) and having means for producing physical effects counteracting the occurrence of an unsteady motion phenomenon depending on the operation condition, characterized in that said control system includes nozzles (23) for supplying a jet flow in the casing upstream of the inlet guide vanes (6), said nozzles (23) are connected through valves (24) to a compressed fluid supply (25) and said sensors are pressure sensors (26) provided upstream of the inlet guide vanes (6) and said control means (26) are provided for regulating the valves (24).

Inventors: ISHII HIROSHI, KASAHARA MASAYUKI, KASHIWABARA YASUSHIGE, KATOH YASUHIRO, SASADA TETSUO, ISHII, HIROSHI, KASAHARA, MASAYUKI, KASHIWABARA, YASUSHIGE, KATOH, YASUHIRO, SASADA, TETSUO, ISHII, HIROSHI, C-203, GREEN AVENUE, KATOH, YASUHIRO, 243, NIKKEN MORIYAMA
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1993
Main classification: F04D27
IPC codes: F04D27