TURBO-MACHINE

Abstract: A turbo-machine provided with impellers (1 to rotate in the casing (3), in which means (nozzle(4)) for forming an annular fluid layer flowing along the inner surface of the casing (3) is provided to detect the generation of instability characteristics or sign thereof in the vicinity of a flow rate range where a pump head curve of the turbo-machine rises rightward to indicate instability characteristics, so as to control said fluid flow layer to be generated continuously or intermittently.

Description: Technical Field The present invention relates to a turbomachine and, more particularly, to a turbomachine which is arranged to prevent occurrence of positively-sloped head-capacity characteristics, which would otherwise be observed in the head-capacity curve during the operation in a partial capacity range, or to shift the onset of the positively-sloped characteristics toward the smaller capacity side, thereby improving the instability of the turbomachine. Further, the present invention relates to a method of stabilizing the operation of such a turbomachine. Background Art Figs. 3(a) and 3(c) are sectional views each showing the impeller part of a conventional turbomachine. Fig. 3(a) shows the impeller part of a turbomachine having an open impeller without a front shroud, while Fig. 3(c) shows the impeller part of a turbomachine having a closed impeller with a front shroud. Figs. 3(b) and 3(d) are sectional views taken along the lines C-C and D-D in Figs. 3(a) and 3(c), respectively. As is illustrated in the figures, as an impeller 1 rotates inside a casing 3 about an axis 2 of rotation, a fluid is sucked into the casing 3 from a suction port (not shown) and discharged into a discharge port (not shown). In the conventional turbomachinery of the type described above, a large-scale separation of flow occurs owing to an unstable high-loss fluid, that is, a low-momentum fluid, on the blade surface, the casing and/or the shroud. As a result, a head-capacity curve having a positive slope appears in a partial capacity range, as shown by the broken line 9 in Fig. 6. Such positively-sloped characteristics of the head-capacity curve are also known as stall phenomenon, which may induce surge, that is, self-induced vibration of a turbomachine piping system, and which may also cause vibration, noise and damage to the apparatus. Thus, the stall phenomenon is a serious problem to be solved for a stable operation of turbomachinery. Means for solving such a problem may be roughly divided into passive means that are supplied with no energy input from the outside of the turbomachine, and active means that are supplied with some energy input from the outside of the turbomachine. Known passive means include a means in which grooves, which is called casing treatment, are provided in the inner wall of the casing, and a means in which an annular passage with straightening vanes is provided inside a part of the casing at an impeller inlet part (see the teaching material for the 181st course sponsored by the Kansai Branch of the Japan Society of Mechanical Engineers, pp. 45-56). These means suffer, however, from the problem that if it is intended to enhance the effectiveness of improvement during the operation in the partial capacity range, the efficiency during the normal operation lowers accordingly. Further, a means in which a fluid is bypassed from the discharge side toward the inlet side during the operation in the partial capacity range is widely employed. However, this means increases the actual capacity of the fluid flowing through the turbomachine, and it inevitably causes a marked reduction in the pump head of the turbomachine. In addition, since a large amount of fluid flows back through the bypass, a great deal of power is consumed disadvantageously. On the other hand, the conventional active means may be roughly divided into the following four types: (1) Means for externally supplying energy to the low-momentum fluid on the blade surface, the casing and/or the shroud; (2) Means for removing such a low-momentum fluid; (3) Means for giving a prerotation to the impeller inlet flow, rotating in the direction of the impeller rotation, to thereby prevent blade stall; and (4) Means for actively generating disturbances to dump a wave mode of unstable fluid oscillation that appears in the flow field before stall occurs. As one example of the means (1), Japanese Patent Application Public Disclosure No. 55-35173 (1980) discloses a means as a method of expanding a surge margin in a compressor, in which part of the high-pressure side fluid is introduced to the tip part of the impeller and/or the area in between each pair of adjacent blades, thereby injecting it in the form of a high-speed jet. According to this literature, the direction of the jet may be any of the radial direction, the direction of rotation of the impeller and the direction counter to the impeller rotation, and the jet injection is equally effective in any of the three direction. Since the function of the jet in this prior art is to supply energy to the unstable low-momentum fluid on the blade surface and to thereby prevent boundary-layer separation, the direction of injection need not particularly be specified. As another known example, Japanese Patent Application Public disclosure No. 45-14921 (1970) discloses a means in which high-pressure air is taken out from the discharge side of a centrifugal compressor and it is jetted out from a nozzle provided in a part of the casing that covers the rear half of the impeller to thereby stabilize the operation during the partial capacity range. The function of the jet in this means involves a turbine effect whereby pressure is supplied to the low-pressure region at the blade rear part (blade suction surface side), and a jet flap effect whereby the effective passage width at the impeller exit is reduced. Accordingly, the jet needs to have a circumferential velocity component in the direction of the impeller rotation and also a velocity component in the direction perpendicular to the casing wall surface. As one example of the means (2), Japanese Patent Application Public Disclosure No. 39-13700 (1964) discloses a means in which a fluid is returned from the high-pressure stage side to the low-pressure stage side in an axial flow compressor to suck a low-momentum fluid which is present inside the boundary layer along the casing wall at the high-pressure stage side, thereby stabilizing the flow. In this prior art, the return fluid in the low-pressure stage acts in the form of a jet so as to supply momentum to the fluid in the vicinity of the wall surface, thereby also providing the same function as that of the above-described means (1). As one example of the means (3), Japanese Patent Application Public Disclosure No. 56-167813 (1981) discloses an apparatus for preventing surge in a turbo-charger, in which air is injected from an opening facing tangentially to the direction of rotation in the impeller inlet part. It is stated in this literature that the function of the injected air is to give prerotation to the flow so as to reduce the attack angle of the flow to the blades, thereby preventing separation on the blade surface. Accordingly, the direction of injection of air is defined as being the same as the direction of rotation of the impeller and tangential to it. This means necessitates giving prerotation over a relatively wide range of the blade height in order to prevent stall over a wider partial capacity range and inevitably results in a reduction of the pressure head. As one example of the means (4), UK Patent Application GB 2191606A discloses a means in which an unstable, fluctuating wave mode in the flow field is measured and, while doing so, the amplitude, phase, frequency, etc. of the wave mode are analyzed, and a vibrating blade, vibrating wall, an intermittent jet, etc. are used as an actuator to actively give the fluid such a wave disturbance as cancels the above-described unstable wave mode, thereby preventing rotating stall, surge, pressure pulsation, etc. This means is based on the assumption that there is an unstable wave motion as a precursor of stall, surge, etc., and hence cannot be applied to turbomachinery in which such a wave motion is not present. The inventors of this application conducted detailed studies of turbomachinery of the type described above and, as a result, has clarified the fact that the occurrence of the positively-sloped characteristics (i.e., the occurrence of stall) depends not simply on the magnitude of the flow loss but also on the pattern of distribution of such a high-loss fluid, that is, a low-momentum fluid, inside the impeller. A high-loss fluid that is generated inside the impeller accumulates in a corner region between the blade suction surface and the casing (or the shroud) by the action of the secondary flow inside the impeller. In mixed flow turbomachinery wherein a relatively strong passage vortex 31 is generated, the above-described high-loss fluid accumulates in a corner portion 33 closer to the blade suction surface. [see Figs. 3(a), 3(b), 3(c) and 3(d)]. In either type of turbomachinery, a large-scale separation occurs in such a corner region, causing positively-sloped characteristics to be induced. In view of the above-described circumstances, it is an object of the present invention to provide a turbomachine which is basically different from the above-described prior arts, wherein only the pattern of distribution of the high-loss fluid inside the passage is changed by controlling the secondary flow inside the impeller, thereby suppressing accumulation of the high-loss fluid in the above-described corner regions, and thus making it possible to prevent occurrence of positively-sloped head-capacity characteristics, which would otherwise be observed in the head-capacity curve of the turbomachine, and hence possible to prevent occurrence of surge. Disclosure of the Invention The present invention provides a turbomachine having an impeller 1 with or without a shroud, which rotates inside a casing 3, as shown in Fig. 1, which is characterized by providing means (nozzles 4) for forming an annular flow layer flowing substantially at right angles to the impeller inlet flow and circumferentially along the inner wall of the casing 3, detecting occurrence of unstable characteristics or a precursor thereof in a capacity range in which the head-capacity curve of the turbomachine shows positively-sloped, unstable characteristics, and forming the above-described annular flow layer continuously or intermittently in the flow field to thereby control the secondary flow inside the impeller. The present invention is also characterized in that the direction of rotation of the annular fluidized layer is made counter to the direction α of rotation of the impeller in accordance with the flow condition (secondary flow pattern) inside the impeller. The present invention is also characterized in that a specific means for forming the above-described annular flow layer 36 in the flow field is a means for injecting jets along the inner wall of the casing 3 from nozzles 4 which are provided inwardly of the inner wall of a part of the casing at the impeller inlet part, thereby generating a vortex sheet at the boundary between the inlet flow and the annular flow layer 36. Thus, according to the present invention, a means for forming an annular flow layer flowing along the inner wall of the casing in the vicinity of a capacity range in which the head-capacity curve of the turbomachine shows positively-sloped, unstable characteristics is provided to change the above-described secondary flow pattern so as to suppress accumulation of a high-loss fluid in the above-described corner region and to prevent occurrence of a large-scale separation inside the impeller, thereby avoiding occurrence of positively-sloped characteristics in the head-capacity curve or improving the head characteristics and hence preventing occurrence of surge, and thus enabling a stable turbomachine operation over the entire capacity range. This will be explained below more specifically. In the present invention, as a specific means for forming an annular flow layer, jets are injected in the impeller inlet part, thereby generating a vortex sheet at the boundary between the inlet flow and the annular flow layer. The improving effectiveness of the above-described active means (1), which employs the supply of energy to the unstable flow, depends on the total energy (the kinetic energy of the jet multiplied by the flow rate of the jet) that is supplied to the flow field by the jet, and it is considered to be proportional to the cube power of the jet velocity. In contrast, the present invention aims at improving the head characteristics by introducing a vortex sheet, and it has been experimentally confirmed that the effectiveness thereof is proportional to the intensity of the vortex layer, that is, to the first power of the jet velocity. Thus, the function of the present invention is clearly different from that of the active means (1). Further, the present invention differs from the active means (1) in that the direction of jet injection is specified, for example, jets are injected substantially at right angles to the inlet flow and circumferentially along the casing inner wall, in order to form the vortex sheet most effectively. The prior arts include a disclosure that is accompanied with a drawing showing an arrangement in which nozzles 41 extending through the casing 3 are used to inject jets at a certain angle (ε) to the inner wall surface of the casing 3, as shown schematically in Fig. 20. In this case, the jets are injected away from the casing inner wall surface. In the present invention, as will be explained later, a flow layer that flows counter to the direction of rotation of the impeller 1 is formed along the inner wall of the casing 3 in accordance with the secondary flow pattern inside the impeller 1 [Fig. 1(b)], and a vortex sheet having a specific direction of rotation is generated at the velocity discontinuity along the flow layer, as shown in Fig. 16. In contrast to this, in the prior art shown in Fig. 20, vortex sheets 42 and 43 which have different direction of rotation are simultaneously generated at both sides of the jet. Therefore, one vortex sheet 43 inevitably acts so as to deteriorate the flow field, thus making it impossible to expect an advantageous effect such as that obtained in the present invention. In addition, a jet that does not flow along the inner wall surface of the casing 3 as in the case of Fig. 20 disturbs the inlet flow 6 and further increases the incidence angle of the flow to the blades, which may induce a separation of the flow. Thus, the means according to above-described prior art may deteriorate the performance by contraries. In the active means (2), the low-momentum fluid itself is removed, whereas, in the present invention, only the distribution of low-momentum fluid in the flow passage is controlled. In the active means (3), the inlet flow is prerotated in the direction of rotation of the impeller. According to the present invention, however, it is impossible to improve the positively-sloped characteristics of mixed flow turbomachinery, in which a strong passage vortex is generated, unless an annular flow layer rotating counter to the direction of rotation of the impeller is formed and a vortex sheet counter to the direction of rotation of the impeller is generated. In the present invention, an annular flow layer flowing in the direction of rotation of the impeller was formed and a vortex sheet having a rotation component in the direction of rotation of impeller was introduced tentatively. As a result, the positively-sloped characteristics and the stall characteristics deteriorated to a considerable extent. Accordingly, the gist of the present invention resides in that an annular flow layer flowing in a direction counter to the direction of the impeller rotation is formed in accordance with the flow condition inside the impeller, and in this point the present invention differs markedly from the conventional active means in which the direction of prerotation is specified as being the same as the direction of the impeller rotation. In addition, it is possible according to the present invention to obtain adequate effect simply by forming a very thin annular flow layer along the casing inner wall. Therefore, there will be no reduction in the pump head due to prerotation as in the conventional means. Whereas the active means (4) is based on the assumption that there is a wave mode of an unstable flow, as stated above, the present invention does not need the presence of such a wave mode. Many of general turbomachines have no fluctuating wave mode as a precursor of occurrence of positively-sloped characteristics or stall, and the present invention can be effectively applied to these turbomachines. This is an advantageous feature of the present invention. Thus, the present invention is a fifth active means that is clearly different from the technical idea of any of the active means (1) to (4) described in connection with the prior art. The present invention also has the advantageous feature that the characteristics in the partial capacity range can be improved without impairing the turbomachine efficiency during the normal operation in the same way as in the case of the other active means, and the present invention is superior to the conventional passive means. In this type of conventional mixed flow turbomachinery, phenomena such as those shown in Figs. 3(b) and 3(d) occur inside the impeller 1. That is, in the open impeller without a shroud, shown in Fig. 3(b), the tip leakage vortex 30 that flows through the clearance between the blade tip of the impeller 1 and the casing 3 interferes with the passage vortex 31 flowing from the blade pressure surface toward the suction surface, so that the high-loss fluid inside the impeller 1 accumulates in a region 32 of interaction of these vortices. As the capacity decreases, the clearance flow 7, which flows backward toward the upstream direction through the clearance between the blade tip of the impeller 1 and the casing 3, becomes stronger, resulting in an increase in the inlet boundary layer thickness (high-loss region) on the casing 3 due to the interaction of the clearance flow 7 with the inlet flow 6. Consequently, the passage vortex 31 develops. Figs. 4 and 5 show results of numerical simulation of the above-described situation by numerical computations of a three-dimensional viscous flow. It is observed in Fig. 5 that the clearance flow 7 between the blade tip of the impeller 1 and the casing 3 induces a reverse flow 7' in the vicinity of the casing 3 (see Fig. 4), and hence the boundary layer (high-loss region) on the casing 3 rapidly develops in this region (see the part B in Fig. 5). It should be noted that LE in Fig. 4 represents the blade leading edge. As the capacity decreases and hence the pressure difference between the blade pressure and suction sides increases, the clearance flow 7 becomes stronger, and consequently the passage vortex 31 develops, causing the high-loss fluid 32 to move to the corner region 33 between the blade suction surface and the casing 3, resulting in a flow pattern in which a large-scale corner separation is likely to occur. In the closed impeller with a shroud, shown in Fig. 3(d), there is no tip leakage vortex 30 to act counter to the passage vortex 31. Therefore, the high-loss fluid on the shroud 35 is present in the corner region 33 between the blade suction surface and the shroud 35 from the beginning, thus forming a flow pattern in which a large-scale corner separation is likely to occur in a larger capacity region than in the case of the open impeller. In the conventional axial flow turbomachinery, on the other hand, a phenomenon such as that shown in Fig. 19 occurs. That is, in the axial flow turbomachinery, the fluid mainly flows substantially parallel to the axis of rotation. Therefore, the action of Coriolis force is relatively weak, so that the intensity of the passage vortex 31 is considerably lower than in the case of the mixed flow turbomachinery. In the meantime, the intensity of the blade tip leakage vortex 30 increases as the capacity decreases. As a result, the high-loss fluid 32 moves to a corner region 39 defined between the blade pressure surface and the casing 3, thus forming a flow pattern in which a large-scale corner separation is likely to occur. As has been described above, the occurrence of positively-sloped characteristics is closely related not only to the magnitude of the flow loss but also to the flow pattern that shows where the high-loss fluid accumulates in the passage. If a large-scale corner separation such as that shown by A in Fig. 3(a), 3(c) or 19(a) occurs in the corner region 33 or 39 in the turbomachine impeller 1, the head-capacity curve shows positively-sloped characteristics as shown by the broken line 9 in Fig. 6, which is considerably inconvenient for the achievement of a stable operation of the turbomachinery. Under these circumstances, the present invention provides the following arrangements: In the case of a mixed flow turbomachine, it is provided with means for forming an annular flow layer flowing counter to the direction of rotation of the impeller 1 along the inner wall of the casing 3 so as to generate a vortex sheet in a direction counter to the direction of rotation of the impeller 1 at the boundary between the inlet flow 6 and the annular flow layer, thereby suppressing the development of the passage vortex 31 in the direction of rotation of the impeller 1 and accumulating the high-loss fluid at a position away from the corner region 33, and thus preventing occurrence of a large-scale corner separation. In the case of a mixed flow open impeller without a shroud, the vortex sheet that is introduced by the present invention promotes the development of the tip leakage vortex 30 which rotates in a direction counter to the impeller rotation. Therefore, the high-loss fluid that accumulates in the interaction region 32 between the passage vortex and the tip leakage vortex 30 moves to a position which is even more away from the corner region 33. Thus, occurrence of a corner separation can be prevented even more effectively. In the present invention, as a specific means for introducing a vortex sheet, an annular flow layer is formed by using jets in the inlet part of the impeller 1. Fig. 16 is an enlarged view of an annular flow layer formed along the casing near the impeller inlet part as viewed from the suction port side, showing a mechanism for introducing a vortex sheet into the flow field. The figure shows one example in which the inlet flow is perpendicular to the plane of the drawing, and a jet 5 that is injected counter to the direction of rotation of the impeller 1 forms an annular flow layer 36 which is perpendicular to the inlet flow. In this case, at the boundary surface 38 of the annular flow layer 36 the velocity varies discontinuously, thus forming a vortex sheet. To evaluate the intensity of vortices present along the boundary 38, circulation dΓ is integrated along a closed curve C that surrounds a boundary part of length dx to obtain an intensity γ of vortices per unit length as follows: In the above expression, the velocity V je is the flow velocity inside the annular flow layer 36, which has become lower than the velocity V j of the jet 5 immediately after the injection because of the decay of the jet. In a case where an inlet guide vane or a suction casing is present upstream of the impeller, the impeller inlet flow enters the impeller with a circumferential velocity component. In this case, the intensity of vortices generated at the boundary between the inlet flow 6 and the annular flow layer 36 is proportional to the velocity component of the jet 5 perpendicular to the inlet flow 6. Accordingly, it is necessary in order to maximize the intensity of vortices generated to form the annular flow layer 36 so as to be substantially perpendicular to the inlet flow 6. When the inlet flow 6 has a circumferential velocity component, the flow layer, which is formed along the casing inner wall surface according to the present invention, forms not a ring shape but a spiral shape. However, there is no difference in the effectiveness of a thin flow layer formed along the casing inner wall surface to generate a vortex sheet. The effectiveness of the present invention is proportional to the intensity of the vortex sheet generated, that is, the first power of the jet velocity, as stated above. This point has been confirmed by the experimental results obtained in an example described later. The main results will be described below. The effectiveness of the vortex sheet increases in proportion to the width of the jet. When the flow layer is not perpendicular to the inlet flow 6, the effectiveness decreases correspondingly to the extent to which the flow layer goes off from the direction which is perpendicular to the inlet flow 6. With these points taken into consideration, Γ is defined as a parameter for evaluation of the effectiveness of the vortex sheet by the following expression: Γ = (B·γ·sinβ)/(L·U 1t ) In the above expression, B is the jet width, and β is the injection angle of the jet measured from the axial direction. The blade length L at the blade tip is employed as a reference length to make Γ a dimensionless quantity, and the peripheral velocity U 1t of the blade inlet tip is employed as a reference velocity. Experiments were carried out by using various jet angles, jet widths, numbers of nozzles, jet velocities, etc., to determine the relationship between the measured critical capacity at which positively-sloped head-capacity characteristics occurred and the jet evaluation parameter Γ at the critical capacity. The results are shown in Fig. 21. It will be understood from the figure that the effectiveness of improvement by the jet injection can be evaluated by the parameter Γ, and it is proportional to the first power of the jet velocity. As is shown by this fact, the present invention improves the positively-sloped head-capacity characteristics by introducing the vortex sheet, and it is basically different from the prior art that is based on the supply of energy (the effectiveness in this case is proportional to the cube power the jet velocity). As has been described above, vortices spread all over the boundary 38 of velocity discontinuity forming a vortex layer 37, and the effectiveness of the present invention is proportional to the intensity of the vortex sheet generated, that is, the velocity V je in the annular flow layer. Fig. 17 expresses three-dimensional view of the interaction between vortices 34 introduced into the flow field and the flow inside the impeller 1 in a mixed flow open impeller. The vortices 34, which are introduced by the vortex sheet 37, are carried into the impeller 1 by the main stream. The vortices 34 interact with the blade tip leakage vortex 30 rotating in the same direction as the vortices 34 to thereby promote it. On the other hand, the vortices 34 interact with the passage vortex 31 rotating counter to the direction of rotation of the vortices 34 to thereby suppress it. Consequently, the high-loss fluid accumulating in the vortex interaction region 32 is moved to a position away from the corner region 33. Brief Description of the Drawings Fig. 1 is a sectional view showing the inlet part of the turbomachine according to the present invention, in which Fig. 1(a) is a sectional view taken along a meridional plane, and Fig. 1(b) is a sectional view taken along the line E-E in Fig. 1(a); Fig. 2 is a developed view of a stream surface in the vicinity of the casing in Fig. 1; Fig. 3 is a view showing a flow in the vicinity of the inlet in conventional turbomachinery, in which Fig. 3(a) is a sectional view, Fig. 3(b) is a sectional view taken along the line C-C in Fig. 3(a), Fig. 3(c) is a sectional view, and Fig. 3(d) is a sectional view taken along the line D-D in Fig. 3(c); Fig. 4 shows a result of numerical simulation by a three-dimensional viscous flow computation in the case of the turbomachinery shown in Fig. 3; Fig. 5 shows a result of numerical simulation by a three-dimensional viscous flow computation in the case of the turbomachinery shown in Fig. 3; Fig. 6 shows the head-capacity curve (pump head-capacity) of turbomachinery; Fig. 7 shows results of an experiment in which jets were injected for a predetermined time under conditions in which surge had already occurred in the pump piping system; Fig. 8 is a view showing the configuration of a nozzle employed in the turbomachine according to the present invention, in which Fig. 8(a) is a vertical sectional view, Fig. 8(b) is a front view, and Fig. 8(c) is a horizontal sectional view of the nozzle head; Fig. 9 shows one example of jet injection control in the turbomachine according to the present invention; Fig. 10 shows another example of jet injection control in the turbomachine according to the present invention; Fig. 11 shows one example of the arrangement of the turbomachine according to the present invention; Fig. 12 shows another example of the arrangement of the turbomachine according to the present invention; Fig. 13 shows the relationship between the number of nozzles provided in the inlet part of the impeller of the turbomachine according to the present invention and the effectiveness thereof; Fig. 14 shows the relationship between the direction of jet injection and the effectiveness thereof; Fig. 15 shows one example in which the head-capacity curve falls markedly; Fig. 16 is a view for explanation of a mechanism for introducing a vortex sheet into the flow field of a turbomachine; Fig. 17 is a view three-dimensionally expressing the interaction between vortices introduced into the flow field of a turbomachine and the impeller internal flow in an open impeller; Fig. 18 shows a vorticity (vortex intensity) distribution in the impeller passage simulated by a viscous flow computation at a position equivalent to that shown in Fig. 3(b) (C-C section); Fig. 19 is a view showing a phenomenon occurring in a conventional turbomachine, in which Fig. 19(a) is a sectional view taken along a meridional plane, and Fig. 19(b) is a sectional view taken along the line E-E in Fig. 19(a); Fig. 20 shows one example of injection of jets in a conventional turbomachine; and Fig. 21 shows the relationship between the critical capacity and the evaluation parameter Γ. Best Mode for Carrying Out the Invention One embodiment in which the present invention is applied to a mixed flow pump apparatus will be described below with reference to the accompanying drawings. Fig. 1 is a sectional view showing the inlet part of the pump apparatus according to the present invention, and Fig. 2 is a developed view of a stream surface in the vicinity of the casing in Fig. 1, showing a method whereby jets of water are injected from nozzles, which is employed as a means for forming an annular flow layer flowing along the casing counter to the direction of the impeller rotation. This embodiment will be explained below in detail. In the pump apparatus according to the present embodiment, nozzles 4 are provided in the vicinity of a part of the casing 3 at a pump inlet part to inject jets 5, which are supplied from a high-pressure source, along the inner surface of the casing counter to the direction α of rotation of the impeller 1 from the vicinities of the casing 3. The jets flowing along the casing 3 form a surface of discontinuity of velocity (38 in Fig. 16). As a result, a vortex sheet having a rotation component rotating counter to the rotation direction α is generated. Vortices (34 in Fig. 17) introduced in this way have a rotation component rotating counter to the passage vortex 31 shown in Fig. 3(b) or 3(d) and hence suppress the passage vortex 31 and prevent the high-loss fluid 32 from accumulating in the corner region 33. Thus, it is possible to prevent occurrence of a large-scale corner separation (stall of the impeller) such as that shown by A in Fig. 3(a) or 3(c). Consequently, it is possible to avoid occurrence of positively-sloped characteristics, as shown by the solid line 10 in Fig. 6. Thus, the unstable region 9, shown in Fig. 6, can be stabilized by the present invention, and it is therefore possible to attain stable pump characteristics over the entire capacity range. Fig. 7 shows results of an experiment in which jets 5 were injected from the nozzles 4 (jet injection) for a predetermined time under conditions in which surging had already occurred in the pump piping system. As will be clear from the figure, even in an unstable operation condition 11 under a state surge in which the discharge pressure is largely fluctuating with time, it is possible to recover the pump out of the state of surge to a stable operating condition 12. Fig. 8 is a view showing an example of the configuration of nozzles 4, in which Fig. 8(a) is a vertical sectional view, Fig. 8(b) is a front view, and Fig. 8(c) is a horizontal sectional view of the nozzle head. The nozzle head 4a is rounded in a hemispherical shape to prevent the flow from being disturbed by the head of nozzle 4 projecting from the inner surface of the casing 3. A high-pressure fluid supplied from a high-pressure source 13 is jetted out from an nozzle outlet 4b in a direction β along the inner surface of the casing 3, with a velocity component counter to the direction α of rotation of the impeller 1. The nozzle 4 which is used in the present embodiment has a sectorial configuration, as shown in Fig. 8, so that a jet 5 is injected divergently. With such a nozzle configuration, the effectiveness can be enhanced. It should be noted that reference numeral 14 in Fig. 8(a) denotes an O-ring for preventing water leakage through the area between the nozzle 4 and the casing 3. A jet blowing off from such a nozzle diverges as it goes downstream while mixing with the surrounding fluid and diffusing. The angle of divergence is about 6 degrees at one side (Trentacoste, N. and Sforza, P.M., 1966. An experimental investigation of three-dimensional free mixing in incompressible turbulent free jets. Rep. 81, Department of Aerospace Engineering, Polytechnic Institute of Brooklyn, New York.). Accordingly, it is considered that even in a case where the direction of jet injection extends downwardly at about 6 degrees to the direction along the wall surface, the jets reattach to the casing inner wall again to form a flow layer flowing along the inner wall. Therefore, there will be no large adverse effect such as that shown in Fig. 20. On the other hand, when jets are injected toward the casing inner wall, the jets collide against the inner wall surface and then form a flow layer flowing along the wall surface. Therefore, no large adverse effect will be produced unless the jets are injected with such a large angle that the jets disperse and fail to form a flow layer. Accordingly, the jets need not be injected strictly parallel to the casing inner wall surface. The above-described effectiveness of the present invention can be obtained as long as the jets are injected substantially parallel to the inner wall surface. Figs. 9 and 10 show examples of injection control of the jets 5. As illustrated, the most easiest and simplest operating method is to inject the jets 5 continuously when surge C occurs, as shown in Fig. 9. It is also possible to execute intermittent control as shown in Fig. 10. That is, when a precursor D of stall (large-scale separation of flow) of the impeller 1 or a surge phenomenon, which will cause unstable pump characteristics, is detected (or when occurrence of such a phenomenon is detected), jets 5 are injected for only a predetermined period of time to avoid occurrence of unstable characteristics, and no jets 5 are injected until another precursor D of similar unstable characteristics is detected. With this intermittent control, it is possible to minimize the energy consumed. The precursor D of unstable characteristics may be detected by various methods that use a pressure sensor installed on the casing 3 or other pump passage surface or inside the nozzle 4, or fluid noise, abnormal noise of the machine, vibration of the machine, or a change in the velocity in the passage. Figs. 11 and 12 show examples of the arrangement of the turbomachine according to the present invention. In Fig. 11, a nozzle 4 is supplied with a fluid from an external fluid source (e.g., tap water) through a booster pump 17 and a solenoid valve 18. A signal from a pressure sensor 15 on the casing 3 is analyzed in a data processor 16. When occurrence of unstable characteristics is predicted, jets are injected intermittently or continuously by controlling the booster pump 17 and the solenoid valve 18. Fig. 12 shows an embodiment in which a fluid source is supplied from the pump discharge part, and the discharge pressure of the pump itself is employed in place of the booster pump 17. This embodiment is seemingly similar to the conventional method in which the flow is bypassed from the pump discharge part. In the conventional bypass method, however, occurrence of unstable characteristics is avoided by increasing the actual operating capacity, and the pump head inevitably lowers by a large amount. On the other hand, in the present invention, the total jet capacity required is about 1% of the pump discharge capacity, so that there will be no lowering in the pump head. Thus, the function of the present invention is basically different from that of the conventional method in which a large amount of discharge flow is bypassed. In addition, the present invention enables the pump operation to be stabilized by energy consumption much less than in the conventional method in which occurrence of an unstable condition is avoided by bypassing. Although the examples shown in Figs. 11 and 12 employ the pressure sensor 15, the stabilization of the pump operation can be realized without using such a pressure sensor 15. That is, if head characteristics (for example, see Fig. 15) measured in advance are stored in the memory of the data processor 16, jets can be injected continuously only when the pump is operated in the range 23, shown in Fig. 15, in which control is needed, by monitoring the capacity. Fig. 13 shows the relationship between the number of nozzles provided in the inlet part of the impeller 1 of a turbomachine and the effectiveness thereof. In this experiment, 12 nozzles, each having a valve, were equally spaced around the suction port (inner diameter: 250 mm), and capacities at which positively-sloped characteristics occurred were measured for various numbers of nozzles by opening and closing the valves. As the number of nozzles increases, the critical capacity at which positively-sloped characteristics occur shifts toward the lower capacity side, that is, the effectiveness of the jets is enhanced. In the case of this experiment, there is no change in the effectiveness of the present invention any longer when the number of nozzles exceeds 6. Fig. 14 shows the relationship between the direction of jet injection and the effectiveness thereof. It will be understood from the figure that the jet injection is effective only when the jets are injected with an angle in the range of 0 to 180 degrees measured from the axial direction, that is, only when the jets are injected with a velocity component counter to the direction of rotation of the impeller; particularly, when the jet injection angle is 90 degrees, that is, when the jets are injected counter to the direction of the impeller rotation, the largest effectiveness is obtained. The direction of jets in which a vortex layer having a rotation component rotating counter to the direction of the impeller rotation can be introduced into the flow field most effectively is a direction perpendicular to the inlet flow, as has been stated in the description of "function" in connection with Fig. 16. In this embodiment, the inlet flow enters in the axial direction. Therefore, in the experiment shown in Fig. 14, the largest effectiveness was obtained at a jet angle of 90 degrees. Fig. 18 shows a vortex intensity distribution in the impeller passage simulated by analysis of a viscous flow at a position equivalent to that shown in Fig. 3(b) (C-C section). In the figure, the vorticity (intensity of vortex) having a rotation component rotating in the same direction as the direction of the impeller rotation are shown by contours of solid lines, while the vorticity having a rotation component rotating counter to the direction of the impeller rotation are shown by contours of dot-dash-lines. Fig. 18(a) shows the vorticity distribution in a conventional impeller, while Fig. 18(b) shows the vorticity distribution in an arrangement in which an annular flow layer is formed in the impeller inlet by injecting jets in the vicinity of the casing 3. Regions of the passage vortex 31 that have the same vorticity are hatched. It will be confirmed that the intensity of the passage vortex 31 is suppressed considerably by introducing a vortex sheet having a rotation component rotating counter to the direction of the impeller rotation by the mechanism shown in Fig. 16. As has been described above, it is possible according to the embodiment to suppress development of the passage vortex 31 and avoid a large-scale separation of flow in the corner region 33. As a result, the positively-sloped characteristics 9, which have heretofore occurred during the pump operation in a partial capacity range, are completely eliminated, as shown in Fig. 6, and the pump can be operated stably without being captured by a state of surge over the entire capacity range. When the head-capacity curve falls markedly as shown by 20 in Fig. 15, the positively-sloped region cannot be completely eliminated, but the critical capacity 21 at which unstable characteristics occur is shifted toward the lower capacity side by injection of jets. In this case, there is a possibility of the pump showing unstable characteristics again. However, if the injection of jets is stopped at this point of time, the pump characteristics move to the point 22 on the original, stable head-capacity curve. Therefore, the pump will not run into a state of surge. Accordingly, the region in which stabilization by jets is required is limited to the capacity range shown by 23 in Fig. 15, in which the head-capacity curve shows positively-sloped characteristics. In addition, the pump whose operation in the region shown by 23 in Fig. 15 has been stabilized by the present invention has stable characteristics over the entire capacity range. Thus, it is possible to form a surge-free pump piping system. Although in the foregoing embodiment the present invention has been described by way of one example in which it is applied to a mixed flow pump, it should be noted that the present invention is not necessarily limited to such a mixed flow pump and that it can be applied to general turbomachines including axial flow type turbomachines, as a matter of course. As has been described above, according to the present invention, an annular flow layer flowing circumferentially along the casing inner surface in the impeller inlet part is formed, whereby it is possible to control the secondary flow inside the impeller, and avoid occurrence of positively-sloped characteristics of the head-capacity curve of a turbomachine or improve the characteristics and hence possible to prevent occurrence of surge and enable a stable turbomachine operation over the entire capacity range. Industrial Applicability Thus, the present invention provides a turbomachine which is provided with means for forming an annular flow layer flowing along the casing inner wall in the vicinity of a capacity range in which the head-capacity curve of the turbomachine shows positively-sloped, unstable characteristics, thereby changing the flow pattern of the secondary flow, suppressing accumulation of a high-loss fluid in the corner region, and preventing generation of a large-scale separation inside the impeller, and thus making it possible to prevent occurrence of positively-sloped characteristics in the head-capacity curve of the turbomachine and hence prevent occurrence of surge.

Claims: A turbomachine comprising: a casing (3) and an impeller (1) disposed in said casing (3), said casing (3) defining an inlet through which a fluid is introduced, said casing (3) including a casing wall having an inner surface defining a space in which an inlet flow (6) is confined to flow from the inlet to said impeller (1), said impeller (1) having an inlet end at which the inlet flow (6) is first received by the impeller (1); and injecting means (4) for injecting, at a location adjacent the inlet end of said impeller (1) in the direction of flow of said inlet flow (6), at least one jet (5), characterized in that said injecting means (4) being adapted to inject said at least one jet (5) in a direction counter to the direction of rotation of the impeller (1) and so parallel to the casing wall that said at least one jet (5) forms an annular layer of fluid flowing along the inner surface of said casing (3) in a direction substantially perpendicular to and bounding said inlet flow (6). A turbomachine as claimed in claim 1, wherein said injecting means (4) comprises at least two nozzles (4) each projecting from the inner surface of said casing wall and having an outlet (4b) located adjacent said inner surface, the outlet (4b) of each of said nozzles (4) being so oriented that the vector of the velocity of the jet (5) injected from said outlet (4b) has a major component extending along the inner surface of said casing wall. A turbomachine as claimed in claim 1 or 2, wherein said casing (3) defines a discharge port located downstream of said location at which the jet (5) is injected and communicating with the interior of said casing (3), and a bypass passage connecting said discharge port to said injecting means (4). A turbomachine as claimed in any of claims 1 to 3, and further comprising a source of high-pressure fluid (13, 17) disposed outside of said casing and connected to said injecting means (4). A turbomachine as claimed in any of claims 1 to 4, and further comprising sensor means (15) for sensing operating conditions of the turbomachine indicative of an unstable operation of the turbomachine, and control means (16, 17, 18) operatively connected to said sensor means (15) and said injecting means (4) for processing information sensed by said sensor means (15) and for controlling, based on the processing of said information, the frequency at which the injection of said at least one jet (5) by said injecting means (4) is carried out. A method of stabilizing the operation of a turbomachine having a casing (3) defining an inlet through which fluid is introduced and including a casing wall having an inner surface defining a space through which an inlet flow (6) of the fluid is confined to flow from the inlet, and an impeller (1) disposed in the casing (3) and having an inlet end at which the inlet flow (6) of fluid is first received by the impeller (1), said method comprising: injecting, at a location adjacent the inlet end of the impeller, at least one jet (5), characterized by injecting said at least one jet (5) in a direction counter to the direction of rotation of the impeller (1) and so parallel to the casing wall that said at least one jet (5) forms an annular layer of fluid flowing along the inner surface of said casing (3) in a direction substantially perpendicular to and bounding said inlet flow (6). A method of stabilizing the operation of a turbomachine as claimed in claim 6, wherein the at least one jet (5) is injected continuously. A method of stabilizing the operation of a turbomachine as claimed in claim 6, wherein the at least one jet (5) is injected intermittently. A method of stabilizing the operation of a turbomachine as claimed in any of claims 6 to 8, and further comprising sensing operating conditions of the turbomachine indicative of an unstable operation of the turbomachine, and cqntrolling the frequency at which the at least one jet (5) is injected based on said sensing. A method of stabilizing the operation of a turbomachine as claimed in any of claims 6 to 9, and further comprising detecting a precursor (D) of conditions giving rise to the occurrence of a positive slope, indicative of unstable operation, in the head-capacity curve of the turbomachine, and controlling the frequency at which the at least one jet (5) is injected based on results of said detecting.

Inventors: AOKI MASANORI, GOTO AKIRA, KATSUMATA TATSUYOSHI, AOKI, MASANORI, GOTO, AKIRA, KATSUMATA, TATSUYOSHI, AOKI, MASANORI EBARA RESEARCH CO., LTD., GOTO, AKIRA EBARA RESEARCH CO., LTD., KATSUMATA, TATSUYOSHI EBARA RESEARCH CO., LTD.
Applicants: EBARA CORP, EBARA CORPORATION
Application year: 1992
Main classification: F04D15
IPC codes: F04D29, F04D27