1. Field of the Invention
The invention relates to a multi-zone diffuser for an axial-flow turbomachine
in which the kink angles of the diffuser inlet both at the hub and at the cylinder of the turbomachine are fixed exclusively for the purpose of evening out the total pressure profile over the duct height at the outlet from the last rotor blade row,
in which means, in the form of streamlined struts, are provided within the deceleration zone of the diffuser for the removal of swirl from the swirling flow,
and in which flow guide rings subdivide the diffuser in multi-duct fashion.
2. Discussion of Background
Such multi-zone diffusers for turbomachines are known from EP-A 265 633. In order to meet the requirements there for the best possible pressure recovery and swirl-free diffuser outlet flow at part load, a straightening grid is provided within the diffuser and this grid extends over the complete height of the flow duct. These means for the removal of swirl involve cylindrical streamlined struts with thick straight profiles arranged uniformly around the periphery. These profiles are designed according to the knowledge available for the construction of turbomachines and are intended to be as insensitive as possible to oblique incident flow. The leading edges of these struts subjected to the incident flow are located relatively far behind the trailing edge of the last rotor blades in order to avoid excitation of the last blade row caused by the pressure field of the struts. This distance is dimensioned in such a way that the leading edge of the struts is located in a plane in which a diffuser area ratio of preferably three is present. This first diffusion zone between the blading and the streamlined struts is therefore intended to remain undisturbed because of the total rotational symmetry. The fact that no interference effects are to be expected between the struts and the blading may be attributed to the fact that the struts only become effective in a plane in which there is already a relatively low velocity level.
Because the opening angle of conventional highly-loaded blading of turbines far exceeds that of a good diffuser, the known diffuser is subdivided into a plurality of partial diffusers by means of flow guide rings in order to support the flow in the radial direction. These guide rings extend from a plane directly at outlet from the blading to a plane at which a diffusion ratio of three is reached, i.e. over the whole of the first diffusion zone. For vibration reasons, these guide rings should preferably be configured in one piece. This leads to a solution without a split plane, which is disadvantageous for assembly reasons. In addition, the guide rings lead to large diameters so that transport problems can arise.
A second diffusion zone extends from the leading edge of the thick streamlined struts to the maximum profile thickness of the struts. The de-swirling of the flow is intended to take place to a major extent in this second zone and, in fact, substantially without deceleration. In a third, subsequent diffusion zone in the form of a straight diffuser, the flow--which at this time is practically swirl-free--is further decelerated.
In addition to maximum pressure recovery, particularly at part load, all these measures are also intended to achieve a reduction in the design length of the installation.
In conventional gas turbines, the flow onto the diffuser at idle has a velocity ratio c.sub.t /c.sub.n of approximately 1.2, where c.sub.t is the tangential velocity and c.sub.n is the axial velocity of the medium. This oblique incident flow leads to a reduction in the pressure recovery C.sub.p.
In other types of machines, such as steam turbines or gas turbines for fluidized bed firing, it is quite possible for the volume flow to be reduced down to 40% so that c.sub.t /c.sub.n ratios of up to 3 occur. In such types of machine, fixed diffuser geometry is not a possibility because the pressure recovery could even be negative. This applies even in the case where the pitch/chord ratio of the streamlined struts is 0.5. Streamlined struts with pitch/chord ratios of approximately 1, which would provide a somewhat larger pressure recovery at full load, i.e. c.sub.t /c.sub.n =approximately 0, cannot be used at all in such machines.
The large drop in pressure recovery may be attributed to the fact that a strong vortex forms between the outlet rotor blades and the streamlined struts in the case of the extreme relationships quoted. The vortex is bounded by the streamlined struts at which the tangential component of the velocity is dissipated. If solid particles (for example in gas turbines) or water droplets (for example in steam turbines) are entrained in the resulting reverse flow, there is an acute danger of root erosion on the blades of the last rotor row.
A known remedy in a turbomachine of the axial type, from EP 0 417 433 A1, is to arrange at least one row of variable guide vanes in the diffuser between the means for swirl removal and the outlet rotor blades. The means for removing the swirl within the diffuser are, in this case also, streamlined struts arranged evenly around the periphery with a straight camber line and symmetrical profile and with a pitch/chord ratio between 0.5 and 1 in the center section of the flow duct. These streamlined struts extend conically in the radial direction. The intention is that the part-load behavior of the machine should be further improved by these measures for designing the diffusion.