Patent Publication Number: US-7905703-B2

Title: Centrifugal compressor return passages using splitter vanes

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to centrifugal compressors and, more particularly, to structure in a multistage centrifugal compressor that serves to minimize or eliminate regions of reverse or separated flow, resulting in improved compressor performance. 
     The return passage in a multistage centrifugal compressor normally consists of a number of similar vanes or airfoils of similar geometry. For certain flow conditions, these vanes are required to turn the flow significantly, possibly resulting in flow separation in the return vanes and degradation of compressor performance. 
     In low flow coefficient multistage compressor designs, the flow angle going into the return vanes can be very low due to high tangential components of flow. Regardless, the vanes must eliminate the tangential component of velocity before the flow goes into the following stage. Diffuser ratios also tend to be low. 
     The elimination of the tangential or whirl component must therefore take place over a relatively short distance. Because of the high tangential component, this results in turning the flow from a mostly tangential flow to a radial flow in a very short distance. These requirements can lead to flow separation and performance degradation. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In an exemplary embodiment, a return section in a multistage centrifugal compressor includes a plurality of circumferentially positioned flow vanes, and a plurality of splitter vanes disposed between the flow vanes. 
     In another exemplary embodiment, a return section in a multistage centrifugal compressor includes a plurality of circumferentially positioned flow vanes, and flow modifying structure interposed between each of the flow vanes, the flow modifying structure serving to minimize regions of reverse or separated flow. 
     In yet another exemplary embodiment, a return section in a multistage centrifugal compressor includes a plurality of circumferentially positioned airfoil shaped flow vanes, and a plurality of airfoil shaped splitter vanes interposed between the flow vanes, the splitter vanes being of a different geometry than the flow vanes and serving to minimize regions of reverse or separated flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a return section in a multistage centrifugal compressor including conventional return vanes; 
         FIG. 2  shows a return section including splitter vanes; 
         FIGS. 3 and 4  are meridional velocity plots showing a comparison between the conventional design and the design including splitter vanes; and 
         FIGS. 5 and 6  are meridional velocity fringe plots at an inlet of the stage following the return vanes with the conventional design and the design including the splitter vanes, respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a return section in a multistage centrifugal compressor including conventional return vanes RV. In a typical design, the return section includes several, e.g., fifteen or so, circumferentially positioned flow vanes RV. With reference to the background described above, it would be desirable to improve compressor performance by eliminating or minimizing separated flow regions to thereby improve performance of a multistage compressor. 
     With reference to  FIG. 2 , in an exemplary embodiment, flow modifying structure is interposed between each of the flow vanes, which structure serves to better control the flow turning and minimize regions of reverse or separated flow. The return section includes a plurality of circumferentially positioned flow vanes  12 , and the flow modifying structure includes a plurality of splitter vanes  14  disposed between the flow vanes  12 . The splitter vanes  14  are preferably but need not be formed of a different geometry than the flow vanes  12  and are disposed in alternating positions between the flow vanes  12 . The number of vanes  12 ,  14  and corresponding geometry are selected and analyzed to ensure that regions of flow separation in the return section are minimized or eliminated. In one exemplary preferred embodiment, the return section includes thirteen flow vanes  12  and thirteen splitter vanes  14 . 
     With reference to  FIGS. 3-6 , computational fluid dynamics calculations have been performed to verify that this structure achieves an improvement over conventional approaches.  FIG. 3  shows a plot of the meridional velocity on the vane surface without a splitter present. A separation bubble is clearly visible on the upper surface of the vane. With reference to  FIG. 4 , the insertion of the splitter eliminates this bubble almost completely and thus eliminates a potential source of instability and poor performance. 
       FIG. 5  shows a fringe plot of the meridional velocity approaching the next stage impeller without the splitters present.  FIG. 6  has the splitters.  FIG. 6  shows that the flow approaching the next stage is much more uniform, a desirable feature for good performance. 
     From a comparison of the plots, it is shown that the splitter vanes  14  have produced minimal re-circulation compared with the standard design. Moreover, the velocity distribution at a next stage impeller inlet is more uniform. The flow angle calculations (mass averaged) at the outlet of the return vanes show that the return vanes with splitters provide about 5° more turning of the flow than the standard return vane. As a consequence, regions of reverse or separated flow are minimized or eliminated, resulting in improved compressor performance. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.