Martin N. Goodhand e-mail: mng24@cam.ac.uk Robert J. Miller Whittle Laboratory, University of Cambridge, Cambridge CB3 0DY, UK The Impact of Real Geometries on Three-Dimensional Separations in Compressors This paper considers the effect of small variations in leading edge geometry, leading edge roughness, leading edge fillet, and blade fillet geometry on the three-dimensional sepa- rations found in compressor blade rows. The detrimental effects of these separations have historically been predicted by correlations based on global flow parameters, such as blade loading, inlet boundary layer skew, etc., and thus ignoring small deviations such as those highlighted above. In this paper it is shown that this may not be the case and that certain, engine representative geometry deviations can have an effect equivalent to an increase in blade loading of 10%. Experiments were performed at the stator hub of a low-speed, single-stage compressor. The results show that any deviation which causes suction surface transition to move to the leading edge over the first 30% of span will cause a large growth in the size of the hub separation, doubling its impact on loss. The geometry deviations that caused this, and are thus of greatest concern to a designer, are changes in leading edge quality and roughness around the leading edge, which are characteristic of an eroded blade. DOI: 10.1115/1.4002990 1 Introduction Three-dimensional separations always occur on compressor blades in the corner between the suction surface and the endwall 1see Fig. 1. Blades can be designed such that these are rela- tively small and benign; however, as loading or incidence is in- creased, their size and thus detrimental effect can increase signifi- cantly. In practice, it is these separations which limit the total blade loading by their impact on loss, blockage, and deviation. Traditionally, the onset of three-dimensional separations has been correlated with global blade parameters i.e., inlet and exit flow angles and pitch to chord ratio and global flow parameters i.e., inlet boundary layer skew and shroud leakage. Examples of such correlations are given by deHaller 2, Koch and Smith 3, and Lei et al. 4. These provide simple criteria for use in the early stages of compressor design where detailed information about specific geometries is not known. These correlations are useful in giving the designer an idea of the important parameters, but can sometimes be misleading by encouraging the view that small ge- ometry deviations are insignificant. In this paper, it is shown that this is not always the case. The small geometry deviations of particular interest to engine designers are those that are likely to occur in real machines. These include deviations that occur through design choice e.g., the fillet radius, manufacture e.g., the accuracy of leading edge manufac- ture close to the endwalls, or in-service erosion e.g., changes in leading edge geometry or surface roughness. These are shown in Fig. 1 and are investigated sequentially. There is very little published research which shows the effect of small geometry deviations on three-dimensional separations. The one piece of work that shows a large effect is by Gbadebo et al. 5. They found that very high levels of roughness Re k =56 ap- plied on the early suction surface near the endwalls, dramatically increased the size of the separation and caused a corresponding 5% reduction in pressure rise coefficient. The authors attributed this change to the roughness prematurely thickening the early suc- tion surface boundary layer. It is thus likely that any other process that thickens the early suction surface boundary layer may also increase the size of three- dimensional separations. One example would be the premature transition caused by a “poor” leading edge geometry. This effect was demonstrated at midspan by Wheeler et al. 6. They tested two leading edges; the first was a 3:1 ellipse where the flow re- mained attached and laminar and the second was a circular arc where the flow separated at the leading edge and reattached tur- bulent. The premature transition with the circular leading edge increased boundary layer growth on the early suction surface; this resulted in a 30% increase in trailing edge momentum thickness, increasing profile loss by 30%. The above literature raises the question of whether leading edge transition, caused either by poor leading edge geometry or surface roughness, may cause a change in the size of three-dimensional Contributed by the International Gas Turbine Institute IGTI of ASME for pub- lication in the JOURNAL OF TURBOMACHINERY. Manuscript received June 28, 2010 final manuscript received July 12, 2010 published online June 22, 2011. Editor: David Wisler. Fig. 1 Three-dimensional separations: traditional view and scope of current investigation Journal of Turbomachinery MARCH 2012, Vol. 134 / 021007-1 Copyright © 2012 by ASME Downloaded From: http://turbomachinery.asmedigitalcollection.asme.org/ on 09/10/2017 Terms of Use: http://www.asme.org/about-asme/terms-of-use