Experimental Analysis of the Pressure Field Inside a Vaneless Diffuser From Rotating Stall Inception to Surge

Abstract

An accurate estimation of rotating stall is one of the key points for high-pressure centrifugal compressors, as it is often connected with the onset of detrimental subsynchronous vibrations which can prevent the machine from operating beyond this limit. With particular reference to vaneless diffuser rotating stall, the most common practice in industrial machines is to make use of a limited number of dynamic pressure probes to reconstruct the stall characteristics after an ensemble averaging approach. In this study, a 1:1 model of an industrial compressor stage was tested in a dedicated test rig and equipped with 24 pressure probes properly distributed along the diffuser circumference with the scope of providing a real-time visualization of the spatial pressure distribution within the diffuser. The results allowed the assessment of some important characteristics of the stall cells that were historically supposed based on averaged data, e.g., the cells rigidity. Moreover, the present study confirmed the existence of a stall pattern with two almost axisymmetric lobes. Finally, the transient analysis of both the stall inception (SI) and the surge onset (SO) was carried out, highlighting the flow field evolution in the diffuser under these conditions

Experimental and Computational Analysis of the Unstable Flow Structure in a Centrifugal Compressor with a Vaneless Diffuser

“…While since the pre-stall point, many small peaks appeared within this frequency region, and the overall energy amplitude increased gradually. Since this frequency region is related to the flow instability [12], the amplitude rise of the frequency components in this region indicates that the internal flow deteriorates. Until the surge point, in the low frequency region, the frequency component of surge waves (f surge = 6 Hz) with the energy amplitude of 6 and the frequency component of modal stall waves (f stall = 282 Hz) with the energy amplitude of 3 stand out one after another.…”

“…Thus, the stall form at this time is an approximately axisymmetric three-lobe stall pattern, as shown in Figure 7. It is also generally believed that the number of stall lobes in a centrifugal compressor with vaneless diffuser was two or three [12]. Three stall cells rotate along the same direction with the same rotating speed, so as to ensure that the signal phases of any two adjacent points at the impeller outlet were almost reverse.…”

“…Meanwhile, according to the signals of different radial measurement points at the stall inception condition, the stall originated at the impeller middle-rear region, resulting in a comprehensive stall and surge. However, some researchers, through the experiments and simulations, suggested that the vaneless diffuser always played a more important role in the overall instability process than the impeller [12,20]. Also, many researchers began to focus on the effects of tip leakage vortex and flow in the impeller on the overall instabilities of centrifugal compressors [4,28].…”

“…The flow structure at the tip clearance is closely related to the stability of the compressor [7], thus some specific control methods [8,9], such as casing treatment [10,11], can greatly enlarge the stable operating range. In particular, according to some published experimental and computational results, the distorted pressure field in the tip region during rotating stall may greatly promote the flow instability, and even lead to surge [12][13][14][15][16].…”

“…Then, the pressure lobes rotate at a certain speed in the direction of the impeller rotating, resulting in a circumferential stall mode [18,19]. In the centrifugal compressor with a vaneless diffuser, the number of lobes is always two or three [12]. In recent years, a PIV experiment was carried out on a vaneless diffuser region of a centrifugal compressor by Ohuchida et al [20], in which the velocity fields at different span height were obtained and the diffuser stall was analyzed qualitatively.…”