Emergence and Propagation of Aerodynamic Instabilities in Centrifugal Compressors with Vaned Diffusers: Effect of Solidity and Stagger

Abstract

Centrifugal compressors are widely employed in turbochargers and small gas turbines due to their ability to achieve high compression per stage. These compressors consist of a rotating impeller and a static diffuser, which can be either vaned or vaneless. During off-design operation or throttle transients, aerodynamic instabilities such as stall and surge can occur, limiting the operating range and potentially causing mechanical damage, resulting in power loss and possible mission abandonment. This study specifically aims to investigate the effect of diffuser geometries on such instabilities and provide insights for early detection, avoidance, and control. While most previous studies have focused on centrifugal compressors with vaneless diffusers, this work concentrates on centrifugal compressors with vaned diffusers, particularly focusing on experimentally studying the effect of solidity and vane angle of the diffuser on the emergence and propagation of aerodynamic instabilities using unsteady pressure measurements.

To conduct the experimental study, a new test setup was required. The methodology involved the development of a novel rotating centrifugal compressor test facility, which was designed and established to facilitate the present experimental investigations. This test facility features a centrifugal compressor mechanically coupled to a radial turbine driven by compressed air, which is used to drive the compressor. The compressor and turbine have separate flow circuits, and the flow through them can be independently throttled to achieve the desired back pressure for the compressor and to control its speed by regulating the turbine mass flow rate. The facility is equipped with pressure and temperature measurement devices to establish the steady-state performance of the compressor, along with fast-response pressure transducers to study its stalling behaviour, focusing on the influence of vaned diffuser geometry. The diffuser of the centrifugal compressor has been designed as a replaceable vane ring, enabling the study of different configurations, starting initially from a baseline diffuser. The solidity of the baseline diffuser is progressively reduced, and its effects on the emergence of aerodynamic instabilities are explored. Additionally, a vaneless configuration is also studied for comparison. Furthermore, the vane angle (stagger) is varied, and its effect on stalling has also been investigated and presented in this study.

Steady-state Computational Fluid Dynamics (CFD) simulations have been done to complement the experimental results and provide detailed spatial insights into the flow behaviour inside the compressor. The steady-state performance and flow conditions are analysed using time-averaged CFD, enabling the examination of the aerodynamic characteristics of the vaned diffuser configurations. The simulations enabled a detailed study of the compressor configurations with all the diffuser geometries considered in the experiment. The analysis is conducted at various speeds for different mass flow rates and pressure ratio conditions to establish the steady-state compressor characteristic map. The simulations help to understand the mechanisms underlying the observed instabilities and provide good, steady three-dimensional flow field information within the compressor.

The experimental steady-state characteristic map of the compressor is generated by maintaining a constant speed and gradually closing the outlet valve in incremental steps, leading to a decrease in mass flow rate and an increase in pressure ratio until the stable operation is compromised. Reducing compressor flow rate alters airflow incidence angles, causing flow separation and rotating stalls that can combine and lead to a complete flow breakdown and surge, characterised by high-frequency pressure fluctuations recorded at various spatial locations. The present study explores the influence of diffuser solidity and stagger angle on the route to surge, considering a baseline diffuser with a 65° stagger and fifteen vanes, two diffusers with lower solidity, a vaneless diffuser, and two diffusers with higher and lower vane angles.

Investigating the unsteady pressure measurements at 38\% part speed (30000 rpm) reveals that the compressor undergoes stall inception through spike-type instabilities, which are short in length scale and time scale. The study explores the features of these spikes, including their amplitude, time scales, and length scales, and finds that the vaned diffuser's solidity and stagger influence these characteristics. The spikes exhibited distinct behaviour during emergence, when the spike count is lower than one spike per revolution. Then they grow in count and amplitude which is called as propagation phase and they eventually transition to rotating stall-type instabilities and which further leads to surge in most cases.

From the compressor characteristics with diffusers of different solidities, it is observed that as the solidity reduces, the slope of the compressor characteristics becomes positive. This change in the slope is associated with an improvement in the flow range, which is the difference between the mass flow at the choke point and the mass flow at the surge point. Also, the flow range from stall inception to surge also expands as solidity is reduced. The analysis of spike stall characteristics indicates that there is a link with the number of diffuser vanes of the centrifugal compressor. However, the number of stall cells and the speed at which they propagate appear to be relatively constant with changes in solidity. About surge, it is noted that mild-surge precedes the onset of deep surge at lower solidity values. This precursor to deep surge is indicative of the system’s susceptibility to changes in solidity. The 7-vaned diffuser particularly exhibits pronounced flow fluctuations during the surge, showing the impact of the reduction in solidity. In contrast, the behaviour of the vaneless diffuser stands out as distinct, not aligning with the patterns observed in configurations with vanes.

The effect of changes in stagger on the emergence and propagation of stall and surge have also been studied. The examination of compressor characteristics reveals that a decrement in stagger results in a marginal reduction in the pressure rise as well as the choke and surge mass flow rates while preserving the invariant nature of the characteristic curves. The stall inception is initiated by spikes that evolve into a rotating stall. Similar to the effect of solidity, the spike stall characteristics are found to be influenced by diffuser stagger angle, yet the number of stall cells and their velocity remain unaffected by changes in it. With respect to surge, though the temporal spans pertinent to instability growth, blowdown, and recovery are observed to be similar, with a reduction in stagger, mild surge is typically encountered preceding a deep surge, particularly in the configuration with the lowest stagger, where the surge cycles are predominantly characterised by Helmholtz waves. An enhancement in the flow fluctuation during limit cycle oscillation is associated with an increase in stagger.

In summary, this study provides a comprehensive analysis of the impact of diffuser solidity and stagger on compressor stalling dynamics and overall stability. The findings reveal that spike-type instabilities are influenced by the diffuser's solidity and stagger broadly. Increasing solidity led to a higher spike count, lower spike period, and lower spike amplitude. With relation to changes in vane angle, the baseline design case exhibited the highest spike count, lowest spike period, and lowest amplitude. The spike characteristics of the diffuser with 60 degrees of stagger were affected by surge, as it underwent surge before stalling. Thus, it is evident that the diffuser's solidity and stagger impact spike count, amplitude, and period, given their short length and timescale. Subsequent to spike, rotating stall appears, and it seems to be insensitive to diffuser geometry variations as seen from the number of cells and their speed. This is likely due to the larger time and length scale of the rotating stall and the small diameter of the impeller at the inlet. In the case of surge, a notable difference in the time signature of pressure was observed, with the amplitude of pressure fluctuation being higher for the diffuser with the lowest number of vanes and the diffuser with the highest vane angle. The number and orientation of vanes appear to influence pressure signatures during surge, with Helmholtz resonance frequencies dominating more in the low solidity case. Mild surge was encountered for cases with low solidity before the compressor entered deep surge. In conclusion, this study elucidates the differences in the route taken by the compressor from stall inception to surge due to the effects of vaned diffuser geometries.