Performance evaluation of a micro gas turbine centrifugal compressor diffuser

This report examines the influence of inlet flow conditions, including Mach number, flow angle, blockage, and axial flow non-uniformity, on the performance and operating range of a straight channel centrifugal compressor diffuser. The research was carried out in a unique facility specifically developed to provide the diffuser with a controlled inlet flow. The tests were carried out for inlet Mach number up to values greater than unity and for a range of inlet flow angles up to the onset of rotating stall. It was found that expressing the overall diffuser pressure recovery coefficient, defined using availability or mass averaged inlet total pressure, as a function of momentum averaged diffuser inlet flow angle yields a relationship which is essentially independent of diffuser inlet flow distortion, blockage, or Mach number. Further, the operating range of the diffuser was limited by the onset of rotating stall at a momentum averaged diffuser inlet flow angle (oc = 70.5 0.5), which was also independent of the inlet flow field axial distortion and Mach number. The straight channel diffuser was designed to be comparable to a previously tested discrete passage diffuser and the performance of the two was compared; the overall pressure recovery of the former was found to be roughly 10% higher than that of the latter. Both diffuser types, straight channel and discrete passage diffuser showed similar behavior regarding the insensitivity of the performance and operating range to inlet flow axial non-uniformities and Mach number. The report also presents information on recent developments in the area of centrifugal compressor diffusers together with a detailed review of the open literature.

Effects of inlet conditions on centrifugal diffuser performance

Design and Optimization of Centrifugal Compressors

Subjected to an oscillating flow rate, a compressor may feed additional (excitational) energy into the attached piping system. The relation between this additional energy input and the instantaneous behavior of a centrifugal compressor stage is deal in a first part. Modeling the stage behavior by taking into account either inertia of the enclosed fluid mass or a first-order transient element or transiem stall in any component leads to a different energy input. The energy input at a flow rate oscillation of given frequency and amplitude was calculated as a function of the slope of the characteristic and the reduced frequency applying a previously published model to describe the instantaneous behavior of the stage. In this model transient stall in the diffuser is taker into account. At reduced frequencies above unity the energy input of the diffuser was reduced by a considerable amount due to the specified instantaneous behavior of the diffuser. This indicates a potential to reduce the additional energy input of the diffuser either by increasing the time constant of the stall process or by increasing the mild surge frequency. For the investigated diffuser size the required reduced frequencies imply mild surge frequencies in a range being too high for industrial application (>200 Hz), Still, this method turned out to give useful insight into the link between the instantaneous behavior of the compressor and its energy input. In a second part for the same centrifugal compressor the energy contribution of several stage segments during mila surge oscillations was determined from detailed instantaneous measurements. As a result, the contribution of each stage segment to the conservation of the mild surge pulsation emerges. Although at the investigated mild surge frequencies the stage segments no longer behave strictly quasi-steadily. their contribution to the additional energy input is found to be mainly determined by the slope of their quasi-steady characteristic.

Energy Input of a Centrifugal Stage Into the Attached Piping System During Mild Surge

High-pressure ratio centrifugal compressors usually adopt vaned diffusers to reach high efficiency. Nevertheless, the compressor operating range might be narrow on account of the diffuser stall res...

Effects of a slotted diffuser on the aerodynamic performance of a highly loaded centrifugal compressor

This paper contains a theoretical and experimental investigation of the flow in the vaneless and semi-vaneless space of a transonic radial compressor diffuser, especially for working points corresponding to the mass flow below choking values, in the efficient operating regimes. A type-dependent finite difference calculation has been developed for the inviscid flow. This method has been combined with a shock-boundary layer-interaction theory, which allows a more realistic calculation of the velocity distribution, taking into account the boundary layer variation along the sidewalls. In parallel with this analytical study, several diffuser geometries were also performance tested. A special data-reduction program allowed calculation of the throat blockage and the velocity distribution throughout the diffuser based on static and total pressure measurements. Schlieren pictures were also taken in order to have a better insight in the real flow phenomena at the diffuser entrance. From this experimental data, a detailed comparison with the analytical study was made. This combined theoretical and experimental study made it possible to show the influence of the geometrical parameters of the vaneless and semi-vaneless space on the velocity distribution, and on the throat blockage. Together with the data available in literature for channel diffusers, it allows a better prediction of the overall performances of diffuser systems.Copyright © 1978 by ASME

G. Verdonk

Papers

Vaned Diffuser Inlet Flow Conditions for a High Pressure Ratio Centrifugal Compressor