Detection and Analysis of an Alternate Flow Pattern in a Radial Vaned Diffuser

Abstract

The flow in the vaned diffuser of an aeronautical centrifugal compressor designed by Safran Helicopter Engines is analyzed through steady and unsteady pressure measurements at different rotation speeds. The analysis leads to the identification of different operating zones thanks to a new variable, the alternate rate ?. It allows the characterization of a specific behavior of the vaned diffuser consisting of an alternate stall pattern in two adjacent channels of the diffuser. While it is close to zero at low speed, the alternate rate reaches a maximum value at a higher speed before collapsing with a further increase in the rotation speed. Depending on the value reached by the alternate rate, three distinct regimes of the flow within the diffuser can be distinguished. For low ? values, the regime is the most common one with an equivalent flow pattern in each channel of the diffuser. For moderate ? values, a mild difference of the flow fields which develop in two adjacent channels can be observed but it remains time independent. Finally, for high values of ?, the alternate pattern is amplified and becomes time dependent, pulsating together with the mild surge of the entire compressor.

Low frequency stall modes of a radial vaned diffuser flow

“…Its intensity also increases with the reduction of the flow rate. Previous studies [15] linked this mode to the occurrence of mild surge inside the whole compressor. As described by Fink [16], it consists of longitudinal fluctuation of the mass flow without its complete reversal.…”

“…Based on the circumferential static (steady and unsteady) pressure distribution, an indicator À the alternate rate A has been developed to determine the establishment and magnitude of the alternate pattern. More information on the calculation of the alternate rate A and the thresholds used below is given in [15] (A is named asymmetry rate instead of alternate rate in [15]). The superimposition of this indicator on the performance map, shows a very localized area of the setting of the alternate pattern.…”

Effects of Inlet Guide Vanes on the Performance and Stability of an Aeronautical Centrifugal Compressor

“…2. Indeed, a particular flow topology spread on two channels of the radial diffuser has been previously observed numerically [12] and experimentally [13] at an IGV stagger angle of 0°. The impact of the IGV stagger angle on this alternate flow topology is studied in the last part of the paper.…”

“…Previous work on the compressor at an IGV stagger angle of 0° has been carried out by Moënne-Loccoz et al [13,15]. Rotating instabilities at the impeller inlet have been observed, the impact of the IGV stagger angle on the development of these rotating instabilities is discussed in the last part.…”

Experimental and numerical analysis of impeller recirculation: inlet guide vanes stagger angle effects and rotating disturbances interaction

“…First the radial and the tangential velocity components are averaged with area weight and then the angle α2 is computed from these values. The definition and the meaning of the alternate rate A, an original indicator introduced because of a particular flow topology in the radial diffuser of this compressor stage, is given by Moënne-Loccoz et al (2020). A high alternate rate corresponds to a flow topology different over two adjacent channels of the radial diffuser: one channel is stalled whereas the other is free (pattern steady in time and in space).…”