An experimental study on the development of a reverse flow zone in a vaneless diffuser

Abstract

This study presents the measured rotating stall signal patterns in a vaneless diffuser in a centrifugal compressor with a radial bladed impeller. Unsteady flow and rotating stall in the vaneless diffuser were investigated by measuring unsteady velocity fluctuations at several different diffuser radius ratios and axial distances using a hot wire anemometer. The flow characteristics in terms of the radial and tangential velocity components and the flow angle distribution in the vaneless diffuser during rotating stall were investigated using phase-locked averaging techniques. The results clearly identified abrupt rotating stall at several different impeller rotational speeds. According to the experimental results, two different mechanisms exist for the development of the reverse flow zone in a vaneless diffuser. One is dominated by the extension of the reentering flow from the diffuser exit, and the other is dominated by the growth of the local flow separation zone on the hub and shroud side. The fluctuation in the flow direction increases as the diffuser radius ratio increases and is dominated by the strength of the reverse flow. At the onset of rotating stall, the radial velocity for one period of the rotating stall slowly increased to a maximum value and then decreased quickly to a minimum value with an intermediate peak. However, at lower flow rates, this intermediate peak did not occur.