Phase-averaged measurements of unsteady flow in a vaneless diffuser

Abstract

The unsteady, periodic flow inside a vaneless diffuser was investigated experimentally. Flow through the model diffuser was delivered by an actual impeller and exited to room conditions. The mass flow rate through the diffuser was chosen to be high enough so that an unstalled diffuser flow was obtained in spite of the presence of a shroud-side recirculation ring. A phase-locked ensemble-averaging technique was used to resolve the unsteady, periodic velocities inside the diffuser. This was accomplished by monitoring the radial velocity at the diffuser inlet using a single hot wire, thus providing the required reference phase of the diffuser flow. The flowfield was measured using a cross wire and the ensemble-averaged, time-resolved distributions of flow angle, radial and circumferential velocities, and their respective statistics were calculated and reported. The recirculating flow was measured using a one-component laser Doppler velocimeter. These results indicate that the recirculation ring starts just downstream of the diffuser inlet and covers about 80% of the diffuser in the radial direction. At its maximum, the axial extent of the recirculation ring is about 35% of the diffuser width. Furthermore, the radial decay of the circumferentially asymmetric character of the flow is relatively slow, and the periodic behavior persists even at the diffuser exit. Finally, the measurements serve as a valuable data set for the validation of diffuser calculation models.