ABSTRACT The unsteadiness of the flow at the leading edge of a vaned diffuser represents a source of low efficiency and instability in a centrifugal turbomachine. Furthermore, the internal flow of the impeller can be affected by asymmetric downstream conditions, which results in extra flow unsteadiness and instabilities. Numerical and experimental data are obtained. The simulation of impeller diffuser interaction is performed using CFXTascflow. A frozen rotor simulation is used for the steady calculation and a rotor-stator simulation is used for the unsteady calculation using the steady results as an initial guess. The unsteady simulation is done not only for one impeller and diffuser blades, but also for the whole impeller and diffuser blades using Unix workstation. For the experimental work, a transparent fan is design and tested at The Turbomachinery Laboratory of SJTU. The test rig consists of a centrifugal, shrouded impeller, diffuser and volute casing all made of plexiglass. A particle image velocimeter (PIV) is used to measure the 2-D instantaneous velocity in the interaction region between impeller, vaned. A series of performance measurements were carried out at different speeds. The first trial of measuring the instantaneous flow field in a part of the impeller and vaned diffuser together at different relative locations between them is presented in this work at different flow rates. Obtaining detailed measurements in the interaction region between the impeller and diffuser can help in understanding the complex flow phenomena and improving centrifugal fan and compressor performance. Finally, the comparison between the unsteady measurements and unsteady calculations showed that the Rotor/Stator Model can predict the basic characteristics of unsteady flow in centrifugal fan but still need improvement to satisfy the true transient simulation for unsteady impeller diffuser interaction INTRODUCTION The improvement of machine performances can only be achieved if there is a progress in the comprehension of the nature of the complex flow that develops at the gap between rotor and stator. During the design of a turbomachine, the flow is considered steady and uniform at the entry of each element. For a centrifugal fan with a vaned diffuser, satisfying this assumption requires a large interface between the rotor and the stator so that the mixing process of the flow leaving the impeller can take place. Otherwise, the unsteady flow that enters the diffuser represents a source of low efficiency. Furthermore, the internal flow of the impeller can be affected by asymmetric downstream conditions, which results in extra flow unsteadiness and instabilities. A number of authors have treated the problem of the interaction of the impeller and its surroundings. Inoue and Cumpsty [1], Sideris [2] and Arnd
