The flow in a centrifugal compressor stage is dominated by the geometry of the impeller, the diffuser and the exit collecting chamber. Asymmetries of the flow field in the annular space behind the impeller may create a circumferential pressure distortion, influencing the energy transfer by the impeller in a negative way, creating variable force on the impeller blades and a radial force on the shaft. Experimental investigations of the circumferential static pressure variations have been carried out on a large radial compressor test stand. Measurements with various impellers, diffusers and collecting chambers show the influence of geometrical modifications on the flow and compressor characteristics. It is shown how a concentric exit chamber results in a very large pressure distortion in the diffuser. Although the amplitude of this distortion is increasing towards the diffuser inlet, it has a negligible effect on the pressure distortion upstream of the impeller. It is further shown how the asymmetry of the flow in the diffuser can be reduced either by using a throttling ring at the diffuser exit, a vaned diffuser or by replacing the constant cross section collector by a volute with circumferentially increasing cross section.
This paper presents the experimental and numerical investigation of an outward volute of rectangular cross section. The investigation is carried out at the level of stage performance, volute performance, and detailed flow field study at selected peripheral positions for various operating points. The objective of the investigation was to gain further knowledge about the flow structure and loss mechanism in the volute. Simultaneously with the experimental investigation, a numerical simulation of the flow in the volute was carried out. A three-dimensional Euler code was used in which a wall friction term and a tuned artificial dissipation term account for viscous effects. A reasonable agreement between the experimental and numerical results is observed. As a result a good and detailed knowledge about the pressure recovery and loss mechanism in the volute is obtained. [S0889-504X(00)00301-9]
This paper is aimed to analyze the experimental results obtained by Kassens and Rautenberg (1998) and Kassens (1997) concerning the overshoot of the rotating velocity field behind the IGV. Since the IGV used in the present investigation has the form of a disc in the closed condition of the blades, the vorticity field of the rotating flow behind the outlet edge of the IGV can be determined by means of the streamline deflection in this IGV, whose function as an actuator disc does not need to be used. The potential-vortex field generated by these streamlines out of the IGV is calculated with good accuracy compared with the experimental results. The further development of the vorticity field mentioned into the distribution of the tangential velocities can be derived from the diffusion of the vorticity, but with an inaccuracy caused by the overshoot of the Rankine vortex. This overshoot originates from the potential vortex field and travels towards the vortex core, which acts as a rigid body rotation. The overshoot degenerates downstream from the outlet of the IGV due to strong damping incorporated in its unusual velocity field. Its disappearance introduces the formation of the orderly Lamb-Oseen vortices, whose theoretical property coincides well with the experimental result. Thus, the overshoot plays an active role for the loss in the total pressure in the central region of the rotating flow generated by the IGV.
The present analysis of the experimental results obtained by Kassens (1997), and Kassens and Rautenberg (1998) about the variation of the axial velocity profile of the rotating flow travelling downstream the inlet guide vane revealed the controlling effect of the decaying overshoot of the Rankine vortex in this process. The disappearance of the overshoot indicates the stop of its excessive drive on the rotating flow. It will be shown that this decay process is directly connected to the braking effect of the impeller blades on the rotating flow generated by the IGV. For the case of an IGV setting angle of 60°, the flow with a very strong swirl component generated by the IGV outlet cannot be admitted by the inlet of the impeller without shock. This shock acts as a braking force of the impeller blades on the swirl component of the approaching flow, a process that is expressed as the spin-down of the swirling flow. A secondary toroidal ring vortex in the frontal zone of the impeller will be generated by the fundamental flow. The recirculation velocity of this toroidal ring vortex has an axial component, which directs towards the impeller in the outer zone of the suction duct, but turns away from the impeller in its central zone. At the same time, the field of the uniform tangential velocity of the IGV outlet is getting into the influence sphere of the spin-down mentioned. As a consequence, it will be converted into the flow field of a potential vortex, which is accompanied by a field of axial velocity having a normal value uniformly distributed in its outer potential zone and a strongly peaked value over its central core. This process is similar to that of the flow field in a bathtub vortex during draining its fluid out of the central pipe. The new flow field consists of a secondary toroidal ring vortex superimposed on the fundamental flow. The rotating sense of this new toroidal ring vortex is just opposite to that formed in front of the impeller. We then have a pair of toroidal ring vortices staying one behind another between the IGV outlet and the impeller inlet. Each member of the pairs serves as the border for the other one. It can thus be concluded that the pair is induced simultaneously by the spin-down of the swirling flow of the IGV outlet brought about by the braking effect of the impeller blades.
This paper describes a new model for the analysis of the flow in volutes of centrifugal compressors. It explicitly takes into account the vortical structure of the flow that has been observed during detailed three-dimensional flow measurements. It makes use of an impeller and diffuser response model to predict the nonuniformity of the volute inlet flow due, to the circumferential variation of the pressure at the volute inlet, and is therefore applicable also at off-design operation of the volute. Predicted total pressure loss and static pressure rise coefficients at design and off-design operation have been compared with experimental data for different volute geometries but only one test case is presented here. Good agreement in terms of losses and pressure rise is observed at most operating points and confirms the validity of the impeller and diffuser response model.
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