Predicted and measured surface velocity and pressure distributions in the internal flow channels of a centrifugal fan impeller are presented for volume flow rates between 80 and 125 percent of design flow rate. Predictions are based on a fully three-dimensional, finite element analysis of the inviscid, incompressible blade channel flow. Additional predictions using a conventional quasi-three-dimensional analysis are presented for comparison. Experimental results were developed using extensive blade and sidewall surface pressure taps installed in a scale model of an airfoil-bladed centrifugal fan impeller designed for heavy industrial and power generation applications. The results illustrate the ability of both flow analyses to predict the dominant features of the impeller flow field, including peak blade surface velocities and adverse gradients at flows far from the design point. In addition, the experimental results provide valuable insight into the limiting channel diffusion values for typical centrifugal cascade performance, and the influence of viscous effects as seen in deviations from the ideal flow predictions.

Efforts to quantify degradation of the performance of a centrifugal fan subjected to distorted inflows are presented. The study centered on examination of pressure rise, efficiency, and onset of stall for a backwardly-inclined, airfoil-bladed fan. Nonuniform flow patterns were generated in an adjustable countervane damper assembly installed upstream of the inlet box, and systematic families of performance curves were generated. Results of these tests show that significant degradation in efficiency and pressure rise—as much as 10 to 15 percent—may result from moderately to severely distorted inflow patterns. Onset of stall was significantly influenced by severely distorted inflows.

A rotating stall associated with massive flow separation on the pressure surface (p-stall) of a centrifugal fan blade is identified. The stall cell rotates in the same sense as the rotation of the impeller. The frequency of pressure pulsation is greater than the running frequency and approaches 4/3rds the running frequency. It is proposed that the mechanism of p-stall propagation is similar to that of the classical rotating stall caused by flow separation on the suction surface (s-stall). However, it is emphasized that the cause of p-stall is different from s-stall and any corrective treatment applied should take its nature into consideration. In the present work, pressure surface stall was induced by pre-rotating the inlet flow using inlet guide vanes and operating the fan against a low system resistance. Heuristic models are proposed for the inception and propagation of p-stall, and computational predictions of the flow fields, along with experimental results, are presented to support the model.

We consider the case of transient three-dimensional, incompressible and isothermal channel flow of a newtonian fluid past a symmetrically confined obstacle of square cross section at zero incidence. The blockage ratio (ratio of cylinder diameter to channel width) is fixed at 1/4. Results from both Laser Doppler (LDV) experiments and direct simulations upto Reynolds numbers (Re ≤ 250) have been reported.Starting from the onset of instability (Re cr ≈ 58), close examination of the apparent region of influence that envelopes the cylinder reveals a clear vortex shedding signature which extends up to about three diameters upstream. While the fundamental vortex shedding frequency displays a fairly steady monotonic increase with Reynolds number, the Strouhal number (St) shows an inflexion point around Re ≈ 115. No signs of hysteresis were detected around this region. Furthermore, incommensurate harmonics observed for Reynolds numbers in the range 127 ≤ Re ≤ 175 suggest a quasi-periodic transition to three-dimensionality. This is shown to be followed by an intermediate periodic window starting around Re ≈ 180. Results from direct simulations performed using OpenFOAM (the Open Source CFD Toolbox) are used not only to enhance confidence in the accuracy of the reported LDV measurements, but also to help explain the observed phenomena through frequency power spectra and spanwise velocity correlations. Finally, it is concluded that only a parametric variation of transverse and spanwise blockage ratios can bring closure to the subject of bluff-body wake transitions.

A rotating stall associated with massive flow separation on the pressure surface (p-stall) of a centrifugal fan blade is identified. The stall cell rotates in the same sense as the rotation of the impeller. The frequency of pressure pulsation is greater than the running frequency and approaches 4/3 the running frequency. It is proposed that the mechanism of p-stall propagation is similar to that of the classical rotating stall caused by flow separation on the suction surface (s-stall). However, it is emphasized that the cause of p-stall is different from s-stall and any corrective treatment applied should take its nature into consideration. In the present work, pressure surface stall was induced by prerotating the inlet flow using inlet guide vanes and operating the fan against a low system resistance. Heuristic models are proposed for the inception and propagation of p-stall, and computational predictions of the flow fields, along with experimental results, are presented to support the model.