The effects of circumferential outlet distortion of a centrifugal pump diffuser on the impeller exit flow were investigated. A fence with sinusoidal width variation was installed at the vaneless diffuser exit. The flow field was measured at the impeller exit with and without the fence, using a hot film probe and an unsteady pressure sensor. Flow parameters varied with the circumferential position and the mean flow parameters plotted against the local flow rate at each circumferential position showed loops along the quasi-steady curves, which were obtained from the result without the fence. Simple theoretical calculations were used to predict the velocity components at the impeller exit with the relative flow angle or total pressure assumed. Good result was obtained when the relative flow angle was assumed to vary quasi-steadily, not constant with the local flow rate. The radial velocity was also reasonably predicted when the total pressure was assumed to vary quasi-steadily. A simple method is proposed to predict the impeller exit flow with downstream blockage in two-step sequence: the first step deals with the diffuser alone to obtain static pressure distribution at the diffuser inlet, while the second step deals with the impeller alone to obtain velocity components distribution at the impeller exit.
In the present study, a computational analysis of the flow in a centrifugal blower is carried out to predict a performance and to explain noise characteristics of the blower. Unsteady, 3D Navier-Stokes equations were solved with k-ε turbulence model using CFX software. CFD results were compared with the experimental data that is acquired from an experiment conducted with the same blower. The pressure fluctuation in the blower was transformed into the frequency domain by Fourier decomposition to find the relationship between flow behaviors and noise characteristics. Sound pressure level (SPL) which is obtained from wall pressure fluctuation at impeller outlet represents relative overall sound level of the blower well. Sound spectra show that there are some specific peak frequencies at each mass flow rate and it can be explained by flow pattern.
The experimental study investigates the cause of prestall, which the magnitude increases of the shaft frequency near stall. This prestall phenomenon is related to the geometric non-uniformity of the axial compressor, which can be classified into blade non-uniformity and casing non-uniformity. The instant static pressure was decomposed into several signal components to investigate this phenomenon. To verify the blade non-uniformity, the dimensionless revolution aperiodic component (Ψ) distribution, which was measured at one arbitrary circumferential location, was analyzed, and to verify the casing non-uniformity, the amplitude of Ψ was analyzed at 8 equally spaced circumferential locations of the first stage. The measurements showed that the blade non-uniformity directly caused the increase of the magnitude of the shaft frequency near stall but that the casing non-uniformity induced the increase of the magnitude of the shaft frequency in the Seoul National University compressor.
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