Velocity profile influence on spiral vortex breakdown in diffuser

abstrakt

angličtina

Flow in diffuser with circumferential velocity component is accompanied by spiral vortex breakdown for large diffuser divergence angles. This phenomenon, although being investigated for almost fifty years1,2, is not fully understood yet. Study of this problem is essential for example for design of hydraulic turbine draft tubes operating in part load conditions, where vortex breakdown causes severe pressure pulsations. It is well known that tangential velocity profile plays important role in origin and character of the vortex breakdown3. Aim of this paper is to find influence of tangential velocity profile on spiral vortex breakdown, not only in terms of swirl number, but also in terms of velocity profile shape. Experiment was carried out for diffuser divergence angle 11.6 degrees. Inlet circumferential velocity profile was provided by swirl generator with fixed blades. Velocity field was measured with LDA and pressure was recorded on the diffuser walls to obtain frequency spectra of the vortex rotation. This set of data was used for the initial verification of the computation. Analysis of the vortex breakdown for different inlet velocity profiles was carried out numerically with finite volume method. The results point to relationship between vortex breakdown characteristics (rotational frequency, helix slope, pressure decrease within the vortex) and inlet velocity field. The results are also correlated with other quantities like vorticity and helicity. This outcome is not only important for deeper insight into vortex breakdown phenomenon, but also brings important knowledge for the turbine design.

spiral vortex breakdown, velocity profile, stability, diffuser, draft tube

RUDOLF, P.

15. 9. 2008

University of Manchester

Manchester

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