The internal flow structures in a vaneless diffuser of a radial pump are experimentally and numerically investigated. Numerical simulations are used for investigating the prominent features of the fluid flow in the vaneless diffuser upon a change of the flow rate and of the impeller-to-diffuser radial gap. Comparisons between experimental and numerical results on the overall pump and diffuser performances are presented and discussed. The influence of inlet diffuser leakages on the development of flow instabilities is specifically addressed, owing to the remarkable impact that a small impeller-to-diffuser radial gap can have on the diffuser flow. Highlights on the rotating instabilities in terms of stall cell number and propagation velocity at design and off-design conditions are the main focus of this study and will be investigated using the numerical and experimental results post-processed by fast Fourier transform (FFT) and wavelet analysis.

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