CFD simulations and verification with respect to head curves and NPSHr

Abstract

The purpose of this thesis is to study the difference between simulation and test of a particular centrifugal pump design by Frank Mohn Flatøy AS (FRAMO). The pump has previously been tested and data from NPSH test and Q-H test is provided for comparison with simulations. An analysis of the test facility has been performed to ensure proper inlet and outlet conditions for the simulations, then a steady state NPSH simulation and a steady state and transient Q-H curve simulations has been performed. To better understand the hydraulic instabilities, a literature study has been carried out. Analysis of frequencies from pressure pulsations are then introduced as a way of identifying these instabilities. In the final analysis the results from simulations and empirical NPSH test and Q-H test have been compared, and the pressure pulsations has been evaluated. Based on these analysis the presents of instabilities are discussed. The thesis first conclude that the instability of the Q-H curve simulation is due to recirculation at the inlet, and that recirculation is not present at test. Further it is found that errors in the NPSH drop curve, if caused by temperature differences, are too small to result in large errors. It was also observed a correspondence between NPSH drop curve from simulation and the development of cavitation. In the analysis of the hydraulic instabilities it was discovered that rotating stall can have frequencies near rotational frequency, making it difficult to detect. Finally, comparisons between empirical data and simulations show considerable variation in results. Possible sources of error are proposed.