Performance of new diffuser design for an axial flow pump
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In an axial-flow pump unit, energy is lost from the pump system due to the swirl of the liquid between the vanes and leaving the stator. A combined stator-diffuser has been designed for the purpose to reduce this swirl and the corresponding loss of energy. Previous experimental investigations indicated that the new stator-diffuser design does reduce the swirl still present in the flow exiting from the pump unit when compared with a conventional design wherein the stator is followed by a separate diffuser. However the efficiency of the modified pump unit was slightly reduced. One of the reasons for this reduction in efficiency has been identified where a sudden enlargement at the outlet of the diffuser introduces secondary flow creating large fluid swirl. To reduce the secondary flow dimensional, theoretical, computational fluid dynamics (CFD) and experimental measurements have been performed to determine the performance effects of the new design diffuser combined with an additional tail piece. The tail piece is fitted to the outlet of the new diffuser to eliminate the sudden enlargement. Theoretical calculations determined that the five vane impeller did not match the new diffuser, whereas the eight vane impeller did match the new diffuser design. Impellers with five and eight vanes were used during laboratory testing. CFD and laboratory experiments were performed to verify that the tail piece addition would produce improved performance. The theory was confirmed from the experiments that the five vane impeller is not the correct impeller for use with the new diffuser with the tail piece however testing with the eight vane impeller efficiency improved by 3.9%, when compared to the standard pump test.
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