In this paper the flows of viscous oil in a centrifugal pump impeller are measured accurately by using two-dimensional laser Doppler velocimeter in the best efficiency and part-loading points, respectively, as the pump handling the viscous oil whose kinematic viscosity is 48mm~2/s. The measurements show that there is a wide wake near blade suction side and not a jet near blade pressure side so that the flow in the centrifugal pump impeller dose not conform the well-known wake/jet flow model, but a primary flow/wake model. The wake is located at two corners of shroud/suction and hub/suction, and covers the whole suction side. When the pump operation condition moves from the best efficiency point to the part-loading point the wake is enlarged. The impeller does not transfer the same rotating kinetic energy to the viscous oil particles in different streamlines.
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