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This paper treats the flow instabilities in a mixed flow pump with a vaned diffuser. Tested pump has a positive slope of the head-flow performance curve at about 65% flow rate of BEP (Best Efficiency Point). In order to investigate this phenomenon, pressure fluctuation measurements and dynamic PIV (Particle Image Velocimetry) measurements have been performed. As a result, it is found that unstable performance is caused by a rotating stall in the vaned diffuser. The flow from impeller exit is influenced on the large scaled backflow generated periodically from the vaned diffuser. Moreover, the backflow is induced by the strong vortex developed at the leading edge of diffuser vane. Based on PIV pictures, the relationship between the static pressure at the inlet of diffuser vane and the internal flow conditions is clarified.

A rotating stall phenomenon in radial vaneless diffuser of centrifugal pump has been investigated experimentally as the first stage of developing a method to suppress the stall. The test impeller is two-dimensional having six blades with outlet angles of 18.1 degrees and the diffuser is parallel walled with the outer diameter ratio of 1.90 to the impeller outlet and the height of 30 mm, which is the same of the impeller. Measured results are shown and discussed in the present paper as follows. The onset of rotating stall is triggered by a reverse flow occurring on the hub surface near the diffuser outlet. At that time the average angle of absolute flow velocity becomes less than 11.9 degrees at the section just downstream of the impeller. As the observed behaviors of rotating stall, change of stall cell numbers, amplitude and frequency of pressure fluctuation are clarified.

The heart diseases are one of the major causes of death in recent years. Most of them are related to the atherosclerosis of coronary artery, whereas the mechanism of atherosclerosis is still unknown and the only known is that the atherosclerosis occurs under the influence of blood flow and the wall stresses on vessel walls. In the present study, in order to obtain the information of unsteady flow in the blood vessels, the flow structures of pulsatile flows in an elastic tube modeled as a blood vessel are investigated. Firstly, to validate our PIV (Particle Image Velocimetry) measurement technique, we apply it to sinusoidal flows with no net flow rate in a rigid straight tube, for which the exact solution with low Reynolds number exists. Then, two types of imitated blood flow in arteries are simulated in a rigid and an elastic straight tubes, and the results are compared together with the numerical simulation, which is also constructed in the present study, using SMAC type fractional step method with ALE (arbitrary Lagrangian and Euler) moving grids treatment. Finally, the effects of elastic boundaries on local flows as well as the wall shear stresses are discussed.

Gas accumulation and its elongation in a centrifugal impeller of liquid/gas two-phase flow pump cause head degradation. In order to decrease the head degradation, a special type of open impeller has been designed on the basis of the following concepts. A designed impeller consists of two stages of rotating and stationary circular cascades to create a dispersive effect of each cascade inlet for gas-phase flow. The outer rotating cascade has a large blade outlet angle to keep the pump head high even at high void fraction. Holes made through the rotating disk, through which recirculating liquid flows may blow off the gas accumulating region, are equipped in each flow passage. This newly conceptualized impeller was examined experimentally. Results are discussed in the present paper on how to improve the two-phase flow performance.

Experiments on air/water two-phase flow performance were carried out using a centrifugal pump with five kinds of closed impellers, each of which has a different outlet or inlet blade angle. Results are described as follows. (1) A sudden head drop due to gas accumulation in the impeller occurs at a low inlet void fraction with high inlet or outlet blade angle of impellers. (2) A degraded pump head at a high inlet void fraction becomes higher with increasing outlet blade angle. These results are discussed in terms of numerical calculations of one-dimensional two-phase flow.