開放型ポンプの研究
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概要
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Introduction of open impeller construction in recent years was one of major features contributing to improvements in pump performance. An open impeller, which has no front shroud, has many advantages in which it is easy to be manufactured and its passage is easily cleaned. An open impeller produces a higher axial thrust than a closed impeller does. Radial ribs on its back shroud are the only practical means for the reduction of its axial thrust. However, little accurate information is available on power loss due to the radial ribs. It is well known that decline of pump performance occurs when cavitation takes place in an impeller, and that, in addition, undesirable effects, such as noises and vibration, are caused as cavitation develops. However, almost no information is available on cavitation and suction performance of open impeller pump. The authors, therefore, try to investigate the effects of the ribs upon the axial thrust and the relation between the cavitation aspects and the suction performances of two open type impeller pumps operated at different specific speeds. The axial thrust due to the ribs was measured with the use of particular arrangements connected to the pump axis. Through this axial thrust experiment, the relations in a wide range between the thrust due to the ribs and each of the following items : the total head, discharge flow-rate and shaft power, is discussed. The decrement of the shaft power varied from 12% to 4%, while the decrement of the ribs stayed constant. This means that power loss due to the ribs increases rapidly with diameter of the rib. Therefore, it seems that a rib diameter should be no greater than that needed for reducing the thrust by the desired amount. In the cavitation experiment, the flow accompanied by cavitation in the open impeller of a centrifugal pump is discussed on the basis of an experiment conducted in a wide range for the discharge flow-rate and the suction static pressure by means of a volute pump having an intake of 36 mm diameter. Cavitation in the open impeller was classified into five zones. The results of observations by the strobolight made on the process of development of each cavitation zone after its inception alternately with the excess, partial and small flow-rate were described, with the photographs of cavitation aspects in the open impeller passage. The cause of inception and development of each cavitation zone was interpreted with consideration for the relation between the cavitation zone and the flow pattern of water in the open impeller. Then, the pressure-drop coefficient λ, was obtained from the NPSH which was found through an experiment with two impellers with blades of different thickness. λ varies with the discharge flow-late, being a minimum near than η_<max'> but increasing of the both sides of this point. The value of λ is about 0.5 for a thick-blade made of special carbon, and about 0.2 for a thin-blade made of cast iron. The relation between cavitation aspects in the pump and bubble-flow in the suction pipe in case of the wide range discharge flow-rate was discussed in connection with the results of the observations by the strobolight, with the photographs showing these relations. From these observational results some interesting phenomena were discovered that the inception of cavitation in the impeller occurred at the same time that a bubble-flow appeared in the suction pipe. This result indicates that, if the suction pipe is transparent, we might be able to predetermine a cavitation in a pump by observing bubble-flow in a suction pipe.
- 明治大学の論文