Risaburo Oba

High-speed observation of ultrahigh-speed submerged water jets

Some peculiar phenomena occur around ultrahigh-speed submerged water jets accompanied by very severe cavitation erosion. Using the flow visualization technique with a xenon flash, the water jets were carefully observed, and the spatial distributions of highly erosive impulsive pressures around the jets were measured by means of a pressure-sensitive film technique. The effects of the injection pressure and the nozzle configuration are systematically clarified. Thus, the characteristics and structures of ultrahigh-speed submerged water jets are clearly shown.

Scanning electron microscopy observation on the incubation period of vibratory cavitation erosion

Abstract The behaviour of the vibratory cavitation erosion on typical machinery material, SUS 304 stainless steel, is investigated using a scanning electron microscope and a profilometer. The tests are carried out under conditions of uniform cavitation nuclei size distribution. It is observed that the polishing Unes act as weak points for the formation of pits. The diameter and the shape of these pits do not change with the test time (t

Fatigue failure of SUS 304 caused by vibratory cavitation erosion

Abstract The mechanism of cavitation erosion is investigated in detail for SUS 304 stainless steel, a typical erosion-resistant material. Systematic observations of eroded surfaces and dislodged particles show that the predominant failure mode in cavitation erosion is fatigue. Characteristic features of such failures include (1) packets of slip bands, (2) cracks that nucleate at regions of high stress (strain) concentration, (3) cracks that propagate first at an inclination to the surface and then parallel to the surface, and (4) fatigue striations and ‘tire tracks’ on the rupture surfaces. These last two features are the most indicative of a fatigue process. It is also deduced that massive voids accompanying pressure shock-rings are the most erosive kind of cavitation.

A Study on Peening by Submerged Ultra-High-Speed Water-Jets

To clarify cavitation behaviours of submerged high-speed water-jets, systematic experimental studies were carried out by using a flow visualization technique. The cavitation behaviours were studied for both the free jet and the impinging jet at various stand-off distances x from the outlet edge of the nozzle. Effects of the injection pressure P 1 and the nozzle structure on the behaviours were also examined. The behaviours of the jets are largely dominated by explosive cavity clouds and by remarkably developed vortex cavitation. Therefore the structure of the submerged jet differs from that of jet in still air. The distribution of impulsive pressure P sh , which is measured by pressure sensitive films, clearly shows the existence of two pressure peaks along the jet. At the stand-off distance where P sh shows the “1st peak,” the impinging jet results in the severe erosion on the ambient solid surfaces. Because of this, the performance of cutting- or drilling- fabrications is considered to be remarkably improved when jet impinging is conducted at x 1 of the “1st peak.” On the other hand, at x 2 of the “2nd peak” downstream from the “1st peak,” numerous pressure pulses are produced on the solid surfaces, but mass loss due to cavitation erosion is barely detected. Clearly, x 2 of the “2nd peak” is believed to be suitable for precise peening fabrications.

Peening on Mechanical Parts of Complex Shape by High-Speed Submerged Water-Jet Accompanied by Uniformly Large Bubble-Cloud (1st Report, The Cloud-Effect-Confirmation for Parts of Complex Shape)

Recently, the high-speed submerged water-jet peening using the high impact pressures being accompanied by the bubble collapse is attracting attention. In this study, therefore, in order to improve such a peening effect and to make rapid progress of the applicable range, we take up a typical water-jet accompanied by uniformly large bubble-cloud, the time dependent change of residual stress is systematically measured on the gear surface by means of the X-ray diffraction method for a complex mechanical part of complicated shape (single helical gear of SCr420H3V2L2 carbon steel), and the definite peening effectiveness has been verified. We show on the surface that the residual tensile stress can be effectively improved into the residual compressive stress. It is also confirmed that uniform peening effect well works even for such very complex parts.

Pulp Fluidization and Limited Consistency for Operating the MC-Centrifugal Slurry Pump.

In order to clarify the detailed behavior of the pulp fluidization in the operation of an MC pump, some effective factors (consistency, degassing pressure, rotational speed and diameter of the booster blade) were discussed. In the medium consistency field, a successful method for deciding the limited consistency as well as the maximum possible consistency was also found.

Base-Vented Performance Simulation for Supercavitating Hydrofoils with Pseudo-Kirchhoff Nose Operating within the Subcavitating Region.

In order to clarify supercavitating hydrofoil performance numerically, especially in the base-vented state at a low incidence, we try to make numerical simulations for three typical thin super cavitating hydrofoils with pseudo-Kirchhoff noses by means of the QUICK method. The precise water-tunnel tests on the supercavitating steady lift- and drag-performance for the foils are examined, after the marked effects of leading edge and the peculiar behavior of disc drag have been clarified. We find that the analyses agree well with the experiments, especially in the base-vented flow at low incidences, showing several interesting facts.

Erosive Area Comparison by Means of Four Cavitation Detecting Methods.

To find a useful method for monitoring cavitation erosion arising in turbomachinery, several methods have been compared. There are mainly four typical methods : pure Al plate, soft Al foil, pressure-sensitive film and painting methods. However, the mutual relationship among them is yet unsettled. In this paper, therefore, the erosive area, where the erosion mainly takes place, was precisely detected by means of the four methods under the same flow conditions in typical separated flows around a slot. It was found that a large difference between the erosive areas is recognized, especially from the threshold levels, since each method inevitably has such an inherent characteristic threshold level against cavitation attacks.

Suppression of Cavitation Erosion on a Low-Erosion Butterfly Valve with a Tunnel-Type Buffer.

In order to improve the reliability of butterfly valves, it is absolutely imperative to prevent fatal cavitation erosion. In our previous studies, we developed a new butterfly valve with a tunnel-type buffer, and showed that it was effective in suppressing cavitation erosion on a specific valve opening when the erosive vortex cavitation occurred predominantly on a typical valve. Practically speaking, when the valve is operated near valve closure, erosion is an especially big problem. In this paper, therefore, the cavitation shock pressures and the highly erosive vortex cavitation arising around the butterfly valve with the tunnel-type buffer are compared with a typical valve counterpart by means of pressure-sensitive films and high-speed photography. It is found that the tunnel-type buffer is also useful in preventing erosion near valve closure.

Behavior of the Erosion-Standoff-Distance Curves for High-Speed Water-Jets in Air.

In order to clarify the mechanism of water-jet erosion, we systematically investigate the erosion-standoff-distance curves, the erosion-time curves, and detailed aspects of the eroded surface for two typical types of nozzles, i.e., conical and cylindrical nozzles. Clearly, the erosion-time curves are markedly different at the 1st peak, at the valley and at the 2nd peak. Hence, the mechanism of erosion is significantly changed with an increase in the standoff distance χ.