グローブ形調節弁のキャビテーションに関する研究 : 第3報,コンタードプラグにおける渦放出とキャビテーション初生

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Cavitation inception in control valves should be accounted for in terms of not only the time-averaged distribution of pressure but also the instantaneous structure of vortices formed in the flow, since the pressure of the vortex core can easily attain vapor pressure as velocity gradient of the shear layer increases. In a range of low and intermediate opening of contoured plug valves, instantaneous cavitation pictures indicate that the cavitation bubbles are first formed around the leading edge of the plug seating rim and then along the outer shear layer of the throttled jet. The inception of the cavitation bubbles may be related to formation and shedding of vortices in the flow immediately before the inception. Such cavitation-free structures of the valve flow, the growth rate of the vortex and boundary layer separation at the seating rim are studied by means of a 2-D numerical analysis of unsteady Navier-Stokes equations. By replacing the separation bubbles at the rim with Rankine vortices where the vapor pressure is first reached, a criterion of the cavitation inception is presented in a similarity law form in terms of the Strouhal number, the boundary layer thickness, and the valve discharge coefficient. The prediction of the inception agrees very well with the experimental data at valve inlet pressurs between 0.4 and 5 MPa for the valves of diameter between 2 and 6 inches.
1993-06-25