Abstract
Numerical and experimental analyses of the static and dynamic characteristics of liquid annular seals with axially partial helical grooves were carried out to investigate the effects of the helix angle γ of grooves. In this analytical model, the effect of γ corresponded to that of the number of thread grooves n because the helical groove pitch along the axial length was set to be constant. The numerical solution and experimental procedures were applied in the same manner as in the previous study on the partially helically grooved seal. The numerical results qualitatively agreed with the experimental results, demonstrating the validity of the numerical analysis. The leakage flow rate Q for a large γ was higher than that for a small γ because an increase of γ diminished the effect of the fluid energy loss at the steps between the groove and the land regions on the reduction of Q. It is noted that all types of the partially helically grooved seals produced smaller Q than the smooth surface seal. For a small concentric whirling motion of the rotor, the tangential dynamic force Ft for the partially helically grooved seals was lower than that of the smooth surface seal and it became lower with increasing γ, resulting a large magnitude of the tangential reaction force for a rotor forward whirling motion. This was expected because a large γ enhances an effect equivalent to the negative inlet swirl motion induced by the helical grooves, which facilitate spiral flow in the direction opposite to that of the spinning rotor. The radial dynamic force Fr became lower with increasing γ, which was attributable to the fact that a large γ enhances the “Lomakin effect” produced at the downstream end of the helical grooves and causes an increase in the magnitude of the radial reaction force. The results obtained within the present analysis showed that the helical grooves with a combination of a large γ (= 29.6°) and a small gal/L (=0.2), in which gal is the axial length of a helically grooved section in a seal stator and L is the seal length, yielded a better dynamic characteristics for the stability margin of pumps because of the suppression of the forward whirling motion of the rotor with large radial and tangential dynamic reaction force.
