It has been showed directly or indirectly that the lubricant is possible to be sucked back into the mechanical seal with dimples or grooves in the existing studies.This phenomenon,termed "outlet suctio" in this paper,may decrease the leakage.Further studies are still required for this phenomenon and its mechanism.A three-dimensional fluid numerical simulation,based on the multiphase flow cavitation model in Fluent,was developed for the mechanical seal with inclined elliptical dimples.The causes of outlet suction and the influence of the outlet suction on leakage were studied by changing the differential pressure between the inner and outer diameter and the rotation rate.Firstly,in order to ensure the accuracy of calculation model,the fluid factor was calculated for the flow pattern check,the grid independence was tested for the grid size,and the simulation result was compared with the reference.Secondly,in the numerical simulation experiments,the phenomenon of outlet suction was observed directly in the streamline plots on the r-θ section of calculation unit beside the outlet section and the velocity distribution contours in radial direction on the outlet section.Thirdly,the pressure and radial velocity distribution on r-θ section was discussed.The results showed that the outlet suction occurs because the pressure close to the seal outlet falls below the external atmospheric pressure.This pressure decrease is caused by the hydrodynamic effect near the divergence gaps of dimples.The increase of hydrodynamic effect enhances the outlet suction.When the decreased pressure reaches the cavitation pressure,the cavitation occurs.The cavitation effect hinders the increase of outlet suction.Finally,the effect of Δp and n on outlet suction,and the effect of outlet suction on the leakage decrease were studied based on the analysis of the out flow rates and outlet suction flow rates.The leakage decreases because of the outlet suction,even to zero.The outlet suction caused by inclined elliptical dimples can be employed to decrease the leakage of mechanic seals.

The accurate calculation of boundary shear stress is very important to the deep understanding of sediment transportation and river evolution. The current research is limited to straight and mildly curved channels, while for a sharply-curved channel, the flow is complex accompanied by transverse circulation and large transverse slope which was affected by the dual effect of gravity and centrifugal force. As the boundary shear stress was affected by many factors, the applicability of differentcalculation methods requires further study. The distribution of three-dimensional flow velocity and dynamic-staticpressure difference were monitored by ADV as well as Preston tube in a 180° sharply-curved flume under the subcritical flow condition. The typical features of transverse flow circulation, redistribution of longitudinal flow velocity and turbulence kinetic energy were analyzed. Based on the analyses, four empirical formulae and k-ε model were selected to calculate the boundary shear stress on the control section of the flume. The computational results of turbulence kinetic energy method and Preston tube method agree well with the k-ε numerical simulation not only in distribution pattern but also in magnitude. Therefore, the above three methods are feasible for the calculation of boundary shear stress in a sharply-curved channel. The boundary shear stress in bed and bank slope of the whole flume was calculated by numerical simulation. The results show that the value of boundary shear stress is small and evenly distributed in thestraight section following the flow entrance, then gradually increases and exhibits non-uniformity in the bending section. Finally it reaches the maximum near the bank slope in the straight section in front of the outlet. On the transverse section, the boundary shear stress is well distributed along the bed while strongly fluctuated near the toe of slope in which the flow is complex with large circulation. Under the effect of the bend, the maximum boundary shear stress in the transverse section gradually shifted from convex bank to concave bank, in order to keep up with the mainstream. The maximal boundary shear stress of the flume is located in the convex bank of 110° cross-section of the bend and the concave bank of the cross-section 0.5 m downstream off the bend outlet. By changing the rate of flow but keeping the downstream water depth constent, the overall distribution of the boundary shear stress is similar and shows the characteristic of strong water flow along straight line while weak water flow along curve line. These researches provided support for flow shear transport mechanism, forecast of river evolution and safety management in the sharply-curved channel. © 2017, Editorial Department of Advanced Engineering Sciences. All right reserved.