Research on Two-Phase Flow in the Gap and Sealing Performance of Single Metal Seals

Based on thermal-fluid-solid coupling results of single metal seals, the three-dimensional non-uniform liquid film models for the gaps of single metal seals were established under various conditions. Liquid film characteristics and mud intrusion behavior on end faces of single metal seals were studied by using multiphase model (Volume of Fraction, VOF), User-defined functions (UDF) and dynamic mesh technique. The influences of environmental pressures and rotational speeds on two-phase flow and sealing performance of single metal seals were discussed under steady and dynamic conditions. The mechanism of vibration conditions on the mud intrusion characteristics of the sealing interface was explored. The results showed that the liquid film pressures of the wedge angle area and chamfer area were basically equal to the lubricating oil pressure and environmental pressure under steady state conditions respectively. The differential pressure of liquid film in the sealing area gradually decreased in the radial direction. The differential pressure of liquid film at the same radial position decreased with the increase of environmental pressures, but changed little with the rotational speeds. Because of the chamfer structure and the relative motion between the two-phase fluids, the fluid vortex generates in the chamfer area so that the mud on the outer diameter side was easily affected by the vortex and rotational effects and invades the chamfer area and the outside of sealing area. The intrusion degree of the mud on the outer diameter side decreased with the increase of the environmental pressure or the decrease of the rotational speeds. The volume fraction of the mud increased first and then decreased along the direction of film thickness due to the variation of the fluid rotational speeds and the viscosity resistance near the wall of the rotor. As a consequence, the outer side of the end face of the rotor was more susceptible to mud invasion and wear. Under the steady state conditions, the leakage rate of lubricating oil and the opening force of the liquid film increased with the environmental pressure, while the friction force of the liquid film decreased. With the increase of rotational speeds, the leakage rate of the lubricating oil and the friction force of the liquid film increased, and the opening force of the liquid film was almost unchanged. Affected by axial periodic vibration of the stator, a high-pressure area (higher than the lubricating oil pressure) and a low-pressure area (lower than the environmental pressure) were alternately changed in the sealing area under low environmental pressures. This would cause the mud on the outer diameter side to invade and extend into the sealing area and gradually accumulate, so the possibility of mud entering the bearing system through the sealing gap was greatly increased during long-term operation. Under high environmental pressures and periodic vibration, the mud intrusion area did not change significantly, and the amount of mud intrusion and extrusion in the sealing area tended to be equal after short time oscillation with the increase of rotational speeds. As the environmental pressures increased, the mean values of dynamic leakage rate and opening force increased, and the mean values of friction force decreased. The mean values and amplitudes of dynamic opening force did not change with the increase of rotational speeds, while the mean values and amplitudes of friction force increased. This research can provide a theoretical basis for the design and development of high-reliability sealing system of drill bit bearing.