Multi-Physics Modelling of Wet Seals in Two-Phase Flow

Pumps are a crucial technology to modern society as they ensure a constant access to one of our most essential resources: water. Everything from production of goods and services to transportation to basic living conditions are dependent on a constant supply of water from these pumps. To keep up with growing demands, pumps must constantly improve in terms of efficiency and reliability. One such improvement that has received increasing attention over the last decade is the mechanical face seal. Data from major pump manufacturers suggest that up to 50% of pump failures are attributed to seal failure, and as a result this represents a critical component in the pump that must be improved.

In this work a steady-state one-dimensional axisymmetric numerical model of mechanical face seals in two-phase flow is developed, which can describe the behavior of mechanical seals under single- and two-phase flow conditions. The mathematical model is developed with a monolithic approach, coupling the complex thermal interactions between fluid film, seal rings, and phase change in a coupled non-linear system of equations. In this coupled thermal model the temperature fields of the seal rings and fluid film, the heat fluxes between seal rings and fluid film, the enthalpy of the fluid film, and the vapor mass fraction of thefluid film are linked and solved using Newtons method.

This method represents two primary novel contributions: 1) a semi-analytical approachto the thermal coupling between fluids and solids, and 2) implementation of saturation functions to couple temperature, enthalpy, and pressure. The  coupled thermal model is linked to the flow characteristics and force equilibrium, making it possible to simulate leakage, film thickness, and stability of the seal under single- and two-phase flow conditions. The modelshows a strong coupling between two-phase flow and seal performance. Vaporization can significantly increase the opening forces, create unstable regions of operation with a negative,film stiffness, and affect the temperature field throughout the seal. These findings are inagreement with other results from literature on mechanical face seal, suggesting the model works well under the assumptions used to derive it. By studying the boiling locus and stability curves it is used to describe the mechanisms of seal instabilities such as puffing. Finally the application of the model is demonstrated through case study on a seal in two phase flow with varying closing forces.