Phase Change in Liquid Face Seals II—Isothermal and Adiabatic Bounds With Real Fluids

Abstract

Phase change effects in parallel and tapered liquid face seals are studied analytically. Both an isothermal and adiabatic model of low Reynolds number flow are considered by numerical integration of the descriptive equations for a real fluid. Real fluid thermodynamic properties are calculated for each step, using a computer program for the steam tables or thermodynamic properties of the fluid considered. Examples are presented for water. The general conclusions are: 1. For low leakage rate the isothermal model is more accurate and for high leakage rates the adiabatic model is more accurate. 2. Both parallel models, ordinarily neutrally stable with a liquid, yield the same general conclusions about stability. If the sealed fluid is near enough to saturation conditions, there will exist generally two values of the film gap, h, which yield the same separating force under a given set of operating conditions. For a given speed, face excursions about the larger value are stable, but excursions about the lower value are unstable, either growing to the larger h if displaced apart or collapsing if displaced together. 3. The transient of collapse is described by the adiabatic model which predicts a catastrophic collapse and then either failure or explosive return to a larger value of h. 4. Converging seals (ordinarily stable with a liquid at some given value of h) may become unstable, the phase change effect dominating the behavior and giving rise to collapse as described above. 5. The mass leakage rate is reduced significantly below the all liquid value when boiling occurs.

Temperature Measurements by Infrared Thermography in the Interface of a Radial Face Seal

“…Boiling can occur in such zones where liquid is saturated (Hughes et al, 1978;Hugues and Chao, 1980). The complex coupling of thermal distortions of the faces, heat dissipation in the film and phase change affect greatly the load capacity and the leakage rate of mechanical seals.…”

Study of the fluid film vaporization in the interface of a mechanical face seal

“…Thus a given film thickness can be reached for two different load values (or balance ratio). However, this situation does not necessarily correspond to an unstable situation, as discussed in previous papers [5][6][7][8] where only steady-state simulations were performed. Brunetière and Apostolescu [30] showed that the dimensionless temperature rise in a liquid lubricated mechanical seal is a function of a dimensionless thermal loading parameter, the sealing number Se:…”

“…Several authors [5][6][7][8] studied phase change in steady state and demonstrated that the opening force due to the fluid pressure in the interface, when plotted versus the film thickness, can exhibit an unstable zone. This means that this force can increase with the film thickness, leading to an opening of the seal.…”

“…If the leakage is small enough, the liquid is assumed to turn instantaneously to vapor at a given radius [5]. Otherwise, it is necessary to consider a continuous boiling where the mass fraction of vapor in the fluid varies gradually from 0 to 1 [6]. This second method is obviously more accurate but more complicated.…”

Frictional heat transfer regularity of the fluid film in mechanical seals

“…Li investigated the temperature field of the fluid film by means of FEM on the assumption that the properties of the fluid film were constant along the sealing dam [1] . Hughes and Chao analyzed the phase change of the fluid film in the liquid face seals and neglected the heat transfer from the outer surfaces of sealing rings into their surrounding sealed liquid [2] . Lebeck regarded the heat convection coefficient from the surfaces of the seal rings to the high pressure sealed liquid as a constant and built an actual seal model [3] .…”