Predictions of the thermo elastic deformation of dry gas seal rings in the hydrodynamic lubrication regime

Dry gas seals represent a significant advancement in turbo machinery due to their ability to handle high pressures and speeds without the use of external sealing fluids, such as oil or water, thus reducing contamination and increasing reliability. Despite their widespread use, internal working mechanisms are not fully understood to date, in particular regarding fluid film thickness prediction, which is an essential component of the seal design. The axial deflection of the rotating and stationary rings in a dry gas seal affects the development of the fluid film formed between the ring faces of the seal, influencing the performance of the seal during its operation, as well as leakage of the seal when it is at rest. The hydrodynamic and hydrostatic pressure fields of the fluid film, together with temperature gradients in the rings, induce axial deflection of these components. This in turn modifies the pressure field developed in the film. This paper focuses on establishing a methodology to couple the deformation field and the dynamic behavior of the fluid film (pressure and temperature fields) through numerical computations. Analytical relationships are employed to obtain the thermo-elastic deflection of the seal rings in the axial direction and this distortion is used in the numerical methodology to accelerate the prediction of the seal behavior. The coupled seal ring-fluid film dynamic system with 11° and 15° spiral angle is stable because the axial deflection calculated from numerical analysis produces a converging radial taper in the direction of the flow (producing a net opening force). An important result of this work is that the predicted magnitude of the axial deflection (as a result of pressure and temperature effects) under thermal and pressure loads on the stationary and rotating rings is smaller but of the same order of magnitude as the fluid film thickness.

Nonlinear Control of Hydrostatic Thrust Bearing Using Multivariable Optimization

This research work is focused on the nonlinear modeling and control of a hydrostatic thrust bearing. In the proposed work, a mathematical model is formulated for a hydrostatic thrust bearing system that includes the effects of uncertainties, unmodelled dynamics, and nonlinearities. Depending on the type of inputs, the mathematical model is divided into three subsystems. Each subsystem has the same output, i.e., fluid film thickness with different types of input, i.e., viscosity, supply pressure, and recess pressure. An extended state observer is proposed to estimate the unavailable states. A backstepping control technique is presented to achieve the desired tracking performance and stabilize the closed-loop dynamics. The proposed control technique is based on the Lyapunov stability theorem. Moreover, particle swarm optimization is used to search for the best tuning parameters for the backstepping controller and extended state observer. The effectiveness of the proposed method is verified using numerical simulations.

Quantitative Analysis of Reynolds and Navier–Stokes Based Modeling Approaches for Isothermal Newtonian Elastohydrodynamic Lubrication

When simulating elastohydrodynamic lubrication, two main approaches are usually followed to predict the pressure and fluid film thickness distribution throughout the contact. The conventional approach relies on the Reynolds equation to describe the thin lubricant film, which is coupled to a Boussinesq description of the linear elastic deformation of the solids. A more accurate, yet a time-consuming method is the use of computational fluid dynamics in which the Navier–Stokes equations describe the flow of the thin lubricant film, coupled to a finite element solver for the description of the local contact deformation. This investigation aims at assessing both methods for different lubrication conditions in different elastohydrodynamic lubrication (EHL) regimes and quantify their differences to understand advantages and limitations of both methods. This investigation shows how the results from both approaches deviate for three scenarios: (1) inertial contributions (Re > 1), i.e., thick films, high speed, and low viscosity; (2) high shear stresses leading to secondary flows; and (3) large deformations of the solids leading to inaccuracies of the Boussinesq equation.

Lubrication and Fluid Film Study on the Vane Tip in a Vane Pump Hydraulic Transmission Considering a Fluid and Structure Interaction

A mathematical modeling approach to determine fluid film thickness on the vane tip in a vane pump transmission is developed. The transmission is based on a double-acting vane pump with an additional output shaft coupled to a floating ring. Owing to the floating ring design, the internal viscous friction helps to drive the output shaft, whereas the friction is turned into heat in a conventional vane pump. To study the mechanical efficiency, it is crucial to investigate the fluid film thickness between the vane tip and the ring inner surface. The modeling approach in this study takes the interactions between vane radial motion and chamber pressure dynamics into consideration, without using a computational fluid dynamics approach. The lubrication on the vane tip is considered as elasto-hydrodynamic lubrication and the fluid film thickness calculation is based on the Hooke lubrication diagram. Results show that the developed simulation model is capable of revealing the fluid film thickness change and vane radial motion in different operation regions. Sensitivity studies of several parameters on the minimum fluid film thickness are also presented.

The Challenge of Feedback in Fluid Power Tribotronic Control Systems

The hunt for increased efficiency and reliability of fluid power components have entailed a great deal of research on loss and wear mechanisms by means of computational tribology simulation in the last decades. The vast amount of theoretical work within tribology necessitates validation of the simulation models. The aim to validate such models has in recent years increased interest in sophisticated tribological measurement technology. A next level, for tribology in fluid power, is the integration of modelling, analysis and measurement technology with control to achieve active tribology, namely fluid power tribotronics. In this paper the challenge of feedback for tribotronic control loops in fluid power are addressed. Fundamental questions are in this regard to what extend do we need information? Is it necessary to measure fluid film thickness absolutely or dynamically, or do we even need in-situ information on the tribological joints? These questions are analysed in this paper by use of a dynamic lubricated journal-bearing model. From this analysis a conclusion on the oil whirl limit is found, which reveal an interesting perspective and potential risk in the application of feedforward in tribotronic control of lubricated journal-bearings. Finally, a discussion is concluded with the submission of relevant open research directions for fluid power tribotronic control systems.

Research on the Dynamic Characteristics of Mechanical Seal under Different Extrusion Fault Degrees

In order to explore the dynamic characteristics of the mechanical seal under different fault degrees, this paper selected the upstream pumping mechanical seal as the object of study. The research established the rotating ring-fluid film-stationary ring 3D model, which was built to analyze the fault mechanism. To study extrusion fault mechanism and characteristics, different dynamic parameters were used in the analysis process. Theoretical analysis, numerical simulation, and comparison were conducted to study the relationship between the fault degree and dynamic characteristics. It is the first time to research the dynamic characteristics of mechanical seals in the specific extrusion fault. This paper proved feasibility and effectiveness of the new analysis method. The fluid film thickness and dynamic characteristics could reflect the degree of the extrusion fault. Results show that the fluid film pressure fluctuation tends to be more intensive under the serious extrusion fault condition. The extrusion fault is more likely to occur when the fluid film thickness is too large or too small. Results illustrate the opening force is affected with the fluid film lubrication status and seal extrusion fault degrees. The fluid film stiffness would not always increase with the rotating speed growth. The seal fault would occur with the increasing of rotating speeds, and the leakage growth fluctuations could reflect the fault degree.

A Study of Hole-Entry Grooved Surface Hybrid Spherical Journal Bearing Operating With Electrorheological Lubricant

The performances of the tribo-pairs are greatly influenced by introducing the grooved surfaces. Developments of the newer type of lubricants have made a great impact on the performance of fluid film bearings. This article investigates the non-Newtonian behavior of electrorheological lubricant on the performance of grooved hybrid spherical journal bearing. The effect of different arrangements of grooves, i.e., partially grooved or fully grooved on the bearing surface, has been studied. The finite element method is used to numerically simulate the results. Furthermore, a parametric study is performed for optimizing the groove attributes. The present work demonstrates that the different grooved arrangements have a substantial influence on the bearing performance. It is revealed that the provision of grooves on the bearing surface decreases frictional losses and enhances the stiffness coefficients of the bearing. Furthermore, numerically simulated results indicate that the electrorheological lubricant enhances the value of minimum fluid film thickness and the stiffness coefficients (S¯xxandS¯yy) of spherical hybrid journal bearing. Improved bearing performance can be achieved by using the optimized grooved attributes together with the electrorheological lubricant.

Molecular dynamics simulations of diffusive properties of stressed water films in quartz and clay grain contacts

<p><strong>Molecular dynamics simulations of diffusive properties of stressed water films in quartz and clay grain contacts</strong></p><p>Floris S.R. Teuling<sup>1</sup>, Marthe G. Guren<sup>2</sup>, François Renard<sup>2</sup>, Martyn R. Drury<sup>1</sup>, Suzanne J.T. Hangx<sup>1</sup>, Helen E. King<sup>1</sup>, Oliver Plümper<sup>1</sup>, Henrik A. Sveinsson<sup>2</sup></p><ol><li>Utrecht University, Department of Earth Sciences, Princetonlaan 8a, 3584 CB Utrecht, the Netherlands</li> <li>University of Oslo, Departments of Geosciences and Physics, The Njord Centre, box 1048, Blindern, 0316 Oslo, Norway</li> </ol><p>Hydrocarbon extraction can increase effective normal stresses in geological reservoirs, potentially inducing deformation and seismicity<sup>1</sup>. The kinetics of time-dependent creep processes that could persist long after production has ended, such as pressure solution and stress corrosion, are poorly quantified. These processes can be limited by diffusion efficiency at stressed grain contacts, which depends strongly on fluid film thickness as well as interfacial and surface energies. The diffusive properties of stressed fluid films between various crystallographic surfaces of the rock forming minerals clay and quartz are critical to predict long term deformation of reservoir. Due to the small length scales of grain contacts, experimental data on these quantities are difficult to acquire. Therefore, we use molecular dynamic simulations to elucidate the physico-chemical behaviour of fluid films at different mineral interfaces.</p><p>We apply large-scale classical molecular dynamics in LAMMPS to numerically resolve fluid film behaviour in grain contacts. The silicate-water system is modelled using a modified ClayFF force field<sup>2</sup>. A β-quartz block was placed within a water-filled nanopore with either hydroxylated  β-quartz or basal illite clay surfaces as walls.  This geometry was built using the software packages Atomic Simulation Environment, Ovito and Packmol. The system was first equilibrated using an NVT thermostat and an NPT barostat for tens of picoseconds under conditions of 8 MPa fluid pressure and a temperature of 100°C. Then, a force was applied on the quartz block, corresponding to 10-200 MPa normal contact stress, such that a thin water film is squeezed at the interface between two grains. Self-diffusion constants were calculated by mean square displacements and velocity autocorrelation in films at steady state thicknesses.</p><p>Simulations reach a steady state after several nanoseconds run time. Under reservoir conditions, fluid film thicknesses are reduced to less than one nanometre. Two to three layers of adsorbed water remain in the grain contact, a result consistent with reported fluid film properties for grain contacts in upper crustal systems. Our results quantify how various juxtaposed quartz surfaces and quartz-clay interfaces influence fluid film thickness, self-diffusion and the dynamics of the water layer, which allows for constraining the kinetics of pressure solution creep in sandstone reservoirs.</p><p>Acknowledgements</p><p>This project received funding from the DeepNL programme</p><ol><li>Pijnenburg, R. P. J., Verberne, B. A., Hangx, S. J. T., & Spiers, C. J. (2019). Intergranular clay films control inelastic deformation in the Groningen gas reservoir: Evidence from split‐cylinder deformation tests. Journal of Geophysical Research: Solid Earth, 124.</li> </ol><p> </p><ol><li>Cygan, R. T., Liang, J. J., & Kalinichev, A. G. (2004). Molecular models of hydroxide, oxyhydroxide, and clay phases and the development of a general force field. The Journal of Physical Chemistry B, 108(4), 1255-1266.</li> </ol>

Tribological Behavior of Hydraulic Cylinder Coaxial Sealing Systems Made from PTFE and PTFE Compounds

Current trends concerning hydraulic cylinder sealing systems are aimed at decreasing energy consumption which can be materialized by minimizing leaks and reducing friction. The latest developments in the field of materials and sealing system geometries as well as modern simulation possibilities allow maximum performance levels of hydraulic cylinders. Reducing friction is possible by hydro-dynamic separation of the sliding and sealing points already at very low velocities and by using materials, such as plastomers, from polytetrafluoroethylene (PTFE) (virgin PTFE and filled PTFE). It is within this context that this paper discusses a theoretical and experimental study focused on the tribological behavior of coaxial sealing systems mounted on the pistons of hydraulic cylinders. It presents a methodology for the theoretical determination of the lubricant film thickness between the cylinder piston and the seal. The experimental installation used for measuring fluid film thickness is presented, and the results obtained under various working conditions are compared to the theoretical ones. For the analyzed working conditions related to pressure, speed, and temperature, the paper concludes with a set of criteria for the selection of the optimum seal material so as to maximize energy efficiency.