Multi-scale multi-physics model and hybrid computational framework for predicting dynamics of hydraulic rod seals

In this research a multi-scale multi-physics (MSMP) seal model is developed. It solves macro-scale deformation mechanics, macro-scale contact, micro-scale contact of surface asperities, micro-scale fluid mechanics in the sealing zone and micro-scale deformation mechanics of the sealing edge in a strongly coupled manner. The model takes into account the surface roughness, mixed lubrication, cavitation and two phase flow, transient squeeze film effects, dynamic operation with temporally varying sealed pressure and rod velocity as well as the effect of macro/micro/nano scale viscoelasticity of the seal. A hybrid finite element-finite volume-statistical computational framework is developed to solve the highly coupled multi-physics interactions of the MSMP model efficiently.;To investigate the micro/nano scale viscoelastic response of surface asperities, atomic force microscopy (AFM) experiments are conducted on polyurethane seal samples. The micro/nano scale elastic moduli for polyurethane produced a surprising finding showing several local moduli to be varying within 2 orders of magnitude (as high as 3000 MPa) higher than the bulk of the polymer (43 MPa). The phase plots of these micro-scale regions showed significant differences in local stiffness and adhesion. To extract the viscoelastic response of the individual asperities, time resolved interaction force measurements were performed. Significant differences in both, the magnitude of the moduli and the relaxation time scales, of individual asperities were observed. Understanding the nano-scale viscoelastic properties of the seal asperities is believed to be a valuable addition to the fundamental understanding of multi-scale sealing dynamics.;With the comprehensive MSMP model developed, key seal performance characteristics were analyzed. The results confirmed the mixed lubrication in the sealing zone, and that seal roughness plays an important role in determining seal behavior. Critical seal roughness, above which the seal leaks, was found for the transient operation. During instroke, fluid films were found to be thicker than those during the outstroke, which promotes non-leaking. Cavitation was shown to take place during outstroke which also helps in reducing leakage. It was found that the shear stress on the rod is primarily due to contacting asperities. With time changing sealed pressure, it was seen that the time variations in fluid pressure distributions are governed by a complex dual dependence on changes due to time varying sealed pressure boundary condition and those due to hydrodynamic effects produced by time varying rod velocity.;Comprehensive viscoelastic MSMP model was also used to analyze high pressure - high frequency sealing applications. A new phenomenon of "secondary" contact was observed in the rod seals. A surprising finding of increase in the dry contact pressures in the secondary contact with time at a constant operating pressure is attributed to the viscoelastic creep induced pressures surrounding the sealing edge. Sealing characteristics in such applications are found to be significantly different than those observed previously. Viscoelasticity induces significant differences in Poiseuille flow and friction force from one cycle to the next. It is also found that with reduction in cycle frequency, the differences in fluid flow and contact variables from cycle to cycle are reduced. This is because, the viscoelastic seal gets more time to relax and come to internal equilibrium before its next cycle. Leakage characteristics were found to be very different for the same seal if it is operated in an application with different cycle frequency. This shows that the seals need to be designed by taking the relaxation time scales of the seal's polymer and cycle frequencies of application into consideration.;The study also showed that reducing temperature or increasing the relaxation time constant of the polymer, would reduce the tendency of secondary contact for a given frequency of operation. It revealed that for a given frequency, there is a "critical temperature" above which secondary contact will occur and the consecutive cycles may not be considered repetitive. Similarly, for a given operating temperature, there exists a "critical frequency" of operation below which the seal will exhibit secondary contact and the contact pressure distributions will change significantly. These findings show that the seals performance can be manipulated by controlling its temperature or relaxation time constants. With this, the new seal designs where local temperature control is achieved by embedding resistive heating circuits can then be envisioned for controlling the seal response. (Abstract shortened by UMI.)...