PERFORMANCE OF ELECTRORHEOLOGICAL (ER) FLUID LUBRICATED HYBRID JOURNAL BEARING SYSTEM

Abstract

Turbomachinery plays a vital role in industries worldwide. Due to competitiveness and rapid technological advancements, turbomachines received considerable attention from the scientific and industrial community in order to extract better performance and higher reliability. The dynamic behavior of these machines is greatly dependent on the fluid film bearings. The fluid film bearing is used for its long life expectancies, low power consumption levels, and versatile dynamic behavior. This resulted in many new developments in the area of fluid-film bearing technology. As a consequence, the non-recessed hybrid journal bearing system, which combines both hydrostatic and hydrodynamic action, have come up as an alternative to self-acting hydrodynamic and rolling element bearings. The non-recessed (hole-entry type) hybrid journal bearings provide excellent performance characteristics such as improved load carrying capacity at low and high speeds, increased minimum fluid-film thickness, large fluid film damping and relative simplicity in manufacturing. These features make them more suitable for engineering applications such as high speed turbo-machinery, machine tool spindles, precision grinding spindle, reactor coolant pumps, liquid rocket engine, etc. Despite the significant advancement in lubrication technology and advent of meticulous deign procedures, bearings do fail while in operation with serious consequences, particularly in large installations such as power plants, rolling mills etc. On-line control and in-situ remedy is essential in such real situations. Accordingly many tribologists and practicing lubrication experts around the world are now a days involved in design of the journal bearing system using an entirely new design approach based on electrorheological (ER) fluids to suit the requirement of high speed and heavy load operation. A very recent method for doing this is the creation of 'smart' journal bearings lubricated with electrorheological (ER) fluids. An electrorheological fluid is a type of colloidal suspension consisting of dielectric particles dispersed in insulating oil. The marvelous feature of an ER fluid is that it can solidify into a jelly-like state almost instantaneously when subject to an externally applied field with moderate strength, say 1 kV/mm, moreover its stiffness can vary proportionally to the strength of the electric field and the change is reversible, once the applied field is removed, the original flow state can be recovered immediately. If such a fluid is used to lubricate a journal bearing system, it is expected that the imposition of an electric field between the rotor and the stator will cause an alteration in the dynamic properties of the journal bearing and makes it possible to control the journal bearing at faster rate. Perhaps the most exciting benefit of ER fluid technology is that it makes possible a whole new generation of machines with adjustable rotor dynamic characteristics. In other words, adjustments to the machine without removing it from service and physically modifying the rotor and associated items. The ER Fluid in Bearing permits changes to rotor dynamic properties through relatively simple means, such as adjusting the lubricant supply pressure or viscosity (via electric field control). This bearing also allows independently adjustable damping and stiffness characteristics, providing more flexibility for machinery designers. A scan of available literature indicates that the use of ER fluid for lubrication in journal bearing are mainly confined to plain hydrodynamic journal bearing system. A study dealing with the effect of ER fluid lubrication for non-recessed hole-entry hybrid journal bearing system may be quite useful to the bearing designer and to the best of author knowledge no such study has yet been reported in literature. Therefore, in order to bridge the gap in the literature, a study on the performance of non-recessed hole-entry hybrid journal bearing lubricated with ER fluid is undertaken in this thesis. The present study covers the different flow control devices such as capillary, orifice and constant flow valve. Fluid film journal bearings are normally manufactured circular in geometry. Circular film journal bearings suffer from fluid induced instability at high speed. Fluid induced instability is a condition that is caused by rotor interaction with the surrounding fluid. It can produce large amplitude, self-excited vibrations capable vi of damaging many machine components. These fluid instabilities are known as whirl and whip phenomena. A significant improvement in the instability threshold speed margin was obtained by making changes in the geometry of the fluid film journal bearings, subsequently the non-circular fluid film journal bearing were developed to overcome the problem of fluid induced instability such as whirl and whip. The available literature on the analysis of non-circular journal bearing system is quite an extensive one and is by and large limited to hydrodynamic journal bearing. It has been observed that very few or limited studies exists for hydrostatic/hybrid journal bearing systems. Further the bearing and journal surfaces are generally assumed to be smooth. However, in actual practice, the geometric shape of any engineering surfaces is controlled by the characteristics of the finishing processes employed to produce them. Close analysis of these surfaces reveals that, even after the most careful polishing, they are still rough to a microscopic scale. In reality, the surface roughness heights are typically of the same order of magnitude as that of the fluid-film thickness in a journal bearing system and hence the performance of journal bearing system gets altered. Therefore, for a realistic prediction of the journal bearing performance characteristics data, consideration of surface roughness effects in the analysis is more appropriate. Thus, the inclusion.of the surface roughness effect in the analysis of journal bearing problems attracted the attention of many researchers and has become as an important activity over the last few years. Fluid film journal bearings often operate in a misaligned condition that can substantially alter the bearing performance. The misalignment of a bearing can result from non-central loading, elastic deflection of the shaft under load, thermal distortion of the shaft, and bearing housing supports and manufacturing errors. Therefore, the importance of the misalignment effect on the bearing performance can hardly be overlooked. Generally, misalignment can have a considerable and usually detrimental effect on the performance of a hybrid journal bearing. It is rare condition that a hybrid journal bearing is not misaligned. Thus, to generate an accurate bearing characteristic data consideration of journal misalignment effect in the analysis becomes more appropriate. vii During the last few decades many investigations, pertaining to the performance characteristics of non-recessed hybrid journal bearings have been carried out and reported in literature. An exhaustive survey of non-recessed journal bearings reveals that the studies dealing with non-recessed hybrid journal bearings focus attention on different aspect such as effect of misalignment, flexibility, non-Newtonian behavior of lubricant, hole size and selection strategies, turbulence, fluid inertia, surface roughness, running in wear etc. To the best of author's knowledge, there is no literature available which addresses the combined influence of ER fluid lubrication, surface roughness and journal misalignment effects on the performance of non-recessed hybrid journal bearing systems. Therefore, the present work is planned to bridge this gap in literature and evaluate the performance of non-recessed hybrid journal bearing system with the following objectives: I. To study the influence of ER fluid lubrication on the performance characteristics of orifice, capillary and constant flow valve compensated hole-entry hybrid journal bearing system. II. To study the combined influence of ER fluid lubrication, surface roughness and misalignment effects on the performance characteristics of a hole-entry hybrid journal bearing system. Ill. To evaluate the comparative performance of a two-lobe four recessed hybrid journal bearing vis a vis the to circular four recessed hybrid journal bearing and the effect of hole-size in hole-entry hybrid journal bearing. The effects of ER fluid lubrication have been studied by considering the different values of ER fluid parameter 'B' and voltage on the ER fluid. The surface roughness orientations such as transversely, isotropically and longitudinally oriented roughness patterns (i.e. the effect of surface pattern parameter, y) and the effect of surface roughness parameter(A), when it is present on both of the surfaces (i.e. P-6 =0.5) are studied in the present work. The journal misalignment is accounted by defining a pair of non-dimensional misalignment parameters ro and 1/7. The bearing performance results have been presented for circular, two-lobe and three-lobe symmetric/asymmetric hole-entry hybrid journal bearing viii configuration considering orifice, capillary and constant flow valve as flow control devices. A theoretical model is used to account and analyze the individual and/or combined influence of surface roughness, journal misalignment, and non-Newtonian behavior of the ER lubricant. The flow of an incompressible, isoviscous and Newtonian lubricant through the clearance space of a smooth journal bearing system is governed by the Reynolds equation. The ER fluid under the influence of electric filed behaves as a non-Newtonian fluid. The generalized Reynolds equation governing the flow of lubricant with variable viscosity is used in this study to account for the ER fluid lubrication between the bearing surfaces under applied electric filed. Continuous Bingham-like fluid model, a rheological equation which is continuous and can be applied over the entire flow domain, proposed by Christopher and Tichy is used in this analysis to account for the behaviour of ER fluid under applied electric field. The generalized Reynolds equation is applicable for the smooth bearing surfaces. In order to include the rough bearing surfaces analysis the average Reynolds equation for three-dimensional area distributed surface roughness effects as proposed by Patir and Cheng is used by in the present analysis The average Reynolds equation in the present analysis is expressed in terms of flow factors, viscosity functions and average and nominal fluid-film thickness. A non-dimensional factor known as offset factor(oL ) has been defined to account for the change in profile of the bearing (two-lobe and three-lobe). Also a pair of non-dimensional parameters ()and (F) are defined to account for the journal misalignment effect. The fluid film pressure field is obtained by solving the modified Reynold's equation. In the present work the finite element method has been used to solve the problem with suitable iterative schemes. The modified Reynold's equation is solved by taking the flow of lubricant through restrictor as a constraint, along with relevant boundary conditions. The system equation corresponding to the modified average Reynold's equation, after adjustment for the continuity of flow through the restrictors becomes non-linear for an orifice restrictor for both Newtonian and non-Newtonian lubricants. This non-linear equation, in the present work, is solved using the Newton-Raphson iterative method. A computer program has been developed using an appropriate solution algorithm and suitable iterative schemes. Converged solution for pressure for Newtonian and non-Newtonian ER fluid problem is achieved by solving modified average Reynolds equation. The results for the bearing performance characteristics of symmetric and asymmetric hole-entry journal bearing system compensated with orifice, constant flow valve and capillary restrictors have been presented so as to draw conclusions regarding individual and combined influence of ER fluid lubrication, surface roughness and journal misalignment effects on the bearing performance. The performance characteristics of two-lobe four recessed hybrid journal bearing and influence of size of hole on the performance of hole-entry hybrid journal bearing systems are also presented. Bearing static performance characteristics parameters in terms of load carrying capacity (To ), nominal minimum fluid-film thickness (Tin un) and bearing flow (a), and dynamic performance characteristics parameters which includes stiffness SS (i, j = x, z) and damping L7,7 (i,j = x, z) coefficients and stability threshold speed margin (eV have been computed for the generally used representative values of bearing operating and geometric parameters. The variation in the bearing performance characteristics parameters have been presented for various values of ER fluid parameter (B), voltage (V), surface roughness parameter (A), surface pattern parameter (7), journal misalignment parameters (ro,(7) and offset factor (63. The results presented in this study indicates that the performance characteristics of hole-entry hybrid journal bearing configurations are significantly affected by the ER fluid lubrication. The present study further reveals a significant interaction between the influences of ER fluid, surface roughness, journal misalignment and effect of geometry on the performance characteristics of a symmetric/asymmetric non-recessed hybrid journal bearing configurations. Thus, the influences of these effects should be studied together rather than individually for the realistic prediction of bearing performance characteristic data