On the tribological characteristics of dynamically loaded journal bearing with micropolar fluids

The addition of the additives to the lubricant oil to enhance the characteristics of the lubricant will influence the performance of the bearings. Based on the theory of micropolar fluids, the tribological characteristics of a dynamically-loaded journal bearing are numerically studied. Comparisons are made between the Newtonian fluids and the micropolar fluids. It is shown that for a dynamically-loaded journal bearing, the micropolar fluids yield an increase not only in the friction force, but also in the friction coefficient. In addition, the oil film pressure and the oil film thickness are obviously higher than that of Newtonian fluids.     

Heat transfer over a stretching surface with variable heat flux in micropolar fluids

Heat transfer over a stretching surface with uniform or variable heat flux in micropolar fluids is investigated in this Letter. The boundary layer equations are transformed into ordinary differential equations, and then they are solved numerically by a finite-difference method. The effects of the material parameter K, Prandtl number Pr, velocity exponent parameter m, and heat flux exponent parameter n on the heat transfer characteristics are studied. It is found that the local Nusselt number is higher for micropolar fluids compared to Newtonian fluids.     

Lubricant film flow and depletion characteristics at head/disk storage interface

The characteristics of lubricant film at head/disk interface(HDI) are essential to the stability of hard disk drives. In this study, the theoretical models of the lubricant flow and depletion are deduced based on Navier–Stokes(NS) and continuity equations. The air bearing pressure on the surface of the lubrication film is solved by the modified Reynolds equation based on Fukui and Kaneko(FK) model. Then the lubricant film deformations for a plane slider and double-track slider are obtained. The equation of lubricant film thickness is deduced with the consideration of van der Waals force, the air bearing pressure, the surface tension, and the external stresses. The lubricant depletion under heat source is simulated and the effects of different working conditions including initial thickness, flying height and the speed of the disk on lubricant depletion are discussed. The main factors that cause the lubricant flow and depletion are analyzed and the ways to reduce the film thickness deformation are proposed. The simulation results indicate that the shearing stress is the most important factor that causes the thickness deformation and other terms listed in the equation have little influence. The thickness deformation is dependent on the working parameter, and the thermal condition evaporation is the most important factor.     

Starved elastohydrodynamic lubrication theory: application to emulsions and greases

In classical fluid lubrication the film thickness is mainly determined by entrainment velocity and oil viscosity. At high pressure, elastic deformation occurs changing the local geometry: this is the elastohydrodynamic lubrication regime (EHL). When a limited amount of lubricant is available to the contact, a component failure due to lubricant starvation can result. A new starvation model is presented, using the amount of oil on the surfaces as the key parameter controlling starvation. It is successfully applied to describe the contacts lubricated with multi-phase fluids such as emulsions and greases, which combine starvation with a very complex rheology.     

Experimental investigation of time-dependent cavitation in an oscillatory squeeze film

The occurrence of time-dependent cavitation and tensile stress in an oscillatory oil squeeze film were investigated experimentally. The test apparatus was a simple thrust bearing consisting of two parallel circular plates separated by a thin viscous oil film. During the test, one plate was at rest while the other (transparent) oscillated in a direction normal to its surface. This test configuration was chosen to avoid the rotational motion and complicated geometry of a squeeze film journal bearing. The frequency of oscillation was in the range of 5 to 50 Hz and was controlled by an electro-magnetic exciter. The process of cavity formation and its subsequent development was recorded by a high-speed video camera. Concomitant pressure in the oil film was measured both within and without the cavitation region. It was found that both tensile stress and cavities existed in a squeeze film under certain working conditions.     

Experimental investigation of time-dependent cavitation in an oscillatory squeeze film

The occurrence of time-dependent cavitation and tensile stress in an oscillatory oil squeeze film were investigated experimentally. The test apparatus was a simple thrust bearing consisting of two parallel circular plates separated by a thin viscous oil film. During the test, one plate was at rest while the other (transparent) oscillated in a direction normal to its surface. This test configuration was chosen to avoid the rotational motion and complicated geometry of a squeeze film journal bearing. The frequency of oscillation was in the range of 5 to 50 Hz and was controlled by an electro-magnetic exciter. The process of cavity formation and its subsequent development was recorded by a high-speed video camera. Concomitant pressure in the oil film was measured both within and without the cavitation region. It was found that both tensile stress and cavities existed in a squeeze film under certain working conditions.     

Exploring micropolar effects in thin film lubrication

The micropolar theory for fluids is adopted to probe the effect of the microstructure and microrotation on lubrication features in thin film lubrication. Micropolarity will give rise to an increase in the equivalent viscosity that subsequently leads to a better lubrication. The effective viscosity grown with micro polarity is very close to that of experiment in a preceding work.     

Exploring micropolar effects in thin film lubrication

The micropolar theory for fluids is adopted to probe the effect of the microstructure and microrotation on lubrication features in thin film lubrication. Micropolarity will give rise to an increase in the equivalent viscosity that subsequently leads to a better lubrication. The effective viscosity grown with micro polarity is very close to that of experiment in a preceding work.     

Degeneration of flow pattern in acousto-elastic flow through sharp-edge microchannels

Acoustic streaming (AS) is the steady time-averaged flow generated by acoustic field, which has been widely used in enhancing mixing and particle manipulation. Current researches on acoustic streaming mainly focus on Newtonian fluids, while many biological and chemical solutions exhibit non-Newtonian properties. The acoustic streaming in viscoelastic fluids has been studied experimentally for the first time in this paper. We found that the addition of polyethylene oxide (PEO) polymer to the Newtonian fluid significantly altered the flow characteristics in the microchannel. The resulting acousto-elastic flow showed two modes: positive mode and negative mode. Specifically, the viscoelastic fluids under acousto-elastic flow exhibit mixing hysteresis features at low flow rates, and degeneration of flow pattern at high flow rates. Through quantitative analysis, the degeneration of flow pattern is further summarized as time fluctuation and spatial disturbance range reduction. The positive mode in acousto-elastic flow can be used for the mixing enhancement of viscoelastic fluids in the micromixer, while the negative mode provides a potential method for particle/cell manipulation in viscoelastic body fluids such as saliva by suppressing unstable flow.     

酸度、温度对液压油和润滑油荧光特性的影响

通过分析HyJet V磷酸酯液压油的环境(润滑油等)及干扰因素(酸度和温度等)的影响,研究荧光法检测飞机用液压油泄漏的可行性。利用荧光分光光度计,获得不同酸度和温度时HyJet V磷酸酯液压油、Jet Oil II润滑油、2197润滑油的荧光特性,实验结果：HyJet V磷酸酯液压油、Jet Oil II润滑油、2197润滑油的荧光峰分别在362,405和456 nm。温度对HyJet V磷酸酯液压油影响较小;Jet Oil II润滑油、2197润滑油荧光强度随温度升高逐渐减小。酸度增加时,HyJet V磷酸酯液压油荧光峰从370 nm渐变为362 nm,荧光强度逐渐减小;Jet Oil II润滑油荧光峰保持405 nm不变,荧光强度逐渐减小;2197润滑油荧光峰由456 nm处红移至523 nm,出现双荧光峰,直至荧光特性消失。结果表明：在环境及干扰因素的影响下,HyJet V磷酸酯液压油的荧光特性基本保持不变,并且区别于Jet Oil II润滑油、2197润滑油。因此,实验证明利用荧光法检测HyJet V磷酸酯液压油泄漏是可行的。     

刮膜蒸发器内非牛顿流体流场特性数值模拟

刮膜蒸发器是通过旋转刮板强制成膜,可实现高黏度非牛顿流体类物料平稳蒸发的新型高效蒸发器.蒸发器内流体的流动、分布与传输机制直接决定了蒸发器的蒸发效率与功耗.不同于现有研究主要基于牛顿流体开展,本文针对不同黏度的非牛顿流体,建立蒸发器三维计算流体动力学模型,系统研究了蒸发器内的流场分布特性和成膜机理.结果表明:低黏非牛顿流体的流场分布特性和牛顿流体类似,物料可在壁面形成均匀且连续的液膜;随着黏度的增加,液膜的均匀性和连续性逐渐变差.通过对流场分布与传输形式的研究,结合液膜分布、速度分布、剪应变率分布,以及黏度分布进行对比分析发现,蒸发器内部结构与运行状态形成的剪切场与黏度分布是蒸发器良好成膜的关键.此外,提出对刮板前缘进行弯折可辅助高黏流体液膜铺展,并对最佳弯折角度进行探索.本研究为刮膜蒸发器的设计和应用提供了理论指导与依据.     

刮膜蒸发器内非牛顿流体流场特性数值模拟

刮膜蒸发器是通过旋转刮板强制成膜,可实现高黏度非牛顿流体类物料平稳蒸发的新型高效蒸发器.蒸发器内流体的流动、分布与传输机制直接决定了蒸发器的蒸发效率与功耗.不同于现有研究主要基于牛顿流体开展,本文针对不同黏度的非牛顿流体,建立蒸发器三维计算流体动力学模型,系统研究了蒸发器内的流场分布特性和成膜机理.结果表明:低黏非牛顿流体的流场分布特性和牛顿流体类似,物料可在壁面形成均匀且连续的液膜;随着黏度的增加,液膜的均匀性和连续性逐渐变差.通过对流场分布与传输形式的研究,结合液膜分布、速度分布、剪应变率分布,以及黏度分布进行对比分析发现,蒸发器内部结构与运行状态形成的剪切场与黏度分布是蒸发器良好成膜的关键.此外,提出对刮板前缘进行弯折可辅助高黏流体液膜铺展,并对最佳弯折角度进行探索.本研究为刮膜蒸发器的设计和应用提供了理论指导与依据.     

Linear and non-linear control techniques applied to actively lubricated journal bearings

The main objectives of actively lubricated bearings are the simultaneous reduction of wear and vibration between rotating and stationary machinery parts. For reducing wear and dissipating vibration energy until certain limits, one can use the conventional hydrodynamic lubrication. For further reduction of shaft vibrations one can use the active lubrication action, which is based on injecting pressurized oil into the bearing gap through orifices machined in the bearing sliding surface. The design and efficiency of some linear (PD, PI and PID) and a non-linear controller, applied to a tilting-pad journal bearing, are analysed and discussed. Important conclusions about the application of integral controllers, responsible for changing the rotor-bearing equilibrium position and consequently the “passive” oil film damping coefficients, are achieved. Numerical results show an effective vibration reduction of unbalance response of a rigid rotor, where the PD and the non-linear P controllers show better performance for the frequency range of study (0-80 Hz). The feasibility of eliminating rotor-bearing instabilities (phenomena of whirl) by using active lubrication is also investigated, illustrating clearly one of its most promising applications.     

Study on the structures of fluid flows in the annular space formed by a submerged tilting pad journal bearing

In this study, the Taylor vortices of the film flow in the bearing clearance and so-called cavity flow between pads in a submerged tilting pad journal bearing were visualized by means of a tracer method. The effects of pad arc extent and pad inclination (from leading to trailing edges) on fluid flows, especially on the structures of Taylor vortices and cavity flow were investigated. The critical Taylor number of the film flow increased with an increase in pad inclination slightly. The pitch of array of the Taylor vortex rings at the critical Taylor number was, however, scarcely influenced by the pad inclination. The pitch was likely fixed by the mean clearance over the pad. Two-dimensional cavity flow field (in the central section perpendicular to the rotation axis) between pads was measured by a Particle Image Velocimetry to investigate the interaction of film flow and cavity flow between pads. The Taylor vortices out of the preceding pad were almost carried over the cavity region into succeeding pad, and hardly mixed with the cavity flow. This phenomenon is important in relation to the oil exchange between the film and cavity flows.     

交变流动下间隙密封耦合气体轴承的特性研究

气体轴承是回热式热机的一项关键技术,它是利用气体代替润滑油作为润滑剂,在轴与轴承套之间构成气膜,是避免运动面与静止面直接接触的较为理想支撑元件。将间隙密封与气体轴承相结合,可以在实现密封的同时消除接触磨损。本文利用ANSYS Fluent对具有77 kW(声功)设计输出能力的活塞进行其气体轴承与间隙密封耦合特性的数值模拟与分析,指导该新型气路结构的优化设计,并验证其在大功率自由活塞斯特林发动机中应用的可行性。     

荧光法检测磷酸酯液压油外泄漏平台设计与实现

通过采用LS-55型荧光分光光度计对HyJet Ⅴ磷酸酯液压油以及Jet Oil Ⅱ、2197润滑油的荧光特性进行分析,提出了一种采用荧光法来检测磷酸酯液压油外泄漏的方法.通过选取合适的光源、滤光片、倍增管等器件,搭建了磷酸酯液压油外泄漏检测平台.并且通过实验验证了平台的可行性,为荧光法检测磷酸酯型液压油外泄漏的实际应用奠定了基础.     

Second law analysis of the flow of two immiscible micropolar fluids between two porous beds

This work investigates the effect of entropy generation rate within the flow of two immiscible micropolar fluids in a horizontal channel bounded by two porous beds at the bottom and top. The flow is considered in four zones. Zone IV contains the flow of viscous fluid in the large porous bed at the bottom, zone I and zone II contain the free flow of two immiscible micropolar fluids, and zone III contains the flow of viscous fluid in the thin porous bed at the top. The flow is assumed to be governed by Eringen’s micropolar fluid flow equations in the free channel. Darcy’s law and Brinkman’s model are used for flow in porous zones, namely, zone IV and zone III, respectively. The closed form expressions for entropy generation number and Bejan number are derived in dimensionless formby using the expressions of velocity, microrotation and temperature. The effect of physical parameters like a couple stress parameter and micropolarity parameter on velocity, microrotation, temperature, entropy generation number and Bejan number are investigated.     

ANALYSIS OF THE MULTIPLE-SOLUTION RESPONSE OF A FLEXIBLE ROTOR SUPPORTED ON NON-LINEAR SQUEEZE FILM DAMPERS

The multiple-solution response of rotors supported on squeeze film dampers is a typical non-linear phenomenon. The behaviour of the multiple-solution response in a flexible rotor supported on two identical squeeze film dampers with centralizing springs is studied by three methods: synchronous circular centred-orbit motion solution, numerical integration method and slow acceleration method using the assumption of a short bearing and cavitated oil film; the differences of computational results obtained by the three different methods are compared in this paper. It is shown that there are three basic forms for the multiple-solution response in the flexible rotor system supported on the squeeze film dampers, which are the resonant, isolated bifurcation and swallowtail bifurcation multiple solutions. In the multiple-solution speed regions, the rotor motion may be subsynchronous, super-subsynchronous, almost-periodic and even chaotic, besides synchronous circular centred, even if the gravity effect is not considered. The assumption of synchronous circular centred-orbit motion for the journal and rotor around the static deflection line can be used only in some special cases; the steady state numerical integration method is very useful, but time consuming. Using the slow acceleration method, not only can the multiple-solution speed regions be detected, but also the non-synchronous response regions.     

Experimental results on a hydrostatic bearing lubricated with a magnetorheological fluid

Magnetorheological fluids can be used as a smart lubricant as a result of the fact that its properties can be changed with the use of a magnetic field. Local flow resistance and local pressure can be generated by applying a local magnetic field. This work presents a hydrostatic bearing in which the pressure profile of a conventional hydrostatic bearing is recreated with solely the use of a magnetic field and a magnetorheological fluid. The magnetic field is applied only locally at the outer edges of the bearing with the use of an electromagnet. The principle is demonstrated with the use of an experimental setup and a model from literature.     

Adhesion and fingering in the lifting Hele-Shaw cell: Role of the substrate

The lifting Hele-Shaw cell (LHSC) is used to study adhesion as well as viscous fingering. In the present paper we report a series of observations of development of the interface for different viscous fluids, both Newtonian and non-Newtonian, in a LHSC operated at a constant lifting force. Glass and perspex are used as the plates in two different sets of experiments. The objectives are 1) to measure the time required to separate the plates as a function of the lifting force and 2) to note the force above which viscous fingering appears. We find that for the Newtonian fluids, the plate separation time follows a universal power law with the lifting force, irrespective of fluid and substrate. The non-Newtonian fluids too, with proper scaling obey the same power law. The appearance of fingering, however, depends on the properties of the fluid as well as the substrate. We suggest a modified form of the capillary number which controls the onset of fingering; this new quantity, termed the “fingering parameter” involves the dielectric constants of the substrate and fluid in addition to the viscosity and surface tension.