Zhansheng Liu

Interface characterization of a β-SiC whisker-Al composite

The nature of the interface, the orientation relationship of β-SiC whisker (β-SiCw)-Al combination, and the misfit dislocation structures at the β-SiCw-Al interfaces in a β-SiCw-Al composite have been observed by a high-resolution transmission electron microscopy (HRTEM). It was shown that quite a good bonding between the whisker and the aluminium was achieved due largely to the lattice match between SiC and aluminium at the interfaces. The orientation relationship between the whisker and the aluminium was {002}SiC∥{111}Al; 〈110〉SiC∥〈110〉Al. The interface was clean, faceted and semicoherent. The misfit dislocation cores were located in the whisker side away from the β-SiCw-Al interfaces.

Performance analysis of multi-leaf oil lubricated foil bearing

The traditional bearing applied in the turbo-pump for the hydraulic servo system is rolling element bearing. To satisfy the demand of the high rotating speed for turbo-pump, the oil lubricated foil bearing can be employed in the rotor system. For the working liquid of the servo system is oil and the rotor for the turbo pump is submerged in the hydraulic oil, the bearing has to operate in an oil-rich environment, where the air bearing cannot be employed. The theoretical analysis and numerical simulation are carried out in this study to investigate the static and dynamic characteristics of multi-leaf oil lubricated foil bearing. For the structure form of the multi-leaf foil bearing with five symmetrical arrangements, the foil deformation equation and the Reynolds equation are solved coupled by successive over relaxation method, where the Reynolds boundary condition is employed. Then the load capacity, lift-off speed and static equilibrium position are acquired. By deriving the dynamic deformation equation of the foil, the dynamic stiffness coefficients and damping coefficients are obtained based on the perturbation method. The effect of the rotating speed and perturbation frequency on dynamic characteristics is analyzed. It indicates that the load capacity of the multi-leaf foil bearing is smaller than that of the fixed geometry oil bearing without foil deformation, whereas the stability of the bearing is increased.

The Effect of Coulomb Friction in Foil Structure on the Static and Dynamic Performance of Bump Foil Journal Bearings

The Coulomb friction model for foil structure of bump foil journal bearing was developed by using finite element mothod. Through changing friction coefficient between top foil and bump foil, bump foil and bearing housing, the stiffness of foil strucure, the Coulomb friction model and linear spring model in earlier literatures were compared under various load distributions, and the effects of Coulomb friction on stiffness of bump foil was studied. The developed structural model was also integrated into foil bearing prediction code to evaluate the effects of the friction on the bearing static and dynamic performance.Copyright

Study on Dynamic Characteristics of Large Ship Rotor-Bearing System Based on an Advanced Water-Lubricated Rubber Bearing Model

The main point of this study is to propose an advanced model for large length-diameter ratio stern water-lubricated rubber bearing and conduct comparisons with the conventional water-lubricated rubber bearing model. The assumption of the advanced model was to divide the whole stern bearing into 4 sections equidistantly along the axial direction. Among each small section, the water film pressure distribution was solved and water film forces were generated. The dynamic model of the rotor-bearing system was built based on Finite Element Method (FEM) and solved by Newmark Method. It indicated that the predicted shaft centerline locus of the nodes located at the propeller and left water-lubricated rubber bearing rise significantly when using the new water-lubricated rubber bearing model.Copyright

Numerical Vibration Analysis of Supersonic Mixed-Compression Intake

Vibration analysis of a supersonic mixed compression intake is carried out under periodic back pressure by using one-way fluid structure interaction method. Steady flow and transient flow in the intake are simulated with finite volume method. The total pressure recovery and the exit flow distortion of the steady flow solution are very close to Anderson and Wong’s experiment results. The frequency components of pressure transferring to the intake cowl wall are analyzed using FFT method for the unsteady flow, and are compared with the natural frequencies of the intake structure to avoid the resonance of the intake structure. The dynamic response of the intake structure is simulated with finite element method considering geometric nonlinearity. The displacement history of structure is obtained, and the region with large amplitude of vibration is identified. It indicates that the forced vibration occurs in the intake under the excitation of the oscillating back pressure. The static displacement is two times of the cowl wall thickness, while the peak value of the dynamic displacement is 10% of the cowl wall thickness.Copyright

Analytical Forces Parameters Identification of Hybrid Journal Gas Bearing Based on Transfer Function

A mathematical model for the flow in the hybrid gas journal bearing is depicted by the transfer function in this paper. The average and modified parameters from the pressure distribution of multi-orifices aerostatic journal bearing are acquired to describe the characteristics accurately. The transient gas film forces of multi-orifices hybrid bearing is derived in the generalized form by analytical means, and then the dynamic characteristics coefficients are got by employing the Laplace transform. It indicates that the obtained forces can calculate the dynamic characteristics coefficients simply, quickly and accurately, which provide an efficient means for designing rotor-bearing systems.Copyright

Performance of a novel foil journal bearing with surface micro-grooved top foil

To improve the performance of foil bearing including load capacity and stability, the surface micro groove structure on top foil is proposed, which is possible with the development of surface micro machining process. The micro groove structure on top foil can be processed by laser surface process, chemical etching, mechanical grinding, etc. This paper proposes a novel bump-type gas foil journal bearing with surface micro-grooved top foil and investigates the influence of micro groove depth on bearing performance theoretically. A modified pressure governing equation is established with the consideration of gas rarefaction, and the performance of the bearing is analyzed based on numerical simulation. By considering the variation of top foil thickness for surface micro groove, the load capacities with and without gas rarefaction considered are obtained by finite difference method, where the 2D thick plate model is adopted for the top foil. By employing the perturbation method, the force coefficients of this type foil bearing are calculated. The results indicate this novel foil journal bearing with surface micro-grooved top foil can decrease the end leakage and increase pressure around load domain efficiently. The load capacity and dynamic properties are improved. Moreover, with the increment of micro groove depth, the load capacity and direct stiffness are reinforced further. For the foil journal bearing with a micro groove depth of 8 µm on top foil, the load capacity and direct stiffness increase by about 11.89% and 11.87%, respectively, compared with traditional foil journal bearing.

Lock-up characteristics of floating ring seals considering rotor whirling motion

This paper presents a lock-up condition with rotor whirling motion to predict the lock-up area for floating ring seals in liquid turbopumps. To calculate the hydrodynamic forces with arbitrary rotor motion, bulk flow model is modified with explicitly considering the squeeze velocity and temporal inertia. Finite element method is implemented to solve the transient nonlinear model. The effect of lock-up position on leakage and rotordynamic coefficients of floating ring seals is studied and shows insensitivity to eccentricity within moderate eccentric area (<0.5), which indicates that predicting lock-up area with the lock-up condition is feasible for application. With circular trajectory of rotors, the effects of rotor whirling radius and friction force on lock-up area are investigated. The results show that the lock-up area shrinks toward rotor whirling center, with the increase in the rotor whirling radius and decrease in the friction force. Further, a coupled method of rotor trajectory and floating ring seal lock-up area is presented to consider the interaction between rotors and floating ring seals. The results show that the effect of the interaction on the lock-up area is almost negligible, as lock-up position of floating ring seals has little effect on rotor trajectory. In addition, the effect of the seal configuration on the lock-up area is studied.

Effect of bump structural friction on the performance of bump foil bearing and rotor dynamic behavior: Experimental study:

Air foil bearings are a kind of self-acting lubricated bearing, which have potential applications in high-speed turbomachinery. Air foil journal bearings have simple structure that consists of top foil, bump foil, and bearing sleeve, and use gas as working fluid. However, the relative motion of top foil and bump foil, bump foil and bearing sleeve occurs when dynamic pressure is generated as long as there is spinning of shaft. Thus, the friction between each part of bearing should be considered when modeling. Many papers have theoretically shown the effect of Coulomb friction between top foil and top foil, bump foil and bearing sleeve on the static and dynamic performances of air foil journal bearing by developing many bump foil structural models. The results show that this foil structural Coulomb friction can significantly make bump foil stiffer. However, the improvement of the Coulomb friction effect through experiments is quite difficult and there is no study regarding this so far. The purpose of this paper is to certify the Coulomb effect on bearing performance by using experimental method. Two bump foil journal bearings are manufactured with different bearing sleeve surface roughnesses. Foil structure stiffness, bearing lift-off speed, and rotor dynamic behavior supported by two sets of bearings are measured and compared.

Feasibility study of a turbocharger rotor supported by air foil bearings with diameter of 17 mm focusing on rotordynamic performance

This paper presents the feasibility study of an oil-free turbocharger with journal bearing diameter of 17 mm. Rotordynamic performance of a turbocharger rotor, supported by two identical bump-type foil journal bearings and a pair of foil thrust bearings, is predicted and tested. High-pressure cold nitrogen is adopted to drive the turbocharger rotor. In rotordynamic analysis, the critical speeds and logarithmic decrement of the turbocharger rotor are predicted by employing the finite element method, in which the stiffness and damping coefficients of foil journal bearings and aerodynamic cross-coupled stiffness of the turbine are taken into account. Compared with experimental results, the accuracy of the prediction for rotordynamic analysis is verified for 7.82% marginal error of the critical speed. During the experiment, three foil journal bearings with different nominal clearances are manufactured and tested. The maximum stable operating speed reaches 105,000 r/min for this 17-mm-diameter oil-free turbocharger rotor system. Test results indicate the nominal clearance has a negative influence on threshold speed of sub-synchronous motions. When the nominal clearance is relatively small, the foil journal bearing could not lift off due to a large starting torque, while sub-synchronous motions would emerge under a large nominal clearance because of the reduced stiffness and damping coefficients of foil journal bearings.