Thomas Hagemann

Measurement and Prediction of the Static Operating Conditions of a Large Turbine Tilting-Pad Bearing Under High Circumferential Speeds and Heavy Loads

The identification results for the static performance characteristics of a large tilting-pad bearing in load between pad configuration are presented for specific bearing loads between 1.0 and 2.5 MPa and for circumferential speeds up to 79 m/s. The bearing is lubricated by spray-bars and can be described by the following specifications: Five pads, 0.23 nominal preload, 60% offset, 56° pad arc angle, 500 mm inner diameter, 350 mm pad length and 1.28 per mille relative bearing clearance. The axial oil flow is reduced by a fixed seal on both bearing edges which has a nominal radial clearance of 1 mm. The film pressure and the gap width are determined in the whole area of the sliding surface by an axial shift of the shaft. The bearing temperatures are measured by means of 100 thermocouples located 5 mm behind the sliding surface. The experimental results indicate that significant pad deformation occurs in circumferential and in axial direction. Also the effective supply temperatures are much higher than the nominal ones. According to the lubricant flow the sensor temperatures close to the spray-bars at the sliding surface rise about 20 K for 7 l/s and 40 K for 3.5 l/s at 3000 rpm. The temperatures are nearly constant between all pads and depend only on speed and not on load.The theoretical analyses of the bearing performance was accomplished with the bearing calculation software COMBROS. This code models the transition between laminar and turbulent flow and solves an extended Reynolds equation, the three dimensional energy equation of the film and the heat conduction equations of the shaft and the pads considering various boundary conditions due to the judgment of the user. Concerning the minimum film thickness, the maximum temperature on the sliding surface and the maximum film pressure only poor agreement was reached if the influence of the axial pad deformation was neglected. In advanced analyses a co-simulation between COMBROS and a structural mechanics software shows that an improvement of the prediction was achieved. The comparison of the measurement data and the advanced simulation shows very good agreement for the characteristic bearing parameters as well as for the local distributions of film pressure, temperature and film thickness in the whole operating range of the bearing. Further, the applied inlet mixing model for the lubricant supply process proves to be very suitable.Copyright

Predictions for run-up procedures of automotive turbochargers with full-floating ring bearings including thermal effects and different bearing setups

Automotive turbochargers feature a variety of boundary conditions influencing the operating behaviour during run-up procedures. As a novel aspect the influence of the thrust bearing and load on lateral vibration phenomena is investigated in detail for this application. The run-up predictions show that the thrust bearing and load have significant influence on the frequencies as well as on the magnitudes of the vibrations. In particular, the amplitudes of conical vibrational modes are damped. Further, the consideration of the hot gas boundary conditions is identified as decisive in order to predict the bearing parameters and consequently the vibrations precisely.

The Impact of Convective Fluid Inertia Forces on Operation of Tilting-Pad Journal Bearings

This paper presents a combination of experimental data, CFD analyses, and bearing code predictions on emergence of convective inertia fluid forces within the lube oil flow of tilting-pad journal bearings. Concordantly, experimental data and CFD analyses show a significant rise of local pressure at the transition between inlet and leading edge of tilting-pad, especially for high-speed applications with surface speeds up to 100 m/s. This effect can be related to convective inertia forces within fluid flow as cross-sections and flow character rapidly change at the pad entrance. An energy-based approach is implemented in the bearing code in order to provide enhanced boundary conditions for Reynolds equation considering this effect. As a result, predictions of bearing code achieved significant improved correlation with measured pressure distributions and CFD-data. Further, beside the local influence, a nonnegligible impact on characteristic parameters of bearing operation such as maximum temperature and stiffness and damping coefficients is observed. Finally, the results are critically analyzed and requirements to gain more distinct and reliable data are specified.

A Theoretical Study on Frequency Effects on Tilting-Pad Journal Bearing Dynamic Coefficients

The quantitative impact of pivot position, pivot support stiffness, and deformation on frequency dependency of a stiffness (K) and damping (C) KC matrix model for tilting-pad journal bearings is investigated. A theoretical model for prediction of frequency influences is introduced neglecting inertia effects. Predictions based on this model are validated with test data taken from literature. The results achieve good agreement with experimental data. A sensitivity analysis is performed providing the following key results: (i) all three investigated parameters possibly exhibit non negligible frequency influences, (ii) significant misinterpretations are possible if single influencing parameters on the operation conditions are disregarded or only roughly estimated, (iii) a frequency independent KCM model is suitable for off-center pivot position and sub-synchronous frequency ratios, (iv) a state-space model separating rotor and pad movement is necessary to model all theoretical possible effects by a linear model with frequency independent coefficients for a certain rotor speed.

A Study on Energetic and Hydraulic Interaction of Combined Journal and Thrust Bearings

A theoretical algorithm for the analysis of bidirectional interaction of combined journal and thrust bearings is presented. While many theoretical and experimental investigations on the operating behavior of single journal and thrust bearings can be found only few results for combined bearings are available. However, combined bearings interact by exchanging lubricant and heat which can affect significant changes of boundary conditions compared to a single bearing application. Therefore, a novel procedure is developed to combine two separate codes for journal and thrust bearings in order to iteratively determine the coupling boundary conditions due to the special design of the entire bearing unit. The degree of interaction strongly depends on the type of lubrication. In a first step predictions are verified by measurement data for a combined bearing with a fixed-pad offset-halves journal bearing and a directed lubricated tilting-pad thrust bearing. Experiments were conducted on a high speed test rig up to sliding speeds of 107 m/s at the mean radius of the thrust bearing. As expected the interaction of the two oil films is comparably low in the investigated speed and load range for this bearing design because of the active lubrication of both bearings and the low hydraulic resistance of the thrust bearing. In order to theoretically investigate interaction of thrust and journal bearings in more details a combined bearing with fixed-pad thrust parts lubricated exclusively by the side flow of the journal bearing is studied. A variation of modeling level, pocket design of the journal part, thrust load and rotating frequency provides the following results: (i) hydraulic and energetic interaction have to be modelled in details, (ii) the axial flow resistance of the pockets strongly influences flow rates and the pressure level at the interfaces (iii) the level of interface pressure rises with increasing thrust loads and decreasing rotor speed, (iv) the axial bearing clearance is rather of minor importance for the investigated bearing. Finally, improvements in order to predict operating conditions more precisely are comprehensively discussed.Copyright

Measurement and Prediction of the Dynamic Characteristics of a Large Turbine Tilting-Pad Bearing Under High Circumferential Speeds

The identification results for the linear dynamic coefficients of a K-C model for a large tilting-pad bearing in load between pad configuration are presented for specific bearing loads between 1.0 and 2.0 MPa and circumferential speeds of 39 m/s and 78 m/s. The bearing with a double tilting support is lubricated by spray-bars and can be described by the following specifications: Five pads, 0.23 nominal preload, 60% offset, 56° pad arc angle, 500 mm inner diameter, 350 mm pad length and 1.28 per mill relative bearing clearance. The test rig and the test procedure are described in detail. For the determination of the dynamic coefficients, a harmonic force is induced by two unbalance-vibration generators being attached to the frame of the rig. The relative movement between bearing and shaft is detected by proximity probes between bearing housing and shaft. The bearing forces are identified by measurements of the entire film pressure distribution in both circumferential and axial direction. In the post processing of the data, the dynamic force components are determined by a Fourier-analysis. This procedure is well-established for fixed-pad bearings. However, the uncertainties of its capabilities for tilting-pad bearings are investigated and discussed in this study. The theoretical analyses with the code COMBROS are based on a calculation of linear perturbations for the predicted static properties. The measurement and the calculation procedures show very good agreement for fixed-pad bearings. For a tilting-pad bearing the results differ with increasing frequency ratio and rotational speed. The experimental results show very poor frequency dependence in load direction and a very high one in the orthogonal direction. Theoretically, the influence of the frequency ratio is comparable in both planes and pretty low due to the pivot offset and the high effective preload. While good agreement for the measured and predicted K-C model can be observed at the lower rotational and vibrational frequency the correspondence becomes worse with the increase of both. The identification procedure uses the fluid film force to determine the dynamic coefficients and assumes that this is equal to the load on the bearing in every time step. The results indicate that the experimental identification is uncertain due to the elasticity of the double tilting bearing support and the initiated dynamic effects of it. An improvement of the measurement that also identifies the limitation of the current procedures as well as simplifications in the theoretical analyses are discussed.Copyright