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Design and Analysis of CFD Experiments for the Development of Bulk-Flow Model for Staggered Labyrinth Seal
Get PDFNowadays, bulk-flow models are the most time-efficient approaches to estimate the rotor dynamic coefficients of labyrinth seals. Dealing with the one-control volume bulk-flow model developed by Iwatsubo and improved by Childs, the “leakage correlation” allows the leakage mass-flow rate to be estimated, which directly affects the calculation of the rotor dynamic coefficients. This paper aims at filling the lack of the numerical modelling for staggered labyrinth seals: a one-control volume bulk-flow model has been developed and, furthermore, a new leakage correlation has been defined using CFD analysis. Design and analysis of computer experiments have been performed to investigate the leakage mass-flow rate, static pressure, circumferential velocity, and temperature distribution along the seal cavities. Four design factors have been chosen, which are the geometry, pressure drop, inlet preswirl, and rotor peripheral speed. Finally, dynamic forces, estimated by the bulk-flow model, are compared with experimental measurements available in the literature
Numerical Modeling of Thermally-Induced Vibration in Rotor Caused by Light-Rub Against Brush Seal
No full textClearance is of paramount importance for turbomachinery
manufacturers to meet today’s aggressive power output,
efficiency, and operational life goals. To minimize leakages,
there are various seal types used, and new sealing concepts are
in development. Because of their inherent flexibility and
compliance, brush seals are capable of significantly reducing the
leakage, and allow sufficient geometrical margins to
accommodate design and operational variations of
turbomachines.
Brush seals can be assembled at very tight or zero radial
clearance or even with interference on the rotor to minimize the
leakage. This means that the risk of contact between the rotor and
the seal bristles exists, especially in case of zero clearance or
interference. If the contact occurs, a hot-spot develops on the
rotor and this may cause the vibration to diverge, resulting in a
synchronous instability, the so-called Newkirk effect. The
friction forces generated by rotor-to-stator rubs often cause a
shaft thermal bow whose main effect on the machine dynamic
behavior is a progressive change of the synchronous (1X)
vibration.
The development of analytical tools able to model this
phenomenon is therefore important to assess the rotordynamic
stability during the design phase and avoid excessive vibrations
which may have severe impact on the operability and on the
mechanical integrity of the machine.
The objective of this paper is the development of a
numerical model to analyze the dynamic behavior of real
turbomachines subject to thermally-induced vibration caused by
light-rub of the rotor against brush seals. The model developed
in the paper is based on the work of Bachschmid et al. [1]: the
dynamics is analyzed in the frequency domain using the standard
rotordynamic model, whereas the heat transfer analysis, to
calculate the temperature distribution and the associated thermal
bow, is studied in the time domain. The contact analysis has been
deeply revised, aiming at estimating suitable normal and
tangential force and the friction heating generated by the contact.
The bow determined by the thermal conditions has been
reproduced using suitable bending moments, which have been
applied to the beam elements in the rotordynamic model
Unconventional Techniques for the Analysis of Experimental Spiral Vibrations
No full textThe occurrence of spiral vibrations in rotating machines is a well-known but not very common phenomenon. However, this kind of shaft vibration, usually caused by light rubs between rotating and stationary parts, may give rise to a slow considerable increase of the amplitude of the synchronous (1X) vibration. Owing to the rubs, the normal contact forces cause a friction-induced thermal bow, which in turn determines rather slow changes in amplitude and phase of the 1X vibration vector. The curve described in a polar plot by the tip of the vibration vector is a spiral. The occurrence of expansive vibrations may cause serious damage.
Many studies about this malfunction are focused on the stability analysis of spiral vibrations. Simplified or rather rigorous thermal models can be used to evaluate the friction-induced thermal bow of the shaft and the slow continuous migration of the hot spot generated on the external surface of the rotor. However, owing to the complexity of the problem, some basic parameters of the thermal models can be affected by a significant uncertainty.
This paper shows some unconventional techniques that can provide useful information for optimizing the rotor-to-stator contact modelling as well as for tuning some critical parameters of the thermal models that affect the velocity with which the hot spot moves around the circumferential surface of the shaft. The effectiveness of these techniques is shown by means of the analysis of the experimental spiral vibrations detected in a steam turbine power unit
Experimental results on condition monitoring of railway infrastructure and rolling stock
No full textThis paper presents the results of a research project in which concepts were developed
and demonstrated for the health monitoring of the rolling stock (traction equipment) and
of the railway infrastructure (track and overhead equipment). A prototype monitoring
system was installed on a e464 locomotive, and results have been gathered across a time
span of 28 months from December 2014 to March 2017
Behavior of a Tilting–Pad Journal Bearing With Different Load Directions
No full textTilting–pad journal bearings (TPJBs) are widely used in
rotating machinery to support the rotors efficiently at elevated
speeds under light/heavy loadings. Because of the importance
of this machine component, many authors have published
several theoretical and experimental studies, to evaluate the
influence of clearance, lubricant temperature, oil flow-rate and
thermal effects on behavior of TPJB. However, the
investigations of the influence of loading direction on
properties of TPJB are very limited. In bearing models as well
as in experimental tests, the load is often assumed along the
vertical direction only and the geometry of the bearing is the
same for each pad, which corresponds to an axial symmetry of
the bearing. This paper presents first a theoretical analysis of
the influence of the load direction on both the static and the
dynamic behavior of a five-pad TPJB with a non-uniform
clearance: that is, the different bearing configurations in the
range between load-on-pad (LOP) and load-between-pads
(LBP) are investigated. Then, the analytical results are
compared with experimental measurements. The tests were
performed with a nominal diameter of 100 mm and a length–
to–diameter ratio of 0.7, using a suitable test-rig, in which it is
possible to apply the static load in any direction. The procedure
for the estimation of the bearing geometry from experimental
measurement of the non-uniform clearance profile is also
described. The results show that the load direction has
considerable effects on both the static and the dynamic
characteristics of the TPJB. Besides, the influence of load
directions is stronger on the dynamic characteristics of tilting
pad bearing than on the static ones
Analysis of Dynamic Behavior of a Non-Nominal Five-Pad Tilting Pad Journal Bearing
No full textBecause tilting-pad journal bearings (TPJB) are more stable and efficient than conventional
bearings, they have been commonly applied to many rotating machinery applications. Most of
the studies about steady state and dynamic characteristics of TPJBs are usually evaluated by
means of thermo hydrodynamic (THD) models assuming nominal dimensions for the bearing.
However machining errors could lead to actual bearing geometry and dimensions different from
the nominal ones. In particular for TPJB the asymmetry of the bearing geometry is the principal
cause of unexpected behavior. In this paper a theoretical analysis on dynamic characteristics of a
five-pad TPJB is investigated with non-nominal geometry, that is, different thickness for each
pad. The dynamic coefficients of a five-pad TPJB with a nominal diameter of 100mm, length-todiameter
ratio (L/D) of 0.7 are evaluated versus rotor rotational speed, load direction and static
load. Then, the analytical results of the non-nominal bearing are compared to those of the
nominal one
Diagnostics of Rolling Element Bearings by Means of the Higuchy Fractal Dimension
No full textIn the field of rolling element bearing, the degradation of
bearing health could be detected by means of suitable damage
indexes. Band-Kurtosis index, that is the kurtosis value of the
band-filtered signal, is often assumed. The critical point of this
approach is the selection of a suitable filter band. In the paper,
the use of a chaos metrics, namely the Higuchi fractal
dimension as damage indicator is described. The trend of this
index is compared with the common approach of band-kurtosis
indicator for an experimental case of a rolling element bearing
in which the defect developed until a permanent failure
Effect of energy equation in one control-volume bulk-flow model for the prediction of labyrinth seal dynamic coefficients
No full textThe influence of sealing components on the rotordynamic stability of turbomachinery has become a key topic because the oil and gas market is increasingly demanding high rotational speeds and high efficiency. This leads the turbomachinery manufacturers to design higher flexibility ratios and to reduce the clearance of the seals. Accurate prediction of the effective damping of seals is critical to avoid instability problems; in recent years, ‘‘negative-swirl” swirl brakes have been used to reverse the circumferential direction of the inlet flow, which changes the sign of the cross-coupled stiffness coefficients and generates stabilizing forces. Experimental tests for a teeth-on-stator labyrinth seal were performed by manufacturers with positive and negative pre-swirl values to investigate the pre-swirl effect on the cross-coupled stiffness coefficient. Those results are used as a benchmark in this paper. To analyse the rotor-fluid interaction in the seals, the bulkflow numeric approach is more time efficient than computational fluid dynamics (CFD). Although the accuracy of the coefficients prediction in bulk-flow models is satisfactory for liquid phase application, the accuracy of the results strongly depends on the operating conditions in the case of the gas phase. In this paper, the authors propose an improvement in the state-of-the-art bulk-flow model by introducing the effect of the energy equation in the zeroth-order solution to better characterize real gas properties due to the enthalpy variation along the seal cavities. The consideration of the energy equation allows for a better estimation of the coefficients in the case of a negative pre-swirl ratio, therefore, it extend the prediction fidelity over a wide range of operating conditions. The numeric results are also compared to the state-of-the-art bulk-flow model, which highlights the improvement in the model
An Experimental Study of Nonlinear Oil-Film Forces in a Tilting-Pad Journal Bearing
No full textJournal bearings have been widely used in high-speed
rotating machinery. The dynamic coefficients of oil-film force
affect the machine unbalance response and machine stability.
The oil-film force of hydrodynamic bearing is often
characterized by a set of linear stiffness and damping
coefficients. However the linear oil-film coefficients with
respect to an equilibrium position of the journal are inaccurate
when the bearing system vibrates with large amplitudes due to a
dynamic load. The study on nonlinear oil-film forces is still rare
and most papers are confined to theoretical analyses. The
purpose of this paper is to derive some new non-linear force
models (28-co., 24-co. and 36-co. models) to identify these
dynamic coefficients based on experimental data. The
fundamental test model is obtained from a Taylor series
expansion of bearing reaction force. Tests were performed with
a nominal diameter of 100mm and a length–to–diameter ratio
of 0.7 using a suitable test rig in which it is possible to apply
the static load in any direction. The results show that these three
models are feasible to identify the oil-film forces in which the
second-order oil-film coefficients received from the 24-co.
model are more stable compared to those of other two nonlinear
models
Dynamic characterization of milling plant columns
No full textBuilding structures the most rigid as possible is a common practice in order to guarantee extremely precise tool spindle positioning. However, very rigid structures are usually very lightly damped and this could lead to some dynamic problems in case of resonance conditions. In the case of milling machines, due to the variety of milling programs and of cutting tool types, the machine tool is often forced by broad frequency spectrum excitation, quite impossible to be predicted at a design stage. Thus a near-resonance working condition could easily appear.
In recent years, to increase damping whilst maintaining very rigid structures, metal foam sandwiches have been used. However, it is quite rare to find comprehensive experimental studies demonstrating the effectiveness of this solution on a complete structure, such as a column of a milling machine.
In this paper, two columns of a milling machine built using different technologies, i.e. classic metalworking and metal foam sandwiches, are compared. On this purpose experimental modal analysis was applied
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