Edge Groove Research Articles

Extended Three-Dimensional Thermo-Hydrodynamic Model of Radial Foil Bearing: Case Studies on Thermal Behaviors and Dynamic Characteristics in Gas Turbine Simulator

Environment-friendly microturbomachinery has its broad current and future applications in fuel cells, power generation, oil-free industrial blowers and compressors, small aero propulsions engines for missiles and small aircrafts, automotive turbo chargers, etc. Air foil bearings (AFBs) have been one of the popular subjects in recent years due to ever-growing interests in the environment-friendly oil-free turbomachinery. AFBs have many noticeable attractive features compared to conventional rigid-walled air/gas bearings such as improved damping and tolerance to minor shaft misalignment and external shocks. In addition, the low viscosity of air or gas allows very low power consumption even at high speeds. A turbine simulator mimicking 50 kW power generation gas turbine was designed. The turbine simulator can generate the same thermodynamic conditions and axial thrust load as the actual gas turbine. In this paper, the 3-D thermo-hydrodynamic (THD) model developed for single radial AFB was further extended to the turbine simulator configuration by extending the solution domain to the surrounding structures including two plenums, bearing sleeve, housing, and rotor exposed to the plenums. In addition, a computational fluid dynamic (CFD) model on the leading edge groove region was developed for better prediction of inlet thermal boundary conditions for the bearing. Several case studies are presented through computer simulations for hydrodynamically preloaded three-pad radial AFB in the hot section. It is found that both bearing and rotor should be provided with cooling air to maintain the temperature of both the rotor and top foil below 300 °C. It is also found that the higher thermal contact resistance between the rotor and hot impellers reduces the axial temperature gradient of the rotor. Dynamic performance of the bearing was evaluated using the linear perturbation method for operation at elevated temperature. The softening effect of the bump foil at elevated temperature results in a decrease of both stiffness and damping coefficients compared to the values at room temperature.

Rotordynamic Characteristics of a Five Pad, Rocker-Pivot, Tilting Pad Bearing in a Load-on-Pad Configuration; Comparisons to Predictions and Load-Between-Pad Results

Rotordynamic data are presented for a rocker-pivot tilting pad bearing in load-on-pad (LOP) configuration for (345–3101 kPa) unit loads and speeds from 4000 rpm to 13,000 rpm. The bearing was directly lubricated through a leading edge groove with five pads, 0.282 preload, 60% offset, 57.87 deg pad arc angle, 101.587 mm (3.9995 in.) rotor diameter, 0.1575 mm (0.0062 in.) diametral clearance, and 60.325 mm (2.375 in.) pad length. Measured results were reported for this bearing by Carter and Childs (2008, “Measurements Versus Predictions for the Rotordynamic Characteristics of a 5-Pad, Rocker-Pivot, Tilting-Pad Bearing in Load Between Pad Configuration,” ASME Paper No. GT2008-50069) in the load-between-pad (LBP) configuration. Results for the LOP are compared with predictions from a bulk-flow Navier–Stokes model (as utilized by San Andres (1991, “Effect of Eccentricity on the Force Response of a Hybrid Bearing,” STLE Tribol. Trans., 34, pp. 537–544)) and to the prior LBP results. Frequency effects on the dynamic-stiffness coefficients were investigated by applying dynamic-force excitation over a range of excitation frequencies. Generally, the direct real parts of the dynamic-stiffness coefficients could be modeled as quadratic functions of the excitation frequency, and accounted for by adding a mass matrix to the conventional [K][C] model to produce a frequency-independent [K][C][M] model. Measured added-mass terms in the loaded direction approached 60 kg. The static load direction in the tests was y. The direct stiffness coefficients Kyy and Kxx depend strongly on the applied unit load, more so than speed. They generally increased linearly with load, shifting to a quadratic dependence at higher unit loads. At lower unit loads, Kyy and Kxx increase monotonically with running speed. The experimental results were compared with predictions from a bulk-flow computational fluid dynamics analysis. Stiffness orthotropy was apparent in test results, significantly more than predicted, and it became more pronounced at the heavier unit loads. Measured Kyy values were consistently higher than predicted, and measured Kxx values were lower. Comparing the LOP results to prior measured LBP results for the same bearing, at higher loads, Kyy is significantly larger for the LOP configuration than LBP. Measured values for Kxx are about the same for LOP and LBP. At low unit loads, stiffness orthotropy defined as Kyy/Kxx is the same for LOP and LBP, progressively increasing with increasing unit loads. At the highest unit load, Kyy/Kxx=2.1 for LOP and 1.7 for LBP. Measured direct damping coefficients Cxx and Cyy were insensitive to changes in either load or speed, in contrast to predictions of marked Cyy sensitivity for changes in the load. Only at the highest test speed of 13,000 rpm were the direct damping coefficients adequately predicted. No frequency dependency was observed for the direct damping coefficients.

Comparison of power loss and pad temperature for leading edge groove tilting pad journal bearings and conventional tilting pad journal bearings

A study was undertaken to compare power loss and pad temperature characteristics between LEG (leading edge groove) tilting pad journal bearings and conventional tilting pad journal bearings with and without a seal tooth. All test bearings were double tilting type with six-pad LOP (Load On Pad), 300.6 mm inner diameter, and 120.0 mm effective length. Pad temperatures and power losses were compared and evaluated versus rotor rotational speed, oil flow rate, and static load. Four kinds of tilting pad journal bearings were evaluated, conventional tilting pad journal bearings with and without a seal tooth and LEG tilting pad journal bearings with and without a seal tooth. Test results indicate that tilting pad journal bearings without a seal tooth have lower power loss and pad temperature than tilting pad journal bearings with a seal tooth. Especially, conventional tilting pad journal bearing without a seal tooth has the lowest power loss and pad temperature among the test bearings.

Poster - Thurs Eve-26: Influence of MLC leaf edge and tongue and groove effect on IMRT dose distributions.

An important consideration when using multileaf collimators (MLCs) for IMRT delivery is the correct account of leaf edge effects. To study these effects, two maps were constructed from a clinical beam's step and shoot delivery sequence: a time-weighted leaf edge position (LEP) map where the pixel intensity was proportional to the length of time that a leaf edge defined the edge of any segment within the field, and a (TG) map where pixel intensity was proportional to the length of time that adjacent segments matched along a leaf edge. We investigated the correlation between LEP or TG maps with dose error maps (obtained by subtracting calculated from either measured (CM) or from re-calculated (CC) data). Re-calculated data were obtained by modifying selected MLC photon modeling parameters from their commissioned values. We calculated the correlation coefficient between corresponding regions of CM and CC maps with TG and LEP maps. A NAT analysis of the CM maps indicated that the NAT index was minimized for tongue and groove width at the commissioned value of 0.1cm. A higher correlation coefficient was seen between CM and LEP maps (0.62±0.11) than between CM or CC and TG maps across all MLC modeling parameters used. The low correlation between both LEP or TG maps and CC maps suggests that the higher correlation observed between both LEP or TG maps and CM maps cannot be attributed to the choice of MLC modeling parameters alone. Further work is needed to pinpoint the cause of this correlation.

New insights into the collapsing of cylindrical thin-walled tubes under axial impact load

The current paper presents further investigations into the crushing behaviour of circular aluminium tubes subjected to axial impact load. Experiments prove that in order to achieve the real collapsing shape of tubes under axial loads in numerical simulations, an initial geometric imperfection corresponding to the plastic buckling modes should be introduced on the tube geometry before the impact event. In this study, it is shown that the collapsing shape of tube is affected by this initial imperfection and consequently it is shown that by applying an initial geometric imperfection similar to the axisymmetric plastic buckling mode, the tubes tend to collapse in a concertina mode. This phenomenon is studied for circular tubes subjected to axial impact load and two design methods are suggested to activate the axisymmetric plastic buckling mode, using an initial circumferential edge groove and using one- and two-rigid rings on the tube. In each case the broadening of the concertina collapsing region is estimated using numerical simulations. Experimental tests are performed to study the influence of cutting the edge groove on the collapsing mode. In order to optimize the crashworthiness parameters of the structure such as the absorbed energy, maximum deflection in axial direction, maximum reaction force, and mean reaction force, a system of neural networks is designed to reproduce the crushing behaviour of the structure, which is often non-smooth and highly non-linear in terms of the design variables (diameter, thickness, and length of tube). The finite-element code ABAQUS/Explicit is used to generate the training and test sets for the neural networks. The response surface of each objective function (crashworthiness parameters) against the change of design variables is calculated and both single-objective and multi-objective optimizations are carried out using the genetic algorithm.

Discussion on the problems related to NC machining of toroid-shaped taper cutter with constant angle between cutting edge and the cutter axis

Defining the helical angle as the angle between the cutting edge and revolving axis of the cutter presents the designing models of the cutting edge and helical groove of a toroid shaped taper cutter. Under the condition of a certain revolving speed, the relative radial and axial feeding speeds of the grinding wheel in two-axis NC machining of the cutter are deduced. The models for calculating the actual obtained groove and the simulation-method of computer are also provided.The surface near to the top of the end cutter has no helical cutting edges and infinite feeding speeds of NC machine. It is necessary to adopt a supplementary cutting edge. A supplementary cutting edge is designed as a cutting edge with a constant pitch. This paper provides valuable reference for the design and NC machining of this kind of toroid shaped taper cutter.

Finite Element Thermo-Hydrodynamic Analysis of a Circumferentially Grooved Floating Oil Ring Seal

This paper describes a finite-element solution of the nonlinear and coupled hydrodynamic and thermal equations for pressure and temperature profiles in an oil seal including the influence of circumferential grooves. Different models of grooves, considering and neglecting the entrance loss as the fluid passes over the groove, are analyzed. Perturbation analysis is used to determine the static and dynamic characteristics. The hydrodynamic forces are calculated by integrating the pressure distribution along and around the seal. This analysis shows that the smooth and rounded edge grooves tend to increase the leakage, and the size of the grooves significantly influences the axial and circumferential pressure variation in the groove. The results of grooved and ungrooved seals are compared and discussed. Presented at the 49th Annual Meeting in Pittsburgh, Pennsylvania May 1–5, 1994

Linear decorations defining edges of an internal octahedron within a natural diamond: observations and an explanation

A 2.13 carat weight natural diamond, a slightly flattened octahedron with roughly-pitted surfaces, exhibits an internal framework of dense black “lines” located a short distance below the present surface. Most of the lines lie closely parallel to 〈110〉 and connect up to delineate edges of an octahedron concentric with and slightly smaller than the present body. The lines are agglomerations of thin black discs (thickness too small to be measured in situ by optical microscopy), diameters ranging from 1 to 40 μm (but mainly in the 5 to 20 μm range), equantly distributed on diamond {111}, and consistent with graphite. Special optical techniques were developed for examining this specimen and they provided control of relative visibility of internal versus superficial features. Optical micrographs taken in all eight 〈111〉 directions are discussed. Synchroton X-ray topography detected the internal linear framework with good contrast, and helpfully displayed spatial relations between its segments. Findings from cathodoluminescence topography are described. An explanation of the phenomenon is outlined. It proposes a sequence of pressure and temperature conditions and of resulting stresses between diamond and encasing solid matrix whereby graphitization was caused to occur locally along edges of an octahedron, which was subsequently enclosed by further diamond growth. Implications for the origin of the edge grooves frequently exhibited by diamond octahedra are suggested.

Embryonic development of the shell in biomphalaria glabrata (Say).

During embryogenesis of the fresh water snail Biomphalaria glabrata (Say) (Pulmonata, Basommatophora) shell formation has been studied by light and electron microscopical techniques. The shell field invagination (SFI), the secretion of the first shell layers, the development of the shell-forming mantle edge gland and spindle formation have been investigated. During embryonic development at 28 degrees C environmental temperature, the shell field invaginates after 35 h. After 40 h the SFI is closed apically by cellular protrusions and scale-like precursors of the periostracum. The first electron translucent layer of the periostracum stems from electron dense vesicles of the cells which lie at the opening of the SFI. A second electron dense layer appears some hours afterwards. When the shell appears birefringent in the polarizing microscope (45 h of development) calcium can be detected in it using energy dispersive x-ray analysis. As calcification occurs the intercrystalline matrix appears under the periostracum and the SFI begins to open. In embryos of 60 h the mantle cavity appears at the left caudal side. When the mantle edge groove develops (65 h of development) lamellate units are added to the outer layer of the periostracum, but no distinct lamellar layer is formed in B. glabrata. In addition to the lamellar cell and the periostracum cell, a secretory cell can be observed in the developing groove. After 65 h of development, spindle formation starts and the shell begins to coil in a left hand spiral. After 5 days of development the embryos are ready to leave the egg capsules.

On-Line Estimation of Groove Wear in the Minor Cutting Edge for Finish Machining

The paper investigates the effective desection and estimation of groove wear at the minor cutting edge, which has a vital influence on surface quality in finish-machining. During bar-turning experiments with different cutting conditions, a miniature 3-D accelerometer, mounted in close vicinity to the tool tip, was used to measure the multivariate vibration signal produced by the turning process. The stochastic signal was modelled as autore gressive vector difference equations, and multiple dispersion analysis was used to quantify the complex interactions among various variables. It was found that the characteristic ingredient of vibration in the thrust direction, complemented by that in the main cutting direction, signifies effectively the critical point at which grooves are being “chiseled out”, surface roughness is deteriorating rapidly, and subsequently the finishing-tool needs to be replaced. The results are interpreted on a physically sound basis. The result also show that the algorithm developed is a feasible approach to on-line monitoring of minor cutting edge groove wear in finish-machining.

A new pressure sensor with innercompensation for nonlinearity and protection to overpressure

An improved silicon diaphragm pressure sensor with twin rectangular isles is presented. The twin rectangular silicon isles are formed by anisotropic etching and pattern compensation technology for convex corner undercutting. Two pairs of diffused piezoresistive gauges are located at the top surface of the edge grooves and the center groove formed by the twin isles respectively. Innercompensation of nonlinearity can be carried out by using this structure. The optimum compensation conditions have been investigated. An effective design of protection to overpressure is also presented. Its protection range is larger than 25 times the full-scale of the standard pressure range.

Finite element modelling of Bourdon tubes used in pressure and temperature gauges

A finite element program has been developed for the elastic analysis of Bourdon tubes used in both pressure and temperature sensing. The theory developed succeeds in enabling the three-dimensional Bourdon tube to be treated as a two-dimensional problem, by assuming that material properties and geometry do not change in one direction. The program was applied to several Bourdon tubes. Results for end-deflection data given by Bourdon tube manufacturers are compared with the finite-element-program results. These agree to within 10%. Also, stress patterns for two temperature Bourdon tubes are presented; one with edge grooves and one without. The post-processing showed that reduction in the level of stresses can be achieved by incorporating edge grooves. The use of such a model for Bourdon tube design is discussed briefly.

The Leading-Edge-Groove Tilting-Pad Thrust Bearing: Recent Developments

This paper compares the leading edge groove and pressurized supply (flooded) lubricant supply methods, and analyzes their influence on the performance of equalizing tilting pad thrust bearings. This paper presents new experimental data on 6-shoe, 267 mm (10 1/2 in.) O.D. bearings, operating at shaft speeds up to 14000 rpm, with loads ranging up to 3.45 MPa (500 psi) for two different lubricants. The data presented details the power loss and babbitt temperature performance of two versions of the leading-edge-groove bearing design and contrasts the results with a pressurized supply bearing design.

A Discussion on Tool Life Criteria and Total Failure Causes

Summary Criteria based on crater wear and flank, wear (wear land) often mask the influence of other types of wear like chip notching on the rake face, primary groove (outer diameter groove) on the side cutting edge and secondary groove (oxidation) on the end cutting edge, all three of which are reported in this paper. Total failure is often wrongly correlated with the size of flank wear or crater wear. Our observations of optical and scanning electron micrographs show that total failure occurs either when the crater breaks into the secondary groove or when the chip notch breaks Into the primary groove, resulting in severe burnishing of the workpiece surface. Amongst the established criteria, crater index and surface roughness Ra are fairly reliable indicators of total failure. This paper highlights work done with HSS, cemented tungsten carbide, cemented titanium carbide and ceramic tools.

Differential etching of ion-implanted garnet

A new technique for generating precise surface structures in single-crystal garnet materials is described. The crystals are ion implanted in localized areas using photoresist as an implantation mask. The photoresist is removed and the crystals are etched in phosphoric acid. The damaged volumes produced by the ion implantation etch at a substantially greater rate than the undamaged material, thereby producing surface structures. Groove depth can be precisely defined and reproduced by controlling the implantation parameters: ion species, energy, and dose. Undercutting is minimal and etching conditions are not stringent. The method is superior to both standard chemical etching and ion milling. The process has been characterized for H, He, and Ne implantations. It is shown that the temperature dependence of the etching rates for damaged and undamaged material is the same. The increase in etching rate is proportional to the damage concentration. The etching rate profiles are therefore synonymous with the damage profiles produced by the implantation. Etching rates more than three orders of magnitude greater than that of undamaged material have been observed. The etching rates for the three elements used correlate fairly well with the theoretical nuclear stopping power, and the groove depths obtained correlate with the theoretical ion ranges. This method has been used to produce serrated edge grooves which serve as rails for magnetic bubble propagation in conductor-groove bubble propagation circuits.