Budi Chandra

Study of Gas/Liquid Behaviour Within an Aeroengine Bearing Chamber

Aeroengine bearing chambers typically contain bearings, seals, shafts and static parts. Oil is introduced for lubrication and cooling and this creates a two phase flow environment that may contain droplets, mist, film, ligaments, froth or foam and liquid pools. Some regions of the chamber contain a highly rotating air flow such that there are zones where the flow is gravity dominated and zones where it is rotation dominated.The University of Nottingham Technology Centre in Gas Turbine Transmission Systems, is conducting an ongoing experimental program investigating liquid and gas flow behavior in a relevant highly rotating environment. Previously reported work by Chandra et al [1, 2] has investigated film thickness and residence volume within a simplified chamber consisting of outer cylindrical chamber, inner rotating shaft and cuboid off-take geometry (termed the generic deep sump).Recently a more aeroengine relevant bearing chamber offtake geometry has been studied. This geometry is similar to one investigated by Chandra [3] at Purdue University and consists of a “sub-sump” region approached by curved surfaces linked to the bearing chamber.The test chamber consists of an outer, stationary cylinder with an inner rotating shaft. The rig runs at ambient pressure and the working fluid (water) is introduced either via a film generator on the chamber wall or through holes in the shaft. In addition to visual data (high speed and normal video), liquid residence volume within the chamber and film thickness were the two numerical comparators chosen. Data was obtained for a number of liquid supply rates, scavenge ratios and shaft rotation speeds.The data from the current model is compared to that from the earlier studies [1, 2, & 3]. The data shows that in contrast to the previously reported generic deep sump study, the residence volume of the curved wall deep sump (CWDS) designs is far less sensitive to shaft speed, liquid supply rate and scavenge ratio. The method of liquid supply only makes a significant difference at the lowest scavenge ratios. Residence volume data for the Nottingham CWDS is comparable, when appropriately scaled, to that for the Purdue design.The film thickness data shows that at the lower shaft speeds investigated the flow is gravity dominated whereas at higher shaft speeds shear dominates.© 2012 ASME

Factors Affecting Oil Removal From an Aeroengine Bearing Chamber

Aeroengines incorporate various bearing chambers that house the shaft bearings and the oil used to cool and lubricate these bearings must subsequently be recovered from these chambers. Effective oil removal (scavenge) is essential to avoid heat generation through unnecessary working of the oil which can lead to excessive heat generation and reduced overall efficiency. Therefore the design of the scavenge region (sump) in a bearing chamber, as well as the ability to assess its performance is very important. An ongoing research program into bearing chamber scavenge comprising experimental and computational components is being conducted at the University of Nottingham Technology Centre in Gas Turbine Transmission Systems. This program is enhancing understanding of sump performance and design. In this paper an experimental study into a simplified but representative scavenge is reported. This experimental work helps to further understanding of the complex two-phase flow physics in a bearing chamber, particularly in the scavenge region, by means of various measurements and flow visualization. For the study a bespoke test rig has been built. It consists of a simplified, generic bearing chamber with simple sump geometry constructed entirely of Perspex to allow visualization. A shaft in the centre of the chamber capable of rotating up to 15,000 rpm is employed to introduce a windage flow in the chamber. Water (the working fluid) is fed to the chamber via an inlet pump and an outlet pump removes liquid from the chamber, closing the circuit. Several pneumatic pinch valves are installed in the flow circuit to allow residence volume measurement. A completely air-tight reservoir with internal baffle functions as a simple liquid-gas separator, allowing measurement of gas volumetric flow rate in the off-take pipe; hence the scavenge ratio (ratio of total exit volume to liquid volume) can be obtained. Residence volume measurements highlight the importance of sump geometry as an ill-designed sump can lead to an undesirable increase in residence volume.Copyright

Experimental investigation into Gas Turbine Oil Scoop Capture Efficiency

Experimental research was conducted into a scooped rotor system that captures oil from a stationary jet and directs it through passages within the shaft to another axial location. Such a system has benefits for delivering oil via under-race feed to aeroengine bearings where direct access is limited. Oil capture efficiency was calculated for three jet configurations, a range of geometric variations relative to a baseline and a range of operating conditions. Flow visualization techniques yielded high-speed imaging in the vicinity of the scoop leading edge. Overall capture efficiency depends on the amount of oil initially captured by the scoop that is retained. Observation shows that when the jet hits the tip of a scoop element, it is sliced and deflected upwards in a ‘plume’. Ligaments and drops formed from this plume are not captured. In addition, some oil initially captured is flung outwards as a consequence of centrifugal force. Although in principle capture of the entire supply is possible over most of the shaft speed range, as demonstrated by a simplified geometric model, in practice 60% to 70% is typical. Significant improvement in capture efficiency was obtained with a lower jet angle (more radial) compared to baseline. Higher capture efficiencies were found where the ratio of jet to scoop tip speed was lower. This research confirms the capability of a scoop system to capture and retain delivered oil. Additional numerical and experimental work, is recommended to further optimise the geometry and increase the investigated temperature and pressure ranges.

Mechanisms for Residence Volume Reduction in Shallow Sump

© 2018 by ASME. Gas turbine aero-engines employ fast rotating shafts that are supported by bearings at several axial locations along the engine. Due to extreme load and heat, oil is injected to the bearings to aid lubrication and cooling. The oil is then shed to the bearing chamber before it is extracted out by a scavenge pump. Scavenging oil from the bearing chamber is challenging due to high windage induced by the fast rotating shafts as well as the two-phase nature of the flow. A deep sump has been found to increase scavenge performance due to its ability to shelter the pooled oil from the bulk rotating air flow thus minimizing two-phase mixing. However, in many cases, a deep sump is not an option due to conflicting space requirements. The space limitation becomes more stringent with higher bypass ratio engines as the core becomes smaller. Therefore, it is imperative to have a high performing shallow sump. However, shape modification of a shallow sump is too constrained due to limited space and, therefore, has minimal impact on the scavenge performance. This research presents several alternative concepts to improve scavenge performance of a generic baseline shallow sump by augmenting it with attachments or inserts. These augmentations attempt to exploit two known mechanisms for reducing the residence volume: momentum reduction and sheltering. The experimental results show that some augmentations are able to reduce the residence volume of a shallow sump by up to 50% or more in some cases.

Experimental Optimization of Rolls-Royce AE3007 Sump Design

The original sump design of Rolls-Royce AE3007 central sump is a tangential sump. An experimental program at Purdue University was conducted to investigate the characteristics of a tangential sump design. The research employed a bespoke experimental rig consisting of modular transparent chamber to allow unprecedented view of the two-phase flow inside the bearing chamber. It was found that a persistent liquid pooling near the drain entrance of a tangential sump obstructs the outflow to the scavenge off-take. While space requirement is minimal, the scavenge performance of a tangential sump is poor. A prototype advanced sump was proposed and built with features to address various issues found in the tangential sump. In this paper, further experimental work on the refinements of the advanced sump is presented. This includes experimental study on the effect of a fence and splitter plate, off-take hole diameter and its location, the effect of gravity by tilting, upstream wall fairing, as well as the depth of the sump itself. The experiments showed that the faired upstream wall modification can reduce residence volume significantly across different operating conditions, indicating improvement in scavenge performance. The other modifications help to reduce residence volume only in certain operating conditions. The results of this study helped towards the birth of an optimized sump design, known as Indy sump. The Indy sump has been accepted as a superior sump design and has been used as a benchmark in many sump design studies.

Computational Study of a Customised Shallow-Sump Aero-Engine Bearing Chamber With Inserts to Improve Oil Residence Volume

An aero-engine bearing chamber is a structure that is used to contain and collect oil used in lubricating and cooling the bearings supporting the high-speed engine shafts. There are various bearings in an aero-engine. Within the bearing chambers, there are typically the bearings, rotating shafts, seals and gears (in some designs). The walls of the bearing chamber are stationary and there are vents and sumps to take out the oil, via an offtake pipe, and the sealing air. The oil collected via the sump and vents is recycled and used again in the loop. To prevent oil degradation and reduce chance of coking in the chamber, it is desired that all of the oil goes through the recycling loop, with no oil staying longer than necessary in the chamber. The sealing air is used to maintain a positive pressure to keep the oil within the chamber. The flow inside a bearing chamber is highly turbulent and consists of a rotating mixture of oil and air. A smaller amount of the oil, mostly as oil-droplets, exits at the vents and is separated from the air using de-aerators [1]. It is expected that by gravity, most of the oil collects at the sump and can be easily scavenged. This is provided the sump can be large enough. The geometry of a bearing chamber is, however, complex largely because of space limitations. It is very important that oil is not resident longer than necessary to prevent over-heating and therefore deterioration or coking. Experimental observations by Chandra & Simmons [2], have shown that bearing chambers with deep sumps perform better that those with shallow sumps. Since shallow sumps are inevitable, a number of innovative studies have been done to improve bearing chamber designs. The presence of air in the oil (e.g. as bubbles) reduces the efficiency of the scavenging pump. Other factors such as oil momentum and windage can take oil away from the off-take pipe potentially increasing oil residence volume. Chandra & Simmons [2] placed inserts such as grille cover, perforated plate, etc, on a side of the bearing wall and improvements in the residence volume were seen. In this work, we are looking at a detailed computational fluid dynamics (CFD) simulation of one of the inserts that performed well. This will aid understanding of the flow characteristics of using an insert to improve oil residence in a bearing chamber.

Innovative Shallow Sump Customizations for Aero-Engine Bearing Chambers

Aero-engines incorporate various bearing chambers and these typically contain bearings, seals, rotating shafts, stationary walls and struts, and sometimes gears. Oil is supplied for lubrication and cooling and is removed (scavenged) from the sump region of the chamber (note that in some parts of the world the entire bearing chamber is referred to as the sump). Depending on the location and function of the bearing chamber, the sump region may be deep or shallow. Effective oil removal is essential as unnecessary working of the oil can lead to excessive heat generation and reduced overall efficiency. Therefore the design of the scavenge region in a bearing chamber, as well as the ability to assess its performance is very important. Previous work, much of which was conducted at the University of Nottingham Technology Centre in Gas Turbine Transmission Systems (UTC) suggests that oil often does not flow cleanly into the off-take due to a combination of several factors: oil momentum, windage, three-dimensional air flow that blocks the off-take flow or transports oil away from the off-take, and pooling because of separated air flow that acts on the oil once oil momentum is dissipated. Experimental research at the UTC found that scavenge performance is highly affected by the sump geometry, especially its depth. Variations of shallow sumps, although some are better than the others, cannot offer the same level of performance as a deep one. However space limitation in an engine often only allows for a shallow sump. This paper presents some experimental exploration on new design ideas. They are in the form of various inserts and attachments that were designed to improve scavenge performance of a shallow sump. These “custom” sumps were tested on the UTC’s scavenge test facility at various flow settings (wall film/flying droplets, liquid flow rates, scavenge ratios, shaft speeds). The residence volumes were measured and compared to a baseline configuration with reduction in residence volume desirable. The inserts tested were a Grille Cover, a Stepped Spillway, a Perforated Plate and a Porous Insert. Both the Porous Insert and the Perforated Plate showed reduced residence volumes in the demanding droplet/windage dominated flow condition with the Perforated Plate offering the best improvement over baseline.

Experimental Investigation Into the Performance of Shallow Aeroengine Off-Takes

Oil removal (scavenge) from aeroengine bearing chambers is an ongoing challenge for aeroengine designers. Effective scavenging of oil is necessary to avoid excessive heat generation, degradation of oil properties and deterioration in heat transfer functionality. However the task of oil removal is not trivial. Oil is entrained in a highly rotating environment induced by the rotating shafts. Simply removing a larger volume of fluid from the chamber with the scavenge pump can create higher air flowrates but lead to oil becoming trapped in the chamber and so no reduction in residence volume (the amount of oil present in the chamber during operation).The University of Nottingham Technology Centre in Gas Turbine Transmissions Systems has been conducting experiments investigating two phase behaviour within a simplified aeroengine bearing chamber operating at ambient pressures with water as the working liquid. The rig is constructed from polycarbonate enabling good visual access. In the chamber offtake region a number of behaviours can be observed relating to the hydraulic uplift and general flow behaviour as gas and liquid exit the chamber. Chandra et al [1] reports a parametric study using a design of experiments approach into geometrical variants of a shallow offtake region defined by curved approaches and a small offtake volume. Phenomenlogical factors were quantified and used to identify the best performing geometry. Previous work at the UTC [2, 3] has used residence volume as the primary comparative performance parameter.In this paper residence volume data is obtained for two sumps for which phenomenological data exists. The paper compares performance on the basis of these visual factors with performance on the basis of residence volume and concludes that although both frameworks have value, they do not lead to identical conclusions for all operating conditions. In film dominated cases significant hydraulic uplift usually corresponds to larger residence volume but for droplet dominated cases this is not necessarily so.Copyright

Transient Two-Phase Effects in an Aeroengine Bearing Chamber Scavenge Test Rig

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Performance Comparison for Aeroengine-Type Sump Geometries

Oil removal from aeroengine bearing chambers presents a challenge for aeroengine designers. Effective scavenging of oil is necessary to avoid excessive heat in the bearing chamber as this may lead to degradation of oil properties and deterioration in heat transfer functionality. However the task of oil removal is not trivial. Oil is entrained in a highly rotating environment induced by rotating shafts. An aeroengine scavenge pump must remove both air and oil from the chamber and the ratio of the two fluids depends on pump operating point as well as the geometry of the exit region. The University of Nottingham Technology Centre in Gas Turbine Transmission Systems is conducting an on-going experimental programme investigating liquid and gas flow behaviour in a relevant highly rotating environment. Previously reported work [2, 4] in combination with work conducted by Purdue University [3, 5] suggests that a deeper sump may help to reduce residence volume. More recently a study has been conducted that investigates geometries incorporating features sometimes included in aeroengine scavenge. Three very different exit geometries were investigated experimentally using chamber residence volume as a parameter of comparison. Data for one of these, the so called Curved Wall Deep Sump (CWDS), has already been reported in [4]. This paper presents residence volume data for all three sumps over a range of shaft speeds, inlet flow rates and scavenge ratios. Trends are analysed and presented and areas for future work identified. Copyright