Two-Phase Flow Heat Transfer and Pressure Drop in Horizontal Scavenge Pipes in an Aero Engine

In modern aero engines the lubrication system plays a key role due to the demand for high reliability. Oil is used not only for the lubrication of bearings, gears or seals, but it also removes large amounts of the generated heat. Also, air from the compressor at elevated temperature is used for sealing the bearing chambers and additional heat is introduced into the oil through radiation, conduction and convection from the surroundings. The impact of excessive heat on the oil may lead to severe engine safety and reliability problems which can range from oil coking (carbon formation) to oil fires. Coking may lead to a gradual blockage of the oil tubes and subsequently increase the internal bearing chamber pressure. As a consequence, oil may migrate through the seals into the turbo machinery and cause contamination of the cabin air or ignite and cause failure of the engine. It is therefore very important for the oil system designer to be capable to predict the system’s functionality. Coking or oil ignition may occur not only inside the bearing chamber but also in the oil pipes which carry away the air and oil mixture from the bearing chamber. Bearing chambers usually have one pipe (vent pipe) at the top of the chamber and also one pipe (scavenge pipe) at the bottom which is attached to a scavenge pump. The vent pipe enables most of the sealing air to escape thus avoid over-pressurization in the bearing compartment. In a bearing chamber sealing air is the dominant medium in terms of volume occupation and also the in terms of causing expansion phenomena. The scavenge pipe carries away most of the oil from the bearing chamber but some air is also carried away. The heat transfer in vent pipes was investigated by Busam [1], [2]. Busam has experimentally developed a Nusselt number correlation for an annular flow in a vent pipe. For the heat transfer predictions in scavenge pipes no particular Nusselt number correlation exist. This paper intends to close the gap in this area. As part of the European Union funded research programme ELUBSYS (Engine LUBrication System TechnologieS), an attempt was done to simplify the oil system’s architecture. In order to better understand the flow in scavenge pipes, high speed video was taken in two sections of the pipe (vertical and horizontal). In the vertical section the flow was a wavy annular falling film whereas the flow in the horizontal section was a an unsteady wavy stratified/slug flow. Heat transfer has been investigated in the horizontal section of the scavenge pipe, leaving the investigation on the vertical section for later. Thanks to the provided extensive instrumentation, the thermal field in, on and around the pipe was recorded, evaluated and also numerically modeled using ANSYS CFX version 14 [23]. Brand new correlations for two-phase flow heat transfer (Nusselt number) and for pressure drop (friction coefficient) in horizontal scavenge pipes are the result of this work. The Nusselt number correlation has been developed in such a way that smooth transition (i.e. no discontinuity) from two-phase into single phase flow is observed. This work was funded and conducted within the 7th EU Frame Programme for Aeronautics and Transport (AAT.2008.4.2.3).

Transient Thermal Stress Analysis of the Typical Structures in Turbine Rotor Based on Thermomechanical Analysis

Thermal stress is one of the most important monitoring parameters in turbine rotors during the transition of work conditions. It has significant influence on the safety and life of turbine rotors. In order to obtain the thermal stress in turbine rotors during the transition process conveniently, the transient heat conduction process in typical structures of turbine rotors is analyzed based on heat conduction equation and finite element simulation. According to thermomechanical principle, thermal stress distributions in the typical structures of turbine rotor are investigated. The solutions of thermal stress in a solid cylinder are derived. A corner formed by shaft and disc is modeled and analyzed by finite element method, and the influences of structure parameters on thermal stresses are studied. The results on thermal stresses in cylinder and corner structures could provide a continent method to estimate thermal stress of turbine rotor on early design stage.

Computational Investigation of Flows and Pressure Fields Associated With Spur Gear Meshing

The efficiency of power transmission systems is increasingly targeted with a view to reducing parasitic losses and improving specific fuel consumption (SFC). One of the effects associated with such parasitic losses is the successive compression and expansion of fluid within the cavities between teeth of a meshing gear pair as they rotate. This process is cyclic and there are multiple cavities compressed and expanded at the same time. During the meshing process the volume of the cavity between the teeth suddenly contracts and as a result pressure rises. The fluid is therefore expelled primarily in the axial direction (for spur gears) since this area is considerably larger compared to the backlash area. Once the cavity starts to expand fluid is drawn into the cavity between the teeth by the negative pressure. Besides the air flow in the gear box, the meshing point is of particular interest to the oil flow, since oil is typically injected at or upstream of the meshing point. Good understanding of such flows can be used to balance lubrication needs with the need to minimise the required oil volumes and parasitic losses. This paper proposes the use of Computational Fluid Dynamics (CFD) as a means to investigate the phenomenon. A simplified two-dimensional CFD approach has been developed to study flows and pressure fields associated with spur gear meshing. The influence of the rotational speed has been investigated. Good validation is shown for the transient pressure variation within the tooth space. The limitations and potential applications of the modelling strategy are then discussed.

Development of a Conjugate Heat Transfer Simulation Methodology for Prediction of Steam Turbine Cool-Down Phenomena and Shell Deflection

Shell deflection during shutdown, cool-down process is a phenomenon well known to the steam turbine community. The main reason for this phenomenon is slower cooling of the top half shell and a relative faster cooling of the bottom half shell. There are multiple reasons for such thermal behavior of the two half casings, including natural heat convection from the bottom half to the top half, asymmetrical distribution of mass, dissimilar behavior of thermal insulation over the top and the bottom halves, etc. Shell deflection poses considerable challenge to the clearance engineer in terms of configuring operating clearance which ensures rub free operations. Understanding the cool-down process for the rotor is also equally important as the allowable steam inlet temperature during the hot or warm restart will depend on prevailing local temperature of the rotor. This paper describes an exemplary physics-based cool-down prediction methodology capable of accurately capturing the rotor cool-down process. The methodology involves development of a full 3D rotor casing thermal model, integrated conjugate heat transfer FE model and validated with measured field data.

Characterization of Advanced Combustor Cooling Concepts for Metallic Walls and Oxide Ceramic Matrix Composites in a Reacting Flow

Effusion or full-coverage cooling is a promising approach to cooling especially the walls of lean combustors where the cooling air consumption is to be reduced significantly. Due to typical velocity distributions and cooling air pressure drop in a combustor the effectiveness can be further increased by reducing the cooling air momentum. Double skin designs like impingement effusion cooling offer a significant improvement but at the drawback of a complex and expensive manufacturing process. In this contribution different advanced cooling concepts offering a similar reduction of the cooling air jet momentum in a single skin design for metal and ceramic walls are characterized under realistic conditions. Lateral trenches as well as effusion holes with 90° turns are used. Their total cooling effectiveness is compared to a plain single skin effusion cooling concept. A configuration with cooling air flowing parallel to the surface into lateral trenches revealed the highest and most uniform distribution of the cooling effectiveness. The metallic samples are manufactured using additive manufacturing offering additional degrees of freedom in the cooling design in comparison to conventional manufacturing techniques.

A Theoretical and Experimental Analysis of the Sealing Capability of a Membrane Seal

Improvements in turbine performance are increasingly being driven by the need to control leakage both in the main gas path as well as secondary air flow systems. Membrane seals have long been established as a method of sealing in some of the harshest of environments found in gas turbines. The membrane seal has a wide usage in gas turbines for stationary component interface sealing. The geometry is of plate construction with bulbous ends, the seals are assembled vertically and are retained by the component grooves. The grooves allow relative sliding and rotation against their surfaces a necessary feature, since during operation the seal needs to withstand relative movements due to thermal growth, vibratory forces, excitation and assembly loads. However, more accurate leakage estimates are required. Thus, in order to evaluate the complete performance characteristics of the seal for a wide range of working conditions, a theoretical and experimental campaign was undertaken. The membrane seal performance curves were created based on a series of tests performed in a specially designed rig. The rig utilised an actuation system that allowed for the precise adjustment of the seal’s relative position in two directions while performing the tests at a given working condition. It was noted that not only the movement and deformation of the membrane but also, assembly clearances and surface condition of the components have an impact on the seal’s performance. To assist in the understanding of the influence of the changing parameters on the performance of the seal an FEA study was undertaken employing known data to aid the understanding and improve the knowledge of how the seal behaves under specific engine conditions. The evaluation gives confidence in the experimental test results.

Performance of a Finned Turbine Rim Seal

In gas turbines, rim seals are fitted at the periphery of the wheel-space between the turbine disc and its adjacent casing; their purpose is to reduce the ingress of hot mainstream gases. A superposed sealant flow, bled from the compressor, is used to purge the wheel-space or at least dilute the ingress to an acceptable level. The ingress is caused by the circumferential variation of pressure in the turbine annulus radially outward of the seal. Engine designers often use double rim seals where the variation in pressure is attenuated in the outer wheel-space between the two seals. This paper describes experimental results from a research facility which models an axial turbine stage with engine-representative rim seals. The radial variation of CO2 gas concentration, swirl and pressure, in both the inner and outer wheel-space, are presented over a range of purge flow rates. The data are used to assess the performance of two seals: a datum double-rim seal and a derivative with a series of radial fins. The concept behind the finned seal is that the radial fins increase the swirl in the outer wheel-space; measurements of swirl show the captive fluid between the fins rotate with near solid body rotation. The improved attenuation of the pressure asymmetry, which governs the ingress, results in an improved performance of the inner geometry of the seal. The fins also increased the pressure in the outer wheel-space and reduced the ingress though the outer geometry of the seal.

CFD Simulation of an Aeroengine Bearing Chamber Using an Enhanced Volume of Fluid (VOF) Method: An Evaluation Using Adaptive Meshing

In aero engines, the rotating shafts are supported by a set of bearings, which are enclosed in bearing chambers. Cooling and lubrication oil escapes from the bearings and these chambers are designed to capture and recycle it. A good understanding of the oil behaviour inside bearing chambers is therefore desirable in order to limit the oil volume involved and minimize transmission losses whilst managing the engine core heat in the best possible manner. This study is focused on the simulation of the oil behaviour inside such a chamber and special attention is given to the so-called KIT bearing chamber. The oil phase in the chamber can take different forms e.g. sprays, droplets, thin films or a combination of those. Assuming the oil we want to track remains dominantly as a film and large droplets/filaments, the Volume of Fluid (VOF) method is used in order to track the oil and the oil/air interface in the chamber, hereby investigating the feasibility and merits of such an approach and extending the earlier work carried out by the authors and colleagues. An Enhanced VOF approach coupled with level-set is used here unless stated otherwise. The simulated pump outlet condition, proposed by the University of Nottingham, is also employed in this study, to replicate an engine displacement pump. Since the use of VOF requires a refined mesh in the oil region, an adaptive mesh approach based on the volume of fluid gradient is developed and validated to control the total cell count for some of the cases reported here and limit simulation costs. The Adaptive Mesh Approach (AMA) can allow a better resolution of critical interfaces, better compute the oil break-up (within the limitation of the physical models used) and then track the droplets and filaments. Therefore, not only the CPU time cost might be reduced compared to a fixed mesh approach but significant physical aspects of the problem should be better accounted for. In order to inform the set up and parameters used with this method, and appraise its value for the proposed application, the experimental study of Fabre is used before the approach is applied to the KIT chamber. Good insight is obtained in terms of run time acceleration for such problem when combining the proposed VOF setup with adaptive meshing. Key set up parameters are quantified. The simulations carried out with the proposed set up are proving to be fairly robust and stable. Qualitative (physical) evidence is also encouraging and confirms the value of such an approach to the study of aeroengine bearing chambers.

Pore-Level Numerical Simulation of Open-Cell Metal Foams With Application to Aero Engine Separators

In aero engines, the oil and air interaction within bearing chambers creates a complex two-phase flow. Since most aero engines use a close-loop oil system and releasing oil out is not acceptable, oil-air separation is essential. The oil originates from the engine transmission, the majority of which is scavenged out from the oil pump. The remainder exits via the air vents, where it goes to an air oil separator called a breather. In metal-foam-style breathers separation occurs by two physical processes. Firstly the largest droplets are centrifuged against the separator walls. Secondly, smaller droplets, which tend to follow the main air path, pass through the metal foam where they ideally should impact and coalesce on the material filaments and drift radially outwards, by the action of centrifugal forces. Although these devices have high separation efficiency, it is important to understand how these systems work to continue to improve separation and droplet capture. One approach to evaluate separation effectiveness is by means of Computational Fluid Dynamics. Numerical studies on breathers are quite scarce and have always employed simplified porous media approaches where a momentum sink is added into the momentum equations in order to account for the viscous and/or inertial losses due to the porous zone [1]. Furthermore, there have been no attempts that the authors know of to model the oil flow inside the porous medium of such devices. Normally, breathers employ a high porosity open-cell metal foam as the porous medium. The aim of this study is to perform a pore-level numerical simulation on a representative elementary volume (REV) of the metal foam with the purpose of determining its transport properties. The pore scale topology is represented firstly by an idealized geometry, namely the Weaire-Phelan cell [2]. The pressure drop and permeability are determined by the solution of the Navier-Stokes equations. Additionally, structural properties such as porosity, specific surface area and pore diameter are calculated. The same procedure is then applied to a 3D digital representation of a metallic foam sample generated by X-ray tomography scans [3]. Both geometries are compared against each other and experimental data for validation. Preliminary simulations with the X-ray scanned model have tended to under predict the pressure drop when compared to in-house experimental data. Additionally, the few existing studies on flow in metal foams have tended to consider laminar flow; this is not the case here and this also raises the question that Reynolds-averaged turbulence models might not be well suited to flows at such small scales, which this paper considers.

CFD Investigation of a Fluidic Device for Modulation of Aero-Engine Cooling Air

The quantity of cooling air delivered by the secondary air system to various engine components is usually fixed by cooling requirements at the most arduous operating condition in the flight cycle. Modulation of cooling air would allow optimization of cooling supply at different flight cycle conditions, giving significant performance benefits. Switched vortex valves (SVV) have been proposed for control of air systems [1]. An important characteristics of this device is the absence of any moving part. This offers advantages compared to other systems. This report discusses the numerical study of a typical SVV. The study includes comparison of predicted results with available experimental data and prediction of switching characteristics of the device. In this study two turbulence models namely the Spalart-Allmaras model (SA) and Reynolds stress model (RSM) were used. The RSM showed a good agreement with measured mass flow rate and qualitative agreement with other experimental observations.