CFD study of oil-jet gear interaction flow phenomena in spur gears

Oil-jet lubrication and cooling of high-speed gears is frequently employed in aeronautical systems, such as novel high-bypass civil aero engines based on the geared turbofan technology. Using such oil-jet system, practitioners aim to achieve high cooling rates on the flanks of the highly thermally loaded gears with minimum oil usage. Thus, for an optimal design, detailed knowledge about the flow processes is desired. These involve the oil exiting the nozzle, the oil impacting on the gear teeth, the oil spreading on the flanks, the subsequent oil fling-off, as well as the effect of the design parameters on the oil flow. Better understanding of these processes will improve the nozzle design phase, e.g. regarding the nozzle positioning and orientation, as well as the nozzle sizing and operation. Most related studies focus on the impingement depth to characterize the two-phase flow. However, the level of information of this scalar value is rather low for a complete description of the highly dynamic three-dimensional flow. Motivated by the advancements in numerical methods and the computational resources available nowadays, the investigation of the oil-jet gear interaction by means of computational fluid dynamics (CFD) has come into focus lately. In this work, a numerical setup based on the volume-of-fluid method is presented and employed to investigate the two-phase flow phenomena occurring in the vicinity of the gear teeth. The setup consists of a single oil-jet impinging on a single rotating spur gear. By introducing new metrics for characterizing the flow phenomena, extensive use of the possibilities of modern CFD is made, allowing a detailed transient and spatially resolved flow analysis. Thus, not only the impingement depth, but also the temporal and spatial evolution of wetted areas on the gear flanks, as well as the evolution of the oil volume in contact with the gear flanks are extracted from the simulation data and compared in a CFD study. The study consists of 21 different simulation cases, whereby the effect of varying the jet velocity, the jet inclination angle, the jet diameter, and the gear speed are examined. Consistent results compared to a simplified analytical approach for the impinging depth are obtained and the results for the newly introduced metrics are presented

Heat Transfer by Impingement Cooling of Spur Gears

Lower specific fuel consumption as well as noise reduction are the main goals in the sector of civilaeronautics engineering nowadays. One prominent concept of achieving these goals is the gearedturbofan engine, in which a planetary gear box is installed between the low pressure spool and thefan. This allows the low pressure turbine as well as the fan to rotate at optimum speeds. This way, thesame power can be generated by fewer stages in the faster rotating turbine, which in turn compensatesthe additional weight of the gear box. The main advantage of the geared turbofan is the possibilityto further increase the fan diameter and therefore improve the propulsion efficiency by means of ahigher bypass ratio. One crucial feature of the gear box is the cooling system needed to ensure safeoperating conditions during all phases of the flight envelope. For an efficient cooling system, optimizedwith respect to weight and cost, the heat transfer between the cooling fluid and the gears needs tobe understood thoroughly. In this study, the impingement cooling of spur gears by oil jets is forthe first time examined analytically and compared to experimental results. This provides knowledgeabout the evolution of the heat transfer coefficient distribution resulting from the cooling fluid flowrate and the gear speed, as well as a deep understanding of the underlying phenomena causing thisbehavior. The analytical solution process comprises of two calculation steps. First, the size of the oilfilm is calculated and secondly, the heat transfer across this surface is evaluated while the oil film isflung off the tooth flank by centrifugal forces. The parameters varied in this study were the oil flowrate, the rotational speed of the spur gear and the oil jet angle. The theoretical results are in goodagreement with the experimental data. The theoretical approach can therefore be applied as a newand efficient tool to estimate the global heat transfer coefficient of impingement cooled spur gears.Furthermore, the validated tool can be used as boundary condition for thermal models of spur gearsand help optimize the impingement cooling oil systems

Numerische Simulation der Ölstrahl-Zahnrad-Interaktion bei Flugtriebwerken mit hohem Nebenstromverhältnis: Stand der Forschung

Anforderungen aus der Flugtriebwerksentwicklung erfordern eine noch effizientere Gestaltung des Ölsystems zur Kühlung und Schmierung von schnelllaufenden Untersetzungsgetrieben. Deshalb unternehmen Wissenschaftler weltweit Anstrengungen, um die bei hohen Drehzahlen angewandte Ölstrahlschmierung und -kühlung zu erforschen. Fortschritte bei den CFD Methoden und gestiegene Rechenkapazitäten führen zu einem gestiegenen Einsatz numerischer Methoden zur Untersuchung der Ölstrahl-Zahnrad-Interaktion, wobei hauptsächlich die Volume-of-Fluid und die Smoothed Particle Hydrodynamics Methode eingesetzt werden. In diesem Übersichtsartikel wird der Stand der Forschung in diesem Bereich dargestellt und die Vor- und Nachteile der eingesetzten Methoden skizziert

Extensive Investigations on Radial Crack Formation in Labyrinth Seals of Aircraft Engines

Labyrinth seals are due to their non-contact function and their thermal resistance the state of the art in sealing technology for thermal turbomachinery such as jet engines. The purpose of labyrinth seals is to adjust or minimize the leakage between rotor and stator. However, because of transient flight manoeuvres and the resulting mechanical and thermal loads on the components, contact between rotating and stationary parts cannot be ruled out. The aim of this study is to extend the basic understanding of the mechanisms of hot crack formation in the components of a labyrinth seal during the rubbing process. Previous investigations at the Institute of Thermal Turbomachinery have shown that the thermally induced stress caused in connection with the thermal change of the material properties can lead to formation of the cracks in the seal fin. In order to get a deeper understanding of the complex crack initiation mechanisms, it is necessary to carry out fundamental experiments on simplified geometries. For this purpose, the stress conditions of a loaded labyrinth seal are applied to a cylindrical test specimen. It is fixed on both sides and subjected to thermo-cyclic load. To model a rubbing process of a labyrinth seal, the initial temperature of the cycle corresponds to the operating temperature of the rotor in a jet engine and the final temperature corresponds to the contact temperature during the rubbing process. The temperature change causes stress in the specimen due to the rigid mounting. In literature there are analytical considerations regarding the stress-strain curve in the seal fin and a possible pre-damage of the components by remaining tensile stress after the rubbing process. In this paper, these analytical approaches are compared with the test data. Also a residual tensile strength of the seal fin which is still to be tolerated is worked out depending on the temperature load. The stress reduction during heating is very significant and is considered in detail. Due to the high mechanical and thermal loads, the stress reduction is superimposed by relaxation and plastic deformation. For a separate consideration of these basic effects, FE simulations are carried out. The main findings are that stress and strain in the component depends on both the initial and the final temperature of the specimen. The final rubbing temperature mainly influences the remaining tensile stress. With the thus calculated remaining strength of the seal fin a probable crack formation could be determined. The evaluation of this data contributes significantly to the understanding of hot crack formation in labyrinth seal fins and is therefore the basis for the optimization of the sealing system with regard to robustness against rubbing processes

Experimental study of the pressure loss in aero-engine air-oil separators

The results of extensive experimental testing of an aero-engine air-oil separator are presented and discussed. The study focuses on the pressure loss of the system. Oil enters the device in the form of dispersed droplets. Subsequently, separation occurs by centrifuging larger droplets towards the outer walls and by film formation at the inner surface of a rotating porous material, namely an open-cell metal foam. The work described here is part of a study led jointly by the Karlsruhe Institute of Technology (KIT) and the University of Nottingham (UNott) within a recent EU project. The goal of the research is to increase the separation efficiency to mitigate oil consumption and emissions, while keeping the pressure loss as low as possible. The aim is to determine the influencing factors on pressure loss and separation efficiency. With this knowledge, a correlation can eventually be derived. Experiments were conducted for three different separator configurations, one without a metal foam and two with metal foams of different pore sizes. For each configuration, a variety of engine-like conditions of air mass flow rate, rotational speed and droplet size was investigated. The experimental results were used to validate and improve the numerical modelling. Results for the pressure drop and its dependencies on air mass flow rate and the rotational speed were analysed. It is shown that the swirling flow and the dissipation of angular momentum are the most important contributors to the pressure drop, besides the losses due to friction and dissipation caused by the flow passing the metal foam. It was found that the ratio of the rotor speed and the tangential velocity of the fluid is an important parameter to describe the influence of rotation on the pressure loss. Contrary to expectations, the pressure loss is not necessarily increased with a metal foam installed