The development and limitations of a numerical modelling framework applied to an aero-engine air/oil separator are presented here. Oil enters the device in the form of dispersed droplets and primary separation occurs by centrifuging larger droplets towards the outer walls, whereas secondary separation occurs by partially coalescing and centrifuging smaller droplets within a porous material, namely an open-cell metal foam. The work described here is part of a study led jointly by the University of Nottingham (UNott) and the Karlsruhe Institute of Technology (KIT) in the Engine Breakthrough Components and Subsystems (E-BREAK) project. The main objectives for UNott have been to define a CFD methodology able to provide an accurate representation of the air flow behaviour and a qualitative assessment of the oil capture within the air/oil separator. The feasibility of using the current state-of-the-art modelling framework is assessed. Experimental measurements of the overall pressure drop and oil capture performed at KIT are used to validate the simulations. The methodology presented here overcomes some limitations and simplifications present in previous studies. A novel macroscopic model for the secondary oil separation phenomena within metal foams is presented. Experiments and simulations 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 variation of air flow, shaft speed and droplet size was conducted. The focus was on the separation of droplets with a diameter smaller than 10 μm. Single-phase air flow simulation results showed that overall pressure drop increases with both increased shaft speed and air flow, largely in agreement with the experiments. Oil capture results proved to be more difficult to be captured by the numerical model. One of the limitations of the modelling set-up employed here is not capable of capturing droplet re-entrainment due to accumulation of oil inside the metal foam, which is believed to play a significant role in the separation phenomena.

ABSTRACT 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.

In this research paper, the cooling process of an impingement cooled spur gear is examined by means of an analytical model. The process is modeled as a coolant film, which is flung off a rotating gear tooth flank by centrifugal forces. During the process, heat is transferred from the isothermal gear tooth flank to the coolant film. With a numerical solution to the analytical model, a formulation for the transient local Nusselt number is derived. The evaluation of the numerical solution revealed that the heat transfer is dominated by heat conduction in the coolant film. The heat transfer process ends when the thermal capacity of the coolant film is reached. The transient Nusselt number is used to derive a time-averaged and a global heat transfer coefficient. Furthermore, the influence of the initial coolant film height is examined by using a modified version of the analytical model. The global heat transfer coefficient decreases toward smaller initial cooling film heights. The analytical model is then extended to include the temperature dependency of the viscosity of the coolant. A viscosity that decreases with increasing temperature yields a moderate decrease in heat transfer. A discussion is presented regarding the applicability of the analytical model toward impingement cooled spur gears. The effect of the simplifications made in the derivation of the analytical model is outlined and assessed with regard to the heat transfer mechanism.

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The transport of a flow from a static system into a rotating system can be realized by means of orifices in the rotating wall. In this paper the experimental study of a liquid flowing through a radial, sharp-edged orifice with a l/d-ratio of 1.56 in a rotating shaft is presented. The discharge coefficient cd for (circumferential) orifice speeds of up to 24 m s−1 and Reynolds numbers ranging from Red = 1.2 × 104 to 3.4 × 104 is evaluated for an oil with a density of 920 kg m−3 and a kinematic viscosity of 5.3 × 10−6 m s−2. A modular test rig was designed, consisting of two concentric rotating shafts forming an annular duct. The outer shaft is fitted with the orifices through which the liquid passes from the static into the rotating system. The modularity allows the exchange of the shaft element containing the orifices. For this study two shaft elements with 5 or 12 radial, cylindrical, sharp-edged orifices were used. Thus, a wider range of flow velocities through a single orifice was achieved. This study is the first to illustrate the effect of cavitation in a rotating orifice. Outside the cavitation regime a change of the approaching flow represented by the velocity ratio causes a change of the discharge coefficient while within the cavitation regime cd additionally depends on the cavitation parameter. A relationship for the flow contraction in the cavitation regime depending on the orifice velocity and the pressures upstream and downstream of the orifice is derived. For a second set of orifices, where the liquid exits the rotor into the surrounding air, a significant regime change depending on the ratio of orifice rotational speed and flow velocity occurs. For higher flow velocities through the orifice this change occurs for lower orifice speeds. A likely cause is the onset of cavitation.

Since every new scientific work starts with a comprehensive literature study about the state of the art, this paper should give an overview about previous, but still useful, and more recent publications to describe a secondary air system (SAS) for gas turbines. Nevertheless, in current times it is impossible to present a complete research, but the well-disposed reader will get a meaningful insight and may use the references as a start for their own detailed search. The first part will give a description of a typical set up of a SAS and the common flow elements that can be found along the flow path. It also includes an explanation of the elements operating behavior, especially regarding the pressure and temperature changes as well as the resulting air mass flow. In the second part the authors describe the plans at the German Aerospace Centers (DLR) Institute of Test and Simulation for Gas Turbines in the SAS’ field of research.