Limitations of a State-of-the-Art Numerical Modelling Framework for Two-Phase Flow in Aero-Engine Air/Oil Separators

Abstract

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.

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

“…First, the experimental results are presented and discussed. Second, the observed pressure loss behavior will be analysed and described phenomenologically using simple analytical modelling and Computational Fluid Dynamics (CFD) results obtained Experimental study of air-oil separators... by Carvalho et al (14) . This approach allows both identification and quantification of the dominating effects.…”

“…Knowing the Table 2 Top . Static pressure contour plot for configurations without and with a fine metal foam without rotation at 20 g/s air mass flow rate (14) . Both outlet shafts have been shortened.…”

“…The corresponding values of the radius, tangential velocity and of the resulting coefficients C and n for the non-rotating test cases are summarised in Table 2. They were determined assuming incompressible flow conditions, as Ma < 0.3 according to the CFD results obtained by de Carvalho et al (14) . Using the respective inlet velocity v φ0 as a reference, the values for C and n only depend on the respective inlet conditions.…”

“…Equation (5) applies to the rotating case as well, as will be discussed in the next section.

Figure 6. Static pressure contour plot for configurations without and with a fine metal foam without rotation at 20 g/s air mass flow rate (14) . Both outlet shafts have been shortened.

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“…Numerical results are used to support and explain the experimental findings. The numerical set-up and results were presented by de Carvalho et al (14) .…”