Within large civil aeroengines a significant contributor to parasitic power loss (manifest as increased heat-to-oil) is the internal gearbox (IGB). An IGB typically contains high speed shafts, a spiral bevel gear pair, bearings and seals as well as the complex geometry of the stationary components. The University of Nottingham Technology Centre in Gas Turbine Transmission Systems (UTC) has conducted experimental and computational projects to enhance understanding of two phase flow behaviour. Validated single phase modelling capability for an unmeshed shrouded crown gear has been established [1–3] and discrete phase modelling [3] has been applied to investigate the oil path under the shroud. Experimental work on shroud configurations [5] and two-phase flow [6] has also been conducted at the UTC using a rig with representative but simplified geometry relative to an aeroengine.
Recent modelling activity has focussed on the region behind the gear. In an aeroengine this region includes a large shaft location bearing that sheds oil into the rear chamber. This oil, combined with the high speed and complex airflows generated by proximity to the gear, makes this region particularly challenging to model. In the experimental test rig at the UTC this zone does not contain a bearing and so as yet no validation data exists.
In this paper an axisymmetric sector model of the rig back chamber is presented. Two phase flow behaviour was modelled using the Volume of Fluid (VOF) and Eulerian models within the CFD software ANSYS-Fluent. A comparison between these two multiphase models is made and their suitability to model the oil behaviour in the back chamber is discussed. Oil flow behaviour in this region is also reported.
The CFD results show that the VOF model is insufficient for predicting oil flows in this environment. Although there is a significant amount of liquid present as wall film, the liquid not on the walls appears important and is not adequately modelled by VOF, which is well-known as being most suitable where there is a definite interface between liquid and gaseous phases. The Eulerian model shows significantly more likely flow behaviour with results indicating a non-uniform distribution of oil across the axial length of the rear chamber with a bias towards the rear (bearing side). The air jet entering in the rear chamber from between the gear and shroud strongly influences flow behaviour in the rear chamber. The computed flow field is such that a full 360° model is recommended for future work of this nature.
Computational study of two phase flow in pressure swirl atomizer using entirely Eulerian model